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ORDNANCE DEPARTMENT DOCUMENT No 2037 
 
 THE 
 
 MANUFACTURE OF OPTICAL 
 
 GLASS AND OF OPTICAL 
 
 SYSTEMS 
 
 A WAR-TIME PROBLEM 
 
 May, 1921 
 
 WASHINGTON 
 
 GOVERNMENT PRINTING OFFICE 
 1921 
 

 
 
 ORDNANCE DEPARTMENT 
 
 Document No. 2037 
 Office of the Chief of Ordnance 
 
TABLE OF CONTENTS. 
 
 Page. 
 
 CHAPTER I . Introduction 5 
 
 The manufacturing problem 
 
 Production statistics 14 
 
 CHAPTER II. The characteristics of optical glass 16 
 
 The functions of the eye 16 
 
 Intensity of illumination 18 
 
 Field of view 19 
 
 The lens system of a telescope 39 
 
 The quality of the image 22 
 
 Monochromatic aberrations 24 
 
 Axial spherical aberration 24 
 
 Coma, sine condition 25 
 
 Astigmatism; curvature of field, distortion 26 
 
 Chromatic aberrations 26 
 
 Chromatic aberration or axial chromatism 26 
 
 Chromatic differences of magnification; lateral chromatism 27 
 
 The characteristics of optical glass 28 
 
 Homogeneity 29 
 
 Uniformity in chemical composition 29 
 
 Strise and cords 29 
 
 Bubbles and seeds. 33 
 
 Stones 35 
 
 Crystallization bodies, cloudiness 37 
 
 Uniformity in physical state 37 
 
 Freedom from strain; state of annealing 37 
 
 Refractivity and dispersion 41 
 
 Composition-refractivity relations 58 
 
 Freedom from color 74 
 
 Transparency 76 
 
 Treatment of polished glass surfaces to reduce amount of light reflected 76 
 
 Weather stability 79 
 
 CHAPTER III. The manufactu re of optical glass 81 
 
 The organization of an optical glass plant 81 
 
 Raw materials 83 
 
 Sand 84 
 
 Potassium carbonate 85 
 
 Sodium carbonate, calcium carbonate, barium carbonate, lead oxide, 
 
 boric acid, etc 86 
 
 Specifications for raw materials 86 
 
 Melting pots 87 
 
 Open versus closed pots 89 
 
 The bleaching of pots 90 
 
 Furnaces 92 
 
 Measurement of furnace temperatures 93 
 
 in 
 
WAK DEPAKTMENT, 
 OFFICE 'OF THE CHIEF OF ORDNANCE, 
 
 January 3, 1921. 
 
 This work on the Manufacture of Optical Glass and of Optical 
 Systems was prepared at the request of the artillery division of this 
 office by Lieut. Col. F. E. Wright, Ordnance Reserve Corps. Col. 
 Wright was in charge of production and inspection of optical glass 
 at the plant of the Bausch & Lomb Optical Co. from April, 1917, 
 to May, 1918; the Army representative on the military optical glass 
 and instrument section of the War Industries Board from March 15, 
 1918, until after the armistice; chairman of the Army commodity 
 committee on optical glass and instruments under the Director of 
 Purchase, Storage and Traffic from April, 1918, until after the armi- 
 stice; and in charge of optical systems in the fire-control section of 
 the engineering division of this office from August 4, 1918, to June 
 16, 1919. 
 The treatise is approved for publication. 
 
 C. C. WILLIAMS, 
 Major General, Chief of Ordnance, 
 
 United States Army. 
 3 
 
THE MANUFACTURE OF OPTICAL GLASS AND 
 OF OPTICAL SYSTEMS. 
 
 A WAR-TIME PROBLEM 
 
 Chapter I. 
 INTRODUCTION. 
 
 War has been called a contest of brains and brawn. Among the 
 ancients emphasis was placed on brawn, on individual prowess and 
 skill; but with the passing centuries and the development of science 
 and technology, the engineering side of warfare has gradually gained 
 the ascendency. At the present time much of warfare is applied 
 engineering. In the recent war Germany had in this respect a great 
 advantage because many of its scientists and technical industrial 
 forces had been engaged for years before the war in the design and 
 manufacture of war materiel, whereas no efforts of like magnitude 
 had been put forth by the allies. In certain branches of industry, 
 moreover, the Germans had established practically a world monopoly. 
 
 In industries of a highly technical nature, such as the chemical 
 dye industry and the optical glass and instrument industry, the 
 Germans had established such effective control that at the beginning 
 of the war we were seriously embarrassed because we did not manu- 
 facture these commodities, and did not know how to make them. 
 The Germans had been at the task for more than a quarter century 
 and had developed a personnel trained and competent for the tasks. 
 Their best scientists and engineers had originated and followed 
 through the factory processes and were thoroughly familiar with 
 them; with us it was a serious question whether we would be able to 
 accomplish anything adequate in time to be of service on the field of 
 battle. The records show that in the short period of 19 months, we 
 did accomplish much to overtake the decades of German experience. 
 In certain instances, as in optical glass and instruments, there has 
 been developed in this country an industry which more than suffices 
 for our own needs and which will render us independent of Europe 
 if we so desire. 
 
 What is optical glass and what is its connection with modern war- 
 fare? At the time of the Civil War it was not used to any extent 
 and why should it now have become such a vital necessity ? These 
 
 5 
 
D c : ^ INTRODUCTION. 
 
 questions are best answered by a comparison of the Civil War meth- 
 ods of artillery warfare with those of the present time. 
 
 During the Civil War artillery was used extensively by both sides 
 and, judged by the standards of that period, was effective and served 
 the purpose well; but, had the same sort of artillery been employed 
 in the recent European war, it would have been of little value, and, 
 in many instances, a positive hindrance to the troops in the field. 
 
 The cannon of the Civil War were, for the most part, of cast iron 
 and smooth bore, and were loaded at the muzzle. Spherical cast- 
 iron projectiles weighing 6 pounds, 12 pounds, and up to 42 pounds, 
 were used effectively at ranges between one-half mile and 1 mile; 
 shrapnel or spherical case shot between 500 and 800 yards; grape 
 and cannister shot for less distances even down to 150 yards. High- 
 explosive and gas shells were unknown. The cannon were fired 
 point blank at the visible enemy. Both direct and ricochet firing 
 were employed; indirect firing against an invisible target, which 
 plays such an important role in modern artillery practice, was un- 
 known. Distances were estimated by the eye and by observing the 
 fall of projectiles. Range-finding devices were considered to be of 
 little value and were not employed. 
 
 The guns were pointed by means of. open metal sights attached 
 directly to the cannon barrel ; the sight resembled in many respects 
 the sights on the modern rifle. The gunner pointed the gun by 
 looking through a peephole in the rear sight and aligning the front 
 sight near the muzzle of the gun with the target. After having 
 leveled the gun by a simple spirit level, obtained the line of sight 
 and estimated the distance to the target, the gunner elevated his 
 cannon to the proper angle, which he measured by means of a gunner's 
 quadrant that consisted essentially of a wooden frame with plumb 
 line and bob combined with a graduated circle, or by means of a 
 graduated vertical bar on which the peep sight was arranged to 
 move. 
 
 Optical instruments for the direction and control of firing were 
 unknown in the Civil War and would have served little purpose with 
 the ordnance then available. During the past half century, however, 
 artillery construction and practice have developed at an unprece- 
 dented rate. The power and precision of the guns have been in- 
 creased to the extent that at the present time relatively small targets 
 may be successfully attacked at ranges up to 40,000 and 60,000 yards. 
 Most of the firing by modern artillery is directed at objects either 
 below the horizon or hidden by intervening obstacles and hence not 
 visible to. the gunner, but whose positions have been accurately located 
 on the map, either by land or aerial reconnaissance. Firing under 
 these conditions becomes an engineering problem and the method 
 of instrumental aim known as indirect fire control is employed. 
 
FIRE-CONTROL INSTRUMENTS. 7 
 
 This method requires the use of surveying instruments and serves to 
 determine the line of sight and the distance to the target with refer- 
 ence to fixed points that are visible to the battery. This method is 
 used not only in firing at hidden targets located miles away, but also 
 in laying down a barrage, or deluge of missies, in front of an advancing 
 line of troops, thus clearing the way for them and preventing counter- 
 attacks by the enemy. 
 
 The essential difference between the old and the new artillery 
 practice is one of distance and degree of precision attained. At the 
 time of the Civil War the artillery was fired point blank in the general 
 direction of the enemy, and the effort was made to overwhelm him 
 with shots at close range. The gunners sighted their fieldpieces 
 hastily and banged away, trusting to hit some vital spot. Modern 
 artillery is concerned chiefly with much longer ranges and the aiming 
 must be accurate, otherwise the target is missed altogether. This 
 degree of accuracy is attainable only by means of optical instruments 
 of high precision which serve both as observational instruments for 
 detecting details of distant objects, noting the accuracy of the firing, 
 and as surveying instruments for measuring angles correctly. These 
 instruments are not only exact but also sufficiently rugged to with- 
 stand the violent concussion of rapid artillery fire. The French 
 75-millimeter gun, for example, which proved so effective in the late 
 war, has a range of 8,300 yards, and can be fired at the rate of 30 shots 
 per minute, as against the 2 shots per minute of the Civil War. The 
 gun, moreover, is an instrument of high precision and the shots can be 
 placed by the gunner practically where he wants them. The best 
 breech-loading musket rifle in Civil War days could be fired 10 times 
 per minute as against 600 shots per minute by a modern machine gun. 
 
 The gunner of to-day who is not equipped with proper fire-control 
 apparatus is almost helpless in the presence of the enemy; he can 
 not see to aim properly without these aids and his firing serves little 
 purpose. The effectiveness of modern artillery and, with it, that of 
 the active army in the field, depends to a large extent on the quality 
 and the quantity of its fire-control equipment. This is only one of a 
 number of the fundamental requisites of the complex army of to-day. 
 Yesterday many of the essentials of to-day were unknown; and yet, 
 when war comes upon us, we can not fight the war of to-day with the 
 tools of yesterday, nor, unfortunately, the war of to-morrow with the 
 tools of to-day. The fundamental principles of warfare and combat 
 remain ever the same; but the methods and means of attaining the 
 ends desired are in a state of constant change and flux. It behooves 
 us, therefore, to keep in touch with modern developments, so that, 
 when a crisis comes, we may know definitely what is required and 
 also what is not essential. Had this been done during past years 
 this country would have saved vast sums of money, and much waste 
 
8 INTRODUCTION. 
 
 effort would have been avoided. Production under emergency con- 
 ditions is always expensive. 
 
 The lessons to be learned from our recent experiences along these 
 lines are so obvious that comment is unnecessary. Such lessons are 
 soon forgotten by the country at large. It is, however, essential 
 that a written record be made of the war-time development of certain 
 of the manufacturing problems as they confronted us and were 
 solved. The record may serve a useful purpose and be of value in 
 case of a future emergency. 
 
 The present report seeks to outline, in a general way, some of the 
 factors which we encountered in the manufacture of optical glass 
 and of lenses and prisms for fire-control and observational instru- 
 ments for the Army and Navy. The presentation is necessarily 
 general in character, emphasis being placed on underlying principles 
 rather than on details. The report presents in substance the expe- 
 rience of the writer, who, with fellow scientists, entered as strangers 
 into a new branch of endeavor to solve certain problems of a tech- 
 nical nature. Between the lines of the written record may be read 
 many interesting experiences for the scientist in technical work 
 and the gradual change in the attitude of practical factory men 
 toward him. In all problems of high precision, whether technical 
 or laboratory, the scientifically trained observer, who has learned to 
 appreciate* the factors essential to the attainment of high precision 
 and is competent to control these factors, is able to undertake the job 
 with better chance of success than the rule-of-thumb man of the 
 shop who has the factory experience, but lacks the necessary training 
 with which to meet new and radically different problems precipitated 
 by war. In war-time organizations as in peace-time organizations the 
 question of competent personnel is always the fundamental factor 
 and the most difficult to meet satisfactorily. The experience of the 
 recent war proved this statement time and again, and demonstrated 
 the need for a closer and more effective touch between the Army and 
 Navy and the scientific as well as the manufacturing forces of the 
 country. 
 
 THE MANUFACTURING PROBLEM. 
 
 At the time of the entrance of the United States into the war, the 
 methods and instruments of indirect artillery fire were already uni- 
 versally employed on the battle fronts and our troops had to be 
 supplied with these instruments if they were to combat the enemy 
 successfully. We were brought, in short, face to face with many 
 technical manufacturing problems which had to be solved and placed 
 on a production basis quickly if our Army and Navy were to get 
 what they wanted when they needed it. Many of these problems 
 arose because manufacturers before the war had been accustomed 
 
THE MANUFACTURING PROBLEM. 9 
 
 to import supplies of this nature from Europe, especially Germany, 
 and had never felt the need for developing the manufacturing proc- 
 esses in their own plants. Such development meant large expense 
 and it was easier and cheaper to place an order for the commodity 
 in Germany. This applied especially to highly technical products, 
 such as dyes and optical glass. 
 
 Military fire-control apparatus includes instruments of high pre- 
 cision, and, as one of the integral parts of such instruments, optical 
 glass must measure up to the same standards of high precision. Upon 
 it the quality of the image formed and the precision of each setting 
 of the sighting instrument is dependent. The lens designer computes 
 the shapes and positions of the several different lenses and prisms in 
 an optical instrument and arranges them along the line of sight in 
 such a way that the particular and inevitable defects, or aberrations, 
 are reduced to a minimum. The degree to which these aberrations 
 can be made negligible depends in large measure on the kinds and 
 quality of glass available to the designer. It is important, therefore, 
 that the quality of the glass be 4 of the best and that a sufficient 
 number of different glass types be at hand. 
 
 Before the war the optical industry in this country was in the 
 hands of a few firms. Several of these were under German influence 
 and one firm was directly affiliated with the largest manufacturer of 
 optical instruments in Germany; the workmen were largely Germans 
 or of German origin; the kinds and design of apparatus produced 
 were for the most part essentially European in character; optical 
 glass was procured entirely from abroad and chiefly from Germany. 
 Educational and research institutions obtained a large part of their 
 equipment from Germany and offered no special inducement for 
 American manufacturers to provide such apparatus. Duty-free im- 
 portation favored and encouraged this dependence on Germany for 
 scientific apparatus. 
 
 With our declaration of war the European sources of supply for 
 optical glass and for optical instruments were cut off abruptly. 
 Even before our entry into the war and especially after hostilities 
 began in 1914, manufacturers of optical instruments realized that 
 the European supply of optical glass might be stopped and they 
 began experiments on its manufacture. In 1912 the Bausch & Lomb 
 Optical Co., of Rochester, N. Y; in 1914 the Bureau of Standards 
 at Pittsburgh, Pa.; in 1915 the Keuffel & Esser Co., of Hoboken, 
 N. J., and the Pittsburgh Plate Glass Co., of Pittsburgh, Pa.; in 1916 
 the Spencer Lens Co., of Buffalo, N. Y., started work and produced 
 some optical glass of fair quality. The quality of glass obtained was 
 not, however, entirely satisfactory and by the time we entered the war, 
 the shortage of optical glass of high quality was so serious that unless 
 something were done speedily to relieve the situation, the Army and 
 Navy would not be equipped with the necessary optical instruments. 
 
10 INTRODUCTION. 
 
 Such was the situation in April, 1917. The fundamental item for 
 fire-control instruments, optical glass, had not been produced satis- 
 factorily in quantity in this country. The methods for its manu- 
 facture on a large scale had still to be developed. After the supply of 
 optical glass had been assured the manufacturing capacity for pre- 
 cision optics had. to be increased to a scale commensurate with the 
 needs, and finally the design of new instruments required proper 
 supervision with reference both to ultimate field use and to speed of 
 production. 
 
 In this emergency the Government through the Council of National 
 Defense appealed to the Geophysical Laboratory of the Carnegie 
 Institution of Washington for assistance in the manufacture of 
 optical glass. This research laboratory had been engaged for many 
 years in the study of silicate solutions, similar to optical glass, at 
 high temperatures, and had a corps of scientists trained along the 
 lines essential to the successful production of optical glass. It was 
 the only organization in the country with a personnel adequate and 
 competent to render aid in a manufacturing problem of this character 
 and magnitude. Obviously the best plan was to cooperate with 
 manufacturers who had had some experience along these lines. 
 
 In order to ascertain the attitude of one of the manufacturers the 
 writer visited on April 4, 1917, the Bausch & Lomb Optical Co., and 
 found them willing to cooperate. Accordingly, when the request 
 from the Government for aid was made shortly after war was 
 declared, a group of scientists, with the writer in charge, went from 
 the Geophysical Laboratory on April 27 to the Rochester plant and 
 began work. Its men were gradually assigned to the different factory 
 operations and made responsible for them. At this plant much of 
 the pioneer development work was accomplished. The methods of 
 manufacture on a large scale were here developed and placed on a 
 production basis. Drs. Day, Allen, Bowen, Fenner, Hostetter, 
 Lombard, Ferguson, Washington, Hall, Merwin, Morey, and Ander- 
 sen were with us for longer or shorter periods of time to May, 1918. 
 At the Geophysical Laboratory special problems were attacked by 
 Drs. Allen, Zies, Posnjak, Bichowsky, Merwin, White, Roberts, 
 Andersen, Adams, and Williamson. To the director of the labora- 
 tory, Dr. A. L. Day, fell the general executive problems involving 
 laboratory personnel and the several phases of their activity in 
 connection especially with the sources and transportation of suitable 
 raw materials and of fuel. He was also instrumental in securing the 
 cooperation of outside firms to undertake special jobs essential to 
 the successful manufacture of optical glass. 1 
 
 1 For a detailed account of this important part of the task see the paper by A. L. Day on "Optical glass 
 and its future as an American industry," J. Franklin Institute 190, 453-472, 1920. See also Chapter VII 
 by Harrison E. Howe on " Optical glass for war needs" in the book The New World of Science, edited by 
 R. M. Yerkes and published by the Century Co. New York, 1920. 
 
THE MANUFACTURING PROBLEM. 11 
 
 As a matter of record, establishing the connection of Ordnance 
 Department with this task, it may be noted that in April, 1917, the 
 writer received his captain's commission in the Engineer Reserve 
 Corps; but, at the request of officers in the Ordnance Department, 
 was transferred to the Ordnance Department. This direct connec- 
 tion with the Army aided the solution of the problem of glass manu- 
 facture in many ways. War-time experience has proved that direct 
 Army and Navy connection with technical problems of this nature 
 is invaluable. Under war conditions the court of last appeal is the 
 Army and the Navy; in situations which arise requiring prompt and 
 effective action it is useful to have this lever available, even though 
 it may not be necessary to employ it. 
 
 Military optical instruments are for the most part telescopes; for 
 their satisfactory design experience has shown that from three to 
 five different types of optical glass suffice. The problem before us 
 was essentially one of high-speed, quantity production of a relatively 
 small number of glass types. We were not concerned with the 
 development of new types of optical glass, but rather with the manu- 
 facture of only a few kinds, such as ordinary crown, borosilicate 
 crown, light and dense barium crown, light and dense flint. Our 
 task was to reproduce in this country glasses of standard type which 
 had long been produced in Europe. The European methods of 
 manufacture of these glasses, however, were not adequately known, 
 and a large part of our time was devoted to the study and develop- 
 ment of manufacturing processes which eventually enabled us to 
 produce sufficient quantities of optical glass of high quality. The 
 problem before us, although essentially a research problem, differed 
 materially from ordinary research problems in that the properties 
 of the final product were known in detail. After the manufacturing 
 difficulties in connection with this problem had been overcome atten- 
 tion might well have been devoted to the development and manufac- 
 ture of new types of optical glasses; but at that time the need for 
 greater manufacturing capacity for optical instruments was so great 
 that this phase of the situation demanded immediate attention. 
 
 Our first efforts at the Bausch & Lomb plant in April and May, 
 1917, were devoted to a study of the glass-making process and of the 
 several factors involved. At the factory we had the hearty coopera- 
 tion of Mr. William Bausch, to whom credit is due for having started 
 the glass plant in 1912, and of Mr. Victor Martin, a practical Belgian 
 glassmaker, who had placed the plant on a running basis and had 
 produced, even in 1912, some optical glass of fair quality. Our task 
 was to build up on Mr. Martin's experience and to introduce into the 
 manufacturing process the element of high precision and control 
 w^hich were not sufficiently recognized, but which were essential to 
 the manufacture of optical glass of uniformly high quality. Our task 
 
12 INTRODUCTION. 
 
 was not strictly a development of new processes or of a new product, 
 but rather the modification of existing processes as practiced at the 
 plant so as to obtain satisfactory optical glass in large quantities. 
 
 The search for raw materials of adequate chemical purity in ton 
 lots, the preparation and mixing of the batches, the control of the 
 melting furnaces, of the pot arches, and of the annealing ovens; the 
 stirring of the molten glass; the inspection of the product at the 
 several stages; these and other problems had to be studied in detail 
 and in each case exact information gained regarding the best practice 
 to be followed. Soon after our arrival at the plant it was the writer's 
 good fortune to deduce, as a result of a statistical study of the existing 
 chemical analysis of optical glasses, certain relations which enabled 
 us to write down at once the batch-composition for glasses of desired 
 optical constants, especially in the flint series. This deduction freed 
 our minds of the uncertainty regarding our ability to reproduce 
 optical glasses of standard types, the batches of which were held 
 secret by glass makers. Indirectly it had a much more important 
 effect because it changed the attitude of the factory men toward us; 
 they realized at once that the cut-and-try method, by which their 
 few batch formulas had been obtained, was superseded by a more 
 direct method of attack which gave us control over whole series of 
 glasses rather than over a few isolated members of a series. Addi- 
 tional studies and analyses made at the Geophysical Laboratory have 
 borne out the conclusions deduced from these preliminary studies. 
 
 The literature on the details of optical glass making is scant, and 
 the processes have heretofore been considered secret and have been 
 closely guarded. The problem before us was one of intense interest 
 and great importance and we soon realized that what was most 
 needed was precision control over all the steps in the manufacturing 
 process. This was emphasized by the writer in Report No. 1, for 
 the week ending May 5, 1917, and in many of the succeeding reports. 
 Reports were prepared weekly on the progress made at the plant, 
 and will be referred to hereafter simply by date and number. 
 
 By November, 1917, the manufacturing processes at the Bausch 
 & Lomb plant had been developed, mastered, and placed on a pro- 
 duction basis. Large quantities of optical glass of good quality 
 were being produced. In December, 1917, the work was extended 
 and men from the Geophysical Laboratory, who had been connected 
 with the pioneer work at Rochester, took practical charge of the 
 plants of the Spencer Lens Co. and of the Pittsburgh Plate Glass Co. 
 At the Spencer lens plant Dr. C. N. Fenner produced excellent optical 
 glass from the start and was very successful in placing the plant on 
 a production basis and in extending its capacity manyfold. He 
 was assisted at different times by Drs. Allen, Andersen, Bo wen, 
 Morey, and Zies, of the Geophysical Laboratory. In July, 1918, 
 Dr. Morey relieved Dr, Fenner, At the Pittsburgh works Dr. 
 
PRODUCTION OF OPTICAL GLASS. 13 
 
 Hosteller was put in charge and was confronted with a most difficult 
 problem in organization to convert and build up an old plate-glass 
 plant into an effective system for the manufacture of optical glass. 
 In this task he was assisted by Drs. Andersen, Adams, Bowen, 
 Lombard, Morey, Roberts, Ferguson, Williamson, and Wright, of the 
 Geophysical Laboratory; by Messrs. Taylor, Bleininger, and Kiess, of 
 the Bureau of Standards; and by the staff of the Pittsburgh Plate 
 Glass Co. In the course of some months the organization was 
 accomplished. Much credit is due to these men for the success 
 attained. 
 
 These details are given because they show how a highly technical 
 problem of this kind was attacked under war-time conditions and 
 solved successfully. The records show that at these three plants 
 approximately 95 per cent of all the optical glass manufactured in 
 this country during the war was produced. 
 
 In addition to the foregoing plants, optical glass was made by 
 the Keuffel & Esser Co. in quantities sufficient to supply their own 
 needs. Much credit is due Mr. Carl Keuffel, who, on his own initia- 
 tive and before we entered the war, erected a glass-melting furnace, 
 made suitable pots, and produced some glass of good quality without 
 outside help. 
 
 Small quantities of optical glass were also made in furnaces at 
 the Bureau of Standards at Pittsburgh. Dr. Bleininger developed 
 at this plant a poured, porcelainlike crucible which proved to be 
 satisfactory for use with the dense barium crown melts that attack 
 ordinary clay pots vigorously. 
 
 Three other firms, the Hazel-Atlas Glass Co., at Washington, Pa.; 
 the Carr-Lowrey Glass Co., of Baltimore; and the H. C. Fry Co., of 
 Pittsburgh, Pa., experimented on the manufacture of optical glass, 
 but did not reach the stage of quantity production. 
 
 In the solution of the optical glass problem, the expense incurred 
 by the Geophysical Laboratory as a voluntary contribution to the 
 Government amounted to about $200,000; at no time during the war 
 or after, did the Geophysical Laboratory request or receive any money 
 from the Government for these or other expenditures ; but the results 
 attained justified the expenditures. This could not have been done, 
 however, without the hearty cooperatkm of the manufacturers, of the 
 Army, especially of the Army Ordnance Department, and of the War 
 Industries Board. The Bureau of Standards aided in the development 
 of a chemically and thoroughly resistant crucible in which to melt op- 
 tical glass; also in the testing of optical glass and especially in the test- 
 ing of optical instruments. The United States Geological Survey aided 
 in locating sources of raw materials, such as sand, of adequate purity. 
 
 The general situation may be summarized by stating that when 
 we entered the war we not only lacked a supply of optical glass, but 
 3922921 2 
 
14 
 
 INTRODUCTION. 
 
 we lacked information regarding the processes of its manufacture. 
 We had little knowledge of the quality and sources of supply of the 
 raw materials required. We lacked manufacturing capacity and a 
 trained personnel to handle the problems. The problems to be solved 
 required the cooperation and working together of many diverse 
 elements ; it was a constant source of inspiration for us to witness the 
 whole-hearted spirit of cooperation of the manufacturers in this con- 
 nection and especially of those manufacturers who were not directly 
 concerned with the final product. The quantity production of a 
 few standard types of optical glass of usable quality was our goal 
 rather than the development of new and highly perfect types of optical 
 glass. There is still room for improvement in many details; but the 
 manufacturing processes are known; there are no secrets and the 
 manufacture of optical glass has become one of factory routine. It 
 is with satisfaction that all those who have contributed to this result 
 may look and realize that one more commodity has been added to 
 the list of commodities made in this country. This was accom- 
 plished, however, under high pressure and at large expense/ part of 
 which might have been saved had the problem been attacked 
 under peace-time conditions. 
 
 The records show that there were produced in this country between 
 April, 1917, and November, 1918, over 600,000 pounds of usable 
 optical glass. Not all of this glass was of the best quality, but it 
 was satisfactory for low-power optical instruments, and some of the 
 product was of the highest quality, equal to the best European glass. 
 On an average its quality was fair and satisfactory for war-time 
 purposes. Six different types of glass were manufactured: Ordinary 
 crown, borosilicate crown, barium crown, dense barium crown, light 
 flint, and dense flint. The total monthly production statistics are 
 listed in Table 1, and presented graphically in figure 1. 
 
 TABLE I. Estimated total production, in pounds, of usable optical glass (A and B 
 quality) in the United States from April, 1917, to November, 1918. 
 
 1917 : Pounds. 
 
 April 2,850 
 
 May 4,600 
 
 June 6,500 
 
 July 4,800 
 
 August.. 4,800 
 
 September 10, 775 
 
 October 15, 645 
 
 November 30, 499 
 
 December.. 42,451 
 
 Total for year 122, 920 
 
 1918: Pounls. 
 
 January 35,955 
 
 February 41, 138 
 
 March 41, 842 
 
 April 24,363 
 
 May 43,397 
 
 June 69,328 
 
 July.... 55,355 
 
 August 71,459 
 
 September 67,741 
 
 October... 79,275 
 
 Total for vear 529, 853 
 
 Total 655,773 
 
PRODUCTION OF OPTICAL GLASS. 
 
 15 
 
 Of these quantities the Bausch & Lomb Optical Co. produced over 
 65 per cent, the Pittsburgh Plate Glass Co. nearly 20 per cent; the 
 Spencer Lens Co. nearly 10 per cent; the remaining 5 per cent was 
 produced by the other firms, including the Bureau of Standards. 
 The figures, especially those for 1917, are less accurate than those 
 for 1918, but they are of the correct order of magnitude. The figures 
 
 POUNDS 
 70OOOO 
 
 600000 
 
 500 000 
 
 400000 
 
 300000 
 
 200 000 
 
 100000 
 
 APR. MAY JUNE JULY AU<J. SEP OCT NOV. DEC JAN. FEB. MAR. APR. MAY JUNE JULY AU<* 3ER OCT 
 
 I9<7 1916 
 
 FIG. 1. Curve I indicates the total monthly production of optical glass in the United States from April, 
 1917, to November, 1918; Curve II shows the cumulative total production of optical glass for the same 
 period. 
 
 show that starting with the total monthly production of 1J tons of 
 usable glass in April, 1917, -the monthly production mounted to 
 about 40 tons in October, 1918; the organizations at that time were, 
 moreover, so well coordinated that an increase of 50 per cent in the 
 total monthly production could have been attained had the need 
 arisen. 
 
Chapter II. 
 THE CHARACTERISTICS OF OPTICAL GLASS. 
 
 Ordinary glass, such as bottle glass, decorative glass, etc., was 
 known to the ancients and has for centuries been manufactured in 
 large quantities; certain communities have been engaged in the art 
 of glassmaking for many generations. Optical glass, on the other 
 hand, is a modern development to meet the demand for better tele- 
 scopes, microscopes, photographic lenses, and other optical appara- 
 tus. At first sight optical glass may appear to be simply glass, not 
 greatly different from ordinary glass, but the differences between the 
 two are fundamental and can best be apppreciated by an examina- 
 tion into the functions of optical glass as an integral part of an optical 
 instrument; from these in turn the characteristics required of the 
 glass which shall satisfy these conditions adequately can be deduced. 
 Tt is not obvious at first thought why optical glass is so difficult to 
 manufacture satisfactorily and why so much emphasis is placed upon 
 its quality. A brief discussion of the factors, on which the per- 
 formance of an optical system depends, will serve to render this clear 
 and prepare the way for further comments on details of similar 
 nature. 
 
 Most optical instruments employed in military operations are of 
 the telescope type and serve not only to aid the observer in detecting 
 details of distant objects, but also to fix with high precision reference 
 lines of sight and thus enable the gunner to direct and to control the 
 fire of his fieldpieces. 
 
 THE FUNCTIONS OF THE EYE. 
 
 The function of a telescopic lens system is similar to that of the eye. 
 We see a distant point because light waves emerge from it or are 
 reflected by it; each point of the object is seen as a point because the 
 eye unites to a point on the retina the waves of light from the object 
 point which impinge on the eyelens. What we actually see is the 
 image formed on the retina of the eye as focused there by its con- 
 verging lens system. (Fig. 2a.) In case the eye is defective such 
 points are not imaged as points on the retina; thus in the eye in repose 
 (adjusted to see distant objects) the retina may be too near the eye- 
 lens (farsighted eye) (fig. 2&), or it may be too far from the eyelens 
 (nearsighted eye) (fig. 2c), or the surfaces of the eyelens may not be 
 spherical but warped so that the converging effect in one meridian is 
 16 
 
FUNCTIONS OF THE FYK. 
 
 17 
 
 different from that in another (astigmatic eye). The farsighted eye 
 is " corrected " by means of a collective or positive spectacle lens (indi- 
 cated by dotted lines in fig. 2.6) ; the nearsighted eye by a dispersive 
 or negative lens (fig. 2c), and the astigmatic eye by a cylindrical lens. 
 The eye resembles a photographic camera in certain respects. The 
 lens system of the normal eye converges rays from a distant point to 
 a point on the retina which in turn responds to the light-wave im- 
 pulses and functions as a receiving transformer converting the light- 
 wave impulses into nerve impulses which travel to the brain and ar^ 
 
 FIG. 2. Diagrams illustrating action of eyelens in forming image of distant object en the retina. Fig. 2a 
 shows action of a normal eye in which the image is focused on the retina itself. In fig. i6 the image 
 plane is located back of the retina (far-sighted eye), while in fig. 2c the image plane is in front of the retina. 
 The dotted lines in fig. 26 and 2cillustrate the action of spectacle lenses in increasing (fig. 26) or decreas- 
 ing (fig. 2c) the convergence of the incident rays. 
 
 there interpreted. The retina is to the eye what the photographic 
 plate is to the camera. 
 
 The eye is an integral part of all optical observing instruments; 
 and these in turn should present to the eye images for observation 
 which approach in characteristics the conditions under which the 
 eye has been accustomed from childhood to function. These funda- 
 mental conditions may be considered under three heads: 
 
 1. Intensity of illumination or brightness. 
 
 2. Resolving power or definition. 
 
 3. Field of view. 
 
18 CHARACTERISTICS OF OPTICAL GLASS. 
 
 (1) Intensity of illumination. For good seeing the intensity of 
 the light emerging from each object point (brightness of object) 
 should not be too weak (twilight illumination) nor too strong. The 
 adaptability of the eye to differences in intensity is most remarkable; 
 it operates satisfactorily over a range of intensities from 1 to 
 10,000,000. We see the details of an object because of differences 
 in brightness (contrast in light and shade) and in color. Under 
 ordinary conditions of illumination the eye is sensitive to a differ- 
 ence of about 2 per cent in brightness between two adjacent points; 
 under very favorable conditions this percentage difference in con- 
 trast may decrease to 1 per cent, but in case the intensity of illumina- 
 tion passes gradually and not abruptly from one detail of an object 
 to another the percentage difference in intensity may increase many 
 per cent before the eye perceives the difference and distinguishes the 
 details. At very low or very high intensities the contrast sensibility 
 of the eye may increase to 10 per cent. For good vision an illumina- 
 tion approaching that of daylight in intensity and distribution is the 
 most favorable. 
 
 (2) Definition. In order to distinguish the details of an object 
 these must exceed a certain size; thus the printed letters on this 
 page are clearly legible at a distance of 1 foot; but they can no longer 
 be read at a distance of 10 feet. Measurements show that the 
 unaided eye can readily distinguish two points as distinct points 
 when the rays from these points subtend at the eye an angle of two 
 minutes of arc (approximately half a mil) ; eyes of high acuity are 
 able to resolve points separated by one minute of arc or less; but, as a 
 general rule, two minutes may be taken as a comfortable limit of 
 resolution under ordinary intensities of illumination. At low 
 intensities (twilight illumination) the ability of the eye to resolve 
 details falls off rapidly with decrease in intensity and the angular 
 separation of points just discernible as distinct points may rise to 
 several degrees. This is readily tested by noting that the headlines 
 of a newspaper can still be read in twilight after the fine print has 
 become completely illegible. It is for this reason especially that in 
 faint illumination a telescope or field glass enables the eye to dis- 
 tinguish details which it can not see otherwise; and this in spite of 
 the fact that a considerable amount of the incident light is lost by 
 reflection and absorption in the elements of the telescope system 
 itself. In the section on the telescope lens system the explanation 
 of this apparent anomaly is given in some detail. 
 
 It is shown in textbooks on physics that each image point formed 
 by a lens such as the eye, the photographic lens, the telescope, or 
 the microscope objective is in reality a diffraction pattern to which 
 the rays from each object point contribute; thus the image of a 
 luminous point, such as a star, is actually a central disk of light 
 
LENS SYSTEM OF A TELESCOPE. 19 
 
 surrounded by a set of concentric dark and light rings. Experience 
 shows that the images from two such points, such as a double star, 
 can be distinguished or resolved when the central disk of, the one 
 image touches the first dark ring in the image of the second. This 
 theoretical limit of resolution depends directly on the wave length 
 of light used and inversely on the aperture of the lens. In the eye 
 the lens aperture is varied by means of the iris which serves also to 
 shield the eye from too intense illumination; the diameter of the 
 iris (pupillary aperture) ranges from 2 to 8 millimeters. 
 
 Any defect in a lens which tends to decrease the theoretical limit 
 of resolution is serious; but such defects are inherent in all lenses 
 and the lens designer aims to reduce these defects or aberrations to 
 a limit at least equal to that of the eye itself. The importance of the 
 eye as an essential part of an observing instrument is obvious; but 
 the need for training the eye to do its part and the desirability of 
 fulfilling certain conditions requisite for the attainment of the best 
 definition are not always realized, especially by observers in the field. 
 
 3. Field of view. The eyes grasp at a glance a certain area or 
 field of view, and thus enable the observer to perceive the relative 
 positions of points and objects in space. The normal eye at rest 
 is focused for parallel rays, i. e., rays from distant points; it accom- 
 modates, however, with extreme rapidity for points distant only 10 
 inches, the distance of near vision. Experiments show that the field 
 of sharpest vision is only J, corresponding to the area of the yellow 
 spot on the retina; but, as a result of persistence of vision and mo- 
 bility of the eye in its socket, the field 'covered satisfactorily is 
 nearly 30 horizontal and 20 vertical; this is surrounded by a field 
 of visual perception but indistinct vision which extends to 150 
 horizontal and 120 vertical in the single eye. In observing instru- 
 ments the apparent field of view should approximate at least the 
 angular area of satisfactory vision and preferably a larger area 
 because the eye by reason of its mobility in changing its line of sight 
 easily covers larger angular fields of view. 
 
 To recapitulate: Three factors, intensity of illumination, definition, 
 and field of view, are fundamental to satisfactory vision. 
 
 THE LENS SYSTEM OF A TELESCOPE. 
 
 In the design of an optical observing instrument these factors are 
 likewise fundamental; in addition, a fourth factor, namely, magni- 
 fication is equally fundamental. The objective of the telescope 
 functions as the eye of the instrument. Its area is much larger than 
 that of the pupil of the eye; hence a correspondingly larger number 
 of light waves from each distant object point impinge on it. The 
 objective should be so designed that it converges the rays which it 
 receives from each distant point to a corresponding point in -the 
 
20 CHARACTERISTICS OF OPTICAL GLASS. 
 
 image plane. (Fig. 3.) The image thus formed is the aggregate of 
 the points of convergence of pencils of rays received by the objec- 
 tive from distant object points. By thus imaging the object points, 
 they are, as it were, brought to a position much nearer the observer, 
 and can there be examined by him with the aid of a magnifying lens. 
 (Eyepiece, fig. 3.) By this means the eye approaches in effect close 
 to the image and the angle subtended at the eye by any two points 
 in the image is correspondingly increased. The ratio of the angle 
 subtended at the eye between two points in the image to the angle 
 at the eye between the corresponding points of the distant object 
 is a measure of the angular magnifying power of the telescope. 
 Thus, if the angle of separation between two distant points, as viewed 
 by the unaided eye, is 2 minutes of arc, this apparent angle of sepa- 
 ration when viewed through a telescope may be 20 minutes of arc; 
 in this case the magnifying power of the telescope is 10. The angle 
 between two object points actually separated by only 12 seconds 
 of arc (0.2 of a minute) at the observer's eye appears through this 
 telescope (10 power) to be separated by 2 minutes of arc; these points 
 
 OBJECTIVE-, IMAGE-PLANE -^ r EYE-PIECE 
 
 ENTRANCE PUPIL-" FIELD OF VIEW DIAPHRAGM I L EX IT PUPIL 
 
 FIG. 3. Sectional view of the optical elements of a telescope lens system, showing paths of rays through 
 
 the system. 
 
 are therefore readily distinguished as separate points with the aid of 
 the telescope. The apparent field of a 6-power binocular is 48; the 
 actual field is 8, only one-sixth as large. 
 
 In a telescope the area of the image is limited by a circular stop or 
 diaphragm located in the image plane ; this is called the field of view 
 diaphragm; it is imaged on the retina and effectively excludes the 
 rays from object points outside of the area imaged. This diaphragm 
 functions similarly to the porthole in a ship's cabin in limiting the 
 field of view, and has been called the entrance port or window of the 
 instrument; it is evident that the farther away the eye is from the 
 porthole or window the smaller is the angular field of view. 
 
 The pencils of rays from distant object points enter the telescope 
 through the objective and are limited in width either by the rim of 
 the objective or by some smaller stop which is called the entrance 
 pupil of the instrument; this, like the iris of the eye, limits the cone 
 of light which a given object point sends through the instrument; 
 the image of this stop (entrance pupil), as seen through the eyepiece 
 end of the telescope, is called the exit pupil. The rays from all points 
 in the image cross at the exit pupil; if the observer's eye is placed 
 
LENS SYSTEM OF A TELESCOPE. 21 
 
 there, the entire image can be seen without lateral shift of the eye; 
 if the eye is placed elsewhere it must be shifted from side to side in 
 order to see the entire field. It is important, therefore, for ease of 
 observation that the iris of the eye coincide in position with the exit 
 pupil of the instrument. If a telescope of the ordinary type is 
 pointed at the sky, the exit pupil can be seen as a disk of light sus- 
 pended in air a short distance back of the eyepiece. 
 
 The telescope is primarily a light-collecting device to concentrate 
 on the pupil of the eye a greater quantity of light from an object 
 point than the eye would otherwise receive. At the same time the 
 angular separation of the object points, as seen in the image, is 
 increased. Mere magnification of the size of the image, however, 
 without corresponding increase in illumination serves little purpose. 
 Hence the general rule that the best power to use is the lowest power 
 which enables the eye to see the details of the object. With this power 
 the size of the exit pupil is larger than with higher powers and the 
 image appears to the eye brighter and more readily seen. 
 
 The quantity of light entering a telescope depends in general 
 directly on the area of its entrance pupil, and this is commonly the 
 objective itself. The relative light-gathering power of two telescope 
 objectives varies accordingly as their areas or as the squares of their 
 diameters. The relative quantities of light from a distant object 
 point, such as a star, flowing into a telescope objective of 1-inch (25.4 
 mm.) aperture and the pupil of the eye of ^ inch (5 mm.) aperture 
 are accordingly as 25 to 1. Similarly a 2-inch objective collects 4 
 times as much light as a 1-inch objective and 100 times as much as 
 the eye; a 3-inch objective, 9 times as much as the 1-inch and 225 
 times as much as the eye. From this it may be concluded that, 
 since in a telescope the ratio of the diameter of its entrance pupil 
 (ordinarily diameter of objective) to that of its exit pupil (eye circle) 
 is a measure of its magnifying power, a linear magnification of 10 
 diameters (one hundredfold magnification of corresponding image 
 areas) is the most favorable in a 2-inch objective. In this case the 
 full resolving power of the eye is utilized. Although an appreciable 
 amount of light is lost on its passage through the telescope lens 
 system, experience has proved that for field purposes this degree of 
 magnification is satisfactory. On the other hand, the magnifica- 
 tion should not be so low that the size of the exit pupil exceeds 
 appreciably that of the eye pupil, which at a maximum is 8 milli- 
 meters in diameter. In the Army type 6 by 30 binocular field- 
 glass the magnification is 6 diameters; the diameter of its objective 
 is 30 millimeters; the diameter of its exit pupil is accordingly 5 
 millimeters. 
 
 To recapitulate: The size of the objective in a telescope determines 
 in general the quantity of light which enters the eye through the 
 
22 CHARACTERISTICS OF OPTICAL GLASS. 
 
 exit pupil. This, in turn, should be approximately equal in area to 
 that of the pupillary aperture of the eye. If it is much larger, some 
 of the light is lost; if much smaller, the subjective brightness of the 
 image is decreased and the resolving power of the eye is not fully 
 utilized. The magnifying power of the telescope should be so chosen 
 that its exit pupil is approximately equal to the pupillary aperture 
 of the eye. The visual brightness of an image can never be greater 
 than that produced by the object itself on the retina; but the fact 
 that the telescope objective concentrates a much larger cone of rays 
 from each object point than does the eye, and at the same time 
 increases the angular separation of these points in the ratio of the 
 magnifying power, accounts for the lack of decrease in apparent 
 brightness which one might expect with increase in magnification. 
 It also explains the fact that many stars invisible to the unaided 
 eye are readily seen through a telescope, and this in spite of the loss 
 of an appreciable quantity of light by .absorption and reflection in 
 the telescope itself. In the case of a fixed star, the star remains a 
 point or diffraction disk even under the highest powers; but, because 
 the telescope gathers a large amount of light, its effect is to produce 
 a correspondingly increased sensation of light on the retina. The 
 luminous stimulus must exceed a certain limit of light energy flux 
 (about 0.001 meter-candle) in order to produce the sensation of light 
 in the eye. For energy fluxes below this limit the eye fails to respond 
 and the luminous point is not visible. The larger the diameter of 
 the telescope lens the greater is its resolving power, and the fainter 
 are the stars which are visible. In twilight illumination details of ob- 
 jects are more readily discerned through a binocular than with the un- 
 aided eye, chiefly because of increased angular separation of the details 
 accompanied by an increase in brightness to offset the increase in 
 size of the retinal image. 
 
 In the design of a telescope lens system the effort is made to 
 obtain an apparent field of view which is comparable to that of the 
 unaided eye. The apparent angular field of view (actual field of 
 view times the magnification) ranges in telescopes from 15 to 50. 
 The higher the magnification the smaller the actual angle subtended 
 at the telescope between points at opposite margins of the field. 
 
 THE QUALITY OF THE IMAGE. 
 
 The ideal image, as formed by the objective of a telescope, is one 
 similar in every respect to the distant object, so that, when viewed 
 through the eyepiece, the image produced on the retina of the eye is 
 a correct and enlarged picture of that received on the retina when 
 viewed by the unaided eye. There arc, however, a number of factors 
 which render it impossible actually to attain this ideal; but in 
 modern lens systems it is possible to approach so closely to it that 
 
QUALITY OF THE IMAGE. 
 
 23 
 
 the definition attained is as good as the eye is capable of perceiving, 
 and for practical purposes this suffices. A brief summary of the 
 several defects of the image will indicate some of the factors with 
 which the lens designer and lens constructor have to contend. It is 
 customary to consider, under separate headings, the defects or aberra- 
 tions affecting image points situated along the axis (line of sight) of the 
 instrument and the defects for points removed from the axis. These 
 aberrations occur for each color of light employed. The defects 
 arising from the use of white light are designated chromatic aber- 
 rations in contrast to the aberrations which are present when light 
 of only one color (monochromatic light) is employed. 
 
 The significance of the several aberrations of a lens is most readily 
 presented by considering first its action on extremely narrow pencils 
 of light rays entering indefinitely near the axis (first order theory) 
 and then deducing the effects produced on wider pencils and larger 
 apertures (third order theory). Gauss showed that for the first 
 
 V 
 
 FIG. 4. Diagramillustrating the principal Gauss points H and H' (planes), and the principal foci, Fand 
 
 F', of a lens for paraxial rays. 
 
 case the effect of the lens may be completely defined by reference to 
 six points on the axis, namely, two focal points, two principal points, 
 and two nodal points. If the lens is surrounded by air and the re- 
 fringence of the medium for the entering rays is the same as that for 
 the emergent rays, the nodal points coincide in position with the 
 principal points, and the lens or lens system may be replaced for 
 purposes of computation by its two foci, F, F', and its two principal 
 points Hj H' \ thus in figure 4, F and F r are the two foci and H and 
 H', the principal points (also nodal points). The equivalent focal 
 length of the lens is F H=F' H' '; its external focal lengths are 
 FV and F'V. If light of different colors be used it is found 
 that rays of different color intersect the axis at points near F ', but 
 not exactly coinciding with it. This variation in the position of the 
 focus or distance of the focus from the rear surface of the lens (ex- 
 ternal focal length) with change in wave length is called chromatic 
 aberration. f It is also found that the position of H' changes slightly 
 
24 CHARACTERISTICS OF OPTICAL GLASS. 
 
 with the color of the light, so that even if the lens were corrected for 
 chromatic aberration the focal distance II' F' varies with the color; 
 this is called chromatic difference of focal length or chromatic differ- 
 ences of magnification. 
 
 If the pencil of light from a distant object point on the axis is not 
 indefinitely narrow but is sufficiently wide to transmit an appre- 
 ciable amount of light, so that at F' there is formed an image point 
 which can be seen, it is found that the marginal rays intersect the 
 axis near F f but not exactly at F ' ; this change in the position of the 
 focus for rays of different aperture is called spherical aberration, and 
 like chromatic aberration means a variation in external focal lengths. 
 In a lens corrected for spherical aberration a distant object point on 
 the axis is imaged as a single point on the axis. If it is desired to 
 produce a single image point of a distant object point situated slightly 
 off the axis, it is essential that the lengths of the optical paths of all 
 rays from the object point to the image point be equal; this will be 
 the case, as Abbe was the first to show, when, for each ray, the ratio of 
 the sine of its opening angle (i. e. angle between axis and ray diverging 
 from object point) to the sine of its closing angle (i. e. angle between 
 axis and ray converging to conjugate image point) is a constant. 
 For a distant object point the entering rays are practically parallel, 
 and the Abbe sine condition is equivalent practically to the state- 
 ment that the focal length, and hence the imagination of the lens 
 for different zones, is constant. The four aberrations, namely, 
 spherical aberration, sine condition, chromatic aberration, and chro- 
 matic differences of magnification are fundamental; but there are 
 other aberrations which are important and merit consideration. 
 In the following paragraphs a description of these aberrations is 
 given, together with a somewhat different treatment of the four 
 aberrations noted above. 
 
 MONOCHROMATIC ABERRATIONS. 
 
 The important monochromatic aberrations are five in number. 
 It is not possible to eliminate them all in any one system and they 
 are not all equally important in any given instrument; the lens 
 designer endeavors, therefore, to reduce to a minimum those aberra- 
 tions that are serious for the special type of lens system he desires. 
 The five monochromatic aberrations are: (1) Aberration of a point 
 on the axis (spherical aberration), (2) aberration of points removed 
 from the axis (coma, sine condition), (3) astigmatism, (4) curvature 
 of field, (5) distortion. The effects of these aberrations are illus- 
 trated in figure 5, a to h. 
 
 1. Axial spherical aberration. In a simple collective lens (fig. 5a) 
 refraction at the periphery causes rays near the margin to converge 
 toward an axial point nearer the lens than the point for central axial 
 
MONOCHROMATIC ABERRATIONS. 
 
 25 
 
 rays. The lens exhibits an excess of convergence for peripheral 
 rays. A simple dispersive lens, on the other hand, exhibits an excess 
 diverging effect for the peripheral. rays. (Fig. 56.) The result in 
 both cases is to produce a gen- a 
 
 eral lack of sharpness in the 
 image. This is called spherical 
 aberration, longitudinal or axial. 
 It is overcome in an objective 
 by combining a collective lens 
 with a dispersive lens as indi- 
 cated by figure 5c so that the 
 excess converging effect of the 
 collective lens is neutralized by 
 that of opposite character in 
 the dispersive lens. The axial 
 point of convergence for a beam 
 of parallel incident rays is called 
 the focal point or the focus 
 of the lens. In correcting for 
 spherical aberration the de- 
 signer may cause the marginal 
 rays to focus at a point beyond 
 the point of convergence of 
 the central rays (fig. 5d) ; the 
 combination is said then to be 
 spherically over corrected. Fig- 
 ure oa illustrates a spherically 
 under corrected lens. An objec- 
 tive may focus both central and 
 marginal rays at one point and 
 fail to do so for intermediate 
 rays (fig. 5e) ; the lens is then 
 said to show spherical zones. 
 Correction of spherical aberra- 
 tion can be effected for only 
 one pair of conjugate planes. 
 2. Coma, sine condition. The 
 
 effect of an UnCOrrected lens On FIG. 5. (a) Spjierical aberration in a single collective 
 
 oblique rays from a point re- 
 moved from the axis is to image 
 the central rays at one point, 
 and the marginal rays nearer or 
 farther away from the axis. 
 (Fig. 5f.) Even though the lens is corrected for axial spherical 
 aberration, it may show lateral spherical aberration for extra-axial 
 points, and the effect is then to draw out the image of the point 
 
 (positive) lens, (b) Spherical aberration in a single 
 dispersive (negative) lens, (c) Doublet corrected for 
 spherical aberration, (d) Spherical overcorrection in 
 a doublet, (e) Spherical zones in a doublet corrected 
 for spherical aberration. (/) Coma in a lens. Lack 
 of fulfillment of sine condition, (g) Astigmatism in 
 a lens. 
 
26 CHARACTERISTICS OF OPTICAL GLASS. 
 
 so that it resembles a comet with its tail directed toward, or away 
 from, the axis. This defect is known as coma. 
 
 Astigmatism. The effect of a lens system on oblique rays is to 
 produce not only coma, but also two sets of images for points re- 
 moved from the axis; in the one set (inner image surface) radial 
 lines (vertical) are imaged, in the second, tangential lines (horizon- 
 tal). (Fig. 5g.) This aberration may be considered to follow as a 
 result of the foreshortening of the lens in the vertical as compared 
 with the horizontal plane for an inclined beam of light. Both coma 
 and astigmatism increase with the obliquity of the incident rays. 
 Astigmatism is removed when the two focal surfaces are brought to 
 coincidence. 
 
 Curvature of field. The correction for astigmatism may result in a 
 curved image surface, so that the image is not entirely in focus over 
 the whole field at any one time. In the lens corrected for flatness of 
 field, however, the image surface is plane. 
 
 Distortion. Even after all of the above aberrations have been sat- 
 isfactorily reduced the image may be distorted so that points on the 
 margin of the field are magnified more than the central area (pin- 
 cushion distortion) or vice versa (barrel-shaped distortion) . In this, 
 as in the foregoing aberrations, the complete elimination is limited 
 to definite distances of the object. 
 
 CHROMATIC ABERRATIONS. 
 
 If instead of an object illuminated by monochromatic light a col- 
 ored object is observed, there is for each color of light an image 
 formed. These images are superimposed and are in different planes; 
 this gives rise to the defects called chromatic aberrations, of which it 
 is convenient to distinguish two cases, namely, axial chromatism 
 and lateral chromatism. 
 
 Chromatic aberration or axial chromatism. The effect of a simple 
 collective lens on a beam of white light is shown in figure 6a. The 
 blue rays converge to a point nearer the lens than the red rays and 
 the lens is said to be chromatically undercorrected. To neutralize 
 this effect a dispersive lens of higher relative dispersion is combined 
 with the collective lens (fig. 6&) and rays of two colors, such 
 as red and blue emerging from a given axial object point, proceed 
 to the same image point on the axis. A lens corrected for two 
 colors is called achromatic; the departure from exact convergence, 
 to an image point, of rays other than the two for which the lens is 
 corrected gives rise to colored borders on the image; these residual 
 color errors are called " secondary spectrum." By the use of optical 
 glasses in which the partial dispersion ratios in the two glasses are 
 nearly identical throughout the spectrum, rays of these colors can 
 
CHROMATIC ABERRATIONS. 
 
 27 
 
 be imaged at practically one image point, and only negligible colors 
 of the tertiary spectrum remain. 
 
 Chromatic differences of magnification; lateral chromatism. For 
 points removed from the axis the corresponding image points may 
 be displaced laterally by different amounts as a result of the differences 
 in magnification with different colors so that the size of the image 
 for blue light is different from that for red light; this gives rise to 
 color fringes toward the margin of the field (fig. 6c). In the 
 microscope this error, which is characteristic of apochromatic ob- 
 
 FIG. 6. (a) Chromatic aberration in a single collective lens. (6) Doublet corrected for axial chromatism. 
 (c) Chromatic differences of magnification or lateral chromatism in a doublet. 
 
 jectives, is neutralized by the use of compensating eyepieces in which 
 the chromatic differences of magnification are of the same magnitude 
 as those of the objective, but of opposite character. 
 
 The materials of which optical lens and prism systems are made 
 must satisfy extremely rigid requirements of high precision; the 
 character of workmanship in the grinding, polishing, and adjusting 
 of the several elements of a lens system must also be good in 
 order to conform to the specifications imposed by the lens designer. 
 In common with other factory operations requiring a high degree of 
 technical skill, a considerable amount of experience is required to 
 ascertain and to maintain the best methods for accomplishing the 
 
28 CHARACTERISTICS OF OPTICAL GLASS. 
 
 ends desired. Under war-time conditions this can be secured only 
 by adequate realization of the principles involved and by a certain 
 inventive adaptability on the part of the men concerned to make 
 the best of the facilities at hand and to develop new facilities at the 
 required speed. 
 
 THE CHARACTERISTICS OF OPTICAL GLASS. 
 
 Optical glass, as used in lenses and prisms, functions as a medium 
 so to refract the rays of light from any distant object point that they 
 will converge to a single corresponding point in the image. This con- 
 dition is extremely difficult to meet and requires that the glass in 
 each lens or prism element be of uniform quality and properties 
 throughout and that its optical constants agree very closely with 
 those of certain standard types of glass. To manufacture, on a 
 large scale, a series of different types of glass of this degree of per- 
 fection requires close attention to details. 
 
 The art of making optical glass consists essentially in melting 
 together certain ingredients at a sufficiently high temperature to 
 insure liquidity so that bubbles which are formed rise to the surface 
 and escape, of mixing the melt thoroughly by vigorous stirring so 
 that its composition is the same throughout, and then allowing the 
 pot of molten glass (600 to 3,500 pounds) to cool down slowly to room 
 temperature. The ingredients that are put into the batch depend 
 on the kind of glass desired; they are essentially the oxides or the 
 salts of the metals that are found in natural rocks, and include silica 
 (as sand), sodium and potassium oxides (as nitrates and carbonates), 
 calcium oxide (as calcium carbonate), and aluminium oxide. In 
 addition to these oxides certain other oxides are used to impart to 
 the glass special properties; these include lead oxide (as red lead or 
 litharge), barium oxide (as barium carbonate), zinc oxide, boric 
 oxide (as hydroxide, or as borax), antimony and arsenic oxide, rarely 
 a little manganese (as MnO 2 ), selenium, cobalt, and nickel (as oxides), 
 and in some glasses fluorine (as a fluoride). A glass that contains 
 an appreciable amount of lead is called flint glass, otherwise it is 
 crown glass; thus we have series of ordinary flint glasses, of barium 
 flints, of borosilicate flints; of ordinary crowns, of silicate crowns, of 
 borosilicate crowns, of barium crowns, etc. These glasses have 
 different refractive indices and different relative dispersions and are 
 used in combination in lens and prism systems to reduce to a mini- 
 mum the aberrations peculiar to the special optical system under 
 design. The quality of performance of the lens system depends on 
 the skill of the lens designer and the lens maker, and also on the 
 quality and variety in types of glasses available. 
 
HOMOGENEITY. 29 
 
 The characteristics of good optical glass are : 
 
 Homogeneity: 
 (a) Uniformity in chemical composition 
 
 1. Freedom from striae. 
 
 2. Freedom from bubbles. 
 
 3. Freedom from inclusions, stones, and crystallites. 
 
 4. Freedom from cloudiness. 
 (6) Uniformity in physical state 
 
 1. Freedom from strains. 
 
 II. Definite refractive indices for different wave lengths: 
 (a) Refractivity. 
 (6) Dispersivity and dispersion ratios. 
 
 III. Freedom from color. 
 
 IV. High degree of transparency. 
 
 V. High degree of chemical and physical stability: 
 
 (a) Resistance to action of weather and certain chemical agents. 
 (6) Toughness and hardness. 
 
 I. HOMOGENEITY. 
 
 A fundamental requirement for optical glass is homogeneity; even 
 a slight departure from a high degree of uniformity in composition 
 is not tolerated because of the effect on the performance of the fin- 
 ished optical instrument. Compared with other kinds of glass, 
 optical glass is a thing of extreme precision; the entire manufactur- 
 ing process of optical glass has been developed with the object of 
 attaining a highly homogeneous product. A number of factors enter 
 into the problem; neglect of any one of these may render the glass 
 unsuitable and useless for optical purposes. These factors will now 
 be considered in some detail; appreciation of their significance is 
 essential to a proper understanding of the several steps of the manu- 
 facturing process. Lack of chemical homogeneity finds expression 
 in stria?, veins, cords, ream; in bubbles, seeds, air bells, boil; in stones 
 and other inclusions, such as crystallites, and in strained glass. 
 
 (a) UNIFORMITY IN CHEMICAL COMPOSITION. 
 
 1. Strise (veins, cords, threads, ribbons, ream, etc.). Stria? are 
 streaks of different composition within the glass mass; they represent 
 either original differences in composition (resulting from insufficient 
 mixing of the batch or from selective settling of batch elements 
 during melting; these differences the stirring process failed to 
 eradicate entirely) or differences arising either from materials 
 introduced into the melt because of solution of the pot or from 
 the volatilization of certain components of the melt whereby local 
 differences in concentration are produced. Striae are generally 
 lower in refractive index than the inclosing glass; the differences in 
 refractive index are commonly limited to the fourth decimal place; 
 but in the case of heavy stria? they may increase to several units in 
 3922921 3 
 
30 
 
 CHAKACTERISTICS OF OPTICAL, GLASS. 
 
 the third decimal place. (Fig. 7.) In one instance the refractive 
 index of a very heavy cord was found to be 0.007 less than that of 
 
 FIG. 7. (a) Heavy striae in a plate of glass. (6) Ribbon striae (central illumination), (c) Ribbon striae 
 (oblique illumination), (d) Cord in a plate of glass; stone with striae streamers near top of plate. 
 (e) Ream in rolled optical glass. (/) Heavy striae in a lens, (g) Cord and pressing defect in a lens, 
 (ft) Ream cut across by the steep curve of a negative lens. 
 
 the surrounding glass. As a result of these differences, the paths 
 of the transmitted light rays are deflected slightly and to this extent 
 the quality of the image is impaired. The effect of very slight 
 
CORDS AND STRIDE. 31 
 
 differences in refractive index, even in the fifth decimal place, is 
 readily seen on a warm day, when hot ascending air currents render 
 distant objects indistinct and destroy sharp definition. 2 
 
 Because of their different refringences striae disturb the paths of 
 transmitted light rays slightly so that these rays no longer hit the 
 exact point of the image they are supposed to hit; but they miss it 
 by a very little; each point of the image may suffer similarly and 
 the result is a decrease in sharpness over the entire field. In the 
 case of high-power instruments the rays converge to an image point 
 under a very small angle and a slight deviation in path seriously 
 affects the quality of the image; in low-power instruments the rays 
 converge to the image points under a larger angle and the same 
 amount of angular deviation may be practically negligible. For 
 this reason it is essential that for optical measuring instruments 
 of precision, such as range finders, panoramic sights, etc., optical 
 glass of the best quality only be used; for low-power visual instru- 
 ments, such as trench telescopes, glass of second quality may not be 
 objectionable for certain lens elements. 
 
 Experience has shown that the optical effect of fine striae or even 
 of heavy striae in optical glass depends on their position and 
 abundance in the particular optical element in which they appear. 
 In the case of heavy cords or ribbons whose composition is noticeably 
 different from that of the adjacent glass, their effect on transmitted 
 light rays is so serious that the glass is worthless for optical purposes. 
 A single heavy cord located near the margin or even at the center 
 of a lens deflects and renders useless only a small fraction of the 
 transmitted light and may have no perceptible effect on the defini- 
 tion; if the cord is in the objective lens, it is imaged near the exit 
 pupil of the instrument and, although not visible, functions as would 
 a piece of thread or wire placed directly in front of the eye. If the 
 stria appears near the image plane, it is seen directly and destroys 
 the definition along its path. In the case of fine striae the effect 
 depends largely on their character; it may be negligible for certain 
 elements in low-power optical instruments. In photographic lenses 
 of precision fine striae in a lens element enlarge the circle of con- 
 fusion for image points to such an extent that sharp definition is 
 destroyed; a single heavy thread, on the other hand, simply deflects 
 a small amount of light and does not cause appreciable deterioration 
 of the image. 
 
 The effect of the presence of striae hi prisms depends on the type 
 of striae and on their position in the prisms, also on the type of the 
 prism. In the case of individual threads which are sharply defined, the 
 
 2 An interesting paper on the " Optical conditions accompanying the striae which appear as imper- 
 fections in optical glass" has recently been published by A. A. Michelsonin Scientific Paper No. 333, U. S. 
 Bureau of Standards, 1919. 
 
32 CHARACTERISTICS OF OPTICAL GLASS. 
 
 effect is simply to cut off a small amount of light and is not sufficient 
 to cause perceptible decrease in illumination or definition. In the 
 case of heavy striae a large part of the light suffers deflection and the 
 image of a distant object examined through the prism and an observ- 
 ing telescope may be doubled or appear to be badly astigmatic; 
 fine-banded striae may give rise also to a general flare, resembling 
 that from coma, over the field. In the case of fine stria? the effect 
 may be negligible or it may be so serious that the definition is spoiled ; 
 if the fine striae are residual remnants of heavy cords which have not 
 been completely dissolved, they may be surrounded by glass whose 
 composition and refringence changes gradually in the vicinity of 
 the striae. A change of this kind in composition and refractivity is 
 serious because it warps a transmitted light wave and renders sharp 
 definition impossible. Under ordinary conditions of test such changes 
 in composition are moreover not readily detected. 
 
 In the testing of prisms one is impressed with the variety and number 
 of striae which may be present and yet have no perceptible effect on 
 definition in the image; whereas in other prisms even very faint 
 striae affect the quality of the image seriously. A heavy stria in an 
 objective prism of a range finder may, if located near the margin, have 
 no perceptible effect on the image; but the same stria situated at the 
 center of the prism affects the definition so seriously that the prism 
 is worthless. In the case of large objective prisms, the incident light 
 rays are practically parallel and it is essential that the quality of the 
 reflecting prism be of the best, otherwise astigmatism, double images, 
 flare, and other defects are introduced into the image. The same 
 holds true of roof-angle and other prisms in which each light ray 
 traverses the prism in more than one plane. Glass for such prisms 
 should be entirely free from striae of any kind, otherwise the resolving 
 power of the instrument containing such prisms may be seriously 
 impaired. 
 
 In the case of fine-banded striae, called "ream" by the plate-glass 
 maker, experience has shown that if the planes of the ream are 
 normal to the line of sight the quality of the image is not appreciably 
 affected by the presence of ream. For example, the protecting win- 
 dows or shields and the reticules of certain fire-control instruments are 
 commonly made of selected plate glass, which is characterized by the 
 presence of bands and ribbons of fine striae approximately parallel 
 with the polished surfaces. The crown lenses of many eyepieces are 
 made of molded rolled glass. All spectacle lenses are made from 
 molded rolled glass, not from stirred optical glass. Experience has 
 shown that in many low-power optical instruments the use of rolled 
 glass for relatively flat lenses of large curvature is permissible, 
 especially if the lenses before grinding be molded to approximately 
 the final shape. For lenses of deep curvature it is not good practice 
 to use average quality rolled glass, because the heavy reams are cut 
 
BUBBLES AND SEEDS. 33 
 
 across and function there as ordinary striae causing double images, 
 etc. (Fig. 77i.) 
 
 The fact that a stria of a certain kind may destroy the usefulness 
 of a prism or lens when located in one position is in general sufficient 
 to bar it out entirely even though it would not be serious were it 
 located in another part of the lens or prism. It is better policy in 
 manufacture to eliminate raw, unworked material which is defective 
 than to discard it later after much labor and expense have been put 
 on it to produce finished optics. The probability that the striae will 
 be favorably located in the finished lens or prism is not sufficiently 
 great to make the risk worth the while. 
 
 A less tangible but more serious defect in optical glass than stria? 
 is the gradual change from point to point in its refringence. This 
 may be present in optical glass in which no stria? are visible. It can 
 only be detected by careful measurement of the relative refringence 
 at different points in a glass plate; this measurement is commonly 
 made by means of an interferometer or a precision refractometer. A 
 gradual change in refractivity in a lens or prism gives rise to an 
 unequal warping of the transmitted wave surfaces and spoils the 
 definition in the image. 
 
 Bubbles (seeds, air bells, vacuum bubbles, boil ). 3 At all stages in the 
 melting and fining process of optical glass manufacture, volatile matter 
 escapes from the melt; in case any of this volatile matter fails to 
 reach the surface before the molten glass cools down, it remains 
 entrapped in the melt as a bubble. (Fig. 8a.) Bubbles vary greatly 
 in size from minute specks hardly discernible to the unaided eye to 
 large bubbles several millimeters and even centimeters in diameter. 
 Illuminated from the side by a strong source of light the bubbles in a 
 piece of glass appear as brightly shining points or stars within the 
 glass mass. Bubbles are not desirable in optical glass; but the effect 
 of a bubble depends largely on its position within the optical system. 
 A bubble is not tolerated in the image plane of a telescope system 
 because it may disturb details in the field of view; but bubbles situ- 
 ated in lenses and prisms distant from the image plane are not in 
 general serious, as they tend chiefly to cut out a negligible percentage 
 of the transmitted light. Bubbles in a telescope objective are imaged 
 in or near the exit pupil of the instrument; if they are large the 
 effect on the observer's eye is the same as though a fine speck of 
 opaque substance were actually placed directly in front of his eye; 
 this holds true for stria? to a much greater degree. In most cases 
 the bubbles are so small that this effect is negligible. In some 
 instances a small bubble marks the position of a former particle of 
 
 3 " Seeds" are small bubbles; "boil" are large bubbles developed toward the end of the fining period or 
 as a result of "blocking"; "vacuum bubbles" are commonly of fair size and develop during the cooling 
 down of the melt; "air bells" are of irregular shape and are formed generally during the pressing or mold- 
 ing operations. 
 
34 
 
 CHARACTERISTICS OF OPTICAL GLASS. 
 
 B 
 
 FIG. 8. (c) Bubbles in a plate of glass; stone with striae streamer near center of plate. (6) Pressing de- 
 fects or "feathers" or "laps" in a lens, (c) "Stones" in optical glass; radial spherulites of cristobalite. 
 (<J) Part of specimen c in polarized light; shows strain-aureole around each stone. " 
 
STONES. 35 
 
 sand or clay which, on solution, set free included gases, thus forming 
 a small bubble. In this case the bubble is apt to mark the end of 
 the short streak or stria in the glass which, if appreciably developed, 
 is a sufficient cause for rejection of the glass. Optical glass should 
 be free from bubbles of this nature. 
 
 Bubbles are hot always easy to avoid entirely; and in some types 
 of optical glass it is practically impossible to produce glass free from 
 small seeds. 
 
 Stones. Stones are included fragments of undissolved material in 
 the glass mass. They may represent coarse particles of the original 
 batch materials (batch stones, such as clusters of sand grains) which 
 failed to be entirely dissolved during the glass-melting operation; 
 more commonly they are pieces of the pot walls (pot stones) which, 
 loosened from the sides or bottom of the pot, find their way into the 
 melt. (Fig. 9a.) They may be fragments of the crown of the fur- 
 nace (crown drops) which have fallen into the melt. Inclusions of 
 any kind are unwelcome guests. The glass adjacent to them is 
 usually in a state of great strain as a result of the difference in rate 
 of contraction between glass and inclusion on cooling; characteristic 
 cone-shaped fracture surfaces may develop in the glass adjacent to 
 such inclusions. 
 
 Stones and included folds (feathers, pressing defects) of dusty 
 material in lenses and prisms may be introduced during the pressing 
 process after the glass has been taken into work. (Fig. 96.) In 
 preparing the glass fragments for pressing into desired shapes it is 
 common practice for the workman to heat them up in a muffle fur- 
 nace on a slab of refractory material. In order to prevent the softened 
 glass from sticking to the plate during this operation, powdered clay, 
 mica, talcum, graphite, or a mixture of these or other materials is 
 spread over the plate. This powder clings to the undersurface of 
 the softened glass fragments. In preparing each glass fragment for 
 his press, the workman paddles it up into a suitable shape. If this 
 operation is done carelessly, he plasters the sides of the fragments 
 with the powder; in the pressing operation these dusty surfaces may 
 be enfolded into the lens or prism blank (fig. Sb) , thereby spoiling it 
 for use in an optical system. Carelessness in this operation may 
 result in large rejections of the finished blanks. Trouble from this 
 source can be greatly reduced by the use of proper refractory base 
 plates and by avoiding the use of excess powder. It is also possible 
 to modify the procedure so that no powder is used and pressing 
 defects are largely eliminated. 
 
 The presence of stones in optical glass is adequate cause for its 
 rejection, because not only are they not tolerated in optical systems, 
 but, as a result of solution, they commonly leave a trail of striae in 
 their passage to and fro in the melt, thus spoiling a much larger 
 percentage of glass than their presence at isolated points within the 
 glass mass might indicate. 
 
36 
 
 CHARACTERISTICS OF OPTICAL GLASS. 
 
 FIG. 9. (c) "Stones" (spherulites of wollastonite) in glass; note also "joint" cracks developed on rapid 
 cooling of the plate of glass. (6) Crystallization bodies in optical glass, (c) Spherulites of cristobalite 
 on surface of barium crown melt, (rf) Crystals of barium disilicate in barium crown glass. 
 
UNIFORMITY IX PHYSICAL STATE. 37 
 
 Crystallisation bodies, (radial spherulites, crystallites, devitrifica- 
 tion stones). On cooling down from a high temperature optical glass 
 behaves like any other solution with falling temperature; the solu- 
 tion becomes supersaturated with respect to certain components 
 and these begin to crystallize out if given time to do so. (Fig. 9a to d.) 
 The homogeneity of the glass melt is thus destroyed; strains of 
 appreciable magnitude are set up in the cooling glass mass adjacent 
 to the crystallites and render it useless for optical purposes. (Fig. 8d.) 
 The presence, in a lens or prism, of an inclusion, however small, is 
 sufficient cause for its rejection. 
 
 Cloudiness. Under certain conditions of manufacture cloudy or 
 milky glass results. Turbidity or opalescence of this kind in optical 
 glass is a very serious defect and renders it useless for optical purposes. 
 The turbidity is easily detected and the glass is rejected before it 
 passes beyond the first melting stage or at worst the lens- and prism- 
 pressing stage of the manufacturing processes. 
 
 b. UNIFORMITY IN PHYSICAL STATE. 
 
 Freedom from strain. Although a piece of glass may be homoge- 
 neous in a chemical sense, yet as a result of improper heat treatment 
 it may be in a state of internal strain. This is to be avoided in opti- 
 cal glass chiefly for one reason. Glass under strain is not in equilib- 
 rium; and even at room temperature the internal stresses seek re- 
 lief by slow movement within the glass block. In a highly strained 
 piece of glass the internal movement may reach some mechanically 
 weak spot in the glass mass, such as a stria or an inclusion that is 
 not able to maintain the stress; the result is then a rapid shearing 
 and consequent fracture. The glass plate or lens cracks without 
 warning. The movement of the strain over portions of a piece of 
 glass is readily followed by examination in polarized light. In 
 optical systems lenses and prisms of definite shapes and sizes are 
 used. A slight departure from the prescribed surfaces gives rise to 
 defects in the image which are readily detected and impair its quality. 
 Strain in optical glass causes the surfaces to warp during the polishing 
 and figuring processes. This tendency toward deformation contin- 
 ues after the several elements of an optical system have been mounted 
 and is a constant source of trouble to the extent of causing an element 
 to crack and thus to render the whole optical system useless. 
 
 A piece of glass under strain is analogous, in its behavior toward 
 transmitted light waves, to a birefracting crystal. This was discov- 
 ered in 1813 by Sir David Brews ter 5 who found that a glass plate 
 under load (compressional stress) behaves as a uniaxial negative 
 crystal, the optic axis being the direction of application of the load. 
 
 The identification of stones in optical glass is discussed by N. L. Bowen in J. Amer. Ceramic Soc., I, 
 594-605, 1918. 
 
38 
 
 CHARACTERISTICS OF OPTICAL GLASS. 
 
 B 
 
 D 
 
 FIG. 10. (o) Well-annealed piece of optical glass. (6) Annealing fair, (c) Annealing poor; note press 
 ing defect in lower half of plate, (d) Annealing very poor, (e) Annealing poor in piece of optical glass ;- 
 note that bubbles (black spots) show no evidence of local strain, whereas the stone to the right of the 
 center of the piece exhibits pronounced local strain. (/) Heavy striae cause strain in a well annealed 
 block of glass, (g) Poorly annealed block of glass. 
 
STRAIN BIREFRINGENCE. 39 
 
 Brewster discovered that the degree of birefringence, as measured by 
 the path difference of the two plane-polarized light waves formed 
 on traversing the strained block at right angles to the direction of 
 the applied load, is proportional to the load itself; in other words the 
 path-difference per unit length of path or the birefringence may 
 serve as a direct measure of the strain. 
 
 It may be inferred that because two rays of different refractive 
 indices are formed as a result of the strain these may seriously affect 
 the quality of the image. In order to test out this inference the 
 degrees of strain in a number of plates of different types of optical 
 glass made by Schott & Genossen, by Parra-Mantois, and by glass- 
 makers in this country were measured. On each plate the actual 
 maximal path difference between the transmitted light waves was 
 determined and this in turn was reduced to path difference per cen- 
 timeter length of glass path traversed. (Fig. 10.) In the best 
 annealed samples the maximal observed path difference for sodium 
 light at the margin of the glass plate was less than 5 millimicrons 
 (millionths of a millimeter) per centimeter glass path. In optical 
 glass of fair quality the path difference reached a value of 20 milli- 
 microns per centimeter; in samples of poor quality a path difference 
 of 40 to 50 millimicrons per centimeter was reached. But even a 
 path difference of 50 millimicrons per centimeter is equivalent to 
 a difference of only 0.000005 between the refractive indices of the 
 two transmitted waves; this difference is negligible even in the best 
 optical systems. Experience has shown that a difference five times 
 this value is within the tolerance limits of the most exacting optical 
 systems. 
 
 There is still a possibility to consider, namely, the change in 
 refractive index of the material under hydrostatic pressure and the 
 change in actual refractive indices for rays vibrating parallel and 
 normal to the direction of an applied load. Measurements by Kerr, 6 
 Pockels, 7 and computations by Adams and Williamson 8 have shown 
 that the index of refraction of glass is increased by compressional 
 load and decreased by tensional load. Kerr found from measure- 
 ments with a Jamin interference refractor that in the case of com- 
 pression both waves are retarded, while in the case of tension both 
 waves are accelerated; that the wave whose vibrations take place in 
 the plane normal to the direction of the applied load is retarded most, 
 its retardation being practically twice that of the wave vibrating 
 along the axis of pressure. 
 
 S. Czapski 9 measured the relative and absolute changes in refrac- 
 tive index in poorly annealed glass rods and plates by a dioptric 
 
 5 Philosophical Transactions, 1814, 1815, 1816. 
 J. D. Kerr, Phil. Mag. (5), 26, p. 321, 1888. 
 
 7 F. Pockels, Ann. d. Phys. (4), 7, p. 745, 1902. 
 
 8 L. H. Adams and E. D. Williamson, Jour. Wash. Acad. Sci., 9, pp. 609-623, 1919. 
 S. Czapski, Ann. d. Phys. u. Chem., 42, p. 319, 1891. 
 
40 CHARACTERISTICS OF OPTICAL GLASS. 
 
 method based on Brewster's and Exner's observation that a cylin- 
 drical glass rod behaves optically as a meniscus lens. Czapski found 
 that the index of refraction for both waves increases from the center 
 of the glass rod or plate ; for crown glass, so strained that the resulting 
 path difference is about 400 millimicrons per centimeter, the increase 
 in refractive index of the wave vibrating parallel with the axis of 
 pressure is 0.0000914 and for the second wave 0.0000468; for a sec- 
 ond plate, so strained that the resulting path difference is about 150 
 millimicrons per centimeter, the increases in refractive indices are 
 0.0000303 and 0.0000155, respectively; for a flint block so strained 
 that the path difference is about 325 millimicrons per centimeter, the 
 increases in refractive indices were 0.0001465 and 0.0001166, re- 
 spectively. 
 
 Adams and Williamson ascertained by computation that the index 
 of refraction of a light flint glass of refractive index n D =1.57, is 
 increased 0.00118 by hydrostatic pressure of 1,000 kilograms per 
 square centimeter; that in the case of a load of 1,000 kilograms per 
 square centimeter (undirectional pressure) the increases in refractive 
 indices of the two waves vibrating normal and parallel with the axis 
 of pressure are, respectively, 0.00049 and 0.00020. Their experimental 
 results show, moreover, that the birefringence resulting from the 
 application of a load of 1 kilogram per square centimeter to a block 
 of glass ranges from 2.5 X 10~ 7 to 3.2 X 10 ~ 7 for the ordinary types of 
 glass. The observation by Brewster that the optical effect produced 
 is directly proportional to the amount of the stress was also found by 
 them to be valid. For extra dense flints and dense barium crowns 
 these birefringence values decrease perceptibly, so that for a very 
 dense flint containing about 74.0 per cent PbO the birefringence is 
 zero irrespective of the state of annealing. This conclusion is in 
 accord with that first reached by Pockels from measurements with a 
 Jamin differential refractor. 
 
 The foregoing results by Adams and Williamson are larger than, but 
 of the same order of magnitude as those obtained by Czapski. They 
 show that a strain birefringence of 10XlO~ 6 (path difference of 10 
 millimicrons per centimeter) , which is about the limit permissible in 
 good quality optical glass, is produced by a load of 40 kilograms per 
 square centimeter and that for this load the change in refractive 
 index of the light flint for the wave vibrating normal to the axis of 
 pressure is 0.000016 or at most 2 in the fifth decimal place, a negligible 
 amount in its optical effect on the quality of the image. In the 
 case of a large telescope objective improperly supported so that its 
 weight is held at a few isolated points, the pressure at these points 
 may greatly exceed 40 kilograms per square centimeter and a serious 
 amount of strain be thereby introduced. 
 
REFRACTIVITY. 41 
 
 Direct measurements of the change in refractive index of strained 
 glass as compared with that of the same piece after annealing were 
 first made, by Schott 10 and later by Czapski 11 who found differences 
 in refractive index up to 0.003, the refractive index of the well- 
 annealed glass being invariably higher than that of the heavily 
 strained glass. Similar series of measurements made at the Bausch 
 & Lomb plant at Rochester with an Abbe-Pulfrich total refractometer 
 led to the same results; the refractive index of a "proof " taken from 
 a pot of molten glass was invariably lower, from 0.001 to 0.004 lower 
 than that finally obtained on the well-annealed plates of glass from 
 the same pot. The same order of magnitude for the effect of strain 
 was obtained at the plant of the Pittsburgh Plate Glass Co. at Char- 
 leroi, Pa., on "dips" or "proofs." For the measurements refractive 
 liquids of known refractive index were employed; the "dip" of 
 glass was immersed in a tank of refractive liquid and its refringence 
 was compared directly with that of the liquid and a standard glass 
 sample. 
 
 The foregoing measurements demonstrate clearly that the chief 
 effect of strain in optical glass is to deform and warp the optical 
 surfaces. Strained glass is not in a state of equilibrium; relief 
 from the internal stresses is sought by internal differential move- 
 ments. Experience has proved that even at room temperatures 
 prisms and lenses made of strained glass do not retain their shape 
 satisfactorily; with the oscillations of room temperature, the accur- 
 ately wrought surfaces of the prisms and lenses undergo constant 
 warping and change; these changes are very slight, but in high pre- 
 cision instruments they are sufficient to render such an optical 
 element useless. If the maximum path difference resulting from 
 strain exceeds 20 millimicrons per centimeter glass path there is 
 danger of surface warping and consequent introduction of astig- 
 matism and other defects in the image. If the strain is uniformly 
 distributed, this defect can be overcome to some extent; but if the 
 strain distribution is irregular, there is no method for counteracting 
 its damaging effect on the quality of the image. Hence the im- 
 portance of proper annealing of optical glass. No glass is entirely 
 free from strain, but if the greatest strain in a plate of optical glass 
 is below a certain limit (resulting maximum path difference less 
 than 5 millimicrons per centimeter glass path) the tendency toward 
 warping of polished glass surfaces is practically nil. 
 
 REFRACTIVITY. 
 
 The function of the lenses and prisms in an optical instrument is to 
 change the directions of propagation of incident light waves so that 
 when they reach the eye of the observer and produce an image on 
 
 10 Zeitschrift Instrumentenkunde, 10, 41, 1890. 
 
 11 Ann. d. Phys. (4) 7, 330, 1902. 
 
42 CHARACTERISTICS OF OPTICAL, GLASS. 
 
 the retina, this image has the desired qualities. This change in the 
 paths of light waves is made possible by the fact that on traversing 
 different substances light waves encounter different degrees of 
 resistance depending not only on the substance, but also on the color 
 of the light itself (wave length) . A measure for the relative rate of 
 travel of a light wave of given color through a substance is the recip- 
 rocal of the refractive index. The refractive index of a substance 
 is in fact the ratio between the velocity of light in free space (vacuum) 
 to that in the substance. Light waves of different color travel at 
 different speeds through a substance (fig. lla) ; as a result, a beam of 
 white light is resolved into its colored components by a prism, the 
 blue rays being deflected the most, the red rays the least (fig. 115); 
 the emerging rays are deflected or dispersed in a definite order 
 
 WHITE 
 
 WHITE 
 
 FIG. 11. (o) Refraction of plane-parallel light at a plane surface. (b) Dispersion of white light in a prism. 
 
 (c) Achromatic prism pair. 
 
 (spectrum). The amount of this deflection, both actual and rela- 
 tive, depends on the refracting substance. Different optical glasses 
 behave differently in this respect; the refractivity of an optical glass 
 is ordinarily specified by its refractive indices for certain definite 
 colors or lines of the spectrum. The spectral lines commonly chosen 
 are A', C, D, F, G r of the solar spectrum and have respectively the 
 wave lengths: 0.7682, 0.6563, 0.5893, 0.4862, and 0.4341 microns 
 (thousandths of a millimeter) . 
 
 In lists of optical glass it is customary for the manufacturer to 
 state the refractive index for the mean of the two D lines only ? 
 and to give the differences in refractive indices between the D and 
 C lines, between F and D, between G' and F, between D and A' , 
 and between F and C. These differences are measures for the dis- 
 persion of the glass in the different parts of the spectrum and suffice 
 to characterize its type. 
 
OPTICAL DISPERSION. 43 
 
 A derived value, namely, the ratio between the difference in 
 
 n v n c 
 
 the refractive indices for any two spectral lines, as for example, 
 ri F and n 0j and the difference, n v n c , in refractive indices for the 
 two spectral lines F and ' C, is called a partial dispersion ratio. The 
 partial dispersion ratios are a measure of the relative lengths of the 
 partial spectra in the different glasses. The differences between the 
 partial dispersion ratios of two glasses express the degree of similarity 
 of their spectra. A second derived value, introduced by Abbe, is 
 in general use and is commonly designated by the Greek letter *>. 
 It has been called the optical constringence, and is the ratio of the 
 refractive index for the D line (sodium light) minus one (effective 
 or excess refractivity) to the difference between the refractive indices 
 for the F and C lines (mean dispersion) ; its reciprocal is a measure 
 of the dispersive power of the glass. In other words the quantity 
 
 i> = -^ '- expresses the effective refractivity as measured in terms 
 
 UY UC 
 
 of the mean dispersion; its reciprocal expresses the mean dispersion 
 in terms of the effective refractivity. 
 
 To correct for color dispersion and yet to obtain the desired deflec- 
 tion of the light rays the lens designer combines an optical glass of 
 weak dispersion with one of high dispersion; thus in a single prism 
 (fig. 116) the rays on emerging from the prism are dispersed at dif- 
 ferent angles and a beam of white light is thereby resolved into its 
 colored components such that a spectrum is formed; in an achro- 
 matic prism combination, however (fig lie), the dispersion of the 
 first prism is neutralized by that of the second with the result that 
 the incident pencil of light is deflected but emerges as a beam of white 
 light. The spectra produced by the two prisms are superimposed in 
 reversed order so that thje dispersing effect of the first glass is neu- 
 tralized by that of the second. In order that this superposition of 
 spectra be effective, it is obviously necessary that the relative dis- 
 persions in the two glasses be the same. If in the first prism the 
 red end of the spectrum is drawn out relatively to the blue, while in 
 the second the blue end extends over a greater relative range than in 
 the first, the superimposed spectra can not be made to fit, and a 
 considerable amount of residual color will be left in the emergent 
 beam. 
 
 In lenses the designer brings together to the same focus rays of 
 one or more colors; he folds the spectrum over, as it were; this pro- 
 cedure is successful to the degree that the relative dispersions in 
 the two glasses of the achromatic objective are similar; to the extent 
 that they are dissimilar there is residual color (secondary spectrum) 
 in the image which can not be eliminated. It is evident that two 
 glasses produce a combination the better adapted for achromatizing, 
 
44 CHARACTERISTICS OF OPTICAL GLASS. 
 
 the less their partial dispersion ratios differ and the more the glasses 
 differ in effective refractivity as expressed in terms of the mean 
 dispersion (V value). It can be proved that the differences in focal 
 length of a telescope objective lens for the different colors in the 
 spectrum decrease with decrease in difference between the partial 
 dispersion ratios and increase with decrease in difference between 
 the v values of the two glasses. In other words, two glasses are the 
 better fitted for achromatizing the more nearly equal are their partial 
 dispersion ratios and the greater the difference in their v values. 
 It can also be shown that the sum of the curvatures of each lens 
 decreases with increase in difference between the v values. 
 
 By combining lens elements of optical glasses of different refractive 
 indices and dispersions it is possible for the designer to obtain 
 much more perfect images and optical performance than with 
 single lenses. In the older types of optical glass, ordinary crowns 
 and ordinary flints, the lead in the flint glasses dominates the optical 
 behavior of the glass with respect both to refractive index and to 
 dispersions, so that in the series of flint glasses the ratio between 
 refractive index and dispersions is practically linear, the dispersion 
 increasing in direct proportion with the refractive index for any 
 given spectral line. This relation between refractive index and 
 dispersion was used to advantage by lens designers to produce 
 images of fair quality; but it also set a limit to the possibilities of 
 results that could be obtained, and experience showed that under 
 such conditions it was impossible to attain the degree of correction 
 required for more exacting lens systems. If these were to be realized 
 it was necessary that new glasses of different relations between 
 refractivity and dispersion be produced. 
 
 The task of obtaining glasses of the desired refractivities and 
 dispersion ratios involves the study of the effects which changes in 
 chemical composition produce on the optical constants of the finished 
 optical glass. The oxides which were used half a century ago in 
 glass manufacture were few and included chiefly the oxides of silica, 
 sodium, potassium, calcium, lead, and aluminium. The first 
 attempts to introduce other elements into glass for the purpose of 
 modifying the optical properties were made by Harcourt, 12 in England, 
 between 1834 and 1860 in collaboration with Stokes; he discovered 
 the effects exerted by boron and barium on the optical constants of 
 glass; his experiments were on a small scale only and did not lead 
 to the commercial production of such glasses. 
 
 In 1880 and the following years detailed studies were made by 
 Schott and Abbe, in Jena, Germany, where, with the financial aid 
 of the Government, a number of new types of optical glass were 
 produced commercially for the first time. Schott introduced ele- 
 
 12 Report of the British Association for the Advancement of Science, 1871, 1874. 
 
NEW TYPES OF GLASS. 45 
 
 ments, such as boron (as oxide and borates), phosphorus (as phos- 
 phates), barium (as carbonate or oxide), fluorine (as a fluoride), and 
 felt assured of a certain degree of success in obtaining glasses of the 
 desired optical qualities because these elements occur in nature in 
 crystallized compounds, which are characterized optically by extremes 
 in refractivity and dispersion. These extremes have never been 
 attained by optical glasses; there is, moreover, no prospect of pro- 
 ducing glasses of these properties chiefly because of the crystallizing 
 tendencies of melts of such abnormal compositions. These melts 
 can not be chilled with sufficient rapidity on a commercial scale to 
 prevent their crystallization. Glasses of other abnormal composi- 
 tions may not exhibit this tendency to crystallize, but the final 
 product may be chemically unstable, so that the glass is readily 
 attacked by the atmosphere and even by the materials used in the 
 grinding and polishing processes; or the finished glass may be so soft 
 as to be of little service in optical instruments. Glasses of these 
 abnormal compositions showing extremes in refractivity or in dis- 
 persion are always difficult to manufacture; they tend not only 
 to crystallize or to be chemically unstable, but they are prone to 
 attack the glass pot and to develop other troubles, such as bubbles, 
 which are difficult to overcome on a large scale in the factory. 
 
 The studies undertaken by Schott and Abbe for the purpose of 
 obtaining new types of optical glass extended over a period of years. 
 These investigators developed a number of new types of optical 
 glass in which the dispersion ratios of certain pairs were more nearly 
 in accord than were those of the older glasses. With the new types 
 of optical glass much better color correction can be obtained in 
 optical lens systems. Their new types of glass include the series of 
 borosilicate crowns, of barium crowns, of barium flints, borate flints, 
 the borate and phosphate glasses. In these glasses the character of 
 the dispersion varies from type to type. 
 
 We shall now consider in some detail and by means chiefly of 
 graphical plots the dispersion relations in optical glasses in order to 
 gain deeper insight into the significance of dispersion in optical glasses 
 and its change with change in chemical composition. 
 
 A fundamental requirement of optical glass is transparency and free- 
 dom from color; this means the absence of an absorption band in the 
 visible spectrum; and this in turn sets a definite limitation to the pos- 
 sible variations in refractivity and greatly restricts the general char- 
 acter of the dispersion relations. With change in color (wave length 
 of light) the refractive indices of optical glass change in the manner 
 illustrated in figures 12a and 126, in which the refractive indices of 
 different kinds of optical glass 13 are plotted for different wave lengths 
 
 : , 
 
 13 Measured by H. Rubens and H. T. Simon on a series of Schott glasses. Ann. d. Phys. u. Chem. N. F. , 
 53, 555, 1894. * 
 
 3922921 4 
 
46 
 
 CHARACTERISTICS OF OPTICAL GLASS. 
 
OPTICAL DISPERSION. 
 
 47 
 
 of light extending from the ultra-violet through the visible spectrum 
 into the infra-red. In figure 12a the refractive indices (ordinates) 
 are plotted against the wave lengths, X, directly (abscissae) ; in figure 
 126 the refractive indices are plotted against the squares of the recip- 
 
 1.9C 
 
 1.8O 
 
 1 7O 
 
 BaO 
 
 1.60 
 
 x FLINT 
 
 -t- BARIUM FLINT 
 L BORATE FL 
 
 INT 
 
 I >* 
 
 CROWN 
 O BARIUM 
 
 CROWN 
 
 ZnO> 
 
 aO 
 
 FLUOR 
 
 ILICATE CROWN 
 CROWN 
 
 GLASS 
 
 -02 
 
 .03 
 
 .04 
 
 .01 
 
 TV r\c * 
 
 FIG. 13. In this figure the mean dispersions, n f n c , of a series of different types of silicate optical glasses 
 are plotted against the refractive indices, n D . The points of projection for the ordinary crowns and the 
 ordinary flints fall on a practically straight line, as indicated in the diagram. 
 
 Is of the wave lengths (1/X 2 , frequency squared) . The glasses 
 represented in figures 12a and 6 include the old types of ordinary 
 crowns and flints and also barium crowns, flints, and borate glass ; thus 
 $204 is a borate glass (n D = 1.51007, *>=58.8); O 1092, a light barium 
 crown (n D = 1.57698, ^=62.0); O 1151, a crown of high dispersion 
 
48 
 
 CHARACTERISTICS OF OPTICAL, GLASS. 
 
 (ri D = 1.52002, i/ =51.8); S 179, a phosphate crown (n D = 1.56207, 
 y = 67.2); O 561, a light flint (n D = 1.57524, ? = 41.2); O 1143, a 
 barium crown (n D = 1.57422, i/ = 57.1); O 469, a dense flint (n D = 
 1.64985, ^=33.7); O 500, an extradense flint (n D = 1.75130, ^=27.6); 
 S 163 densest flint (n D = 1.88995, i>=22.3). 
 
 1.9C 
 
 1.8C 
 
 1.6O 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 /' 
 
 
 
 
 
 
 
 
 
 
 
 o 
 
 4- 
 4. 
 
 / 
 
 
 
 
 & 
 
 > + * 4* 
 
 7 
 
 
 X FLINT 
 -h BARIU 
 l_ BORA" 
 
 VI FLINT 
 E FLINT 
 
 
 Vx 
 
 .',/ 
 
 / 
 
 
 o CROW 
 BARIU 
 
 N 
 * CROWN 
 
 
 " f~ 
 <j^J'/ 
 
 y 
 
 
 
 BOROS 
 FLUOI 
 
 ILICATE < 
 J CROWN 
 
 :ROWN 
 
 -7 
 
 oIlLICA GL 
 
 ASS 
 
 
 
 
 
 
 .020 
 
 DISPERSIVE POWER 
 
 ,030 
 
 .040 
 
 .O5O 
 
 FIG. 14. In this diagram the dispersive powers of a series of optical glasses are plotted against their re- 
 fractive indices, n D . 
 
 It may be noted that the dispersion curves of figure 12& show an 
 inflection point in the visible spectrum ; as a result, the run of disper- 
 sions throughout the visible spectrum is represented by approxi- 
 mately straight lines in the different glasses. These relations have an 
 
OPTICAL DISPERSION. 49 
 
 important bearing on the development of certain dispersion formulas 
 and will be discussed in a later paragraph. 
 
 In the old types of glasses (ordinary crowns and ordinary flints) 
 the dispersion increases with the refractive index (figs. 13 and 14) ; but 
 the dispersion in the blue end of the spectrum increases more rapidly 
 than that in the red and the spectra of different glasses are so dis- 
 similar (irrationality of dispersions) that only a fair correction for 
 achromatism can be attained. The introduction of new types of 
 glasses by Abbe and Schott enabled the lens designer to produce much 
 better lens systems than was formerly possible. 
 
 It has long been known that if the mean dispersions of ordinary 
 crown glasses and of flint glasses be plotted against refractive index 
 the points fall approximately on a straight line (fig. 13). In other 
 words, in these older types of glass the mean dispersion increases di- 
 rectly with the refractive index. It was to overcome this limitation 
 that Harcourt, and later Abbe and Schott, investigated the changes 
 produced in optical glasses by radical changes in the chemical com- 
 position. They found that boron and barium are especially valuable 
 in this connection; in figure 13 the relations between refractive index 
 and mean dispersion in the new Schott and Parra-Mantois glasses are 
 also given and show how far some of these depart from the straight 
 line of the old flints and crowns. 
 
 If the dispersive powers (l/v as defined above) of the Schott glasses 
 are plotted against the refractive index (fig. 14), the old- type glasses 
 fall on a slightly curved line; the fields of the new types of glasses are 
 clearly differentiated on the diagram. This is also true when the 
 y-values of the glasses are plotted against refractive index, although 
 in that diagram the curve of the old-type glasses is much more curved. 
 
 In figure 15 the ratios of the partial dispersions in the red (n D n A f ) 
 and blue (n f n ) ends of the spectrum (relative length of the red 
 to that of the blue) are plotted against the refractive index, n D . 
 This diagram illustrates probably better than the others the refractiv- 
 ity-dispersion relations ; in it the fields of the different glass types are 
 well marked. Thus in the fluor-crown glasses the length of the red 
 end of the spectrum exceeds that of the blue end relatively more 
 than in any other glass type; the borosilicate crowns follow next in 
 order; then the ordinary crowns, the barium crowns, the barium 
 flints, and finally the flints in which the relative dispersion of the 
 blue (n G ' n F ) exceeds that of the red (n D n A '). 
 
 It is possible from figure 15 to select glasses differing appreciably 
 in absolute refringence and at the same tune to state their relative 
 dispersions in the blue and red parts of the spectrum. Chemical 
 analyses of many of the glasses plotted on this diagram are listed 
 in Table 4 (p. 59) ; by combining graphically the information pre- 
 sented in figure 15 and Table 4 it is possible to deduce by interpola- 
 
50 
 
 CHARACTERISTICS OF OPTICAL GLASS. 
 
 tion the approximate chemical compositions of glasses intermediate 
 in optical properties between those which are plotted. The methods 
 for accomplishing this are described in a later paragraph. Figure 15 
 
 19O 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 X 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 I 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ^ 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 1 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 xj 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ^C 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 X *S 
 
 \ 
 
 \ 
 
 _Q 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 *X 
 
 V 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 X 
 
 FLif 
 
 JT 
 
 
 
 
 
 
 X 
 
 +- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 4 
 
 BAR 
 
 IUM 
 
 FLI 
 
 >4T 
 
 
 
 
 % 
 
 \ 
 
 ^f x . 
 
 
 
 
 
 O 
 
 o 
 
 BaC 
 
 
 
 
 
 
 
 L: 
 
 BOR 
 
 ATE 
 
 FL 
 
 INT 
 
 
 
 
 
 nl 
 
 s 
 XN 
 
 -* 
 
 
 Q 
 
 t 
 
 V 
 
 o 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 X 
 v y 
 
 -1- 
 
 
 _^ 
 
 c 
 
 
 
 
 
 
 
 
 
 
 
 CROWN 
 
 
 
 
 
 
 
 
 
 *l 
 
 
 *f 
 
 ^ + 
 
 ? 
 
 t 
 
 
 
 
 
 
 
 
 o 
 
 1 
 BARIUM 
 
 CRC 
 
 >WN 
 
 
 
 
 
 
 
 
 X 
 
 
 X^ 
 
 
 
 
 
 
 
 
 
 
 
 
 BOROSIU 
 
 ICATE ( 
 
 ROV 
 
 /N 
 
 
 
 
 
 
 
 
 
 -k 
 
 
 8 
 
 
 
 
 
 
 
 (A 
 
 m 
 
 
 o 
 
 FLUOR 
 
 CRO 
 
 WN 
 
 
 
 
 
 
 
 
 
 
 
 
 j 
 
 ^ 
 
 O 
 
 B,C 
 
 3. 
 
 
 
 T 
 
 Q 
 
 
 
 PHOSPh 
 
 ATE 
 
 CR 
 
 DWh< 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 " ^ 
 
 * 
 
 
 
 < 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 1- 
 
 , 
 
 _i 
 n 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 O.8O 
 
 050 
 
 1.OO 
 
 1.1O 
 
 1.20 
 
 FIG. 15. In this figure the ratio (w D n A ')/(w G '7i F ), which expresses in effect the length of the red end 
 of the spectrum to that of the blue end, is plotted against the refractive index r? D for a series of different 
 types of silicate optical glasses. 
 
 shows, moreover, the extent to which the glassmaker has succeeded 
 in changing the refractivities of optical glasses. The diagram includes 
 the borate and phosphate glasses in addition to the silicate glasses. 
 
PARTIAL DISPERSION RELATIONS. 
 
 51 
 
 If now we consider only the partial dispersions and plot, as in 
 figure 16, the partial dispersion n F n c , n F n D and n G ' n F against 
 n D WA' for a series of silicate glasses, the result in each case is a 
 straight line; in figure 16 the partial dispersions of all the silicate 
 glasses listed by Schott of Jena, and by Parra-Mantois of Paris (about 
 289 different glasses in all) are included except those of the densest 
 
 .015 
 
 FIG. 16. In this figure the partial dispersions, n f n and n G ' n F of all silicate optical glasses listed by 
 Parra-Mantois and by Schott, are plotted as ordinates against the partial dispersion w D -n A ' as abscissae. 
 The result in each case is a straight line. 
 
 flint S 386 of Schott. This is a remarkable result and states that 
 any partial dispersion of a glass bears a linear relation to any other 
 partial dispersion; the degree of departure from this relation does 
 not exceed one or two units in the fourth decimal place for the 
 glasses plotted. Except for the dense barium crown glasses the dis- 
 
52 CHARACTERISTICS OF OPTICAL GLASS. 
 
 tance of the points from the straight line is commonly only a few 
 units in the fifth decimal place. 
 
 This fact, that in a series of optical glasses the partial dispersions 
 are related by linear functions, proves that once a partial dispersion 
 is given, the entire dispersion curve is fixed irrespective of the type 
 of optical glass. This means that within the limits to which this 
 statement holds, namely, one or two units in the fourth decimal place, 
 if any partial dispersion is given, all other dispersions follow auto- 
 matically; in other words, a change in dispersion at one part of the 
 dispersion curve carries with it definite changes in the curve through- 
 out the visible spectrum. Thus a series of standard dispersion curves 
 can be set up independent of the absolute refractive index. This 
 signifies that if, for any optical glass, two refractive indices be given, 
 its dispersion curve can be written down directly; that in case two 
 optical glasses of very different indices are found to have the same 
 actual dispersion for one part of the spectrum, their dispersion 
 curves are identical to one or two units in the fourth decimal place 
 throughout the visible spectrum. If, for example, the refractive 
 index n D , and the v value of an optical glass be given, its mean 
 dispersion, n n c , can be computed from the equation n F n c = 
 (n D I)/?; its partial dispersions n D n A r , n F n D , and n G ' n F can 
 then be read off directly from figure 16 with a fair degree of accuracy, 
 sufficient, at least, to give an adequate idea of the run of dispersion 
 in the glass. 
 
 From these relations it is possible to build up empirical dispersion 
 formulas containing two or three constants which represent the data 
 in the visible spectrum with a high degree of exactness. 14 
 
 The linear relations between the partial dispersions of an optical 
 glass are valid only for that portion of the dispersion curve which is 
 distant from an absorption band. With the approach to an absorp- 
 tion band the dispersion curve departs from its even course and is 
 no longer comparable with the dispersion curves of other glasses. 
 This is well shown in figure 17 in which the measurements of H. 
 Rubens in the infra-red and H. T. Simon 15 in the visible and ultra- 
 violet of a series of optical glasses are plotted in terms of the partial 
 dispersions. The different types of glasses are named on the diagram 
 and are identical with those plotted on figure 12. The similarity in 
 the course of the partial dispersions is well shown by two glasses 
 in the list, namely, a crown of high dispersion, O 1151, of refractive 
 index n D = 1.52002, and a barium crown, O 1143, of refractive index 
 UD= 1.57422. In Table 2 the partial dispersions n r n D are listed. 
 In this table it is evident that the partial dispersions of the two 
 glasses run along fairly well together from the infra-red at 2 JJL to the 
 
 " F. E. Wright. Journ. Opt. Soc. America, IV, 148-159, 1920. 
 15 Ann. Phys. u. Chem. N. F. 53, 555, 1894. 
 
PARTIAL DISPERSION RELATIONS. 
 
 53 
 
 violet of the visible spectrum. From here on into the ultra-violet 
 the crown with high dispersion, which contains 13.3 per cent of lead 
 
 FIG. 17. In this figure the partial dispersions n r n D between the sodium line and the following wave 
 lengths in microns: 2.4, 2.2, 2.0, 1.8, 1.6, 1.4, 1.2, 1.0, 0.8, 0.7682, 0.6563, 0.5892, 0.5349, 0.5086, 0.4861, 0.4800, 
 0.4678, 0.4340, 0.3610, 0.3466, 0.3403, 0.3261, 0.3133, 0.3081, 0.2980, 0.2880, 0.2837, 0.2763, are plotted as ordi- 
 nates against the partial dispersions, n G 'n , for a series of optical glasses measured by H. Rubens and 
 H. T. Simon. The partial dispersions of the following Schott optical glasses are plotted on the diagram: 
 O 1092, light barium crown, (n D =1.51698); S 204, borate glass, ( D = 1.51007); O 1143, dense barium crown, 
 (n D = 1.57422); O 1151, crown of high dispersion, (n D = 1.52002); O 451, light flint, (n D =1.57524); O 469, 
 dense flint, (n D = 1.64985); O 500 dense flint, (n D =1.75130); S 163, extra dense flint, ( D = 1.88995). 
 
 oxide, approaches an absorption band and its partial dispersions rise 
 accordingly. 
 
54 
 
 CHARACTERISTICS OF OPTICAL GLASS. 
 
 TABLE 2. 
 
 In this table the partial dispersions, n T n D of a crown of high dispersion, O 1151, and a barium crown, 
 O 1143, are given for a series of wave lengths, r, extending from the infra-red at 2 /* to the ultra-violet at 
 0.2980 M- 
 
 Wave length 
 in/*. 
 
 O 1151. 
 n T -n D . 
 
 O 1143. 
 
 r D- 
 
 Wave length 
 in p. 
 
 O 1151. 
 r -n D . 
 
 1143. 
 ft r D . 
 
 r 
 20 
 
 0. 02272 
 
 0. 02272 
 
 r 
 
 0.4861 . 
 
 0. 00713 
 
 0. 00704 
 
 1.8 
 
 .02012 
 
 - .02012 
 
 0.4800 
 
 .00780 
 
 .00766 
 
 1 6 
 
 .01762 
 
 .01772 
 
 0.4678 . . . 
 
 .00901 
 
 .00884 
 
 1.4 
 1.2 
 
 - .01522 
 - .01312 
 
 - .01542 
 - .01312 
 
 0.4340 ' 
 0.3610 
 
 .01310 
 . 02664 
 
 . 01288 
 .02500 
 
 1.0 
 
 .01042 
 
 - .01312 
 
 0.3466 
 
 .03066 
 
 . 02977 
 
 0.8 
 
 0.7682 .. 
 
 - .00692 
 .00634 
 
 - .00692 
 - .00640 
 
 0.3403 
 0.3261 
 
 . 03260 
 . 03768 
 
 .03161 
 . 03623 
 
 0.6563 
 
 - .00290 
 
 - .00302 
 
 0.3133... . 
 
 .04305 
 
 . 04103 
 
 0.5892 
 
 .00000 
 
 .00000 
 
 0.3081.... . . 
 
 . 04556 
 
 . 04103 
 
 05349 
 
 .00325 
 
 .00324 
 
 0.2980... 
 
 . 05091 
 
 . 04791 
 
 0.5086 
 
 .00523 
 
 .00516 
 
 
 
 
 The different effects of lead, barium, boron, and other glass-making 
 oxides are more clearly shown in the infra-red and ultra-violet than 
 in the visible spectrum. The maximal departure from normal dis- 
 persion curves in the visible spectrum caused by the presence of 
 large amounts of barium is approximately two units in the fourth 
 decimal place. 
 
 In the series of flint glasses an increase in lead oxide content 
 raises the refractive index and causes the absorption band in the 
 ultra-violet to shift toward the visible spectrum. This is clearly 
 shown by the flint glasses plotted in figure 12, namely O 451, O 469, 
 O 500, S 163. Simon was unable, because of the presence of this 
 absorption band, to measure the refractive indices of the light flint 
 O 451 beyond the wave length 0.2980 /*, of the medium flint O 469 
 beyond 0.3261 M, of the very dense flint O 500 beyond 0.3403 /u, and 
 of the densest flint S 163 beyond 0.4340 /x. 
 
 Further evidence of the shift of the absorption band with increase 
 in lead-oxide content has been obtained by the direct measurement 
 of the transparency of the flint glasses in ultra-violet light. Data on 
 the transmission of plates of flint and other optical glasses in the 
 ultra-violet are given in the catalogue of optical glasses issued by 
 Chance Bros. The results of their measurements on the flint and 
 other glasses are reproduced in Table 3 in which the limit of trans- 
 parency of a glass plate 1 centimeter thick is indicated by the 
 wave lengths at which the percentage transmissions are 50 and 10 
 respectively. 
 
OPTICAL DISPERSION. 55 
 
 TABLE 3. Transparency of flint and other glasses of Chance Bros, in the ultra-violet. 
 
 T sT 
 
 Name. 
 
 D . 
 
 V 
 
 Approxi- 
 mate per 
 centage 
 PbO 
 
 Wavelengths (*i) 
 for transmission. 
 
 50 per 
 cent. 
 
 10 per 
 cent. 
 
 '321 
 313 
 330 
 332 
 341 
 360 
 295 
 309 
 311 
 309 
 312 
 309 
 307 
 329 
 353 
 335 
 339 
 320 
 316 
 328 
 
 7863 
 6953 
 572 
 360 
 337 
 4480 
 7423 
 646 
 1203 
 9322 
 1066 
 569 
 3463 
 9002 
 9753 
 4873 
 1453 
 5062 
 7983 
 4277 
 
 Extra light flint 
 
 1.5290 
 1.5412 
 1. 6182 
 1.6225 
 1.6469 
 1.7401 
 1.4785 
 1.5087 
 1.5155 
 1.5186 
 1. 5149 
 1. 5152 
 1.5407 
 1.5744 
 1.5881 
 1.6118 
 1.6126 
 1.5515 
 1.5534 
 1.5250 
 
 51.6 
 47.6 
 36.4 
 36.0 
 33.7 
 28.3 
 70.2 
 64.2 
 60.8 
 60.3 
 57.9 
 56.9 
 59.4 
 57.9 
 61.1 
 59.0 
 56.7 
 51.7 
 46.1 
 M.7 
 
 18 
 23.5 
 45 
 46 
 51 
 66 
 
 330 
 316 
 337 
 338 
 347 
 370 
 301 
 315 
 318 
 315 
 323 
 314 
 309 
 338 
 358 
 348 
 350 
 323 
 318 
 337 
 
 Light flint 
 
 Dense flint 
 
 do 
 
 Very dense flint 
 
 do 
 
 Fluor crown 
 
 Borosilicate crown 
 
 
 Hard crown 
 
 
 . do 
 
 
 Zinc crown 
 
 
 Soft crown 
 
 
 Light barium crown 
 
 
 Medium barium crown. 
 
 
 
 Dense barium crown 
 
 do 
 
 
 do 
 
 
 Light barium flint 
 
 
 do 
 
 
 Telescope flint 
 
 
 
 
 With the exception of the first member of this series, which may 
 contain appreciable amounts of zinc or barium oxides that may 
 affect the transparency in the ultra-violet, the absorption band shifts 
 continuously with increase in lead content toward the longer wave 
 lengths and the visible spectrum. 
 
 The yellow color of the very dense flints has been ascribed to the 
 influence of this absorption band in reducing the intensity of the violet 
 and blue of the visible spectrum ; other factors, however, such as 
 the presence of small amounts of iron oxide and possibly also of lead 
 dioxide or other oxide of lead as impurities, may have a pronounced 
 influence on the color. Very dense flint glasses made of materials of 
 high chemical purity and under conditions of thorough oxidation are 
 noticeably less colored than glasses of the same composition whose 
 batches and heat treatment have not been scrutinized carefully. 
 
 There are other approximately straight-line dispersion relations 
 within the visible spectrum which may be noted because on them 
 certain empirical dispersion formulas are based. Thus if the refrac- 
 tive indices be plotted as ordinates against the squares of the fre- 
 quency (1/X 2 ) as abscissae, the course of the dispersion of an optical 
 glass is represented by a curve which departs only slightly from a 
 straight line (fig. 126) ; these departures are commonly less than one 
 unit in the third decimal place. 16 A dispersion formula built up on 
 this relation is the two-constant formula of Cauchy, namely 
 
 The formula may also be written 
 
 n-l=A'+B/\ 2 . 
 
 See Sellmeier, Ann. d. Phys. u. Chem., 143, 272, 1871; also Pulfrich, Ann. d. Phys. u. Chem., 45, 
 1892; and Hovestadt, Jenaer Glas, p. 46-48. Jena. 1900. 
 
56 CHARACTERISTICS OF OPTICAL GLASS. 
 
 In view of the fact that the range of refractive indices in optical 
 glasses over the visible spectrum is relatively limited, any approxi- 
 mately straight-line relation between refractive index and a function 
 of the wave length, such as expressed by the foregoing Cauchy 
 formula, becomes an hyperbola if the reciprocal be taken of the 
 refractive index or of the excess refractivity; but the portion of the 
 curve covered by the visible spectrum is so short that, even in this 
 case, the departure of the hyperbola from a straight line is not great 
 and the dispersion relations are still fairly well represented. Thus, 
 the new formula recently suggested by Nutting 17 is the Cauchy 
 formula, in which l/(n 1) is written for (n1). Nutting's formula 
 represents the dispersions in certain cases better than the Cauchy 
 formula, whereas in other glasses the Cauchy formula is the better. It 
 would lead too far to present data of computation on a series of 
 Gifford glasses which bear out this statement. The conclusion is, 
 however, directly evident from a comparison of figures 18a and 18&, 
 in which for all silicate glasses of Schott the squares of the frequency 
 
 n 1 
 
 (1/X 2 ) are plotted as abscissae against =- and its reciprocal 
 
 n A/ i 
 
 A '_ j respectively, as ordinates. As a result of this method of 
 
 plotting all dispersion curves pass through the unit ordinate for the 
 A'-spectrum line. The dispersion curves radiate from this point as 
 approximately straight lines, the departures from straight lines being 
 greatest in the dense flints and also in the very light crowns and 
 borosilicate crowns. 18 
 
 Another method of expressing these relations is to plot the fre- 
 quency scale on the horizontal line at unit distance from the ab- 
 scissa axis, to draw lines radiating from the origin through the 
 points on the frequency scale, and to find the intercepts of these 
 lines with ordinates equal to the refractive indices. 19 The disper- 
 sion curves under these conditions are approximately straight 
 lines. 
 
 These relations suffice to prove that in any dispersion formula 
 (if carried only over the visible spectrum in a transparent colorless 
 substance, such as optical glass) which expresses the dispersion 
 relations in approximately linear form, the reciprocals may be taken 
 of the refractive index or any function of the same and the new 
 dispersion curve thus obtained will again be approximately a straight 
 line. In the ultra-violet and infra-red these relations may no longer 
 obtain, and they inevitably break down as an absorption band is 
 approached. 
 
 In figure 18a the effective refractivity (n1) for any wave length 
 is expressed for each glass in terms of its effective refractivity for 
 
 " Revisita d'Ottica e Meccanica di Precisione, I, 54-57, 1919. 
 " Compare F. E. Wright, Jour. Opt. Soc. America, IV, 195-204, 1920. 
 
 w For a brief account of this method of plotting reciprocals see F. E. Wright, Jour. Wash. Acad. Sci., 
 10, 185-188, 1920. 
 
OPTICAL DISPERSION. 
 
 57 
 
58 CHARACTERISTICS OF OPTICAL GLASS. 
 
 the A' wave length (n A - I). The curves of this figure demonstrate 
 that with rise in refractive index the dispersion also rises, and that, 
 in the flint series especially, the dispersion increases relatively faster 
 than the refractive index. This fact of increased rate of rise of 
 dispersion with increase in absolute refringence is also clearly shown 
 by a comparison of the dispersion relations in the flint series of 
 glasses after reduction for each glass of all its refractive indices in 
 the ratio n/n D or n/n A ,. This procedure reduces the refractive 
 index of each glass for the ZMine or A'-\me to unity, and thus renders 
 the relations directly comparable. On plotting the ratios n/n D 
 against X or 1/X 2 we find that, in spite of the reduction of all glasses 
 to a common datum level of absolute refringence (W D = 1), the higher 
 the refringence in the flint glass series the greater the slope of the 
 dispersion curve, thus proving the relatively greater dispersion of 
 the heavy flint glasses. 
 
 The foregoing relations, together with other relations, such as are 
 
 shown by graphical plots in which: (a) - -is plotted against 
 
 77/F ^D 
 
 ^-^ > (b) , against the wave length, X r , directly, (c) n r 
 
 against X r , (d) UT against v r , demonstrate that the actual shape of a 
 dispersion curve in optical glasses can be changed only in a definite 
 manner and that the departures from any one of the set of standard 
 dispersion curves do not exceed two units in the fourth decimal place. 
 The effort of the glassmaker is therefore necessarily directed toward 
 the production of glasses of different refringences for the same 
 general run of dispersions. 
 
 In the foregoing paragraphs the dispersion relations in optical 
 glasses are presented on diagrams in some detail and from differ- 
 ent viewpoints purposely, because they < are of fundamental impor- 
 tance to the study of dispersion not only in optical glasses, but also 
 in other colorless substances. They indicate clearly the limits which 
 the glassmaker has attained in his efforts to produce different types 
 of optical glass and demonstrate that the paths which he may follow 
 are narrowly prescribed. 
 
 RELATIONS BETWEEN CHEMICAL COMPOSITION AND REFRACTIVITY. 
 
 The study of the refractivity relations in optical glasses, as illus- 
 trated in the foregoing diagrams, indicates that certain chemical 
 oxides in combination with silica dominate certain fields. To 
 determine these relations, chemical analyses are essential. Unfor- 
 tunately, the available analyses are not all of equal value. Table 
 4 contains the best chemical analyses of optical glasses whose optical 
 constants are given and are at present known to the writer. 20 Many 
 of these " analyses " are synthetic compositions computed from the 
 batch compositions; in the table the sum in each "analysis" of this 
 type is either 100.0 or 99.9. 
 
 > See also list of analyses published by Williams and Rand, J. American Ceramic Soc., 2, 434-441, 1919. 
 
CHEMICAL ANALYSES. 
 
 59 
 
 1 
 
 ts 
 
 S jS 
 
 
 Oi^ ; ;N ; ^^<N ; N c4 
 
 
 5 
 
 *5J||g 
 
 
 NCO^MO-H 
 
 
 -oi-i-not-ttSr-ieo-HOOi^^ocii^NO 
 
 
 CO 
 
 Q*w* 
 
 a ' 
 
 g^g---2 
 
 
 sssssssssssssgssggs 
 
 
 ^ 
 
 c=2 
 
 
 ssssg 
 
 
 gg2-2fe|5SgSSS8S5J2SSS 
 
 
 x 
 
 'Sis 
 
 a 
 ss 
 
 us us >n us ^ 
 
 
 ": "t L ? "r "? ^ t '? ^ ^ u ? L ? 
 
 
 * 
 
 
 
 
 
 
 
 
 "5"? 
 
 3 
 
 iKllI 
 
 
 
 gggoogoggggooogogog 
 
 
 o 
 
 8 
 
 
 h 
 
 
 
 
 
 
 f 1 
 
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 05COCO(NOCC5 
 
 
 233SISS53325S323:: 
 
 
 
 
 ^ |ls 
 
 
 ogSoocs 
 
 
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 9, 
 M 
 
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 22ss= as .s"sS-aa- 
 
 
 
 i S"2g 
 
 
 i i .'to 
 
 
 
 
 
 f Hi 
 
 B 
 
 : : i N 
 
 
 
 
 
 1 I's-g o 
 
 ! 
 
 s^sss 
 
 CROWNS 
 
 000050 05 ^!oo | o j 
 
 S CROWN 
 
 
 I 111 
 
 1 ill i 
 
 '2 -3g, . S 
 
 6 
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 :::::: 
 
 )SILICATE 
 
 
 3 SILICAT] 
 
 o 
 
 i iifi- s 
 
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 3 
 
 
 tf 
 
 O 
 B 
 
 
 S5 
 N3 
 
 
 i jiff 
 
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 d 
 
 
 
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 S : : : o!JoSo : : :S i-i : 
 
 
 
 1 His 
 
 s* S^-* 
 
 | 
 
 r 
 
 
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 7 IIP 
 
 
 is i 
 
 
 XiCOCOOOOOC005U7if5C05iOCt^O 
 
 
 
 ^ ^s^'S 
 
 Q c9 . S O 
 
 i 
 
 o 
 
 
 ^ 
 
 
 
 
 
 CD Tf 
 
 <N CO SC CO CO 
 
 
 S^eo^ooSoSSo-c-S**** 
 
 
 
 
 Hi 
 
 o 
 
 
 
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 Gf 
 
 ill? 
 
 6 
 
 
 
 
 
 
 IsBg 
 
 "S.^ - 
 
 
 .-H C<1 CO * t C 
 
 
 1 j j j I j i jj j: j j I n i j ^ 
 
 
 
60 
 
 CHARACTERISTICS OF OPTICAL GLASS. 
 
 differ 
 
 a 
 
 ^ I 
 
 II 
 
 2 % 
 
 |S 
 
 S ""i 
 
 ^ .LA . ^ t .co -r 
 -O -CO . >v$ iC 
 
 co'co'eo' co'co -co' ico i 'oo !co 
 
 
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 dot 
 
 :o5 
 
 ' 
 
 <Nc5oOiOCO>OoOOOM^HCOoSiCoSo 
 
 OOCOO500^HCoSCiOiOlNi-H 
 
 w5rfdo6o6ddaod5o6 
 
 oo co i 
 dt>, 
 
 CO M rfi CO CO r" CO CO 5 CO O >C O C CO 
 
 O O 
 
 rj id uj ffj id d oo 
 
 OOOOt i 
 
 O t>- 
 t-0005 
 <N M ' d <N f4 (N 
 
 : :,, :g : g 
 : :rf : ' 
 
 
 
 CO* O U3 irf -* irf TJ? CO* ^4 CO* D *' * ri rt M' ^H 
 
 ieidcoco 
 
 COC^C<I^HOOCOe^OIMCo2Tf>eOCOCOCO 
 
 c5 
 
CHEMICAL ANALYSES. 
 
 61 
 
 :38 :fe > 
 
 O C^ O I"* CQ 1C *O C O CD '^ ^t' 
 
 ,c '. : : : 
 
 
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 <N^CO(N-; 
 
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 ::::::::::: : = 
 
 
 
 H 
 
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 BN 
 
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 5 
 
 co co co co co co co co -rococo 
 
 
 
 
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 C ci t^ C> i< arj oc oo ac ac ec c 
 
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 3922921 
 
62 
 
 CHARACTERISTICS OF OPTICAL GLASS. 
 
 I 
 
 , 
 
 r^ H 
 
 i g 
 
 
 c5(No; :fo^s 
 
 
 JcSScc^coS 
 
 
 
 O4 <N CO CO CO CO <N (M 
 
 
 C^ (M CO CO CO CO 
 
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 o^co-^eioso^HO 
 
 
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 o 
 c 
 
 illililll 
 
 
 OS >OCO O O CO 
 
 
 
 
 
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 e 
 
 000000000 
 
 888888888 
 
 
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 M 
 
 J2 j j j j j j j 
 
 
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 o 
 
 C3 
 
 ^ CO 
 
 
 joooo 
 
 
 
 
 
 
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 I] 1 1 1 |s i 
 
 m 
 cc 
 
 
 O 
 
 00000 
 
 O 
 
 
 PH 
 
 
 H 
 
 
 
 
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 PH 
 
 m 10 ic 10 o o 
 
 
 O 
 
 O 
 
 w 
 
 PH 
 
 
 3 
 
 ooooco -<o 
 
 
 OOOiCC I 
 
 o: 
 
 NMjiijj 
 
 
 iO O >C O O O 
 OS OS CO * irf 
 
 t^. to >o >o o *! 
 
 
 JJJJJJJJj 
 
 
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 2 
 
 gSSSSSSKS 
 
 
 COCO CO CO CO CO 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 83S8SS8S= 
 
 
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 CCCMcCOS^COZcOCC 
 I I I I I I. I I I I 
 
CHEMICAL ANALYSES. 63 
 
 ^sx-,' :e3 e 
 
 I 
 
 O o 
 
 si 
 
 et 
 
 -.odd^Q .-f ^ 
 o II sgj ; 'l^f 
 
 <0 < _- 
 
 c i MX: .=3^ b-3 
 
 ^ 
 
 2ooo||||o o 
 ^wwWa 
 
 I 
 
64 
 
 CHARACTERISTICS OF OPTICAL GLASS. 
 
 s. J 
 
 I C 
 
 b 3 
 
 o 
 S 
 
 I- I 
 ' Q 
 
 D . bb 
 
 ^S 
 bS 
 
 v 1 
 
 tf P 5 : 
 
 i 
 
 II 
 So 
 
 
 00 
 
 II 
 
 - - O O 
 
 82 
 
 i 
 
 i bb 6 $ V^J 
 
 y 6 6 - 23 ;- 
 
 -* I I S P P 
 
 g 3 ^ P oq 
 
 ! a I 3 11 
 8 S 2 - 
 
 2 
 
 i 2< 
 
 ^ 
 
 o 
 
 ^ l Q M I QQ ^J 
 66 V. o ' S^ 
 
 ol5o^f N P M ^|o6o|o^|iP^^| 
 l||SSlsSSSSSliliilS^|^^sg^ 
 
 ^^/-S/^S^-t^M^ -/^(^i^^f^N/^^S/^rVy-Vy^S^vy-S^-., 
 
 7 S .-OOISOO ^OOOOOO 
 
 g , j|?i'8tstiiii 
 
 I. I. I. I I I. lf U U U I I I I I. I. I. I I. I. I. I I . 
 
 888.cs .d & si"! s is E 
 IsilleSSS .s? . 
 
 OOOOOOOOOgOOOuO 
 
 U u U t U TTTTT 
 
 3 
 
CHEMICAL ANALYSES. 65 
 
 j ' g 06 
 
 O a d 
 
 1 i ^ ^. 
 
 I I d 11 
 
 5 
 
 It 
 
 3 - 
 
 
 O CO QO 00 00 OQ 
 
 "o o o"o o^o 
 
 ,&&&.. 
 
 - oooooo 
 
 OC0050000 CpCOCBOOKM 
 
 ??? ????? I I. I M. I 
 
 ffa5?f c P ( f i.-b-sA't.?. 
 
 < i s | ) '| ) -' 2? 'i ) * 
 
 ^ '' f ' I ^ ^ "'' i ^ cf c" if o" o" if 
 
 p, C. &. & S-"; 
 
 S22222222 o-o'SS3 
 
 OOOOOCOCO ^3XJ5J5^3J3 
 
66 (nrAKACTETUSTICS OF OPTICAL GLASS. 
 
 Table 4 includes 16 precision analyses of a number of types of 
 foreign optical glass; these analyses were made with the greatest 
 care by Drs. E. T. Allen, E. Zies, and E. Posnjak, of the Geophysical 
 Laboratory, and are interesting not only because they furnish reliable 
 data on the essential components of the glasses, but because they 
 prove that the German glasses contain almost negligible amounts of 
 impurities; in short, that the excellence of these glasses is the result 
 of the use of raw materials of high chemical purity and of crucibles of 
 resistant qualities. The analysis of optical glasses is not an easy 
 task and special methods for such work were developed and per- 
 fected by Allen and Zies. 21 Table 4 contains also a number of 
 selected analyses (largely synthetic and deduced evidently from the 
 batch compositions) from a list published by E. Zschimmer in 
 C. Doelter's Handbuch der Mineralchemie, I, pages 869-888, 1912. 
 Many of the analyses in the Zschimmer list were published first by 
 Winkelmann and others and are given in the book on Jena glass by 
 H. Hovestadt (translation by J. D. and A. Everett, London, 1902) 
 on pages 146-147. The Winkelmann-Hovestadt numbers are in- 
 cluded in Table 4. A number of these synthetic analyses have been 
 checked by chemical analyses of the glasses; the results have been 
 in general in fair accord. Analyses computed from the batches of 
 certain other glasses are also included. In Table 4 the refractive 
 index n D , the z>-value, and the density of each glass are listed; also 
 the dispersions of the standard type glasses cited in the optical glass 
 lists of Schott and of Chance. In each case these dispersions are 
 sufficiently near the actual dispersions of the glass whose analysis 
 is given to be substituted for them. 
 
 The simplest series of optical glasses is evidently the flint series, 
 and for this reason this series was studied first and certain composi- 
 tion-refr activity relations were deduced from it. The chemical 
 relations (weight percentages) were plotted on a triaxial diagram, 
 such as is commonly used in representing the relations in a three- 
 component chemical system. 
 
 The fact that, for the members of the flint series, the refractivity 
 relations are expressed by means of smooth continuous curves in 
 the foregoing diagrams 13, 14, and 15, indicates that this series is 
 analogous in its behavior, so far as the flint glasses are concerned, 
 to a two-component system. If so, the chemical composition of 
 these glasses, when plotted in the triaxial diagram, should be found 
 to fall on a straight line. In figure 19 the weight-percentage com- 
 positions are plotted directly; the three components are silica (SiO 2 ), 
 lead oxide (PbO), and the alkali oxides (Na,O, K 2 O). The points 
 on the diagram include all available compositions of flint glasses. 
 The potash flints are distinguished in the diagram from the soda- 
 
 Jour. Am. Ceram Soc., I, 739-786, 1918. 
 
CHEMICAL COMPOSITIONS. 
 
 67 
 
 potash flints and from the soda flints. The compositions of all the 
 glasses plotted fall on a practically straight line between the composi- 
 tions: Lead metasilicate (PbO.SiO 2 ) and the potassium silicate glass 
 of the composition (K 2 O.6SiO 2 ), or the sodium silicate glass of the 
 approximate composition Na 2 O.4SiO 2 . The optical constants of a 
 synthetic potassium silicate glass of this composition were found to 
 be n D = 1.4836, ^ = 61.0. 
 
 The entire flint series is analogous chemically to a two-component 
 mixture; and, as such, any one of its physical constants such as 
 refractive index, y-value, or density varies continuously with change 
 
 sio 
 
 K 2 O6Si0 2 
 
 Na O 
 
 PbO 
 Na 2 O 
 
 FIG. 19. Triaxial diagram showing the weight-percentage compositions of the potash flint, soda-potash 
 
 flint, and soda-flint glasses. 
 
 in composition. This variation is illustrated in figure 20 in which the 
 variation in the chemical composition is represented along the 
 abscissa axis as weight percentages of lead oxide. The ordinates 
 give then the values of the refractive index for sodium light, the 
 j>-value, and the density for the glasses of the several compositions. 
 Smooth curves passing through these points enable the observer to 
 read off the percentage of lead oxide required in a glass having any 
 desired constant represented on these curves. In figure 21 the changes 
 in other optical constants (partial dispersions, v-value) as well as in 
 lead-oxide content, with change in refractive index n D , are represented 
 for all Schott and Chance flint and ordinary crown glasses. These 
 
68 
 
 CHARACTERISTICS OF OPTICAL GLASS. 
 
 changes are represented in the flint series by smooth continuous 
 curves. The diagram shows that the flint glasses are characterized 
 by higher dispersions and a relatively more rapid rise in dispersion with 
 rise in refractive index than is the case in the ordinary crown glasses. 
 
 r 70t2.OO 
 
 -6O 
 
 -so- 
 
 -4O- 
 
 20 
 
 1.90 
 
 1.80 
 
 1.7O 
 
 -30-H.6O 
 
 I 
 
 o 
 
 1.50 
 
 \ 
 
 V 
 
 ' \ 
 
 L 
 
 10 20 3O 4O 50 
 WEIGHT PERCENTAGE PbO 
 
 60 
 
 70 
 
 80 
 
 FIG. 20. In this diagram there are shown the changes in the density and the optical constants (refractive 
 index, n D , and v) with changes in lead oxide (1'bO) in the flint series of glasses. 
 
 The foregoing three figures represent the sum total of empirical 
 efforts on the part of glassmakers to produce a series of flint glasses 
 w r hich have certain optical properties. It is of interest to inquire, 
 in passing, why the glassmakers, who did not know what figure 19 
 
CHEMICAL OPTICAL RELATIONS. 
 
 69 
 
 clearly demonstrates, should have chosen the particular compositions 
 along the single straight line in the concentration field. The reason 
 is not far to seek. Melts whose compositions lie above the line ap- 
 
 1.75 
 
 1.7O 
 
 1.65 
 
 1.6O 
 
 1.55 
 
 1.50 
 
 FIG. 21. In this diagram the changes in the partial dispersions, n f n c ~, n D n A '; WF n ; n c F , in the 
 y- values and in the lead oxide (PbO) content with changes in refractive index, D , are shown for the 
 flint series of glasses. The ranges of partial dispersions in the ordinary crown glasses with changes in 
 the refractive index, n D , are also illustrated. 
 
 proach pure silica in composition and are extremely viscous and melt 
 at such high temperatures that they can not be produced in furnaces 
 of the ordinary type. Glasses whose compositions are given by 
 points below the line approach either the alkalies or lead oxide in 
 
70 
 
 CHARACTERISTICS OF OPTICAL GLASS. 
 
 composition. Glasses high in alkali melt easily, but are soft and 
 extremely hygroscopic and therefore unsuitable for optical purposes. 
 Glasses high in lead melt readily, but tend to crystallize with great 
 ease and hence are unsuitable from the glassmakers standpoint. 
 The glassmaker is thus forced to adopt those compositions which 
 melt readily, which are not hygroscopic, and which do not crystal- 
 lize too readily on cooling from high temperatures. An extended 
 series of experiments by Mr. Olaf Andersen of the Geophysical Lab- 
 oratory, carried out on a small scale in the laboratory in platinum 
 crucibles, corroborated the above conclusions in detail. The results 
 of his studies are to be published later. 
 
 BOROSILICATE CROWN 
 
 o CROWN 
 
 O BARIUM CROWN 
 
 X FLINT 
 
 + BARIUM FLINT 
 
 L BOROSILICATE FLINT 
 
 AAAAA; 
 
 FIG. 22. Triaxial diagram shoeing the weight-percentage compositions of the ordinary crowns, borosili- 
 cate crowns, barium crowns, flints, barium flints, and borosilicate flints of Table 4. 
 
 In figure 22 a composite diagram of the relations in both the 
 crown and flint silicate glasses of Table 4 is presented. The range 
 of compositions in this diagram, as in figure 19, is restricted to a 
 fairly definite band; the reasons for this narrow belt of compositions 
 are given in the foregoing paragraph. Although there is more lee- 
 way here for the glassmaker in the matter of compositions, there 
 are certain definite limits beyond which he may not pass without 
 inviting trouble and loss. 
 
 At the time we entered the war, a certain few of the optical glass 
 types had become standard and sufficed for the optical instruments 
 
CHEMICAL OPTICAL RELATIONS. 71 
 
 required by the Army and Navy ; our task consisted essentially in repro- 
 ducing these types. The first problem was to devise and to develop 
 manufacturing processes competent to meet the situation. The impor- 
 tance was realized of producing optical glass of uniformly high quality 
 in which the departures from the standard types are negligible. 
 The lens manufacturer can not afford to change his grinding and 
 polishing tools with each pot of glass received in order to cope with 
 large changes in optical properties which may occur from melt to 
 melt. It is the task of the manufacturer of optical glass so to control 
 his manufacturing processes that large departures from the standard 
 type do not occur and the variations in optical constants from melt 
 to melt of the same type are small and negligible; like other tasks 
 of high precision these conditions are not always easy to meet. 
 
 The glassmaker must hold his glass to type within narrow limits. 
 This is obtained from melt to melt by use of raw materials of high 
 chemical purity, by controlling accurately the furnace temperatures, 
 by using chemically and thermally resistant pots in which to melt 
 the glass, by proper stirring methods to insure thorough mixing, 
 also by adding to the molten batch ingredients which change the 
 optical properties (as measured on proofs taken of the glass melt) 
 in the direction required to have the finished product conform to 
 the standard type. 
 
 In Figure 23 are plotted the variations in refractive index n D for 
 series of melts of optical glasses of different types. These glasses 
 were furnished before the war by Schott und Genossen and by 
 Parra-Mantois to an American manufacturer and indicate how 
 closely in these specially favorable examples the melts were held 
 to type. Each point on a curve of refractive indices represents 
 a different melting. In other glasses the melts do not run so closely 
 to type. In general, the glasses manufactured in this country showed 
 at first somewhat greater departures from type than the European; 
 but during the later months of the war after batch materials, quality, 
 and treatment of melting pots and furnace conditions had been 
 improved and placed on a basis of better factory routine the varia- 
 tions were no greater than in the European glasses. 
 
 An interesting series of measurements of the variations in refrac- 
 tive indices of optical glass fragments broken from different parts 
 of the same melt was made by J. W. Gifford, 22 who found differences 
 of 0.00033 in the case of a dense barium crown; as a rule the differ- 
 ences were restricted to the fifth decimal place. 
 
 In general, the refractive index of a glass type may vary from 
 pot to pot by several units in the third decimal place; with certain 
 glasses, such as borosilicate crown, the departure from type is less 
 and restricted practically to 0.001; the rvalue may vary several 
 
 Proc. Roy. Soc., 87, 189, 1912. 
 
72 
 
 CHARACTERISTICS OF OPTICAL GLASS. 
 
 units in the first decimal place. First-quality glass selected from 
 any one pot should be constant in refractive index within several 
 units in the fourth decimal place; in some pots the difference be- 
 tween extreme limits may be 0.0008, but commonly they are less 
 
 I.6IA 
 
 1.617 
 
 1509 
 
 FIG. 23. Curves illustrating the variations in refractive index, n v , from pot to pot of melts of the same 
 type as furnished by 'Parra-Mantois and by Schott and Genossen. These particular types show the 
 s.nallest variations of a number of different types from which the selections were made. In each 
 case all the melts of each typeselected are plotted on the diagram. Each point on a curve represents 
 a different melt. The types represented by the curves are: (1) Medium flint, D =1.61(>8, Parra-Mantois; 
 (II) light flint, n D =1.5587, Parra Mantois; (III) barium light flint, D = 1.5825, TypeO 578, Schott and 
 Gen.; (IV) dense flint, n D = 1.6489, TypeO 102, Schott and Gen.; (V), barium silicate crown, n u =1.5726, 
 TypeO 211, Schott and Gen.; (VI) borosilicate crown, n D =1.5100, Type O 144, Schott and Gen.; (VII) 
 borosilicate crown, n D =1.5163, Type O. 3832, Schott and (Jen. 
 
 than this. The refractive index of a distinctly striated portion 
 of the glass may and commonly does differ from that of the sur- 
 rounding glass only in the fourth decimal place. The refractive index 
 of a stria is commonly lower than that of the adjacent glass. 
 
OPTICAL GLASS TYPES. 
 
 73 
 
 The optical constants of the principal types of glass produced 
 at the several plants in the United States during the war are listed 
 in Table 5. In this table only the more important types are included. 
 The list could be increased manyfold if the variations or departures 
 from type were included. This factor had to be taken into account 
 in the specifications for optical glass furnished to the Army and Navy 
 because in the early months of the war great difficulty was expe- 
 rienced in obtaining satisfactory melting pots which were thermal!} 7 
 and chemically resistant. The difficulty arising from a wide range 
 in refractivities of glass types was overcome to some extent by 
 dividing the glasses of each general type into lots so that the range 
 of refractivities in each lot was relatively small; the variations within 
 the lot were then within the tolerance limits set by the manufacturer 
 of the particular optical instrument for which the glass was intended. 
 Under the stress of war activities this procedure was not always 
 observed, and manufacturers received glass which necessitated 
 changes of tools and consequent retardation of production. In war 
 time the importance of competent personnel to handle such details 
 can not be overemphasized; the lack of appreciation of the signifi- 
 cance of technical details of this nature causes loss of time and money 
 and may become serious in a crisis. 
 
 TABLE 5. List of optical constants of principal glass types manufactured in quantity in 
 the United States during 1918. 
 
 BOROSILICATE CROWN. 
 
 Manufacturer. 
 
 n D . 
 
 " 
 
 nr-Wc- 
 
 We- 
 
 n r . 
 
 Bausch & Lomb Optical Co 
 
 .5164 
 
 64.9 
 
 0. 00795 
 
 1. 51423 
 
 .52218 
 
 Bureau of Standards 
 
 .5190 
 .5171 
 .5100 
 .5244 
 
 64.6 
 64.3 
 63.5 
 64.0 
 
 .00803 
 .00804 
 .00803 
 .0082 
 
 1. 51659 
 1. 51470 
 1.50801 
 1. 5219 
 
 . 52462 
 . 52274 
 .51604 
 .5301 
 
 Keuffel & Esser 
 
 . 51010 
 
 63.8 
 
 .00799 
 
 1 50734 
 
 51533 
 
 Pittsburgh Plate Glass Co 
 
 .50960 
 . 51579 
 
 63.2 
 63.8 
 
 .00807 
 .00807 
 
 1. 50717 
 1. 51345 
 
 . 51524 
 52152 
 
 Spencer Lens Co 
 
 .51611 
 . 51300 
 . 51123 
 
 63.6 
 62.3 
 63.4 
 
 .00812 
 . 00823 
 00806 
 
 1. 51358 
 1. 51056 
 1 50884 
 
 . 52170 
 . 51879 
 51690 
 
 
 .51905 
 . 51358 
 . 51587 
 
 60.7 
 62.5 
 64.0 
 
 .00854 
 .00822 
 .00806 
 
 1.51646 
 1. 51107 
 1. 51340 
 
 .52500 
 .51929 
 1. 52146 
 
 ORDINARY CROWN. 
 
 Bausch & Lomb Optical Co 
 
 
 1.5116 
 
 61 
 
 00839 
 
 
 50923 
 
 
 1 51762 
 
 Bureau of Standards . . 
 
 
 .5143 
 . 51979 
 
 60.7 
 60 9 
 
 .00847 
 00850 
 
 
 .51191 
 51530 
 
 
 1. 52038 
 52380 
 
 Keuffel & Esser 
 
 
 51593 
 
 60 8 
 
 00849 
 
 
 51530 
 
 
 52194 
 
 Pittsburgh Plate Glass Co 
 
 
 . 52495 
 
 59.1 
 
 00887 
 
 
 52170 
 
 
 53057 
 
 Spencer Lens Co 
 
 
 .52400 
 51635 
 
 58.8 
 60 6 
 
 .00890 
 00851 
 
 
 . 52135 
 
 51388 
 
 
 .53026 
 52239 
 
 Bausch & Lomb Optical Co 
 
 
 .52389 
 569 
 
 59.5 
 57 
 
 .00880 
 01000 
 
 
 . 52114 
 56699 
 
 
 .52994 
 57699 
 
 Bureau of Standards 
 
 
 .5721 
 . 5710 
 .5728 
 
 56.8 
 56.5 
 57.7 
 
 .01006 
 . 01010 
 .0099 
 
 
 .56920 
 .56805 
 .5699 
 
 
 . 57926 
 . 57815 
 5798 
 
 Keuffel & Esser 
 
 
 .57485 
 
 57 4 
 
 01002 
 
 
 57198 
 
 
 58200 
 
 Pittsburgh Plate Glass Co 
 
 
 . 57167 
 .57718 
 
 56.9 
 56 1 
 
 .01003 
 01040 
 
 
 .56880 
 57405 
 
 
 .57883 
 58445 
 
 Spencer Lens Co 
 
 
 .56100 
 .59300 
 57222 
 
 57.7 
 54.5 
 57 5 
 
 .00968 
 . 01088 
 00994 
 
 
 . 55816 
 . 58979 
 56925 
 
 
 .56784 
 .59300 
 57919 
 
 
 
 . 56822 
 
 57.1 
 
 .00992 
 
 
 .56529 
 
 
 . 57521 
 
74 
 
 CHARACTERISTICS OF OPTICAL GLASS. 
 
 TABLE 5. List of optical constants of principal glass types manufactured in quantity in 
 the United States during 1918 Continued. 
 
 DENSE BARIUM CROWN. 
 
 Manufacturer. m 
 
 71 D . 
 
 V. 
 
 Hv-nc' 
 
 n c . 
 
 F. 
 
 Bureau of Standards 
 
 1.62149 
 
 53.6 
 
 0.01159 
 
 1.61809 
 
 1.62968 
 
 Spencer Lens Co 
 
 1.59500 
 
 57.2 
 
 . 01040 
 
 1. 59182 
 
 1.60222 
 
 Pittsburgh Plate Glass Co 
 
 1.61100 
 
 56. 7 
 
 . 01077 
 
 1.60796 
 
 1.61873 
 
 
 
 
 
 
 
 LIGHT FLINT. 
 
 Bausch & Lomb Optical Co 
 
 .5619 
 
 44.8 
 
 0. 01253 
 
 1. 55829 
 
 . 570X2 
 
 Bureau of Standards 
 
 .5725 
 .5664 
 .5802 
 
 .58484 
 
 42.7 
 42.6 
 41.4 
 40.5 
 
 . 01342 
 . 01331 
 .01413 
 . 01439 
 
 .56864 
 . 56258 
 . 57615 
 . 58070 
 
 ! 575X9 
 .59028 
 . 59509 
 
 Pittsburgh Plate Glass Co 
 
 . 57220 
 
 42.1 
 
 . 01365 
 
 .56817 
 
 . 58182 
 
 Spencer Lens Co 
 
 .56900 
 .57900 
 .58069 
 
 42.5 
 41.1 
 41.1 
 
 . 01337 
 . . 01408 
 . 01414 
 
 .56508 
 .57499 
 .57668 
 
 .57845 
 .58907 
 .59082 
 
 
 
 
 
 
 
 MEDIUM FLINT. 
 
 Bausch & Lomb Optical Co 
 
 1.6150 
 
 36.8 
 
 0. 01779 
 
 1.60998 
 
 . 62667 
 
 Bureau of Standards 
 
 1.6180 
 1. 6218 
 1.6274 
 
 36.5 
 36.4 
 36.4 
 
 . 01692 
 .01806 
 .0172 
 
 1.61331 
 1. 61695 
 1. 6225 
 
 . 63023 
 . 63401 
 .6397 
 
 Keuff el & Esser . . . 
 
 1.61252 
 
 37.9 
 
 . 01615 
 
 1. 60799 
 
 .62414 
 
 Pittsburgh Plate Glass Co 
 
 1. 61573 
 1. 61660 
 1. 61800 
 
 37.6 
 37.5 
 36.6 
 
 . 01634 
 . 01646 
 .01686 
 
 1.61101 
 1. 61209 
 1. 61321 
 
 1.62735 
 1. 62855 
 .63007 
 
 Spencer Lens Co 
 
 1.61100 
 1.62700 
 1. 60536 
 
 37.3 
 35.6 
 37.9 
 
 . 01640 
 . 01760 
 . 01596 
 
 1. 60638 
 1. 62192 
 1. 60079 
 
 1. 62278 
 1. 63952 
 1. 61675 
 
 
 1. 61098 
 1. 62037 
 1. 63317 
 
 37.0 
 36.4 
 35.1 
 
 .01650 
 .01704 
 .01808 
 
 1. 60630 
 1. 61545 
 1. 62797 
 
 1. 62280 
 1. 63259 
 1. 64605 
 
 DENSE FLINT. 
 
 Bausch & Lomb Optical Co 
 
 1.6465 
 
 33.9 
 
 0. 01904 
 
 .64095 
 
 .65999 
 
 Bureau of Standards 
 
 1.6495 
 1. 6545 
 .65555 
 
 33.7 
 33.3 
 34.4 
 
 . 01928 
 . 01963 
 . 01903 
 
 . 64428 
 .64906 
 . 6.5019 
 
 .66356 
 . 66869 
 . 66922 
 
 Keuffel & Esser 
 
 . 65174 
 
 33.8 
 
 . 01925 
 
 .64628 
 
 . 66553 
 
 Pittsburgh Plate Glass Co 
 
 .63500 
 
 34.9 
 
 .01818 
 
 .62982 
 
 .64800 
 
 Spencer Lens Co 
 
 .64500 
 .66100 
 .64015 
 
 34.1 
 32.9 
 34.6 
 
 .01883 
 . 02013 
 . 01853 
 
 . 63961 
 . 65527 
 1. 63489 
 
 . 65854 
 . 67539 
 . 65342 
 
 
 
 
 
 
 
 BARIUM FLINT. 
 
 Bureau of Standards 
 
 1 55000 
 
 52 6 
 
 01040 
 
 1. 54700 
 
 1.55740 
 
 Pittsburgh Plate Glass Co 
 
 1.60300 
 
 44.0 
 
 . 01370 
 
 1.59910 
 
 1. 61280 
 
 Spencer Len^ Co 
 
 1.60900 
 1 62210 
 
 43.4 
 38 2 
 
 .01405 
 01628 
 
 1. 60495 
 1 61744 
 
 1.61900 
 1 63372 
 
 
 
 
 
 
 
 EXTRA DENSE FLINT. 
 
 Spencer Lens Co 
 
 1 75614 
 
 27 2 
 
 02778 
 
 1 74839 
 
 1. 77617 
 
 
 1 
 
 
 
 
 
 III. FREEDOM FROM COLOR. 
 
 Color in optical glass is caused by the presence of certain coloring 
 agents, which are generally difficult to eliminate from the raw 
 materials, especially from the sand that goes into the glass batch. 
 Iron in appreciable quantities may also be introduced as a result of 
 solution of the walls of the pot. Copper, nickel, cobalt, chromic 
 
FREEDOM FROM COLOR. 75 
 
 oxide, vanadium, or manganese may also be responsible for some of 
 the coloration. A very small amount of certain of these oxides 
 suffice to produce relatively intense coloration. Iron oxide is the 
 chief source of trouble in this respect. In the ferrous-ferric state it 
 produces intense green coloration, in the pure ferric state the result- 
 ing color is pale yellow. 
 
 Chemical analyses show that the best European glasses contain 
 less than 0.02 per cent Fe 2 O 3 ; if the iron oxide content exceeds 0.05 
 per cent the glass is colored noticeably green or yellow green. The 
 exact hue produced by a given amount of iron oxide is different for 
 different glasses; in the barium crowns the color is green to bluish 
 green; in the flints yellow to greenish yellow; in borosilicates a 
 relatively large amount of iron is required to produce decided colora- 
 tion, which is then gray green or gray blue. The amount of iron 
 contributed by the pot is a serious matter and may equal or exceed 
 that contained in the raw materials. Analyses show that the clay 
 used in the German pots contains less than 1 per cent; in the clays 
 used in this country it is commonly over 2 per cent. It is probable 
 that in some of the glasses slight traces of a coloring agent, such as 
 cobalt, may be present and affect appreciably the color* and espe- 
 cially the transmission of the glass; the definite percentage effects 
 on transmission of alumina and of the colorless oxides, such as the 
 alkalies, alkaline earths, boron oxide, and silica on the light-trans- 
 mission is unknown. 
 
 Experience with glass containing small quantities of iron oxide has 
 proved that the resultant color of the glass is largely dependent on the 
 state of oxidation of the iron; thus a decidedly green colored glass, 
 if melted in a platinum crucible heated in an electric resistance 
 furnace, becomes practically colorless under these conditions of high 
 oxidation. In case manganese be present, the color may even shift 
 to a decided purple. The effect of different degrees of oxidation 
 may occasionally be observed in glass broken from a large melting 
 pot, especially near the top margin of the glass mass. In this periph- 
 eral portion the color of the glass may change rapidly from decidedly 
 green to colorless to purple. The following analyses of two samples 
 taken from a large pot of flint (calculated synthetic composition: 
 SiO 2 46.4, PbO 45.8, K 2 O 2.7, Na 2 O 4.7, B 2 O 3 0.33, MnO 0.075) in 
 which these color differences appeared, prove that the amount of 
 manganese oxide present is the same throughout the mass; the man- 
 ganese is calculated in both samples as MnO; the state of oxidation 
 was not ascertained. 
 
 1 MnO 0. 065 
 
 MnO . 063 
 
 1. A small piece of glass from the top surface of the melt ranging in color from nearly 
 colorless to decidedly purple. E. T. Allen, analyst. 
 
 ?. Sample taken 5 centimeters from the top surface of the melt; decidedly green in 
 color. E. T. Allen, analyst. 
 
76 CHARACTERISTICS OF OPTICAL GLASS. 
 
 The chief objection to a slight amount of color is the fact that it 
 indicates a glass of relatively low transmission. Glass intended for 
 prisms, in which the optical glass path is necessarily long, should be 
 relatively less colored than glass intended for thin lens elements. 
 The quality of optical glass with respect to color is best ascertained 
 as it is broken from the melting pot and can be observed in pieces a 
 foot or more thick. Under these conditions all glass is appreciably 
 colored. The same phenomenon can be seen on window or plate 
 glass which appears through the plate to be colorless, but, when 
 viewed through the edges, it is relatively dark green or yellow or 
 blue green. 
 
 IV. HIGH DEGREE OF TRANSPARENCY. 
 
 This implies freedom from coloring agents which tend to absorb an- 
 appreciable portion of the incident light. In the best European 
 glasses the light-absorption ranges from 0.3 to 0.6 per cent per 
 centimeter thickness of glass path. High transmission is attained 
 primarily by the use of raw materials of high chemical purity and 
 by melting the glass in chemically resistant pots under conditions of 
 high oxidation. The use of decoloring agents is not to be recom- 
 mended, because they commonly function by contributing to the 
 glass a color complementary to the color which would otherwise be 
 obtained, the resultant effect being neutral gray. By this method 
 relative freedom from color can be had, but only at the expense of 
 the light transmission. 
 
 High transparency in optical glass for military instruments is 
 important because on it the brightness of the visual image depends 
 and with it the ability of the eye to detect details of distant objects. 
 The better the Army and Navy can see the more effective are they 
 in the presence of the enemy. 
 
 The treatment of polished glass surfaces to reduce the amount of light 
 reflected. Two factors contribute toward loss of light in a given 
 optical system for a given intensity of illumination of object. These 
 are: (1) Absorption of light by the glass itself; (2) reflection of light 
 from the surfaces of the lens and prism elements. Theoretically the 
 second factor has a definite limiting value which is fixed by the 
 refractive index of the reflecting glass. For vertically incident light 
 
 this value is expressed by the Fresnel equation | - , | , in which n 
 
 [ n + i J 
 
 is the refractive index of the glass. 
 
 In 1892 H. D. Taylor 23 discovered that old photographic lenses 
 which had become slightly tarnished were faster than new lenses of 
 the same aperture. Evidently the exposed surface was modified in 
 some way such that it reflected less light than before. Taylor 
 
 " The Adjustment and Testing of Telescope Objectives, published in 1896 by T. C'cok, of York, England . 
 
TRANSPARENCY AND REFLECTING POWER. 77 
 
 experimented with the problem and found that by the use of certain 
 chemicals he was able to decrease the amount of light reflected by a 
 given glass surface. He did not, however, reveal the actual chemicals 
 which were used except to state that hydrogen sulphide and alkaline 
 sulphides reduced the reflecting power appreciably. Recently F. 
 Kollmorgen 24 has been able, by treatment of glass surfaces, to decrease 
 the amount of light lost in an ordinary flint or barium crown lens 
 from 8 to 10 per cent to 3 or 4 per cent. Experiments of similar 
 nature were also made by Dr. H. Kellner 25 and similar results were 
 obtained. 
 
 In view of the importance of this matter for range finders, peri- 
 scopes, and other military optical instruments a series of experiments 
 was begun by Dr. J. B. Ferguson and the writer during the early 
 months of the war. Unfortunately other matters prevented the 
 completion of this task, but the results thus far attained are of 
 interest. 
 
 Polished specimens of light flint glass of refractive index n D = 1.570 
 were immersed in solutions of different concentrations and held ordi- 
 narily for 18 hours at 80 C. The experiments proved that with a 
 given concentration of solution the surface change is a gradual proc- 
 ess, and that, for the best results, time-temperature-concentration 
 relations are required for each solution with each type of glass. 
 With the light flint specimens the greatest reduction in reflecting 
 power was obtained with a 1 per cent solution of acid sodium phosphate 
 (NaH 2 PO 4 ) acting for 18 hours at 80 C. Solutions containing , 2, 
 and 20 per cent of the salt were tried, but these reduced the reflecting 
 power less. Other solutions nearly equal in effectiveness are: Phos- 
 phoric acid (H 3 PO 4 ), 1 per cent; copper sulphate (CuSO 4 ), 2 per cent; 
 nickel sulphate (NiSO 4 ), 2 per cent; ferric sulphate (Fe 2 (SO 4 ) 3 ), 2 per 
 cent with a little free H 2 SO 4 ; potassium dichromate (K 2 Cr 2 O 7 ) , 2 per 
 cent; less effective are solutions of potassium arsenate, sodium arse- 
 nate, copper chloride, zinc chloride, nickel chloride, cobalt chloride, 
 potassium iodide, copper nitrate, acetic acid, potassium chromate; 
 little, if any, effect was obtained with solutions of ferric nitrate, 
 magnesium sulphate, zinc sulphate, copper chlorate, potassium 
 chlorate, potassium sulphocyanide, potassium fluoride. Solutions of 
 alkali salts, such as sodium carbonate, sodium bicarbonate, potassium 
 carbonate, ammonium carbonate, etch the surfaces but do not 
 decrease the reflecting power to any extent. In solutions of sodium 
 sulphide and potassium sulphide a sulphide film is formed on the 
 polished surface. 
 
 It is an interesting fact that the light reflected from a treated 
 surface is in most cases appreciably colored; this color is commonly 
 
 2 < Trans. Soc. Illuminating Engineers, 2, 220-234, 191fr. 
 2 > Private communication. 
 
 3922921 6 
 
78 
 
 CHARACTERISTICS OF OPTICAL GLASS. 
 
 a faint blue or blue violet, but in the case of samples treated in solu- 
 tions of potassium bichromate, copper nitrate, borax, potassium 
 arsenate, the color of the reflected light is noticeably yellow. 
 
 Specimens of borosilicate crown glass were treated in similar man- 
 ner and showed similar decreases in reflecting power. Good results 
 were obtained with solutions of ferric sulphate, 1 and 2 per cent; 
 copper sulphate, 2 per cent; potassium bichromate, 4 per cent; less 
 satisfactory are solutions of copper nitrate, acetic acid, borax, potas- 
 sium binoxalate, nickel sulphate, acid sodium phosphate. The time 
 of exposure in all these experiments was 18 hours at 80 C. 
 
 Samples of light barium crown glass were found to be readily 
 attacked. Weak solutions of nickel sulphate, acid sodium phosphate, 
 copper sulphate, ferrous sulphate, phosphoric acid, acetic acid, cop-^ 
 per chloride were tried; in all cases a decided decrease in reflecting 
 power was observed, but the surfaces were noticeably etched, indi- 
 cating too long exposure. 
 
 To account for this phenomenon of decreased reflection below the 
 theoretical limits three tentative hypotheses are suggested : 
 
 1. A thin surface film of very low refractive index is deposited on 
 the reflecting surface (adsorbed film) . 
 
 2. There is selective solution at the surface such that the refractive 
 index of the exposed residual surface is greatly lowered. 
 
 3. In the process of etching by the attacking solution the surface 
 becomes covered with minute pits which are small compared with 
 the wave length of light. Light waves impinging on the reflecting 
 face encounter a plateau surface consisting of the remnants of the 
 original polished surface with the intervening air pockets above the 
 etch pits. As a result the etched surface behaves optically in its reflect- 
 ing power as a material whose reflectivity is equal to the sum of the 
 reflecting powers respectively of the exposed plateau elements of the 
 glass, and of the intervening air spaces which are so small that they 
 do not cause appreciable diffraction of the light waves. 
 
 The changes in the intensity of normally incident light on reflec- 
 tion with change in refractive index of the reflecting medium as 
 computed by means of the Fresnel equation are listed in Table 6. 
 
 TABLE 6. Percentage of optically incident light reflected from a single polished surface. 
 
 Refractive index of glass 
 surface. 
 
 Per cent. 
 
 1.222 
 
 j 
 
 1.329 
 
 2 
 
 1.418 
 
 3 
 
 1.502 
 
 4 
 
 1.575 . .. 
 
 
 I 648 
 
 6 
 
 1.720 
 
 7 
 
 1 790 
 
 8 
 
 1.857 
 
 9 
 
 1 927 
 
 10 
 
 
 
, CHEMICAL AND PHYSICAL STABILITY. 79 
 
 These computations indicate that a glass surface reflecting only 2 
 per cent of vertically incident light must have a refractive index 
 1.329, which is less than that of water. 
 
 The fact of the lowering of reflecting power by treatment in solu- 
 tions is established beyond question. The change produced is 
 so permanent that it does not disappear on ordinary rubbing or 
 cleaning the surface or after several years' exposure to the air. Such 
 surfaces show a tendency to appear faintly colored (blue or violet) 
 in reflected light depending on the solution employed and also on 
 treatment (polishing) just before immersion in the solution. If 
 the refractive indices of original and treated samples are measured by 
 total reflection methods no difference in refr activity between them 
 can be detected. 
 
 The three hypotheses cited above are not of equal probability. 
 The fact that the reflecting power can be lowered by immersing the 
 glass surface in solutions of widely different character, and that the 
 surface can not be rubbed off is difficult to explain by the first 
 hypothesis. The refractive index required theoretically to give such 
 low reflecting power, and the fact that the index of silica glass is 
 about 1.460 is an argument against the second hypothesis. In favor 
 of the third hypothesis is the observed change in refractive index 
 of the zeolites .and some other water-bearing compounds, namely, 
 that on loss of water, the refractive index of the mineral is lowered 
 and not raised as one might possibly expect. 
 
 During the war this process of treating polished surfaces to reduce 
 the reflecting power was not developed to the point where it could be 
 adopted as a routine factory operation; before this can be done more 
 experimental data are required. The possibilities, from both a 
 theoretical and a manufacturing viewpoint, are, however, great, and 
 warrant further detailed investigation of this subject. 
 
 V. HIGH DEGREE OF STABILITY, BOTH CHEMICAL AND PHYSICAL. 
 
 The glass should be of such composition that it is weather resistant, 
 does not tarnish readily, and retains an optical polish well. For 
 most types of glasses this condition obtains; but in a few types, 
 such as certain dense barium crowns and the borate and phosphate 
 glasses, the lack of stability may cause trouble if the lens elements 
 are not adequately protected from attack. Glasses high in alkalies 
 are much less stable than glasses low in alkalies. Hardness and 
 toughness are also a function of the chemical composition and oi 
 the state of annealing of the glass. The glass should be hard and 
 tough, so that under ordinary field conditions the exposed surfaces 
 are not easily scratched and damaged. 
 
 Experience with glasses in the Tropics has shown that the high 
 temperature and humidity existing there are potent factors in 
 
80 CHARACTERISTICS OF OPTICAL GLASS. 
 
 attacking exposed surfaces of optical glass in instruments. In 
 optical instruments used in the Tropics a brown-colored film or 
 coating may form which gradually dims the image and finally renders 
 the instrument useless. Investigation of this coating has proved 
 that in certain crown glasses crystals of sodium carbonate are formed 
 on the polished lens surfaces; in other glasses an organic mold or 
 growth or film scum appears, especially on instruments which have 
 been sealed and made water-tight. This film may consist of many 
 small liquid drops and occurs especially on the reticules of field 
 glasses and fire-control instruments; in most cases it appears to 
 avoid the area immediately adjacent to the etched lines of the 
 reticule; or it may be much finer and spread uniformly over the 
 polished surface. Experiments have shown that film of this nature 
 is for the most part not an inherent defect of the glass itself, but 
 is organic matter which has evaporated from lacquer and grease and 
 dirt left in the instrument during the assembling process. The 
 remedy for such film is meticulous care and cleanliness in the assembly 
 of optical instruments. In certain cases the glass surfaces are badly 
 attacked and the glass itself is then of faulty composition. 
 
 The stability of optical glasses is a difficult property to define 
 satisfactorily. In all cases a glass is desired which shall remain 
 unchanged on exposure to all kinds and conditions of weather. To 
 test the stability of the glass under actual conditions is obviously 
 a time-consuming process. The attack on the glass is, however, 
 accelerated by elevating the temperature and the pressure. As a 
 measure of the stability the behavior has been taken of the glass 
 under arbitrarily fixed but reproducible conditions of high tempera- 
 ture and high pressure in acid, neutral, and alkaline solutions of 
 known composition. There is danger, however, that the character 
 of the reactions which take place at higher temperatures and pressures 
 may be, and probably is, different from that of the reactions at 
 room temperature and atmospheric pressure, with the result that 
 the conclusions drawn from the behavior of the glass at high tem- 
 peratures, high pressures, and concentrated solutions may be much 
 in error when applied as criteria of the weathering stability of glass. 
 For this reason it has been deemed better to devise tests of extreme 
 sensitiveness which may be used at ordinary temperatures and 
 atmospheric pressure. These and other tests will be described in 
 Chapter IV on the inspection of optical glass. 
 
Chapter III. 
 THE MANUFACTURE OF OPTICAL GLASS. 
 
 THE ORGANIZATION OF AN OPTICAL GLASS PLANT. 
 
 Optical glass has been shown to be a product which must meet the 
 strictest demands of extremely high precision and throughout all 
 stages of its manufacture this fact must be recognized. One of the 
 chief difficulties encountered in the rapid development of optical 
 glass manufacture in this country for war purposes was the lack of 
 appreciation on the part of manufacturers, of superintendents, of 
 foremen, and of workmen of this fundamental fact; to educate them to 
 a proper realization of their several responsibilities required time and 
 this necessarily retarded production during the first months of the 
 war. High precision means careful control over all steps of the 
 manufacturing processes and a personnel competent to establish such 
 control; the problem is essentially one of physical and chemical 
 engineering, and the manufacture of optical glass to be successful 
 must be directed by men thoroughly trained along these lines. Tech- 
 nical control of this kind soon pays for itself in the quality and the 
 quantity of the product obtained. 
 
 An optical glass plant should be so organized that effective con- 
 trol is exercised over each step of manufacture from the raw materials 
 to the finished product. The raw materials should be of uniformly 
 high chemical purity. They should be ordered from manufacturers 
 under definite specifications and with provision for adequate chemical 
 control at the factory to insure the necessary quality. The melting 
 pots require close attention. If the size of the glass plant warrants 
 it, each plant should have its own pot house; this obviates the inevi- 
 table jolts and jars and rapid changes in temperature and in humidity 
 incident upon transportation and makes it possible to develop pots 
 which are best suited to special types of optical glass. The melting 
 and annealing furnace temperatures must be held to schedule. The 
 batches must be properly mixed and filled into the pots. The crucible 
 after removal from the furnace must be so cooled that properly fissured 
 glass results. The inspection of the raw glass must be carefully 
 supervised, otherwise much good glass is butchered and wasted, and 
 much poor glass is taken into work which later has to be discarded. 
 The molding and pressing of the glass into blocks must be properly 
 directed, otherwise serious losses are incurred. 
 
 81 
 
82 MANUFACTURE OF OPTICAL GLASS. 
 
 In the present chapter the several sections of the glassmaking 
 process will be considered briefly and somewhat in the order of actual 
 manufacture. In the analysis of these steps we shall find that there 
 are many factors involved, and that only with proper regard to their 
 several effects can optical glass of uniformly high quality be produced. 
 In other words, the making of modern optical glasses does not con- 
 sist solely in the mixing together of a secret batch, handed down 
 from father to son, in melting this down in a furnace, and in allowing 
 the melt to cool properly. In many branches of the glass industry 
 there is still much that harks back to the days of the alchemist; 
 many glassworkers are highly skilled artisans trained to their tasks 
 from childhood ; but in the making of optical glass there is little room 
 for the deftness and whim of the artist. The problem is essentially 
 one of precision and factory control; and although the glassmaker's 
 experience is not to be disregarded, optical glass of high quality can 
 not be produced by it alone. The training and the viewpoint of the 
 chemist and physicist are required in addition to attain and to main- 
 tain the high quality of product essential for use in optical instru- 
 ments for military purposes. A physicist or chemist is in a better 
 position to undertake the manufacture of optical glass than is the 
 plate or window glass maker of many years' experience who has fixed 
 ideas on the subject of all glassmaking and can not realize that optical 
 glass is different from the product which he has been accustomed to 
 make. 
 
 In the management of an optical glass plant, as in all other indus- 
 trial organizations, it is highly essential that each man and woman 
 connected with the plant realize the importance of his or her work, 
 and that the service rendered is contributing directly to the success 
 of the plant and to the welfare of each individual in it. In time of 
 war the spirit of active and wholehearted cooperation should domi- 
 nate the entire plant and is easy to attain by proper appeal to patri- 
 otic motives and to the feeling that each worker is contributing his 
 share toward the weal of the country in the emergency. In order 
 that the organization function properly it is essential that the duties 
 of every position be adequately prescribed, together with the respon- 
 sibility and authority pertaining to the. position. Workers should 
 be selected and assigned with reference to their several capacities; 
 those who show special aptitude and ability should be given oppor- 
 tunity to advance and to realize that their efforts are appreciated. 
 Tact is required at all times to keep the organization running smoothly 
 and effectively. During war time there is liable to be more or less 
 continuous increase in personnel and equipment to meet the demands 
 for more rapid production; also frequent changes in details of fac- 
 tory practice and routine to improve the manufacturing processes; it 
 is necessary therefore to keep the organization of the plant so flexible 
 
RAW MATERIALS. 83 
 
 that, without fundamental 'changes in its structure, it can take care 
 of the changing demands made upon it. This means, among other 
 tasks, the training of some of the workers for more than one par- 
 ticular task. 
 
 There are many factors which enter into the problems confronting 
 the management of an organization of diverse human elements and 
 which have to be considered if the desired end is to be attained, 
 namely, high-speed production, together with a satisfied and effec- 
 tively cooperating and loyal group of workers. It would lead too far 
 in this report to consider further this most important of all factors, 
 namely, the human factor, without which nothing can be done in 
 spite of the best and most improved processes of manufacture. Suf- 
 fice it to state that in an emergency, such as war, nearly everyone is 
 eager to do his part, or may be compelled to do so, and to that extent 
 the problems are simplified. 
 
 RAW MATERIALS. 
 
 In the foregoing pages reference is made to the many factors which 
 enter into the manufacture of optical glass; of these not the least is 
 purity of raw materials. Except for certain gases, such as carbon 
 dioxide, nitrogen oxide, oxygen, water vapor, etc., which are liber- 
 ated during the fusion process, and for the small amounts of certain 
 somewhat volatile oxides, such as lead oxide, which escape during 
 the melting and fining stages, whatever is put into the batch appears 
 in the final product and can not later be eliminated; this fact makes 
 it imperative for the glassmaker to use batch ingredients of high and 
 definitely known purity. Optical glass is unfortunately sensitive 
 both in color and in refractivity to slight changes in chemical com- 
 position. If the glassmaker neglects to insure proper quality and 
 uniformity of raw batch materials, not only is the glass produced 
 variable and unsatisfactory, but the furnace schedule may be seri- 
 ously affected and production all along the line to the finished optical 
 instrument is impeded and rendered uncertain. 
 
 The best procedure to reduce to a minimum the variations in com- 
 position and quality of the raw materials is to establish direct con- 
 tract relations with the producer of the raw materials (not with the 
 jobbers), each contract to state explicitly the kinds and amounts of 
 impurities which may be tolerated. Each shipment of raw material 
 should be analyzed at the plant with reference to impurities and to 
 water content. The batches are then computed on the basis of this 
 analysis; if this procedure is not followed, an element of uncertainty 
 is introduced which, in certain types of glass, may cause a departure 
 from the desired optical constants sufficiently large to render the 
 glass useless for the purpose for which it was intended. 
 
84 MANUFACTURE OF OPTICAL GLASS. 
 
 Much time and effort was spent by the Geophysical Laboratory in 
 canvassing the raw material situation throughout the country, in 
 developing rapid routine methods for the testing of such materials, 
 and in educating manufacturers to a realization of what high chem- 
 ical purity in large lots means. To obtain raw materials of adequate 
 purity, it was necessary for the Geophysical Laboratory to make 
 many tests and analyses of samples submitted, to send men to dif- 
 ferent places to examine the methods of production, to suggest 
 changes in method in order to insure proper quality, and to encourage 
 manufacturers to install special apparatus to meet the demands for 
 high chemical purity. The standards required for optical glass are 
 extremely high, and it means very accurate and direct analytical 
 control at the plant to obtain a product of proper quality. In par- 
 ticular, the sand situation and the potassium carbonate situation 
 required close attention. 
 
 Sand. The requirements of sand for optical glass are high chem- 
 ical purity and uniform, small size of grain. High chemical purity 
 is essential because silica constitutes the major part of most optical 
 glasses. The iron content of the sand should be less than 0.02 per 
 cent iron oxide. The water content of each sand shipment should 
 be determined. Other coloring agents such as chromium, manganese, 
 vanadium, copper, cobalt, etc., should not be present. Most of the 
 sands in this country contain more than 0.02 per cent iron oxide; 
 but with careful selection of the sand at the quarry it is possible in 
 several of the localities to obtain sand of a degree of purity better 
 than 0.02 per cent iron oxide. Experience has shown that sands 
 suitable for optical glass are located at Rockwood, Mich., Hancock, 
 Md., and Ottawa, 111. These deposits are all of friable sandstone 
 deposited under wind-blown, desert conditions during early geologic 
 times (Lower Paleozoic). Of these three deposits, the Rockwood 
 sand is the purest; the best grade contains on an average about 
 0.015 per cent iron oxide. The sand is pure white and remarkably 
 uniform in size of grain. Optical glass of good quality can be made 
 from the sand of any one of these three localities. l 
 
 An extended series of experiments was carried out by the Geo- 
 physical Laboratory with a view to extracting the iron from the sands 
 and chiefly from the glass melting pots by chemical means. For 
 this purpose the sands and pots were submitted at high temperatures 
 to an atmosphere of chlorine gas; under these conditions the chlorine 
 attacks the iron oxide and forms iron chloride, which is volatile. 
 This reaction takes place much more rapidly and completely if, in 
 place of chlorine, phosgene gas (carbonyl chloride) is used. The final 
 outcome, however, of these experiments, which were intensely 
 
 i See Notes on American high-grade glass sands, by P. G. H. Bos well, Trans. Soc. Glass Technology I, 
 147-152, 1917. 
 
POTASSIUM CARBONATE 85 
 
 interesting from a laboratory viewpoint and extended over many 
 months, has been that it is not worth the while in war times to attempt 
 the purification of sands on a large scale by such methods. The 
 expense of the chlorination process is, moreover, relatively high, and 
 sufficiently pure sand is available in the open market. Methods 
 for the determination of iron in glass sand are described by J. B. Fer- 
 guson, jn 2 
 
 Potassium carbonate. Before the war the better grades of potas- 
 sium carbonate were imported from Germany. It was not a difficult 
 task at that time to obtain potassium carbonate of the desired degree 
 of purity. This source of supply, however, was shut off, and it was 
 necessary for manufacturers to develop new sources and to arrange 
 for the purification of potassium carbonate on a large scale. At the 
 earnest solicitation of the Geophysical Laboratory, the Armour 
 Fertilizer Works, of Chicago, 111., patriotically undertook to produce 
 the quantities of potassium carbonate of the required purity for 
 optical glass. This they were able to do after some months of 
 experimentation, and the supply of potassium carbonate after that 
 time was assured. During the early months of the war it was neces- 
 sary to obtain the potassium carbonate in small, odd lots from 
 jobbers and to analyze each cask. Much of this potassium carbonate 
 contained appreciable quantities, up to 5 per cent, of potassium and 
 sodium chloride and sulphate. The presence of these salts was most 
 unwelcome in the light flint-glass batches, and at first a number of 
 melts were lost because of the fact that the glass turned milky on 
 cooling, thus rendering it useless for optical purposes. It was found 
 necessary in each lot of potassium carbonate to determine the 
 amounts of water, of sulphates, of chlorides, and of iron present in 
 addition to potassium and sodium content. Methods of analysis 
 were developed which enabled these determinations to be made 
 rapidly and accurately. The potassium carbonate furnished by the 
 Armour Co. was in crystallized form (K 2 CO 3 , 2H 2 O) and contained 
 about 18 per cent water of crystallization. The SO 3 content averaged 
 between 0.2 and 0.4 per cent, the chlorine content between 0.1 and 
 1 per cent, that of iron oxide from 0.01 to 0.02 per cent. 
 
 In view of the scarcity and high cost of pure potassium carbonate, 
 efforts were made to reduce its quantity in the batches and to sub- 
 stitute for it sodium carbonate and to use a larger quantity of 
 potassium nitrate which was readily obtainable in a relatively pure 
 state. These efforts were entirely successful and it was found that, 
 if in the batches, sodium oxide (as sodium carbonate) is substituted 
 in amounts equal in weight percentage to that of the potassium 
 oxide, the optical constants (refractive index, n D , and v) of the glass 
 are changed but little; the refractive index is raised slightly and the 
 
 2 J. Ind. and Eng. Chem., 9, 941, 1917. 
 
86 MANUFACTURE OF OPTICAL GLASS. 
 
 v is lowered slightly. The glass, however, tends to be somewhat 
 duller in appearance and at high temperatures is slightly less viscous 
 than the original potassium melt; for this reason greater care is 
 required in the annealing of soda-rich glasses. This substitution 
 of sodium carbonate for potassium carbonate and the use of relatively 
 larger amounts of potassium nitrate materially reduces the cost of 
 the batches. 
 
 Sodium carbonate. Experience has shown that sodium carbonate 
 sufficiently pure for all purposes can be obtained in the open market. 
 It should contain less than 0.01 per cent Fe 2 O 3 , 0.03 per cent SO 3 , 
 and 0.4 per cent H 2 O. 
 
 Calcium carbonate. Precipitated calcium carbonate appears to 
 be the best and purest source of calcium oxide. Certain marbles, 
 such as that from Rutland, Vt., are also satisfactory. The Fe 2 O 3 
 content should not exceed 0.05 per cent and preferably be less than 
 0.01 per cent. 
 
 Barium carbonate. Precipitated barium carbonate of high purity 
 and with an iron content less than 0.01 per cent is produced by several 
 firms in this country. The presence of sulphur compounds in this , 
 material is not desirable as they tend to color the glass. Some 
 chlorine may be present, but it should be less than 0.05 per cent in 
 amount. 
 
 Lead oxide. Lead oxide in the form of red lead (Pb 3 O 4 ) or of 
 litharge (Pb O) containing less than 0.02 per cent iron oxide was 
 obtained only with difficulty. Sublimed litharge was found to 
 contain the least amount of iron oxide. In flint glasses there is 
 some danger of reduction of the lead oxide to metallic lead; the 
 furnace conditions must accordingly be oxidizing and abundant 
 nitrate should be present in the batch. 
 
 Boric acid. Boric acid and borax are obtainable without difficulty 
 and in a sufficiently pure state; also zinc oxide, alumina, and 
 arsenious oxide. 
 
 Specifications for the degree of purity of the raw material for 
 optical glass are based on the observations that for glasses of high 
 transparency the amount of iron oxide Fe 2 O 3 present should not exceed 
 0.02 per cent; chlorine, 0.6 per cent, and SO 3 , 0.1 per cent; nickel oxide 
 should not be present. Part of the iron oxide in the glass is derived 
 from the batch materials and part from the solution of the walls of the 
 melting pot. A series of analyses, by J. B. Ferguson, of the iron oxide 
 in the raw batches for different types of optical glass and the finished 
 glasses made from these batches proved that in pots of good quality 
 from 15 to 30 per cent of the total iron oxide present in the glass is 
 contributed by the pot; in pots of inferior quality the percentage 
 contributed by them is of course higher. The batch should there- 
 fore contain less than 0.015 per cent iron oxide and perferably less 
 
MELTING POTS. 87 
 
 than 0.01 per cent. For many of the batch materials, such as KNO 3 , 
 K,CO 3 , NaNO 3 , Na 3 CO 3 , B(OH) 3 , ZnO, a specification of 0.01 per cent 
 Fe 2 O 3 in the oxide itself is easy to meet but in others this is not the 
 case and for these materials reasonable limits should be set which 
 the manufacturers can meet without difficulty. In no case, however, 
 should the percentage of Fe 2 O 3 exceed 0.05, otherwise the total per- 
 centage of Fe 2 O 3 in the glass may exceed 0.02 and the resulting trans- 
 parency be unsatisfactory. To determine the percentage of Fe 2 O 3 , 
 allowable in a salt, such as sodium carbonate Na 2 CO 3 , in order that 
 the Fe 2 O 3 in the oxide Na 2 O shall not exceed 0.01 per cent the ratios 
 in Table 7, page 106, are convenient; thus for Na 2 CO 3 the Fe 2 O 3 content 
 should not exceed 0.0058 per cent. 
 
 One of the chief results flowing from this need for high chemical 
 purity of raw materials was the realization by certain manufacturers 
 of what must be done in order to obtain the desired chemical purity, 
 and the establishment in their works of methods of high precision. 
 Our experience along these lines proved clearly in many instances 
 that the need for such factory control had not been realized by manu- 
 facturers. To meet the situation, these manufacturers adopted 
 scientific methods of procedure and their products were then equal 
 in every respect to the best European products. 
 
 MELTING POTS. 
 
 The art of pot making, like that of glassmaking, has been until 
 recently in the hands of a chosen few, who learned their trade in 
 childhood from their fathers and who have guarded most jealously 
 their acquired knowledge. In recent years, however, the character- 
 istics of the different clays and other materials which enter into the 
 composition of pots have been studied in ceramic laboratories and 
 there is now available a considerable amount of information regarding 
 the clays of this country and their thermal and chemical behavior. 
 At the time when we entered the war great difficulty was experienced 
 in obtaining pots of proper quality and a number of melts were lost 
 because the mol'ten glass dissolved its way through the pot. The 
 clays, which were then used, contained on an average about 2 per 
 cent of iron oxide with the result that, no matter how pure the batch 
 materials were, the solution of a thin film of the inner walls of the 
 pot 1 millimeter thick increased the percentage of iron in the melt 
 from 0.02 to 0.04 per cent, and the finished glass had a decidedly 
 green or yellow color. Through the efforts of several pot makers, 
 especially the Laclede-Christy Co., the Buckeye Clay Pot Co., the 
 Gill Clay Pot Co., the Willetts Clay Pot Co., and, in particular, Dr. 
 A. V. Bleininger, of the Bureau of Standards, 3 with the cooperation 
 of the United States Geological Survey, clays and kaolins relatively 
 
 3 Jour. Am. Ceram. Soc., I, 1-23, 1918. 
 
88 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 free from iron and highly resistant have been located in this 
 country and pot mixtures have been developed which approach pure 
 kaolin in composition. They contain little if any free quartz and 
 are satisfactory in nearly every respect. 
 
 The qualities desired in a glass melting pot are resistance to 
 chemical corrosion by the molten glass, freedom from iron oxide, and 
 ability to withstand exposure for a day at least to temperatures of 
 1,450 to 1,500 C without deformation even when filled with heavy 
 molten glass. A clay pot is necessarily heavy and weighs from 500 
 pounds to a ton or more; its walls range from 2 to 6 inches in thickness, 
 depending on its size and material. Extreme care is taken during 
 
 FIG. 21. Clay pots in the making by the hand "building-up" process. (Photograph by J. Harper Snapp 
 at the plant of the Bausch & Lornb Optical Co.) 
 
 the drying-out process to avoid the development of small shrinkage 
 cracks because these weaken its strength and furnish entrance 
 channels for the molten glass batch to attack its walls, thereby invit- 
 ing disaster. The same degree of care is exercised on heating a pot 
 to a high temperature; this operation requires ordinarily three to 
 five days. At all stages of this process the glassmaker seeks to 
 attain uniform heating of the entire pot and to avoid direct contact 
 of the heating flame at any one point which then would become 
 superheated and eventually cause the pot to crack. 
 
 Glass melting pots can be made up either by the old building-up 
 process (fig. 24) or they can be cast or poured into molds. The cast 
 pots have an advantage over the ordinary type of pot in that they 
 
MELTING POTS. 89 
 
 are more dense and dry out more quickly. The ordinary type of pot 
 requires from three to six months to season, whereas the cast pot of 
 the porcelain-kaolin type is ready for use within two months or less. 
 It is outside the domain of the present report to enter into a detailed 
 discussion regarding the manufacture of these pots. They can now 
 he had commercially from several different firms and there is no 
 reason to fear that in the future such pots will not be available. In 
 war time it is, of course, essential that the output be regulated 
 properly and that standard types of pots only be used. 
 
 The pots commonly employed for optical glassmaking during the 
 war period were open pots and measured 36 inches outside diameter 
 by 32 inches high (27 inches inside diameter and 27 inches deep). 
 Still smaller pots, 30 inches outside diameter and 23 inches high, 
 were used at first as a makeshift; still larger pots, 49 inches outside 
 diameter and identical in size and shape with the standard ton 
 plate-glass casting pot were successfully employed, especially by the 
 Pittsburgh Plate Glass Co. 
 
 A method for casting pots of the porcelain-kaolin type was devel- 
 oped during the war by A. V. Bleininger 3 at the Bureau of Standards 
 in Pittsburgh. In these pots broken waste bisque of white-ware 
 potteries, which is relatively free from iron, served as "grog" and the 
 composition of the pot as a whole approached kaolin with only a 
 small amount of iron oxide present; in them dense barium crown 
 glass was melted successfully. Unfortunately these pots were not 
 available in quantity during the war and were not used to any extent 
 in the production of optical glass for war purposes. -The advance 
 which this new process marks is, however, great and much credit is 
 due Dr. Bleininger for having placed it on a satisfactory basis. 
 
 Open versus closed pots. Manufacturers are not in accord regarding 
 the use of closed pots in the manufacture of optical glass. The greater 
 part of the optical glass in this country is made in open pots, averaging 
 36 inches in diameter and holding about 1,000 pounds of ordinary 
 crown glass or 1,500 pounds of dense flint glass. So far as can be 
 ascertained from the literature, European practice favors the use of 
 closed pots. The reason for this is obvious. In the open pot the 
 glass melt is exposed to the direct gases and vapors of the furnace; 
 these may be reducing in action, even though the general run of the 
 furnace has an excess of oxygen. This is especially true on windy 
 days when the air supply is more or less irregular. On such days 
 gusts of smoky gas full of soot can be seen sweeping over the pot. 
 There is consequently danger in open pots of reduction phenomena 
 which may seriously affect the quality of the product. A still further 
 objection to the use of open pots is the fact that there plays across the 
 top of the pot an incessant current of gas; this gas consists of several 
 
 3 . Jour. Am. Ceram. Soc. f I, 1-23, 1918, 
 
90 MANUFACTURE OF OPTICAL GLASS. 
 
 kinds of vapors which are more or less soluble in the glass melt; 
 moreover, a current of gas passing over the exposed surface of the 
 melt blows away the vapors of alkalies or lead oxide and this increases 
 their rate of volatilization: on turning off the gas to allow the pot to 
 cool down, the vapors over the pot are entirely changed; certain 
 absorbed gases may then tend to escape and thus give rise to bubbles 
 which are difficult to eliminate at this stage of the process. Further- 
 more the uncovered pot is open to a larger space of the furnace, and 
 vapors, absorbed or indigenous, on escaping, may tend only slowly to 
 reach a vapor pressure comparable to their saturation pressure for 
 the given temperature and compositions. 
 
 The covered pot is protected from the reducing influences of heating 
 gases; there is no circulation of gases over the surface of the melt 
 which tend to sweep the components of the melt away and to accele- 
 rate their rate of evaporation; there is no abrupt change in vapor 
 pressure and in composition of the vapors above the surface of the 
 melt on turning off the gas, or on removing the pot from the furnace. 
 The covered pot tends toward uniformity in the cooling of the melt as 
 well as toward keeping the melt homogeneous; the tendency of the 
 uncovered pot is in the opposite direction. 
 
 The uncovered pot on the other hand is heated in furnaces of the 
 regenerative and recuperative type, chiefly by radiation from the 
 crown of the furnace, and as a result heats up more rapidly and more 
 efficiently than the covered pot in which the heat has to penetrate 
 through the walls of the pot in order to reach the melt. This rapid 
 rate of heating is not to be underestimated, because by it the time 
 required to melt and to finish the glass is much less than that required 
 when covered pots are used. The glass-melting process even in open 
 pots of ton size in the plate-glass industry takes less than 24 hours. 
 In the manufacture of optical glass the complete melting process may 
 also be finished within 24 hours, thus allowing a melt to be finished 
 in the furnace in one day. With covered pots, such a time-schedule 
 of furnace operations is not possible. 
 
 During the war open pots or semicovered pots were used in this 
 country even for the production of extra dense flint and dense barium 
 crown glasses. The results prove that the open pot is satisfactory 
 for the melting of any of the ordinary types of optical glass, such as 
 are required in military optical instruments. 
 
 The bleaching of pots. During the early months of the war the avail- 
 able pots were of poor quality and relatively soluble in the optical 
 glass melts; they contained several per cent of iron oxide; some of this 
 iron was dissolved by, and entered into the rnelt, coloring the glass 
 green and lowering its transparency; the absorption of the glasses 
 ranged from 2 to 5 per cent per centimeter as compared with 1 per cent 
 or less of the European glasses. It was realized that if the iron oxide 
 
BLEACHING OF POTS. 91 
 
 could be removed from the walls of the clay pot in contact with 
 the melt, other conditions remaining the same, one source of serious 
 trouble would be eliminated. An extended series of experiments was 
 accordingly made by the Geophysical Laboratory on the action of chlo- 
 rine gas on the walls of an empty pot at high temperature; this 
 method was based on the known fact that at high temperatures iron 
 chloride is very volatile. Similar methods had moreover been used 
 before for removing iron from enamels 4 and also from small crucibles 5 
 and from other materials. 
 
 To convert the iron contained in the walls of the clay pot into vola- 
 tile iron chloride both chlorine and phosgene gas were tried; phosgene 
 gas is chemically much more active than chlorine gas, especially at 
 its dissociation temperature (slightly below 600 C.). The pot to be 
 bleached was placed in a pot arch or melting furnace and in most in- 
 stances was covered with a clay lid; a continuous stream of chlorine 
 gas was then allowed to flow into the crucible for a given period of time 
 (one-half hour to five hours) at temperatures ranging from 400 to 
 1 ,250 C. Much iron (up to 80 per cent) was removed by this treatment 
 and the walls of the crucible were bleached to a depth of 15 millimeters ; 
 the rate and effectiveness of the reaction increased with the tempera- 
 ture; crystals of iron oxide (hematite) were deposited in great num- 
 bers along the outer edge of the pot where the escaping iron chloride 
 gas entered the furnace chamber. 
 
 In spite of the marked decrease in iron content of the pot walls, a 
 corresponding decrease in the iron content of the glass melted in the 
 treated pot was not observed and, in one instance at least, the glass 
 was full of pot stones. The chlorine-gas treatment, by dissolving the 
 iron oxide which formed part of the bond of the clay pot, had evi- 
 dently rendered the walls of the pot more porous and less resistant 
 to attack. It is possible that subsequent baking of the pots after the 
 chlorination treatment would restore the compact structure of the 
 walls as a result of sintering and incipient melting. This situation 
 and the rapid improvement in the quality of the pots at this time 
 stopped further experimentation along these lines. It is better to use 
 pots having a low initial iron content, to bake them thoroughly 
 at very high temperatures, and to shorten the melting period than to 
 attempt to purify the clay in the finished pots and thus to run the risk 
 of spoiling an entire melt because of stones derived from the loosened 
 pot wall texture. 6 
 
 * Bole and Howe, Trans. Am. Ceram. Soc. 17, 125 (1915). 
 
 * G. C. Stone, cited in Jour. Am. Ceram. Soc. 2, 360 (1919). 
 
 e A detailed description of the experiments on the use of chlorine gas for the volatilization of iron from 
 optical glass pots is given by Hostetter, Roberts, and Ferguson in Jour. Am. Ceram. Soc. 2, 356-372, (1919). 
 
92 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 FURNACES. 
 
 For the manufacture of optical glass three different types of melt in j. 
 furnace are in common use; of these two are of the reverberatory type 
 and operate either on the regenerative (fig. 25) or recuperative prin- 
 ciple, the air in them being preheated by the hot products of gas com- 
 bustion escaping from the furnace chamber. The third type is a non- 
 regenerative, stackless furnace, heated by a blast; in it the air-gas 
 mixture is injected into the furnace under pressure. In the regenera- 
 tive and recuperative furnaces the heating of the crucible is accom- 
 plished chiefly by radiation from the crown of the furnace. This in- 
 
 FIG. 25. Batch about to be filled by means of long "scoop : ' into glass-melting pot inside the furnace. 
 (Photograph by J. Harper Snapp at the Bausch & Lomb Optical Co.) 
 
 sures a more even distribution of heat than in any type of furnace in 
 which the combustion of the gases takes place within the furnace 
 itself as in the blast furnace. Experience has shown, however, that 
 good optical glass can be produced in any one of these types of fur- 
 naces, provided they are properly regulated, both with respect to tem- 
 perature control and to gas combustion. Whatever type of furnace 
 is available, two factors are fundamental to the production of good 
 optical glass, namely, maintenance of a definite temperature over a 
 long period of time, and constancy of temperature distribution within 
 the furnace chamber so that the melting pot is uniformly heated. 
 
 Because of the careful regulation of furnace conditions required 
 in optical glass making, it is common practice to use single-pot melt- 
 
TYPES OF FURNACES. 93 
 
 1 2; furnaces of dimensions adapted to fit the size of pot used. Dur- 
 
 i the war pots 36 inches outside diameter were commonly used and 
 
 generative furnaces with a chamber measuring 4 by 5 by 5 feet, 
 
 side dimensions, were found to be satisfactory. (Fig. 25.) In these 
 
 iniaces a heavy door (tuille), suspended by chains and capable of 
 
 mg raised by a hand crank, closes the front of the furnace. The 
 .n'le in turn has a smaller opening through which the batch is intro- 
 
 -I'd and later the stirring rod enters. Ordinary fire clay serves to 
 ia>tor up the cracks between the tuille and the brickwork of the 
 
 < mice. At one of the plants (Bausch & Lomb) several two-pot 
 'MM i aces of the regenerative type were erected and proved to be 
 
 i.vessful; these furnaces were equipped with tuilles at each end. 
 
 operating a two-pot furnace special care must be taken to insure 
 iiuformity of furnace conditions. Batches of the same composition 
 
 .d pots of the same size must of course be used in any given run in 
 
 two-pot furnace. On the whole there is no special advantage 
 irained by the two-pot furnace except that it occupies less space than 
 the two single-pot furnaces. 
 
 It is not germane to the present report to discuss the details of 
 construction of optical glass making furnaces. It is assumed that 
 the builder of such a furnace has considered the type of furnace, the 
 materials for its construction, the best design and size of chamber, the 
 kind of fuel available (natural or artificial gas) , the size of stack, and 
 the best arrangements for the combustion of the gas. Suffice it to 
 state that for ease of regulation and general efficiency the regenerative 
 and recuperative types of furnace are superior to the other types. 
 The single-pot regenerative type is considered to be the best. 
 
 It is the task of the furnace man, assigned to a furnace of given 
 type, so to run it that the desired temperatures are attained and main- 
 tained and that within the furnace chamber an oxidizing atmosphere 
 is always present, especially in the vicinity of the pot of molten glass. 
 Low, lazy, sooty flames sweeping over the surface of the molten 
 metal are to be avoided, as they indicate a reducing atmosphere. 
 On windy, gusty days such flames are almost inevitable, but the fur- 
 nace operator should be on the alert to reduce their occurrence to a 
 minimum. 
 
 In the regenerative type of furnace the flow of gas should be 
 reversed at definite intervals in order to insure uniform temperatures; 
 if, through carelessness, the furnace gets out of balance and one side 
 is colder than the other, fluctuations of 50 C. with each reversal of 
 the flow of gas within the furnace may arise. The balance is restored 
 by allowing the cooler checker work of the furnace to be heated for a 
 longer period of time than that of the hotter side. 
 
 Measurement of furnace temperatures. Different methods are in 
 use for ascertaining furnace temperatures. Formerly this was done 
 3922921 7 
 
94 MANUFACTURE OF OPTICAL GLASS. 
 
 by the furnace operator who, from long practice and often with no 
 protection for his eyes, estimated with remarkable accuracy the fur- 
 nace temperature by direct inspection. To the untrained observer 
 the furnace at the melting temperatures is at " white heat/' and 
 no details are distinguishable within it; however, with the aid of 
 dark glasses to reduce the intensity of the light and heat radiations, 
 he can readily see the details, but can not estimate the temperatures 
 satisfactorily. The human eye is not infallible and even the expert 
 furnace man may make costly mistakes which would not have 
 occurred had better methods for temperature measurement been used. 
 
 In the ceramic industry Seger cones are employed, while in the 
 glassmaking industry thermoelements are in general use and are 
 placed in the rear wall of the furnace where they indicate continu- 
 ously the temperature of the hot junction of the thermocouple; the 
 cold junction of the thermocouple is kept at constant temperature 
 either by burying it from 6 to 10 feet in the ground at some distance 
 from the furnace or by immersing it in a pail of water attached to 
 the rear wall of the furnace where it is held at the constant tempera- 
 ture of boiling water. The thermoelement in this position does not 
 indicate the actual furnace temperature, but the temperature of the 
 point within the furnace wall to which it extends. As a result, there 
 is an appreciable lag in its readings of the fluctuations of tempera- 
 ture within the furnace. The exposure, moreover, to the furnace 
 gases which penetrate into the retaining tube causes the thermo- 
 element to deteriorate and to give readings which are too low and 
 inclined to be somewhat erratic. More satisfactory are pyrometers, 
 optical and electrical, which enable the observer to ascertain the 
 actual temperature of any point within the furnace. 
 
 A portable standard thermoelement of platinum and platinum 
 rhodium with compensating leads can be used if properly shielded 
 from the furnace gases. A device of this sort for purposes of inves- 
 tigation was devised by the writer (Report No. 4 for week ending 
 May 26, 1917) and was constructed by leading the thermoelement 
 wires through a water-cooled iron pipe 10 feet in length to a porce- 
 lain tube 18 inches long, closed at one end, and attached to the end 
 of the pipe; the porcelain tube is made of highly refractory material, 
 and is about 1 centimeter in outside diameter. (Fig. 26.) 7 The 
 cold junction of the thermoelement is held in ice and the electro- 
 motive force of the couple is read off on a direct-reading millivolt- 
 meter. By means of this water-cooled thermoelement rod, tempera- 
 tures at different points within the melting furnace and pot arches 
 were ascertained and compared with the temperatures measured at 
 the same tune and on the same points by the Morse optical pyrometer 
 
 * Reproduced from article on optical pyrometers by C. N. Fenner in Bull. Am. Inst. Min. and Met. Eng., 
 1006, 1919. 
 
FURNACE TEMPERATURES. 
 
 95 
 
 (Leeds and Northrup type) and the Fery radiation pyrometer (Taylor 
 Instrument Co., type). To illustrate the distribution of temperature 
 in one of the single-pot furnaces, the following readings were taken 
 along the line extending from a small opening in the furnace tuille 
 (door) to the rear wall of the furnace and over the crucible about 5 
 inches above the surface of the metal. 
 
 FIG. 26. Sectional view of end of water-cooled temperature-measuring device. A isthehot junction of 
 platinum-rhodium thermoelement; B, a refractory porcelain tube; C, inner iron pipe; D, asbestos packing; 
 .Band F, iron pipes of water-cooling system; G, specially turned iron terminus of water-cooling system. 
 
 Distance from out- 
 side of door. 
 
 Temperature. 
 
 Feet. 
 0.5 . 
 
 C. 
 
 ,222 
 ,273 
 ,287 
 ,365 
 I373 
 ,386 
 ,396 
 ,403 
 ,403 
 ,403 
 1,398 
 1,395 
 
 o p 
 
 2,232 
 2,323 
 2,349 
 2,489 
 2,503 
 2,527 
 2,545 
 2,557 
 2,557 
 2,557 
 2,548 
 2,543 
 
 1 
 
 1.5 
 
 2.0 
 
 2 5 
 
 3.0 . 
 
 35 
 
 4.0 
 
 4.5 
 
 5.0 
 
 5.5 
 
 6.0 ,.. 
 
 
 These readings show that the temperature distribution above the 
 pot itself is fairly uniform; near the furnace door there is of course an 
 appreciable drop in temperature. 8 (Fig. 27.) 
 
 The thermoelement rod is satisfactory for exploratory purposes, 
 but the porcelain tube at the end is too fragile for factory adaptation. 
 
 The optical pyrometer, on the other hand, is a satisfactory works 
 instrument. It is essentially a low-power telescope, in the image 
 plane of which a heated metal filament (small electric lamp) is viewed , 
 together with the object whose temperature is to be ascertained. 
 (Fig. 28.) Both image and filament are viewed through a red- 
 colored glass, and the intensity of illumination of the lamp filament 
 
 8 These measurements were taken by Dr. C. N. Fenner and the writer and are cited in the article on 
 "The use of optical pyrometers for control of optical glass furnaces, by C. N. Fenner, Bull. Am. Inst. 
 Min. and Met. Eng., 1006, 1919. 
 
96 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 is changed until it practically disappears against the illuminated field. 
 Since the intensity of luminous radiation varies with the fourth power 
 
 I350 C 
 
 1300* 
 
 IZ50' 
 
 )200 ( 
 
 t 
 
 I 2 3 4- 5 <o 7 
 
 FEET - 
 
 FIG. 27. Cross section of melting furnace, showiu.; points at which temperature measurements were 
 made with the water-cooled rod of fig. 26. Below is the corresponding distance-temperature curve. 
 
 of the temperature (Stefan's law), a slight change in temperature 
 suffices to produce a great change in the brightness of the furnace. 
 The instrument is remarkably sensitive, and with it concordant 
 
FURNACE TEMPERATURES. 
 
 97 
 
 results are easily obtained by different observers, provided certain 
 precautions are taken. 
 
 The instrument should be calibrated at stated intervals, because 
 with use the characteristics of its electric lamp change and with them 
 the temperature-indicating scale of the instrument. The calibration 
 is readily accomplished by means of a standard thermoelement to read 
 the temperature of the stoppered end of a porcelain or clay tube 
 heated to definite temperatures either in the glass-melting furnace or 
 in a laboratory electric-resistance furnace; a tube of this sort approxi- 
 
 FIG. 28. Photograph of operator measuring the temperature of optical glass-melting furnace with 
 an optical pyrometer. The operator is regulating the resistance in order to match the intensities 
 of lamp filament and furnace illumination. (Photograph by J. Harper Snapp at the Hamburg 
 plant of the Spencer Lens Co.) 
 
 mates a " black body" in its characteristics and serves the purpose 
 well. The heated inside of this tube is imaged by the telescope and 
 the current reading of the lamp for this temperature is ascertained. 
 A series of such readings for different temperatures suffices for the 
 calibration. 
 
 Experience with the optical pyrometer has proved that it is well 
 adapted for furnace work. At temperatures above 1,350 C. (2,450 
 F.) its readings, after proper calibration, are commonly in error less 
 than 5 C. At temperatures from 950 to 1,050 C. (1,750 to 
 1,900 F.) there is a great difference in temperature between the 
 
98 MANUFACTURE OF OPTICAL GLASS. 
 
 arch of the furnace, which is in direct contact with the burning gases, 
 and the furnace walls which are heated by radiation; at these tem- 
 peratures the readings of the optical pyrometer are invariably too 
 high and average about 35 C. above the correct value. The walls 
 reflect the light from the hotter crown of the furnace and conse- 
 quently appear brighter and hotter than they actually are; the tem- 
 perature of the rear wall of the furnace as determined by the optical 
 pyrometer is moreover not so high by 10 or 20 C. as the side walls 
 which have a better chance to reflect the light. These differences in 
 apparent temperature between the rear and side walls of the furnace 
 at 1,000 C. are readily detected by the unaided eye; on looking into 
 the corner of such a furnace near the end of the stirring period of 
 the glass the observer notices a distinct difference in the brightness 
 of the side and rear walls at their junction. At these temperatures 
 care must be taken by the observer to sight upon the same spot in 
 the furnace each time in order to attain uniformity in the readings 
 from melt to melt. Experience has shown that the most satisfactory 
 results are attained at this stage of the glassmaking process, which 
 immediately precedes the removal of the pot from the furnace, by 
 sighting on the molten glass adjacent to the moving stirring rod. 
 The glass at this point is hottest, and its surface is uneven and 
 reflects light from all parts of the furnace. 
 
 In the radiation pyrometer the focal length of the gold-plated 
 reflecting mirror is so short that at the focus not only is the object 
 sighted upon imaged, but in the near vicinity (fraction of a millimeter) 
 points several feet away are also imaged with the result that the 
 radiation pyrometer tends to integrate the temperatures of points 
 along the line of sight at some distance in front of and behind the 
 actual point sighted upon. If the furnace approaches a black body 
 in its temperature distribution, this is not a disadvantage; but where 
 this is not the case difficulties arise which are apparently less easy to 
 allow for than in the optical pyrometer. 
 
 It is convenient in all cases to use thermoelement pyrometers in 
 conjunction with the optical pyrometer because they aid the furnace 
 operator in following his schedule and serve as a check on the readings 
 of the optical pyrometer. 9 
 
 Furnace men after a certain amount of practice learn to estimate 
 with some precision the temperatures within the furnace; but in all 
 cases a careful record should be kept from hour to hour of the meas- 
 ured temperatures within each melting furnace in order that large 
 fluctuations do not occur and seriously affect the quality of the 
 glass. 
 
 9 A detailed discussion of the methods found useful during the war for the measurement of the tempera- 
 tures of optical-glass furnaces is given by C. N. Fenner, in Bulletin Am. Inst. Min. & Met. Eng., No. 151, 
 pp. 1001-1011 (1919). 
 
BATCH COMPOSITIONS. 99 
 
 THE BATCHES. 
 
 The glassmaking industry has been one of slow development through 
 the centuries from the time of the Egyptians and Phoenicians to 
 the present day. It has been cultivated in certain restricted areas 
 and has been handed down as an art from father to son, with the 
 result that even at present it is enveloped in an air of mystery and 
 secrecy. The formulas for making the different types of glass are 
 held highly confidential and may not be divulged except to a chosen 
 few. As a result the glassmaking industry is conducted with few 
 exceptions by rule-of-thumb methods and the manufacturing proc- 
 esses are controlled by men of special experience who occupy unique 
 positions in the factory. In Europe the optical-glass industry is no 
 exception to this rule; all details of actual manufacture are closely 
 guarded; but little has been published on the subject, and it is 
 difficult to obtain reliable information of any kind. The batches for 
 the several types of optical glass required by optical-glass manufac- 
 turers are not available, except for a few of the older types of optical 
 glass. 
 
 In undertaking the manufacture of optical glass in this country it- 
 was necessary therefore to gather together such chemical informa- 
 tion as could be found and to have analyses made of the more 
 urgently needed types of optical glas^ which happened to be in stock. 
 A statistical study of the available analyses was first made. This 
 study, the results of which are given in Chapter II, was supplemented 
 by experiments on the changes, during the melting process, in the 
 relative quantities of the several chemical elements in the batch as a 
 result of volatilization and of solution of the pot. Certain relations, 
 particularly in the series of ordinary flint glasses and of the barium 
 crowns and flints, were discovered which enabled us, by interpolation, 
 to write down at once the batch for any member of their series such 
 that the finished glass should have approximately the desired refrac- 
 tive indices and ^-values. Experimental batches were first made up 
 in small quantities and melted down in pots containing 10 to 50 
 pounds. In this way we soon arrived at definite control over the 
 entire series of desired optical glasses and were in a position to repro.- 
 duce any one of the silicate glasses which was needed. 
 
 As a result of this statistical study, carried on under definite sci- 
 entific principles, we became independent of all secret batches and 
 were able to proceed to a scientific study of the relations between 
 chemical composition and optical constants of the resulting glass and 
 to determine in a general way the effect of any individual chemical 
 element on the optical constants. This proved to be of great value, 
 as it allowed us to devote our entire attention to the other factors on 
 which the quality of optical glass depends. 
 
100 MANUFACTURE OF OPTICAL GLASS. 
 
 Computation of batches from chemical analyses of glasses.--ln pre- 
 paring a batch to reproduce a glass of given optical constants it is 
 necessary for the operator to take into consideration the losses 
 incurred by selective volatilization of the batch components. The 
 amounts lost during the melting and fining processes depend on the 
 size and character of the batch, the character and temperatures of 
 the furnace, the method of filling in the batch, the type of the melting 
 pot, whether open, semiclosed, or closed, the duration of the opera- 
 tions, and the character of the stirring, so that it is not possible to 
 give more than rough percentage estimates of these factors. A few 
 experiments suffice, however, to determine the relations for a given 
 set of operating conditions; once these have been ascertained the 
 glassmaker endeavors to adhere to them strictly and thus to insure 
 uniformity in the final product. In the computation of the batch 
 mixtures the following allowances are made for losses by selective 
 volatilization: PbO or Pb 3 O 4 , 0.5 to 5 per cent and even higher; 10 
 boric oxide, 1 to 5 per cent, in case boric acid rather than borax 
 is used in the batch; the alkalies, K 2 O and Na 2 O, up to 5 per 
 cent. Little definite information is available on the losses incurred 
 by selective volatilization and in a given case much depends on 
 the factors already mentioned. At the present time it is a matter 
 largely of experience and of actual trial to make proper allow- 
 ance for these factors. As a result of this volatilization the batch 
 becomes relatively richer in silica and the refractive index of the 
 finished glass is lowered. In the case of extra dense flint or dense 
 barium crown melts volatilization becomes serious when melts are 
 made in open pots. In this case the greatest care must be taken 
 to keep the furnace conditions similar from melt to melt, other- 
 wise large fluctuations in refractive index occur from pot to pot 
 and these work a hardship on the manufacturer of lenses for 
 which these glasses are used. If there is an appreciable solution, of 
 the walls of the pot, the alumina and silica thus dissolved tend to 
 lower the refractivity of the glass. An effort should be made in all 
 cases to reduce this uncertainty by employing melting pots which 
 are both thermally and chemically resistant. 
 
 From the relations presented in foregoing paragraphs it is possible 
 to prepare a plot from which the batch composition for any member of 
 the flint series can be read off directly. (Fig. 29.) This plot is con- 
 structed on the basis of sand 100 units of weight (pounds or kilo- 
 grams) . Thus from the diagram the batch composition in kilograms 
 for the flint glass of refractive index n D = 1.640 we read: Sand, 100; 
 lead oxide (PbO), 119, or (Pb 3 O 4 ), 122; anhydrous potassium car- 
 bonate (K 2 CO 3 ), 20.6; potassium nitrate (KNO 3 ), 7. Sodium oxide 
 
 w See O. Andersen, The volatization of lead oxide from lead silicate melts. Jour. Am. Ceram. Soc., 
 2, 784, 1919. 
 
BATCH COMPOSITION^ ' 
 
 101 
 
 in the form of sodium carbonate or sodium nitrate may be substi- 
 tuted for potassium oxide provided the amount of Na 2 O (weight 
 percentage) equals in weight the amount of K 2 O which it replaces; 
 substitution of Na 2 O for K 2 O raises the refractive index slightly, 
 decreases the v slightly and changes the viscosity relations noticeably. 
 Flint glasses high in soda exhibit a tendency to be duller, less trans- 
 parent, and more noticeably colored than the corresponding pure 
 potash flints. The density and v of any glass of given PbO or Pb 3 O 4 
 content are indicated on the plot (fig. 29) by the intersections of the 
 PbO or Pb 3 O 4 ordinate with the curves for density and v respectively; 
 thus in the foregoing example the density of the flint glass n n = 1.640 
 
 \ 
 
 \ 
 
 FIG. 29. Batch-composition diagram for the series of ordinary flint glasses. The diagram is prepared on 
 the basis of sand 100. From the diagram the approximate batch composition for any flint glass of given 
 refractive index, n D , or v- value, or specific gravity, d, can be read off directly. 
 
 is d = 3.78 and its v = 34.5. The batch composition for a flint glass of 
 ? = 46.0 is, in kilograms: Sand, 100; PbO, 44.5, or Pb 3 O 4 , 45.5; 
 K 2 CO 3 , 7.6; KNO 3 , 2.2. Its density is 2.93 and its n D = 1.549. 
 
 The compositions of glasses of other types can be ascertained by 
 means of the compositions given in Table 4 (p. 59) together with the data 
 plotted especially on figures 15 and 22, Chapter II. It would lead 
 too far to consider these systems in detail; in some of them the infor- 
 mation at hand is meager and hardly sufficient for satisfactory inter- 
 polation; but in most instances the glassmaker, with the aid of these 
 methods and the data now available, is able to write down batches 
 which approach very closely the batch desired, so that with a few 
 
102 MANUFACTURE OP OPTICAL GLASS. 
 
 experimental melts in small crucibles he can determine the correct 
 batch composition within very narrow limits. 
 
 The foregoing diagrams indicate clearly the fields which are domi- 
 nated by the different oxides; in view of these relations it is, there- 
 fore, fitting to name the silicate glasses as had been done: Fluor 
 crowns, borosilicate crowns, ordinary crowns, and barium crowns; 
 flints, barium flints, and borosilicate flints. Zinc-bearing glasses 
 do not require separate designation, because zinc oxide in optical 
 glasses does not impart special optical properties to the glass; it 
 serves chiefly to change the concentration and viscosity relations 
 in optical glass melts, especially to render the melt easier to work 
 and to decrease the tendency to crystallize on cooling. In the flint 
 series the light, medium, and dense flints are distinguished. The 
 barium flints are flints in which part of the lead is replaced by barium. 
 In all types of silicate glasses the amount of silica present is rela- 
 tively large and the optical characteristics of each glass are in effect 
 a blend of characteristics between those of silica and some other end 
 member or members with silica commonly dominating. 
 
 A study of the above diagrams, especially figure 15, page 50, 
 proves that in the silicate optical glasses, lead and barium oxides 
 on the one hand, silica and boron oxide on the other, exert the most 
 profound influence on the optical constants of the glass. Thus the 
 highly refracting glasses contain abundant lead oxide or barium 
 oxide; the low refracting glasses contain abundant silica or boron 
 oxide. Of all the glassmaking elements, lead has the most pro- 
 nounced effect on both the refractive index and the dispersion; it 
 increases especially the blue end of the spectrum relatively to the 
 red. 
 
 If in a flint glass of given refractivity, a high p-value (lower dis- 
 persive power) is desired, part of the lead oxide is replaced by barium 
 oxide (introduced in the batch as barium carbonate). Compared 
 with lead oxide, barium oxide produces less high refractive index and 
 very much weaker dispersion (high y-value) ; glasses high in BaO 
 and PbO are called barium flints or baryta flints. Zinc oxide is 
 intermediate in its action between calcium oxide and lead oxide 
 on the one hand, and barium oxide on the other; like calcium oxide 
 it tends to raise the refractive index and the dispersion slightly. 
 In the series of flint glasses the relative dispersion increases with 
 increase in refractive index; in other words, if, for the sake of com- 
 parison, the refractive indices n A ', n c , n F , n G ', for each member 
 of the series are divided by the refractive index n D , then the ratios 
 increase with rise in refractive index n D ; the characteristic feature 
 of the dispersion of lead glasses is the rapid rise in refractivity toward 
 the blue and violet end of the spectrum. The same relations are 
 
BATCH COMPOSITIONS. 103 
 
 T 1-1 j_- n D n f t na' n-v 
 
 clearly shown in a diagram in which the ratios - - and 
 
 HF-UD n -n D 
 
 are plotted as abscissae against the refractive indices as ordinates. 
 
 Increase in silica, boron oxide, or fluorine tends, on the other 
 hand, to lower the refractive index, to increase relatively the dis- 
 persion of the red end of the spectrum and at the same time to de- 
 crease the total dispersion. This lengthening of the red end -of the 
 spectrum is especially true of boron oxide and apparently also of 
 fluorine 11 in the fluor crown glasses. The addition or substitution 
 of small amounts of boron oxide raises the refractive index of certain 
 glasses slightly. This behavior is remarkable because by itself 
 the refractive index of boric oxide glass is only n D = 1.463 and 
 *> = 59.4; that of silica glass is n D = 1.4585 and ? = 67.9. Evidently 
 borates are formed which impart different properties to the glass 
 from those which might be inferred from the characteristics of the 
 individual components. 12 Similarly alumina (A1 2 O 3 ) which, in the 
 crystallized state has a very high refractive index (1.76), produces 
 with silica a glass of unexpectedly low refractivity. Alumina and 
 magnesia raise the viscosity of most glass melts and tend thereby 
 to prevent crystallization. 
 
 Phosphorus, although formerly used in appreciable amounts, 
 especially in the series of phosphate glasses, has now been discarded 
 because of the poor weather-resistant qualities of the phosphate 
 glasses. Figure 15 shows that these glasses differ only slightly 
 from the borosilicates and the barium crowns; this difference does 
 not outweigh the practical disadvantage of weathering instability. 
 
 The essential differences between the ordinary flints and ordinary 
 crowns are the higher refractivity and the greater dispersion, both 
 actual and relative, especially in the blue end of the spectrum, in 
 the flint glasses. With these two types of glasses it is not possible 
 therefore to compensate exactly the dispersive effects of a positive 
 crown element by a negative flint element and secondary spectrum 
 results. By the use of glasses in which the relative dispersions are 
 more nearly similar than between the crowns and flints, it is possible 
 to correct more perfectly for achromatism; the presence of the two 
 chemical elements, barium and boron, in optical glass shifts the 
 relative dispersions in the crowns and flints so that they are more 
 nearly in accord. By the use of these elements in the flint glasses the 
 extreme dispersions of the blue end of the spectrum in the normal 
 flint glasses are reduced relatively and the course of the dispersion 
 throughout the visible spectrum is rendered more nearly like that of a 
 crown glass. 
 
 The statement made by Hovestadt, Jenaer Glas, p. 11, 1900, that boron oxide tends to lengthen the 
 red end of the spectrum, whereas fluorine has the opposite effect and tends to decrease the red end relatively 
 to the blue is apparently not borne out by the fluor crown glasses, which, however, contain abundant 
 boric oxide and this may veil the effect of the fluorine. 
 
 14 See also Zschimmer, Zeitschr. Elektrochemie, 11, 632, 1905. 
 
104 MANUFACTURE OF OPTICAL GLASS. 
 
 In writing down the batch composition for a glass of specified 
 refractive index and dispersion the glassmaker has a number of 
 factors to consider, such as chemical composition and the changes 
 in composition resulting from selective volatilization and from pot 
 solution. In the series of ordinary crowns and flints, silica (sand), 
 alkalies (potassium and sodium oxides), lime (calcium oxide), and 
 lead oxide are the essential constituents; in these the proportions of 
 the different elements may not exceed certain limits. If the per- 
 centage of silica is above 75 per cent the melt is so viscous that it 
 can not be properly melted in the furnace; the percentage of alkalies 
 may not exceed 20 per cent, otherwise the resultant glass is hygro- 
 scopic and chemically unstable; over 13 per cent of lime may not 
 be used because of the tendency of melts high in lime to crystallize, 
 and because of the difficulty of fusing such melts properly; lead 
 oxide may be used up to 70 per cent or more, but in glasses containing 
 a large percentage of lead the danger from crystallization and from 
 .attack on the melting pot is serious. In the new series of glasses, 
 boron, barium, zinc, and aluminium oxides are the most important 
 additional constituents which are employed; up to 50 per cent barium 
 oxide may be used, but then ordinarily together with boron and 
 alumina; melts high in barium attack the crucible seriously, espe- 
 cially if any free silica is present in the clay. The dense barium 
 glass melts require, moreover, special furnace treatment to produce a 
 glass free from bubbles and other defects such as crystallization 
 nuclei. 13 Boron oxide may be used up to 20 per cent or more; it 
 replaces in a measure silica. In general the use of small quantities 
 of boric acid in lead glasses is not to be recommended, because 
 experience has shown that its presence favors the development of 
 opalescence in the glass on cooling. Zinc in quantities above 12 per 
 cent is likely to cause crystallization of the glass. More than 5 
 per cent of alumina tends to render most glass melts exceedingly 
 viscous and practically unworkable at the melting temperatures; 
 in most glasses the presence of alumina decreases the danger from 
 crystallization and renders the glass tough and resistant. In the 
 densest barium crown glasses up to 10 per cent A1 2 O 3 may be used; 
 it aids not only in preventing crystallization of barium disilicate, but 
 it also improves the working qualities of the melt and glass. The 
 presence of chlorine or sulphur in the batch materials is to be avoided 
 because of the danger of opalescence in the finished glass. In 
 case these elements are present, it is advisable to run the melt at a 
 very high temperature. 14 
 
 " Compare N. L. Bovven. Jour. Wash. Acad. Sci., 8, 26.5-268, 1918; Jour. Am. Ceram. Soc., 2, 261-281, 
 1919. 
 
 " See J. D. Camvood and W. E. S. Turner. Jour. Soc. Glass Techn., 1, 187, 1917; C. N. Fenner and J. B. 
 Ferguson, Jour. Am. Ceram. Soc., 1, 468, 1918: C. N. Fenner, Jour. Am. Ceram. Soc., 2, 106, 1919. 
 
BATCH COMPOSITIONS. 105 
 
 The presence of arsenic in most glasses is favored by many glass- 
 makers because it tends to increase the transparency and brilliancy 
 of the glasses; certain experiments indicate that the presence of 
 arsenic in the melt probably sets up an oxidizing action at high 
 temperatures 15 and thus reduces the effect of iron as a coloring 
 agent. Although some arsenic is volatilized in the melt yet an 
 appreciable amount remains in the solution. Careful analytical 
 work by Allen and Zies 16 of the Geophysical Laboratory has demon- 
 strated the presence in optical glasses of arsenic in both states of 
 oxidation, as arsenic trioxide and arsenic pentoxide. They consider 
 that the chief function of arsenic may be to cause a "boil" and thus 
 to sweep out small entrapped bubbles which otherwise rise with 
 extreme slowness to the top; they found that in green plate glass 
 in which no nitrate is used much more arsenic escapes from the melt 
 than from a melt such as spectacle crown glass containing niter. 
 R. L. Frink 17 observed crystals of arsenious oxide in bubbles in 
 glass, thus proving that at the high temperatures this oxide is vol- 
 atrilized from the melt. Allen and Zies conclude "that arsenic 
 trioxide is oxidized at low temperature and the product formed 
 is stable enough to remain until a high temperature is reached and 
 the glass becomes fluid, when it slowly dissociates into oxygen and 
 arsenic trioxide. both of which aid in the fining." 
 
 Fenner 18 adds the suggestion that the large bubbles of arsenic 
 vapors may in addition collect potential bubbles by functioning as 
 vacuum chambers into which volatile substances may evaporate at a 
 rapid rate. Be the effect what it may the general practice is to add a 
 little arsenic to the batch and the results attained by its use warrant 
 its continuance as a component of optical glass batches. 
 
 In the preparation of batches the nitrates and carbonates of the 
 alkalies are used in proportions ranging from 1 :5 to 1 :2, depending on 
 the type of glass; nitrates alone produce too active a melt, while car- 
 bonates alone do not furnish the desired oxidizing agents. Melts high 
 in alkalies and made from batches containing alkali carbonates, but 
 no nitrates, are difficult to fine properly. The chief function of alka- 
 lies in optical glass is to produce melts which are easily workable; they 
 influence the viscosity of the melt; the viscosity of a potassium flint 
 glass melt changes very slowly with the temperature; that of a so- 
 dium flint glass changes fairly abruptly at a temperature somewhat 
 above the softening point. Increase in the total alkali content of a 
 glass commonly raises its refractive index slightly; thus in the me- 
 dium flint glasses an increase in alkalies with corresponding decrease 
 
 fc Sec Doelter, Handbueh der Mineralchemie (Leipzig), 1, 861, 1912; also E. T. Allen and E. G. Zies, 
 Jour. Am. Ceram. Soc., 1, 787, 1918. 
 Jour. Am. Ceram. Soc., 1/767-794, 1918. 
 u Trans. Am. Ceramic Soc., 17,798, 1915 
 !8 Jour. Am. Ceramic Soc., 2, 123-124, 1919. 
 
106 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 in silica, the percentage of lead oxide remaining the same, raises the 
 refractive index. Both alumina and magnesia may exert a pro- 
 found effect on the viscosity of the melt. In the computation of 
 batches from chemical analysis the data of Table 7 are useful. 
 
 TABLE 7.- Table of molecular weights and ratios between molecular weights for use in the 
 computation of glass batches from chemical analyses. 
 
 MOLECULAR WEIGHTS. 
 
 A1 2 O 3 ; 102.2 
 
 As 2 O 3 197. 9 
 
 BaO 153.4 
 
 Bi 2 O 3 464.0 
 
 B 2 O 3 70.0 
 
 CdO 128. 4 
 
 CaO 56.1 
 
 Cr 2 O 3 152.0 
 
 CoO 75.0 
 
 CaO 56. 1 
 
 FeO... . 71.8 
 
 K 2 O 
 
 94.2 
 
 SnO 2 
 
 141 
 
 Li 2 O 
 
 29.9 
 
 TiO 2 
 
 70. 1 
 
 MgO 
 
 40.3 
 
 U 2 O 3 
 
 524 4 
 
 MnO . . . 
 
 70. 9 
 
 V 2 3 ... 
 
 .. 150.0 
 
 Na 2 O 
 
 62.0 
 
 ZnO 
 
 81.4 
 
 NiO... 
 
 .... 74. 7 
 
 CO 2 ... 
 
 44. 
 
 PbO. 
 
 223.2 
 
 N 2 O-> 
 
 108. 
 
 SeO->. . . 
 SiO 2 . . . . 
 
 111.2 
 60.3 
 
 CljOj 
 
 SO 3 .. 
 
 150.9 
 80.0 
 
 SrO... 
 
 .. 103.6 
 
 
 
 Ratio of oxide to salt 
 
 I. 
 
 II. Re- 
 ciprocal 
 of I. 
 
 Ratio of oxide to salt. 
 
 I. 
 
 II. Re- 
 ciprocal 
 of I. 
 
 BjO 3 to2B (OH) 3 ... 
 
 0.564 
 
 1.77 
 
 MgO to MgCO 3 
 
 0.478 
 
 2.09 
 
 Na 2 B4O 7 10H 2 O 
 
 .366 
 
 2.73 
 
 MnO t o MnO-2 
 
 .816 
 
 1 23 
 
 Na 2 B 4 O 7 
 
 .713 
 
 .40 
 
 Na 2 O toNa 2 CO 3 . . 
 
 .585 
 
 1.71 
 
 BaO to BaCO 3 . 
 
 .777 
 
 .29 
 
 Na 2 SO < 
 
 .437 
 
 2.29 
 
 BaSO 4 
 
 657 
 
 52 
 
 2NaNO 3 
 
 365 
 
 2 74 
 
 CaO to CaCO 3 
 
 .560 
 
 .78 
 
 Na2B 4 O 7 lOH 2 O 
 
 .162 
 
 6.11 
 
 CaF 2 
 
 .718 
 
 .39 
 
 Na 2 B 4 O 7 
 
 .307 
 
 3.26 
 
 KjO to KjCO 3 . . . 
 
 .682 
 
 .47 
 
 PbO to Pb 3 O 4 
 
 .977 
 
 1.02 
 
 KiCO 3 2H 5 O 
 
 .541 
 
 1.85 
 
 PbO 2 
 
 .933 
 
 1.07 
 
 K 2 SO<.. 
 
 .541 
 
 1.85 
 
 
 
 
 2KNO 3 
 
 .466 
 
 2.15 
 
 
 
 
 2KC1O 3 
 
 384 
 
 2 61 
 
 
 
 
 
 
 
 
 
 
 PRACTICAL APPLICATIONS. 
 
 As an illustration of the use of the foregoing diagrams the batches 
 for several different types of optical glasses will now be deduced. 
 
 (a) Ascertain the batch for an optical glass of refractive index, n D = 
 1.649 and v = 33. 7. 
 
 By definition v= (n D l)/(n F n c ); accordingly the mean disper- 
 sion of this glass is 0.649/33.7 = 0.01925. From figure 13 we find 
 that a glass of these properties is a member of the flint series; 
 we may therefore turn directly to figures 19 to 22 and 26 for the 
 desired information; the batch may be read off directly from figure 26 
 or ob tamed less directly from figures 19 and 20. The refractive 
 index curve of figure 20 shows that the percentage of lead oxide, 
 PbO, contained in a glass of refractive index n D = 1.649 is 52; from 
 figure 19 in turn we find that the approximate composition of a 
 flint glass containing 52 per cent PbO is SiO 2 , 41; PbO, 52; and the 
 alkalies (K 2 O, or Na 2 O, or ra K 2 O plus n Na 2 O) , 7. By means of the 
 conversion factors listed in Table 7 we now convert the K 2 O into a 
 mixture of K 2 CO 3 and KNO 3 such that the total amount of K 2 CO 3 is 
 three times that of KNO 3 . A simple algebraic computation shows 
 that this will be the case if 0.814, or, roughly, four-fifths of the total 
 
BATCH COMPOSITIONS. 107 
 
 amount of K 2 O, is assigned to K 2 CO 3 and the rest to KNO 3 ; the 
 corresponding factor for Na 2 O is 0.827. The batch then without 
 allowance for volatilization is: Sand, 100 kiligrams or pounds; 
 litharge (PbO), 126.8; potassium carbonate (anhydrous K 2 CO 3 ), 20.4; 
 potassium nitrate, 6.8. To correct for volatilization the lead oxide 
 should be increased a little; as a first approximation subject to test 
 and slight modification after .a trial melt we may write: Sand, 100; 
 litharge, 127; (or red lead Pb 3 O 4 , 130); potassium carbonate, 20.5; 
 potassium nitrate, 6.8; and arsenious oxide (As 2 O 3 ), 0.6. 
 
 The batch may also be read off directly from figure 29: Sand, 100; 
 litharge, 127; K 2 CO 3 , 20 ; KNO 3 , 7. In the batch diagram, as given, 
 arsenious oxide is not included; it is, however, common practice 
 among glassmakers to add from 0.2 to 1 kilogram As 2 O 3 per 100 
 sand. Too much arsenious oxide especially in the flint glasses may 
 cause the glass to turn milky on cooling; but a little is considered 
 to aid in producing a colorless glass of high, brilliant luster. 
 
 In case the analysis of the raw materials shows the presence of an 
 appreciable amount of water in any one of the substances, such as 
 sand or potassium carbonate, proper correction for this should be 
 made in computing the actual batch to be used. 
 
 It should be understood that these batch figures are of the correct 
 order of magnitude only; that the nice adjustment of the batch 
 depends on a number of factors which are best ascertained by actual 
 trial; these factors include size and type of melting pot and of melting 
 furnace, resistance of the pot to attack by the glass melt, furnace 
 schedule, and treatment of the glass batch and melt. A departure 
 of one or even two units in the third decimal place in the refractive 
 index and of one or two tenths in the y-value may be found on actual 
 trial. A slight modification of the relative quantities of the batch 
 substances suffices commonly to produce the desired results. 
 
 (b) State the batch compositions of a glass of refractive index n D = 
 L517 and v=6J h 3; n -n c = 0.00804. 
 
 The p-value of this glass is so high that it is evidently a borosilicate 
 crown. Reference to Table 4 of analyses indicates that the optical 
 constants of glass No. 17 are closely similar to those desired. In 
 general it may be stated that for a borosilicate glass of such high 
 y-value, the refringence is unusually high, whereas for a crown glass 
 of this refractive index, the v-value is too high. The presence of 
 boron oxide in the glass raises its rvalue; but, if added in quantity, 
 it lowers the refractive index; barium oxide, on the other hand, 
 tends to raise both the refractive index and the rvalue. It is 
 evident, therefore, that in order to attain the higher refractive index 
 together with high y-value, barium oxide should be substituted for 
 the lime of the crown glasses and boron oxide should be present in 
 appreciable quantities in order to approach the type of glass desired. 
 
108 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 The r-value of glass No. 17 is too low; in order to raise it slightly the 
 relative amount of boron oxide should be increased somewhat. A 
 batch computed on the basis of analysis 17 and modified in the manner 
 indicated is the following: Sand, 100; B(OH) 3 , 31; K 2 CO 3 , 31; 
 Na 2 CO 3 , 21; KNO 3 , 5.2; BaCO 3 , 6; As 2 O 3 , 0.3. 
 
 In case a slightly lower Vvalue, as 64.1 or 64.0, is desired, the amount 
 of B(OH) 3 should be reduced to 30 or even 29. The interplay of 
 boron oxide and barium oxide in optical glasses, as affecting their 
 dispersion and refringence, is a most important factor for the glass- 
 maker to realize. 
 
 (c) State batch composition of a borosilicate crown of refractive 
 index n= 1.511, v=63.5. 
 
 In "this glass the refractive index is low and the rvalue high; 
 there is no necessity, therefore, of substituting BaO for CaO. Analyses 
 9, 10, and 11 may serve as a basis from which to deduce a batch 
 composition for this glass; thus an appropriate batch would be: 
 Sand, 100; B(OH) 3 , 12; Na 2 CO 3 , 22.6; K 2 CO 3 , 15; KNO 3 , 9; CaCO 3 , 
 4.5; As 2 3 ,0.4. 
 
 (d) Select two glasses which in combination will produce a telescope 
 objective nearly free from secondary spectrum. 
 
 Secondary spectrum in a well-constructed telescope objective 
 consisting of two glasses in combination results from the dissimilarity 
 of dispersion in the two glasses. It can be readily proved that, 
 other conditions being the same, the amount of secondary spectrum, 
 for any spectrum interval, present in a doublet depends directly on 
 the partial dispersion ratios of the two glasses and inversely on the 
 difference between the ^-values. 
 
 The dispersion relations referred to in the foregoing pages enable 
 us to select glasses which meet these requirements. They may in 
 fact be selected directly by inspection of figure 15. As an illustration 
 let it be required to select a suitable glass which may be used in 
 combination with the zinc crown glass No. 26, of Table 4 of glass 
 analyses. The optical constants of this glass are: n D = 1.5128; 
 v = 57.3 ; n v -n c = 0.00894 ; n D - n A/ = 0.00575 ; n F - n D = 0.00630 ; 
 n Q ,-n F = 0.00508. The ratio (n D -n A ,)/(nQ,-n F ) = 1.132. From 
 figure 15 we find that glass No. 45, a dense barium crown, has approxi- 
 mately the same ratio between the two partial dispersions, namely, 
 (n D n A/ )/(n G/ n F ) = 1.127. Its optical constants are: n D = 1.6098; 
 y = 58.8; n F -n c = 0.01037; n-n A , = 0.00665; n -n D = 0.00730; 
 n G/ n F = 0.00590. 19 The partial dispersion ratios for the two 
 glasses are: 
 
 Nn 
 
 7l D WA' 
 
 Tip WD 
 
 Tie' n? 
 
 
 n v n c 
 
 n r n c 
 
 n f -n c 
 
 26 
 
 0.641 
 
 0.704 
 
 0.569 
 
 45 
 
 .642 
 
 .704 
 
 .569 
 
 
 
 
 
BATCH COMPOSITIONS. 109 
 
 From the analyses of these two glasses it is not a difficult matter 
 to write, down appropriate batches. Thus the batch for the zinc 
 silicate crown, No. 26, is approximately: Sand, 100; zinc oxide, 17; 
 sodium carbonate (Na 2 CO 3 ), 34; and sodium nitrate (NaNO 3 ), 11.3, 
 arsenious oxide (As 2 O 3 ) 0.3. A suitable first trial batch for the glass 
 No. 46 is sand, 100; boric acid (B(OH) 3 ), 69.5; alumina (A1 2 O,), 25.7; 
 barium carbonate (BaCO 3 ), 199.5; arsenious oxide, 0.3. The function 
 of the alumina in this batch is to lessen the, tendency of the barium 
 oxide to form crystallization nuclei of barium .disilicate and also 
 to improve the working qualities of the melt itself. 19 
 
 From diagram 15 we note that a third glass, namely, the barium 
 crown No. 27, has almost the same dispersion relations; its ratio 
 (nr> n A ')/(n^ ?I F ) is 1.132. Its partial dispersion ratios are 0.640, 
 0.703, and 0.565 for the intervals D to A', F to D, and G' to F 
 compared with the mean dispersion for the interval F to C. From 
 analysis 27 a first trial batch composition may be computed by the 
 methods outlined and found to be: Sand, 100; boric acid B (OH) 3 , 9; 
 zinc oxide, 8.4; barium carbonate, 41.7; sodium carbonate, 9; 
 potassium carbonate, 18.5; potassium nitrate, 9.5; arsenious oxide, 
 0.5. The addition of more barium carbonate to this batch would 
 raise the refractive index. As the amount of BaO is raised, that of 
 the alkalies is lowered in order partly to reduce the corrosive action 
 of the melt on the pot. 
 
 These examples suffice to indicate that the problem of batch com- 
 positions of optical glasses at the present time, until more data have 
 been made available, is one of interpolation together with a certain 
 amount of experience which enables the glassmaker to determine 
 what the behavior of the melt will be under the conditions at his 
 plant. In all cases it is advisable to prepare small melts, 5 to 50 kilo- 
 grams in weight, to stir these properly, and to ascertain the optical 
 constants of the finished glass. Good quality glass can not be pro- 
 duced by this procedure, and the conditions are distinctly different 
 from those in the melt of a large charge of 500 to 1,000 kilograms; 
 but the order of magnitude of the optical constants obtained is correct. 
 It is also advisable to hold the small charge melt for a relatively 
 long period of time at temperatures somewhat below the final stirring 
 temperatures in order to ascertain the tendency of the melt to crystal- 
 lize or to become milky and opalescent; the batch can then be modi- 
 fied accordingly. 
 
 It may be of interest to note that this part of the general problem 
 of optical glass manufacture proved, during the war, to be one of 
 
 19 The fact that the f values of these two glasses are so nearly alike is unfavorable from a practical stand- 
 point because under these conditions the power of the resulting combination is weak and steep curves are 
 required to attain even a low power. Two glasses, for which the difference in > values is less than 15.0, 
 are not generally considared acceptable for achromatic doublets. 
 
 3922921 8 
 
110 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 the least of our troubles and indicated the futility and uselessness 
 of secrecy in this particular phase of optical glass manufacture. 
 As a research problem, the general problem of optical glass manu- 
 facture in war time differs from ordinary research problems because 
 the properties of the final product are definitely known and the task 
 is to reproduce glasses of known characteristics rather than to develop 
 new types of glasses. In the above paragraphs no consideration is 
 given to the more fundamental problem of computing the optical 
 constants of a glass from its chemical composition. The information 
 at hand was not adequate for this purpose and our war-time interest 
 was not concerned with this problem, which still awaits satisfactory 
 solution. 20 
 
 Among the batches for different types of optical glass which 
 originated with the Geophysical Laboratory, 21 , those listed in Table 
 8 are typical: 
 
 TABLE 8. Batch compositions of optical glasses. 
 
 - 
 
 (A) 
 Light 
 crown. 
 n D = 1.516 
 ,= 60.0 
 
 (B) 
 
 Boro-sili- 
 cate crown. 
 
 W D = 1.511 
 ^=64.1 
 
 (C) 
 Boro-ili- 
 cate crown. 
 n D = 1.511 
 y=63.4 
 
 (D) 
 Boro-sili- 
 cate crown. 
 D =1.517 
 >=64.3 
 
 (E) 
 Barium 
 crown. 
 TOD= 1.570 
 ,,= 57.0 
 
 Barium 
 crown. 
 n D = 1.571 
 f=56. 7 
 
 Pand.. 
 
 100.0 
 
 100.0 
 
 100.0 
 
 100.0 
 
 100.0 
 
 100.0 
 
 BOsHs 
 
 
 21.0 
 
 21.0 
 
 31.0 
 
 13.6 
 
 11.3 
 
 A1 2 O 3 
 
 
 
 
 
 2.1 
 
 
 As 2 O 3 
 
 1.0 
 
 .4 
 
 .2 
 
 .3 
 
 .6 
 
 1.0 
 
 Pb 3 O4 
 
 
 
 
 
 
 
 ZnO 
 
 
 
 
 
 18.3 
 
 20.0 
 
 CaCOs 
 
 20 
 
 5 
 
 4 
 
 
 
 
 BaCO 3 
 
 4.7 
 
 
 
 6.0 
 
 75.3 
 
 76.7 
 
 K 2 CO 3 
 
 10.0 
 
 15.6 
 
 15.0 
 
 15.0 
 
 5.2 
 
 22.0 
 
 KNO 3 ... 
 
 12.4 
 
 14.0 
 
 3.0 
 
 5.2 
 
 11.3 
 
 5.0 
 
 NajzCOs 
 
 19.3 
 
 18.2 
 
 22.6 
 
 21.0 
 
 15.1 
 
 
 
 
 
 
 
 
 
 
 Light 
 flint. 
 W D - 1.580 
 
 K=41.1 
 
 (H) 
 Light 
 flint. 
 n D = 1.570 
 i>=42.0 
 
 (I) 
 Medium 
 flint. 
 TO D = 1.605 
 *=37.6 
 
 (J) 
 Dense 
 flint. 
 D = 1.640 
 r=34.6 
 
 (K) 
 Dense 
 flint. 
 W D = 1.649 
 -=33.5 
 
 (L) 
 Barium 
 flint. 
 n D = 1.619 
 f=37.6 
 
 Sand 
 
 100.0 
 
 100.0 
 
 100.0 
 
 100.0 
 
 100.0 
 
 100.0 
 
 BO 3 H 3 
 
 
 
 
 
 
 
 A1 2 O 3 
 
 
 
 
 
 
 
 As 2 O 3 
 
 .7 
 
 .1 
 
 .6 
 
 .7 
 
 .7 
 
 .6 
 
 Pb 3 O<. 
 
 68.1 
 
 62.0 
 
 90.5 
 
 124.1 
 
 129.3 
 
 89.1 
 
 ZnO 
 
 
 
 
 
 
 7.0 
 
 CaCO 3 
 
 
 
 
 
 
 
 BaCO? 
 
 
 
 
 
 
 19.0 
 
 K 2 CO 3 .. 
 
 9.4 
 
 14.2 
 
 10.3 
 
 5.3 
 
 20.0 
 
 21.3 
 
 KNO 3 .. 
 
 8.8 
 
 8.5 
 
 7.8 
 
 10.3 
 
 6.7 
 
 9.6 
 
 Na^Os 
 
 20 5 
 
 9 5 
 
 15 4 
 
 16 4 
 
 
 
 
 
 
 
 
 
 
 20 The first five papers of a series of articles on "The development of various types of glasses" have re- 
 cently been published by C. J. Peddle in the Jour. Soc. Glass Technology, 4, 3-107, 1920; see also "The 
 optical properties of some lime-soda glasses," by J. R. Clark and W. E. S. Turner, Jour. Soc. Glass Tech- 
 nology, 4, 111-115, 1920. 
 
 21 Batches b, e, g, i, k, I are taken from the article by C. N. Fenner, Jour. Am. Ceram. Soc., 2, 143, 1919. 
 
MIXING OF THE BATCH. 
 
 Ill 
 
 TABLE 8. Batch compositions of optical glasses Continued. 
 PERCENTAGE EQUIVALENTS. 
 
 
 Lfght 
 crown. 
 D = 1.516 
 *=60.0 
 
 (B) 
 Boro-sili- 
 cate crown. 
 n D =1.511 
 K=64.1 
 
 (C) 
 Boro-sili- 
 cate crown. 
 D =1.511 
 >=63.4 
 
 (D) 
 Boro-sili- 
 cate crown. 
 n D =1.517 
 f=64.3 
 
 (E) 
 Barium 
 crown. 
 n D =1.570 
 >=57.0 
 
 (F) 
 Barium 
 crown. 
 n D =1.571 
 
 c=56.7 
 
 SiO 2 . . 
 
 71.6 
 
 70.0 
 
 70.9 
 
 67.9 
 
 48.9 
 
 49.0 
 
 BjO 3 
 
 
 8.3 
 
 8.4 
 
 11.9 
 
 3.8 
 
 3.1 
 
 AhOa"" 
 
 
 
 
 
 1.0 
 
 
 As 2 3 
 PbO 
 
 .7 
 
 ,3 
 
 .1 
 
 .2 
 
 .3 
 
 .5 
 
 ZnO 
 
 
 
 
 
 8.9 
 
 9.8 
 
 CaO 
 
 8 
 
 1 9 
 
 1 6 
 
 
 
 
 BaO 
 
 2.6 
 
 
 
 3.2 
 
 28.6 
 
 29.2 
 
 K 2 O 
 
 9.0 
 
 12.0 
 
 8.2 
 
 8.5 
 
 4.3 
 
 8.4 
 
 Na 2 O 
 
 8.1 
 
 7.4 
 
 10.8 
 
 8.3 
 
 4.3 
 
 
 
 
 
 
 
 
 
 
 (G) 
 Light 
 flint. 
 W D = 1.580 
 =41.1 
 
 (H) 
 Light 
 flint. 
 D = 1.570 
 >=42.0 
 
 (I) 
 Medium 
 flint. 
 WD= 1.605 
 v=37.6 
 
 (J) 
 Dense 
 flint. 
 D =1.640 
 r=34. 6 
 
 (K) 
 Dense 
 flint. 
 TOD= 1.649 
 v=33. 5 
 
 Barium 
 flint. 
 w D =1.619 
 *=37.6 
 
 SiO 2 
 
 52.7 
 
 55. 6 
 
 47 9 
 
 41.7 
 
 41.3 
 
 43.8 
 
 B 2 O 3 
 
 
 
 
 
 
 
 Ai 2 o;" 
 
 
 
 
 
 
 
 
 As 2 O 3 
 
 4 
 
 
 3 
 
 3 
 
 3 
 
 .3 
 
 PbO... 
 
 35.0 
 
 33.7 
 
 42.4 
 
 50.5 
 
 51.6 
 
 38.1 
 
 ZnO.. 
 
 
 
 
 
 
 3.1 
 
 CaO 
 
 
 
 
 
 
 
 BaO . . . 
 
 
 
 
 
 
 6.5 
 
 K 2 O 
 
 5 5 
 
 7 5 
 
 5 1 
 
 3 5 
 
 6 8 
 
 8.3 
 
 Na 2 O 
 
 6 3 
 
 3 1 
 
 4 3 
 
 4 
 
 
 
 
 
 
 
 
 
 
 MAGNETIC SEPARATOR. 
 
 A number of experiments were made with a view to eliminate so 
 far as possible the iron-bearing compounds which might be present 
 in the raw batch materials, particularly the sand. For this purpose 
 a magnetic separator of the Dings type proved to be the most effi- 
 cient. By means of this separator the magnetic particles from the 
 sand and from the cullet were extracted. It was found, however, 
 that in the long run the amount of magnetic material obtained by 
 this method was hardly sufficient to warrant the expense and trouble 
 involved in the operation. In case, however, the raw materials are 
 not of the highest quality a magnetic separator should render 
 valuable service. 
 
 MIXING OF THE BATCH. 
 
 The raw materials which enter into the composition of the batch 
 mixture must be thoroughly mixed before they are filled into the 
 pot, in order that the composition before melting is practically uni- 
 form throughout. In smaller plants the mixing of the batch is done 
 by hand after the manner of quartering in assay plants. The mate- 
 rials are weighed out accurately and passed through a sieve of four 
 or six meshes per inch into a long, open wood box, large enough to 
 hold the entire batch. (Fig. 30.) The batch is then thoroughly 
 
112 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 worked over, back and forth, by shoveling the materials from one 
 end of the box to the other; in this operation the effort is made to 
 mix the materials by turning and spreading each shovelful. The 
 workmen soon become skilled at this task, and analyses show that a 
 highly uniform composition throughout the mass can be obtained by 
 this method. It has, however, certain disadvantages. The shifting 
 about of the powdered materials raises an appreciable amount of 
 dust, which is breathed in by the workmen. The workmen are sup- 
 posed to wear aspirators, but they are lax and may fail to regard 
 this precaution; they inhale the dust of lead oxide, arsenic, and 
 strong alkalies, and this in time seriously affects their health. This 
 situation is somewhat improved by the use of strong ventilators 
 
 FIG. 30. Mixing and sifting the materials for the raw batch by hand. (Photograph 
 by J. Harper Snapp at the plant of the Spencer Lens Co.) 
 
 placed about the mixing boxes; such ventilators remove the fine 
 dust from the room fairly well, but if the suction is too violent the 
 lighter portions of the batch are apt to be carried away. 
 
 Mechanical mixers are in general use at large plants. (Fig. 31.) 
 Experience has shown that these are efficient, the chief objection to 
 them being that, if any metal parts, such as iron, are exposed these 
 are abraded, and iron is thereby introduced into the batch. In 
 the case of optical glass this may seriously affect the color of the 
 glass. Of the mechanical devices the cube mixers lined with wood 
 and equipped with paddles have proved satisfactory; also a conical 
 mixer equipped with paddles. (Fig. 31.) Mixing by mechanical 
 means is of course more rapid than by hand, but in the case of an 
 optical glass plant the capacity is never so large that, were it not for 
 
GULLET. 
 
 113 
 
 the danger to the men's health, mechanical mixers would probably 
 not be used. 
 
 Gullet. The quality of glass required for optical purposes is so 
 high that the percentage of good glass finally obtained from each pot 
 rarely exceeds 25 per cent. Part of this wastage results from the 
 several operations through which the glass has to pass from the pot 
 stage to that of final acceptance; but a considerable portion of the 
 pot glass is rejected on first inspection because of striae, bubbles, 
 shape and small size of fragments, and other defects. Rejects of pot 
 glass are called "cullet," and are commonly remelted together with 
 
 FIG. 31. Mechanical batch-mixer. (Photograph by J. Harper Snapp at the plant of the Bausch & 
 
 Lomb Optical Co.) 
 
 fresh batch. Two purposes are thereby served valuable material 
 is saved and the melting pot is protected in a measure from the active 
 corrosive action set up on the melting down of the raw batch mate- 
 rials. 
 
 The amount of cullet which may be used is more or less indeter- 
 minate. Experience has shown that good results can be obtained 
 from melts free from cullet; also from melts containing 50 per cent 
 of cullet. Good practice is to use up the available cullet and not to 
 allow it to accumulate. Cullet intended for a batch is broken up 
 with a hammer or in a jaw crusher into pieces measuring not over 2 
 
114 MANUFACTURE OF OPTICAL GLASS. 
 
 inches in diameter. In this form it is easy to mix with the batch 
 materials and also to handle in a shovel or scoop. Iron may be in- 
 troduced during this operation, and the cullet fragments should be 
 passed over an electromagnet before mixing with the batch materials. 
 In the handling of cullet the most scrupulous care must be taken to 
 keep the cullet from a particular pot of glass separate from all others. 
 The object in all cases is to obtain optical glass of predetermined 
 optical constants, and the admixture of any cullet of abnormal optical 
 constants has a pronounced effect on the optical constants of the final 
 product. Furthermore, cullet, after having been once through the 
 melting process, is commonly richer in iron and other impurities 
 than the raw batch materials. Cullet which is strongly colored 
 should therefore be used only in limited quantities. In case the 
 optical constants of available collet are slightly different from those 
 of the glass desired it is the task of the manager so to adjust his raw 
 batch composition that the resultant glass will have the correct val- 
 ues. If large quantities of cullet are used, it is considered to be 
 good practice to increase relatively the amount of nitrates in the 
 batch in order to produce an actively oxidizing melt. In no case 
 should cullet containing stones or fragments of pot wall be used. 
 
 FURNACE OPERATIONS. 
 
 We come now to the most spectacular and to the novice the most 
 interesting period of the glassmaking process. The batch is moved 
 from the batch room to the furnace hall, where it is to pass through a 
 fiery furnace and there to be transformed into glass. The glass- 
 making process consists not of a single operation, but of a series of 
 operations which are carried out one after the other and for which a 
 definite schedule is commonly arranged. These operations include: 
 
 (a) Preheating, in a pot arch, of the pot in which glass is to be melted ; 
 
 (b) transference of the pot from the pot arch to the melting furnace 
 and setting of pot on even keel in the furnace ; (c) baking or burning 
 the pot at a very high temperature; (d) glazing of the pot; (e) filling 
 in the batch; (/) melting and fining; (d) skimming and stirring; (h) 
 cooling of melt in furnace; (i) removal of pot from furnace; (j) 
 cooling of pot to room temperature. These steps will be considered 
 in the order named. 
 
 Preheating of the pot in the pot arch. To be effective the furnace 
 hall of the optical glass plant requires not only melting furnaces in 
 which to melt the batch but also a number of pot arches in which to 
 preheat the melting pots. The pots, as they are received from the 
 pot-maker, are thoroughly well dried and free from cracks. Al- 
 though massive in appearance and several inches thick, they are in 
 fact very fragile and must be handled with the utmost care as they 
 
SETTING OF THE POT. 115 
 
 are in reality built-up clay masses which, if jarred severely, may crack 
 and fall apart. 
 
 The clay of the crucible contains even in the dry state a large 
 amount of water in chemical combination as well as adsorbed water; 
 this must escape during the heating of the crucible. It is the task of 
 the glass maker to heat his crucible so slowly and so uniformly that 
 during the operation it does not crack. The heating is commonly 
 done in a simple type of gas-heated furnace, equipped with a short 
 stack, in which gas flames mount over the breast w r all, pass along 
 the arch or crown and are drawn out through openings in the floor 
 of the furnace. A pot arch of this type is not difficult to regulate, 
 but it requires constant care to keep it properly regulated. If the 
 gas flames play directly on the green pot a crack is certain to develop 
 along the path of the flame. 
 
 FIG. 32. Preheated pot in process of transfer from pot arch to melting furnace. Furnace 
 door is raised and furnace siege (floor) has been made ready toreceive pot. Pot is trans- 
 ferred by a "pot wagon." (Photograph by J. Harper Snapp at the plant of the Bausch & 
 Lomb Optical Co.) 
 
 The melting pot is placed on three firebrick supports 6 or 8 inches 
 off the ground in the pot arch; the arch is sealed and the heat turned 
 on slowly. The period of slow preheating depends on the size and 
 kind of crucible; but ordinarily three to five days is about right; by 
 this time a temperature of 800 to 1,050 C. (1,475 to 1,900 F.) has 
 been reached and the hot pot may be transferred to the melting fur- 
 nace which is heated to about the same temperature. The pots are 
 transferred by means of a pot wagon, which may be described as a 
 huge pair of tongs (mounted on wheels and with adjustable counter- 
 weights) which grasp the pot beneath the outer flange of the pot. 
 (Fig. 32.) Half a dozen men can handle without difficulty a 36-inch 
 pot even when filled with glass and weighing 1,500 pounds. 
 
 Setting of the pot.- During the period of intense heat developed in 
 the melting furnace its floor or " siege" becomes somewhat soft, the 
 
116 MANUFACTURE OF OPTICAL GLASS. 
 
 pot of molten glass sticks to it and on removal has to be pried up 
 and loosened from it. This operation leaves the siege uneven. Be- 
 fore introducing the preheated, empty pot the glassmaker accordingly 
 uses long iron rods and pushers to level the siege. He scrapes off 
 protruding high spots as well as possible and then fills in the cavities 
 with fresh sand and spreads a final thin layer of sand over the pot 
 area in order that the pot may rest on a firm, level foundation and 
 not be subjected to unnecessary strains. Care should be taken not 
 use too much fresh sand in this operation, otherwise the siege be- 
 comes too high in the course of time. 
 
 The pot is set on the level siege, the furnace door (tuille) is lowered 
 and sealed, and the fire is turned on. 
 
 Baking oj tlie pot. In the course of several hours a furnace tem- 
 perature of 1,400 to 1,500 C. is attained; the empty pot is baked 
 at a temperature of 1,425 to 1,450 C., depending on the kind of pot, 
 generally for an hour or more to allow the clays to sinter together 
 and even locally to show incipient melting, so that the texture of 
 the pot walls becomes dense and is then not readily attacked by 
 the molten glass. The pot is purposely superheated above the fining 
 temperature of the glass, in order to render its inner walls dense and 
 chemically resistant. The duration of the burning differs with the 
 kind of pot employed, but it should always be sufficiently long to 
 insure proper quality of walls. The burning may be done in a pot 
 arch providing a sufficiently high temperature can be reached in the 
 arch; in this case the pot, after burning, is transferred to the melting 
 furnace, thus shortening the glass-melting period. 
 
 Glazing of the pot. After thorough baking of the pot the furnace 
 temperature may be lowered to the fining temperature of the glass 
 or even slightly lower, to 1,325 C. (2,300 to 2,400 F.). Charges of 
 40 to 50 pounds of cullet are now filled into the pot at intervals of 
 half an hour by means of a long-handled scoop or ladle; more cullet 
 may be used if desired. In filling in the cullet from his scoop, the 
 glassmaker endeavors to spread the material around the inside upper 
 walls of the pot, whence it flows to the bottom, which it should cover 
 to a depth of at least an inch. The function of the cullet glaze is to 
 protect the walls of the pot from the batch. If the glazing is done 
 at a very high temperature there seems to be a tendency for the thin 
 molten glass to enter into the pot walls and leave them dry and 
 unprotected. The glazing period may extend over one to three hours, 
 depending on the amount of cullet used and on the furnace time- 
 schedule. The furnace has now been held for several hours at a high 
 temperature; during this period its walls and regenerative chambers 
 have been heated thoroughly and a reserve amount of heat has been 
 stored up to aid in the rapid melting down of the cold batch and 
 thus to obtain a satisfactory fining period 
 
MELTING AND FINING. 117 
 
 Filling in the batch. After the glazing of the pot, the batch is 
 filled in at intervals. This operation should proceed in such a man- 
 ner that the batch is introduced in quantities sufficiently small that 
 they do not seriously cool down the melt in the pot and yet large 
 enough to insure an economical use of fuel and of time. (Fig. 24.) 
 With each fill of the batch and consequent opening of the furnace 
 port, there is a perceptible cooling down of the furnace; with the 
 introduction of each ladle of the batch there is furthermore an appre- 
 ciable amount of the batch lost, which rises as a cloud of dust, enters 
 into the furnace chamber, and is carried away by the gases; the 
 dust settles in part on the walls and arch of the furnace and shortens 
 its life appreciably. During the filling-in period the temperature of 
 the furnace should be kept fairly high. If the temperature is held 
 too low, the reactions within the melt proceed slowly and uneco- 
 nomically; if too high, the reactions proceed too violently and the 
 pot may boil over and be seriously attacked by the chemicals. Each 
 successive fill of batch-mixture is made before the preceding fill has 
 been completely dissolved and while undissolved grains of sand are 
 still present in the melt. Experience has shown that the filling may 
 be done at high temperatures without serious attack of the body 
 walls. It is the task of the furnace operator to hold the temperature 
 of the furnace sufficiently high that the melting down proceeds fairly 
 rapidly without serious danger of boiling over and without serious 
 attack on the walls of the crucible. In short, the filling-in should 
 not be done in such large quantities that the pot is appreciably 
 cooled; nor at too frequent intervals, nor in too small quantities; 
 otherwise there is serious loss by volatilization and escape of dust. As 
 the filling-in proceeds, the temperature of the furnace should be raised 
 to the fining temperature which ranges from 1,375 to 1,425 C. for 
 the ordinary kinds of optical glass. The pot is filled finally with 
 metal to within an inch of the top. 
 
 Melting and fining. There are certain features of the glass-melting 
 process which have not yet been definitely established and which can 
 only be approximately determined because of the number of varia- 
 bles involved. Raw materials of high purity are essential; freedom 
 from iron is necessary because it is the chief coloring agent. The 
 danger of iron coloration from pot solution can be reduced by using 
 pots of high chemical resistance and low iron content, by a thorough 
 baking of the crucible at a temperature considerably above the fining 
 temperature of the glass, and by shortening the glass-melting period 
 as much as possible. Experience has shown also that the iron in the 
 ferrous-ferric state produces a maximum amount of coloration with 
 a given percentage of iron; that iron in the ferric state produces 
 relatively much less coloration. It is essential, therefore, that care 
 be taken to run the furnace with an appreciable excess of hot air; 
 
118 MANUFACTURE OF OPTICAL GLASS. 
 
 if the furnace construction is good the atmosphere above the batch 
 is oxidizing and long lazy reducing flames do not sweep across the 
 top of the pot and tend to reduce the ferric state of the iron. In the 
 regenerative type of furnace a reducing atmosphere is avoided by 
 first shutting off the gas on one side of the furnace, opening the 
 butterfly, and then waiting for half a minute before turning on the 
 gas on the opposite side of the furnace. This plan enables the fur- 
 nace to get into proper balance and draft, and eliminates the pres- 
 ence of black sooty flames every time the direction of gas flow is 
 reversed (10 or 20 minute intervals). Experience with furnaces indi- 
 cates that the regulation of the temperature of the glass-melting 
 furnaces is one of the most important factors in the manufacture 
 of optical glass; this applies not only to preheating and baking of 
 the empty pots, but also to the filling-in and the glass-melting 
 temperatures. 
 
 The filling in of the batch and the complete melting and solution 
 of the batch components is accompanied by an evolution of the vola- 
 tile components of the batch; the final product is a solution of silicates 
 free from bubbles. Experience has shown that agitation or vigorous 
 stirring of the melt during, and especially toward the end of the filling- 
 in period accelerates the solution and melting down of the batch and 
 tends furthermore to reduce the differences in concentration in dif- 
 ferent parts of the melt. In particular it prevents the heavy lead 
 oxide components of the batch from sinking to the bottom and thus 
 forming a heavy layer which later is difficult to eliminate. Moreover, 
 the stirring of the melt during the period of intense chemical reaction 
 tends to favor the escape of any bubbles which may be formed. 
 The stirring should not be so violent that the foam which forms on 
 on the surface of the glass melt is stirred into the glass mass. Stirring 
 at this period decreases the time of melting and shields the pot from 
 attack by the solutions and cuts down the amount of gas used. Asa 
 result of the intensive study of these factors Dr. G. W. Morey, 22 of 
 the Geophysical Laboratory, was able to shorten the melting period 
 from 36 hours to 24 hours. 
 
 At the end of the filling-in period, the glass is fairly well melted, 
 but the chemical reactions, which take place and which mean the 
 replacement of carbonates and nitrates by silicates and borates (if 
 boron be present) and the driving off of the volatile gases, require 
 some hours for completion; during most of this period the molten 
 glass is filled with small bubbles and the escaping gases and chemical 
 reactions tend to keep the temperature of the melt down. Toward 
 the end of the process, the volatile gases have for the most part es- 
 caped. The temperature of the melt may now rise. The evolution of 
 
 M G. W. Morey, An improved method of optical glass manufacture, Jour. Am. Ceram. Soc., 2, 146-150, 
 1919. 
 
GLASS MELTING PROCESS. 119 
 
 gas becomes more pronounced, the melt enters the stage of the "open 
 boil," and then passes into the "fine or plane" stage at which it is 
 relatively free from bubbles and seeds. 
 
 In the glassmaking industry the fining of the glass melt is a most 
 important factor. The fining is evidently a stage attained by the 
 melt in which the saturation limit for the gases (CO 2 and N 2 O 5 ) is 
 relatively low and presumably becomes progressively lower as the 
 carbonates and nitrates are eliminated from the melt. There is some 
 evidence that the batch containing only nitrates and no carbonates 
 fines at a lower temperature than the nitrate-carbonate batch; there 
 is also evidence that at high temperatures arsenic pentoxide dissociates 
 into oxygen and arsenic trioxide and causes an evolution of gas which 
 tends to sweep the glass metal clear of small seeds toward the end of 
 the fining period. Because of its importance to the whole glass manu- 
 facturing industry a careful study of the gas evolution-time-tempera- 
 ture relations in different glass batch types should be undertaken; 
 preliminary investigations along these lines have been undertaken 
 by Dr. E. G. Zies, of the -Geophysical Laboratory. The data obtained 
 by these studies should give a clearer insight into the glass-melting 
 process than has heretofore been possible and will enable glass- 
 makers to establish scientific control over their melts. The problem 
 is fundamental in character. The study of the chemical reactions 
 at high temperatures which take place on the conversion of the 
 carbonate-nitrate solutions into a solution of silicates is essentially 
 that of the glassmaking process, and the more we know of these 
 reactions and their rates, the better can they be controlled. 
 
 The optimum temperatures for fining the glass are different for 
 different glass types. In general the crown glasses (light crown, 
 borosilicate crown) fine well at 1,400 C. or a little higher. Medium 
 flint melts, on the other hand, are less viscous and fine well at 1,370 
 C. In case there is danger of milkiness, because of the presence of 
 small amounts of chlorides or sulphates, or too much arsenic or other 
 opalescence-producing compound, the melt should be fined at a 
 higher temperature, 1,425 C. or still higher, if the pot will stand it. 
 At high temperatures the pot may become fairly soft and be attacked 
 by the metal, and stones or leakage may result; furthermore, because 
 of increased volatilization, the optical constants of the melt change 
 rapidly. 
 
 The ease with which molten glass rids itself of bubbles depends 
 somewhat on the type of glass. The flint glasses are relatively liquid 
 and commonly fine readily. The crown glasses are in general more 
 viscous and may cause trouble during the fining stage. To facilitate 
 and to expedite the fining of the glass, "blocking" of the melt is 
 frequently resorted to; this process consists in introducing into the 
 melt a small amount of some volatile substance, such as water, which 
 
120 MANUFACTURE OF OPTICAL GLASS. 
 
 on escaping forms large bubbles and sets the melt in violent agitation. 
 The details of this process are described below in the section on 
 stirring. 
 
 At different periods during the melting and lining stages of the 
 glass, proofs or dips are taken to ascertain the condition of the metal. 
 For this purpose an iron rod flattened at the end, with a shallow cup 
 , attached to the bent end of the rod, is inserted into the metal and 
 a sample of molten glass is ladled out; or a small quantity of glass 
 is gathered on the end of a small iron pipe and blown into a spherical 
 flask. The operation is done as quickly as possible; the rod with 
 its dip of attached glass is withdrawn and examined during cooling. 
 In the case of proofs the base of the cup is cooled in a pail of water; 
 the hemispherical proof is then removed and placed on top of the 
 melting furnace, where it cools down slowly. 
 
 SJcimming. Toward the end of the fining period bubbles of fair 
 size escape freely and the stage of "open boil" begins; with the close 
 of this period the melt is reasonably free from seeds and bubbles and 
 is said to be "fine" or "plane." During the fining process scum, 
 stones, and other materials which are specifically lighter than the 
 metal rise to the top and can be seen floating on the surface; these 
 are removed by "skimming." A long iron rod with a cross plate 
 attached to the end is passed over the surface of the metal and the 
 froth is skimmed off. Care is taken in this operation not to submerge 
 any of the floating islands of scum because they reappear only slowly; 
 also not to remove any more of the good glass than is necessary 
 because of the changes in optical constants of the glass which may 
 result therefrom. 
 
 Stirring. Ordinary types of glass consist chiefly of silicates in 
 solution; geological and experimental evidence proves that silicates, 
 such as are found in glass and in igneous rocks, are miscible in all 
 proportions. From the layers which are sometimes observed in 
 optical glass melts, especially in the heavy flints, one might infer that 
 limited miscibility between certain phases exists; but it is easy to 
 prove that the layers are the result of gravitative differentiation, 
 the heavy, lower-melting lead oxide and other components of the 
 batch settling to the bottom, the light sand particles rising to thi 
 surface. Such stratification in layers of different density is not a' 
 uncommon thing in glass melts and is overcome by stirring and by 
 blocking. Once the melt has been rendered thoroughly homogeneous 
 slight differences in composition may still be introduced as a result 
 of volatilization from the surface of the melt and solution of the pot 
 walls along the sides and bottom. The chief purpose of stirring is 
 to render the melt homogeneous. 
 
 Stirring was first introduced by P. L. Guinand, a Swiss maker of 
 glass, who used a clay stirring rod operated by hand. Guinand pro- 
 
STIRRING OPERATIONS. 121 
 
 duced glass of good quality, some of which was used by Fraunhofer 
 in astronomical telescopes. Hand stirring (fig. 33) is still employed 
 at many factories during the early part of the melting period for the? 
 purpose of thoroughly mixing the melt so that great differences in 
 composition do not exist; the stirring begins while the melt is still 
 active and undissolved sand grains are still present. Stirring at this 
 stage of the process accelerates the chemical reactions within the 
 melt and aids greatly in the attainment of homogeneity. Hand 
 stirring of this kind is best done intermittently in order not to cool 
 down the molten glass appreciably and also to allow bubbles to 
 escape. 
 
 FIG. 33. Stirring an optical glass melt by hand. (Photograph by J. Harper Snapp at the plant 
 of the Bausch & Lomb Optical Co.) 
 
 Following Guinand's practice, the clay stirring tube or thimble is 
 first carefully heated to a bright red in a small gas furnace and then 
 placed on the breast wall of the melting furnace where it attains a 
 white heat; from here it is conveyed to the edge of the melting pot, 
 nserted slowly into the melt and then withdrawn and allowed to 
 Rremain for another hour on the edge of the crucible with its closed 
 end floating on the molten glass. This treatment allows the glass to 
 penetrate into the clay mixture and to drive out gases which would 
 otherwise escape into the melt and be difficult to eliminate. The 
 clay tube is now attached to the end of the water-cooled rod, and 
 stirring by hand or by machine may begin. The clay stirring tubes 
 should be at least 3 inches thick at the bottom and 4J inches at the 
 top with a collar 1 inch thick to give strength to the end of the tube 
 into which the elbow end of the water-cooled stirring rod passes; also 
 
122 MANUFACTURE OF OPTICAL GLASS. 
 
 to furnish a flange with which to support the tube when the stirring 
 rod is removed. The tube should be about as long as the pot is deep. 
 The tubes are commonly made at the plant from the material of raw. 
 broken pots. (Fig. 34.) The end of the water-cooled, iron-pipe 
 stirring rod is preferably a square block of iron about 4 inches long and 
 1 inch thick, set at right angles to the rod; it fits fairly snugly into 
 the square hole at the top of the fire-clay stirring tube and holds it 
 in position during the stirring operation. 
 
 If hand stirring is used, the water-cooled stirring rod passes over 
 a small grooved iron wheel, mounted on a pivot directly in front of 
 the small opening in the furnace door. (Fig. 33.) To relieve the 
 workman of supporting the heavy rod during the stirring operation, 
 it is counterbalanced by weights suspended from pulleys which con- 
 
 FIG. 34. Clay stirring-rod attached to water-cooled stirring rod mounted on a stirring 
 machine. (Photograph by J. Harper Snapp at the plant of the Spencer Lens Co.) 
 
 nect to its cold end; at this end a crossbar, preferably of wood, is 
 attached and with it the rod is held and guided by the workman. 
 Skill and practice are required to stir well by hand. There is danger 
 at first of scraping the sides and bottom of the pot and of disengaging 
 the clay tube, but with practice the motions become routine and 
 workmen find no difficulty in stirring continuously during a 20-minute 
 shift. During actual stirring the eyes of the workman are shielded 
 by proper glasses to cut down the intensity of the light and heat 
 radiated from the furnace. 
 
 Hand stirring is satisfactory for the early stirring operations, but 
 the mechanical stirrer is superior for the long-continued stirring 
 which follows. The mechanical stirrer is an electrically driven device 
 mounted on a heavy framework which runs on wheels and can be 
 
STIRRING PROCESS. 123 
 
 moved from one furnace to another. (Fig. 34.) Different mechan- 
 ical means are employed to impart to the stirring tube the desired 
 motions. A rotating metal plate can be used, to which the cold end 
 of the stirrer is attached ; and by an automatic screw feed this end 
 can be made to describe circles of continuously varying radii if 
 desired ; the middle of the tube is pivoted on a metal pin which slides 
 in a groove that can be inclined at different angles, thus imparting 
 the desired degree of up-and-down movement to the stirring tube. 
 A maximum up-and-down stroke of the stirring tube of 4 to 6 inches 
 is about right. With this arrangement the curves described by the 
 stirrer are approximately circles 23 combined with the up-and-down 
 stroke. The stirring machine is adjusted to proper height of the 
 stirrer by means of small jackscrews permanently attached to the 
 framework. Similar motions of the stirrer can be accomplished by 
 means of systems of gearing (planetary) and have proved satisfactory 
 in practice. The curves described by these systems, especially if the 
 axis, on which the rod is pivoted, is fixed, may depart in shape con- 
 siderably from a circle. The requirements to be met by the mechan- 
 ical stirrer are : Ability to impart to the stirrer approximately circular 
 motions of different diameters; at the same time an up-and-down 
 movement if possible; variable speeds varying from 30 down to 4 or 
 5 revolutions per minute (speed preferably continuously variable or 
 if by steps, by at least four steps) ; stirring rod easily and quickly 
 removable; ease and certainty of manipulation. In all mechanical 
 stirring the stirring tube should never approach nearer than 2 inches 
 to the sides or bottom of the pot. 
 
 The significance of the stirring process is best realized by analogy. 
 In the case of sugar dissolving in hot water or tea the obvious method 
 to expedite the rate of solution and to render the solution homogeneous 
 is to stir it vigorously with a spoon. Similarly, fine or heavy striae 
 can be absorbed and the glass rendered homogeneous by effective 
 stirring; but this stirring must be done in such a way that the 
 different parts of the melt are thoroughly mixed and at sufficiently 
 high temperatures that the rate at which diffusion acts to eliminate 
 differences in concentration is sufficiently rapid to enable the glass 
 to become homogeneous within a reasonable period of time. The 
 higher the temperature the thinner the metal and the more rapidly 
 are the differences of composition eliminated by diffusion. The need 
 for an up-and-down and an in-and-out motion with a stirring rod of 
 sufficient size to be effective is evident. A homogeneous solution of 
 silicates can only be obtained by proper stirring methods applied at 
 proper temperatures. The solutions are not individual units, such as 
 crystal compounds of definite compositions, but they are solutions of 
 silicates mutually soluble which dissolve the one in the other and finally 
 
 23 Williamson and Adams, Jour. Am. Ceram. Soc., 3, 671-677, 1920. 
 
124 MANUFACTURE OF OPTICAL GLASS. 
 
 merge completely to produce a homogeneous mixture. These jr 
 ciples are stated somewhat in detail because they are fundamen 
 to the attainment of good optical glass. This treatment of opti( 
 glass is different from that of any other kind of glass, such as plal' 
 glass, and may therefore not appear to be important to the skilh 
 maker of plate or window glass whose interest centers chiefly K 
 seeds and heavy strire, but not in fine striae which are almost imper-j ,j 
 ceptible to the unaided eye. 
 
 An instructive experiment to illustrate the formation of striaa and 
 the effect of proper stirring on the elimination of striae is to mix, 
 in a beaker glass, glycerin and water, or glycerin and alcohol, orn 
 syrup, honey, or molasses and water; stirring rods of different shapes,, 
 and sizes may be used to ascertain the effects of the different possible a 
 methods of stirring. During the first part of the mixing process the B 
 more viscous liquid (glycerin or syrup) forms a series of veins, 
 strings, ribbons, which disturb the even course of the light rays 
 through the solution and render it semitransparent. As the stirring i 
 continues the heavy cords and threads decrease in distinctness and 
 sharpness and the solution appears to be filled with fine lines. After 
 further stirring the solution becomes clearer and finally attains a 
 state of complete mixing; it is practically homogeneous. These 
 experiments aid the observer in visualizing the stirring process and 
 impress him not only with the significance of striae and their elimi- 
 nation, but also with the relatively long period of time required to 
 render even a relatively thin solution, such as glycerin and water, 
 homogeneous; he realizes at once the importance of the thorough 
 and long-continued stirring of optical glass at high temperatures in 
 order to attain homogeneity. 
 
 In the early stages of the final melting process of optical glass 
 pronounced differences in composition exist in the glass melt; diffu- 
 sion acts to diminish these differences in concentration. The rate at 
 which this is accomplished depends on the concentration differences 
 from point to point in the melt; the more numerous and the greater 
 these differences are between adjacent points, the more rapid is the 
 transfer of material by diffusion and the sooner is homogeneity 
 attained. Elements of different composition are spread out and 
 brought into direct contact by stirring. Although at high temper- 
 atures the pot walls are dissolved more rapidly, yet unless the stirring 
 is carried on at a high temperature it is ineffective, and fine striae 
 are introduced which are not completely digested by the metal. 
 
 The function of the first part of the stirring process which takes 
 place while solution within the melt is still active is to mix the melt 
 thoroughly. The stirring is done rapidly and the melt is vigorously 
 agitated. This kind of stirring should be maintained until all ele- 
 ments in the batch have been completely dissolved and the volatile 
 
STIRRING PROCESS. 125 
 
 .onents have escaped. It is not necessary, however, that the 
 ring be carried on continuously during this period; in fact, it is 
 ter to stir the melt intermittently, because during this stirring 
 iod the temperature of the furnace necessarily falls and the melt 
 cooled somewhat. Thus by stirring for 15 minutes and then 
 sing up the furnace and allowing it to attain a high tempera- 
 are, the chemical reactions take place more rapidly and there 
 is less chance for great differences in concentration to be set up. 
 Such stirring is best done by hand and an effort is made to stir up-and- 
 down and in-and-out with a spiral motion. This part of the process 
 lixes the melt thoroughly and aids bubbles to escape. The stirring 
 -hould not be done so rapidly that escaping bubbles are carried down 
 nto the melt. Toward the end of this process the characteristic 
 ,tage of the reaction known as the "boil" of the molten glass begins 
 and stirring by machine may now commence. 
 
 As an aid to hand stirring, blocking may be used ; this mode of treat- 
 nent aids also in the finishing of the melt. Blocking consists essentially 
 in introducing into the melt a highly volatile substance, such as water, 
 or arsenious oxide, or ammonium nitrate, which produces a sudden 
 evolution of gas; this gas, on escaping through the melt, agitates it 
 violently and tends not only to mix the melt, but also to sweep out 
 any fine bubbles which may be held in it. The method of blocking has 
 been used for many years in the plate and window glass industry and 
 derives its name from the fact that blocks of wood soaked in water and 
 held by a proper clamp are commonly thrust down into the melt; 
 the intense heat causes a violent evolution of steam from the water, 
 which produces the desired effect. Lumps of arsenious oxide answer 
 the same purpose ; also sticks of ammonium nitrate. An iron rod is 
 used almost invariably in this connection and inevitably introduces 
 a certain amount of iron into the melt. For optical glass this would 
 be serious if the amount were appreciable, and care should be taken to 
 employ clean iron rods free from scale. The method of blocking can not 
 entirely replace hand stirring because the whole blocking action lasts 
 for an exceedingly short time; but as an aid in fining the glass and 
 in bringing about an open boil, blocking may be advantageous. The 
 blocking is most effective if introduced at the beginning of the " open 
 boil" stage of the melt. The injection of blocking material should 
 be repeated several times in rapid succession, to be followed after 
 an interval of 10 to 20 minutes by a second series of similar injections. 
 The glass is now fairly homogeneous and the task is to attain still 
 greater homogeneity. This is best attained by mechanical stirring at 
 temperatures slightly below the fining temperatures. During the 
 melting process the glass is in a constant state of change. Volatili- 
 zation of certain components of the melt proceeds at an appreciable 
 rate from the time the batch enters the pot until after the pot has 
 3922921 9 
 
126 MANUFACTURE OF OPTICAL GLASS. 
 
 been removed from the furnace; the walls of the pot are moreover 
 attacked and solution of these walls takes place at a rate dependent 
 on the quality of the pot. The changes in composition which arise 
 from these two sources are restricted, chiefly to the margins of the 
 melt, namely, the bottom, the sides, and the top. In all cases these 
 changes tend toward an increase in the percentage of silica in the 
 melt. Any factor therefore which produces a movement of the 
 margins of the melt toward the center necessarily introduces into 
 the melt streaks of different composition and hence of different opti- 
 cal properties. The factors involved in such transfer of materials 
 are primarily mechanical movement and thermal convection currents 
 set up as a result of differences in temperature between different parts 
 of the metal; in the case of pot solution the lighter, more siliceous 
 material from the pot tends to rise, thus setting up a current and 
 allowing fresh melt to continue the attack on the walls. It is impor- 
 tant that the temperature of the furnace be kept uniform, and that 
 the stirring be done in such a manner that the marginal parts of the 
 melt are disturbed as little as possible. In the case of pots which 
 are chemically resistant, the danger of trouble from the walls of the 
 pot is relatively slight and glass of good quality should extend to 
 the margin of the pot. The stirring rod during mechanical stirring 
 should not approach nearer than 2 inches to the sides or bottom of 
 the pot, the object being to insure homogeneity in the central part 
 of the glass mass and to shield this by leaving an undisturbed shell 
 of molten glass between it and the pot walls. At the beginning of 
 the period of steady mechanical, stirring the rate of stirring should 
 be fairly rapid (25 to 30 strokes per minute) ; there is, however, no 
 special reason for a pronounced up-and-down movement at this stage, 
 because by this time the differences in concentration between different 
 parts of the melt are small and the function of the stirring is simply 
 to eliminate these small differences and to obtain a melt uniform in 
 composition throughout. Machine stirring at fairly high temper- 
 atures is continued for some hours and a high degree of homogeneity 
 is attained thereby in the melt. 
 
 Were it now possible to have the molten glass acquire instan- 
 taneously room temperature in a well- annealed state, much of the 
 glassmaker's troubles would be eliminated; but this is not the case, 
 and the furnace operator endeavors during the cooling-down period 
 to retain the homogeneity which the metal has acquired. This 
 period is critical, and much glass may be lost in this operation unless 
 extreme care be taken. 
 
 The gas is turned off and furnace and melt are allowed to cool 
 slowly. Stirring is continued, but at a decreased rate and with 
 shorter stroke, and the vertical motion is eliminated. The function 
 of the stirring from now on is defensive only and seeks to efface the 
 
STIRRING PROCESS. 127 
 
 inhomogeneities introduced by convection currents into the melt from 
 the walls and bottom of the pot and from the surface of the melt at 
 which selective volatilization is ever active. These sources of inho- 
 mogeneity do not produce great changes in the total composition of 
 the melt, but, if allowed to be carried by convection currents through 
 the melt, they leave a trail of very slightly different composition and 
 this means striae in the final product. When we consider that the 
 refractive index of a heavy stria or cord differs from that of the inclos- 
 ing glass by only one or two units in the third decimal place, we 
 realize how slight the differences in chemical composition actually 
 are and how essential it is to aid diffusion in smoothing out these 
 minute differences by persistent stirring. 
 
 The rate at which to stir the molten glass during the cooling-down 
 period is a matter to be learned by experience. It is desirable at 
 all times to stir it as rapidly as possible but with certain limitations. 
 As the melt cools the glass becomes stiffer, and the stirrer tends to 
 carry before it a wave of glass which becomes higher and more pro- 
 nounced the more viscous the glass and the faster the stirrer travels 
 through it. The stirring should not be so rapid that part of this 
 wave is at any time infolded into the wall because bubbles and the 
 lower refracting surface film are thereby introduced; the stroke 
 should, moreover, be so slow that the glass at the margin of the pot 
 is left undisturbed. 
 
 During this part of the process there is little chance for the bot- 
 tom of the pot to cool off, and it tends to function as a heating plate 
 and to set up convection currents within the melt. The effect could 
 be eliminated, if there were some easy method available for raising the 
 pot and setting it on fire-clay supports at this stage, but no satisfactory 
 method for accomplishing this is known to the writer. It is well to 
 keep the door (tuille) of the furnace raised slightly during this period 
 so that through the opening, several inches high, along the bottom, 
 cool air can enter and cool the base of the pot. In case of necessity 
 a cold air blast may be directed against the base of the pot; this 
 has been tried, but the results attained do not seem to warrant the 
 extra trouble involved. 
 
 Stirring of a cooling-glass melt can not be continued indefinitely 
 because the glass becomes so stiff that further movement is impossible. 
 It is the task of the glass worker to determine when stirring shall 
 cease and the pot be removed from the furnace. It is desirable to 
 continue the stirring as long as possible in order to reduce to a mini- 
 mum the danger from convection currents, which move very slowly 
 in an extremely viscous melt; on the other hand, if the stirring is 
 continued to too low a temperature, the glass becomes so stiff that 
 the mass adhering to the stirring tube grows in size and sweeps far 
 
128 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 out into the marginal shell of the metal, thus introducing striae 
 which the steady stirring seeks to avoid. 
 
 The temperatures at which different types of glass should be 
 removed from the furnace range from 1,150 C. in certain barium crown 
 glasses to 900 C. in dense flint glasses. Having once ascertained the 
 best temperature for removal of a given type of glass in a given size 
 of pot, the glassmaker endeavers to remove other pots of the same 
 type at the same temperature. To accomplish this, he plots on a 
 chart the temperature readings, and by extrapolation of the time- 
 temperature curve determines the exact time for removal of the pot. 
 This question of pot removal has been closely studied by Dr. C. N. 
 Fenner; 24 the procedure which he adopted for two of his glass types 
 is illustrated in figure 35. A record of the several operations in 
 his treatment of the two glasses is also reproduced from his article. 
 
 1400 
 
 1300 
 
 ,1200 
 
 1000 
 
 Nxl 
 
 Si- 
 
 6 
 TIME 
 
 IO 
 A.M 
 
 FIG. 35. Time-temperature curves illustrating the procedure followed by C. N. Fenner in furnace operation 
 during the stirring operations of two different types of glass, namely, medium flint (Curve I) and ordinary 
 crown (Curve II). 
 
 EXAMPLES OF PROCEDURE IN STIRRING (AFTER C. N. FENNER). 
 MEDIUM FLINT (n D = 1.605, F=37.6). 
 
 Pot 25 inches inside diameter at bottom, 27 inches deep. 8.03 a. m., started stirring 
 machine at 13 revolutions per minute. Radius of stirring circle 6| inches; vertical 
 motion 4 inches. (NOTE. A stirring circle of greater radius would have been pref- 
 erable.) 
 
 8.07 a. m. Changes speed to 18 revolutions per minute. 
 
 11.00 a. m. (Temperature 1,300 C.) Shut off gas and air and lowered stack damper. 
 11.30 a. m. (Temperature 1,250 C.) Took off vertical motion (A fig. 35). 
 12.30 p. m. (Temperature 1,125 C.) Reduced radius of stirring circle to 5J inches. 
 
 (NOTE. Might have been kept a little larger (B fig. 35). 
 
 1.05 p. m. (Temperature 1,080C.) Speed reduced to 13 revolutions per minute. 
 (C fig. 35). 
 
 2 < The technique of optical glass melting, Jour. Am. Ceram. Soc., 2, 133-138. 1919. 
 
REMOVAL OF POT. 129 
 
 1.27 p. m. Radius of stirring circle reduced to 4 inches (D fig. 35). 
 1.45 p. m. (Temperature 1,030C.) Radius of stirring circle reduced to 3^ inches 
 (E fig. 35). 
 
 2.01 p. m. (Temperature 1,013 C.) Radius of stirring circle reduced slightly 
 
 (F fig. 35). 
 2.13 p. m. (Temperature 1,003 C.) Radius of stirring circle reduced to H inches 
 
 (G fig. 35). 
 
 2.35p.m. (Temperature 975 C.) Stirring stopped. 
 2.40 p. m. Pot out. (NOTE. A temperature of 950 C would have been preferable 
 
 for this type of glass. 
 
 LIGHT CROWN (71 D = 1.516 J/=60) . 
 
 Pot 25 inches inside diameter at bottom, 27 inches deep. 
 
 8.03 a. m. Stirring machine started at 13 revolutions per minute. No vertics. 
 motion. Radius of stirring circle 6^ inches. 
 
 8.25 a. m. Changed speed to 18 revolutions per minute. 
 
 8.55 a. m. Changed speed to 13 revolutions per minute. 
 
 9.50 a. m. Reduced radius of stirring circle to about 5 inches. (NOTE. A proof 
 taken just before this had shown numerous bubbles; speed of travel was 
 reduced to avoid danger of stirring air into the metal.) 
 
 2.35 p. m. Shut off gas, air, etc. 
 
 3.02 p. m. Temperature 1,259 C. 
 
 3.25 p. m. Radius of stirring circle reduced to 4 inches (A'' fig. 35). 
 
 3.35 p. m. Temperature 1,197 C. 
 
 3.45 p. m. Temperature 1,182 C. 
 
 3.53 p. m. (Temperature 1,168 C.) Radius of stirring circle 2^ inches (B' fig. 35). 
 
 4.02 p. m. Temperature, 1,156 C. 
 
 4.13 p. m. (Temperature 1,138 C.) Stirring stopped (C' fig. 35). 
 
 4.18 p. m. Pot out. 
 
 Fenner finds that for the borosilicate crown (analysis 10, Table 4) 
 and for the barium flint (analysis 101, Table 4) the best temperatures 
 at which to remove the pot from the furnace are 1,050 C and 975 C, 
 respectively. In the ordinary flint series the higher the lead con- 
 tent of a glass, the lower the temperature at which it is advisable to 
 remove the pot of molten glass. 
 
 As soon as the optimum temperature for the removal of the pot 
 has been reached, a definite procedure is followed in order to get the 
 pot out of the furnace as quickly as possible; each man is given defi- 
 nite duties to perform and does these day after day. The stirrer is 
 stopped; the clay stirring tube is brought slowly by means of the 
 screw feed of the stirring machine to the side of the pot, where it is 
 grasped and held in position by a forked tool while the water-cooled 
 stirring rod is disengaged. The stirring rod is removed, the stirring 
 machine pushed out of the way, the clay-stirring tube hooked against 
 the side of the pot in an upright position by a heavy U-shaped iron 
 rod, one end of which is inserted into the tube opening while the other 
 end hangs down over the outer edge of the pot. An alternative 
 method is to withdraw the stirring tube entirely from the melt by 
 lifting it with the forked tool very slowly until it slips over the rim of 
 
130 MANUFACTURE OF OPTICAL GLASS. 
 
 the pot. The tuille is now raised about a foot; the pot is commonly 
 so tightly stuck to the siege that it has to be pried loose from it by 
 means of a heavy iron bar acting as a lever on a low iron-block ful- 
 crum placed on the siege in front of the pot; in this operation care is 
 taken not to jerk or tip the pot violently, but rather to raise it slowly. 
 The tuille is now raised high, the pot wagon is wheeled into place, 
 the pot is grasped by the* tongs or tine of the pot wagon, the counter- 
 weights are slid along the pot-wagon arm until the pot is practically 
 counterbalanced. The pot is lifted gently and without jerks and 
 jarring from the siege and wheeled out (fig. 36) on the floor of the 
 furnace hall where it is placed on a support of fire-clay blocks. (Fig. 
 37.) 
 
 FIG. 36. Removal of pot of molten glass from the melting furnace. Note the "whiskers " 
 on the bottom of the pot; these are from the siege (floor) of the furuace to which the pot 
 was stuck. (Photograph by J. Harper Snapp at the plant of the Bausch & Lomb Opti- 
 cal Co.) 
 
 The siege of the furnace is now scraped and leveled preparatory to the 
 introduction of a fresh pot which is removed from a pot arch where it 
 has been gradually heating for several days. (Fig. 32.) The new 
 pot is removed from the pot arch by the, pot wagon and placed in 
 correct position in the melting furnace; the furnace tuille is lowered 
 and sealed and the gas is turned on preparatory to baking the new 
 pot at a very high temperature. 
 
 In the meantime the pot of molten glass has been cooling down in 
 the open air. (Fig. 37.) Although the molten glass has not changed 
 its appearance noticeably and is apparently still red hot, a hard crust 
 has formed on the surface which may show an incipient shrinkage 
 crack. It is moved either into the empty hot pot arch from which 
 the fresh pot was removed or it is covered with an insulating 
 cap which retards the rate of cooling, so that when cooled to room 
 temperature the glass is fissured properly and is not highly strained. 
 
SCHEDULE OF FURNACE OPERATIONS. 
 
 131 
 
 This part of the glass-melting process, like the pouring of glass and 
 rolling the molten mass into long sheets in the plate-glass industry, 
 is the most spectacular part of the manufacturing process. The 
 red hot, fuming pot of molten glass radiating such in tense heat that the 
 novice is fearful to approach very near to it creates an impression 
 which is not soon forgotten. 
 
 SCHEDULE OF FURNACE OPERATIONS. 
 
 In the factory production of optical glass a definite schedule of 
 the operations and temperatures is followed for each type of glass; 
 
 FIG. 37. Pot of molten glass cooling down after removal from melting furnace 
 and before insertion into cooling arch or being covered by insulating cap. 
 Note the marks left by the molten glass which has spilled over the pot 
 during the hand-stirring operations. (Photograph by J. Harper Snapp at 
 the plant of the Spencer Lens Co.) 
 
 the schedules are different for different types of glass and a detailed 
 record is kept of the treatment accorded each melting pot and each 
 glass batch from the time it entered the furnace hall as raw batch 
 to that of its delivery as raw glass to the inspection room. The 
 schedule of furnace operations depends somewhat on the size of the 
 glass plant and the attitude of the manager. Ordinarily it is con- 
 venient under present-day labor conditions to arrange the schedule 
 so that the pots are removed from the furnaces during the afternoon of 
 each day; this means either a 48-hour or a 24-hour schedule. If the 
 plant is a large one and labor is always at hand, an intermediate 
 schedule of 27, 30, or 36 hours may be followed to advantage. 
 
132 
 
 MANUFACTURE OY OPTICAL GLASS. 
 
 In figure 38 five different schedules are presented in graphical form. 
 In this figure the curves are not superimposed and referred to the 
 same zero ordinate; but for the sake of clearness, the temperature 
 scale for each curve is shifted 100 C. (one scale unit) above the curve 
 next below it. Good glass can be, and has been, produced by each 
 schedule; in fact, considerable leeway is permissible in the furnace 
 schedule, providing certain fundamental principles are not violated. 
 
 The curves of figure 38 are self-explanatory and are taken to illus- 
 trate different types of practice at the different plants where different 
 
 1500 
 
 1000 
 
 40 44 4& 
 
 FIG. 38. Curves showing different melting-furnace schedules for a medium flint glass as followed at differ- 
 ent times at different plants. In this figure the plan has been followed of separating each curve from the 
 next lower curve by one scale interval of 100 C. instead of superimposing the five curves. For this reason 
 ihe temperature 1,400 C. is repeated five times on the vertical scale. 
 
 labor and furnace conditions prevailed. The curves represent the 
 schedule for a flint glass of refractive index about 1.61 and contain- 
 ing about 45 per cent lead oxide (PbO) . 
 
 The 24-hour schedule of melting optical glass. During the latter 
 months of the war, from September, 1918, a shortened glass- 
 melting schedule was introduced by Dr. G. W. Morey, of the Geo- 
 physical Laboratory, at the plant of the Spencer Lens Co. This sched- 
 ule is based on certain logical improvements in glass-melting practice, 
 and is described here in a special section in order that the principles 
 on which it is based may receive adequate emphasis. 
 
SCHEDULE OF FURNACE OPERATIONS. 133 
 
 Melts in open pots are heated chiefly by radiation from the crown 
 of the furnace; the raw batch is melted accordingly, from the top 
 downward; the easily fusible materials, such as the alkalies and lead 
 oxide, melt first and trickle downward, leaving the sand and less 
 fusible materials to sinter together in the upper layers and thus im- 
 peding their rapid solution. The result is an accumulation of 
 extremely active chemicals, as alkali carbonates and nitrates, and of 
 heavy fluxes, as lead oxide, on the bottom and lower sides of the 
 pot which are thereby energetically attacked. The melting of the 
 highly siliceous and viscous upper layers becomes under these condi- 
 tions a slow and hampered process. In spite of the care taken in the 
 batch room to mix the batch thoroughly, inhomogeneity is thus 
 introduced, by differential melting, at the outset of the melting process. 
 These differences in composition between top and bottom of the 
 melt are, of course, eliminated later by stirring, but they can be 
 avoided to a large extent by stirring the melt during the latter half 
 of the pot-filling period when there is enough material in the pot to 
 support the stirrer in an upright position during hand stirring. The 
 melt should be stirred after each fill in order to insure uniform and 
 rapid distribution of the fresh raw batch through the melt, and thus 
 to expedite solution and to prevent segregation of the fluxes by 
 gravitative differentiation. This is the first change in procedure 
 adopted by Morey; it increases the rate of solution of the batch arid 
 lessens to a marked degree the inhomogeneity arising from differen- 
 tial melting and gravitational settling of' the readily fusible materials. 
 Incidentally some hours are saved by this procedure and the melting 
 pot is attacked less than under the old schedule. 
 
 The second improvement applies to the "fining" of the glass and 
 seeks to accelerate the escape of bubbles and seeds from the melt by 
 continuous stirring during the fining period. This is best accom- 
 plished by machine stirring with a combined circular or spiral and an 
 up-and-down movement. The bubbles result from the decomposi- 
 tion especially of the alkali-carbonates and nitrates. It is common 
 practice to hold the melt at a high temperature during the fining 
 period to increase its fluidity and thus to facilitate the rise of the es- 
 caping bubbles to the surface. In the plate-glass industry blocking 
 is used near the end of the fining period to aid in washing out the small 
 bubbles in the melt. In optical glass manufacture the usual pro- 
 cedure is to employ intermittent hand stirring with subsequent rest 
 periods during which the melt attains a high and fairly uniform tem- 
 perature throughout. The attainment of adequately high and uni- 
 form temperatures is extremely difficult with continuous stirring. 
 
 The 24-hour schedule requires for its successful application highly 
 efficient furnaces and gas of good heating quality; the preliminary 
 baking of the pot is best done in a pot arch specially constructed for 
 the attainment of high temperatures. 
 
134 MANUFACTURE OF OPTICAL GLASS. 
 
 With the shortened period of exposure of the pot to the furnace 
 temperatures, there is less pot solution and hence greater transpar- 
 ency and freedom from color of the glass and also less chance for the 
 presence of pot stones. The new schedule represents an appreciable 
 saving in time and operating costs and also a marked increase in the 
 rate of production per melting furnace. 
 
 The following schedule for the melting of a medium flint of n D = 
 1.617 and v = 36.5 is given by Morey 25 as an illustration. In this 
 schedule the time is recorded only from that of the first cullet fill. 
 Slightly different schedules are followed for other types of glasses. 
 
 Schedule for M F 2 glass. 
 Hours. 
 
 0.00 Add cullet (1,390 C.). 
 
 1.00 Fill pot three-quarters full of batch. 
 
 2.30 Fill pot with batch. 
 
 4.00 Hand stir, fill pot with batch! 
 
 5.30 Hand stir, fill pot with batch. 
 
 7.00 Hand stir, fill pot with batch. 
 
 7.30 Stirring machine on. 
 
 15.00 Gas off. 
 
 In the practical application of the schedule for this and other types 
 of glass strict attention to details must be given. The schedules are 
 closely timed and the furnaces must be kept in the best running 
 condition 
 
 EXPERIMENTS WITH STIRRED AND UNSTIRRED POTS OF OPTICAL GLASS. 
 
 The following experiments are of interest because of their bearing 
 on the general functions of stirring in optical glass. (Experiments 
 described in Report No. 4 for the week ending May 26, 1917.) Four 
 small open pots were filled each with a light flint batch of the following 
 composition in kilograms: Sand, 3.000; red lead, 1.920; potassium 
 carbonate, 0.783; potassium nitrate, 0.237. These pots were held 
 for different periods of time at about 1,400 C. The first pot was not 
 stirred and was removed after exposure to this temperature for 8 
 hours; the second pot, also not stirred, was removed after 12 hours; 
 the third pot was removed after 15J hours, including a stirring period 
 of 1J hours; the fourth was removed after 19 hours, including a 2-hour 
 stirring period. The glass produced by this treatment in small pots 
 is of course valueless optically. It abounds in bubbles, is not highly 
 transparent, and is only fairly wiiite in color. Especially interesting 
 are the two unstirred pots. The first pot taken out after 8 hours 
 melting is full of small bubbles (seeds) and stones. Each stone and 
 some of the bubbles are seen to have left, in their upward passage 
 through the glass, a tail such that in the aggregate they resemble a 
 
 *> Jour. Am. Ceram. Soc. 2, 160, 1919. 
 
RESULTS ATTAINED BY STIRRING. 
 
 135 
 
 company of tadpoles marching in parallel columns toward the upper 
 surface. The stones are evidently silica (sand) in process of assim- 
 ilation. (Fig. 39.) 2C 
 
 Geologically this phenomenon is interesting because it presents the 
 phenomenon of gravity differentiation in place, thus giving rise to a 
 separation into a top layer rich in silica and poor in lead (about 20 
 per cent lead) and a bottom layer rich in lead (about 46 per cent lead) . 
 The distribution of the layers, as determined by refractive index 
 measurements, is shown in figures 40er to d. A comparison of pots 1 
 and 2 (fig. 406) shows that the sharp gradations from the top surface 
 to the central portion and also from the bottom layer up to the center, 
 
 'H, ;,; v-n, fflfffMfS 
 Jmjjijf 
 '''^wmiff 
 
 w 
 
 FIG. 39. Photograph of sand grains in process of solution in a small melt of optical glass. 
 
 as represented graphically in figure 40e, are less pronounced in pot 2. 
 In other words, diffusion in the course of four hours additional heat- 
 ing has tended to reduce the sharp differences in concentration in the 
 melt. Convection probably aided to a certain extent; but the dis- 
 tribution of the material in the pot does not indicate pronounced con- 
 vection currents. 
 
 In the stirred pots the distribution of uniform glass is remarkable. 
 The stirring was poorly done; the stirrer scraped the bottom and 
 sides repeatedly; and yet, except for a lower refracting surface film 
 not over 1 millimeter thick, there is little variation in composition 
 from top to bottom or from side to side. The persistence of the lower 
 refracting surface film proves that there is appreciable volatilization 
 
 2 6 Experiments carried out by N. L. Bowen and the writer. See N. L. Bowen. Jour. Wash. Acad. 
 Sci. 8, 88-93, 1918. 
 
136 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 of lead and alkalies and that the upper layer is richer in silica. The 
 unstirred pot of glass is not unlike a layer cake in its refractive index 
 distribution. The surfaces of equal refractive index are approxi- 
 mately planes parallel with the upper surface. This being the case 
 
 563 
 
 0-1.565 
 
 /I.5Z8 
 J 
 
 1.560-0 
 0-1565 1.565^ 
 
 1565- 
 
 1568^ ^/|.567 I' 
 ^L5aO L600\ ^1.560 </' 
 
 fY-H.618 rrH.615 \ j^-lieOJ I.; 
 
 
 ^r.535,l.530xL535 
 
 0-1.565 
 o-t.567 
 
 ^1.572 
 2x1-579 
 
 ^>l.59g 
 
 0-1.555 f> 
 crl-560 1.560 
 
 N.565 \\ 
 
 o-J.568 
 
 0-1.565 j.566 
 oH.573 0-L575 
 cr\.593 rr-1.596 l.603y. 
 
 DISTANCE FROM BOTTOM 
 ^coSSSSScJl 
 
 I 
 
 ' 
 
 -=^ 
 
 ^X 
 
 
 
 
 
 
 
 
 
 
 
 ^\\ 
 
 
 
 
 
 
 
 
 
 
 |1 
 1 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \v 
 \\ 
 
 
 
 
 
 
 
 
 
 
 X: 
 
 ^_ 
 
 
 
 
 
 520 .. 
 
 J30 1.5- 
 
 <M 1.5 
 
 50 1.! 
 
 560 1.! 
 
 '70 L 
 
 580 1. 
 
 $90 Hi. 
 
 ~ 
 500 1. 
 
 _ I 
 BIO 1.6 
 
 REFRACTIVE INDEX 
 
 FIG. 40. Diagram illustrating the changes in refractive index, n D , in small pot melts of a light flint batch- 
 Pot 1 was heated at 1,400 C. for 8 hours, but was not stirred; pot 2 was heated at 1,400 C. for 12 hours, 
 also not stirred; pot 31 was heated at 1,400 C. for 15 hours, including a stirring period of 1% hours; pot 4 
 was heated at 1,400 C. for 19 hours, including a 2-hour stirring period. The curves I and II illustrate 
 the changes in refractive index, n D , from top to bottom of the melts of pots 1 and 2. 
 
 it appears that an energetic up-and-down stirring during the first part 
 of the melting process is essential in order to render the mass homo- 
 geneous. The bottom of the pot should be thoroughly swept by cur- 
 rents of the molten glass during this period. 
 
COOLING OF THE MELT. 
 
 137 
 
 An illustration (weekly report No. 6, June 9, 1917) of the degree 
 of uniformity attained in a large pot (No. 472) of light flint 
 (tt D = 1.579, *> = 41.0, PbO = 36.5 percent) after two 1 5-minute periods 
 of hand stirring during each two hours is given in figure 41. After 
 these periods of stirring the pot. began to leak and was removed from 
 the furnace and allowed to cool in the open air. Samples were taken 
 from the different parts of this pot of glass and were measured by the 
 immersion method. It is remarkable that so short a period of stirring 
 should result in so thorough mixing of the melt. Except for the top- 
 film layer which is always lower in refractivity and the sides and 
 bottom which were contaminated with dissolved pot, the refractive 
 indices do not vary more than one or two in the third decimal place. 
 
 1.562 
 1.567 
 A57S 
 
 1*0- \te~* 
 
 1578 \580 
 
 578 01578 
 
 1.577 
 
 1.575 
 578 
 
 ,. 
 
 LA 
 
 I.575 
 
 0-1.579 
 
 1.576-c 
 
 80^1.560 
 
 -V573 
 
 0-1.580 
 
 ^1580 1.580^5-1.560 1580^ 
 
 0-I.58O 1^75o 
 
 FIG. 41. Diagram illustrating changes in refractive index, n D , at different parts of a pot of light flint glass 
 after two hand-stirring periods of 15 minutes each Pot No. 472, Bausch & Lomb Optical Co. 
 
 In view of the fact that fine striae are caused by only slight differences 
 in refractive index, these measurements indicate that the function of 
 long-continued stirring is chiefly to remove the slight differences in 
 composition which are still present and are constantly arising because 
 of volatilization and of pot solution, which diffusion smooths out 
 only slowly. 
 
 THE COOLING OF THE MELT. 
 
 If the operations up to this point have been successful, the pot of 
 molten glass, on removal from the furnace, is sensibly homogeneous 
 except for the peripheral portions where, as a result of selective 
 volatilization at the surface and of pot solution along the sides and 
 bottom, the melt contains more silica (and possibly alumina from the 
 pot) and is more viscous. The temperature of the melt is, moreover, 
 fairly uniform except for the bottom in contact with the thick, over- 
 
138 MANUFACTURE OF OPTICAL GLASS. 
 
 heated base of the pot which has had little chance to cool except by 
 radiation through the melt. Measurements of the temperature of 
 the furnace floor (siege) immediately after removal of the pot show 
 that its temperature, and hence that of the base of the pot, is from 50 
 to 100 C. higher than that of the metal in the pot. There is, of 
 course, a slight temperature difference between the center and margins 
 of the melt as a result of its continued cooling. The molten glass 
 has now the consistency of thick heavy syrup ; and with further fall 
 in temperature its viscosity increases rapidly. 
 
 The problem which confronts the glassmaker at this stage is to 
 avoid any tendency which may cause the marginal portions to stream 
 into the body of the melt; also to avoid crystallization phenomena 
 and the formation of bubbles. A lesson learned from experience 
 and not adequately realized by many glassmakers is the fact that 
 during the time interval between the cessation of stirring and the 
 cooling of the melt to 600 C. (temperature fall from 1,000 to 600 
 C. approximately) much glass is needlessly lost. Were it possible to 
 maintain, through the temperature drop to 600 C., the degree of 
 homogeneity attained in the melt at the end of the stirring period 
 much more glass would be saved than is at present the case. An 
 analysis of the several factors involved will render this clear and sug- 
 gest the precautions to be taken. 
 
 Convection currents. On removal of the pot from the furnace the 
 melt has the consistency of thick, heavy syrup. With further fall 
 in temperature its viscosity increases so rapidly that any appreciable 
 difference in temperature between different parts of the melt produces 
 a distinct difference in density. Convection currents are set up as a 
 result of the downward flow of cooler and denser portions of the melt, 
 and are serious because they generate heavy striae, cords, and ribbons 
 in the melt, thereby rendering much of it useless. The fact, moreover, 
 of- excess heat at the base of the pot favors an upward trend of the 
 bottom layers of the melt, and their stream lines may pass into the 
 central core of the melt and thus cause stria?. 
 
 In a viscous melt the rate of transfer of material by convection 
 is relatively slow and decreases with fall in temperature and conse- 
 quent rise in viscosity. The obvious method to reduce convection 
 currents is to cool rapidly and uniformly from 1,000 to 600 C. 
 from the margins of the melt toward the center. The bottom of the 
 pot is, however, much hotter and thicker than the sides; it acts 
 somewhat as a heat reservoir and cools less slowly than the sides 
 or the surface of the melt. An effort should be made to expedite 
 the cooling of the base by allowing a free or forced circulation of air 
 to play around it. The surface of the melt, on the other hand, chills 
 more rapidly in direct contact with the air than either the sides or the 
 bottom; a tendency is thereby set up for its cooled, and hence denser, 
 
CRYSTALLIZATION PHENOMENA. 139 
 
 material to sink toward the bottom and thus to introduce striae. 
 Its rate of cooling should accordingly be retarded by some method 
 of heat insulation, such as covering it with a layer several inches 
 (commonly 4) thick of light insulating material, as diatomaceous 
 earth. 27 
 
 Fine siliceous powder of this nature shows no tendency to sink 
 into the melt and insulates most effectively. Much good glass has 
 been produced, it is true, without the use .of an insulating surface 
 layer; but in this case the pot with melt is allowed to remain in the 
 open air for a short period only, 15 to 30 minutes, and is then inserted 
 into a previously heated pot arch from which the empty pot was taken 
 to replace the finished pot removed from the melting furnace. The 
 temperature of the heated air in the pot arch is high; hence the tend- 
 ency for the surface to cool with extreme rapidity is practically 
 annulled. 
 
 Vacuum bubbles. There is still another reason for retarding the 
 rate of cooling of the surface of the melt. Glass, on cooling, shrinks, 
 and at low temperatures its viscosity becomes so great that it behaves 
 practically as an elastic solid. If the surface of the melt cools 
 rapidly in the open air, a hard crust forms in the course of half an 
 hour while the center remains nearly as hot as it was when it left the 
 furnace. The surface contracts on cooling, and cracks may begin 
 to form. On further cooling the center tends to draw away from the 
 unyielding crust. If the tensional stresses are not then relieved by 
 cracks and fissures, both horizontal and vertical, large bubbles, 
 called vacuum bubbles, may form and ruin an appreciable quantity 
 of glass. This phenomenon is not so common in large pot melts 
 because in them cracks generally do develop and resemble then in all 
 details the jointing phenomena of lava flows. In small experimental 
 pot melts cracks are not so likely to form and in them vacuum 
 bubbles are of common occurrence. The bubbles can be avoided either 
 by insulating the surface of the melt with a layer of diatomaceous 
 earth or by breaking through the surface crust with a pointed iron 
 rod, thus puncturing the seal established by the crust. 
 
 Crystallization phenomena. The phenomena of crystallization 
 in optical glasses are so important and so unwelcome to the glassmaker 
 that a practical understanding of the principles involved is essential 
 if crystallization or other precipitation is to be avoided. At high 
 temperatures optical glass is a mobile liquid; with rise in temperature 
 its fluidity increases and it behaves in all respects like an ordinary 
 liquid or like molten metal; with fall in temperature its viscosity 
 
 This method was first applied by the Geophysical Laboratory and is described in detail by H. S. Roberts 
 in " The cooling of optical glass melts." Jour. Amer. Ceram. Soc., 2, 543-563, 1919. 
 
 K These phenomena are discussed in detail by N. L. Bowen in "Devitrification of glass," Jour. Amer. 
 Ceram. Soc., 2, 261-281, 1919. 
 
140 MANUFACTURE OF OPTICAL GLASS. 
 
 increases so rapidly (doubling for each drop of 8 to 10 C. in tem- 
 perature) that at room temperatures its viscosity is nearly infinite 
 and it behaves practically as an elastic solid. 
 
 Glass has no definite melting temperature, but rather a temperature 
 range over which it softens rapidly and becomes a fluid in the ordinary 
 sense of the word. At all temperatures glass is a solution; at high 
 temperatures a mobile or fluent solution, at low temperatures an 
 immobile solution. As a solution glass is subject to the general laws 
 of solutions. A solution such as sugar and water is able at a given 
 temperature to dissolve a certain quantity of sugar; if now the tem- 
 perature be raised, the solution is found capable of dissolving still 
 more sugar. The saturation limit rises, in this case, with rise in 
 temperature; a solution of a given composition, saturated with respect 
 to a given substance at a high temperature, may be greatly super- 
 saturated with it at a lower temperature. The solution is then not 
 in equilibrium and seeks to attain equilibrium by the precipitation 
 of a certain amount of the phase which is present in excess. 
 
 Experience with silicate melts of the general type of glass melts 
 has shown that the silicate components are miscible in all proportions 
 and that, on cooling from a high temperature, the solution becomes 
 supersaturated, in general, first with respect to one phase, then to 
 two, and so on. As soon as the saturation limit of any one phase is 
 reached a tendency is set up for this phase to crystallize out. There 
 are, however, certain factors which tend to counteract this tendency. 
 Crystallization in each case means the orderly arrangement of atoms 
 or molecules in space; the rate of building up of each crystal struc- 
 ture depends on a number of factors, such as degree of supersatura- 
 tion of the phase in the solution, the viscosity of the solution (function 
 of composition and temperature), rate of transfer of material in the 
 solution, crystallizing ability of the crystal phase, etc. The molecules 
 in the solution must wander (diffuse) to the crystal nucleus or grow- 
 ing crystal and this takes time; if the viscosity of the solution at this 
 temperature is high, the rate of transfer of the molecules is slow; 
 furthermore the tendency toward crystallization is nil above the 
 temperature at which the saturation limit is reached ; not far below 
 it, the crystallizing tendency increases rapidly, reaches a maximum, 
 and then, because of the greatly increased viscosity, grows less and 
 finally practically disappears when the glass becomes hyperviscous. 
 The power of crystallization of different substances varies greatly. 
 The usual measure for the power of crystallization at a given tem- 
 perature is the number of crystal nuclei formed in unit time in unit 
 volume. A substance of high crystallizing power can not be cooled 
 much below its saturation limit before crystallization sets in; one of 
 low crystallizing power is readily undercooled and may only with diffi- 
 culty be made to crystallize even under the most favorable conditions. 
 
CRYSTALLIZATION PHENOMENA. 
 
 141 
 
 The primary object in optical glass manufacture is to obtain 
 homogeneous, colorless glasses of definite optical constants. In 
 seeking to attain these ends batch-compositions may be tried out 
 which are greatly supersaturated with respect to one of the phases, 
 and this, on the cooling of the melt, crystallizes out and ruins the 
 product. 
 
 In setting up trial batches for glasses of a given composition, it is 
 advisable in each case to make small trial melts and to hold these at 
 different temperatures between 800 and 1,100 C. in order to ascer- 
 tain their crystallizing tendencies. In the case of the crystallization 
 
 FIG. 42. Fracture section across a pot of light flint glass. Pot No. 594, B. & L. White rim around edge 
 of glass consists of sillimanite. Pot shows little evidence of attack by glass melt. 
 
 of one or more phases these can be determined by petrographic 
 microscope methods. 
 
 Ordinarily the primary phase to appear is silica (in the form of 
 tridymite or cristobalite) , or calcium metasilicate (as wollastonite) . 
 In glasses very high in lead, lead metasilicate may be precipitated- in 
 glasses high in barium, barium disilicate has been observed to crys- 
 tallize in the form of skeleton crystals hexagonal in shape. Near 
 the margins of the glass melt, adjacent to the clay pot walls, a thin 
 white layer is not uncommon (fig. 42) ; it consists generally of an 
 interlacing aggregate of crystallized aluminium metasilicate (needles 
 of sillimanite) . 
 
 3922921 10 
 
142 MANUFACTURE OF OETICAL GLASS. 
 
 In case crystallization (devitrification) of the glass melt occurs 
 within the time-temperature limits of the glassmaking schedule, 
 either the amount of the excess phase must be reduced in the batch 
 or the viscosity of the melt should be changed by the addition of a 
 small amount of alumina (rarely magnesia) or by the substitution 
 of potassium for sodium, or a smaller melting pot may be taken in 
 order to expedite the rate of cooling of the melt. The fact that a 
 melt can be successfully made in a 36-inch pot, holding half a ton 
 of glass, does not signify that the same batch composition will be 
 satisfactory in a 49-inch pot, holding a ton of glass. 
 
 The time factor has a most important bearing on the crystalliza- 
 tion of glass melts. Molten silicates have relatively large heat 
 capacities and are poor conductors of heat; large masses can not 
 therefore be cooled at a rapid rate even under the most favorable 
 conditions. This means that the larger the pot, the more care must 
 be taken to avoid crystallization. The experimental melts made for 
 the purpose of testing out a batch composition should be held at 
 given temperatures for lengths of time corresponding to those actu- 
 ally obtaininig in the glass pots used. 
 
 The types of crystallization which develop in the melt depend not 
 only on the kind of substance which is precipitated, but also on the 
 temperature and the composition of the melt. Thus single isolated 
 crystals of barium-disilicate were formed 29 toward the end of the 
 stirring period (1,100 C.) as crystal skeletons in a melt of light 
 barium crown. These were avoided in later melts by reducing the 
 percentage amount of barium oxide in the batch to the extent that 
 the temperature at which the melt became supersaturated with re- 
 spect to barium disilicate was lowered below that of the final stirring 
 period. The amount of reduction was computed, after the melting 
 temperature of pure barium disilicate had been found by measure- 
 ment to be 1,426 C., by assuming, as a first approximation, that the 
 lowering of the saturation temperature was directly proportional to 
 the amount of barium disilicate present. As the required change of 
 composition was only slight and sufficient to lower the saturation 
 limit from 1,100 to about 1,030 C. this assumption was justified. 
 Thus the saturation limit was reduced from 1,426 (pure barium 
 disilicate) to 1,100 C. by a reduction of barium disilicate from 100 per 
 cent to 57 per cent. On the assumption of a linear relation between 
 composition and temperature at the saturation limit, this signifies a 
 lowering of about 7 C. in the temperature of saturation per reduction 
 of 1 per cent barium disilicate. But since all saturation-temperature 
 curves which have been determined in silicate melts are concave 
 toward the origin in a temperature-concentration diagram, the 
 gradient of the curve is likely to be somewhat steeper than a straight 
 
 N. L. Bowen. ^Jour. Wash. Acad. Sci., 8, 265-268, 1918. 
 
CRYSTALLIZATION' PHENOMENA. 143 
 
 line so far away from the pure compound with the result that a 
 reduction of 1 per cent in barium disilicate would probably lower this 
 saturation limit more than 7 and possibly as much as 15 C. Actual 
 test showed that a reduction of 5 per cent in the amount of barium 
 disilicate present in the batch eliminated the presence of its crystals 
 from the melt under the given conditions of melting. Similar 
 methods for the adjustment of batch composition are followed with 
 melts of different compositions in which crystallization may appear. 
 
 In ordinary types of optical glass silica is present in excess with the 
 result that it is first to crystallize out from the melt usually in the 
 form of radial spherulites of cristobalite or tridymite. Crystalliza- 
 tion in pots of optical glass begins ordinarily at the top surface and 
 sides and proceeds inwards. The surface of the melt, because of 
 volatilization, becomes richer in silica which is then the primary 
 phase to crystallize out. The surface of a cooled pot of glass, such as 
 borosilicate crown or barium crown, is commonly covered with fine, 
 exceedingly thin crystallites, visible only under a hand-lens and 
 resembling hexagonal snowflakes. The surface, if examined closely, 
 is seen to be covered with a hexagonal network of lines which are 
 obviously the directions of tenuous crystal growth in the thin sur- 
 face film. 
 
 If the cooling of the glass pot is not conducted with sufficient 
 rapidity the crystallites in the surface film extend inward into the 
 glass mass; white radial spherulites are formed. In the case of 
 borosilicate crown, a crust of radial spherulites of crystallized silica 
 1 to 2 millimeters thick, occurs almost invariably around the margins 
 of the surface of the melt. Crystallization of this kind is not serious 
 from the glassmaker's standpoint because of the relatively small 
 amount of glass wasted. In the case of the molding of glass, surface 
 crystallization may be serious because the crystallized crust is much 
 harder than the glass itself and offers serious resistance to the grind- 
 ing wheels during the plate-grinding operations, so serious in fact that 
 every effort should be made to regulate the temperature in the 
 molding kilns so that crystalline crusts are not formed on plates of 
 borosilicate or barium crowns. In the flint glasses the danger from 
 surface crystallization is much less. 
 
 Other substances which may crystallize out of certain melts are 
 calcium metasilicate (in the form of wollastonite) , lead metasilicate, 
 and aluminum metasilicate as sillimanite. The first two occur com- 
 monly as radial spherulites, like rounded pellets up to 1 centimeter 
 in diameter throughout the melt. Because of the differences in their 
 rates of contraction as compared with that of the enveloping glass, 
 a large amount of strain is set up in the glass; conical cracks may 
 develop and extend for a short distance from the radial spherulite into 
 the glass. The presence of a crystallization body in a lens or prism 
 is sufficient cause for its rejection. 
 
144 MANUFACTURE OF OPTICAL GLASS. 
 
 Cloudiness or opalescence. Still another phenomenon, allied to 
 precipitation, may arise during the cooling process and ruin the entire 
 pot of glass. In certain types of glass, especially in the flint series, 
 there is a tendency for the melt to turn milky or cloudy during the 
 cooling-down period. Turbidity of this kind in optical glass is a very 
 serious defect and renders it useless for optical purposes. The cloud- 
 iness ordinarily develops at the sides and top of a crucible of glass and 
 proceeds inward from the margins. The factors involved in this 
 problem are not entirely clear, but the following facts are significant: 
 Proofs, taken at high temperatures, of a melt which later becomes 
 turbid, are perfectly clear and show no trace of milkiness; these proofs 
 may develop cloudiness, however, on reheating to temperatures some- 
 what above the softening point of the glass (800 to 1,000 C.). If 
 held for long periods of time (several days) at this temperature, 
 crystallites of a low refracting substance, possibly silica in the form 
 of tridymite or cristobalite, develop. The presence of these crystal- 
 lites does not definitely prove that the substance which causes the 
 milkiness is excess silica. In the milky glass the precipitated material 
 is held in suspension and the particles, whatever their nature may be, 
 whether silica, lead sulphate, lead chloride, boron silicate, or arsenic 
 oxide, may serve as nuclei around which the radial spherulites of 
 crystallized silica cluster when the glass is maintained at 900 to 
 1 ,000 C. for a long period of time. A thin plate of opalescent light flint 
 examined under the ultra microscope showed the presence of innu- 
 merable particles suspended in the glass. The phenomenon is there- 
 fore one of precipitation either of colloidal particles or of submicro- 
 scopic crystallites. A chemical analysis of a fragment of milky glass 
 showed the presence of 0.146 per cent SO 3 ; this is equivalent to 0.553 
 per cent lead sulphate. 30 
 
 Factory experience proves that the presence of sulphates and 
 chlorides in the raw materials, especially in the potassium carbonate, 
 favors the formation of opalescence. In England 31 the same trouble 
 with cloudiness in flint glass for tableware purposes was experienced 
 during the war and was ascribed to the presence of sulphates and chlor- 
 ides in the poorer grade of available potassium carbonate. Experience 
 has proved that a slight change in the composition of the batch may 
 greatly decrease the probability of the occurrence of milkiness; thus 
 light flint containing 2 per cent boron oxide is especially liable to turn 
 milky; it is possible that the presence of this oxide favors the develop- 
 ment of cloudiness in this flint. It has been found that fining at a 
 high temperature (increased volatilization of certain components), 
 thorough stirring, and rapid cooling of the melt thorough the tempera- 
 
 so See article by Fenner and Ferguson "On the effect of certain impurities in causing milkiness in 
 optical glass." Jour. Am. Ceram. Soc., I, 468, 1918. 
 i Cauwood and Turner, Jour. Soc. Glass Technology, I, 187, 1917, 
 
OPALESCENT GLASS. 145 
 
 ture range, in which precipitation is liable to occur, are advisable. 
 The addition of 1 or 2 per cent of alumina to the glass aids as a pre- 
 ventative; this oxide tends to increase the viscosity of many glass 
 melts and its presence then necessarily raises their fining temperatures. 
 Long-continued heating (24 hours), 32 at 950 C. in a platinum resist- 
 ance furnace, of a light flint glass containing 2 per cent alumina which 
 was fined at the usual fining temperature of the light flints, proved that 
 glass of this composition did not become milky; whereas the same 
 light flint without the addition of alumina did become milky under 
 the same treatment, thus proving that alumina tends to hinder to some 
 extent the milky precipitation. Observations have proved that by 
 reheating milky flint to a temperature of 1,100 C. it can be rendered 
 clear, but that under these conditions bubbles develop and render the 
 glass useless. 
 
 In this problem of milky glass we are confronted with the pre- 
 cipitation of some substance possibly colloidal in nature; it is 
 probable that the opalescent effect in the light flints may be produced 
 by different substances. The presence of sulphates and chlorides in 
 the batch favors its formation; thus, light flint glass made from 
 potassium carbonate containing 0.1 per cent SO 3 was clear and of 
 good quality; glass similar in composition, but made from potassium 
 carbonate containing 0.75 per cent SO 3 , turned milky on cooling; 
 while glass made from potassium carbonate containing 0.4 per cent 
 SO 3 became milky only at the margins of the crucible. In medium 
 flints the presence of a relatively large amount of arsenic oxide may 
 also cause cloudiness and should be avoided. That the rate of cooling 
 is an important factor is proved by the fact that large pots containing 
 a ton of optical glass are more liable to become milky on cooling 
 than small pots half this size. 
 
 Whatever, the precipitate is, the solubility relations are such that 
 at a high temperature the solution is not supersaturated, but on 
 cooling it becomes saturated with respect to some substance and, 
 with still further lowering of the temperature, precipitation begins if 
 sufficient time be allowed for it during the cooling process; the tem- 
 perature range within which precipitation is liable to occur is 500 to 
 1,000 C. It is possible that the substance in the light flints is silica 
 or lead sulphate, or lead chloride which is only slightly soluble in 
 silicates. On fining the glass at high temperatures the volatiliza- 
 tion of the sulphates and chlorides increases; such heating may also 
 inhibit the formation of the colloidal particles which on cooling pro- 
 duce opalescence; the presence of small amounts of boron oxide seems 
 to favor the formation of such clusters; the presence of alumina tends 
 to hinder their formation. 
 
 32 Experiment by C. N. Fenner. 
 
146 MANUFACTURE OF OPTICAL GLASS. 
 
 In other branches of glass manufacture milky glass is produced 
 purposely by the addition of certain substances, such as phosphates 
 and fluorides, which are relatively insoluble in the glass melt. When 
 chilled quickly, these glasses may remain clear, but on reheating and 
 cooling down slowly through a temperature range from 800 to 400 C. 
 they become cloudy. The precipitation in opalescent glasses indi- 
 cates the grouping of certain constituents of the melt into particles 
 of at least colloidal size such that they have an appreciable diffracting 
 effect on light waves in the visible spectrum. 
 
 Closely allied to the development of milky glass in the light flints 
 is the behavior of red and yellow glasses at the annealing tempera- 
 tures. If a glass, colored with cadmium sulphide, selenium, copper 
 ruby, or gold ruby, be chilled rapidly from a high temperature the 
 intensity of its coloration is relatively slight; but if the glass be 
 cooled slowly from high temperatures or be reheated after chilling, 
 it becomes deeply colored, the more intense colors being deep red. 
 This behavior indicates a shift of a strong absorption band from the 
 ultra-violet into the violet or blue. There is evidently a shift in 
 molecular grouping or aggregation within the solution such that the 
 grouping stable during the annealing range absorbs the blue end of 
 the spectrum; this regrouping takes place while the glass is still 
 relatively rigid. 
 
 Blue and green glasses do not show this pronounced change in 
 color absorption in the visible spectrum on change in heat treatment. 
 In all such cases involving change in color, or the development of 
 opalescence and milkiness, there is probably a selective grouping and 
 aggregation of certain of the atoms or molecules into particles which, 
 though still submicroscopic in size, have an effect on transmitted 
 light waves; the rate and character of this selective grouping is 
 dependent, moreover, on the temperature conditions; the grouping 
 can be practically suppressed by cooling down through the critical 
 temperature range so rapidly that sufficient time is not available 
 for the completion of the process. 
 
 The fact that the intensity of coloration in the yellow and red 
 glasses is markedly dependent on the heat treatment may have an 
 important bearing on the transmission in optical glasses. Ferric 
 iron colors glasses yellow and absorbs a large part of the ultra-violet; 
 with increase in ferrous iron content the color of the glass shifts to 
 the green and even to the blue in the barium glasses; glasses colored 
 with ferrous iron oxide are good infra-red absorbers. There are 
 indications that the intensity of coloration of glasses containing iron 
 in the ferric state varies with the heat treatment and that the reheat- 
 ing of such glasses for annealing or pressing tends to lower the 
 transmission. 
 
IDENTIFICATION OF CRYSTALLITES. 147 
 
 Little is definitely known regarding the phenomena involved in 
 the formation of milky and opal glasses and of red and yellow colored 
 glasses. In the milky glasses swarms of particles of some substance 
 or substances, probably excess silica either in the colloidal state or as 
 embryonic crystals of tridymite or of cristobalite, are precipitated 
 and are held in suspension in the molten glass. It has been found 
 that the presence of minute quantities of alkali sulphates or alkali 
 chlorides or both tend to favor this precipitation, acting in this respect 
 after the manner of " catalyzers/' Be the precipitate and the causes 
 therefor what they may, the factory practice to be followed to avoid 
 the occurrence of milkiness is to use materials of high chemical 
 purity, especially potassium carbonate containing less than 0.3 per 
 cent SO 3 and 2 per cent Cl, to fine at high temperatures somewhat 
 above 1,400 C., to stir the melt thoroughly for as long a period 
 as possible, and to cool rapidly through the temperature range of 
 precipitation. 
 
 The identification of crystallites in optical glass. The identification 
 of crystallization bodies in optical glass is best accomplished by use 
 of the petrographic microscope. Methods have been devised for 
 the measurement of the optical constants of crystals in fine grained 
 aggregates and have been employed for many years in routine work 
 of this nature at the Geophysical Laboratory. 33 For the determina- 
 tion of any given " stone" in a glass it is broken out of the glass and 
 crushed, by tapping with a pestle in an agate mortar, to a fine powder. 
 A few particles of the powder are immersed in a small drop of liquid 
 of known refractive index on a microscope object glass. A small glass 
 cover slip is placed on the drop and the preparation is examined 
 under the microscope. By the use of refractive liquids of different 
 known refringences it is possible to ascertain with the petrographic 
 microscope, the principal refractive indices of the substance or 
 substances, in case more than one be present; also the principal 
 birefringences of each substance, the general shape of its optical 
 ellipsoid, its optical character, its optical axial angle, its pleochroism, 
 etc. These properties enable the observer in most instances to iden- 
 tify the crystals. In view of the relatively small number of crystalli- 
 zation bodies, " stones, " which may possibly occur in optical glass 
 the measurement of the refractive indices alone generally suffices to 
 identify the crystal. The optical properties of the several more 
 common crystals in glass are the following: 34 
 
 Silica (SiO 2 ) may appear in any one of three different forms, all 
 of which are found in natural minerals, namely, quartz, tridymite, 
 and cristobalite. 
 
 33 F. E. Wright, The methods of petrographic microscope research. Publication 158, Carnegie Institu- 
 tion of Washington, 1911. 
 
 M See also N. L. Bowen, "The identification of 'stones' in glass." Jour. Amcr. Ceram. Soc., I 594-60% 
 1918. 
 
148 . MANUFACTURE OF OPTICAL GLASS. 
 
 Quartz appears rarely and only as undissolved sand grains from 
 the original batch. They occur as irregular rounded grains, are 
 optically uniaxial and positive, birefringence medium with refractive 
 indices, e= 1.554, co= 1.545. 
 
 Tridymite is similar in every respect to the natural mineral and 
 appears in thin hexagonal-shaped plates of weak birefringence. 
 Examined on edge these plates show parallel extinction and weak 
 negative elongation. The refractive indices are: a = 1.469, 7 = 1.473. 
 Interference figures are difficult to obtain because of the weak bire- 
 fringence. 
 
 Cristobalite is the form stable 35 between 1,420 C, and its melting 
 temperature 1,710 C. It occurs commonly in the form of skeletal 
 crystals resembling the octahedral growths observed in copper, gold, 
 and common salt. Its crystal aggregates are terminated in many in- 
 stances with spear-shaped, octahedral endings and are not lath- 
 shaped as in the case with tridymite. Its refractive indices are 
 a =1.484, 7-1.487. The birefringence is exceedingly weak. The 
 higher refractive indices and different crystallographic development 
 suffice to distinguish cristobalite from tridymite. 
 
 Calcium metasilicate (CaSiO 3 ) occurs commonly in the form of 
 radial spherulites of the mineral wollastonite. Its needles extinguisn 
 parallel to the direction of elongation and show either positive or 
 negative elongation. The plane of the optic axes is normal to the 
 elongation; optic axial angle small, optical character negative. 
 Refractive indices are: a = 1.620, 7 = 1.633; birefringence, medium. 
 
 Sillimanite (Al 2 SiO 5 ) occurs in radial spherulites and interlacing, 
 lath-shaped aggregates. The individual crystals show parallel ex- 
 tinction and positive elongation. The refractive indices are rela- 
 tively high, = 1.660, 7 = 1.681; the birefringence is moderately 
 strong. The crystals frequently exhibit a lower refracting core 
 which may be the result of skeletal development of the laths. 
 
 Barium disilicate (Ba Si 2 O 5 ) . Crystals of this compound were first 
 identified by Bo wen 36 as orthorhombic. They occur as thin six- 
 sided plates; extinguish parallel with negative elongation. Refrac- 
 tive indices a =1.598, 7 = 1.617; birefringence moderately strong. 
 
 THE ANNEALING PERIOD. 
 
 On cooling to a dull red heat, the pot of molten glass becomes 
 increasingly stiffer and with falling temperature acquires more and 
 more the properties of a rigid solid. The rate of cooling is, however, 
 not equal over different parts of the pot and the rates of contraction 
 are correspondingly different; as a result internal stresses and strains 
 
 C. N. Former, The stability relations of the silica minerals. Jour. Wash. Acad. Sci., 2, 471, 1912; Am. J. 
 Sci. (4), 86, 331-384, 1913. 
 Jour. Wash. Acad. Sci., 8, 265-268, 1918. 
 
THE ANNEALING PERIOD. 149 
 
 are set up. At higher temperatures from 700 C. down to 400 C. or 
 lower the internal stresses are relieved by actual flow of the glass 
 if it is allowed sufficient time to flow. The rate of release of the 
 internal stresses is a function of the magnitude of the stresses and 
 these in turn are caused by differences in the rates of cooling of 
 different parts of the pot of glass. At higher temperatures the 
 stresses are relieved by internal flow as rapidly as they are developed ; 
 at these temperatures the glass behaves as a viscous liquid and is 
 unable to maintain shearing stresses for any length of time. Shear- 
 ing stresses are relieved by actual flow of the melt. With falling 
 temperatures the rate of release of internal stresses becomes slower 
 and practically ceases at low temperatures; it fails, in short, to keep 
 step with the temperature drop and hence the stresses persist and, 
 unless relieved by stresses in the opposite direction, may continue 
 indefinitely at room temperatures. 
 
 The factors which enter into the problem of the cooling and frac- 
 turing of a pot of optical glass are complex and at the present time 
 are not adequately known to permit of its complete solution. The 
 available information suffices, however, for the establishment of a 
 fairly satisfactory manufacturing routine. The more important 
 factors are: 
 
 (a) Change of viscosity of the different glasses with temperature. 
 
 (6) Hates of relief or relaxation times of internal stresses of differ- 
 ent magnitudes at different temperatures. 
 
 (c) Changes in expansion coefficient with temperature. 
 
 (d) Temperature distribution within a cooling solid of the shape, 
 dimensions, and thermal characteristics of an optical glass mass 
 cooling in a clay pot. 
 
 These factors will now be considered in the order given. 
 
 (a) The most important recent investigation on the change of 
 viscosity of glass with temperature is that of F. Twyman 37 whose 
 experimental results showed that the mobility of most glasses through 
 the critical range from 400 C. to 600 C. doubles for each 8 rise in 
 temperature. In the form of an equation this statement reads 
 
 or 
 
 in which M is the mobility (the converse of the viscosity) , 6 the tem- 
 perature, and K and , constants dependent on the kinds of glass. 
 Still more recent experiments by M. So, 38 Tool and Valasek, 39 Littleton 
 and Roberts, 40 and Adams and Williamson 41 have corroborated the 
 
 37 The annealing of glass, Jour. Soc. Glass Technology, I, 61-73, 1917. 
 
 39 Phys. Math. Soc. Japan, 2, 113-116, 1920, Math. Phys. Soc. Tokyo, Proc. 9, 425-441, 1918. 
 
 Bur. Standards Bull. No. 358, 1919. 
 
 < Jour. Opt. Soc. America, IV, 224-229, 1920. 
 
 < l Jour. Franklin Institute, 190, 597-632, 835-870, 1920. 
 
150 MANUFACTURE OF OPTICAL GLASS. 
 
 results of Twyman and found that the mobility changes differently 
 with different glasses and that it doubles for a rise in temperature of 
 7.9 C. to nearly 11 C., depending on the type of glass, the value 
 being commonly higher for the crown glasses than for the flint series. 
 These measurements of mobility were made by observing the rate of 
 stretching, or of bending, or of twisting of glass rods under constant 
 load at the different temperatures. As the temperature falls the 
 mobility decreases so rapidly that mechanical methods are no longer 
 practicable and recourse is had to optical methods. 
 
 Strain in glass signifies physical inhomogeneity and this finds 
 expression in the effects which strained glass exerts on transmitted 
 light waves that encounter different degrees of resistance according 
 to the direction of their transmission. This difference gives rise 
 to the phenomena of double refraction and these can be detected 
 and measured by the use of polarized light and suitable accessory 
 appliances. Early in the last century Brewster 42 discovered that 
 in strained glass the resulting birefringence is proportional to the 
 load; in other words, stress in a glass can be measured in terms of 
 birefringence. Methods, based on the measurements of the bire- 
 fringence in a piece of optical glass, have long been used and afford 
 simple and satisfactory means to study strain in glass from ordinary 
 temperatures up to the softening region of glasses. Commonly the 
 path-difference between the two plane-plorized light waves is meas- 
 ured by means either of a graduated wedge, or of a Babinet com- 
 pensator, or of other device of similar nature, either in white or 
 monochromatic light; or the ellipticity of the emergent beam is 
 determined in monochromatic light and the path difference is then 
 computed from the ellipticity. Measurements by these methods 
 have recently been published by Twyman, 43 Tool and Valasek, 39 
 Adams and Williamson, 44 and Littleton and Roberts, 45 and prove that 
 the law given for the change of viscosity with temperature is valid 
 for the change of rate of relief of stresses with temperature; except 
 that in the case of birefringence the temperature interval required 
 for a doubling of the rate is found to be somewhat smaller than 
 that necessary for a doubling of the mobility when measured by 
 mechanical methods. This discrepancy may, however, be due to 
 the fact that measurements of the mechanical deformation are 
 made at higher temperatures than those of the change in birefringence. 
 The equation expressing the change in rate of relief of stress or the 
 
 s Bur. Standards Bull., No. 358, 1919. 
 
 Philosophical Transactions, 1814. 
 
 Jour. Soc. Glass Technology, I, 61-73, 1917. 
 
 Jour. Opt. Soc. America, IV, 213-223, 1920. 
 
 Jour. Opt. Soc. America IV, 224-229, 1920; Jour. Franklin Inst. 190, 597-fi32, 835-870, 1920. 
 
THE ANNEALING PERIOD. 
 
 151 
 
 change in relaxation 
 accordingly 
 
 time as measured by optical methods is 
 
 ^ =K ' 2 
 
 -0/8 
 
 00-0 
 
 0.0376 -(0Q-0) -0 
 
 (2) 
 
 or 
 
 log T= C. (0 -d) = M 2 -M i e (2a) 
 
 This equation of Twyman, which states in effect that the logarithm 
 of the relaxation is a linear function of the temperature, is important 
 and will be referred to again. 
 
 (b) In a theoretical presentation of this general subject, Maxwell 47 
 suggested as a tentative hypothesis that at a given temperature the 
 rate of disappearance of stress in a strained body is proportional 
 to the stress itself. Twyman states that his experimental results 
 confirm this statement. Tool and Valasek 39 found that the relaxa- 
 tion time increases perceptibly with decrease in the stresses and 
 resulting birefringence. Adams and Williamson 48 deduce from 
 their measurements that the rate of relief of the stress varies, not 
 directly, but as the square of the stress. Expressed in terms of 
 decrease in birefringence this leads to the equation 
 
 1 
 
 10 7 -An 10 7 -An 
 
 T-t 
 
 (3) 
 
 in which An is the initial birefringence (stress) , An the birefringence 
 after the time, t, and T 7 , the relaxation time. 
 
 Further experimental evidence will be required to explain these 
 discrepancies, namely: (1) The differences in the rates of change 
 of viscosity as determined by optical and by mechanical methods, 
 respectively, and (2) the rate of relief of stress, as a function of 
 the stress at a constant temperature. 
 
 TABLE 9. Constants 
 
 ^ C, N of equations 2 and 2 a, and annealing temperatures 
 (centigrade) for different times. 
 
 
 
 
 
 
 Time to reduce strain-birefringence from 50.10~ 7 
 
 
 Mnr 
 
 
 M 2 
 
 
 to 5.10-7. 
 
 Kind of glass. 
 
 
 M 2 
 
 or 0o 
 
 N 
 
 | 
 
 | 
 
 1 
 
 
 
 
 
 . 
 
 
 
 
 
 
 a 
 1 
 
 a 
 i 
 
 d 
 
 i 
 
 A 
 
 | 
 
 | 
 
 ' 1 
 
 3 
 
 
 
 
 
 
 
 iO 
 
 
 
 
 
 - 
 
 -" 
 
 ** 
 
 
 
 
 C 
 
 
 c. 
 
 a 
 
 c 
 
 c. 
 
 c. 
 
 C 
 
 r. 
 
 c 
 
 Borosilicate crown. . . 
 
 0.030 
 
 18.68 
 
 624 
 
 10.00 
 
 598 
 
 575 
 
 565 
 
 539 
 
 515 
 
 493 
 
 464 
 
 444 
 
 Ordinary crown i . 029 
 
 17.33 
 
 CQO 
 
 10.31 
 
 573 
 
 548 
 
 538 
 
 511 
 
 487 
 
 464 
 
 434 
 
 414 
 
 Light barium crown. . . 032 
 
 20.10 
 
 628 
 
 9.41 
 
 605 
 
 583 
 
 574 
 
 549 
 
 527 
 
 506 
 
 480 
 
 461 
 
 Heavy barium crown. 
 Barium flint 
 
 .038 
 .028 
 
 24.95 
 16.28 
 
 657 
 582 
 
 7.92 
 10.75 
 
 637 
 555 
 
 619 
 530 
 
 611 
 519 
 
 590 
 491 
 
 572 
 466 
 
 554 
 442 
 
 532 
 412 
 
 516 
 390 
 
 Light flint 
 
 .033 
 
 15.92 
 
 482 
 
 9.12 
 
 460 
 
 439 
 
 429 
 
 406 
 
 385 
 
 364 
 
 338 
 
 320 
 
 Medium flint 
 
 .038 
 
 18.34 
 
 483 
 
 7.92 
 
 463 
 
 445 
 
 437 
 
 416 
 
 398 
 
 380 
 
 358 
 
 342 
 
 Heavy flint 
 
 037 
 
 17.51 
 
 473 
 
 8.13 
 
 453 
 
 434 
 
 426 
 
 405 
 
 386 
 
 368 
 
 345 
 
 329 
 
 Extra heavy flint 
 
 .033 
 
 15.03 
 
 456 
 
 9.12 
 
 433 
 
 412 
 
 403 
 
 379 
 
 358 
 
 337 
 
 312 
 
 292 
 
 39 Bur. Standards Bull., No. 358, 1920. 
 
 Adams and Williamson, Jour. Opt. Soc. America IV, 219, 1920. 
 
 " Phil. Mag. (4), 34, 129, 1868. 
 
 Jour. Opt. Soc. America, IV, 219, 1920. 
 
152 MANUFACTURE OF OPTICAL GLASS. 
 
 In Table 9 49 are listed the relaxation times required for the reduc- 
 tion of the birefringence from 50 X 10~ 7 to 5 X 10~ 7 or, in other words, 
 the path difference per centimeter glass path from 50 millimicrons 
 to 5 millimicrons for different types of glass at different temperatures. 
 The chemical compositions of these glasses are given in Table 11 on 
 page 160. The numbers listed in columns 2, 3, 4, and 5 of Table 9 
 are the values of the constants M = C, M 2 / M l = , and N of the 
 foregoing equations 2 and 2a. The time required for the relaxation 
 of any given percentage of a stress at a given temperature is stated 
 by the empirical equation (3) in terms of the resulting birefringences. 
 Thus the times required to reduce the birefringences from 500 10~ 7 
 to 50 10~ 7 and from 50 10~ 7 to 5 10~ 7 are in the ratio (equation 3). 
 
 J_ 1 
 
 50 500__ 1 
 
 I J_ ]0 
 
 5 ~ 50 
 
 From this it is evident that the smaller the stress the longer it takes 
 to relax it. The values listed in Table 9 may therefore be taken as 
 indicating the order of magnitude of the relaxation times for different 
 glasses at different temperatures. 
 
 (c) Experiments 50 by Peters and Cragoe have shown that the 
 expansion of optical glasses is practically linear up to about 500 C 
 after which the rate increases perceptibly as shown in Table 10 
 reproduced from their paper. Somewhat above this temperature, 
 moreover, the glass becomes soft and behaves as a strictly viscous 
 liquid. Heat measurements 51 by Tool and Valasek have shown 
 furthermore a perceptible heat absorption in these glasses at this 
 temperature on heating the glass and an evolution of heat on cooling. 
 This heat effect has been ascribed to a change in the molecular group- 
 ings within the liquid. Be the cause what it may, there seems to 
 be definite proof of a change in the behavior of the glasses within this 
 temperature range, and in critical work this change must be taken into 
 account. 
 
 The first recorded observation on a heat effect of this kind in a 
 glass is that of Day and Allen 52 on borax glass in the heating of 
 which "a slight but persistent absorption of heat appeared in the 
 same region (490-500 C) and continued over some 20 C, after 
 which the original rate of heating returned." 
 
 49 Adams and Williamson, Jour. Opt. Soc. America, 4, 219, 1920. 
 
 60 C. G. Peters and C. H. Cragoe, Measurements of the thermal dilatation of glass at high temperatures 
 Jour. Opt. Soc. America, IV, 105-144, 1920. 
 
 A. Q. Tool and J. Valasek, Bur. Standards Bull. No. 358, 1920. 
 
 52 The Isomorphism and thermal properties of the feldspars, Carnegie Institution of Washington, Pub. 
 No. 31, p. 34. 
 
THE ANNEALING PERIOD. 
 TABLE 10.- Mean coefficient of linear expansion of optical glasses. 
 
 153 
 
 Kind of glass. 
 
 Tempera- 
 ture in- 
 terval C. 
 
 CXHM. 
 
 Tempera- 
 ture in- 
 terval C. 
 
 cxio. 
 
 Light crown 
 
 22-426 
 
 0.102 
 
 502-522 
 
 0.555 
 
 
 22-498 
 
 090 
 
 539-562 
 
 393 
 
 Barium crown 
 
 23-499 
 
 .090 
 
 589-610 
 
 .649 
 
 Light flint 
 
 22-451 
 
 .076 
 
 495-511 
 
 292 
 
 Medium flint 
 
 23-402 
 
 .097 
 
 452-478 
 
 .396 
 
 Barium flint 
 
 22-494 
 
 .088 
 
 519-550 
 
 .331 
 
 
 
 
 
 
 (d) Measurements of the changes in temperature at different points 
 in a pot of cooling glass have been made by H. S. Roberts 53 by 
 inserting bare base-metal thermoelements (alumel-chromel) into the 
 melt directly after the removal of the pot from the melting furnace. 
 Time-temperature readings were taken for different points within 
 the mass during the entire period of cooling; for each pot of glass a 
 record was also kept of the quality of annealing and of the character 
 of the fracture surfaces. From a series of such data Roberts was 
 able to formulate cooling schedules and methods of treatment of the 
 pot of glass during cooling that insured the kind of fracturing and the 
 quality of annealing desired. 
 
 General investigations into the temperature distribution of cooling 
 solids and the stresses arising from changes in temperature gradient 
 have been made by Hopkinson, 54 C. Neumann, 55 Winkelmann and 
 Schott, 56 Lord Rayleigh, 57 Schulz, 58 and more recently by Williamson, 59 
 and Adams and Williamson, 60 and others. 61 Adams and Williamson 
 have computed tables giving numerical values of several factors 
 involved in the annealing of glass bodies of different shapes and sizes. 
 
 We shall now consider the temperature-strain distribution in a 
 pot of cooling glass and shall, for the present, neglect the changes in 
 expansion coefficient just below the softening region of the glass; 
 and assume with Roberts 62 that the coefficient of linear expansion 
 is practically constant throughout the range from room temperature 
 to that at which the glass begins to soften. Glass on heating expands 
 and on cooling contracts; if different parts of the same glass body 
 cool at different rates, these contract at correspondingly different 
 rates and, as a result, internal stresses are produced. The rate at 
 
 53 Jour. Amer. Ceram. Soc., 2, 543-563, 1919. 
 
 5< Messenger of Mathematics, 8, 168, 1879. 
 
 Theorie der Elasticitat, Leipzig, p. 112, 1885. 
 
 56 Ann. d. Phys. 51, 745, 1894. 
 
 " Phil. Mag. (6), 1, 169, 1901. 
 
 i8 Sprechsaal, 47, 460, 478, 1914. 
 
 59 Jour. Wash. Acad. Sci., 9, 209, 1919. 
 
 60 Physical Rev. N. S. IV, 99-114, 1919; Jour. Wash. Acad. Sci., 9, 609-623, 1919; Jour. Opt. Soc. America, 
 4, 213-223, 1920. 
 
 61 Byerly, Fourier Series and spherical Harmonics, Ginn & Co.; Carslaw, Fourier Series and Integrals, 
 Maemillan & Co.; Ingersoll and Zobel, Mathematical Theory of Heat Conduction, Ginn & Co. 
 
 62 Jour. Am. Ceram. Soc., 2, 546, 1919, 
 
154 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 which stresses of this nature are introduced depends on the rate of 
 change, with time, of the temperature gradient, which produces the 
 differences in the rates of contraction. At higher temperatures 
 internal shearing stresses of this kind are relieved by actual flow of 
 the material while at lower temperatures the viscosity becomes so 
 great that such flow is extremely slow and stresses may persist for 
 long periods of time. 
 
 At the time of removal of the pot from the melting furnace the 
 temperature throughout the pot is sensibly uniform, as shown by 
 measurements of Roberts. The temperature at this time differs 
 
 
 50 100 HOURS 
 
 FIG. 43. Time-temperature curves for melts of optical glass cooled under different conditions. Curves 
 A, B, C are for a melt cooled in sand; curves D and -E for a melt cooled in the open air without any 
 added insulating material. 
 
 somewhat for the different types of glasses but ranges between 950 
 and 1,100 C. On cooling down either in a pot arch or under an 
 insulated casing the temperature of the peripheral portions of the 
 glass melt falls more rapidly than that of the center. The maximum 
 temperature difference is reached generally between 500 C. and 600 
 C. and may amount to 100 C. or more; but it is commonly less. At 
 600 C. practically all glasses except dense barium crowns are so 
 soft that any stress arising from a change in the temperature gradient 
 is relieved by flow so rapid that the glass mass remains in an essentially 
 unstrained condition. If it were possible to maintain the tempera- 
 
THE ANNEALING PERIOD. 
 
 155 
 
 ture gradient established at 600 C. as an approximately steady 
 state, by linear cooling down to the point at which the outside of the 
 glass mass reaches room temperature, no strain would be introduced 
 by this procedure. But from here on the central portion. cools down 
 to room temperature, thereby contracting and tending to pull away 
 from the margins and introducing radial tensional stresses which in 
 turn set up tangential compressive stresses in the peripheral portions 
 of the glass mass. The stresses, thus set up because of the changes 
 in temperature gradient, depend on the temperature difference 
 between the center and the margin of the glass mass and also on its 
 shape, size, and elastic character. 
 
 The rates of cooling of the center (curve A) and the side (curve 
 B) of a 36-inch pot of medium flint (n D = l.Ql) are shown in figure 
 43, reproduced from Roberts' paper. 63 Curve C shows the rate of 
 
 c 
 200 
 
 u 100 
 
 
 
 \ 
 
 200 
 
 400 600 800 
 TEMPERATURE AT CENTER 
 
 1000' C 
 
 FIG. 44. Temperature differences between center and margin of melt for different temperatures of the 
 center of three different melts. Curve A is for a melt cooled in air without any added insulation; 
 curve B for a melt cooled in a pot-arch; curve Cfor a melt cooled in sand. 
 
 cooling of the outside of the pot. In figure 44 the differences in tem- 
 perature between the center and the sides of the glass mass are given 
 for different temperatures (curve 0). This diagram shows that a 
 maximal difference of about 60 C. is reached at 600 C. and that 
 between 600 and 400 C. the decrease in temperature difference is 
 about 20 C. Curve A of figure 43 shows that at 600 C. the melt is 
 cooling at the rate of 17 C. per hour; at 500 C. at the rate of 10 C. 
 per hour; and at 400 C. at the rate of 6 C. per hour. Table 9 of 
 Adams and Williamson, reproduced on page 151, states that at 416 
 C. the time required to anneal a medium flint is one hour; at higher 
 temperatures the annealing time is much less. The conclusion is 
 therefore justified that at a temperature of 400 C., or slightly above 
 400 C., the glass mass is free from strain, all stresses resulting from 
 a temperature gradient having been relieved by flow. The meas- 
 urements by Roberts indicate that with appropriate shift of the 
 temperature limits similar relations obtain for all glasses; and that, 
 
 "Jour. Am. Ceram. Soc.. 2, 559, 
 
156 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 when the glass mass alone is considered without reference to the 
 clay-pot container, the stresses, introduced on further cooling fror i 
 the softening temperature region down to room temperature, result 
 from the gradual decrease of the temperature gradient between the 
 center and the margins; in other words, from this temperature region 
 down to room temperature the center of the glass mass is cooling 
 faster than the periphery. Radial tensional stresses arise as a result 
 of this change in temperature and tend to produce spherical cracks 
 and shells not unlike, in general appearance, the concentric outer 
 shells of an onion. This type of fracture is designated " exfoliation " 
 in geology and is of common occurrence in the field. 
 
 FIG. 45. Well annealed melt showing plane cracks. (Photograph by H. S. Roberts at the 
 Chirleroi plant of the Pittsburgh Plate Glass Co.) 
 
 It is possible to increase temporarily the temperature difference 
 between the center and the margins by chilling the margins. In 
 this case the outside is chilling at a faster rate than the center and, 
 as a result, stresses of radial compression and tangential tension in 
 the outer shells are introduced, which tend to cause the glass mass 
 to crack or split along radial planes normal to the boundary surfaces, 
 whereas, in the case of the decreasing thermal gradient, cracks paral- 
 lel with the boundary surfaces tend to be produced. 
 
 A cooling glass melt is similar in many respects to cooling lava. A 
 lava flow, after it has cooled to the temperature of its surroundings, 
 
THE ANNEALING PERIOD. 
 
 157 
 
 is seen to be filled with cracks and fissures running approximately 
 parallel and normal to its surface. These cracks develop as a result 
 of the contraction of the cooling lava and are a characteristic phe- 
 nomenon not only in lavas but also in intrusive igneous rocks and in 
 sedimentary rocks. Similar cracks occur, as mud cracks, in clay or 
 argillaceous material on drying out. Cracks and jointing phenomena 
 of this nature are so well known that further description is unnec- 
 essary. Suffice it to state the glassmaker desires to find each pot of 
 glass, when cold, cracked into large rectangular, well-annealed blocks 
 bounded by relatively plane and smooth surfaces. (Fig. 45.) Under 
 unfavorable or improper rates of cooling, Assuring and jointing of this 
 
 FIG. 46. Poorly annealed melt showing spherical cracks. (Photograph by H. S. Roberts at the 
 Charleroi plant of the Pittsburgh Plate Glass. Co.) 
 
 kind does not develop, but spherical cracks abound and shells of the 
 glass mass are found split off and away from a compact, rounded 
 central core. (Fig. 46.) Large " marbles " or " onions " of this type 
 are unwelcome, because they do not split regularly, but into sharp, 
 irregular, and wedge-shaped chunks, with the result that much glass 
 is lost. The glass which is obtained is, moreover, hi fragments unsat- 
 isfactory for molding or pressing; its fracture surfaces are rough 
 and so uneven that the glass can not be satisfactorily inspected. In 
 the properly cooled melt the spherical cracks (analogous to the exfoli- 
 ation cracks in rocks) do not affect a large amount of glass. The 
 body of the glass is fissured by vertical and occasional horizontal 
 
 3922921 11 
 
158 MANUFACTURE OF OPTICAL GLASS. 
 
 plane cracks, through which the quality of the glass can be readily 
 seen, as through a polished plane surface. The modes of formation 
 of these radial, plane fissures as contrasted with the concentric 
 spherical cracks (exfoliation) are evidently different and require 
 explanation. In this connection the following facts are significant : 
 
 The temperature measurements of Roberts cited above prove that, 
 in general, the maximum temperature difference between the center 
 and the margins of a pot of molten glass on cooling from 1,100 or 
 1,000 C. is reached at approximately 600 C.; at this temperature, 
 all shearing stresses arising from the changes in the temperature gra- 
 dient are eliminated by actual flow of the viscous glass. From this 
 temperature region down to room temperature, the temperature differ- 
 ence between the center and the margins gradually decreases; the 
 center cools more rapidly than the margins and in consequence tends 
 to contract and to pull away, thereby setting up tensional radial 
 stresses. Analysis of the stresses produced in a glass mass cooling 
 under these conditions 64 proves that the radial tensional stresses in 
 a cylinder or a sphere increase from zero at the periphery to the maxi- 
 mum value at the center while the accompanying tangential stresses 
 are compressive at the margins, gradually decrease toward the center, 
 become zero, and then increase as tensional stresses attaining a 
 maximal value equal to the radial stress at the center. 
 
 In a body cooling under these conditions, the tangential stresses 
 at the margins are compressive arid therefore unfavorable to the for- 
 mation of radial cracks. Some other cause must be sought to account 
 for the observed type of fracture. 
 
 It was noted during the early months of the war that a change in 
 the make of a pot (change in materials and in thickness of pot walls) 
 resulted in unfavorably fractured pots of glass; this was remedied by 
 a change in the cooling schedule. Experience taught that in general 
 an average cooling rate of 8 C. or less per hour from 600 to 350 C. 
 resulted in satisfactorily fractured and annealed glass whereas cooling 
 at double this rate or 15 C. per hour resulted in poorly annealed and 
 exfoliated " bowlders" or " marbles" of glass. A further proof of the 
 influence of the pot on the character of the fracture of the glass was 
 furnished later by H. S. Roberts, who reheated a " marble" very 
 slowly by insulating it well in sand and then cooled it somewhat more 
 slowly than the cooling rate of the ordinary pot. The result was a 
 well-annealed block without cracks, thus indicating the influence of 
 the pot in producing cracks in the glass mass cooled under these 
 conditions. 
 
 The coefficient of linear expansion of the material of optical glass 
 pots varies with the ingredients of the pot. As an average we may 
 assume its coefficient of linear expansion to be about 50* 10~ 7 . The 
 
 * E. D. Williamson, Jour. Wash. Acad. Sci. 9, 209-217, 1919. 
 
THE ANNEALING PERIOD. 159 
 
 coefficients of linear expansion of optical glasses range from about 
 6010- 7 to 100 10 7 . 65 Thus it is 97-1Q- 7 for a medium flint glass 
 (Table 10, page 153). 
 
 The significant fact is that in all cases the pot walls contract less on 
 cooling than does the glass adhering to them, with the result that the 
 glass is put under elastic tension, whose magnitude depends on the 
 difference in expansion coefficients between the glass and the contain- 
 ing pot. 
 
 The stresses which arise as a result of these relations can be com- 
 puted on the assumption that the pot of glass is cylindrical in shape 
 and that for diametral plane sections near the center of the pot a 
 plane normal to the axis of the cylinder remains a plane throughout 
 the contraction. The stresses, which arise as a result of linear cooling 
 at such a rate that a difference in temperature between the center and 
 the periphery of 100 C. is attained in the steady state, are for a pot of 
 medium flint glass of radius, r = 40 centimeters (diameter of pot about 
 30 inches) in kilograms per square centimeter: 66 
 
 /o j- i\ T> 
 (Radial) P. - 
 
 lft[ 
 (Tangential) P.- 
 
 in these equations P 2 andP 3 are the radial and tangential stresses, respec- 
 tively, at the point r; a = 97 10~ 7 is the expansion coefficient for medium 
 flint; ^ = 0.001C., temperature increase or decrease per second to 
 maintain a constant difference of 100 C. between center and margin 
 of glass cylinder 80 centimeters in diameter; K = 0.004, the diffusivity; 
 
 O 77" _p_ ~D O J7" _ O D 
 
 e = -Qn-jr' and/= IQP if are e l as ^ic constants, e being the elongation 
 
 of glass rod or bar of unit length and area caused by a load of 1 
 kilogram, and / the accompanying contraction in the direction 
 normal to the direction of the load and axis of the rod; Young's 
 modulus is 1/e and Poisson's ratio, f/e; the constants K and R are, 
 respectively, the modulus of compressibility (bulk modulus) and the 
 modulus of rigidity. Values of the elastic constants for different 
 glass types are listed in Table 11 of Adams and Williamson 67 ; in 
 this table the weight percentage chemical compositions, and data 
 on birefringence, on thermal diffusivity, and on linear expansion are 
 also listed. 
 
 Peters and Cragoe, Jour. Opt. Soc. America, IV, 105-144, 1920. 
 
 E. D. Williamson, Jour. Wash. Acad. Sci., 9, 216, 1919. 
 
 " Jour. Wash. Acad. Sci., 9, 613, 1919; Jour. Franklin Institute, 190, 607, 1920. 
 
160 MANUFACTURE OF OPTICAL GLASS. 
 
 TABLE 11. Chemical and physical constants of different types of optical glasses. 
 
 Kind of glass. 
 
 Chemical composition (approx.) wt. per cent. 
 
 Si0 2 . 
 
 A1 2 3 . B 2 3 . 
 
 PbO. 
 
 ZnO. 
 
 BaO. 
 
 CaO. 
 
 K 2 0. 
 
 Na 2 0. 
 
 BorosilicatG crown 
 
 67 
 73 
 47 
 40 
 46 
 54 
 45 
 42 
 28 
 
 12 
 
 
 
 4 
 
 
 8 
 1 
 5 
 
 9 
 14 
 3 
 
 Ordinary crown . . . 
 
 
 
 
 
 12 
 
 Light barium crown 
 Dense barium crown 
 
 1 4 
 3 6 
 
 
 11 
 
 8 
 8 
 
 29 
 43 
 15 
 
 
 Barium flint 
 
 24 
 35 
 
 48 
 52 
 69 
 
 
 4 
 
 5 
 4 
 3 
 3 
 
 3 
 6 
 3 
 3 
 
 Light flint 
 
 
 
 Medium flint . 
 
 
 
 
 Dense flint 
 
 
 
 
 Extradense flint 
 
 
 
 
 
 
 
 
 Kind of glass. 
 
 Optical properties. 
 
 Mechanical properties. 
 
 Thermal properties. 
 
 Refr. 
 Index 
 D . 
 
 V 
 
 Bi-efrin- 
 genee due 
 to 1 kg/cm 2 
 1-iad. 
 B 
 
 Modulus of 
 compres- 
 sibility 
 kg/cm 2 . 
 K 
 
 Modulus of 
 rigidity 
 kg/cm 2 . 
 
 R 
 
 CoefT. of 
 linear ex- 
 pansion. 
 
 a 
 
 Thermal 
 diffusiv- 
 itv. 
 cm 2 /scc. 
 
 K 
 
 Boiosilicate crown 
 
 .516 
 .523 
 .574 
 .608 
 .606 
 .573 
 .616 
 .655 
 .756 
 
 62 
 59 
 57 
 57 
 44 
 42 
 37 
 33 
 27 
 
 -2.85X10-7 
 -2.57X10- 7 
 -2.81X10-' 
 -2.15X10- 7 
 -3.10X10-7 
 -3.20X10-7 
 -3.13X10-7 
 -2.67X10-7 
 -1.22X10-7 
 
 0.43X10" 
 0.46X10* 
 0.52X10 6 
 0.53X10 6 
 0.42X10 6 
 0.35X10 6 
 0.34X10* 
 0.34X10* 
 ' 0. 32X 10* 
 
 0. 29X 10* 
 0.28X10* 
 0.30X10* 
 0.29X10* 
 0.26X10* 
 0.24X10* 
 0.22X10* 
 0.22X10* 
 0.20X10* 
 
 7.5X10-* 
 8. 9X 10-* 
 7.4X10-* 
 6.4X10- 
 7.7X10-6 
 8.4X10-* 
 8.1X10 
 8.2X10-* 
 8. 5X 10-* 
 
 0. 0045 
 0.0047 
 0. 0039 
 0.0040 
 0.003X 
 0.0040 
 0.0038 
 0.0036 
 0.0033 
 
 Ordinary crown 
 
 Light barium crown 
 
 Dense barium crown 
 
 Barium flint 
 
 Light flint 
 
 Medium flint 
 Dense flint 
 
 Extradense flint, 
 
 
 Substituting the appropriate values for the medium flint in the 
 above equation, we find for the marginal shell (r = a) the stresses 
 
 97-10 7 X 0.001X1600X2 
 3 ~ 16X0.004X (1.842-10 6 -0.431'10 re ) 
 
 -343.72 kg. cm 2 . 
 
 For a rate of linear heating ^ = 0.005 per second, the tangential com- 
 pressive stress 68 is 171.86 kg. cm 2 . The stresses at the center (r = 0) 
 are 
 
 aha 2 97-10 7 X 0.001x1600 
 
 16 X 0.004 Xl.411-10- 6 
 
 171.86 kg. cm 2 . 
 
 Both stresses are tensional. 
 
 The flint glass mass at a temperature 400 C. or higher is practically 
 without strain in spite of the temperature gradient existing at that 
 temperature. On cooling the temperature gradient gradually dis- 
 appears, but in so doing gives rise to stresses which persist except for 
 that part of the stresses which is eliminated by viscous flow. As a 
 first approximation we may consider the above stresses, tangential 
 and radial, computed for the margin and center of the mass as repre- 
 sentative of the order of magnitude of the actual stresses existing in 
 the glass mass at room temperature. 
 
 68 Tensional stresses are considered positive, compressive stresses negative. 
 
THE ANNEALING PERIOD. 161 
 
 As a result of the difference in coefficients of expansion of the glass 
 mass and of the containing pot to whose walls the glass adheres, 
 elastic tensional stresses are set up and superimposed on the stresses 
 induced by the disappearance of the temperature gradient. These 
 stresses can be computed if, as before, we assume the pot to be cylin- 
 drical in shape and consider only central diamentral planes located at a 
 distance from the ends. The total linear contraction of the glass 
 per unit length from 420 to 20 C. is then 400' 97'10 7 ; the total 
 linear contraction of the pot is 400' 50*10 7 ; and the difference in 
 elongation is 400' 47'10 7 = 0.00188. If the pot walls were perfectly 
 rigid the linear stretching per unit length of the adhering glass mass 
 would be 0.00188, but as this is not the case, we may, as a first 
 approximation, assume that the walls are pulled in as much as the 
 glass is stretched out, in which case the elongation per unit length 
 in the glass mass is 0.00094. Let P lt P 2 , and P 3 be the axial, radial, 
 and tangential stresses, respectively, at a given point. The equations 
 for the elongations E, F, and G in the directions P lt P 2 , and P 3 are 
 then 
 
 6=-fP,-fP 2 
 But, under the assumptions made, P l = and P 2 = P 3 . Therefore 
 
 P 0. 00094 0. 00094 
 P * = P * = -<^ = 1.411 XlQ- = 665 ' 19 ^ Cm 
 
 Under these conditions there are superimposed on the glass mass, 
 as a result of the interaction between pot and glass on cooling, tensile 
 stresses both radial and tangential which approach the elastic limit 
 of the glass in magnitude. The radial tension on the marginal layer 
 of glass is then 665 kg. cm 2 ; the tangential tension is 665-344 = 321 
 kg. cm. 2 for the larger temperature difference and 493 kg. cm 2 for the 
 smaller temperature interval (^ = 0.0005 per s0c.) Both the tan- 
 gential and the radial tensional stresses increase from the periphery to 
 the center; at the center their order of magnitude is 665 + 172 = 837 
 kg. cm. 2 The tensional stresses at the center are hydrostatic. 
 
 The foregoing values are only first approximations; but they indi- 
 cate that in the glass mass itself tensional stresses of considerable mag- 
 nitude are present such that, if a mechanically weak point were to 
 develop in the glass, the system is mechanically unstable and so 
 near failure that cracks of considerable magnitude would probably 
 develop. The pot walls are for the most part, moreover, under 
 heavy tangential compression and show little tendency to crack 
 radially. 
 
162 MANUFACTURE OF OPTICAL GLASS. 
 
 In case the pot of glass were to cool more rapidly than usual so 
 that for example & = 0.002 per second, then P 3 = -687.44 kg. cm. 2 
 Under these conditions the final tangential stress in the marginal layer 
 is still compressive 665 687 = 22 kg. cm. 2 and there is no tendency 
 for radial cracks to form. Under these conditions of rapid cooling the 
 glass is not only poorly annealed, but is not cracked transversely, and 
 a " marble" or " onion" is the result. 
 
 It is a matter of factory observation that the pot walls at the 
 margins of a cold pot of glass are cracked but little, and that it is in 
 many cases difficult to find a pronounced crack on the outside of the 
 pot without examination of the glass itself. 
 
 The foregoing explanation of the formation of cracks is not complete 
 and does not account adequately for the fact that in a properly cracked 
 pot of glass a master vertical plane fissure divides the part into 
 halves. The formation of the other joint cracks, roughly perpen- 
 dicular to the master joint-plane, is readily deduced from symmetry 
 relations and the existing tensional shearing stresses. 
 
 Roberts 69 found that rapid chilling of the pot of glass at about 
 300 C., induced by removal of the insulation, or by turning off the gas 
 and opening the dampers in the case of melts cooled in the pot arch, 
 favored the formation of radial cracks. Tangential tensional forces 
 are introduced by this procedure, and the general cooling schedule of 
 the glass is thereby disturbed so that a mechanically weak spot 
 might give way and thus introduce the radial cracks. Once radial 
 cracks have begun to appear the tendency for the spherical cracks 
 to form is diminished. 
 
 It may be noted that the plane cracks (joints), produced as de- 
 scribed above, form slowly and with reference to the distribution of 
 the shearing stresses involved; in other words they form with refer- 
 ence to the symmetry of the pot. Similar phenomena are of common 
 occurrence in nature as, for example the columnar jointing of certain 
 lava flows in which the joint columns are normal to the cooling 
 boundary surfaces. For the formation of plane joint cracks, slow 
 and fairly uniform cooling is essential. In the case of stresses de- 
 veloped by the sharp blow of a hammer, the distribution of the elastic 
 stresses is entirely different, and characteristic wavy or conchoidal 
 fracture surfaces develop in well-annealed glass or other homoge- 
 neous material. If, however, there exist other stresses, as in a block 
 of poorly annealed, badly strained glass, in addition to those devel- 
 oped momentarily by mechanical means, the fracture surface resulting 
 from the hammer blow is uneven and commonly hackly in nature; 
 it is rough and torn as though the glass had been split across some 
 internal structural lines, as in a block of wood. 
 
 69 Jour. Amer. Ceram. Soc., 2, 543-563, 1919. 
 
POT- ARCH COOLING. 163 
 
 There is a noticeable difference between the types of fracture 
 developed in a mass of glass or other homogeneous material on cooling 
 and on that produced on heating. As a rule the cooling cracks tend 
 to be plane surfaces intersecting at angles ranging between 60 and 
 90 and to be approximately normal to, and parallel with, the bound 
 ary surfaces. Cracks developed on rapid heating are commonly 
 warped surfaces, rarely plane and rarely showing any tendency 
 toward regularity in the mode of their intersections. This criterion 
 was successfully applied during the war to detect and to locate 
 sabotage by enemy aliens in the grinding and polishing department 
 of one of the plants devoted to the manufacture of optical munitions. 
 
 POT-ARCH COOLING. 
 
 During the war different methods of cooling the melt were employed 
 and served the purpose well. During 1917 the method first described 
 by Schott in 1888 70 was followed; it had been in use at the Bausch 
 & Lomb plant before our arrival. The pot after removal from the 
 furnace was allowed to stand in the open air on fire-clay supports 
 and to cool for a period of 15 to 45 minutes. It was then placed on 
 similar fire-clay supports in the heated pot arch from which the new 
 empty pot had just been removed and set in the melting furnace. 
 During the interval occupied with the exchange of pots the door of 
 the pot arch was left open and its heating chamber allowed to cool to 
 700 or 800 C. The pot arch was then closed, its burners lighted 
 and the pot temperature allowed to fall to a dull-red heat (500 
 to 600 C.) in about 20 hours, after which the burners and stack 
 drafts were either closed completely and the pot arch was sealed, 
 or the burners were turned gradually lower so that by the end of the 
 next day (24 hours) the temperature had fallen to about 350 C. 
 It was found by measurement of the pot-arch temperatures that if 
 the cooling rate between 600 and 350 C. averages 8 C. drop per 
 hour satisfactory annealing and transverse fissuring result; but if 
 the temperature falls at a rate of 12 to 15 C. per hour poor 
 annealing and a poor quality of fracturing are obtained. Between 
 350 to 300 C. the burners are turned off and the furnace is allowed 
 to cool at a normal rate. By this time the transverse fissuring has 
 probably begun. Cold-air drafts should be avoided in the .pot arch, 
 as these may supercool the glass pot locally and thereby induce local 
 irregularities. 
 
 At the Spencer Lens plant C. N. Fenner modified this schedule 
 somewhat. He found that satisfactory annealing and fracturing 
 results if the pot of molten glass after removal from the melting 
 furnace and an exposure of about 15 minutes to the open air is placed 
 
 70 Ueber Glasschmelzerei fur optische und andere wissensch. Zwecke. Verein zur Beforderung des 
 Gewerbefleisses, 4 June, 1888. 
 
164 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 in the heated pot arch and the burners are so adjusted that the tem- 
 perature drops to a dull-red heat (500 to 600 C.) in about 16 hours. 
 The burners are then either turned off completely and the pot arch 
 is sealed so that its heat is lost solely by conduction through the 
 furnace walls, or the burners are so adjusted that the pot-arch tem- 
 perature above the pot drops to 400 C. in about 24 hours. After 
 this the burners are turned off and the pot arch is allowed to cool. 
 
 At the Pittsburgh Plate Glass Co. the pot-arch method used by 
 Roberts 71 was not greatly different from the foregoing. The melt 
 was cooled fairly rapidly (in 20 hours) to about 500 C., but still 
 
 MELT COOLING IN 
 HEATED AIR 
 
 600 
 
 GOOD ANNEALING 
 
 400 
 
 200 
 
 20 40 60 80 HOURS 
 
 FIG. 47. Time-temperature curves for optical glass melts cooled in a pot arch. The temperatures are 
 those of the air over and close to the pot. The quality of the annealing and fracture in each case are 
 indicated by the symbols chosen to represent the curve. 
 
 sufficiently slowly that the temperature difference between the center 
 and the sides of the glass mass did not exceed 75 C. The melt 
 cooled from 500 C. to 300 C. in the next 30 hours when the heat was 
 turned off, the rate of cooling of the pot being thereby temporarily 
 accelerated. In all cases Roberts insulated the top surface by a layer, 
 3 to 4 inches thick, of diatomaceous earth. 
 
 The results of the three different methods of cooling are summarized 
 in figure 47 reproduced from Roberts article. 72 These curves show 
 that slow cooling through the high temperature range serves no 
 purpose; whereas slow cooling through the annealing range from 
 
 n Jour. Am. Ceram. Soc., 2, 553, 1919. 
 72 Jour. Amer. Ceram. Soc., 2, 556, 1919. 
 
THE KATE OF COOLING. 
 
 165 
 
 500 to 300 C., is essential to successful annealing and to transverse 
 fracturing of the glass mass. 
 
 INSULATION TO REGULATE RATE OF COOLING. 
 
 Because of the fact that the temperature range, through which 
 careful regulation of the rate of cooling is required, is below 600 C., 
 it is possible to envelop the pot of molten glass with heat-insulating 
 material and thus to retard its cooling rate as effectively as in a pot 
 arch. This method of pot cooling was developed chiefly at the 
 Pittsburgh Plate Glass Co. and was placed on an effective routine 
 basis by Mr. H. S. Roberts. Either sand or a better heat-insulating 
 material, such as diatomaceous earth (kieselguhr) , may be used and 
 the pot surrounded by it. A layer of sand 8 inches thick suffices 
 for pots 30 to 50 inches in diameter. The moving of the sand is a 
 laborious operation and the final form of insulating device, as devel- 
 oped both at the Pittsburgh Plate Glass Co. and at the Bausch & 
 Lomb Optical Co., was essentially a hollow, double-walled, sheet-iron 
 cap in the form of a cylinder closed at one end and equipped with 
 handles for lifting. The space, 3 to 4 inches wide, between the walls 
 and the ends of this cap are filled with light, diatomaceous earth. 
 A drum of this design is easily handled and, when placed in position 
 over the cooling pot, provides adequate heat insulation. The 
 schedule followed by Roberts for this method is briefly: On removal 
 of the pot of molten glass from the melting furnace place it on three 
 fire-clay blocks at least 8 inches high to allow adequate circulation 
 of air along base of pot. Cover surface of melt with a layer 3 to 4 
 inches thick of diatomaceous earth. Four hours later for 36-inch 
 pots and eight hours later for 49-inch pots apply insulation in the 
 form either of an 8-inch layer of loose sand or of the sheet-iron in- 
 sulating cap. In all cases shovel loose sand or kieselguhr as insula- 
 tion beneath pot. Remove insulation from 36-inch pot three days 
 later and from 49-inch pot five days later. Pot can be broken up 
 two or three days later. The temperatures at the different stages 
 of this operation were measured by Roberts and are listed in Table 
 12 reproduced from Roberts article. 73 
 
 TABLE 12. Approximate temperatures at center of 36-inch pot melts cooled in sand. 
 
 Type of glass. 
 
 n D . 
 
 Tem- 
 perature 
 set out. 
 
 Tem- 
 perature 
 after 4 
 hours. 
 
 Anneal- 
 ing tem- 
 perature. 
 
 Tem- 
 perature 
 after 72 
 hours. 
 
 Light flint. . . 
 
 57 
 
 C. 
 1 038 
 
 C. 
 
 800 
 
 C. 
 
 465 
 
 C. 
 320 
 
 Medium flint 
 
 61 
 
 '996 
 
 780 
 
 455 
 
 300 
 
 Dense flint 
 
 66 
 
 968 
 
 760 
 
 445 
 
 290 
 
 Ordinary crown 
 
 52 
 
 1 093 
 
 850 
 
 570 
 
 350 
 
 Borosilicate crown 
 
 52 
 
 1 116 
 
 860 
 
 590 
 
 360 
 
 Light barium crown 
 
 1 57 
 
 1*016 
 
 790 
 
 580 
 
 310 
 
 Dense barium crown 
 
 1.61 
 
 i'oi6 
 
 790 
 
 600 
 
 310 
 
 
 
 
 
 
 
 7 * Jour. Am. Ceram. Soc., 2, 561, 1919. 
 
166 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 BREAKING UP A POT OF GLASS. 
 
 The pot of glass, after having cooled to room temperature, is set 
 out on the floor of the furnace hall where it is broken apart. (Fig. 
 48.) In this operation care is taken to preserve intact the large 
 blocks of glass bounded by joint planes. Commonly a vertical 
 master joint divides the pot into halves; the pot is split along this 
 plane by means of the chisel edge of a crow bar; the blocks of glass 
 are then jarred loose from the pot by tapping it with a sledge hammer. 
 The glass blocks are not hit directly with the hammer because of the 
 
 FIG. 48. Breaking apart pots of optical glass. Note the sheet-iron insulating cap on left, also 
 storage box for pot of optical glass on right of photograph. (Photograph by J. Harper Snapp 
 at plant of Bausch & Lomb Optical Co.) 
 
 shattering and loss which would be thereby incurred. All pot 
 fragments are separated from the glass and the entire mass of raw 
 glass is transferred to a box of standard size and divided into several 
 compartments to hold the small and large pieces of glass. (Fig. 48.) 
 Labels are attached to each box giving the number and date of the 
 melt and the type of glass. The boxes are made with raised bottoms, 
 so that a special lifting truck can be used to move them from 
 place to place as needed without tilting. Boxes of this kind, made 
 of heavy lumber and of standard shape and adequate size are con- 
 venient; they can be stored three or four deep and were found to 
 be satisfactory in every respect. It is essential that each pot of 
 glass be treated as a unit in subsequent operations, otherwise there 
 is danger of mixing one type of glass with another and this leads 
 
ROLLED OPTICAL GLASS. 167 
 
 to disastrous results in later factory routine. Meticulous care should 
 be exercised to avoid mixing different glass melts. 
 
 Before transferring the box of raw glass to the storage vault, it is 
 examined while still on the floor of the furnace hall by means of a 
 strong electric light 74 for striae, stones, color, bubbles and seeds, 
 character of fracture, size of blocks, and state of annealing; a rough 
 estimate is made of the general quality and percentage yield of 
 usable glass in the melt. A record of this preliminary inspection, 
 together with the optical constants measured on pieces, selected at 
 random from the broken fragments is filed for reference and enables 
 the manager to form an estimate of the kinds and quality of raw 
 glass on hand and available for further operations. 
 
 The box of glass is now transferred from the furnace hall to the 
 storage vault and passes out of the hands of the glassmaker to the 
 trimmers and thence through the pressing and molding stages to 
 the grinders and polishers, to the inspectors and thence back to the 
 storage vault. These stages of the manufacturing process will now 
 be considered briefly. Before leaving the furnace hall, however, a 
 modification of the standard process of optical glass manufacture 
 may be discussed because it was introduced during the war as a 
 time-saving method and proved in practice to be well adapted for 
 certain purposes. 
 
 CASTING OF OPTICAL GLASS. 
 
 Spectacle lenses are made from ordinary crown glass rolled into 
 sheets similar to plate glass. This process of manufacture is dis- 
 tinctly different from that of optical glass and has many advantages 
 in its favor. The casting process, as practiced in the plate-glass 
 industry, is briefly the following: The pot of molten glass, after it 
 has fined properly and cooled somewhat, is removed from the melting 
 furnace and lifted by a traveling crane to a heated, flat, iron casting 
 table, 14 feet wide and 22 feet long, on which the molten glass is 
 poured and then rolled out into a sheet of the desired thickness by 
 means of a heavy cylindrical iron drum 20 inches in diameter. The 
 sheet of glass is then pushed into a heated annealing oven or lehr 
 where it cools down to room temperature in the course of a day or 
 so, depending on the thickness of the sheet. The empty melting 
 pot is returned at once to the melting furnace and is gradually 
 refilled with raw batch for a new run. Casting pots may be used 
 for 10, 20, and even 30 or more runs. The quality of glass has been 
 found to improve after the first two or three runs because the walls 
 of the pot become more tightly sintered and baked, and offer greater 
 resistance to the metal. 
 
 74 A portable automobile head or search light answers the purpose well. 
 
168 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 This procedure eliminates many of the troublesome operations 
 connected with the cooling-down process, and also with the molding 
 and pressing, annealing and grinding, and polishing of the blocks of 
 pot glass preparatory to the final inspection. During the rolling 
 operation the striae in the molten glass are spread out as thin sheets 
 and bands or ribbons parallel with the surfaces of the plate and hence 
 do not appear when the plate, after polishing, is examined through 
 the " flats," as for instance in a window pane of plate glass. If, 
 however, the plate is examined through the edges, it resembles a pile 
 or ream of sheets of paper viewed edgewise; hence the plate-glass 
 maker's name, "ream," for this kind of striae. Striae arranged uni- 
 
 FIG. 49. Casting a small pot of light crown optical glass. (Photograph by J. Harper Snapp at 
 plant of Bausch & Lomb Optical Co.) 
 
 formly in this manner are readily detected when the glass plates are 
 examined through the fracture surfaces obtained on cutting a sheet 
 of glass into small plates by means of a diamond point or glass-cutter's 
 wheel. 
 
 Were it possible to produce optical glass of good quality by this 
 method much time and expense would be saved and production 
 expedited materially. Before the war rolled spectacle crown glass 
 had been successfully used for certain lens elements, such as eyepiece 
 field lenses, graduated scales (reticules), and prism shields in certain 
 fire-control instruments. Experiments were accordingly tried both 
 at the Pittsburgh Plate Glass Co. and at the Bausch & Lomb Optical 
 Co. to pour and to roll optical glass after the manner of plate glass. 
 
ROLLED OPTICAL GLASS. 
 
 169 
 
 The pot of optical glass is removed from the furnace at a temperature 
 at which the melt is still fairly liquid or fluent and is transferred as 
 quickly as possible to the casting table. Experience is required to 
 pour glass properly, and for the purpose the services of an experienced 
 hand at casting plate glass are valuable. (Fig. 49.) The tempera- 
 tures at which 49-inch pots should be removed from the furnace for 
 casting are, for the different types of glass, the temperatures at which 
 in the stirring operation it is necessary to reduce the size of the 
 stirring circle and are approximately 75 the temperatures listed in 
 Table 13. 
 
 TABLE 13. Castinj temperatures for different types of optical glass. 
 
 
 
 Temper- 
 ature of 
 
 " 
 
 
 Temper- 
 ature of 
 
 
 
 melt on 
 
 
 
 melt on 
 
 
 
 removal 
 
 
 
 removal 
 
 Type of glass. 
 
 D . 
 
 from 
 furnace 
 
 Type of glass. 
 
 n D . 
 
 from 
 furnace 
 
 
 
 for 
 
 
 
 for 
 
 
 
 casting 
 
 
 
 casting 
 
 
 
 pur- 
 
 
 
 pur- 
 
 
 
 poses. 
 
 
 
 poses. 
 
 
 
 C. 
 
 
 
 C. 
 
 
 1 52 
 
 1 200 
 
 Light flint 
 
 1.575 
 
 1,160 
 
 
 1 515 
 
 1 200 
 
 Medium flint 
 
 1.615 
 
 1.140 
 
 Barium crown 
 
 1 57 
 
 1 175 
 
 Dense Hint . 
 
 1.65 
 
 1,100 
 
 do 
 
 1 61 
 
 1 200 
 
 Barium flint 
 
 1.61 
 
 1, 175 
 
 
 
 
 
 
 
 The results obtained with rolled optical glass prove that it is possi- 
 ble with careful selection to produce optical glass of good quality by 
 this method. For most lens elements rolled optical glass is satisfac- 
 tory, because in these lenses the light passes approximately normal 
 to whatever ream may be present. Striae and ream are present in 
 sheets of glass rolled from well-stirred melts, because even in these 
 melts it is impossible to eliminate entirely the differences in composi- 
 tion between the margins and the center of the melt resulting from 
 pot solution and from selective volatilization of the melt. These 
 peripheral portions are spread through the homogenous central por- 
 tion of the melt by the rolling operation and give rise locally to 
 "ream." 
 
 Experience with rolled optical glass has proved definitely that the 
 casting method is not only feasible but, in many respects, superior to the 
 ordinary method, especially as a war-time measure. It is an entirely 
 American development and its adoption during the war resulted in 
 an appreciably increased rate of production. For large prisms, 
 especially pentaprisms and roof-angled prisms in which the light rays 
 traverse the prism in different directions, glass practically free from 
 striae is required and is best obtained by the standard method of 
 manufacture. But for ordinary, low-power visual instruments and 
 
 75 Taken in part from Hostetter and Roberts, Jour. Am. Ceram. Soc., 3, 1920. 
 
170 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 photographic lenses the quality obtained by pouring and rolling is 
 satisfactory. 
 
 All types of glass may be cast if certain precautions are observed. 
 The flint series, and even extra dense flints, are of course most readily 
 cast because of their viscosity relations and ease of annealing. Sheets 
 of glass 2 inches thick may be cast by experienced hands under prop- 
 erly regulated conditions. 
 
 For many purposes inspection through the edges of the small plates 
 cut with a diamond from a large sheet suffices for the detection of 
 imperfections, such as striae, bubbles, stones, strain, and color. For 
 more critical work the edges should be ground and polished so that rec- 
 tangular plates are obtained. 
 Inspection of plates of rolled 
 glass polished on the sides only 
 (flats) may fail to reveal the 
 presence of heavy ream. 
 
 PREPARATION OF RAW POT 
 GLASS FOR PRESSING OR 
 MOLDING. 
 
 The operations described in 
 this and the next few sections 
 apply only to raw pot glass 
 and not to rolled optical glass. 
 From a production standpoint 
 one of the advantages of rolled 
 optical glass over ordinary 
 optical glass is the reduction of 
 the number of factory opera- 
 tions required to produce the 
 rolled stock and the conse- 
 quent saving of time and ex- 
 FIG. so. The inspection of optical glass in rough pense which result therefrom. 
 
 chunks. (Photograph by J. Harper Snapp at plant Each pot of raw DOt glass as 
 of Bausch & Lomb Optical Co.) . . \ 
 
 it is received from the storage 
 
 vault or from the furnace hall is inspected in the rough (fig. 50) and 
 the portions of each block which contain striae or other imperfections 
 are trimmed off. The large blocks are broken into smaller pieces suit- 
 able for molding and pressing. For this purpose a heavy hammer and 
 a sharp chisel with a cutting edge 1 to 2 inches long are used. (Fig. 51 .) 
 The block of glass is placed on several thicknesses of thick, compact 
 felt which shields the glass from bruises; the block is struck a sharp 
 quick blow. With practice the operator can break a block of glass 
 into nearly rectangular pieces with approximately flat sidesj To 
 
TRIMMING OF GLASS. 
 
 171 
 
 accomplish this, well annealed glass is essential; poorly annealed 
 glass breaks so irregularly and with such rough surfaces that it is 
 exceedingly difficult to prepare smooth pieces from it. 
 
 For the direct inspection of the glass blocks a source of illumina- 
 tion interrupted by dark areas is advisable; thus skylight entering 
 through a window of many small panes or through a wide lattice 
 work placed in front of a large window pane is better than uninter- 
 rupted skylight, because faint imperfections, such as fine striae, can be 
 
 FIG. 51. Trimming raw optical glass. (Photograph by J. Harper Snapp at plant of Bausch & 
 
 Lomb Optical Co.) 
 
 seen more readily in half-shadow, oblique illumination than in direct 
 illumination . The pieces containing imperfections are discarded as cul- 
 let ; the pieces free from obvious imperfections are trimmed preparatory 
 to the pressing or molding operations. Reentrant angles are broken 
 off; bruises and blemishes from the chisel are trimmed off; irregular 
 and sharp angles are ground off. The glass blocks are best trimmed 
 by the use of tough steel chipping-blocks; these are essentially square 
 pieces of hard steel, one-fourth to one-half inch thick and 3 or more 
 inches on a side, clamped upright by a screw bolt into a heavy iron 
 
172 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 support. (Fig. 52.) The edges of the plate are ground sharp; the 
 piece of glass to be trimmed is placed against the edge or corner of 
 the steel plate and the glass block is struck a blow on the opposite side 
 with a weighted but light hammer of fiber, leather or celluloid. A 
 hammer of this kind does not bruise the glass and enables the trimmer 
 to chip off pieces of glass of almost any size and shape from the block. 
 With a little practice trimmers become expert and prepare pieces 
 of glass for pressing in a short time. Unless closely supervised, how- 
 ever, they may become careless and waste a considerable amount 
 
 FIG. 52. Trimming defects from pressed plates of optical glass. 
 (Photograph by J. Harper Snapp at plant of Spencer Lens Co.) 
 
 of good glass. The trimming tables are brushed off and cleaned thor- 
 oughly after the preparation of each pot of glass in order that glass 
 from different pots may not be mixed. 
 
 THE INSPECTION OF RAW GLASS BY THE IMMERSION METHOD. 
 
 For the careful inspection of optical glass it is necessary that the 
 surface imperfections be removed either by grinding and polishing 
 plane-parallel surfaces or by immersing the glass fragment in a liquid 
 of the same refractive index. The latter method was suggested and 
 tried out by the writer in May, 1917 (Weekly Report No. 3 for week 
 ending May 19, 1917), but was not adopted as a factory method 
 
INSPECTION OF RAW GLASS. 173 
 
 because of the difficulty of obtaining suitable refractive liquids. 
 Half a year later the method was tried and adopted at the Pitts- 
 burgh Plate Glass Co. A mixture of carbonbisulphide and benzol 
 was used, in spite of the danger from fire and of the possibility of 
 distress to the workman caused by the carbonbisulphide fumes. 
 
 Faint striae in optical glass are detected because their refringence 
 is slightly different from that of the surrounding glass. Two methods 
 are in common use to render faint striae visible; both depend on the 
 deviations, produced by the striae, in the paths of transmitted light 
 rays; these in turn give rise to differences in intensity of field illumin- 
 ation which under favorable conditions can be readily seen. Appro- 
 priate methods for this purpose are described in detail in the next 
 chapter. A simple method for rendering striae visible in a block of 
 glass immersed in a liquid of the same refringence is to examine the 
 block against a background 6 feet away, consisting of a lattice work 
 or a sheet iron plate, in which a series of rows of half-inch holes have 
 been drilled, placed directly in front of a frosted or opal sheet of 
 glass illuminated from behind by an electric light. In the half 
 shadows of the field illuminated in this manner striae stand out dis- 
 tinctly as faint shadows or lines of light. 
 
 A second, more sensitive method was also used at Pittsburgh and 
 was developed especially by Mr. W. H. Taylor, of the Bureau of 
 Standards. In place of white light, monochromatic light obtained 
 by prismatic refraction (carbonbisulphide dispersion prism with 
 collimator and telescope) was used; it enabled the observer, by 
 proper shift of the spectral color, to obtain a very accurate match 
 in refractive index between the liquid .and the immersed block, thus 
 causing the surface markings of the block to disappear altogether and 
 hence rendering the field illumination uniform in the case of glass 
 free from striae. Lenses are used in this method to render the 
 transmitted rays parallel and thus to increase its sensitiveness. 
 
 The refractive liquid tank, as developed at the plant of the Pitts- 
 burgh Plate Glass Co., is made of a piece of iron plate one-eighth to 
 one-fourth inch thick and bent into a flat-bottom U -shape; this forms 
 the bottom and two of the sides of the tank. The two remaining 
 parallel sides are of plate glass cemented to the iron plate by a mix- 
 ture of glue and plaster of Paris, or of lime and zinc oxides in sodium 
 silicate. Tanks of different sizes are useful. The blocks of glass 
 are held on a simple wire holder and immersed with it into the 
 refractive liquid. The refractive liquid tank is kept covered as 
 much as possible in order to prevent losses by evaporation and to 
 hold the refractive index of the liquid constant. In place of carbon- 
 bisulphide which has many unfavorable properties, a-monobromnaph- 
 thaline 76 may be used. It is an oily liquid and leaves an oily film on 
 
 7 6 Halowax oil, a cheap, impure form of monochlornaphthaline, was also tried. 
 3922921 12 
 
174 MANUFACTURE OF OPTICAL GLASS. 
 
 each piece of glass examined. The carbonbisulphide volatilizes com- 
 pletely, leaves no film, and is ideal in this respect. 
 
 A simple method for adjusting the refractive index of the liquid 
 mixture to that of an immersed glass fragment is to sight through 
 any wedge-shaped (prismatic) edge of the piece of glass toward a 
 narrow slit of light, such as a single filament of an electric bulb. 
 Compare the position of the light filament when observed through 
 glass and liquid and then through the liquid alone. In case, on 
 interposing the glass fragment, the filament appears to move toward 
 the thick end of the wedge-shaped glass piece, the index of the liquid 
 is too low, and vice versa. If the liquid and glass prism have the same 
 refractive index for the central part of the spectrum (yellow to green) 
 there is no appreciable shift of the filament on insertion of the glass 
 fragment in the path of light. If viewed under oblique illumination 
 under the conditions of equal refractivity for yellow light the edges 
 of the glass fragments show red and blue and purple colors. 
 
 This method of liquid inspection has many features in its favor and 
 was operated successfully at the Pittsburgh Plate Glass Co.; but at the 
 Bausch & Lomb plant, in spite of the most improved system of venti- 
 lation and special booths for the purpose, the workmen refused to 
 use the method after a few days trial. In case a suitable high 
 refracting liquid were available which is not poisonous and evil- 
 smelling, the method would be adopted in all glass plants and much 
 labor and expense thereby saved. The immersion method is useful 
 for the rapid determination of the refractive indexes of glass samples 
 and is valuable in separating plates or fragments of different glasses 
 which may have been mixed through error. 
 
 From one- third to one-half of the pot glass is commonly discarded 
 as cullet or is lost by the breaking and trimming operations pre- 
 paratory to flattening. 
 
 THE MOLDING AND PRESSING OPERATIONS. 
 
 The trimmed, but still irregular, blocks are now flattened into place 
 of different thicknesses either by molding or by pressing. In 
 European practice has always been to mold the glass, but in this 
 country the pressing method was largely used during the war. Expe- 
 rience with both methods indicates that for small blocks of glass the 
 pressing method is preferable to the molding method, but that for 
 large blocks or thick plates the latter is superior and that less glass 
 is wasted by this method than by the pressing method. 
 
 The pressing process. The prepared blocks are put into a preheat- 
 ing kiln and are slowly heated over night to 450 to 500 C v depend- 
 ing on the kind of glass. They are then placed as needed into the 
 muffle, which is heated by a gas-air blast playing from a side entrance 
 over the arch of the muffle. Heat radiates from the crown of the 
 
MOLDING AND PRESSING OPERATIONS. '175 
 
 muffle to the base plate on which the pieces of glass rest. Soon the 
 glass begins to soften and the operator prevents it from sticking to 
 the base plate by moving it along and by spreading over the slab a 
 mixture of some refractory powder, such as clay and mica, or alumina, or 
 diatomaceous earth or talc or fine graphite or a mixture of these; the 
 fine powder sticks to the glass like flour to molasses taffy or to dough 
 and shields it from the plate. The base plate itself is generally made 
 of fire clay, but it may be of porcelain or of fused silica with a little less 
 binding, material, in which case practically no additional powder is 
 required. Specially prepared plates of graphite, mica, and clay are also 
 employed. The object desired is to prevent the molten glass from 
 sticking and at the same time to avoid, so far as possible, a heavy coat- 
 of powder over the glass surface, which on pressing may be infolded 
 and produce feathers, laps, and folds. For some purposes it is ad- 
 vantageous to heat the base-plate from beneath in order to avoid a 
 one-sided heating of the glass pieces. Heating by radiation from the 
 crown of the muffle liquefies the upper part of the glass block so that 
 it flows down and spreads out beyond the cooler bottom resting on the 
 base plate. 
 
 As the glass becomes softer the operator paddles it into shape with 
 iron rods, flattened at the ends, and may thereby in certain pieces 
 infold some of the dust-covered surfaces into the glass mass. The 
 operator endeavors to shape the glass pieces so that they fit into the 
 press mold properly. As soon as the glass has attained the proper 
 shape and temperature (750 to 900 C.) it is transferred to a pre- 
 heated iron mold and then placed under the heated plunger of a foot 
 or pneumatic or hydraulic press and flattened to the desired thickness 
 (fig. 53). The flattening operation takes less than a second; during 
 this time the molten glass mass, which is of the consistency of thick 
 tar, must flow under forced pressure; it does so somewhat after the 
 manner of lava, in waves, the troughs of which are liable to be engulfed 
 in the flowing mass and to appear then as " pressing defects/' 
 " feathers," "folds," "laps" in the finished glass plate. With care- 
 less manipulation a very large percentage of all glass can be rendered 
 useless during this operation. In place of a mechanical press, the work- 
 man may shape the glass block to proper size by use of the iron paddles 
 alone. This is, of course, a slow operation, but there is less danger 
 from defects with this method. The rapid action of the plunger of 
 the press is necessary because the outer surface of the glass block 
 chills rapidly in the open air and becomes so stiff that soon the block 
 can no longer be pressed into shape. 
 
 If the iron mold or plunger is not hot enough, the surface of the 
 pressed plate is chilled too rapidly and becomes filled with fine 
 transverse cracks; in the case of a cold mold the crackling may be so 
 pronounced that it resembles the "craze" of ornamental glassware. 
 
176 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 In the pressing process an iron or steel mold of the desired depth is 
 is used. The plunger reaches the rims of the mold and presses the 
 glass to this thickness; in order to accommodate the mold to blocks 
 of different sizes it is closed on the sides and at one end only. A 
 plunger from the other end fits nicely into the mold and slides into it 
 just after the vertical plunger acts and presses the glass into square 
 ends. This operation enables the operator to obtain plates of con- 
 
 FIG. 53. Block of glass in mold and ready to be pressed into plate by plunger of hydraulic press. 
 (Photograph by J. Harper Snapp at plant of Bausch & Lomb Optical Co.) 
 
 stant thickness and width but of variable length; the side plunger 
 introduces, however, further infolding of the molten glass and may 
 occasion serious loss if n<?t handled properly. In many cases it is 
 better not to use the side plunger, even though the ends of the plate 
 do not then come out square. The advantage of the pressing process 
 is that plates of uniform thickness and width are obtained; its chief 
 defect is the loss of glass occasioned by pressing defects. An advan- 
 tage of the pressing process is that the number of the pot, the refrac- 
 
THE MOLDING PROCESS. 177 
 
 tive index and i>-value, and other desired information may be im- 
 printed on the block during the pressing operation. 
 
 A plan to avoid the danger from pressing defects and yet to retain 
 the advantages of the pressing method was proposed by Capt. H. C. 
 Fry, jr./ 7 and put to practical test on a small scale at the optical- 
 glass plant of the Bureau of Standards in Pittsburgh. His method 
 is essentially an adaptation of the method, used in ordinary glass 
 factories, of gathering the desired amount of molten glass on the end 
 of an iron rod (punty) and transferring it then to the press. In the 
 case of optical glass a block of glass is first slowly heated up to near the 
 softening point and is then stuck at one end to a bleb of molten glass, 
 of the same kind at the end of a punty; on the punty it is heated and 
 reheated carefully in the direct flames of a glory hole and is paddled 
 with proper shaping tools from time to time to approximately the 
 desired shape. The actual pressing operation does not then involve 
 much deformation of the glass mass and the danger of infolding is 
 thus reduced to a minimum. Plates, prisms, and lenses pressed by 
 this method have clean surfaces like the glassware produced in ordi- 
 nary glass factories and are free from feathers. Compared with the 
 ordinary method, this method is much slower and only an actual 
 factory test can demonstrate whether its advantages of better quality 
 of product outweigh the disadvantage of greatly decreased rate of 
 output. 
 
 The molding process. In this method the irregular pieces of glass 
 are taken as they come from the trimmer (practically in the shape 
 as they are broken from the pot because trimming is less essential 
 in the molding process than in the pressing process) and are placed 
 in molds of proper sizes, which are then set at the cool end of a tunnel 
 about 20 feet long, 2 feet wide, and 10 or 12 inches high, inside 
 dimensions; the molds are pushed gradually, one behind the other, 
 toward the hot muffle end of the furnace; about 6 feet from the hot 
 end of the tunnel a clay partition or curtain extends to within 4 or 
 5 inches of the floor and serves to confine the heat to a certain extent. 
 The molds on reaching this part of the furnace have attained a temper- 
 ature of 500 or 600 C. and may then be thrust into the hot muffle 
 chamber where the temperature rises rapidly to 1,100 and even to 
 1,200 C. This abrupt rise in temperature is valuable for several 
 reasons; the heat in the muffle is derived from the heated arch by 
 radiation; the glass is therefore heated largely from the top down. 
 By melting down the glass from the top the liability to formation of 
 bubbles is largely avoided; furthermore the glass is taken rapidly 
 through the critical temperature range of devitrification with respect 
 to any one of the components; within this range, precipitation may 
 
 " J. Am. Ceram. Soc.,'2, 432, 1003, 1919. 
 
178 MANUFACTURE OF OPTICAL GLASS. 
 
 occur either as crystallization and the formation of a devitrified 
 crust, which is very difficult to remove, or as a milky, colloidal develop- 
 ment which is even more serious. In fact, by this process opalescent 
 glass becomes in some instances clear and satisfactory for optical 
 purposes. Once the molds enter the heated chamber the glass melts 
 down rapidly. The molds are gradually pushed forward from the 
 cool end; the hot molds are removed from the muffle end. 
 
 The time required for the entire process is about two and one-half 
 hours; approximately 25 molds are kept in line; the output per 
 tunnel per day of 24 hours is from 400 to 500 molds. The molds are 
 transferred to a cooling arch, where they are stacked and allowed to 
 cool down slowly for several days. It would be a great improvement 
 if the molding tunnel were heated electrically and made much longer 
 so that the annealing of the molded plates could be accomplished in 
 the same tunnel, thus avoiding the necessity of transfer to an anneal- 
 ing kiln or lehr. In this case careful regulation of the temperatures 
 would be required and also probably an automatic feed to carry the 
 moulds through the tunnel at the prescribed rate. 
 
 The tunnels are simple in construction and can be built side by 
 side, each offset, echelon fashion, to give room for the muffles. The 
 wear on clay molds is severe; the life of a mold averages about three 
 runs through the tunnel; cast-iron and other metal molds have been 
 used with some success. The temperatures ^required in the muffle 
 furnace vary with the type of glass; the workman learns readily from 
 the behavior of the glass in the molds to regulate the gas blast so that 
 the optimum conditions for each type of glass are obtained. 
 
 The molding tunnels may well be designed so as to have a curtain 
 chamber beyond the muffle in which the molds can cool down to, say, 
 600 C, or lower in the case of flint glasses ; the glass plates may then be 
 dropped from the molds and transferred to the annealing kiln. At 
 these temperatures neither the glass nor the molds are so brittle as at 
 ordinary temperatures, and ,they can be handled roughly without 
 danger of breaking. The advantages of this procedure are obvious; 
 the glass plates in sliding into the cooling chamber are not tilted, and 
 consequently cool down on even keel; the removal of the mold from 
 the glass greatly increases the capacity of the annealing furnace. 
 The danger of cracking in the molded glass. plate, because of the dif- 
 ferent rate of contraction of the clay mold, is also eliminated by this 
 operation. In the barium crown glasses cracking of plate^ cooled 
 down in the molds may be serious. In all work of this kind the an- 
 nealing furnace should not be heated so hot that the inserted plates 
 bend out of shape. 
 
 Before introduction into the tunnel the molds with glass blocks 
 may be preheated by placing them above the molding tunnels, thus 
 expediting the actual tunnel treatment. The chief difficulties and 
 
ANNEALING OF GLASS. 179 
 
 losses in the molding process arise from the glass plates sticking to 
 the molds; in removing the glass plate either it is broken or the mold 
 is broken, or both. To prevent the glass plates from sticking, the mold 
 is lined with a slip consisting of mixtures of clay and mica or sand, or 
 of alumina, talcum powder, graphite, or other refractory material. 
 If the slip is too thick, some of the powder may be carried up into the 
 molten glass. In the molding operations the best conditions of treat- 
 ment for each type of glass are ascertained only by experience with 
 each kind of mold in each type of tunnel. Porcelain molds have been 
 found to be more satisfactory than the ordinary clay molds, which 
 have a tendency to produce bubbles in the molded plates. 
 
 Examinations of many hundreds of plates of molded glass, both 
 foreign and domestic, proves that the molding process produces 
 plates freer from defects than are pressed plates. It is therefore less 
 wasteful of glass than the pressing process and is nearly as efficient 
 and rapid, especially for larger plates; for small lens and prism blanks, 
 the pressing method is obviously superior. 
 
 The molding process is most valuable for the molding of large 
 blocks from which large prisms can be sawed directly. The quality 
 of the glass blocks can be approximately determined as they are 
 taken from the pot; the molding process tends to confine existing 
 defects; the pressing process tends, on the other hand, because of the 
 rapid flow of glass, to cause existing striae to spread out into the good 
 glass and also to introduce infolded portions of the clay and dust 
 between surfaces of the paddled piece of glass. 
 
 One disadvantage of the molding process is the fact that by it 
 the plates can not be automatically labeled with pot number etc. as 
 they can by the pressing process. 
 
 In both pressing and molding processes the workmen must be 
 trained to the tasks and carefully supervised, otherwise much good 
 glass may be wasted as well as time and expense. 
 
 THE ANNEALING OF MOLDED OR PRESSED PLATES. 
 
 The general principles, on which methods for annealing optical 
 glass are based, have already been stated in the section on the cooling 
 down of pots of optical glass. These principles were known in 
 part when we entered the war, but the details, especially the tem- 
 perature to which each type of glass should be heated for annealing 
 and also the time-temperature relations during the cooling of the 
 several types of glass, had to be ascertained by actual experiment. 
 Accordingly a simple arrangement was adopted which enabled us 
 to observe the course of annealing at all stages of the process. (Fig. 
 54.) Most of the experimental work was done in a small, gas- 
 heated kiln through the back of which a hole was drilled to allow a 
 beam of plane-polarized light to traverse the furnace. A beam of 
 
180 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 plane-polarized light was obtained by reflection of rays from an in- 
 tense light source by a plane polished plate of opaque glass mounted 
 at the proper polarizing angle; this beam of polarized light traversed 
 the glass plates in the furnace in a horizontal direction and was ana- 
 lyzed by means of a nicol prism and a sensitive tint plate held outside 
 and in front of the furnace, the end openings in the furnace being 
 protected by thin glass windows. By means of this simple arrangement 
 the exact state of strain in a plate in the furnace could be ascertained 
 at any time and its temperature measured by means of a thermo- 
 element and a millivoltmeter or a potentiometer system. Series of 
 strain-temperature-time measurements on plates of the several 
 different types of glass were made and from these practical annealing 
 schedules were worked out which proved satisfactory in practice. 
 In point of actual time expended and of difficulties encountered the 
 
 FIG. 54. Simple arrangement adopted for the study of strain-temperature-time relations in optical glass. 
 L is an electric bulb; F, a plate of frosted glass; G, a piece of opaque glass placed at the polarization 
 reflecting angle; A, a piece of thin glass; M, the gas-heated annealing oven; P, the glass plate under 
 test; T, the thermoelement; B, a thin glass window; S, a sensitive-tint plate: N, the analyzer. 
 
 general factory problem of annealing proved to be one of the easiest 
 to solve and to put into routine practice. From a theoretical view- 
 point it is, however, an exceedingly complex problem and many of 
 the factors involved are inadequately known even at the present 
 time. A brief summary of the problem will serve to indicate some 
 of the difficulties which it presents. 
 
 At high temperatures glass is a mobile liquid; at room-tempera- 
 ture it is practically an elastic solid. On cooling from a high tem- 
 perature the fluidity of molten glass decreases with fall in temperature 
 until its tendency to flow, in the ordinary sense of the word, ceases; 
 the liquid gradually congeals, as it were, and passes from a viscous, 
 fluent body to a plastico-viscous body; with still further decrease of 
 temperature the elastic qualities begin to. dominate until at room- 
 temperature but little evidence of plasticity or viscosity remains. 
 This passage from the one condition to the other is a continuous 
 
ANNEALING OF GLASS. 181 
 
 process except for a small temperature interval just below the soften- 
 ing region at which the glass begins to flow; in this temperature 
 region 78 there is a rapid rise in the expansion coefficient accompanied 
 by a distinct heat effect (evolution of heat on cooling and heat 
 absorption on heating). The temperature region of special impor- 
 tance in glass annealing has for its upper limit that temperature at 
 which glass is just able to support deformational loads, and hence 
 internal stresses, for appreciable periods of time; from this tempera- 
 ture down to room-temperature the behaviour of glass is interesting 
 to follow. 
 
 Twyman and others have shown that for the first 100 or 200 C. 
 below the softening temperature (the annealing range) the viscosity' 
 
 is doubled with each drop of about 8 C. in temperature, or M= K.2 8 
 in which M is the- measure of the rate of deformation at a given 
 temperature, Kis & constant depending on the kind of glass, and 6 the 
 temperature. If glass were a perfectly elastic body, Hooke's law, 
 that the stress is proportional to the strain, would apply, or S = E B in 
 which S is the stress, B the strain, and E the modulus of elasticity. 
 The change of stress with tune C would be accordingly proportional 
 to the change of strain with tune or dS/dt = E-dB/dt. At the higher 
 temperatures glass is not perfectly elastic and internal stresses, set 
 up by initial strains, are gradually relieved by flow. The simplest 
 assumption, made first by Maxwell 79 is that the rate of relief at a 
 given temperature varies with the stress and the kind of glass or 
 dS/dt=E-dB/dt-S/T'm which T is a constant, the "time of relaxa- 
 tion" as designated by Maxwell. If the strain B is constant, as it 
 is in the case of a body under deformational load at constant tem- 
 perature, or if it changes only slightly with time, we may write as a 
 first approximation 
 
 an equation which represents the course followed in the release of the 
 inner stress; S is the initial stress; $ t the stress after the time t. 
 If on the other hand the stress, S, is constant, as it is in the case of 
 a glass rod under compression or tension, then S = E-TdB/dt. By 
 measuring the rate at which the glass rod is deformed with time for 
 a given load S we can compute the time of relaxation T] this is the 
 time, according to Twyman, required for annealing the block of 
 glass stressed to the given amount and held at the given temperature. 
 Twyman states that his experimental results corroborate the above 
 assumption suggested tentatively by Maxwell. Adams and William- 
 
 " Tool and Valasek, Bureau of Standards, Paper No. 358, 1919; Peters and Cragoe. Jour. Opt. Sec 
 America, IV, 105-144, 1920. 
 Phil. Mag. (4), 34, 129, 1868. 
 
182 MANUFACTURE OF OPTICAL GLASS. 
 
 son, on the other hand, state that their more recent data indicate 
 that the relief of stress by flow at a given temperature is proportional 
 not to the stress but to the square of the stress or 
 
 dS/dT=E-dB/dt-S 2 /T 
 from which we derive for a constant load (dS/dt = 0), 
 
 S 2 =T-E-dB/dt; 
 and for constant strain (dB/dt = 0~) , 
 
 ,,,. S 2 lit 
 db/at= m> or s~sT = ~T' 
 
 In 1912 Zschimmer and Schulz 80 carried out a series of experiments 
 on the amount of strain introduced on the rapid cooling of blocks of 
 glass from different temperatures and found that the effect of tem- 
 perature within the annealing range on the total amount of strain 
 produced is represented by the empirical equation 
 
 (S -S).(e-o ) = C 
 
 in which S , GO, and C are constants, 8, the strain, and 6 the tem- 
 perature from which the glass was chilled. These results of Zschim- 
 mer and Schulz are not, however, directly applicable to the above 
 problem and will not be considered further. 
 
 The coefficient of expansion is practically linear up to a tempera- 
 ture not far below the softening temperature. (Table 10, p. 153.) 
 The temperature distribution in solids of different shapes and sizes 
 during heating or cooling has been investigated repeatedly, espe- 
 cially from a mathematical standpoint; and recently Williamson and 
 Adams have considered the subject with special reference to glass 
 bodies heated either linearly or with the surfaces held at a definite 
 temperature. In the study of strained glass samples optical methods 
 are especially useful and are based on the relation first discovered by 
 Brewster that in a glass block under load the optical effect produced 
 (birefringence or optical path difference per centimeter glass-path) 
 varies directly as the load; in other words, the stress is proportional 
 to the birefringence and the observed changes in birefringence may 
 serve as measures of the relative changes in the stresses. The rela- 
 tions between birefringence and stress in various types of glass are 
 listed in Table 11, as determined by Pockels and Adams and Wil- 
 liamson. Mathematical analysis of the strains and stresses in solids 
 due to temperature gradients was first given by F. Hopkinson and 
 recently has been presented by Williamson with special reference to 
 optical glass solids of simple shape. 
 
 With these relations in mind let us follow the changes in a block 
 of glass roughly spherical in shape and initially heated to a uniform 
 
 so Zeits. f. Inst.rumentenkunde, 38, 303, 1912; Sprechsaal, 47, 460-478, 1914; Ann. d. Phys. (4), 42, 345-396, 
 1913. 
 
ANNEALING OF GLASS. 
 
 188 
 
 500 
 
 temperature slightly below its softening temperature as it cools down 
 to room temperature. The glass adjacent to the surface cools at first 
 more quickly than the interior, and in so doing contracts and tends 
 to squeeze the center thereby setting up stresses of radial compression 
 throughout the mass. As the temperature gradient increases on 
 further cooling of the glass 
 mass the amount of stress 
 introduced increases through- 
 out the mass. 
 
 If given sufficient time, the 
 temperature gradients or the 
 differences in temperature 
 between the center and dif- 
 ferent points along a radius 
 approach a steady state as 
 indicated in (fig. 55) and if 
 the cooling were to continue 
 as linear cooling this tem- 
 perature gradient would per- 
 sist. The strains, thus set 
 up on cooling, produce stress- 
 es in the glass mass which at 
 the higher temperatures are 
 in large measure relieved by 
 actual internal flow. If the 
 cooling were to proceed with 
 sufficient slowness, the entire 
 stress at any high tempera- 
 ture could be relieved. If the 
 normal temperature gradient 
 were established at a suffi- 
 ciently high temperature, all 
 stresses would be relieved by 
 flow, and if this temperature 
 gradient were maintained un- 
 til the surface of the mass 
 reached room temperature no 
 
 Strp<^ would hp m-PSPTit flt FIG. 55. Curves representing the temperature distribu- 
 tion in a snhere of elass of unit, radius noolinir at , a 
 
 that instant. From here on 
 however, strain would be 
 introduced rapidly, because of the cooling of the center and its con- 
 sequent contraction and tendency to pull away from the outer shell, 
 thus setting up stresse of radial tension, and accompanying tangential 
 compressive stresses near the periphery; toward the center these 
 tangential stresses decrease, become zero, and pass into tangential 
 tensional stresses which at the center are equal to the radial stresses. 
 
 20(J o 
 
 |00 <> 
 
 i.o 
 
 0.5 
 
 0.5 
 
 1.0 
 
 tion in a sphere of glass of unit radius cooling at a 
 linear rate from 500 C. to C. The zero abscissa 
 
 represents the center of the sphere ' 
 
184 MANUFACTURE OF OPTICAL GLASS. 
 
 In general the rate of cooling is not linear nor is all the stress 
 relieved by flow. Obviously less stress is introduced and more exist- 
 ing stress is relieved by flow the more slowly the glass mass is cooled. 
 The result is that the temperature gradient becomes a maximum and 
 then decreases with further cooling until it disappears at room tem- 
 peratures. While the temperature gradient during this process is 
 increasing the outside is cooling faster than the center and introducing 
 stresses of radial compression. These are in part relieved by flow, so 
 that the optical effect observed is not so great as it would be for a 
 temperature interval of equal amount in a glass mass in which the 
 stress had been maintained at its full value. On further cooling the 
 temperature gradient is reversed and the center begins to cool more 
 rapidly than the periphery, thus neutralizing the stresses introduced 
 at the higher temperatures. There is this difference, however, that 
 the rate of relief of strain at the low temperatures is exceedingly slow, 
 so that little if any of the stresses introduced are relieved by internal 
 flow. The result is that not only are the stresses which were intro- 
 duced at the high temperatures neutralized, but radial tensional 
 stresses are set up in amount equal to the algebraic sum of the two 
 sets of stresses involved. The normal state of stress in a body at 
 room temperature is accordingly that which results when the center 
 has cooled more rapidly than the outside. This view of the subject 
 was apparently first emphasized by Twyman, and more recently by 
 Williamson and Adams. 
 
 As a further illustration let a block of glass be heated from room 
 temperature to 100 C., a temperature at which stress is not relieved 
 appreciably in a short period of time. Let the glass block be free 
 from strain at room temperature. On heating the glass, strain is 
 introduced into it and can be readily measured by the polariscope. 
 This strain reaches a maximum and then begins to decrease and 
 finally disappears completely with the attainment of the uniform 
 temperature, 100 C. On cooling, the reverse phenomena can be 
 observed. So long as the glass behaves as a strictly elastic body the 
 above relations hold true; but, if at any temperature part of the 
 stress is relieved by viscous or plastic flow, the phenomena cease to be 
 reversible and the statement is then to that extent incorrect. 
 
 Having determined the temperatures for the several types of glass 
 at which strain is relieved within a reasonable time and having ascer- 
 tained the times required at several different temperatures to reduce 
 the stress to a certain limit the observer ..still requires data on: (a) 
 The limits of permissible strain in optical glass when used for different 
 purposes; (b) the time- temperature rate of cooling in order that 
 strains of appreciable magnitude are not introduced during the cool- 
 ing period. For the specification of permissible strain a large number 
 of plates .of optical glass of different types, made by Chance Bros., in 
 
ANNEALING OF GLASS. 185 
 
 England, by Parra-Mantois in France, and by Schott and Genossen, 
 in Germany, were examined in polarized light and the amount of 
 maximum birefringence or the path difference per centimeter glass- 
 path was measured in each piece. This examination showed that the 
 maximum path difference for sodium light per centimeter glass-path 
 rarely exceeded 30 millimicrons and was commonly not over 10 milli- 
 microns. These values were characteristic of glass which before the 
 war had been used for optical instruments of the most diverse kinds, 
 such as range finders, telescopes, field glasses, microscopes, surveying 
 instruments, and had proved satisfactory in actual use. The maxi- 
 mum limit for permissible strain was accordingly arbitrarily set as 
 that which produced a maximum path difference of 20 millimicrons' 
 in a plate when viewed through the edges. This corresponds to a 
 path difference of 10 millimicrons per centimeter glass-path at the 
 center of the plate or a birefringence, 0.000001. The methods for 
 detecting and measuring these quantities are described in the next 
 chapter. It was also stipulated that the strain distribution should 
 be sensibly symmetrical in the plate and that local irregularities 
 should not be present. 
 
 For a determination of the time-temperature cooling rate the law 
 of Twyman was available that the mobility of glass decreases loga- 
 rithmatically with the temperature; also the statement by Twyman 
 that his measurements corroborated the assumption made by Max- 
 well, as a first approximation, that the time of relief of stress at a 
 given temperature is proportional to the stress itself. 
 
 By means of the arrangement described above for the study of 
 strain in optical glasses (fig. 54) the temperatures at which the strain 
 disappears very rapidly was found to be (heating rate 40 to 75 C. 
 per hour) in borosilicate crown 590 C., in light barium crown 600 
 C., in ordinary crown 540 C., in light flint 500 C. At 550 G. the 
 strain disappears from a plate of borosilicate crown in a few hours; 
 at 510 C. over night; at 480 C., 24 hours are not sufficient for the 
 removal of the strain. These and other measurements together with 
 Twyman's work enabled us to set up annealing schedules which pro- 
 duced glass plates as well annealed as the best foreign glass. The 
 annealing schedules were so arranged that, after insertion of the 
 plates into the[annealing kiln, it was held over night at a temperature 
 such' that by morning the entire glass charge would be at a uniform 
 temperature and practically free from strain. This temperature 
 was of course different for different glasses and was commonly 25 
 to 50 C. below the foregoing temperatures, at which the strain dis- 
 appeared very rapidly. The furnace was then cooled down at an 
 increasing rate in order to avoid the development of strain. The 
 ideal type of time-temperature cooling curve for the annealing of 
 glass is convex upward; the natural curve of a furnace is concave 
 
186 MANUFACTURE OF OPTICAL GLASS. 
 
 upward. The time-temperature curve of a cooling kiln may be 
 made to approximate the proper annealing curve by proper regula- 
 tion of the heat. More accurate and detailed data on the relaxation 
 times (time required to reduce strain-birefringence of different types 
 of glasses from 50- 10' 7 to 5- 10~ 7 are given in Table 10 quoted on page 
 153 from Adams and Williamson whose measurements are more recent 
 and were made under accurately controlled laboratory conditions. 
 
 Essential for the proper annealing of optical glass is the annealing 
 furnace. Furnaces of the proper design may be heated either by gas 
 or by electricity. The object to be attained in all designs is uniformity 
 in temperature distribution throughout the entire space occupied by 
 the glass. In the case of a gas-heated furnace this is attained by the 
 use of long perforated gas pipes which serve as gas burners for the 
 gas-air mixture and which extend the length of the furnace and are 
 situated below the two breast walls. Commonly no flue or stack is 
 used to conduct away the products of combustion. In case such 
 flues are used they should be located at intervals in the arch of the 
 kiln and be equipped with dampers so that the flow can be nicely 
 regulated. Any design which sets up a draft in the furnace may 
 introduce currents within the heating chamber and thus give rise to 
 inequalities in temperature distribution. 
 
 A reliable and accurate thermoelement installation for the meas- 
 urement of furnace temperatures is a second necessity. This con- 
 sists of two parts; the thermoelement (base metal thermoelements 
 answer the purpose well) in which an electromotive force is set up 
 between the hot and cold junctions, the magnitude of this force 
 depending on the temperature difference between the two ends; an 
 instrument for the measurement of the electromotive forces thus set 
 up; this may be either a potentiometer or a direct reader. Experi- 
 ence proved that the potentiometer type of instrument is preferable 
 to the direct reader (millivoltmeter) . Because the temperature dif- 
 ference depends directly on the temperature of the cold junction as 
 well as on that of the hot junction, it is essential that the temperature 
 of the cold junction be kept as nearly constant as possible. A 
 practical method to insure this constancy is to bury the cold junc- 
 tion 8 or 10 feet under ground and at some distance from the annealing 
 furnace. 
 
 It is also essential that the thermoelements be tested frequently 
 (at least once a month and as a routine job) and the correctness of 
 their readings ascertained. For this purpose additional thermoele- 
 ments should be kept on hand -so that they may be substituted for 
 thermoelements under test, thus insuring continued operation of the 
 annealing furnaces. A convenient standard temperature for ref- 
 erence is that of melting tin which can be kept in a pure state without 
 trouble. It is best to introduce the thermoelement from the top of 
 
GLASS AXXEALIXG SCHEDULES. 187 
 
 the arch of the furnace and to allow it to extend well into the heating 
 chamber. 
 
 A record should be kept of the quality of the annealing of each lot 
 of glass. The glass may for the purpose be divided into the following 
 classes : 
 
 Class 1. Annealing excellent. Maximum strain-birefringence less 
 than 5- 10~ 7 or 5 millimicrons per centimeter glass path. 
 
 Class 2. Annealing good. Maximum strain-birefringence between 
 5-10- 7 to 12- 10- 7 . 
 
 Class 3. Annealing fair. Maximum strain-birefringence between 
 12- 10' 7 and 20 -10' 7 and symmetrically distributed. 
 
 Class 4. Annealing poor. Strain-birefringence greater than 20- 10' 7 
 or, if less, then not symmetrically distributed. 
 
 The following records of annealing at the Bausch & Lomb plant, 
 supervised by G. W. Morey on January 15, 1918, in routine factory 
 kilns, each with a capacity of more than a ton of glass in pressed 
 plates arranged in the furnace so as to allow some circulation of 
 air, may be of interest. The furnace schedule in the case of the 
 borosilicate was the following: Temperature during filling-in period 
 from 1.40 p. m. to 4.10 p. m. 565 C. (1,050 F.). Temperature 
 maintained at 565 C. (1,050 F.) until midnight, then slowly 
 dropped to 524 C. (975 F.) at 8.30 a. m., .to 432 C. (810 F.) at 
 1.30 p. m. Gas was then turned off and furnace allowed to cool 
 down. Glass removed the following day. Of the 694 plates exam- 
 ined in this lot 82.5 per cent were in class 1, 9.5 per cent in class 2, 
 7.7 per cent in class 3, and 0.3 per cent in class 4. In another lot of 
 1,019 plates annealed at different times and of different kinds of 
 glass, ranging from 9 millimeters to 30 millimeters thickness, 93.5 
 per cent were in class 1; 4.4 per cent in class 2; 1.3 per cent in class 
 3, and 0.8 per cent in class 4. It is difficult to avoid a certain num- 
 ber of poorly annealed plates; these may have rested next to the 
 bottom of the kiln or been near the front and chilled by air currents. 
 
 Annealing schedules for various kinds of glass. In a recent publi- 
 cation on the annealing of glass, Adams and Williamson 81 present in 
 tabular form annealing schedules for glass slabs of different types 
 and thicknesses. These schedules are based on extended mathe- 
 matical and experimental investigations and are the best available 
 schedules at the present time. With slight modifications they can 
 be adjusted for other shaped pieces of glass, such as prisms, lenses, 
 spheres, cylinders, etc. These schedules are listed in Table 14 
 reproduced from the paper by Adams and Williamson. 
 
 81 Jour. Franklin Inst., 190, 850-856, 1920. 
 
188 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 TABLE 14. Annealing schedule for optical glass slabs of different thicknesses; the strain 
 of the annealed plate, as measured optically in polarized light ivhen the slab is viewed 
 through thz edges, not to exceed an optical path difference of 5 millimicrons per centi- 
 meter glass- path. 
 
 [Hold the glass at the proper annealing temperature for the indicated time and cool at the indicated rate.] 
 
 Kind of glass. 
 
 Thickness. 
 
 Annealing temperatures, degrees centigrade. 
 
 1cm. 
 
 2cm. 
 
 5cm. 
 
 10cm. 20cm. 
 
 Borosilicate crown 
 
 561 
 535 
 571 
 608 
 516 
 427 
 434 
 424 
 400 
 
 541 
 514 
 552 
 592 
 494 
 408 
 418 
 407 
 382 
 
 515 
 487 
 527 
 572 
 466 
 384 
 397 
 386 
 357 
 
 495 
 466 
 508 
 556 
 454 
 366 
 382 
 370 
 339 
 
 475 
 445 
 489 
 540 
 423 
 348 
 366 
 353 
 321 
 
 Ordinary crown 
 
 Light barium cr own 
 
 Dense barium crown 
 
 Barium flint 
 
 Light flint. 
 
 Medium flint 
 
 Dense flint 
 
 Extra dense flint 
 
 
 Annealing times.- Cooling rates, degrees centigrade per hour. 
 
 
 50 
 minutes. 
 
 v, 3 * 
 
 hours. 
 
 21 
 hours. 
 
 86 
 hours. 
 
 14 days. 
 
 Initial rate 
 
 96 
 
 24 
 
 3.8 
 
 1.0 
 
 2 
 
 Rate after 10... 
 
 116 
 
 29 
 
 4 6 
 
 1 2 
 
 3 
 
 Rate after 20 
 
 144 
 
 36 
 
 5.8 
 
 1.4 
 
 . 4 
 
 Rate after 30 
 
 184 
 
 46 
 
 7.4 
 
 1.8 
 
 6 
 
 Rate after 40 .. 
 
 240 
 
 60 
 
 9 6 
 
 2 4 
 
 6 
 
 Rate after 50 
 
 319 
 
 80 
 
 13.0 
 
 3.2 
 
 g 
 
 Rate after 60 
 
 432 
 
 108 
 
 17 
 
 4 3 
 
 1 i 
 
 Rate after 70 
 
 591 
 
 148 
 
 24 
 
 5 9 
 
 1 5 
 
 Rate after 80 
 
 816 
 
 204 
 
 33 
 
 8.2 
 
 2 
 
 Rate after 90... 
 
 1 134 
 
 283 
 
 55 
 
 11 
 
 2 8 
 
 Rate after 100 
 
 1 584 
 
 396 
 
 63.0 
 
 16.0 
 
 4.0 
 
 Maximum cooling rate.. 
 
 2 400 
 
 600 
 
 96 
 
 24 
 
 6 
 
 Maximum heating rate 
 
 7 200 
 
 1 800 
 
 288 
 
 72 
 
 18 
 
 
 
 
 
 
 
 Although the factory annealing of plates of pressed lens blanks 
 and of prisms is not difficult, yet it requires constant supervision with 
 reference both to the maintenance of the furnace temperatures and 
 to the furnace schedules for the type and sizes of blocks of glass under 
 treatment. The thicker the plate of glass the slower must be the 
 initial rate of cooling from the annealing temperature. The permis- 
 sible initial rate of cooling decreases about as the square of the recip- 
 rocal of the thickness ; for example, a satisfactory initial cooling rate for 
 a plate of borosilicate crown 1 centimeter thick is 20 F. (11 C.) per 
 hour; for a plate 2 centimeters thick, the proper initial rate would 
 be about 5 F. (3 C.) per hour. 
 
 In the case of a lehr the rate of annealing is of course much more 
 rapid; thus most of the flint glass in plates 9 millimeters thick and 
 also small lenses and field glass prisms were satisfactorily annealed 
 during the war in a lehr in a period of 9 to 18 hours. Here again the 
 temperature distribution and the rate of travel of the glass through 
 the lehr requires proper supervision and regulation to avoid abrupt 
 changes in temperature as well as air currents which may chill the 
 plates. 
 
GRINDING AND POLISHING OPERATIONS. . 189 
 
 THE GRINDING AND POLISHING OF PRESSED OR MOLDED PLATES OF 
 GLASS PREPARATORY TO INSPECTION. 
 
 After removal from the annealing furnaces the plates are inspected, 
 in the rough, through the ends for quality of annealing and are then 
 sent to the grinding and polishing tables where they are made ready 
 for critical inspection for striae, bubbles, stones, and pressing defects. 
 For this purpose the plates are ground either on the sides or on the 
 ends or on both. European optical glass is shipped in plates 
 ground on two opposite ends for inspection. A molded or pressed 
 plate of glass examined critically under these conditions and found 
 to be free from striae is probably free from any striae which would 
 cause trouble in an ordinary optical lens system. If, however, striae 
 are observed, either the plate should be discarded as a whole or the 
 striated portions should be trimmed off; but in performing this 
 operation much good glass may be lost because the exact positions of 
 the striae in the plate are not easy to determine under the conditions of 
 observation. The plate may be rotated about different axes and the 
 position of a stria approximately located and then trimmed off; but 
 after this has been done reinspection of the plate is possible only with 
 the aid of an immersion liquid. 
 
 The grinding and polishing of the plates on the sides (flats) , on the 
 other hand, enables the inspector to mark the striated or otherwise 
 unsuitable portions plainly during the inspection; these can then be 
 readily trimmed off and discarded so that the remainder of the plate is 
 of first quality glass; such a plate can moreover always be reinspected 
 directly, because the trimming away of a striated portion does not 
 destroy the remainder of the plane surfaces. It is true, on the gen- 
 eral law of probability, that inspection through the ends of a glass 
 plate enables the observer to detect fine striae more readily than 
 through the flats chiefly because of the much longer glass path exam- 
 ined by the first method; but the available methods of inspection are 
 sufficiently sensitive in general to inhibit the release of much striated 
 glass by a good inspector. To be thoroughly satisfactory the glass 
 plates should be inspected both through the ends and also the flats. 
 This is evident when plates of rolled or plate glass are inspected for 
 striae; such plates, examined through the flats, appear to be free 
 from striae, but when viewed through the ends, they are seen to be 
 filled with bands of striae or ream running parallel with the flat 
 surfaces of the rolled sheet. 
 
 The polishing of pressed or molded plates on the ends represents 
 a great saving of time and expense and also of the glass itself as 
 compared with polishing on the flats. In general it may be stated 
 that for glass relatively free from striae polishing on the ends is to 
 be recommended. If, however, striae are abundant and it is desired 
 3922921 13 
 
190 
 
 MANUFACTURE OF OPTICAL GLASS. 
 
 to eliminate these by trimming, the plates should be ground and 
 polished on the flats. In case glass of the highest quality is desired 
 the plates should be polished both on the flats and on at least two 
 opposite ends. 
 
 The grinding and polishing of the plates is not a matter of much 
 difficulty. Ordinary factory methods for grinding and polishing are 
 employed (figs. 56 and 57), an effort being made to arrange the 
 machines and the operations in such order that a large quantity of 
 glass can be handled expeditiously and cheaply. Polishing on felt 
 is sufficiently good for the purpose and is much more rapid than 
 polishing on pitch. In the case of pressed plates to be polished on 
 the ends, the blocking of the plates may be done by the use of clamps 
 
 FIG. 56. Rough grinding wheel for grinding plates of optical glass. (Photograph by 
 J. Harper Snapp at plant of Spencer Lens Co.) 
 
 which hold the plates in position between pieces of felt and without 
 the aid of pitch or plaster of paris; much time and expense are saved 
 thereby. 
 
 War-time experience showed that, in the case of rolled glass, in- 
 spection was possible for ordinary requirements through the fairly 
 smooth fracture surfaces obtained by cutting the sheet of glass into 
 squares by means of a glazier's diamond or a glass cutter's wheel. 
 In this case the striae or ream are all arranged in parallel sheets and 
 are readily detected. 
 
 The plates after polishing are thoroughly washed and cleaned in 
 hot alkali and soap solutions and are then ready for critical inspec- 
 tion, In case no defects are then found in a given plate it is a finished 
 
PERCENTAGE LOSSES OF GLASS. 191 
 
 product, so far as the optical glass plant is concerned, and is ready for 
 shipment to the maker of lenses and prisms for optical instruments. 
 In case the inspection reveals the presence of defects, such as 
 striae, bubbles, stones, pressing feathers or folds, or other defects in 
 a plate, it is rejected and returned to the trimmers for elimination 
 of the sources of trouble. Striated portions are trimmed off, pressing 
 defects are trimmed and ground off, either by the use of special 
 grinding tools or by a sand-blast. The fragments of good glass still 
 remaining, if sufficiently large, are repressed, reground, and re- 
 inspected; if only in small fragments they are either discarded or 
 reserved for use in second-grade optical lens systems, especially 
 cheap photographic lenses. 
 
 FIG. 57. Grinding (lower row) and polishing (upper row) disks for plates of optical glass. 
 (Photograph by J. Harper Snapp at the Hamburg plant of Spencer Lens Co.) 
 
 PERCENTAGE LOSSES OF GLASS IN THE FACTORY OPERATIONS. 
 
 The yield of good optical glass is not the same for each pot of glass, 
 but varies within wide limits for different types of glass and for 
 different pots of the same type. A yield of 10 to 20 per cent of good 
 glass from a pot is generally considered to be satisfactory; but during 
 the war period after the factory operations had been properly sys- 
 tematized the percentage yield of good glass in pressed plates was 
 increased to 30 and 35 per cent and, in the case of a particular glass, 
 rose to 50 per cent during one month at one of the factories. At 
 first thought it may appear that these losses are excessive and with 
 proper care might easily be reduced. This is no doubt true, but 
 many of the losses are the result of mechanical operations and these 
 can not readily be avoided. They do not necessarily indicate that 
 the glassmaker is at fault. Even if the pot of raw glass were prac- 
 
192 MANUFACTUBE OF OPTICAL GLASS. 
 
 tically perfect appreciable percentage losses would result from the 
 breaking up of the pot and from the several trimming operations, 
 especially in preparing the glass for molding or pressing into plates. 
 Many of the fragments obtained on breaking down a pot of glass are 
 small and of irregular shape and therefore useless. 
 
 Although in actual factory routine the percentage losses caused 
 by any given operation differ from pot to pot and from one type of 
 glass to another, the following general percentage losses may be 
 expected: 
 
 (a) Preparing raw pot glass for pressing or molding into plates, 
 from 30 to 60 per cent. 
 
 (b) Molding or pressing operations, from 3 to 5 per cent. 
 
 (c) Grinding and polishing of pressed plates on flat sides, from 
 5 to 10 per cent. Grinding and polishing on ends of molded plates, 
 from 1 to 5 per cent. 
 
 (d) Trimming of plates polished on the flats, from 10 to 15 per 
 cent. 
 
 In the further preparation of inspected plates as material for lens 
 and prism blanks which are trimmed to pieces of exact weight, there 
 is a loss of 20 to 25 per cent of the weight of the plates or of 8 to 
 10 per cent of the original weight. In the lens and prism pressing 
 operations a loss of about 10 per cent in the weight of glass plates or 
 of 3 to 5 per cent of the weight of the original raw glass may occur. 
 
 At one of the factories a yield of 20.5 per cent of glass in the shape 
 of lens and prism blanks was obtained over a considerable period of 
 time. The yield in the form of inspected pressed and polished plates 
 varied with the different types of glasses, but ranged between 20 and 
 50 per cent. 
 
 SUMMARY. 
 
 In this chapter the effort has been made to describe in a general 
 way the processes involved in the manufacture of optical glass. The 
 outstanding feature of these processes is not their intricacy or de- 
 pendency on special apparatus of unusual or extreme characteristics, 
 but their relative simplicity and dependency on accurate control, 
 especially along chemical and thermal lines. Raw batch materials 
 of high chemical purity, optical pots of high thermal and chemical 
 resistance, accurate thermal regulation of the melting and annealing 
 furnaces, and careful attention to schedules as part of the ordinary 
 daily routine are necessary and essential to success in the manufac- 
 ture of optical glass which has to meet the exacting requirements of 
 high precision in so many respects. There is nothing secret or mys- 
 terious in its manufacture; but the organization which fails to appre- 
 ciate the significance of high precision and of adequate scientific con- 
 trol and regulation can not expect to produce optical glass of uni- 
 
SUMMARY. 193 
 
 f ormly high quality. This statement is emphasized because one of the 
 greatest sources of trouble in the development of the war-time manu- 
 facture of optical glass on a large scale was the lack of appreciation 
 on the part of certain manufacturers of this fundamental fact, and 
 hence their lack of effective cooperation; this, together with a lack 
 of trained personnel, both in the factories and in the inspection and 
 other branches of the Army and Navy, added much to the difficulties 
 of the situation during 1917 and the first half of 1918. During this 
 time, moreover, no information or assistance of any kind was forth- 
 coming from Europe. Fortunately, however, the difficulties were 
 overcome, slowly at first and then more rapidly, so that by the end 
 of 1917 no serious apprehension existed in the minds of the men 
 actually engaged on the tasks regarding the final outcome, nor of our 
 ability to meet all the demands of the military forces in the field. 
 
Chapter IV. 
 THE INSPECTION OF OPTICAL GLASS. 
 
 Inspection has only one purpose wherever emplo3^ed, namely, to 
 eliminate the bad from the good and to grade the objects inspected 
 into classes of different degrees of merit. In the manufacture of 
 optical glass proper inspection is essential, and may not be neglected. 
 In the absence of inspection, material of poor quality may continue on 
 through many expensive and painstaking operations, all of which are 
 then wasted, because eventual rejection is inevitable. This means 
 financial loss; vice versa, the use of best-quality material for inferior 
 and cheap instruments serves no purpose. This again means finan- 
 cial loss. In the case of optical glass this is especially true because 
 of the many different uses to which the glass is put. In high-precision 
 measuring instruments, such as military fire-control instruments and 
 microscopes, the best quality glass is necessary; in low-power instru- 
 ments, such as field glasses, which serve chiefly as an aid to vision, 
 the tolerance limits are not so narrow, and less good glass may be 
 employed; a still lower grade of glass serves the purpose adequately 
 in cheap photographic lenses. Wherever the cost of manufacture is 
 high, inefficient and inadequate inspection methods mean financial 
 loss and a serious wastage and misuse of material. Proper inspec- 
 tion by intelligent observers is an expensive process; but in modern 
 business effort such methods have been found by experience to pay 
 manyfold, because they reduce losses in the final product and enable 
 proper control to be established throughout the plant. 
 
 Experience has demonstrated that in a large organization the 
 inspection department should be operated as a separate and distinct 
 branch, each inspector to report and to be reponsible to the chief 
 of the inspection branch and not to the foreman of the particular 
 shop to which he or she may have been assigned. Unless this is 
 done, each foreman becomes practically the judge of his own product 
 without reference to the assembled instrument as a whole; the in- 
 evitable result is then frequent trouble between the different manu- 
 facturing departments, each department asserting and maintaining 
 that its standards are correct and adequate. Effective organization 
 prescribes that the responsibility for the inspection of the component 
 parts and also of the assembled instrument or article be lodged in a 
 single branch or department which is in a position to establish, in 
 cooperation with the manufacturing departments, proper and ade- 
 104 
 
COKDS AND STRLE. 195 
 
 quate tolerances for the several items in a given instrument and then 
 to insist impartially upon the maintenance of the estahlished toler- 
 ances. 
 
 The inspection of optical glass in molded, pressed, or rolled plates 
 is an essential step in the manufacturing process, because by it the 
 quality of the raw glass, which is used for the lenses and prisms of 
 optical instruments, is definitely established and unsuitable glass is 
 eliminated at the outset. The requirements, which optical glass 
 has to meet, are stated in detail in Chapter II. These are, in brief, 
 chemical and physical homogeneity, high transparency and freedom 
 from color, durability, and definite optical constants. Homogeneity 
 tests include inspection for bubbles, for stones and crystallization 
 bodies, for pressing defects, such as feathers, folds, or laps, for striae, 
 and for strain. Separate tests are made for chemical durability with 
 reference especially to the behavior of the glass on long-continued 
 exposure to air in different climates. These tests of chemical stability 
 are necessarily of long duration and can be applied in general only to 
 representative samples, not to each piece. The optical constants are 
 measured commonly on a refractometer, and rarely by the immersion 
 method. 
 
 For purposes of inspection the glass is ordinarily furnished in plates 
 and blocks which are ground and polished either on opposite sides 
 (flats) or on opposite ends. Simple inspection of these plates under 
 special conditions of illumination enables the observer to detect 
 stones, bubbles, pressing defects, feathers, folds, and heavy striae 
 and cords. For the detection of fine striae and threads, more refined 
 methods have to be employed. 
 
 STRIDE. 
 
 All methods for the detection of striae are based on the fact that 
 striae are of different refractivity (commonly lower) from the sur- 
 rounding glass and, as a result, deflect the directions of transmitted 
 rays of light. In actual inspection work the transmitted light rays 
 are given very definite directions so that the slightest departure from 
 the prescribed paths results in a local difference in intensity of field 
 illumination, thus rendering the striae visible. The striae not only 
 deflect the light, but function also somewhat as a lens so that different 
 sides of a given stria appear unequally bright, especially if observed 
 under conditions of oblique illumination, as indicated in figs. 7c, 7e, 
 and 58. In figure 58 the stria is represented in section as lens-shaped 
 and the deflection of the light rays is shown in much exaggerated 
 form; the inequality of illumination of opposite sides of a stria when 
 obliquely incident rays traverse the field is the chief distinguishing 
 feature of striae. A little practice enables the observer to distinguish 
 between striae and scratches on the polished surface of the plate, 
 
196 
 
 INSPECTION OF OPTICAL GLASS. 
 
 especially if the plate is tilted and turned during the examination. In 
 some cases the striae are not sharply defined and there is only a 
 gradual change in refractivity between different parts of the plate. 
 This gives rise to a disturbed, nonuniform, and even wavy illumina- 
 tion of the field which suffices to render the glass unsuitable for high 
 precision work, but still usable for lower-grade optical systems. Poor 
 annealing may also give rise to disturbed field illumination, especially 
 near the edges of the plate. 
 
 There are a number of different methods available for the detection 
 of striae in optical glass. These may be divided into two general 
 classes, namely, direct- vision methods and projection methods. 
 Both types were used before the war, but not on a scale commen- 
 surate with war-time needs and conditions. It was necessary, 
 therefore, to investigate the several different methods with reference 
 to sensitiveness, simplicity and speed of operation, effect on eyes of 
 inspectors, and general practicability as routine factory methods. 
 
 FIG. 58. Diagrams illustrating the effect of a body, such as stria, of lenticular cross section on transmitted 
 light rays; ni is the refractive index of the stria, n? that of the inclosing glass. 
 
 Experience proved that conditions differed greatly . in different 
 plants, and that a method acceptable to the workman in one district 
 was by no means equally acceptable to the workman in another 
 district. 
 
 In the inspection of optical glass, especially for striae, the personal 
 equation of the inspector plays an important role, and it is extremely 
 difficult to devise methods which insure that the same standard 
 quality of product is passed by different inspectors or even by the 
 same inspector at different times. Certain inspectors, by virtue of 
 keen eyesight and long training and experience in optical work, may 
 detect at a glance fine striae in plates which pass unnoticed by a less 
 skilled inspector. It is also true that in changing from one method 
 of inspection to another, or even from one instrument to another of 
 similar construction, the inspector requires a certain amount of 
 practice before his inspection becomes critical. It is, moreover, 
 well known that heavy striae may be present in a plate and yet 
 escape detection if the plate is examined through one direction only. 
 
CORDS AND STRIJE. 197 
 
 For example, plates of rolled plate glass, which is not optical glass 
 and is characterized by heavy ream when examined edgewise, 
 appear to be free from striae when inspected through the "flats." 
 On the other hand, it is possible, in the case of ribbon striae, that 
 these can not be detected when the plate is examined edgewise, 
 but they are then easily observed through the flats. The merits 
 of the several different available methods are accordingly difficult 
 to appraise properly, and detailed study under factory conditions is 
 necessary before a satisfactory decision can be made. 
 
 Experience and nice discrimination are required to draw the line 
 between glass of first quality, second quality, and cullet. In Chapter 
 II the effects of the presence of striae in glass in the different types 
 of optical instruments are discussed in detail. In high-power instru- 
 ments in which good resolution is required the striae which deflect, 
 even slightly, an appreciable number of light rays from then' normal 
 paths produce a noticeable effect on the resulting image and thereby 
 seriously impair the efficiency of the instrument. For such instru- 
 ments optical glass of the best quality is necessary and should be 
 most carefully inspected for striae. For optical glass intended for 
 low-power visual instruments, less critical inspection suffices. In 
 time of war there is a constant tendency to favor quantity in favor 
 of quality; inspectors are inclined, as a result both of this attitude 
 and of their inexperience, to pass much glass as first class which, 
 after having been worked up into high precision lenses and prisms, 
 must be discarded; also carelessly to assign much first-quality glass 
 to second-rate glass. 
 
 The different methods in common use for rendering visible fire 
 striae in optical glass are illustrated hi figures 59 to 64. 
 
 DIRECT VISION METHODS. 
 
 The modified Toepler method. One of the best methods for detect- 
 ing striae is a modification of that employed many years ago by 
 Toepler for the testing of astronomical objectives. This method is 
 illustrated diagrammatically in figure 59, in which S is a source of 
 light, such as a concentrated tungsten filament bulb, D a diffusion 
 screen of finely ground glass or thin opal glass, A a pinhole or narrow 
 cross slit aperture in the rear focus of the collimating achromatic 
 lens, LI (E. F. 30 to 75 centimeters and diameter 5 to 13 centimeters). 
 The larger the diameter of the lens the better, as it furnishes a larger 
 field. The plate to be inspected is placed at P; behind it is the 
 achromatic field lens L 2 , which in the case of 5-inch condenser lenses 
 of long focal length may be used in combination with a second 
 similar lens to shorten the focal lengths to 30 or 40 centi- 
 meters. At B is a movable pinhole aperture, or better, a cross 
 slit, figure 59a, in the rear focal plane of the field lens. This is 
 necessary because the surfaces on the plate may not be strictly 
 parallel and therefore deflect and shift the light slightly. The 
 
198 
 
 INSPECTION OF OPTICAL GLASS. 
 
 cross slit aperture is held in place by a small spring or dip and 
 can be shifted at will. In practice the eye is moved until the plate 
 to be examined appears in half shadow; in this position the striae are 
 readily seen on tilting and moving the plate across the field. Expe- 
 rience has shown that ribbon striae parallel to the polished surface 
 of the plate are not easily detected by this or any other method. 
 Their presence is indicated by a disturbed illumination of the field, 
 but no striae are seen as such. Plates polished on the sides enable 
 
 FIG. 59. Modified Toepler method for the inspection of optical glass plates. S is the source of light; D, a 
 diffusing screen; A, a small pinhole aperture; L\, an achromatic lens; P, the plate under examination; L 2 , 
 an achromatic lens; B the cross-slit opening illustrated on a larger scale in 59o; E, the eye of the observer. 
 
 the observer to determine just where the striae are, and thus to cut 
 out and trim off the striated portions; plates polished on the ends 
 enable the observer to detect the presence of striae through a long 
 glass path, but do not permit him to locate the exact position of a 
 stria with sufficient exactness, so that the striated portion can 
 be cut out from the plate without wasting good glass. The ideal 
 method is to have the glass polished both on the sides and ends. 
 Glass plates and blocks submitted for final inspection should be 
 
 L. L Z P 
 
 FIG. 60. The knife-edge method for the inspection of striae in a plate of optical glass. S, is source of light; 
 D, a ground glass diffusing disk; A, a pinhole aperture; LI, an objective lens; /^an objective lens;P, a 
 plane parallel plate; K, a thin metal shield (knife-edge); E, the observer's eye. 
 
 polished at least on opposite ends. Felt polish is satisfactory for 
 inspection purposes. 
 
 The arrangement in figure 59 may be modified as shown in figure 60 
 by using well-corrected objectives and placing the plate between the 
 rear lens and the eye, at which a knife edge or adjustable slit is used. 
 Experience with this method has proved that it is highly sensitive, 
 but that a considerable amount of time is lost because of the shift 
 of the focal point; furthermore, the field covered is normally slightly 
 
THE IMMERSION METHOD. 
 
 199 
 
 less than that obtained by the arrangement of figure 59; for prac- 
 tical factory operations the first device is more rapid and therefore 
 preferable. 
 
 The concave-mirror test. Another method to obtain rays of definite 
 direction is to use a concave mirror, M, of 25 centimeters diameter 
 and about 1 to 2 meters radius of curvature (fig. 61 ) ; to place near the 
 center of curvature an illuminated pinhole aperture A and a second pin- 
 hole or cross slit aperture near E, and through it to view the plate placed 
 at P, as indicated in figure 61. As in the first method, the presence 
 of striae is shown by lack of uniformity in field illumination. This 
 method is very sensitive, but has the disadvantage that the plate 
 under inspection is 5 or 6 feet away from the observer and can not 
 be tilted or turned and thus readily marked for the trimming off of 
 the striated portions. It may be used, of course, nearer the observer, 
 but the field may not then be fully covered. The principle on which 
 
 FIG. 61. Concave mirror test for the inspection of striae in plates of optical glass. S is source of light; 
 
 D, ground glass diffusing disk; A , pinhole aperture; M , a concave mirror; P, plane parallel plate of glass: 
 
 E, the observer's eye. 
 
 this method is based is identical with that underlying the Toepler 
 method for the testing of the quality of figuring of an objective lens. 
 The immersion method with monochromatic light. This method was 
 applied by Mr. W. H. Taylor, of the Bureau of Standards, at 
 first to avoid the necessity of polishing plates for inspection. For 
 this purpose the sides of the plates are ground flat. Each end is 
 then covered with a thin film of a liquid of the same refractive index 
 and this is in turn covered with a flat plate. Because of the differ- 
 ence hi dispersion between liquid and optical glass plate a mono- 
 chromatic illuminator of the type shown in figure 62 is used to facil- 
 itate the exact match in refractive index between plate and liquid. 
 Under these conditions the ground surfaces disappear entirely and 
 the plate can be examined critically by any suitable method for 
 striae. This method is, of course, not so rapid as the ordinary 
 methods, which require polished end or side surfaces, but it does 
 avoid the polishing of the plates. 
 
200 
 
 INSPECTION OF OPTICAL GLASS. 
 
 Taylor adopted later the immersion method for the examination 
 of rough irregular chunks of glass and also of plates for striae; for 
 the purpose he used the arrangements shown in figure 02. 1 The part 
 M is essentially a device for obtaining monochromatic light; it can be 
 rotated as a whole about the axis K and different prismatic colors are 
 thus obtained. A monochromatic illuminator furnishes the simplest 
 means of matching exactly the refringence of immersion liquid to that 
 of the immersed block of glass; the procedure for obtaining an exact 
 
 FIG. 62. Immersion method for inspection of plates and irregular fragments of optical glass in monochro- 
 matic light as developed by Mr. W. H. Taylor. <S is the source of light; L\, a condenser lens; A \, pinhole 
 aperture; C, carbon sulfide dispersion prism; M, the board in which the foregoing optical parts are 
 mounted; K, the axis about which M can be rotated; La, L<, L$, condenser lenses; 5T, liquid immer- 
 sion tank; P, the plate or fragment of optical glass; A%, Aa, stops; E, the observer's eye. 
 
 match is first to adjust the liquid mixture until its refractive index is 
 equal to that of the immersed glass for some part of the visible 
 spectrum and then to make the fine adjustment for exact match by 
 means of the monochromatic illuminator. In figure 62, $ is the source 
 of light, a concentrated tungsten filament (electric bulb) , L 17 a condenser 
 lens, AV a small pinhole aperture, L 2 , a collimatorlens, 0, a carbon bisul- 
 phide prism, L 3 , a condenser lens, T, the immersion tank, fitted at oppo- 
 site ends with parallel plates of plate glass and filled with the immersion 
 liquid, L 4 and L 5 , achromatic lenses, ^L 2 'and A 3 , small apertures, ", the eye. 
 In a later arrangement the lens, Z 2 , is placed nearer the aperture B and 
 the emergent rays are slightly convergent. This enables the observer 
 to obtain approximately monochromatic light without the careful cen- 
 tering and adjustment required in the device of figure 62. The immer- 
 sion liquids are mixtures of carbon bisulphide (n D = 1.628) and benzol 
 (n= 1.501). In place of the carbon bisulphide, a-monobromnaph- 
 
 1 Illustrated in article on comparison tests for striae in optical glass, by L. E. Dodd. 
 Soc., 2, 981, 1919. 
 
 Jour. Am. Ceram. 
 
THE IMMERSION METHOD. 
 
 201 
 
 thalene (UD= 1.658) or a-monochlornaphthalene (n D = 1.633) maybe 
 used; a commercial name for the monochlornaphthalene is halowax 
 oil. Carbon bisulphide is unpleasant to use because of its disagreeable 
 odor and toxic properties. Monobromnaph thalene has also an 
 unpleasant odor and does not volatilize as does carbon bisulphide, 
 but remains as an oily film on the inspected glass surfaces. Both 
 
 FIG. 63. Simple projection methods for the inspection of optical glass for striae, (a) S is source of light; A 
 a small circular opening; P, optical glass plate; C, projection screen, (ft) S is the source of light; A, 
 small aperture; D, diffusing disk; L, a dispersive lens; P, optical glass plate; C, the screen, (c) S is the 
 point source of light; P, the optical glass plate; C, a projection screen consisting of thin white or opal glass; 
 E, the observer's eye behind the screen. 
 
 liquid mixtures are not satisfactory for factory purposes and the 
 workmen object seriously to them. The fumes induce headaches 
 and a feeling of debility, so that it is questionable if the quality of 
 inspection by the immersion method can be maintained at a high 
 level. At one of the plants the men after a thorough test refused to 
 continue work with the immersion methods. At the plant at which 
 
202 INSPECTION OF OPTICAL GLASS. 
 
 the method was adopted the men showed evidences of the toxic 
 action of the bisulphide fumes and did not maintain a high standard 
 of critical inspection. As a laboratory method, however, the method 
 is useful especially for the rapid measurement of refractive indices 
 and dispersions. 2 
 
 In the case of an accidental mixing of plates of several different 
 types of glass, these are most readily sorted and separated by use 
 of the immersion method. 
 
 PROJECTION METHODS. 
 
 In these methods the effects produced by the plate under inspec- 
 tion on a uniformly illuminated field are viewed directly or in a 
 photograph. Several of these methods are less critical than the 
 foregoing, but still well suited for rapid, medium grade work. 
 
 The methods illustrated in figures 63 a and 6 were in use before the 
 war. In figure 63a an electric lamp is inclosed in a sheet-iron box 
 with ventilating top. Light from a small hole or slit in the box 
 illuminates a sheet of drawing paper or other even white diffuse 
 reflecting surface a foot or two distant. The opening in the box is 
 sufficiently large that the illuminated surface is surrounded by a fairly 
 wide half -shaded edge. The rays illuminating the penumbra or half 
 shadow have fairly definite directions and if deviated slightly from 
 their normal paths give rise to local unequalities in the illumination 
 of the penumbra. Striae under these conditions are readily detected, 
 if heavy; and if light, can be seen after practice. If the plate be 
 examined in the bright, nonshaded part of the field, the light is not 
 sufficiently unidirectional to disclose slight differences of field illumi- 
 nation, and as a result fine striae pass unnoticed. This method is 
 satisfactory for the elimination of heavy striae and cords. Glass 
 which passes this test is satisfactory for field glasses and ordinary 
 photographic lenses, but it may not be suitable for optical instru- 
 ments of high precision. 
 
 A modification of this method is to use a large opening so that the 
 half-shaded edge is present only on one side of the field. In practice 
 it is well in both these methods to utilize only the half-shadow part 
 of the field and to cover the otherwise fully lighted part of the field 
 with black, matt paper. This cuts down much of the glare present 
 in the field, which is tiring to the eyes. The use of properly colored 
 glasses is helpful in this connection. 
 
 The method of figure 63& is a variation of method 63a and of about 
 the same degree of accuracy. 8 is the tungsten bulb source of light ; 
 A a small, pinhole aperture; L a biconcave lens 3 inches in diam- 
 eter. The rays emerging from L form a cone which passes through 
 the plate P under inspection and impinges on the screen C. If the 
 
 See in this connection a paper by R. W. Cheshire, Phil. Mag., 32, 409, 1916. 
 
PROJECTION METHODS. 
 
 203 
 
 plate P is free from striae, the illuminated field at C is uniform; but 
 if striae be present, they give rise to characteristic lines and threads 
 of less or greater intensity of illumination than the rest of the field, 
 and hence are readily detected. In this method, as in certain of the 
 preceding methods, the plate, as viewed, is magnified two or three fold. 
 In the method 3 of figure 63c, a thin sheet of opal glass (one-fourth 
 millimeter thick) flashed on a sheet of clear glass is substituted for the 
 screen of method, fig. 63a. The shadow effects of transmitted rather 
 than reflected light are noted on the screen. As source of light Dodd 
 recommends a small tungsten light with small V-shaped filament 
 helix of tungsten wire. (Edison Mazda No. 131, 6-8 volts; 3.5 amp.; 
 c. p. 28, filament C.) An equally satisfactory source of light for this 
 and for methods 1 to 4 is the Ediswan pointolite bulb. Dodd's work 
 indicates that with practice this method can be used in place of the 
 methods 1 to 4 ; it is evident, however, that the conditions of illumina- 
 tion in this method are not so critical as in methods 1 to 4 and that 
 the method is correspondingly less sensitive. 
 
 FIG. 64. Projection method for the examination of striae in optical glass. S is the point source of light; 
 L\, a condenser lens; A, pinhole aperture L%, an objective; P, optical glass plate; Lz, projection lens; C, 
 projection screen. 
 
 The method of figure 64 was used by T. T. Smith 4 and others 
 for photographing striae in plates of optical glass. As indicated by 
 the figure a special optical system was adopted and favorable condi- 
 tions were thereby attained for the detection of striae. If for the 
 photographic plate a diffusing screen or a thin plate of opal glass 
 is substituted, this arrangement is satisfactory for the visual detection 
 of striae. In figure 64, S is the source of light, such as a concentrated 
 tungsten filament or pointolite bulb, Z x a condenser lens system, A 
 a small aperture, L 2 a collecting lens, P the glass plate, L 3 a photo- 
 graphic lens, and C the photographic plate or screen. 
 
 In all projection methods the tiring effect of glare on the inspector's 
 eyes should be reduced by proper attention to the arrangement and 
 intensity of the light source and especially to the reflecting or trans- 
 mitting screen 5 on which extremes in contrast of light intensity 
 should be avoided as much as possible. 
 
 L. E. Dodd, Jour. Am. Ceram. Soc., 2, 977, 1006, 1920. 
 
 * T. T. Smith, A. H. Bennett, A. E. Merritt, Bureau of Standards, Scientific Paper No. 373, 1920. 
 
 5 An undeveloped photographic plate is a satisfactory screen for this purpose. In the course of time 
 it becomes dark and should be replaced by a fresh plate. Light buff colored smooth drawing paper 
 commonly used. 
 
204 INSPECTION OF OPTICAL GLASS. 
 
 Practical tests of the majority of above methods led to the adoption 
 of the first method as best adapted to the routine inspection of glass 
 plates, polished either on the ends or on the sides (flats). Most of 
 the glass accepted by the Army and Navy inspectors was inspected 
 by this method, and the results obtained proved its usefulness. The 
 method had become standard even before the war, the only change 
 introduced by the writer being the cross slit. (Fig. 59a.) 
 
 In routine work with the first method there are several pre- 
 cautions to be taken which are essential to rapid, critical inspection. 
 The source of light should not be too strong. A 40-watt tungsten 
 bulb is adequate. In front of the small aperture A a small ground 
 glass or thin opal diffusion plate is introduced in order properly to 
 illuminate the aperture. In place of the diffusion disk a condenser 
 lens, as in figure 64, may be used to image the point source of light 
 in the aperture, but the diffusion disk is perfectly satisfactory and 
 simpler than the condenser lens arrangement. The aperture A 
 should be located at the focus of the achromatic lens Z t . It is 
 essential that this adjustment be carefully made because the degree 
 of parallelism of the rays through the plate P depends directly on 
 the exact location of A. The simplest method for testing the position 
 of A is to sight through the lens Z t toward A with a telescope or 
 field glass focused on a very distant object; A is then moved until 
 it appears in sharp focus through the telescope. Lens L 2 is set up 
 parallel to Z, and about a foot distant. The position of the cross- 
 slit aperture B is located at the position of sharp focus of the aper- 
 ture A. In the normally adjusted instrument the center of the 
 aperture cross B coincides with the small image spot of A. A shield 
 is commonly placed in front of D to cut off stray light from the 
 light S; the observer should be, moreover, shielded from direct sky- 
 light. Total darkness is not necessary nor advisable; the eye func- 
 tions best and with least fatigue if it is not exposed to strong contrasts 
 in illumination. 
 
 A plate which is to be examined for striae is first inspected for 
 cleanness of polished surfaces. The plate should reach the inspector 
 in clean condition; if it is not clean, he should return it for cleaning; 
 it is not his function to clean plates. He should, however, dust 
 off each plate with a soft cloth before critical inspection. 
 
 The clean plate is placed in the position indicated in Fig. 59 be- 
 tween the two lenses L x and L 2 ; the observer's eye views the plate" 
 through the slit aperture B. The eye is placed as near as possible, 
 to this aperture in order to receive light from the entire surface of 
 lens L 2 . The opposite surfaces of the plate P are rarely strictly 
 parallel, with the result that there is a perceptible shift of the point 
 of focus at B. The plate is accordingly turned about the horizontal 
 axis, its surfaces still remaining approximately parallel with the 
 
BUBBLES AND SEEDS. 205 
 
 flat lens surfaces, until the focus crosses one of the slit openings, and 
 the plate and field in consequence appear brightly illuminated; the 
 plate is then rotated a little farther and appears in half shadow or 
 practically dark. In this position striae are most readily detected 
 and with practice can be seen at a glance. At first the inspector 
 may experience difficulty in locating the positions of half shadow; 
 but after a few trials his eye becomes accustomed to the phenomena 
 and he is able to turn the plate without special effort to a sensitive 
 position. 
 
 In case the plate under examination is ground and polished on the 
 flats, the striated areas are marked with colored wax crayon on the 
 plate to be trimmed away by the trimmers. 
 
 Scratches on the flat surfaces may be deceptive at first, but after 
 short practice they are readily distinguished from striae. 
 
 A slight difference in illumination between different parts of the 
 field in which no striae are recognizable, occurs in some plates and 
 is commonly indicative either of fine or ribbon striae so placed that 
 they are not visible when viewed in the direction of observation, or 
 of a gradual change in refractive index from one part of the plate to 
 another. A plate, exhibiting these phenomena, should be inspected 
 through another direction before acceptance. In an occasional 
 plate a curious, concentric system of rings can be seen; these are 
 marks left by the grinding or polishing tool, the plate having been 
 rotated for an appreciable period of time about the center of the 
 concentric rings as axis. 
 
 Experience has shown that for critical work of the highest pre- 
 cision inspection through one direction only does not suffice; the 
 plates should be inspected through different directions to insure 
 freedom from striae. For most purposes, however, critical inspection 
 through one direction suffices to eliminate most of the striae. 
 
 Bubbles, seeds, stones, crystallization bodies; pressing defects, feathers, 
 folds, laps. A plate of optical glass after having passed inspection 
 for striae is examined for "seeds," "stones," and "feathers and 
 folds/' These defects are most readily observed under conditions 
 of illumination, approaching those of dark-ground illumination; 
 these are readily obtained by placing a source of light (electric 
 bulb) under a cardboard or wood cover (fig. 65), thereby screening 
 off all but horizontal rays of light. This arrangement is analogous 
 in its effects to a ray of sunlight which, streaming into a room, 
 renders visible the minute dust particles in the air; foreign particles 
 in the glass, such as minute seeds or stones serve to reflect and 
 diffract the impinging light rays, so that they reach the eye of the 
 observer; each particle, thus illuminated, appears as a bright source 
 of light or a star and is readily visible. In order that the contrast 
 between light and dark be as great as possible, the plate under 
 3922921 14 
 
206 
 
 INSPECTION OF OPTICAL GLASS. 
 
 examination should rest on a sheet of dull black paper or a piece of 
 black felt or cloth and be shielded from light at the sides; the back- 
 ground is then dark and the brightly illuminated points stand out 
 clearly. 
 
 The presence of a small number of bubbles in a plate of optical 
 glass is not sufficient cause for its rejection. All stones and crystalli- 
 zation bodies, also feathers and folds, should be removed either by 
 grinding or sand blasting, if near the surface, and by chipping out, 
 if deeply buried. The presence of a stone or fold or feather in a lens 
 or prism is adequate reason for rejection. 
 
 Inspection for strain. Two factors are important in the inspection 
 for strain, namely, the maximal amount of strain in a plate and the 
 general distribution of the strain. Strain in optical glass is readily 
 recognized by the effect which a strained piece of glass exerts on 
 plane-polarized light waves during transmission. In a strained piece 
 of glass, waves of light are transmitted with different speeds, depend- 
 ing on the direction of transmission and on the plane of vibration. 
 
 FIG. 65. Arrangement suitable for the inspection of glass plates for bubbles, seeds, stones, and pressing 
 
 defects, etc. 
 
 This gives rise to the phenomena of double refraction which can be 
 recognized between crossed nicols by the interference colors which 
 are formed and whose sequence is practically that of the Newton 
 color scale. The particular interference color formed depends di- 
 rectly on the distance between two interfering light waves; this dis- 
 tance in turn depends both on the difference in speeds of the two 
 interfering waves through the strained glass (stresses in the glass) 
 and on the length of the glass-path. A definite and yet easily 
 attainable limit of strain even under war-time conditions of anneal- 
 ing was accordingly set and expressed either in terms of absolute 
 birefringence for a specified color or in terms of path difference per 
 centimeter glass-path. This war-time limit is that of the maximum 
 observable strain in a plate and is, for sodium light, 20 millimicrons 
 path-difference per centimeter glass-path at the margins or 10 milli- 
 microns path difference at the center of the plate; expressed in 
 birefringence these limits are 20'10~ 7 and 10*10~ 7 , respectively. The 
 distribution of the interference colors should, moreover, be sym- 
 metrical in the plate. A more severe and yet easily attainable tol- 
 
QUALITY OF ANNEALING. 
 
 207 
 
 erance limit is half the above or maximal path difference of 10 milli- 
 microns per centimeter glass-path at the margins of the plate; this 
 corresponds to a path difference of 5 millimicrons at the center 
 of the plate and may be considered to be a normal and reasonable 
 peacetime tolerance. 
 
 The practical utilization of these limits is extremely simple. 
 Apparatus arranged as in figure 66a or figure 666 is serviceable. 
 In figure 66a, S is the source of light (electric bulb),Z) a piece of 
 
 K N 
 
 IG. 66. Apparatus suitable for the inspection for strain in optical glass plates. In fig. 660 a diffusing 
 screen of ground glass illuminated from the rear by an electric lamp is used; in fig. 666 the diffusing 
 screen is replaced by a condenser lens system". In both arrangements the lamp is properly housed in a 
 sheet-iron box. (a) S is the source of light; D, a diffusing screen of ground glass; G, plate of opaque 
 glass; P, optical glass plate; K, sensitive-tint plate; N, nicol prism; E, observer's v eye. (6) D, diffusing 
 plate; A , pinhole aperture; L, condenser lens; S, G, P, K, N, and E: are the same as in (a). 
 
 ground glass, G a polished plate of opaque glass so tilted that the 
 angle of incidence is the Brews ter polarizing angle (tangent of the 
 angle is equal to the refractive index of the glass). For ordinary 
 opaque glass n=1.52, and therefore the polarizing angle is 56 40'. 
 This position of the polarizing plate is readily found even without 
 angular measurements by observing through a nicol the light re- 
 flected from the plate inclined at different angles to the line of sight. 
 At the polarizing position the field is practically dark because the 
 
208 
 
 INSPECTION OF OPTICAL GLASS. 
 
 reflected light is practically plane-polarized and is extinguished 
 by the analyzing nicol. Figure 666 is similar to figure 66a, except 
 that the parallelism of the rays incident on the plate G is insured by 
 the use of a point source of light (pointolite bulb or concentrated 
 tungsten filament) at the rear focus of a condenser lens. Either 
 apparatus functions satisfactorily for the purpose. 
 
 To render the low, first-order interference colors more readily 
 apparent a sensitive-tint plate of selenite, or of quartz, or as a last 
 resort, of mica, is inserted in the diagonal position in front of the 
 
 analyzer, thus coloring the 
 entire field a uniform purple. 
 A slight change in the path 
 difference suffices to change 
 this interference color to 
 orange if the path difference 
 is decreased, or to blue if it 
 is increased. The sensitive 
 tint is observed between 
 crossed nicols when the path 
 difference, in air, for sodium 
 light is 555 millimicrons. A 
 shift of 10 millimicrons suf- 
 fices then to cause a per- 
 ceptible change in color. In 
 a plate 10 centimeters wide 
 a path difference of 20 mil- 
 limicrons per centimeter 
 glass-path becomes 200 mil- 
 limicrons total path differ- 
 
 FIG. 67. Illustrating the appearance of the four grades o 
 
 annealing in optical glass plates, 10 centimeters long, when ence; in other Words the in- 
 viewed in polarized Ught with J^ ter f erence co l or a l ong foe 
 through the ends, (a) Annealing excellent; (6) annealing . i i 
 fair; (r) annealing poor; strain evenly distributed; (d) an- margins of the plate IS 
 nealing poor, strain unevenly distributed. The curves in changed f rom fo e sensitive 
 these figures represent simply different interference colors . . 
 (Newton color-scale) as they are observed with the aid of a purple tint to bright yellow 
 sensitive-tint plate; thus in fig. 67a the interference colors or crreenish blue ' the Center 
 may pass from blue at the margin of the plate to purple 
 
 and magenta (sensitive-tint) to red and reddish orange at IS then orange yellow Or blue, 
 
 the center. In the other figures the range of interference depending on the position 
 colors from center to margin of the plate is still greater. 
 
 of the plate. The observed 
 phenomena are indicated in figure's 67 a, Z>, c, d and 10 a, 6, c, d, e,f, g. 
 
 It should be noted that a very large percentage of the glass passed 
 by Government inspectors during the war period was so well annealed 
 that the resulting maximum path difference was less than 10 milli- 
 microns per centimeter, or 5 millimicrons at the center of the plate. 
 
 Once the polarization apparatus is set up the inspector examines 
 three or four plates at a time. Each plate is examined through the 
 
OPTICAL TRANSMISSION. 209 
 
 edges; polished edges are not required. The plate is inserted in the 
 line of sight in the diagonal position or so that its edge includes an 
 angle of 45 with the plane of incidence of the reflecting opaque 
 polarization plate G. A plate which shows distinct green or yellow 
 interference colors is rejected and returned for reannealing. Ac- 
 ceptable are plates which show changes in color ranging from orange 
 red or red to purple to blue and in these the color changes must be 
 symmetrical with reference to the outline of the plate. 
 
 Experience has demonstrated that, although for the exact meas- 
 urement of path differences a graduated quartz wedge or Babinet 
 compensator or other sensitive device is necessary, the above simple 
 apparatus is entirely satisfactory for routine factory inspection. 
 
 The design and construction of both the striae and the strain in- 
 spection apparatus should be so carried out that the inspector can 
 work with a maximum degree of comfort and with the least ex- 
 penditure of energy in lifting and transferring the plates. Strained 
 positions should be avoided because they tire an inspector and re- 
 duce at once his efficiency. 
 
 The transparency oj the plates. The inspector is commonly not 
 required to determine the degree of transparency of optical glass 
 plates. This is, however, an important factor and may well be con- 
 sidered briefly in this section. The methods employed are those of 
 the physical laboratory, modified slightly with reference to the 
 problem of light-transmission in optical glass. A number of different 
 instruments can be used for the purpose; they are photometers and, 
 as such, depend on the ability of the observer to match or balance 
 two contiguous, illuminated fields. Under favorable conditions of 
 illumination it is possible to detect differences of 2 and even 1 per 
 cent in the intensity of illumination between the halves of a 
 photometric field. 
 
 In measurements of the amount of light transmitted by a plane- 
 parallel plate, account must be taken of the light lost by reflection 
 at the plane surfaces. Let p be the amount of light reflected at each 
 boundary surface. Let ft be the amount of light transmitted per 
 unit thickness (1 centimeter) of plate; then I ft is the amount ab- 
 sorbed per unit thickness. For a thickness I the amount of light 
 transmitted is ft 1 . If multiple internal reflection be taken into 
 account, the amount, 7, of light transmitted by a plate of thickness I, is 
 
 From this equation we find 
 
210 INSPECTION OF OPTICAL GLASS. 
 
 If in this equation the value of p, as stated by the Fresnel equation 
 for the intensity of reflected, vertically incident light, namely, 
 
 /nl\ 2 
 P = l ~~~i )> m which n is the refractive index of the glass, be sub- 
 
 stituted, we find 
 
 a quadratic equation in /3 1 in which n and / are known. The equa- 
 tion is, however, based on the assumption that Fresnel's equation is 
 correct. Experiments with glass plates have proved that the 
 amount of light reflected by a glass surface can be appreciably re- 
 duced by short exposure of the polished surface to certain solutions. 
 It is advisable, therefore, to eliminate p from the equations by measur- 
 ing the transmission on different thicknesses of the glass plate. 
 
 The first equation can be written for two different thicknesses Z x 
 and Z 2 of the plate 
 
 From these two equations p can be eliminated. In the resulting 
 equation, I lt 7 2 , l lt and 1 2 are known, so that the value /3 can be ascer- 
 tained. The resulting equation is, however, so complicated that in 
 general the roots can be obtained only by approximation methods. 
 A direct and simple approximate solution can be obtained by neg- 
 lecting p 2 in equation (1) ; in this case we find for two transmissions 
 /!, 7 2 , measured on thicknesses Z x and Z 2 . 
 
 The value of computed from this equation is too low; but it is of 
 the correct order of magnitude. 
 
 If internal reflections be neglected, the ratio of the measured 
 transmissions or the two thicknesses is the transmission for the 
 path Za-Zj. The practical effect of neglecting p 2 is therefore to neglect 
 the internal reflections; but these add at most only 0.1 to 0.3 per 
 cent to the transmitted light. To disregard these in the equations is 
 to obtain too high a transmission and hence too small an absorption. 
 In the case of a thick plate of relatively high absorption this error is 
 not great; but in the case of a thin plate of high transparency the 
 percentage error may be appreciable. 
 
 The above relations show that, in case results of greater accuracy 
 are desired, the only feasible method is to measure the amount of 
 light reflected at vertical incidence. The difference between the 
 original intensity and this reflected amount combined with the 
 amount transmitted gives directly the amount absorbed by the plate. 
 
OPTICAL TRANSMISSION. 
 
 211 
 
 By measuring the intensity ratios for two thicknesses of plate, 
 and by placing each thickness of plate in front, first of the prism set 
 of the photometer and then in front of the second aperture of the 
 photometer, the observer avoids the difficulties arising because of 
 
 2.5 
 
 THICKNESS OF PLATE IN CM > 
 
 FIG. 68. Graphical solution of the equation for light- transmission of plane-parallel plates. The abscissae 
 represent the thickness of the plate in centimeters; the radiating lines, the percentage transmission of 
 the glass plate; the ordinates, the absorption in per cent per centimeter glass-path. 
 
 reflection at the boundary surfaces of the glass plate and because of 
 the decreased field intensity ratio resulting from the prism set. 
 
 The transmission /3 for a plate of unit thickness can be ascertained 
 from the last equation which may be written 
 
212 
 
 INSPECTION OF OPTICAL GLASS. 
 
 In figure 68 a graphical solution of this equation is given; the 
 abscissae represent the thickness of the plate in centimeters, the 
 ordinates the absorption in per cent per centimeter glass-path, and 
 the sloping lines the percentage transmission of the plate. 
 
 During the war there were used for the measurement of the light 
 transmission of optical glass plates and of optical instruments three 
 different types of photometers, namely, a bench photometer, a pho- 
 tometer with neutral tint wedges, and a polarization photometer. In 
 the bench photometer 6 (fig. 69) two shnilar electric bulbs, S 1} S 2 , 
 serve as light sources and are wired on the same circuit; each light 
 illuminates an opal glass plate and these are viewed together by 
 means of a photometer prism-set and eyepiece. The lamp 2 , of part 
 
 r\ 
 G, 
 
 FIG. 69. Apparatus for the measurement of the light transmission in optical glass plates. S\ and S 2 are 
 electric lamp bulbs; GI and O 2 , thin plates of milk glass; Pi, a Lummer-Brodhun photometer prism; 
 L, graduated scale; A, position indicator for the lamp S%; E\, eyepiece. Designed and manufactured by 
 Keuflel and Esser. 
 
 fc, can be moved along a graduated scale L and its distance a from the 
 opal plate G 2 at any position can be read off directly. The fields are 
 brought to uniform illumination throughout and the position of 
 lamp $ 2 is recorded. The glass plate is then inserted between the 
 parts a and 6; the field is again brought to uniform intensity and 
 the position of S 2 recorded (distance a x ). The percentage transmis- 
 sion is then a^/a?. With this instrument in proper adjustment the 
 light transmission can be measured to about 1 per cent. 
 
 In place of the optical bench with a sliding lamp, a photometer 
 with graduated neutral tint wedges may be used and sighted toward 
 a uniformly illuminated field. A device of this nature (fig. 70) was 
 
 Designed and constructed by Keuffel and Esser, Hoboken, N. J. 
 
OPTICAL TRANSMISSION. 
 
 213 
 
 constructed during the war in the Geophysical Laboratory and proved 
 to be satisfactory. The details are illustrated in figure 70; F t and 
 TF 2 are carefully graduated neutral tint wedges, 7 P l and P 2 right angle 
 reflecting prisms; L 17 L 2 , objective lenses; A ly A 2 , diaphragms at 
 rear foci of L v and L 2J respectively; P, the reflecting eyepiece prism 
 consisting of two right angle prisms cemented, as indicated, by optical 
 contact; ", a positive eyepiece. A white uniform source of light is 
 obtained by means either of a white diffusing screen of magnesia 
 illuminated by two electric lamps from the sides, or of a box lined 
 with white paper and illuminated by electric lamps properly placed, 
 or of a cylinder of white drawing paper illuminated at one end by a 
 strong electric lamp and closed at the other by a piece of white 
 Belgian opal glass with matt ground surfaces. The halves of the 
 photometric field are first balanced without the glass plate and then 
 
 FIG. 70. Photometer with neutral tint wedges for the measurement of the light transmission in optical 
 glass plates. W\ and W 2 are graduated neutral tint wedges; PI, P 2 , reflecting prisms; L\, _L 2 objective, 
 lenses; A\, Az, removable stops; P, photometer prism. 
 
 after the plate has been inserted in front either of W l or W 2 . The 
 wedge readings give then directly the percentage ratio of the light 
 transmitted through the plate. 
 
 A third type of photometer is the small Koenig-Martens hand 
 polarization photometer. This was adapted to the purpose by the 
 special attachments shown in figure 71, which were constructed in 
 the Geophysical Laboratory. 8 These served for a large series of 
 measurements and proved to be exceedingly useful. The optical 
 arrangement of the photometer is shown in figure 72a. Its essential 
 feature is the Wollaston cube, W which consists of two calcite 
 prisms so cut that the optic axis in each prism is parallel to the end 
 surfaces of the cube, so that, when cemented together along the 
 sloping surfaces, the optic axis of the first prism-half is at right 
 angles to the optic axis of the second. The effect of the prism is to 
 
 * Supplied by the Eastman Kodak Co. 
 
 * F. E. Wright, Jour. Opt. Soc. America, 2-8, 65-75, 93-96, 1919. 
 
214 
 
 INSPECTION OF OPTICAL GLASS. 
 
 produce two divergent beams of plane-polarized light polarized at 
 right angles, the first to the second. These are reduced to a common 
 plane of vibration by the analyzer N. The intensity of each beam 
 of light, as observed through the analyzer, is proportional to the 
 square of the cosine of the angle a which its plane of vibration includes 
 with the plane of vibration of the analyzer; at each instant, therefore, 
 the intensity of the one field, 7 2 , is proportional to cos 2 a, that of the 
 other, /u to cos 2 (90 a) = sin 2 a; the ratio of the intensities is 
 
 i = C tan 2 . The constant C in this equation is the ratio of intensi- 
 
 M 
 
 FIG. 71 .The Koenig-Martens hand photometer with special attachments for use in measuring light 
 transmission of optical glass plates and optical instruments. A , the photometer; N, a scale- 
 reading lens; B, an attachment supporting the two reflecting prisms, "and P; C, a tube with 
 eccentrically mounted lens M for use in measuring light transmission in optical instruments; 
 D and E, attachments used in measuring the amount of normally incidentlight reflected from 
 a polished glass surface. 
 
 ties of the two beams of light emerging from the Wollaston cube. 
 This ratio is approximately equal to unity. An extended series of 
 computations shows that for a Wollaston prism of slope angle 45, 
 (7=1.0026 in case the two end faces are protected by glass slips 
 cemented to the prism with Canada balsam (n= 1.540); for the bare 
 Wollaston prism C= 1.0006. It is, therefore, permissible in this type 
 of prism to consider that C= 1 and 
 
OPTICAL TRANSMISSION. 
 
 215 
 
 The error made by this assumption is negligible. In every instru- 
 ment, however, the validity of this assumption should be tested and 
 the value of C ascertained from the angular position a of the analyzer 
 for which the two halves of the field are equally illuminated. 
 
 In this photometer the light enters the entrance pupils A and B 
 (fig. 72a) ; these apertures are in the rear focal plane of the lens C 
 cemented to the Wollaston prism W; the rays of light pass from the 
 Wollaston prism through the biprism F whence they pass as parallel 
 beams of light through the analyzer N and the eyepiece lens D, and 
 through the exit pupil at E. 
 
 In order to separate the two apertures (entrance-pupils) of the 
 photometer, two reflecting prisms, P, K, (fig. 71) are commonly 
 employed. For this purpose the attachment shown in figure 71 
 was constructed. It is attached directly to the instrument. The 
 large prism P slides in the grooves indicated in the figure and can 
 
 a. 
 
 b. 
 
 C W F 
 
 FIG. 72. Optical arrangements of Koenig-Martens photometer. In fig. 72o the paths of the rays from the 
 two apertures A, B, are shown; in fig. 726 the paths of the rays when only one aperture, A , is employed. 
 
 be rotated about the axis at Q. The presence of these prisms 
 decreases the intensity of the one field and the position of equal 
 intensity of the two halves is no longer at 45 but at a different 
 angle . 
 
 The procedure in measuring the transmission of a plate is briefly : 
 
 1. Ascertain position ( ) of analyzer for equal intensity of fields 
 when no plate intervenes. 
 
 2. Place plate on flat side (thickness Z t ) in front of prism set and 
 read a lt position of analyzer for equal intensity of fields. 
 
 3. Place plate on end (thickness 7 2 ; a 2 <a l ) and read new position 
 of analyzer for equal intensity of the two fields (the angle a 2 ; a 2 < aj . 
 
 4. Place plate on flat side in front of second aperture of photom- 
 eter and read a 3 , position of analyzer for equal intensity of fields. 
 
 (5) . Place plate on end and read a 4 , ( 4 < 3 ) the position of analyzer 
 for equal intensity of the two halves of the field. For each position 
 take 10 readings. 
 
216 INSPECTION OF OPTICAL GLASS. 
 
 The transmission for a plate of thickness 1 2 1 1} is then 
 
 /= tan 2 <* 4 .cot 2 o! 3 , ( 4 < a a ) 
 
 The transmission /3 for a plate of unit thickness can be ascertained 
 from the standard equation which may be written 
 
 / 72 
 
 log = , 7 --log y = 7 -- r (log tan 2 
 h "~ h *2 fc 2 "" 6 i 
 
 cot 
 
 41* 
 
 43* 
 
 I 
 
 E 
 
 80 
 
 LLLt 
 
 41* 
 
 42 
 
 /I/ 
 
 43 
 
 1 
 
 44 
 
 / 
 
 45- 
 
 FIG. 73. Graphical solution of the equation 7=tan 2 a2 cot 2 ai used in connection with the polarization pho- 
 tometer. The abscissae represent the angles a^; the sloping lines, the angles ai; and the ordinates, the 
 percentage light transmission, 7. 
 
 In case only one thickness of plate is available the reflection formula 
 of Fresnel for vertical incidence may be assumed to be valid, and 
 the equations are 
 
 A n 2 + 1 , 
 
 -^ = tan Q! 4 -cot a 2 --^-> (a 4 <a 2 ). 
 
 In case the amount of light p reflected from a single surface has 
 been measured, the value of /3 can be computed from the quadratic 
 equation ou page 210. 
 
LOSS OF LIGHT BY REFLECTION. 217 
 
 Experience has shown the graphical solution of these equations as 
 represented by figure 73 is adequate for all purposes; in this diagram 
 the vertical lines are the angle 2 , the sloping lines the angle a v and 
 the horizontal lines the transmissions, which, when a^ and 2 are 
 given, can be read off directly. In this diagram only a limited range 
 of angles is given, but for all optical glasses it suffices. A second 
 diagram of the same character was also prepared, giving a much 
 larger range and was used in the measurement of the light transmis- 
 sion of certain optical instruments and of experimental glasses of 
 appreciable absorption. 
 
 Reflection of light. In order to test the validity of the Fresnel 
 equation for reflected, vertically incident light, and also to ascertain 
 the effect of certain solutions in reducing the amount of light reflected, 
 an attachment was used on the Koenig-Martens photometer which 
 proved satisfactory and simple in operation. The attachment is 
 illustrated in figure 71, D and E. It consists of two right-angle 
 prisms R, each rotatable about a horizontal axis. Mounted back of 
 each prism is a lens A, the function of which is to image the reflecting 
 surface, held by the mount D, figure 71, in the image plane of the eye- 
 piece. The cylindrical mounting for the prisms and lenses slips over 
 the end of the Koenig-Martens photometer to such a position that 
 the apertures of the photometer are located at the rear focal planes 
 of the two lenses. This insures the entrance of parallel rays into the 
 prisms. In practice the glass plate is placed on the mount E at F at 
 such a position that its reflecting surface is seen in the field of view. 
 The light reflected by the plate in this position is not strictly vertical, 
 but it is so nearly so (within 10 or less) that no appreciable error is 
 made by assuming strict verticality; this is evident from the general 
 Fresnel expression for the intensity of rays of light reflected at 
 different angles of incidence. 
 
 In the practical measurement of the intensity of reflected light it is 
 convenient first to balance the fields with the reflecting surface in 
 front of the aperture at the right side of the photometer and then to 
 shift to the left side by rotating the prisms each through 90 and to 
 match the fields again. By this method the first values are checked 
 against the second. It is also advisable to mount the photometer 
 at some distance from the source of uniform light in order that local 
 differences of illumination may be eliminated. The uniformity of 
 illumination of the two fields should be tested by observing the fields 
 when both prisms are pointing directly toward the source of light, 
 and also when they both point at the reflecting surface. By this 
 method the intensity ratio C of the two fields under conditions of 
 the same uniform incident illumination is obtained. 
 
218 INSPECTION OF OPTICAL, GLASS. 
 
 THE MEASUREMENT OF THE TRANSMISSION OF OPTICAL INSTRUMENTS. 
 
 For this purpose the attachment of figure 71 C was used. It con- 
 sists simply of a lens mounted at the end of a cylinder which slips over 
 the end of the Koenig-Martens photometer. The lens is so placed 
 that its rear focal plane falls on the aperture of the photometer, thus 
 insuring the entrance of parallel light into the instrument. The 
 optical instrument, whose transmission is to be measured, is placed 
 so that its entrance or exit pupil is imaged by the lenses F (figure 72) 
 and C (figure 71) in the image plane of the eyepiece. It is immaterial 
 whether the exit or the entrance pupil of the optical instrument is 
 used. In the case of telescopes the entrance pupil (objective end) 
 is more convenient to use because it is much larger. The two fields 
 are first balanced with the analyzer at c^ without the optical instru- 
 ment in the path; after which the instrument is placed in the path 
 and the fields are again balanced at a 2 . The transmission ratio is 
 then 
 
 7 = y = tan 2 o! 2 cota 1? (a 2 <a 1 ). 
 
 M 
 
 The transmission can then be read off directly on the graphical plot, 
 figure 73. 
 
 THE WEATHER STABILITY OF OPTICAL GLASSES. 
 
 An essential requirement for optical instruments intended for field 
 use is weather stability or unchangeability of the surfaces of the lenses 
 and prisms when exposed to the action of the atmosphere. Glass 
 which is not stable is attacked by atmospheric agencies, such as 
 water vapor, carbon dioxide gas, etc. ; by finger perspiration and by 
 bacteria and minute plant organisms, which eat into the finely fin- 
 ished optical surfaces. These agencies destroy the polish, and, in 
 many instances, produce a film over the surface which greatly reduces 
 the light transmission and in extreme cases may render the instru- 
 ment useless. 
 
 Many methods have been devised for the purpose of testing the 
 weather stability of optical glass. The procedure generally followed 
 is to carry out a series of laboratory tests under definitely controlled 
 conditions on standard glasses of known composition and of known 
 degree of weather stability and to ascertain their behavior toward 
 pure water and also alkaline, acid, and saline solutions of definite 
 concentrations and at definite temperatures; from data of this 
 nature, the probable weather stability of an unknown glass may 
 then be inferred from its behavior in the standardized laboratory 
 tests. There is, of course, danger in this procedure that any inference 
 drawn from the behavior of a complex .solution, such as glass, at 
 high temperatures and possibly high pressures toward reagents of 
 
STABILITY OF GLASS. 219 
 
 high concentration, may bear no relation whatever to the behavior 
 of the glass toward atmospheric agencies which are dilute agents 
 that attack slowly and at room temperatures. The process is one 
 of decomposition or selective solution and the behavior of the silicate 
 system at low temperatures toward chemically weak atmospheric 
 agents working over long periods of tune does not necessarily bear 
 any direct relation to the behavior of the same system at high 
 temperatures and high pressures toward other reagents of much 
 greater concentration operating for only a short period of time. 
 
 A long series of experiments by Mylius 9 and Foerster 10 bear 
 out these statements and prove that changes in temperature pro- 
 duce very large differences in the ratios of alkalies to silica dis- 
 solved as well as in the total quantities dissolved; also that changes 
 in concentration of the attacking solutions have a pronounced effect. 
 These researches were carried out especially to test the quality of 
 chemical glassware; but in view of the fact that weather stability 
 means in large part resistance to attack by water at ordinary tem- 
 peratures the results obtained are indicative of the relative dura- 
 bility of the glasses when exposed to atmospheric agencies. The 
 lack of resistance to attack by atmospheric agencies finds expression 
 in an optical glass surface in the formation of films, either liquid 
 (in the form of dropg ranging in size from microscopic to easily 
 visible drops) or of crystallized aggregates (commonly as alkali carbo- 
 nates); or of organic growths (algae, bacteria). The surface may 
 become pitted and corroded; it may absorb water and swell, and 
 then devitrify, thereby causing the surface ultimately to flake and 
 crack; isolated spots and specks may appear on the surface, espe- 
 cially in the heavy flint glasses. These phenomena were examined 
 microscopically by E. Zschimmer n on a set of plane polished plates 
 of optical glass of different types which had been stored in a loosely 
 closed metal box for periods up to seven years. 
 
 Dimming test. Zschimmer 12 found that by exposing polished glass 
 surfaces to moist air, nearly saturated with water vapor and heated 
 to 80 C, for a period of two hours or longer , films form on the surfaces 
 which are similar to the films formed at room temperatures on pol- 
 ished surfaces after months' or years' exposure to the atmosphere, 
 Zschimmer 's work was repeated and modified slightly by Elsden. 
 Roberts, and Jones, 13 and accordant results were obtained by them. 
 Constant temperature was maintained by means of a carefully regu- 
 lated thermostat. Test plates, 1? by J by J inches in size with well 
 
 9 Ber. d. deutsch. Chem. Ges. 22, 1092, 1889; 24, 1482, 1891; Zeitschr. f. Instrumentenkunde 9, 117, 1889. 
 11,311, 1891. 
 
 10 Ber. d. deutsch. Chem. Ges. 25, 2494, 1892; 26, 2915, 1893; 26, 2998, 1893; Zeitschr. f. analyt, Chem. 
 33, 299, 1893; 34, 381, 1894. 
 
 11 Chem. Zeitung, 25, 730, 1901. 
 
 12 Zeitschr. Elektrochemie 11, 628-638, 1905; D. Mechan. Zeit., 7, 53, 1903. 
 Jour. Soc. Glass Technology, 3, 52-69, 1920. 
 
220 INSPECTION OF OPTICAL GLASS. 
 
 polished surfaces were first examined, after careful cleaning by a 
 special method, for surface imperfections; these were recorded for 
 each plate. Polished quartz plates served as control plates and were 
 inserted into the apparatus together with the glass plates. In all, 
 operations the greatest care was taken to insure clean glass surfaces 
 free from contamination. The plates were then exposed in the 
 apparatus to a slow current of moist air at 80 C. for the desired time. 
 The thermostat was then cooled to a temperature of 2 C. below room 
 temperature at such a rate that no distinct precipitation (dew) was 
 formed on the quartz plates. The plates were then removed and 
 quickly transferred to dry test tubes and the tubes closed with 
 rubber stoppers. The plates were first examined for dew and a 
 record was made in each case of the time required for the dew to dis- 
 appear. The plates were then examined for "film" or degree of 
 attack of polished surface; detailed examination under the micro- 
 scope revealed the characteristic features of each deposit and from 
 these an estimate was made of the degree of attack on plates held at 
 80 C. for 30 hours. On this basis the glasses were classified into 
 three groups excellent, medium, and poor. 
 
 Unaffected or very slightly affected 1 
 
 Appreciably affected 2 
 
 Seriously affected 3 
 
 The classification of the relative weather stability of different 
 optical glasses by this method appears to be in accord with the actual 
 relative behavior of the glasses toward weathering agencies, and also 
 to agree with the conclusions reached by the iodoeosin test of Mylius. 
 
 The iodoeosin test. Experiments by Warburg and Ihmori 14 proved 
 that glass is attacked by water in such manner that some alkali is 
 leached out; even in the presence of weak acids, it is the action of the 
 water which is the important factor in the attack. The quantity of 
 alkali thus set free on hydrolytic decomposition of the glass is indica- 
 tive of the resistance of the glass to weathering agencies. The most 
 sensitive and reliable method for ascertaining the amount of free 
 alkali present appears to be the iodoeosin test of Mylius. 15 
 
 For the iodoeosin test a saturated solution of water in ether is 
 used in which pure iodoeosin (C 20 H 8 I 4 O 5 ) is dissolved (0.5 g per liter 
 solution). A freshly fractured surface of the glass to be tested is 
 immersed for one minute at 18 C. in this solution; during this time 
 the glass is attacked, a small quantity of alkali is set free, enters 
 into combination with the iodoeosin to form the red alkali salt 
 (soda or potash) which is insoluble in ether and hence is precipitated 
 on the surface tniis coloring it pale red, the intensity of the coloring 
 
 " Wiedemann's Ann., 27, 481, 1885. 
 
 i* Zeitschr. Instrumentenkunde, 1905, 149; Zeitschr, Anorg. Chem., 55, 233-260, 1907; 67, 200-224, 1910; 
 Silikat Zeitschr. I, 2, 25, 45, 1913. See also Hovestadt, Jena Glass, translation by J. D. and A. Everett, 
 London, 1901; Zschimmer in Doelter's Handbuch d. Mineralchemie I, 855-918, 1912. 
 
STABILITY OF GLASS. 
 
 221 
 
 depending on the amount of free alkali available for precipitation. 
 The glass surface, on removal from the iodoeosin solution, is plunged 
 quickly into ether and the excess iodoeosin solution is washed off. 
 The surface is then allowed to dry; the surfaces, except the freshly 
 broken surface of the glass fragment, are wiped clean with a cloth; 
 the iodoeosin salt is then dissolved in a small quantity of water and 
 its quantity determined colorimetrically by matching it against a 
 standard iodoeosin salt solution. The alkalinity of the freshly 
 broken glass surface in terms of milligrams iodoeosin thus abosrbed 
 per square meter area is called the " natural alkalinity" of the glass 
 by Mylius. The " weather alkalinity" is, however, a better indicator 
 of the weather stability of the glass and is in effect the amount of 
 iodoeosin absorbed per square meter area on a freshly broken glass 
 surface after it has been weathered by exposure to moist air for 
 seven days at 18 C. and is then tested in the same manner as a 
 freshly broken surface before such exposure. Mylius found that the 
 " weather alkalinity" in the more resistant and weather stable glasses 
 is not in general greatly different from the " natural alkalinity"; 
 but that in the less stable glasses the "natural alkalinity" may be 
 four times as great as the "weather alkalinity." Certain anomalies, 
 especially in the borosilicates and the dense flint and barium glasses 
 are less disturbing in the "weather alkalinity" experiments than in 
 the "natural alkalinity" tests. 
 
 In the practical use of these tests certain precautions have to be 
 observed which are emphasized by Mylius. Extended tests on 
 glass of known degrees of weather stability have proved the general 
 reliability of the dimming and the iodoeosin tests. In the case of 
 the heavy flint and barium silicate glass the effects of the lead and 
 barium on the iodoeosin may be disturbing if the prescribed pro- 
 cedure is not strictly followed; the presence of boron oxide in a 
 glass may also cause trouble under certain conditions. The iodoeosin 
 tests should be corroborated by other methods wherever possible. 
 
 Mylius divides glasses into five groups on the basis of the alkalinity 
 of the weathered surfaces (weather alkalinity) as follows: 
 
 Class. 
 
 Type of glass. 
 
 " Weather alkalinity, " 
 milligrams per square 
 meter. 
 
 Example. 
 
 I 
 
 Practically insoluble 
 
 to 5 
 
 Silica glass 
 
 II.. 
 
 Resistance glass. 
 
 5 to 10 
 
 Jena Gerate, pyrex. 
 
 Ill 
 
 Hard glass . 
 
 10 to 20 
 
 
 IV... 
 
 Softglass 
 
 20 to 40 
 
 Ordinary crowns. 
 
 V... 
 
 Poor glass 
 
 Over 40 
 
 
 
 
 
 
 Optical glasses should have a surface alkalinity, after weathering, 
 less than 40 milligrams per square meter. 
 3922921 15 
 
222 
 
 INSPECTION OF OPTICAL GLASS. 
 
 The acetic acid test. This test was first used by E. Zschimmer ie 
 and is especially adapted to show the spots which may develop on 
 flint glasses as a result of finger marks (perspiration). A drop of an 
 aqueous solution containing 0.5 per cent acetic acid and 0.05 per 
 cent glycerine is allowed to remain for 24 hours on a polished glass 
 surface. It is then washed off; the intensity of the resulting tarnish 
 or coating is a measure of the tendency of the glass to develop spots 
 as a result of finger marks. Modifications of this test have been devel- 
 oped, but they are of limited application and usefulness. 
 
 Autoclave tests. Tests of the behavior of glass surfaces subjected 
 to attack by water and acid solutions at high temperatures and pres- 
 sures, such as water at 180 C., have been adopted at different plants 
 to ascertain the weather stability of the glass, but the value of the 
 procedure has been seriously questioned. During the war they were 
 applied by Bichowsky 17 to a series of optical glasses and interesting 
 results were obtained. The tests adopted were not quantitative, and 
 the results obtained are subject to the personal judgment of the 
 observer. Three types of solution were employed, namely, water, 5 
 per cent solution of sodium hydroxide, and 1 : 1 hydrochloric acid. 
 Half-inch cubes of the glass were immersed in one of the solutions 
 in a steel bomb and heated for 4 hours at 225 C. in the case of water, 
 for 2 hours in the case of the sodium hydroxide solution. The hydro- 
 chloric acid tests were made in sealed combustion tubes held at 175 
 C. for 6 hours. 
 
 Bichowsky grades each glass cube according to its appearance after 
 treatment in one of the above solutions as follows: 
 
 Grade. 
 
 Appearance wet. 
 
 Appearance dry. 
 
 9. 
 
 Thick slushy film 
 
 Film more than 1 5 millimeter 
 
 8... 
 
 Thick film 
 
 thick. 
 Flakes off when dry. 
 
 7 
 
 Deep etching in stratches and corners 
 
 Thin film flakes off 
 
 6 
 
 do. . 
 
 Thin film does not flake off 
 
 5 
 
 Opalescent film 
 
 Deeply etched 
 
 4 
 
 do. . . 
 
 Not etched. 
 
 3 
 
 Film 
 
 Film 
 
 2 
 
 Clear . 
 
 Do. 
 
 I... 
 
 do 
 
 Clear 
 
 
 
 
 His tests on different types of glass show marked differences in 
 behavior in the different solutions. From the results he was able, 
 however, to infer with some degree of assurance the general stability 
 of the glass. Unfortunately his experiments could not be carried far 
 enough on a sufficiently large number of glasses of known degrees of 
 weather stability to demonstrate the practical usefulness of the 
 method as a test for actual "weather stability." 
 
 ' Deutsch. MechanikerZeit. 1903, 55. 
 
 Journ. Am. Ceram. Soc., 3. 296-308, 1920. 
 
FILM ON GLASS SURFACES. 223 
 
 THE FORMATION OF FILM ON INCLOSED GLASS SURFACES. 
 
 A serious matter during the war was the appearance in many 
 optical instruments, especially binoculars, of a more or less pronounced 
 film on lens and prism surfaces which reduced the light transmission 
 seriously and in many cases rendered the instrument useless. In many 
 instances it was found possible to clean the surfaces, but this neces- 
 sitated in each case reassembly and readjustment of each instrument 
 after cleaning. The "film" problem was attacked by many investi- 
 gators and the source of trouble found to be in many cases lack of 
 cleanliness in the original assembly of the instrument; 18 film of this 
 nature is especially liable to occur on the graduated surfaces of reti- 
 cules, of prisms, and of the field lens of the eyepiece. In certain 
 cases the formation of film was traced back to lack of weather stability 
 in the glass itself. 
 
 A faint film is recognized most readily hi oblique, half-shadow 
 illumination, rather than in direct full illumination. To detect film 
 on a glass surface in an optical instrument, point its eye-piece end 
 toward a strong light; turn the instrument through a small angle so 
 that its axis does not coincide exactly with the line of sight from the 
 observer to the source of light. Under these conditions of indirect 
 lighting, film on a lens or prism surface appears as a faint haze, not 
 unlike the effect produced by a ray of sunlight on suspended dust par- 
 ticles in a room. With proper care and an intense source of light, 
 this test is extremely sensitive and renders visible even faint traces 
 of film. 
 
 18 H. S. Ryland, On the prevention of film in inclosed optical instruments, Trans. Opt. Soc. London, 19, 
 179-181, 1918; L. C. Martin and C. H. Griffiths, Deposits on glass surfaces in instruments, Trans. Opt. Soc 
 London, 20, 135-155, 1918. 
 
Chapter V. 
 THE MANUFACTURE OF LENSES AND PRISMS. 
 
 The operations and processes described in this chapter are for the 
 most part different from those involved in the manufacture of optical 
 glass; they are, moreover, more widely known and are essentially 
 mechanical in nature, and hence do not require to any great extent 
 the services of the chemist and the physicist. The maker of optical 
 instruments on undertaking to manufacture an instrument to be 
 used for a definite purpose ascertains first the exact optical and 
 mechanical conditions, such as magnification, field of view, external 
 dimensions, etc., which the finished instrument has to meet. The 
 lens designer is then called upon to produce an optical system which 
 will not only satisfy the optical requirements, but also fit properly 
 into the given mechanical system. Guided by certain principles he 
 computes the shapes, sizes, and positions of the several elements of a 
 lens system which will best meet the prescribed conditions with the 
 several types of glasses available; in the general design he relies not 
 only on the results of computation, but also on the results of previous 
 computations, and on his general experience with optical systems. 
 
 Having thus specified the shapes and sizes of the several optical 
 elements of the system, it is the task of the manufacturer to fashion 
 the desired lenses and prisms from the plates of glass received from 
 the glass factory. The glass is treated throughout as a material on 
 which certain operations have to be performed to attain certain 
 results, just as brass or other metal which enters into the con- 
 struction of an optical instrument is subjected to other operations 
 to attain other desired results. Each material is characterized by 
 certain properties which necessitate special modes of treatment in the 
 factory in order to attain the desired results. It is the task of the 
 optical engineer to prescribe the modes of treatment best adapted to 
 produce these results with optical glass under given factory condi- 
 tions. It is not the purpose of the present chapter to specify and to 
 describe in detail the manufacturing processes best suited for the 
 production of optical systems required in optical munitions; but 
 rather to state in a general way the factory procedures and to empha- 
 size certain fundamental principles, thereby directing attention to 
 some of the difficulties which may arise in case a great increase in the 
 production of optical munitions is demanded as a result of war-time 
 conditions. 
 224 
 
PREPARATION OF GLASS. 225 
 
 There is need at the present time for a comprehensive treatise on 
 the manufacture of precision optics. The literature on the subject 
 is scant and incomplete. The tendency in the optical trade is to 
 keep secret all manufacturing processes and to overemphasize the 
 difficulties encountered and the skill required to overcome them. 
 
 In the following articles certain phases of the general optical 
 manufacturing problem are described: 
 
 Draper, H. On the construction of a silvered glass telescope 15J inches in aperture, 
 etc. Contributions to Knowledge, Smithsonian Institution, vol. 34, No. 1459, 1904. 
 
 Ritchey, G. W. On the modern reflecting telescope and the making and testing 
 of optical mirrors. Contributions to Knowledge, Smithsonian Institution, vol. 34, 1904 
 
 Therelfall, R. On Laboratory Arts, London. 
 
 French, J. W. Trans. Opt. Soc. London, 17, 24-40, 1916; 18, 8-48, 1917, 
 
 Rayleigh, Lord. Proc. Optical Convention, 1905, pp. 73-79. 
 
 Halle, Bernhard. Handbuch d. praktischen Optik. 
 
 PREPARATION OF GLASS FOR LENSES AND PRISMS. 
 
 Before putting any raw optical glass into work it should be in- 
 spected for defects, such as striae, bubbles, stones, strain, etc. ; other- 
 wise unsatisfactory glass may be put into work only to be discarded 
 later. During peace times, glass which is optically satisfactory and 
 requires no further inspection can be bought in the open market, 
 but in time of war the inspection is necessarily less rigid and poor 
 glass may be passed by inexperienced inspectors. This glass, unless 
 reinspected at the lens factory and rejected, is put into work and 
 eventually causes waste of energy and time. This situation occurred 
 time and again during the recent war, and in at least one instance 
 retarded production seriously. A remedy for this situation is the 
 accumulation, during peace times, by the Government of a sufficient 
 quantity of raw optical glass of good quality to enable manufacturers 
 to use it until new production of raw glass on a scale commensurate 
 with the needs is well under way. 
 
 After the quality of the raw optical glass has been approved by 
 the inspector, its preparation for lenses and prisms depends in part 
 on the attitude of the manufacturer and on the sizes of the lens or 
 prism elements to be made from it. It is possible from a given block 
 of optical glass either to saw out lens and prism blanks or to press 
 blanks of approximately the correct shape. Many manufacturers of 
 optical instruments are not equipped with lens-pressing plants and 
 are required either to have the pressing done elsewhere or to saw 
 out blanks from the plates of raw glass. In some instances the lens- 
 pressing process has been found to be more economical than the cut- 
 ting process; but, in others, the cutting process is superior and 
 cheaper. 
 
226 
 
 MANUFACTURE OF LENSES AND PRISMS. 
 
 THE SIZES OF PRESSED AND SAWED BLANKS. 
 
 In the grinding and polishing operations a certain amount of glass 
 is necessarily wasted, and, in determining upon the sizes of the blanks 
 to be used for the manufacture of lenses and prisms of definite shapes 
 and sizes, the manufacturer makes proper allowance for this wastage. 
 The amount of excess stock needed depends upon the size of the lens 
 and the curvature of its surfaces. In case pressed blanks are to be 
 used, several millimeters (3 to 8 millimeters or 0.1 to 0.3 inch) are 
 added to each prism dimension to allow for irregular surfaces and 
 for pressing or molding defects; the actual allowance increases with 
 the size of the prism. In low-power lenses of large radii of curvature 
 several (3 to 5) millimeters are added to the diameter to allow for 
 centering of the lens ; in higher power lenses of 
 shorter radii of curvature an allowance of 2 
 millimeters to the diameter is sufficient ; 1 milli- 
 meter extra thickness is allowed for grinding on 
 each lens surface. Thus for a lens 20 milli- 
 meters in diameter the lens blank should meas- 
 ure 2 millimeters thicker and have a diameter 
 1.5 to 2 millimeters larger; for a lens 75 milli- 
 meters in diameter the blank should be 3 
 millimeters thicker and have a diameter 4 to 5 
 millimeters larger to allow for centering. In 
 case the blanks are to be sawed from plates of 
 glass an allowance of several millimeters should 
 FIG. 74.-Diagram illustrating be allowed for each prism face. For a given 
 the several elements neces- hickness of glass plate the cutting of the platest 
 
 sary to compute the volume , , , , 11,1 < i 
 
 of a lens. L is the thick- should be so planned that a minimum of glass 
 ness of the lens; a, the semi- j s wa sted. The prisms may be so cut that the 
 
 diameter; n and r a , the radii , . f * . / ,, , 
 
 of curvature; fti and to, hypothenuse lace is normal to, or parallel with ? 
 
 the height of the spherical or includes an angle of 30 or 45 with the flat 
 
 surface of the plate. The size and shape of 
 
 the available plate of glass and that of the prisms to be cut from it 
 are the determining factors in each case. 
 
 In order to ascertain the weight of a lens or prism blank it is nec- 
 essary to compute first its volume and then its weight from the known 
 specific gravity of the glass from which the blank is to be made. In 
 this connection the following formulae and tables are useful: 
 
 The volume, V, of a lens of semidiameter a, thickness Z, radii of 
 curvature r x and r 2 , height of segments, 7i l and h z as indicated in 
 figure 74 is 
 
 V = I [3a 2 (I - h, - Ji 2 ) (3r, - hj h* (3r 2 - Ji 2 ) h 2 *] 
 
 in which the negative sign in the last two members applies to concave 
 lens surfaces, the positive sign to convex lens surfaces. In this 
 
 expresson a 
 
 2 = 
 
 (2r 2 Ji 2 ) . 
 
WEIGHTS OF LENSES AND PRISMS. 
 
 A simpler approximate expression is: 
 
 227" 
 
 in which the negative sign is used for convex surfaces, the positive 
 sign for concave surfaces. The values obtained with this expression 
 are always too low; for a/r = 0.243, 0.341, 0.415, 0.477, 0.532, 0.723, 
 respectively, the percentage errors are 1, 2, 3, 4, 5, and 10. 
 
 In the computations of the volumes and weights of lenses and 
 prisms of different types of glass, Tables 15 and 16 are useful. 
 
 TABLE 15. 
 
 
 
 
 
 
 Weight. 
 
 
 
 
 Cubic 
 
 Cubic 
 
 
 
 Inches to 
 
 Centime- 
 
 centi- 
 
 inches to 
 
 
 
 No. 
 
 centi- 
 
 ters to 
 
 meters 
 
 cubic 
 
 Cubic cen- 
 
 Cubic 
 
 
 meters. 
 
 inches. 
 
 to cubic 
 
 centi- 
 
 timeters 
 
 inches 
 
 
 
 
 inches. 
 
 meters. 
 
 water in 
 
 water in 
 
 
 
 
 
 
 pounds. 
 
 pounds. 
 
 1 
 
 2.54 
 
 0. 3937 
 
 0.061 
 
 16.39 
 
 0.00220 
 
 0. 03613 
 
 2 
 
 5. OS 
 
 0. 7874 
 
 .122 
 
 32.77 
 
 .00441 
 
 . 07225 
 
 3 
 
 7.62 
 
 1.1811 
 
 .183 
 
 49.16 
 
 .00661 
 
 . 10838 
 
 4 
 
 10.16 
 
 1. 5748 
 
 .244 
 
 65.55 
 
 .00882 
 
 . 14451 
 
 5 
 
 12.70 
 
 1.9685 
 
 .305 
 
 81.94 
 
 .01102 
 
 .18064 
 
 6 
 
 15. 24 
 
 2.3622 
 
 .366 
 
 98. 32 
 
 .01323 
 
 . 21676 
 
 7 
 
 17. 78 
 
 2. 7559 
 
 .427 
 
 114. 71 
 
 .01543 
 
 .25289 
 
 8 
 
 20.32 
 
 3. 1496 
 
 .-188 
 
 131. 10 
 
 .01764 
 
 .28902 
 
 9 
 
 22.86 
 
 3. 5433 
 
 .549 
 
 147. 48 
 
 . 019?4 
 
 . 32514 
 
 10 
 
 25.40 
 
 3.9370 
 
 .610 
 
 163. 87 
 
 .02205 
 
 . 36127 
 
 In this table are listed the equivalents of centimeters (col. 2) to 
 inches (col. 1); of inches (col. 3) to centimeters (col. 1); of cubic 
 inches (col. 4) to cubic centimeters (col. 1); of cubic centimeters 
 (col. 5) to cubic inches (col. 1); in columns 6 and 7 the weights in 
 pounds of cubic centimeters and cubic inches of water, respectively 
 
 (col. 1) are listed. 
 
 TABLE 16. 
 
 Specific 
 
 Weight of cubic cen- 
 timeter of glass. 
 
 Weight of cubicinch 
 of glass. 
 
 gravity. 
 
 Grams. 
 
 Pounds. 
 
 Grams. 
 
 Pounds. 
 
 2.3 
 
 2.3 
 
 0.00507 
 
 37.960 
 
 0.08309 
 
 2.4 
 
 2.4 
 
 .00529 
 
 39.329 
 
 .08671 
 
 2.5 
 
 2.5 
 
 .00-551 
 
 40.968 
 
 ..09032 
 
 2.6 
 
 2.6 
 
 .00573 
 
 42.608 
 
 .09393 
 
 2.7 
 
 2.7 
 
 . 00595 
 
 44.245 
 
 .09754 
 
 2.8 
 
 2.8 
 
 .00617 
 
 45.884 
 
 . 10116 
 
 2.9 
 
 2.9 
 
 .00639 
 
 47. 522 
 
 .10477 
 
 3.0 
 
 3.0 
 
 .00661 
 
 49. 161 
 
 . 10838 
 
 3.1 
 
 3.1 
 
 .00683 
 
 50.800 
 
 . 11199 
 
 3.2 
 
 3.2 
 
 .00706 
 
 52.438 
 
 .11561 
 
 3.3 
 
 3.3 
 
 .00728 
 
 .54. 077 
 
 .11922 
 
 3.4 
 
 3.4 
 
 .00750 
 
 55 716 
 
 .12283 
 
 3.5 
 
 3.5 
 
 .00772 
 
 57.355 
 
 . 12645 
 
 3.6 
 
 3.6 
 
 .00794 
 
 58. 993 
 
 .13006 
 
 3.7 
 
 3.7 
 
 .00816 
 
 60.632 
 
 . 13367 
 
 3.8 
 
 3.8 
 
 .00838 
 
 62.271 
 
 . 13728 
 
 3.9 
 
 3.9 
 
 .00860 
 
 63.909 
 
 .14090 
 
 4.0 
 
 4.0 
 
 .00882 
 
 65. 548 
 
 . 14451 
 
228 ^ MANUFACTURE OF LENSES AND PRISMS. 
 
 In this table are listed the weights in grams and in pounds of 1 
 cubic centimeter and of 1 cubic inch of glasses of different specific 
 gravities (col. 1) ranging from 2.3 to 4. In the case of specific 
 gravities given to the second or third decimal place the exact values 
 can be ascertained by adding to the appropriate value of Table 16 
 the desired amounts read off from Table 15. Thus, the weight in 
 pounds of a cubic inch of glass of specific gravity 3.62 is 0.13006 
 (col. 5, Table -16) plus 0.0007225 (col. 6, Table 15), or 0.13078. 
 In this computation careful attention should be paid to the decimal 
 points. As a general rule, it is simpler to state all dimensions in 
 the metric system; on the majority of ordnance drawings, however, 
 the dimensions are given in inches, and in this case the foregoing 
 tables are useful. The following equivalents are also useful in these 
 computations: One ounce equals 28.35 grams; 1 pound avoirdupois 
 equals 453.4 grams; 1,000 grams equals 2.204622 pounds avoirdupois; 
 1 inch equals 2.540 centimeters; 1 centimeter equals 0.3937 inches. 
 
 The volumes of the several different types of prisms in common 
 use are readily deduced and offer no difficulty to the computer. 
 
 The manufacturer having ascertained the volumes and weights of 
 the several different blanks required for a particular instrument 
 commonly adds a safety factor of 50 per cent to the total weights of 
 optical glass required to cover losses from preparation of the glass 
 for blanks, from pressing and other defects, from grinding and 
 polishing, from breakage, and from rejections. 
 
 GRINDING AND MILLING OPERATIONS. 
 
 In case the raw glass plates available are of irregular thickness or 
 are too thick, it is necessary to cut off thin plates or to grind or to 
 mill the plates to the correct thickness. Of these processes grinding 
 is commonly adopted and consists essentially in grinding down by 
 hand one side of each plate with coarse emery or carborundum on a 
 rapidly rotating, plane, cast-iron disk. A series of such plates, 
 ground on one side and of about the same thickness, are mounted 
 on a flat cast-iron plate or disk with blocking pitch (mixtures of 
 pitch, pine tar, rosin, shellac, Venice turpentine, and beeswax in 
 different proportions depending on time of year and size of blocking 
 tool) and then ground with emery or carborundum flat against a 
 horizontal, rotating, plane, cast-iron plate. During the first part 
 of this operation the iron blocking tool is carefully watched and 
 guided somewhat by hand in order to insure equal thickness of all 
 plates. Thicknesses are measured by means of a caliper; the dis- 
 tance between the ground-glass surface and the surface of the iron 
 blocking tool is measured at different points over the plate. The 
 uniformly thick plates are then ground with finer abrasive; if neces- 
 sary they are then remounted and the coarsely ground flat under- 
 
THE CUTTING OF GLASS. 229 / 
 
 surfaces of the plates reground with finer emery. For these grinding 
 operations a large rotating, heavy iron disk 4 to 8 feet in diameter 
 is generally used and serves the purpose well. In case the moistened 
 abrasive does not remain satisfactorily on the rapidly rotating plate 
 the addition of a little glycerine to the water is advantageous. 
 
 An alternative to the grinding process is to mill the plates with a 
 diamond-charged milling tool on a milling machine. The milling 
 tool is a solid or thick- walled, hollow cylinder of copper, or brass, or 
 soft steel, 2 to 4 inches in diameter and 3 to 6 inches long. For 
 coarse cutting, the tool is charged with diamond dust which passes 
 through an 80-mesh but is retained on a 100-mesh seive. The copper 
 cylinder is charged commonly by first cutting with a sharp engraving 
 tool, mounted on a fixture attached to a lathe, a series of closely spaced 
 longitudinal grooves or cuts which extend the entire length of the 
 cylinder and include an angle of about 45 with the radius. These 
 cuts resemble the chisel cuts on the copper disk saws described below 
 and serve the same purpose. The grooves thus cut are filled with 
 diamond dust mixed with thick oil; the cylinder is rotated slowly 
 while a hardened steel roller is pressed against it, and the grooves 
 are thus closed. The millers are recharged by the same method of 
 grooving and filling. Brass and soft-steel cylinders are harder and 
 are commonly charged by direct pressure, the diamond dust of the 
 dust-oil mixture being forced into the metal by the action of a narrow 
 hardened steel roller bearing heavily against the slowly rotating 
 cylinder. 
 
 The diamond-charged milling tools are mounted in a milling 
 machine and the glass plates are carried forward on a moving bed 
 as in ordinary milling operations. Kerosene or some soap com- 
 pound serves as lubricant. The tools are run at high speed from 
 500 to several thousand revolutions per minute and care is taken 
 not to take too deep or too fast a cut otherwise the tool may chatter 
 and in certain instances shatter the plate. 
 
 Glass plates having irregular flat surfaces can be rapidly cut down 
 to proper thickness with plane parallel sides by a miller of this type 
 which is in common use in many optical grinding plants at the present 
 time. It is possible by means of two heavy brass plates one-fourth 
 inch thick mounted on an arbor and charged with diamond dust t 
 both on the end and down the sides to make a straddle mill for 
 cutting blocks with parallel sides. In this case care should be taken 
 to mount the glass blocks securely. 
 
 THE CUTTING OF GLASS. 
 
 From a thick plate of glass slices or strips of glass of given thick- 
 ness are commonly cut by means of saws, specially designed for the 
 purpose. A great variety of such saws are in use; they are commonly 
 
230 J MANUFACTURE OF LENSES AND PRISMS. 
 
 disks of thin metal, such ~as copper, brass, tinned or untinncd soft 
 sheet iron or steel, impregnated with fine diamond dust. The speed 
 and lubrication of the saw depend somewhat on the way in which 
 the diamond dust is embedded in it. 
 
 In one type of diamond saw fine diamond dust (80 to 100 mesh) 
 mixed with thick oil is introduced along the periphery of the saw by 
 filling it into fine crosscuts made with a sharp chisel edge and then 
 closing the cuts by means of an accurately fitting hard-steel roller 
 pressed against the edge of the slowly rotating disk. The cuts are 
 made commonly by hand and in direction are intermediate between 
 radial and tangential cuts. During the cutting operation the disk, 
 which is generally of copper and of stock one-sixteenth inch thick for 
 saws up to 5 or 6 inches in diameter, and correspondingly heavier for 
 larger saws, is held firmly between two plates on an arbor in the 
 lathe, the rim of the copper disk extending one-eighth to one-fourth 
 inch beyond the plates. With this method of mounting the plate 
 retains its shape during the charging process. The edge is commonly 
 rounded slightly and the transverse cuts are made as close together 
 as possible. In operation the saw is rotated against the cuts. A 
 saw of this type, properly used and well lubricated, should last for 
 many days. Kerosene or some soap compound is used as cooling 
 lubricant and, if high speed be used, is squirted under pressure in a 
 steady stream against the saw cut in the glass which is thus kept 
 cool and does not become overheated. Speeds of 1,000 revolutions 
 per minute and even greater with small saws are satisfactory. Saws 
 of this type cut readily through glass plates at the rate of one-half 
 inch or more per minute; as the saw wears down it should be fed more 
 slowly. Too much pressure on the saw cuts out the diamond dust 
 and causes trouble. The saw should always bear against a flat sur- 
 face and not against a sharp edge of glass; otherwise it is liable 
 to be bent or damaged. 
 
 A variation of this type is made of brass disks one-sixteenth inch 
 thick for 4-inch saws and three- thirty-seconds inch thick for 6 or 7 
 inch saws. Except for an outer ring one-eighth inch wide, the thick- 
 ness of the metal disk is reduced slightly by cutting on a lathe in 
 order to afford clearance and thus to reduce friction in the rapidly 
 rotating disk during the glass-cutting operation. Even when 
 no effort is made to insure a thicker rim, the filling with diamond 
 dust by the ordinary process accomplishes the purpose to an appre- 
 ciable degree. The edge itself is flat and is charged with diamond 
 dust (80 to 100 mesh), mixed with heavy oil, by means of a hardened 
 steel roller pressed against the edge while the disk mounted between 
 circular plates is slowly rotated in the lathe. The disk is then 
 mounted against a flat plate on an arbor and the powder is pressed 
 into its rim on each side of the plate by means of the steel roller. 
 
EDGING OF GLASS DISKS. 231 
 
 Small disks of this type in operation may be rotated at from 2,000 
 to 3,000 revolutions per minute and are lubricated along the cutting 
 edge by a stream of kerosene under pressure. 
 
 Diamond-charged saws of the above types may be mounted in 
 series on an arbor of a milling machine and a multiple gang saw thus 
 obtained which cuts from a glass plate a series of strips of definite 
 thickness. These strips may then be mounted in a supporting 
 frame and cut crosswise into small square plates suitable for the 
 manufacture of lenses after the corners have been chipped and ground 
 off to obtain disks of the desired diameter. 
 
 In other saws a thick mixture of coarse carborundum powder and 
 water is fed against the cutting edge of the rotating metal disk 
 (commonly of sheet iron) and the glass plate is thereby cut with fair 
 speed. The abrasive may be fed to the disk by having it dip into 
 the thick carborundum-water mixture contained in a pan below the 
 wheel. 
 
 Thin carborundum wheels running at a fairly high rate of speed 
 have also been successfully employed for cutting glass plates. 
 
 In all cases the speed of the cutting saw and the rate of feed of 
 the cooling lubricant should be so adjusted that a rapid and satisfac- 
 tory cut is obtained without crowding the saw too severely and 
 without chipping the glass seriously. The glass plate should be 
 securely mounted on a support relatively free from vibrations. 
 
 In the case of thin plates of glass the crosscut sawing operations 
 are generally dispensed with and the cuts made either with a glazier's 
 diamond or a steel-wheel glass cutter. 
 
 EDGING OF DISKS. 
 
 Square plates of the desired thickness are made into /disks by 
 cutting and nipping or grinding off the corners. The usual practice 
 is first to cement a number of such plates, cut and nipped to approxi- 
 mately circular shape, with paraffin into a column; an alternative 
 is to use the plates dry or with a thin film of moisture on each surface 
 and to hold them in place by a spring against the end of the column; 
 to mount the column on a rotating axis and to have this play first 
 against a carborundum grinding wheel which removes all corners 
 and produces finally a column of uniform diameter. For the fine 
 grinding operations either a fine grinding wheel or a diamond-charged 
 miller is used. 
 
 An alternative method for the production of glass disks is to cut 
 them directly from the glass plate on a drill press. For this 
 purpose a hollow cylinder of brass or soft steel of the proper diameter 
 and 2 millimeters thick is used. Several longitudinal grooves 4 or 5 
 centimeters long and 2 to 3 millimeters wide are cut into the sides 
 
 / 
 
232 * MANUFACTURE OF LENSES AND PRISMS. 
 
 of the cylinder; along these grooves the thick mixture of carborundum 
 powder (150 mesh) and water is fed to the cutting edge, which may 
 be a smooth or a serrated edge. The glass plate is mounted securely 
 to the bed of the drill press; the cylinder grinding tool is attached 
 to the axis of the drill press and is rotated at about 500 revolutions 
 per minute. The glass, disk is cut out as in an ordinary drilling 
 operation, the rate of cutting being about I millimeter per minute. 
 The diameter of the cutting tool should be slightly larger (1 to 2 
 millimeters) than that desired for the glass disk. In operation a 
 relatively large amount of the thick abrasive paste is used and is 
 continuously worked over by hand and forced down the feed grooves 
 of the rotating tool. Cylindrical tools in which the cutting end and 
 rim are charged with diamond dust may be used in place of the 
 carborundum; but in this case care should be taken to have longitu- 
 dinal grooves both inside and out as well as indentations along the 
 cutting end to provide for adequate circulation of the kerosene or 
 soap-compound lubricant. 
 
 A more primitive method for edging glass disks preparatory to 
 lens grinding is to cut the glass plate into squares with a glazier's 
 diamond point, to cut and chip off the corners of each square with a 
 pair of pliers, to cement it then with sealing wax or shellac on one 
 end of a rod of metal or wood, to mount the rod in a lathe; and 
 then with a mounted diamond point or a sharp three-cornered tool, 
 made by grinding smooth the surfaces of a three-cornered file, to 
 chip off carefully the protruding corners of the disk rotating with a 
 surface speed of not over 10 feet per minute and thus gradually to 
 work it down to a smooth circular edge. During this operation the 
 glass edge should be kept moistened with kerosene or turpentine. 
 A grinding-wheel attachment can also be used and with it the edge 
 can be ground down to a circle of the proper diameter. 
 
 Prisms are commonly sawed from glass plates. The particular plan 
 to be followed in order to waste as little glass as possible is determined 
 in each case by the dimensions of the glass plate and by the shape 
 and size of the prisms to be cut from it. The use of the milling 
 machine for precision work in grinding prism surfaces will be con- 
 sidered in a later section. 
 
 . 
 
 THE PRESSING OF LENSES AND PRISMS. 
 
 In many optical shops the raw glass is molded first into the approxi- 
 mate lens or prism shape and size before it is given to the grinders 
 and polishers. The pressing into blanks thus avoids the cutting and 
 grinding operations described above, and were it not for certain 
 defects which are thereby introduced, such as folds, feathers, and 
 other pressing defects, and poor annealing, there is no doubt that 
 the pressing method would be used to the exclusion of all others. 
 
PRESSING OPERATIONS. 233 
 
 Special furnaces and presses are required, but the great saving both 
 of glass and of grinding operations more than offsets the initial 
 expense. 
 
 Raw-inspected optical glass which is to be pressed into lens or 
 prism blanks of a special size is first cut or broken into fragments 
 of the required weight. For this purpose steel trimming blocks (fig. 
 52, p. 172) are used with weigh ted, soft fiber or celluloid hammers ; the 
 sharp edge of the steel trimming block serves as contact point for 
 the glass plate ; a sharp blow with the celluloid hammer on the upper 
 side of the glass plate at a point between the edge of the steel block 
 and the hand supporting the plate suffices to fracture the glass plate 
 at the desired point. The glass is first cut with a glazier's point into 
 sections of about the correct weight. Each section or fragment is 
 
 FIG. 75. The pressing of small lenses and prisms. (Photograph by J. Harper Snapp at plant 
 of Bausch & Lomb Optical Co.) 
 
 then trimmed to the exact weight, as determined on a small beam 
 balance, on the one pan of which either a pressed blank or its equiva- 
 lent weight is placed. In the trimming operations all sharp angles, 
 especially reentrant angles and bruised or cracked portions of the 
 glass fragment are trimmed off, in order to avoid folds during the 
 melting down of the glass. The finished glass fragments are placed 
 in a properly labeled tray in which they are transferred to the pressing 
 department. 
 
 Large pieces of glass for large lenses or prisms are placed in a 
 preheating kiln and gradually heated over night to a dull red heat; 
 smaller pieces, intended for eyepiece and other small lenses are pre- 
 heated directly in the pressing muffle furnaces. The muffle furnaces 
 
234 v MANUFACTURE OF LENSES AND PRISMS. 
 
 are heated from each side (fig. 75) by a compressed-air gas jet which 
 plays against the refractory arch crown of the heating chamber; 
 the glass fragments to be melted down are placed on a base-plate of 
 refractory material and are heated chiefly by radiation from the 
 crown of the furnace. The operator stands in front of the furnace, 
 which is built at a height convenient for ease of operation, and 
 watches the progress of melting of the glass fragments. In order to 
 prevent these from sticking to the refractory base-plate a thin layer 
 of fine powder (clay, alumina, mica, talc, graphite, or a mixture of 
 these) is spread over the plate; this powder adheres to the molten 
 glass and, like flour on sticky dough, effectively coats its outer 
 surface. The disadvantage of using powder for this purpose lies in 
 the fact that the operator in shaping the irregular glass fragment as 
 it gradually softens is liable, by too much working of the glass, to 
 infold some of the dust-laden surfaces; so that they appear in the 
 finished lens or prism blanks as pressing defects. With careful 
 operators the rejections of pressed blanks because of pressing defects 
 are not high; but a careless or inexperienced operator may spoil a 
 large number of blanks in a short time if left to himself. 
 
 In the muffle furnace the glass is allowed to become reasonably 
 soft. In the majority of muffles the heating is from the top down; 
 but in others the base-plate itself is heated in addition. This results 
 in a more uniform melting of the glass fragment. In all cases, however, 
 the top surface softens first and flows down and tends to spread out 
 over the base-plate. The operator prevents it from doing this by 
 the use of flat iron paddles or rods with flat ends (fig. 75) ; with these 
 the glass mass is molded approximately to the shape of the mold in 
 which it is to be pressed. In the case of lens blanks he endeavors to 
 produce a round disk of a diameter somewhat greater than that of the 
 finished blank; in the case of prism blanks he shapes the soft glass 
 into approximately the desired prism shape. 
 
 The softened glass, properly shaped, is slid along the base-plate 
 to the cast-iron mold and is transferred in it to a plunger operated 
 either by foot power or by compressed air or by hydraulic pressure. 
 Presses for small lenses are commonly pneumatic presses; for larger 
 work hydraulic presses are preferred. The iron mold, in which 
 the softened glass is shaped, is preheated by means of gas burners 
 in order that the glass be not too rapidly chilled. In all pressing 
 operations it is essential that the rate of cooling be not too rapid, 
 otherwise fine surface crackling results which can not later be removed. 
 
 The pressing molds are commonly made of cast iron and serve 
 the purpose well. (Fig. 76.) The chief difficulty encountered with 
 their use is the tendency of the glass blank to stick in the mold ; it is 
 then removed by tapping the mold. In the case of small molds it 
 has been found advantageous to make them of tool steel and to place 
 
PRESSING OPERATIONS. 
 
 235 
 
 in the bottom a closely fitting plunger to which is attached a short 
 pin that extends through the bottom of the mold. These molds 
 can be made more accurately and with better finish than cast-iron 
 molds. The blank after pressing in this kind of mold is removed by 
 tapping the extension pin of the plunger against the furnace plate; 
 this raises the plunger and the glass blank is thereby loosened and 
 drops out. 
 
 The pressing furnaces are made of different sizes depending on the 
 size of blanks to be pressed. In the small muffles provision is made 
 by the addition of an upper compartment to keep the sheet-iron tray 
 or box of small blanks hot until the tray is full and ready to be trans- 
 ferred to the annealing furnace. Provision is also made in most 
 furnaces to shield the operator from the furnace heat. There are 
 different ways of doing this; in one, a series of chains is hung in 
 front of the furnace, but they are interrupted at one point by a sheet 
 of heat-resistant glass through which the operator sees his work; an 
 
 FIG. 76. Molded prism and lens blanks; and the molds for pressing them. Frankford Arsenal.) 
 
 upward flowing, forced current of air conducts the heat away from 
 the chains; in a second type a stream of fresh air is blown against 
 the operator, and passes between him and the furnace; in a third a 
 water curtain is employed. 
 
 In actual pressing operations a number of fragments are kept in 
 work, so that there is always one fragment practically ready for the 
 press. The output varies with the size of the blank. Thus an 
 average output per press per working day is 2,000 or more eyepiece 
 lens blanks or spectacle lens blanks; 800 or more binocular prism 
 blanks, 150 large reflecting prism blanks 2 to 3 inches on a side. 
 Greater skill is also required to produce the larger prism and lens 
 blanks. 
 
 The method proposed by Capt. H. C. Fry, jr., to avoid the appear- 
 ance of dusty surfaces and consequent folds and laps in pressed 
 blanks, by heating the glass fragments on the end of a punty in a 
 glory hole and paddling the softened glass gradually into shape with 
 
236 MANUFACTURE OF LENSES AND PRISMS. 
 
 flat copper or iron tools, has been tried out in a small way by the 
 Bureau of Standards. The method produces clear, clean blanks, 
 but it is slow and obviously best adapted for large blanks and less 
 suitable for small lens and prism blanks. It may, however, be 
 possible to develop a production method based on this principle 
 which will meet the factory needs better than those at present in use. 
 
 In the pressing of large blanks the operator has constantly to 
 guard against a too rapid heating of the outer surface and consequent 
 flowing down and spreading out of the glass, otherwise folds are 
 certain to appear. During the actual pressing operation, the speed 
 of the plunger, which is kept hot by means of a gas flame, should be 
 so regulated that the glass is properly pressed without the appear- 
 ance of heavy surface crinkles or of fine surface cracks in the pressed 
 blank. The pressing operation is in all cases a violent procedure. 
 The soft glass when placed beneath the press has the consistency of 
 thick pitch; the plunger descends and, in a period of time generally 
 less than a second, the irregular mass of glass is forced to flow and 
 to fill the mold. The soft glass tends to flow under these conditions 
 somewhat as a lava flow, namely, in waves ; with the result that in 
 many instances the powdered surfaces are infolded and spread out 
 in the direction of flow. A " feather" results, and the blank is 
 rejected. 
 
 It appears that careful study of the factors involved in the pressing 
 operations should enable the factory engineer to prescribe a better 
 routine practice than the present method and thus avoid a certain 
 percentage of the rejections which occur in pressed blanks. 
 
 The procedure for the annealing of pressed blanks is described in 
 detail in Chapter III and need not be repeated here. Suffice it to 
 state that in the annealing of large blanks careful attention should 
 be given to the attainment of uniform temperature at the prescribed 
 annealing temperature and also to the rate of cooling in order that 
 appreciable strain be not then introduced. 
 
 THE GRINDING AND POLISHING OF PRISMS. 
 
 Prisms intended for use in optical instruments are glass bodies of 
 geometrical shape and bounded by flat surfaces. In precision optics 
 the prisms must be of definite dimensions; their surfaces must be 
 optically flat and the interfacial angles must be correct within a few 
 seconds of arc in certain instances. These conditions are difficult 
 to meet and nice attention to details is essential to produce the 
 desired results. 
 
 The first operation in the grinding of prisms, whether in cut or in 
 pressed blanks, is to grind flat one of the surfaces which in the finished 
 prism is a side or end surface and is left as a frosted, unpolished 
 surface; in a right-angled prism, for example, these surfaces are the 
 
GRINDING OF PRISMS. 237 ' 
 
 triangular side surfaces. This ground surface serves as a surface of 
 reference for future operations. The rough grinding is commonly 
 done by hand on a large rotating, horizontal iron disk, 3 to 5 feet in 
 diameter, and rotating fairly rapidly. Coarse emery, or carborun- 
 dum, or crushed steel either 90 or 150 mesh is used. 
 
 The prism surfaces at right angles to the first base surface are 
 now roughed out in similar manner; the angles between the surfaces 
 are checked by means of accurate angle gauges. 
 
 A number of rough-ground prisms (20 or more) are mounted with 
 the ground faces in contact with a heated flat iron disk ; a thin layer of 
 beeswax, paraffin, or rosin and beeswax serves as cement. (Fig. 77.) 
 The disk is allowed to cool down and the second side surfaces of all 
 
 Fro. 77. The bloQking of prisms for rough grinding. Angle plates into which par- 
 tially finished prisms are fitted for the grinding of additional faces. Prisms 
 cemented to a flat plate preparatory to grinding are also shown. (Frankford 
 Arsenal.) 
 
 prisms are then ground down together, the tool being either held or 
 guided by hand so that the newly ground surfaces are parallel with 
 the first surfaces. (Fig. 78.) These surfaces are first ground with 
 coarse emery or carborundum of 90 mesh; then successively with 150 
 mesh, 220 mesh, and smoothing abrasive. In each case the change 
 from a coarser to a fine abrasive is made when examination of the 
 ground surface shows it to be free from scratches and of uniform 
 granularity; the edges should be free from chips. The coarse grinding 
 is done on a large horizontal iron disk; but for the fine grinding 
 flat iron disks about 18 inches in diameter and rotating at about 
 half the speed of the large roughing plates are used. The smooth 
 grinding of the second side surfaces having been finished, the prisms 
 
 3922921 16 
 
238 * MANUFACTURE OF LENSES AND PRISMS. 
 
 are remounted and the first side surfaces are reground until the prism 
 has the prescribed thickness as measured by a gauge or caliper 
 directly on the mounted prisms. In all grinding operations it is 
 essential that care be taken not to mix the grades of abrasive; for 
 this reason the operators work with uprolled sleeves, and commonly 
 use only one group of abrasives, either the coarse or the fine; thus the 
 rough grinders operate only on the large roughing disks. The plates 
 are scrupulously cleaned before passing to a finer powder, in order 
 that no single coarser grain be left to produce scratches. 
 
 After the two side surfaces have been ground and smoothed so 
 that all prisms are of uniform thickness, the prisms are thoroughly 
 cleaned and are then cemented with beeswax or paraffin, side by 
 
 FIG. 78. The rough^grinding of a block of prisms. (Frankford Arsenal.) 
 
 side, in a special angle tool; the large surface, such as the hypothenuse 
 surface of a right-angle prism, is ground first, the sides of the prism 
 having been fitted into the right-angle grooves of the tool. The ex- 
 posed surfaces are ground with the several grades of emery and finally 
 smoothed, care being taken to grind down all sides of the block 
 uniformly. The side surfaces likewise are mounted in special angle 
 tools and ground as above. Each prism is now bounded by flat 
 surfaces; the side surfaces are perpendicular to the other surfaces; 
 the angles between the remaining surfaces are also approximately 
 correct. 
 
 The prisms are now ready for blocking and polishing. An optically 
 flat, thick iron disk between 8 and 16 inches in diameter is heated until 
 
GRINDING OF PRISMS. 
 
 239 / 
 
 paraffin melts on it. Small flat pieces of glass are first placed around 
 the edge of the tool, then the prisms are grouped over the tool, the 
 face in each prism to be polished being placed down on the tool in 
 the paraffin. The smallest face is commonly the first to be blocked; 
 thus in a right-angled prism the side surfaces are polished before the 
 larger hypothenuse face. The prisms are pressed firmly against the 
 tool and allowed to cool; the exposed surfaces are then painted with 
 a thin layer of beeswax. 
 
 A brass ring or band is now clamped around the tool; its height 
 should exceed that of the largest prism on the block. (Fig. 79.) 
 
 FIG. 79. Prisms in process of blocking with plaster of paris for polishing. On the right there is a 
 block of prisms ready for polishing. (Frankford Arsenal.) 
 
 The basin thus formed is nearly filled with a mixture of water and 
 plaster of Paris.. For some purposes the plaster of Paris is mixed with 
 a little Portland cement in the proportion 10 to 1. A flat iron disk 
 is placed, with four or more holes drilled through it, on top of the 
 cement; the holes are filled with the plaster mixture. The cemented 
 block is allowed to set and harden for about 16 hours. The hardening 
 process can be expedited by placing the blocks in a drying cabinet 
 together with phosphor pentoxide or calcium chloride. 
 
 After the cement has set, the iron plate is heated to the melt- 
 ing temperature of the paraffin and the plaster block is slid off the 
 iron plate. The prism surfaces to be polished are now exposed. 
 
240 * MANUFACTURE OF LENSES AND PRISMS. 
 
 The plaster is cut away to a depth of one-eighth inch from the 
 prisms so that they alone touch the grinding and polishing tools. 
 
 The optically flat, thick iron blocking tool is cleaned and heated 
 to the temperature of melting beeswax. The plaster block with the 
 carefully cleaned prisms is placed face down on the heated iron disk ; 
 in certain instances it is advisable to bear heavily down with a screw 
 press or with added weights on the plaster block in order to insure 
 strict parallelism of all exposed prism surfaces, after the block has 
 cooled.,// The cold block is removed from the tool; the plaster is 
 coated with two coats of shellac in order to shield it from water 
 during the grinding and polishing operations. After the shellac has 
 become thoroughly dry the block is ready for fine grinding. (Fig. 79.) 
 For this purpose a slightly convex grinding plate is used rather than 
 a perfectly flat plate, so that the surfaces after fine grinding will be 
 slightly concave. Experience has show T n that it is easier to polish 
 from slightly concave to piano or from the margin to the center than 
 vice versa. The grinding plate may well be smaller than the prism 
 block. The amount of depression between center and margin in a 
 prism surface after fine grinding should be less than 0.001 inch. 
 
 Flour emery for finishing is obtained by washing emery, which has 
 been used in previous operations, in clean water and allowing it to 
 settle out. The longer the time required for the settling, the finer 
 the emery deposited. For the final finishing, emery that remained 
 in suspension for 10 minutes but had been deposited at the end of 60 
 minutes is commonly used. Different methods are in use at different 
 factories for the grading of emery and of carborundum, all of which 
 depend on the rate of settling of different sized particles of the abra- 
 sive in clear water. Difficulties are encountered because of the ten- 
 dency of thin flakes to float, to remain in suspension, or to adhere to 
 the sides of the settling tanks. It is a good plan to reclassify each 
 fractional settling to insure, so far as possible, the elimination of 
 larger particles which might cause much damage. In the case of 
 carborundum the grading is especially difficult, but with care it can 
 be satisfactorily accomplished. 1 
 
 The fine grinding-tool is rotated relatively slowly; the fine emery 
 and water are rubbed over it, the prism block is placed upon it and 
 is rubbed and rotated evenly over the plate so that all sides of the 
 block are equally ground. Care should be taken to keep the emery 
 wet, because, if too dry, it tends to form spherical aggregates or balls 
 which then scratch the surfaces badly. After grinding for five min- 
 utes the prism surfaces should be cleaned and inspected with a mag- 
 nifying glass. If they appear free from scratches and pits and the 
 surfaces have a uniform velvet finish, they are ready for polishing, 
 
 J W French Trans. Opt. Soc. London, 19, 1918. 
 
POLISHING OF PRISMS. 24 1/ 
 
 after they have been thoroughly cleaned with a brush and every 
 particle of abrasive has been removed from the prism block. 
 
 The polishing tool is prepared by melting clean strained Nor- . 
 wegian pitch to which a little rosin has been added and pouring the 
 viscous liquid on a flat horizontal iron tool to a depth of one-fourth 
 inch. Strips of wet paper placed around the edge of the tool prevent 
 the pitch from overflowing. In cold weather add a little pine tar to 
 the pitch in order to soften it slightly. After the layer of pitch has 
 become cold two series of parallel grooves, mutually perpendicular, 
 about one-eighth inch wide and 1 inch apart, are cut into the pitch 
 
 FIG. 80. Polishing a block of prisms and a block of lenses of large radius. (Frankford Arsenal.) 
 
 surface. The pitch is then reheated sufficiently to soften its upper 
 surface upon which then a cold, flat, or slightly concave iron tool 
 is pressed, which is moistened with a creamy mixture of water and 
 rouge or dilute glycerin to prevent its sticking to the pitch. This 
 imparts to the pitch surface the exact negative of the surface of the 
 iron tool. The iron plate may be pressed against the pitch surface 
 directly after the first heating if desired, and the grooves cut in 
 afterwards. (Fig. 80.) 
 
 Instead of an iron blocking tool Ritchey 2 uses blocks consisting 
 of nicely dovetailed oak pieces, made in layers and impregnated with 
 paraffin to prevent warping. These blocks change their shape to a 
 
 * Contributions to Knowledge, Smithsonian Institution, 34, No. 1459, 1904. 
 
MANUFACTURE OF LENSES AND PRISMS. 
 
 less degree with change in temperature than do the iron blocks and 
 for certain kinds of work are preferable. 
 
 With a slightly convex polisher the tendency to polish a convex 
 surface because of the greater action near the periphery of the 
 polisher is counteracted. A similar effect can also be attained by 
 increasing the width of the grooves toward the margin of the polisher. 
 In the polishing operation a small quantity of a thick, creamy mix- 
 ture of rouge and water is spread over surfaces of the prisms in the 
 blocks mounted on a polishing machine so that its axis of rotation 
 is vertical. Upon it is placed the polisher which is attached above 
 by a pin in a socket at the center of the polishing disk to an arm 
 which moves it back and forth across the rotating plaster block. 
 The polisher is free to rotate and does so in the same direction 
 as that of the rotating plaster block. When short strokes of the 
 polisher are used the greatest wear is at the center of the plaster 
 block and the prism surfaces tend thereby to become concave; 
 when long strokes are employed so that the polisher extends well 
 over the edge of the prism block the greatest wear is along the 
 periphery of both polisher and block, and the prism surfaces tend 
 then to become convex. 
 
 The polishing process is necessarily slow. Precision polishing 
 requires slow movements and well-lubricated surfaces to prevent the 
 heating up of glass surfaces. If the polisher runs hot, as is the case 
 with spectacle lenses, the exact figure is lost and only low precision 
 work can be obtained. In all precision optical work patience is 
 essential. 
 
 The polishing tool is commonly of slightly smaller diameter than 
 that of the block of prisms. A circular stroke of the polisher is 
 generally employed but a straight chordal stroke may be used with 
 success when certain precautions are taken. Corrections for slight 
 curvature of surface are usually made by lengthening the stroke of 
 the polisher in case the surface is slightly concave, and decreasing 
 the stroke in case the prism surfaces are slightly convex. It is 
 possible, however, to use the same length of stroke and to correct for 
 curvature by changing the curvature of the pitch surface of the 
 polisher. 
 
 The time required to polish a block of prism surfaces depends some- 
 what on the size of the block and on other details; commonly four 
 to eight hours suffice. The prism surfaces should be inspected with 
 a magnifier for quality of surface polish, especially for freedom from 
 scratches, small pits, and grayness of surface; and for flatness of 
 surface by means of an optically flat test plate. When a test plate 
 is placed above a prism surface and moved so that it is practically 
 parallel with the prism surface the Newton interference colors of thin 
 air films appear. (Fig. 81.) If the interference bands are perfectly 
 
POLISHING OF PRISMS. 
 
 243 / 
 
 straight to the extreme edge and, when the surfaces are in a more 
 nearly parallel position, only one interference color is seen to extend 
 over the entire plate, the surface is sufficiently flat. The use of a 
 monochromatic light source, such as the sodium flame or the green 
 mercury lamp when viewed through a mercury green filter, facilitates 
 the observation of interference fringes. 
 
 FIG. 81. Spherometer and test-plates for measuring radii of curvature of lenses. The test plates 
 show the Newton color fringes. . (Frankford Arsenal.) 
 
 Toward the end of the polishing process the polisher is run nearly 
 dry in order to give a perfectly grainless polish. 
 
 The prisms are removed from the plaster block by first breaking 
 the block itself away from the iron plate with a chisel, and then by 
 tapping the plaster block with a hammer lightly; it breaks apart 
 easily and the prisms are readily separated from it and are then 
 
244 J MANUFACTURE OF LENSES AND PRISMS. 
 
 cleaned with benzene or gasoline to dissolve off the beeswax or 
 paraffin. 
 
 The next operation is to test and, if necessary, to correct the angle 
 between the polished face and an adjacent finely ground but unpol- 
 ished face. One arm of an angle gauge with the required angle is 
 placed against the unpolished face and slid down it until the second 
 arm comes into contact with the polished face. If the prism angle 
 is greater or less than the prescribed angle, light can be seen between 
 the gauge arm at the near or far side of the polished surface. The 
 angle is corrected then by hand by grinding with smoothing emery 
 on a flat tool and by pressing upon the end that is high. It is not 
 possible to correct prism angles satisfactorily by polishing alone. The 
 correction of angles by handwork must be carefully done, otherwise 
 errors may 'be introduced which can not be remedied by polishing 
 alone. As soon as the prism angle has been satisfactorily corrected 
 the prism face is ready to be polished by the process outlined above. 
 The reason for starting the polishing with the smallest face is now 
 obvious; it is the most difficult face to grind as a flat surface by hand, 
 and its correction by hand for angle is therefore less favorable than 
 that of the larger surfaces. 
 
 The foregoing operations are carried out for all faces of the prism. 
 In the practical operations experience is required to know exactly 
 when to stop one operation and to begin another, as, for Example, 
 when to change from one size abrasive to the next finer grade; also 
 to know the optimum speeds at which to grind or polish with different 
 sizes of blocks and with different types of glass. 
 
 In many instances better results are obtained by polishing the 
 prism in mounted blocks rather than individually. Thus in the case 
 of the Dove reflecting prism, which serves as the vertical rotating 
 prism of the panoramic sight, the end surfaces are relatively small; 
 the prism is in fact the truncated base of a large right-angled reflect- 
 ing prism. Better results are here attained by cementing strips of 
 plane-parallel plates of glass to a series of these prisms and thus by 
 completing the right angle to produce a much larger bearing surface 
 and correspondingly greater accuracy. In case it is not desired to heat 
 up the prisms in the cementing operation, sodium silicate may be, 
 and has been used successfully as a cement; the glass strip cemented 
 with sodium silicate is removed finally by grinding. 
 
 Roof -edge prisms, such as the elbow prism in the panoramic sight, 
 are best polished in pairs. The final corrections for angle are com- 
 monly made by hand on one surface only; a high degree of skill is 
 required to produce this type of prism. 
 
 The chief difficulty encountered in the production of optical flat 
 surfaced is not the grinding and polishing of these surfaces to the de- 
 sired precision in the mounted block, but the fact that on removing 
 
PRECISION MILLING METHODS. 245' 
 
 the prisms from the block certain strains are released, and these 
 cause the optical surfaces to warp and to lose to some extent their 
 optical quality. The release of strain is, moreover, not instantaneous, 
 but takes place gradually, so that the surfaces may continue to 
 change slightly for an appreciable period of time. The cement with 
 which the glass is cemented has a coefficient of expansion different 
 from that of the glass and on cooling strains the glass unequally. 
 Efforts have been made successfully to overcome this difficulty by 
 cementing the flat surfaces to the flat tool, preferably of glass in this 
 case, by optical contact or by capillary attraction .using for the pur- 
 pose a thin film of liquid. 
 
 The plates may also be weighted with pieces of lead and then 
 cement (rosin and oil) placed around each plate. To enter further 
 into the details of these methods, which are especially useful in the 
 preparation of sextant mirrors, would lead too far in the present gen- 
 eral description. Suffice it to state, it is possible by giving proper 
 attention to manipulation to cement many surfaces by optical con- 
 tact and thus to avoid the troubles arising from the use of ordinary 
 cements. It is common practice, for example, to cement by optical 
 contact the surfaces of two roof-edge prisms which are to be polished 
 in pairs. Surfaces to be placed in optical contact must be chemi- 
 cally clean. A very slight warming of an optically flat surface of a 
 plate or prism causes it to warp and become slightly convex momen- 
 tarily; a warm prism or plate placed against the optically flat surface 
 of a tool causes the tool surface to expand, and hence to become con- 
 vex upward. Under these conditions optical contact between the 
 two surfaces is first established at the centers, and on slight cooling 
 readily excludes the thin film of air which under ordinary conditions 
 is liable to be entrapped and thus spoil the optical contact over the 
 entire area. The placing of optically flat glass surfaces into optical 
 contact is, however, an art which can be acquired only by practice. 
 Pressure clamps are applied in certain cases to force the two flat 
 surfaces gradually into optical contact. 
 
 The strains and movements which result on embedding the prisms 
 in a large mass of plaster of Paris have been reduced to a certain 
 extent by using specially constructed iron gratings or ribs. These 
 divide the entire block into a series of small compartments each one 
 of which is filled with the plaster of Paris; the iron walls reduce the 
 range of action of strains which may be set up on the setting of the 
 plaster. 
 
 PRECISION MILLING OF PRISMS. 
 
 During the war an ingenious method for the milling of precision 
 prisms was developed by Capt. W. R. Ham, of the Ordnance Depart- 
 ment, and was successfully used for the production of the elbow, 
 vertical-rotating, and rotating head prisms of the panoramic sight. 
 
246 x MANUFACTURE OF LENSES AND PRISMS. 
 
 For the purpose a precision milling machine is essential. Diamond 
 charged cylinders of brass and of soft steel are used for the milling 
 operations. The brass cylinders are charged with coarse diamond 
 powder (80 to 100 mesh), the soft-steel cylinders are charged with 
 the fine diamond dust which has remained in suspension in water for 
 five minutes. In each case the diamond dust is rolled in by means 
 of a narrow, hardened, steel roller. The charged cylinders, about 2 
 inches in diameter and 3 inches long, are then rolled between three 
 hardened steel rollers, accurately mounted and exerting heavy pres- 
 sure on the charged cylinder, which is thus straightened and rendered 
 of uniform diameter. This is essential for precision work. 
 
 The glass which is to be ground to prism shape is then mounted in 
 a fixture. Accurate settings for position are made commonly by 
 reference to a finished prism of acceptable quality and mounted on 
 the same axis with the prism to be ground. An autocollimating 
 telescope of high resolving power is sighted on a finished prism and 
 determines the amount of rotation necessary to pass from one prism 
 face to a second. The coarse brass cylinder mill is first used, then 
 the fine soft-steel mill for the finishing cuts on each face. The faces 
 thus prepared are remarkably accurate in position and are ready 
 without further preparation for blocking, which if properly done 
 practically insures satisfactory interfacial angles between the finished 
 prism faces. The application of this method means a considerable 
 saving of time and energy in the making of difficult prisms, such as 
 the roof-edge reflecting prism, the penta-prism, and other complex 
 prisms, especially prisms having roof edges. 
 
 THE GRINDING AND POLISHING OF LENSES. 
 
 The methods for grinding and polishing lenses depend to a certain 
 extent on the shape and size of the lenses to be produced. In this 
 section lenses up to 3 inches in diameter only will be considered. 
 The grinding and polishing and figuring of larger telescope lenses is an 
 art in itself which enters but little into the construction of the optical 
 systems required in military instruments intended for field use. In 
 the case of the smaller lenses, 3 inches or less in diamter, flat lenses 
 with surfaces of large radii of curvature are handled differently from 
 lenses with surfaces of shorter radii of curvature, the reason for 
 this being that the flat lenses can be mounted in blocks or shells of 
 the desired radii of curvature whereas the lenses of the second group 
 are commonly of such steep curves that they have to be ground and 
 polished individually. 
 
LENS GRINDING AND POLISHING. 247 
 
 / 
 
 Flat lenses. 
 
 Disks or pressed circular blanks intended for lenses of relatively 
 flat curvature are ground and polished in groups mounted on specially 
 prepared blocks called shells or tools. These tools are made either of 
 cast iron or bronze and are turned to the desired radius of curvature 
 in a lathe or other machine specially constructed for the purpose. 
 For each radius of curvature at least one convex and one concave 
 tool are used and commonly two of each. The convex and concave 
 tools after turning to the desired curvature are ground together with 
 an abrasive in order to render them more accurately spherical. In 
 case the radius of curvature is slightly too long in other words, the 
 tools are too flat they are deepened by grinding with a short rocking 
 stroke during the rotation of the tool and polisher; if the tools are 
 too deep, a wide stroke of the upper tool by which it overhangs part 
 of the time well beyond the periphery of the tool below is used. 
 Furthermore if the concave tool is on top serving as grinding tool the 
 radii of curvature of both tools are shortened; if the convex tool is 
 on top, the curvature of both tools is flattened. Thus a flat grinding 
 tool when worked on top of a second tool tends to become concave. 
 On the convex polishers two sfcts of mutually perpendicular grooves 
 one-fourth inch wide and 1 inch apart are commonly cut across the 
 surface. Generally convex lenses are purposely ground a little flat 
 so that the polishing tool polishes from the margin toward the center 
 of the lens surface. For this reason a slightly larger radius (0.003 
 inch) is used in the curvature of the grinding tools. The reverse is 
 desired for concave lens surfaces. The radii of curvature of the 
 blocking (polishing) tools are commonly made of the lengths desired 
 for the lens surface. 
 
 The lens blanks or flat disks are then cemented or blocked on a con- 
 cave or convex tool of the proper curvature. (Fig. 82.) To block the 
 lenses they are first heated and a layer of the blocking cement (sealing 
 wax, or rosin and beeswax, or other cement) sufficient to cover the 
 entire lens and still have a thickness of not more than 2 millimeters 
 at its center, is melted on each lens. In blocking convex lenses, a con- 
 cave supporting tool is employed; its surface is moistened and the 
 lenses are placed on it with the clean surfaces down. A heated convex 
 tool is then pushed against the cement-backed lenses and the lenses 
 are thereby cemented to it. The cemented lens block is mounted in 
 the lens-grinding machine and fairly coarse emery and a high speed 
 of rotation are used. (Fig. 83.) Finer grades of emery are in turn 
 used on the lens surfaces until these are ready for polishing. The 
 curvature of the lens surface is tested from time to time by means of 
 a gauge and slight corrections for curvature are made by regulating 
 the stroke of the grinding tool relative to that of the lens block. 
 The grinding tool itself is reground, if necessary, by means of a fellow 
 
248 v MANUFACTURE OP LENSES AND PRISMS. 
 
 tool of the correct ra'dius of curvature and mounted in place of the 
 lens block. This tool is gauged repeatedly to insure correct 
 curvature. 
 
 A polishing block of pitch is prepared by flowing pitch over the 
 tool of the required curyature and impressing on it, while still hot, a 
 fellow tool of equal but opposite curvature. The pitch surface is 
 grooved after the manner of the plane polisher. A long stroke of 
 the polisher is purposely maintained in order to polish the periphery 
 of the lenses first. After the lens surfaces have been well polished, 
 their curvatures are commonly tested by means of standard test sur- 
 faces (if possible of quartz) ; the test surface is pressed against the 
 
 FIG. 82. The polishing of lenses of medium curvature. Blocks with lenses mounted for polish- 
 ing are shown in the foreground to the right; also a block of lenses to be polished to a large 
 radius of curvature. (Frankford Arsenal.) 
 
 polished lens surface and its relative curvature is determined from 
 the character of the colored Newton interference rings which appear 
 when the examination is made in reflected light. On surfaces to be 
 cemented a tolerance of 8 to 10 fringes is permissible; on other sur- 
 faces 3 or 4 fringes is the maximum tolerance. An accurate sphero- 
 meter of the ring or three-point type with direct reading scale is also 
 satisfactory. As soon as the curvature and polish of the lenses are 
 satisfactory, the lenses are removed from the block by means of a 
 sharp knife or chisel edge inserted into the blocking cement below the 
 lens. The second surface is ground and polished in similar manner 
 provided it is sufficiently flat. The thickness of the lens is gauged 
 
LENS GRINDING AND POLISHING. 
 
 249 
 
 during grinding; for this purpose the tool to which the lenses are 
 cemented must be accurately made and the cementing properly done. 
 After both lens surfaces have been satisfactorily polished to the 
 desired curvatures, the lens is centered and edged. For this purpose 
 a brass tool of about the same diameter as the lens is required. One 
 end of the tool is turned to a wedge-shaped outer edge, the center 
 being cut to a depth of one-half inch. The lens is warmed and its 
 steeper surface cemented with shellac to this end of the tool, mounted 
 in the axis of a small bench lathe. The tool is rotated and heated by 
 a small flame. The lens is now moved by means of a soft wooden, 
 pronged stick, resting against a support and pressing against the lens 
 surface, until a distant lamp or other object, as seen reflected by the 
 lens surfaces, remains stationary. The lens is now centered and the 
 shellac is cooled by moistening it with water from a sponge. The 
 
 FIG. 83. Rough grinding of lenses of medium curvature by hand. (Frankford Arsenal.) 
 
 edge of the centered lens is turned to a concentric circle by hand by 
 means of a file or diamond point or abrasive lap (fig. 84) or, mechani- 
 cally, by the use of a rapidly rotating, diamond-charged copper cylin- 
 der, similar in character to the milling tools described in a foregoing 
 section. The edging machine is commonly so designed that the dia- 
 mond charged cylinder is set in action by means of a lever and 
 brought to bear against the edge of the rotating lens, which it grinds 
 then automatically to the prescribed diameter. 
 
 LENSES OF STEEP CURVATURE. 
 
 Lens surfaces of short radius of curvature are ground separately. 
 Lens blanks in the form of disks with plane-parallel surfaces undergo 
 first a roughing-out operation by which the lens surface is cut out to 
 the approximate degree of curvature. This can be done as a hand 
 operation by means of a file or of a mounted diamond point supported 
 
250 MANUFACTURE OF LENSES AND PRISMS. 
 
 on a rest, and directed against the slowly rotating glass disk as 
 though it were a piece of metal, or a brass or lead or iron grinding 
 tool of the proper radius of curvature can be used with coarse car- 
 borundum and the surface ground out; or a copper spherical tool 
 charged with fairly coarse diamond dust and mounted in a special fix- 
 ture above an ordinary lens spindle can also be employed and the 
 roughing done with great speed and precision. In the roughing 
 operation care must be taken to keep the edges of the disk of equal 
 thickness; this should be controlled by the use of suitable gauges. 
 The lens after having been roughed out by one of these methods is 
 mounted with sealing wax on a spindle and further grinding and pol- 
 ishing is done on an automatic machine. Bronze grinding tools of 
 the proper curvature are used with the different grades of abrasive. 
 
 FIG. 81. Edging and centering machines for lenses. (Frankford Arsenal.) 
 
 Pitch polishers, made by pressing the correctly smoothed lens surface 
 or a metal surface of the same curvature against the warm pitch on 
 the polishing tool, are used with rouge. The pitch surface is grooved 
 as usual. The lens is commonly polished nearly to dryness after 
 each wetting. A Newton color test or other gauge (fig. 81) is used 
 to ascertain the curvature. If the polished surface is convex and 
 too flat, the stroke of the polisher should be increased; more grooves 
 may also be cut in the center of the polisher to reduce the bearing 
 surfaces at that point. If the sweep is made too wide the edge of the 
 lens becomes too steep and a " bevel" results. To remove this, the 
 stroke of the polisher is shortened and the lens is polished until it is 
 too low, then the stroke is increased until correct curvature is at- 
 tained. In case the surface is too high grooves are cut in the pol- 
 
GRINDING AND POLISHING PROCESSES. 25 lr 
 
 isher near the edge and the length of stroke of the polisher is de- 
 creased. If, in this uase, the surface is polished down too fast, a 
 "hole" results, the surface is polished low in the center, but remains 
 high near the periphery. Increase of the stroke of the polisher 
 remedies this defect. 
 
 Concave surfaces are treated in a similar manner during polishing 
 to correct faults in curvature. Thus, if the radius of curvature is 
 too large, a short sweep is given to the lens and grooves are cut in 
 the margin of the polisher. A longer sweep to the lens and removal 
 of pitch from the central part of the polisher increases the radius of 
 curvature of the concave lens. 
 
 Precision polishing is a slow process. At no time should the tool 
 be allowed to heat up appreciably. The speed of rotation should 
 always be slow. 
 
 In, the absence of automatic spindle machines the grinding tind 
 polishing of single lenses can be done by hand on a small bench 
 lathe. The operations under these conditions are learned best by 
 actual xperience. After some training, operators can produce by 
 hand from 20 to 50 lenses per week. The work is necessarily slow 
 and exacting, and with careless operators accidents may occur at 
 any time to ruin a lens on which a considerable amount of work has 
 been put. 
 
 THE GRINDING AND POLISHING PROCESSES. 
 
 It is of interest to consider briefly the mechanics of the grinding 
 and polishing processes. These have been studied in some detail by 
 J. W. French, 3 who presents a number of important data as well as 
 certain tentative conclusions. French considers the operations under 
 three heads: 4 (1) Rough grinding or forming, (2) smoothing, (3) pol- 
 ishing. To quote from his paper: 
 
 For the first operation a coarse abrasive, such as carborundum, emery, or sand, is 
 employed. The amount of material removed is very great as compared with the 
 pseudo-polish effect. The object of the operation is to shape the glass roughly by the 
 economical removal of material. 
 
 For the second operation finer grades of carborundum or emery are used. As com- 
 pared with the first operation, the amount of material removed is less and the pseudo- 
 polish effect is greater. The object of the operation is to shape the glass by the fine 
 removal of material, both as regards final angle and size. So fine is the surface pro- 
 . duced in practice that the size is not generally reduced in the succeeding polishing 
 operation by more than 0.01 millimeter and the change of angle due to the actual 
 production of the polished layer can be controlled within a very few seconds over a 
 length of 25 millimeters. 
 
 In both first and second operations the abrasive may be used in loose form and 
 applied with a lubricant such as water by means of a surface grinding tool, usually of 
 coarse-grained cast iron, or it may be in the form of a grinding wheel. 
 
 1 Some notes on grinding and polishing. Trans. Opt. Soc. London, 17, 24-64, 1916; 18, 8-48, 1917; see also 
 Lord Rayleigh, Proc. Opt. Convention, 1905; Nature 48, 526, 1893; 54, 385, 1901; W. Rosenhain, Trans. 
 Opt. Soc. London, 11, 112-123, 1910. 
 
 < Trans. Opt. Soc. London, 18, 9, 1917. 
 
252 V MANUFACTURE OF LENSES AND PRISMS. 
 
 Operations (1) and (2) differ only in degree and might be regarded as one stage. 
 
 When manufacturing large quantities of the same piece supplied as molded blocks 
 of approximately the correct size and shape, the finest grades of abrasives may be 
 used in the first operation and the pieces may be formed to the final dimensions both 
 as regards size and angle. As the limits of dimensions imposed may be as small as 
 one-thousandth of an inch, and the limit of angle one minute, that is, one three- 
 thousandth of an inch, it will be evident that modern optical work is comparable 
 with good mechanical work as regards dimension, and that an equipment of precision 
 tools is necessary for the so-called roughing shop. 
 
 For the smoothing operation under the circumstances above mentioned the same 
 fine grade of abrasive may be employed. The purpose of the operation is then to 
 correct any minute irregularities over the surface of the block of prisms after the 
 individual parts have been laid down and cemented. The purpose is not to control 
 the size and angle. 
 
 The grinding process consists chiefly of a chipping out, by con- 
 choidal fracture, of small or large fragments of glass. Certain corners 
 and' edges of the grains of the abrasive on rolling about in the grinding 
 operation bear momentarily the weight of the grinding tool and com- 
 municate it to the glass surface on which they rest. The larger the 
 grains the less the number that bear the load and hence the heavier 
 they press on the supporting glass and the larger the chip broken 
 out from the glass. The momentary application of a heavy load 
 over a small area, such as the edge or corner of a hard grain, sets up 
 in the glass intense shearing stresses which exceed the elastic limit 
 of the glass and cause it to fracture. The area under pressure is so 
 small that during the short time of the action of the load only a 
 small volume of glass is affected and hence only a small chip is 
 broken out. The greater the load per unit area the larger and deeper 
 the chip broken out. Glass is so brittle that there is little, if any, 
 cutting such as occurs in a malleable metal, like copper, when turned 
 in a lathe. It might be considered that the action of the abrasive 
 is in part a scratching action, plowing up furrows or grooves in the 
 glass ; but if scratches on glass surfaces be examined under a micro- 
 scope the effect is seen to be essentially a chattering or succession of 
 conchoidal or shell-like fractures. 
 
 For successful grinding it is essential that the abrasive be hard, 
 that it maintain its sharp corners or edges, and that it does not cleave 
 readily into thin flakes, which obviously are not suitable abrasive 
 agents. 
 
 French 5 found that " abrasion depends quantitatively upon the 
 number of points brought into action in unit time, so that the more 
 frequently the grains are brought into repeated action, the greater 
 will be the material removed. It is customary, therefore, to move 
 the grinding tool relatively to the glass, and the amount of abrasion 
 will depend upon the relative speed of the tool and glass." French 
 determined by a series of experiments that with a given speed and a 
 
 s Trans. Opt. Soc. London, 18, 17, 1917. 
 
GRINDING AND POLISHING PROCESSES. 
 
 253 / 
 
 given size of fresh abrasive applied at intervals of one minute on a 
 given kind of glass surface, the abrasive effect or amount of glass 
 removed in unit time varies directly with the load applied; also that, 
 other things being equal, the amount removed by abrasion is directly 
 proportional to the speed of the tool relative to the glass. French 6 
 states further: 
 
 Water, or other lubricant, plays an essential part in the abrasion of glass. If there 
 is too little water the glass will be deeply cut and possibly crack, and if there is too 
 much water the result will be much the same, at least, so far as cutting is concerned. 
 Under proper conditions the abrasive should be well moistened with water, the 
 grains being separated from one another by a film of liquid. One of the important 
 functions of the water is to equalize the temperature over the surface and thus to 
 prevent rapid local rises at the points of action. 
 
 If there is too little water, the particles gather in lumps which deeply score the glass. 
 Owing to the absence of sufficient water, the heat generated at the point of abrasion 
 may rise to such an extent that cleavage takes place. If there is a large excess of 
 water a continuous layer of liquid may exist between the tool and the glass. All air 
 will then be excluded and the load on the tool will be enormously increased. Some 
 particles of abrasive may be forced into the glass or tool; others will be caught by the 
 obstruction and tear the glass deeply as before. 
 
 The polishing of the finely ground glass surface is done on a flat 
 tool covered with a layer of pitch, wax, paper, or cloth of different 
 degrees of hardness, and coated with moistened polishing material, 
 such as rouge (finely divided ferric oxide), black oxide of iron (ferrous- 
 ferric oxide) , putty powder (tin oxide) , chromic oxide, or manganese 
 dioxide. For all precision work a pitch surface obtained by melting 
 the pitch and pressing an optically flat surface against it during 
 cooling is used. Two different stages are distinguished by French 7 
 in the polishing process, namely, wet polishing and dry polishing. 
 
 "The function of the first stage is to remove material; the function 
 of the second is to fill up sleeks." Sleeks are minute markings in 
 the polished surface. In certain operations it is customary to "dry 
 up each wet"; but in other operations dry polishing is done only at 
 the end of the wet polishing stage. 
 
 French emphasizes the fact first noted by Lord Rayleigh 8 that a 
 few seconds after the finely ground surface has been rubbed on the 
 pitch surface with rouge and water, small, nearly perfectly polished 
 areas appear on the glass surface; in 10 minutes 80 per cent of the 
 surface is polished; but several hours are required to obtain perfect 
 polish over the entire surface. To quote further from French's 
 article : 
 
 If the polished patches are carefully examined under the miscroscope with suitable 
 illumination, there will be observed numerous grooves of different depths, the majority 
 being just invisible. The deeper grooves are typical sleeks, the others possibly 
 
 Loc. cit., p. 18, 1917. 
 
 ' Trans. Opt. Soc. London, 18, 23, 1917. 
 
 On polish, Proc. Optical Convention, 1905. 
 
 3922921 17 
 
254 ' MANUFACTURE OF LENSES AND PRISMS. 
 
 embryo sleeks. Frequently the appearance resembles that of a fresh varnished 
 surface over which a fine brush has been drawn. There seems no doubt that the 
 material from the grooves and sleeks is actually removed and not deposited again in 
 the wet polishing stage. From the average of one large series of mesurements the 
 the reduction of thicknesss of a plate of crown glass during each hour of wet polishing 
 by machine was about seven wave lengths. From similar tests made with a thoroughly 
 cleaned old pitch polisher, using water only without any polishing medium, other 
 than that embedded in the skin of the polisher, the amount removed was 2.5 wave 
 lengths per hour. In the third series of tests made with a new black pitch polisher 
 containing no rouge and supplied only with water as a polishing medium the 
 removal of material per hour was two wave lengths. Throughout all these teste 
 the tool was kept continuously wet. It was never allowed to become dry. Before 
 each of the tests the tools were worked down to the true figure of the surface, so as to 
 insure action over the whole face. 
 
 With regard to the dry polishing it is customary in practice to dry up the wet that is, 
 to allow the water to dry and the rouge to be worked out into the grooves provided for 
 it. The glazed-pitch surface then comes into actual contact with the glass surface, 
 upon which it appears to exercise a considerable liquefactive effect and drag, as is to 
 be expected. All the minute furrows produced in the wet stage are leveled down and 
 the sleeks are filled in. The chief function of the operation is to fill in the sleeks and 
 improve the brilliance of the surface. With one forward and one backward stroke 
 by hand over a length of about 15 centimeters a transverse sleek having a width of two 
 wave lengths can be filled in. Two strokes are required for one of four wave lengths. 
 When each wet is dried up the amount of material removed is slightly greater than in 
 the case of wet polishing alone. Thus, in a series of tests with the old pitch polisher 
 already mentioned, using water only during the wet stage and drying up after each 
 wet, an additional three-fourths wave length was removed per hour. The general 
 conclusion is that most material is removed in the wet stage and that a small amount 
 is also removed in the dry polishing stage. 9 
 
 From the foregoing experimental evidence French concludes that 
 polish consists essentially in a dragging off or removal of glass surface 
 by local surface flow of the glass. The depth of the sleeks or furrows 
 left by a flowing particle of rouge was found on measurement to 
 be about eight wave lengths. This result indicates that the sur- 
 face layer thus affected locally by the polishing action (Beta layer 
 of French) is a film of appreciable depth. Polishing according to 
 this conception signifies the gradual removal of successive surface 
 layers of glass to a plane below the bottom of the deepest pit in the 
 finely ground surface. 
 
 By a series of interesting experiments on surface fracture French 
 has been able to show the influence of the Beta film. More data are 
 required to determine definitely the exact significance and extent of 
 the surface flow in polishing, especially with regard to the ability of 
 the glass surface to flow into existing pits or depressions in the ground 
 surface after the manner of the Beilby layer in metals. Suffice it to 
 state the actual data of observation by Lord Rayleigh and French 
 are fundamental to any mechanical explanation of the glass-grinding 
 and glass-polishing processes. 
 
 J. W. French, Trans. Opt. Soc., 18, 24-27, 1917. 
 
CEMENTING OF LENSES AND PRISMS. 255 r 
 
 THE CEMENTING OF LENSES. 
 
 After the centering operation the lenses are ready for cementing. 
 Canada balsam is the cement usually employed for the purpose. In 
 thin films it is practically colorless and for most lenses is satisfactory 
 for a period of time at least. The fact, however, that Canada balsam 
 is not a chemical compound but a mixture of essential oils (turpen- 
 tines, 24 per cent, and rosins, in part soluble in alcohol, 60 per cent> 
 and in part insoluble, 16 per cent) renders its use a matter of nice 
 manipulation. The turpentine oils, volatile at a low temperature, 
 may be distilled off by heating the Canada balsam in a flat dish 
 over a steam plate with or without the addition of a vacuum chamber. 
 Canada balsam thus hardened is most conveniently kept in short, 
 straight-walled glass tubes, about 1 inch in diameter and 3 inches 
 long, with a flat bottom. A short wooden rod inserted into the 
 Canada balsam while still soft serves as a handle for the balsam 
 stick. 
 
 The cementing of lenses and prisms is best done in a separate 
 room held as dust-free as possible. Electrically heated plates are 
 commonly used for heating both optical parts and the Canada balsam. 
 Pasteboard or paper covers are used to shield the work from dust. 
 
 Before cementing, all glass surfaces are thoroughly cleaned with 
 alcohol and dusted with a fine camel's hair brush. Place the glass 
 elements on a sheet of clean paper on an electric plate and heat up 
 slowly. Do not heat so hot that the paper is scorched. When the 
 glass surfaces are sufficiently hot rub over them the stick of Canada 
 balsam or apply hot Canada balsam which has also been heating on 
 an electric plate. All prism surfaces should be blocked up to a hori- 
 zontal position in order that the cement will spread out evenly over 
 the entire surface. In the case of lens elements both the concave 
 and convex surfaces should be treated with the balsam. The ele- 
 ment with the convex surface is then grasped in a pair of warmed 
 tweezers, with ends bent to conform to the curvature of the lens, 
 and is placed on top of the concave lens element. The two are then 
 pressed and rubbed together to squeeze out as much of the excess 
 balsam as possible and allowed to cool. The cemented lens is 
 cleaned with kerosene or alcohol and ether; a second reheating, 
 sufficient to soften the balsam slightly, is common practice, after 
 which the lens is pressed against an angle block to insure the center- 
 ing of the cemented elements. In place of the angle block an optical 
 system may be used by means of which a distant cross or a cross from 
 a collimator is imaged by the heated lens in the front focal plane 
 of an eyepiece at which a cross hair is mounted. The lens is centered 
 when the imaged cross coincides with the cross hairs. 
 
 For mounting complex prism groups, such as range-finder eyepiece 
 prisms, supporting glass side plates are employed and optical methods 
 
256 MANUFACTURE OF LENSES AND PRISMS. 
 
 serve to indicate when the several prisms are in satisfactory adjust- 
 ment. 
 
 In all work with Canada balsam scrupulous cleanliness is essential. 
 Care must be taken not to heat the Canada balsam too hot, other- 
 wise it discolors badly; also not to heat the lenses too hot, otherwise 
 bubbles may form in the balsam film which are not easily pressed 
 out. The Canada balsam, if used in the natural condition and not as 
 hardened balsam, is generally heated for a period of time at a low 
 temperature to drive off the more volatile turpentine oils; after 
 mounting with soft balsam the lenses are baked at a low temperature 
 for some hours. The softer the balsam and the lower the baking 
 temperature, the longer the baking period. Electrically heated 
 ovens are best suited for the baking operations 
 
Chapter VI. 
 
 THE INSPECTION OF FINISHED OPTICAL PARTS AND 
 
 SYSTEMS. 
 
 The grinding and polishing department of an optical shop is 
 concerned only with the shapes and sizes, but not with the optical 
 performance, of the lenses and prisms which it produces. It is 
 responsible for: The kinds of glass employed, polish and curvatures 
 of lens surfaces, thickness of lenses, centering of lenses and cemented 
 lens combinations, polish and flatness of prism surfaces, prism 
 interfacial angles, dimensions of prisms. The gauges and other 
 testing divices used in the shop serve the purpose only of measuring 
 the external shapes and dimensions of the individual lenses and 
 prisms. In this department the glass is treated throughout, as is 
 brass or other metal in a machine shop, as a substance on which 
 certain mechanical operations have to be performed to produce 
 pieces or parts of prescribed sizes and shapes. 
 
 The assembly department, on the other hand, is interested not 
 only in the mechanical features of the lenses and prisms, but also in 
 their optical performance. The apparatus and methods required 
 for testing the optical qualities of a lens, a prism or an optical system 
 are entirely different from the devices for measuring length, thickness, 
 and curvature as employed in the grinding and polishing depart- 
 ment and merit special description. In the present chapter some of 
 the approved methods for ascertaining the optical qualities of a 
 lens, prism, and complete optical system will be described briefly. 
 A general statement only can be given with special emphasis on the 
 underlying principles. 
 
 During the war it became necessary, in certain cases, for Govern- 
 ment inspectors to pass upon the separate elements of an optical 
 system as well as upon the finished optical instrument. This situa- 
 tion arose because of the fact that here and there throughout the 
 country there were lens-grinding establishments, chiefly spectacle- 
 lens manufacturers, that might be able to produce the optical parts 
 for certain instruments, but not the mechanical parts; on the other 
 hand there were also available precision mechanical shops not fully 
 occupied with war work that might produce the mechanical parts. 
 The situation was not without its troubles, because there was an 
 
 257 
 
258 INSPECTION OF FINISHED OPTICAL PARTS AND SYSTEMS. 
 
 inevitable tendency on the part of each manufacturer, the optical 
 and the mechanical, to shift the blame for lack of proper performance 
 of the optical parts on the other fellow. The manufacturer of the 
 optical parts sought to obtain as large tolerance limits as possible 
 in order to attain maximum production with the least trouble and 
 expense. The manufacturer of the mechanical parts, on the other 
 hand, sought to establish as narrow tolerance limits as possible on 
 the optical parts and thus to avoid trouble and expense in the optical 
 assembly room; in general he did not realize the need for some 
 adjustment in the assembly of the mechanical parts, because of the 
 extreme difficulty in manufacturing interchangeable optical elements. 
 To secure the best results from an optical system slight shifts in 
 relative positions are generally necessary, and this means fitting the 
 mechanical parts to the optical. The ideal system, and the system 
 to which the manufacturer of precision metal parts is accustomed, 
 is to make each part to a standard size with a small tolerance limit 
 definitely fixed by gauges, and then to assemble the different parts 
 without much special fitting. In the case of optical elements small 
 changes in the refractivity or in the degree of curvature of the lens 
 and prism elements may produce relatively large changes in the 
 final result, such that some relative adjustment of the different 
 optical parts is necessary; these necessitate slight changes in the 
 mechanical arrangements. To this kind of work the manufacturer 
 of the mechanical parts is not accustomed; and, if he undertakes 
 to assemble the instruments in final form, a specially trained group 
 of assemblers for the optical and mechanical parts is essential. 
 
 Adjustment and fitting of the mechanical parts of an instrument 
 in order to get the best results from a given optical system is con- 
 sidered necessary even in optical instrument factories of long experi- 
 ence. This is still more essential in the case of spectacle-lens manu- 
 facturers who, in war time, are willing to try to make precision optics; 
 but are not properly equipped to do so and may not have had ade- 
 quate experience to realize the significance of precision work. It 
 requires patience, tact, and nice discrimination on the part of the 
 Government inspectors under such conditions, discouraging alike 
 to the manufacturer of the optical parts and to the instrument maker, 
 to keep up the interest and to establish in a short time production in 
 both factories on a satisfactory routine basis. In general it is good 
 policy to place orders for optical instruments of high precision with 
 firms experienced in their construction, the firm receiving the con- 
 tract to make and to be responsible for the entire instrument. For 
 low power, visual instruments separate contracts may be let for the 
 optical parts and for the mechanical parts, the final assembly to be 
 done preferably by the maker of the mechanical parts. 
 
LENSES AND PRISMS. 259 
 
 In contracts of this kind definite tolerances, both optical and 
 mechanical, for each part of the optical instrument should be stated 
 specifically; these are determined not only by the kind of optical 
 system desired, but also by the mechanical adjustments available 
 for the final mounting of the optics. In certain instruments the 
 mechanical arrangements are such that only small variations in the 
 lenses, especially in the focal length, are permissible, whereas in 
 other instruments large variations in focal length are tolerated and, 
 except for a slight change in the total magnification, do not materially 
 affect the performance of the optical system. In fire-control and 
 other measuring instruments the tolerances are more exacting 
 than in instruments intended for observation purposes only. Nice 
 discrimination and wide experience are required to prepare fair and 
 adequate specifications for the several optical elements of a given 
 lens system. The tolerances should be so set that every optical 
 element, which will function satisfactorily in the complete instru- 
 ment, is passed and each optical element, which will not. so function, 
 is rejected. 
 
 METHODS FOR THE INSPECTION OF THE COMPONENT PARTS OF AN 
 
 OPTICAL SYSTEM. 
 
 There are in general two different groups of methods available for 
 the inspection of an optical system and its component optical ele- 
 ments; these methods may be termed " direct" and " projection.'* 
 Observations by the first group of methods are made with the aid of 
 auxiliary optical instruments and apparatus and the defects of a given 
 optical element or optical system are ascertained by direct inspec- 
 tion. The continued use of optical instruments for inspection is, 
 however, tiring. In many cases projection methods of inspection 
 have been used to advantage, whereby the optical element (lens or 
 prism) or the image, formed by it, is projected on a suitable screen 
 where it can be viewed in enlarged form and the departures from set 
 standards read off directly on easily legible scales. The eyestrain 
 under these conditions is appreciably less than in instrumental 
 observations with auxiliary telescope or microscope and the readings 
 may be made more rapidly. In this chapter a general outline only 
 is given of the several available methods of inspection for detecting a 
 particular optical defect in a lens or prism or optical instrument. No 
 attempt is made at completeness either in citing all available methods 
 or in describing any given method in detail. To do this would 
 require a separate volume. 
 
260 INSPECTION OF FINISHED OPTICAL, PARTS AND SYSTEMS. 
 
 THE INSPECTION OF LENS- AND PRISM-ELEMENTS OF AN OPTICAL 
 
 SYSTEM. 
 
 Inspection of lenses for size and shape. The thickness of a lens 
 through the center is measured by means of a screw-micrometer 
 gauge, a vertical comparator, a spherometer, or some similar device. 
 The diameter of the lens is measured by means of a screw-micrometer 
 gauge or of two ring gauges, the first, a go gauge which determines the 
 maximum permissible diameter, the second, a no-go gauge which 
 prescribes the minimum diameter. The curvatures and sphericity 
 of the lens surfaces are measured by means of one of several types of 
 spherometer or by the use of standard test surfaces, or of circular 
 gauges or templets cut out of sheet metal or of glass. Mechanical 
 tests of this kind are so familiar that further description is unnecessary. 
 
 Inspection for physical defects in a lens. These include: (a) Bub- 
 bles, stones, laps, folds, and other pressing defects; (I) striae; and 
 (c) strain. For the inspection of the first group of defects the lens 
 is placed on 1 a piece of black cloth and is illuminated by a strong 
 light from the side as in figure 65, page 206. In an objective and in 
 other lenses mounted at a distance from the image plane a few small 
 bubbles are tolerated and do no appreciable harm; but in eyepiece 
 field lenses, reticules, and any other optical elements which are 
 located in the image plane or in its conjugate planes, bubbles are not 
 tolerated because they appear in the field of view and may cause 
 trouble in a military optical instrument. Lenses containing stones 
 or crystallization bodies should be rejected. The presence of a 
 pressing defect is commonly sufficient cause for rejection; but in 
 certain cases a small, faint fold at the edge of a lens may not appre- 
 ciably affect its optical ' performance. Heavy ribbon striae in a 
 lens should not be tolerated; objective lenses exhibiting single-thread 
 striae are generally accepted. In most cases of striae, the ultimate 
 criterion is not the presence of striae but rather the optical per- 
 formance of the lens under conditions of test arranged to render 
 its defects easily discernible. Strain is tested by examination of 
 the lens in polarized light. In order to attain uniform illumina- 
 tion of the lens it is customary to employ a point source of light 
 at the rear focal plane of a well-corrected collimator lens and 
 to place a polarizing prism either in front of the small aperture or 
 to reflect the parallel rays from the collimating lens on a flat, opaque, 
 polished glass surface at the polarizing angle. The lens to be ex- 
 amined is placed either at right angles to, or parallel with, the beam 
 of polarized rays and is viewed through an analyzing nicol and sen- 
 sitive tint plate and the strain distribution is thereby ascertained. 
 The lens should not be held in the fingers longer than necessary 
 during the test because of the strains introduced by local heating. 
 
LENSES AND LENS ELEMENTS. 
 
 261 
 
 Under these conditions the lens appears to be best illuminated 
 when the observer's eye is placed at its rear focal point. In the 
 case of a concave lens, it is advantageous to shift the collimator 
 lens or to add a second condenser lens in order to produce sharply 
 convergent polarized light, part of the convergence of which is 
 then neutralized by the negative lens under test. 
 
 Routine inspection, by projection methods, of lenses for certain 
 physical defects has been practiced for many years in certain optical 
 shops. During the war these methods were widely adopted and with 
 much success. The lens under inspection is illuminated by a strong 
 source of light (fig. 85), and is in turn imaged by a good photographic 
 lens on a dull-white flat screen of drawing paper, or of finely ground 
 opal glass; for certain purposes an undeveloped photographic plate 
 or a carefully faced magnesia block serves as a screen. The screen 
 should be of fine texture without markings of any kind. The en- 
 
 FIG. 85. Projection method for the inspection of lenses. S is the source of light; D, diffusing screen; A , a 
 pinhole aperture; L\, condenser lens; LZ, lens to be inspected; L s , projection lens; K, projection screen. 
 
 larged image of a lens illuminated, as indicated in figure 85, shows 
 clearly physical defects, such as bubbles, stones, pressing defects, 
 and striae. By racking the photographic lens in and out, different 
 planes in the lens under inspection can be imaged on the screen 
 and every part of the lens then inspected. At the same time surface 
 defects, such as pits and scratches, and insufficient polish, are imaged 
 on the screen and the lens can be inspected for them. By closing the 
 diaphragm of lens L 3 , figure 85, and shifting the lens L 2 or L 3 slightly, 
 oblique illumination is obtained and aids in rendering faint striae 
 visible. 
 
 A photographic plate and camera may be substituted for the 
 projection screen and a permanent record of the results of inspection 
 of a particular lens can be thus obtained. The defects illustrated 
 in figures 7, 8, and 9 were photographed in this manner. Similar 
 photographs have been taken before; recently Smith, 1 Bennett, and 
 
 i T. T. Smith, A. H. Bennett, G. E. Merritt, Characteristics of striae in optical glass, Bureau of Standards, 
 Scientific Paper No. 373, 1920. 
 
262 INSPECTION OF FINISHED OPTICAL PARTS AND SYSTEMS. 
 
 Merritt have published excellent photographs of striae in optical 
 glass, taken by this method, which is essentially that of a special 
 optical system illuminated properly. 
 
 Mechanical defects. The lens surfaces are examined with a magni- 
 fying glass for pits, scratches, sleeks, grayness, and insufficient polish, 
 chipped edges, and cracks. Illumination from the side by an intense 
 light source is advisable. Wavy or double-curvature polish should 
 not be present; this is commonly first detected in centering the lens. 
 
 Cemented lenses are inspected for bubbles and blisters in the 
 cement film, for color of the Canada balsam, and for the presence of 
 particles of dirt or of fuzz from the cleaning rag in the balsam. 
 
 The projection method of inspection described in the last section 
 is suitable for detecting mechanical defects in a lens. 
 
 Optical qualities. Lenses may be inspected for focal length, 
 accuracy of centering, and quality of image with respect to definition 
 or resolution, spherical aberration, coma, astigmatism, distortion, and 
 achromatism. There are many methods available for ascertaining 
 these properties and, in a particular case, the simplest and most 
 direct method should be adopted which will yield results of the desired 
 degree of precision. In routine inspection the effort is made to 
 ascertain if the several characteristics of the lens come within the 
 prescribed limits so that on assembly in the instrument it will function 
 properly as one of the integral parts. 
 
 In the testing of lenses and optical systems in general, there are 
 certain devices and arrangements which are used in so many methods 
 that it will facilitate the presentation to describe them briefly at this 
 point. These include artificial stars, distant targets and scales, 
 collimators, and collimating devices. 
 
 Artificial stars are set up as distant point sources of light and serve 
 especially in tests of the optical performance of telescope objectives 
 and telescope systems. Small silvered glass globes, globules of 
 mercury, or small illuminated pinhole apertures in opaque screens 
 located 30 or more feet away, depending on the focal length of the 
 objective from the observer, are used for the purpose. For many 
 purposes an artificial star illuminated by a monochromatic illumi- 
 nator is convenient for testing chromatic aberrations or the change 
 in performance of a lens with change in wave length of light. As 
 source of light a lamp with concentrated tungsten filament or tungsten 
 bead or a Nernst filament is convenient. A condenser lens may also 
 be used to advantage. 
 
 Distant targets are commonly painted with black or colored lines 
 on a white background. Scales, vertical and horizontal, of definite 
 intervals are thus prepared; also systems of coordinate lines, mutually 
 perpendicular and definitely spaced; also diagrams of lines or sectors 
 radiating from a common center. Targets of this kind are generally 
 
LENSES AND LENS ELEMENTS. 263 
 
 erected 100 or more feet away from the observer and are useful for 
 testing the performance of military optical instruments. 
 
 Test plates, either of sheet metal with small holes and slit and 
 cross-slit openings cut into them, or of silvered glass plates with sets 
 of parallel, definitely spaced lines cut with a dividing engine, and 
 small circular holes, or a photographic negative with similar sets of 
 lines are especially useful for measuring the optical quality and the 
 resolving power of an objective or a complete telescope. The metal 
 test plate should be several feet square and should be placed in a 
 window 50 or more feet distant; its perforations are illuminated by 
 skylight. The other test plates are much smaller and have much 
 finer lines. They are illuminated from the rear by a strong source 
 of light and a condenser lens, and are placed 30 or more feet away from 
 the observer. 
 
 A collimator is widely used for routine factory inspection and is a 
 convenient substitute for the distant target or test plate. It consists 
 essentially of a well-corrected telescope objective mounted at one 
 end of a telescope tube, at the other end of which is placed an opaque 
 plate with small circular aperture located at the rear focus of the 
 collimator objective and illuminated by a lamp or bulb. For certain 
 purposes an astronomical telescope objective, 4 or 5 inches in diam- 
 eter and of long focal length, is used with a small test plate of silvered 
 glass placed at its rear focus. On the test plate are ruled with a 
 dividing engine sets of definitely spaced lines, both vertical and 
 horizontal; also a group of lines diverging from a common center, 
 and several small circular holes of different diameters. The test 
 plate may be 1 inch in diameter and, if the objective of the collimater 
 is of the best quality, serves for testing most of the defects in an 
 optical system. Collimators are compact testing devices, and, were 
 it not for the fact that the images which they form are encumbered 
 with the defects introduced by the collimator objective, they would 
 be even more extensively used than at present; for many purposes 
 collimating telescopes are entirely satisfactory, not only for the 
 inspection of optical elements, but also for the adjustment of optical 
 instruments. 
 
 Measurement of focal length and external focal length of a lens. 
 The measurement of the equivalent focal length or of the external 
 focal length (back focus) of a lens suffices in many instances to deter- 
 mine the degree of accuracy of the curvature of the lens surfaces. 
 If the focal length is correct, it may be assumed that the lens surfaces 
 are approximately correct. This applies especially to cemented 
 doublets and other combinations. 
 
 Many methods are available for measuring the equivalent focal 
 length of lenses and are described in detail in textbooks on optics. 
 For work of precision an optical bench with the necessary accessories 
 
264 INSPECTION OP FINISHED OPTICAL PARTS AND SYSTEMS. 
 
 is essential; on it the four principal Gauss points, namely, the two 
 foci and the two principal (and nodal) points are commonly deter- 
 mined with reference to the vertexes of the two external surfaces 
 of the lens or lens combination. A distant artificial star illuminated 
 with strong white light or with monochromatic light commonly serves 
 as test object in these measurements. The exact positions of the 
 principal foci are determined by means of an auxiliary microscope; 
 the positions of the nodal points are ascertained by rotating the lens 
 about a vertical axis and noting that when the axis of rotation 
 coincides with a nodal point, the image of a distant object remains 
 stationary during the rotation of the lens through an appreciable 
 angle. 
 
 A critical test for zonal variations in equivalent focal length, for 
 spherical aberration, and for other axial aberrations of a lens is the 
 Hartmann 2 method of extra-focus measurements as modified by 
 
 FIG. 86. Hartmann-Tillyer test for the aberrations in a positive lens or lens combination. 
 
 Tillyer. 3 For the Hartmann-Tillyer test a metal disk perforated along 
 one or more diametral lines by a set of definitely spaced holes (1 milli- 
 meter in diameter and 3 millimeters apart for lenses of fair size) is 
 placed in front of the lens or objective to be tested (fig. 86), so that 
 monochromatic light from a distant artificial star on entering the 
 objective is restricted to a definite number of small pencils. Two 
 shadow photographs are taken, the one at some distance inside the 
 focus of the lens, the second outside the focus. Each ray pencil 
 forms a small disk on the photographic plate. The exact positions 
 of the ray-pencil disks are accurately determined and from these, 
 together with the distance of separation of the photographic plates, 
 the points of intersection of the rays on the axis can be computed. 
 The distance between these points is the spherical aberration. The 
 angle u which a par axial pencil entering the objective at a height li 
 
 2 Zeitschr. Jnstrumontenkunde, 24, pp. 1, 33, 97, 1904. 
 
 Jour. Wash. Acad. Sci., 3,,481, 1913; Nat. Bur. Standards, Sci. Paper No. 311, 1917. 
 
LENSES AND LENS ELEMENTS. 265 
 
 above the axis can also be computed from the data of measurement 
 and found to be 
 
 a + b 
 tan u= -t 
 
 wherein a and & are the distances from the axis of the disks formed 
 by the ray pencil on the two photographic plates separated by the 
 distance d. The equivalent focal length is then 
 
 /= Ji/sm u 
 
 Tillyer has shown that if the positions of the holes on the screen are 
 known to within 0.005 millimeter, the equivalent focal length for the 
 particular zone can be determined by this method with an average 
 error of 0.05 per cent. The variation of the focal length with dif- 
 ferent zones of the objective is a measure of correction for the sine 
 condition. By using lights of different colors (monochromatic 
 illuminator, cadmium spark) it is possible also to measure the 
 chromatic aberration. 
 
 A method for ascertaining the exact focus of a lens is the Foucault 
 or "knife-edge" method; this method is based on the fact that in a 
 well-corrected objective the rays are sharply focussed at a point, 
 and radiate as an evenly illuminated cone on either side of the focus. 
 If now a knife-edge be brought slowly across the line of sight at the 
 exact focus directly in front of the observer's eye the illumination of 
 the objective is cut off abruptly as the knife-edge crosses the axis. 
 In case the lens is not well corrected either spherically or chromati- 
 cally, there is no position at which the illumination disappear; 
 suddenly; either the inner or the outer portion of the illuminated lens 
 disk becomes dark first and the lens appears unequally illuminateds 
 in case white light is used the disk appears differently colored for 
 different positions of the knife-edge along the axis. 
 
 By mounting the lens under test as the objective of a telescope 
 with an astronomical high-power eyepiece and viewing the changes 
 in the image of a distant artificial star when the eyepiece is pushed 
 in and out, the observer can locate the exact position of focus and 
 at the same time determine in a satisfactory manner the extent 
 to which the several aberrations in the lens under test are corrected. 
 In case the lens is well corrected the image of the star appears as a 
 small circular disk of light uniformly illuminated. Its diameter is 
 least at the position of exact focus. In the case of spherical aberration 
 the illumination of the disk is not uniform, the central area of the 
 disk being brighter for one position of the eyepiece, the outer portion 
 for another position. By using different colors of lights and observing 
 the shapes of the disk in different parts of the field the observer can 
 determine the optical performance of the lens with a high degree of 
 accuracy. 4 
 
 * This method is described in detail by H. D. Taylor, in The Adjustment and Testing of Telescope 
 Objectives, published in 1896 by T. Cook, of York, England. 
 
266 INSPECTION OF FINISHED OPTICAL PARTS AND SYSTEMS. 
 
 During the war a great many photographic lenses were inspected 
 by means of a distant artificial star (pin hole aperture in front of a 
 monochromatic illuminator) used in conjunction with a precision 
 optical bench constructed in the shape of a T. The lens is mounted 
 on the bench in a holder of the iris diaphragm type and can be 
 rotated, and also slid back and forth in its carriage which is supported 
 on a pillar mounted on a slider which can be moved along the head 
 or cross arm of the T-shaped bench. Along the vertical arm of the 
 T-bench a second pillar is provided which supports an observing 
 microscope and is mounted in a carriage which slides along this arm. 
 The angle of rotation of the lens in its carriage can be read off on an 
 accurately divided circle; the transverse movement of the lens can 
 be read off accurately by a micrometer screw. The observing 
 microscope consists of an 8-millimeter apochromatic objective with 
 compensating eyepiece and cross hairs; its movement in the sliding 
 carriage can also be read off with high precision by micrometer screws. 
 
 With this bench the Gauss points are first located for the wave 
 length 550 millimicrons (yellow-green color). The character of the 
 image produced by the rays from the distant star (10 meters distant) 
 entering the lens at different angles (in steps of 3 from the axis) 
 is studied and the positions of the radial and tangential astigmatic 
 lines as well as the circles of least confusion are ascertained for dif- 
 ferent colors. From these measurements the astigmatic differences 
 are read off directly; also the degree of flatness of the field, the dis- 
 tortion, and the chromatic differences of magnification. For the 
 oblique rays a correction is- applied to allow for the flat plate. Coma 
 is judged from the character and width of the image of the star for 
 different positions of the lens. From the appearance of the star 
 image alon^, as formed by the photographic lens in different positions, 
 the inspector is able to form a correct estimate of the quality and 
 type of the lens. In the hands of a competent inspector this method 
 proved exceedingly useful and satisfactory. 
 
 For the testing of photographic lenses a chart method 5 may also 
 be used to advantage but it is less rapid than the foregoing. 
 
 For ordinary routine inspection of lenses intended for military 
 optical instruments, which are for the most part instruments of low 
 magnification (4 to 12 power) the foregoing methods are unnecessarily 
 refined. For most purposes it suffices to measure the external focal 
 length (back focus) ; this is the distance from the principal fociis to 
 the vertex of the nearest lens surface. This can be done rapidly 
 and with sufficient exactness by placing the lens under test in a ring 
 mount as the objective of a telescope, by setting in front of the tele- 
 scope a collimator having at its rear focus a small test plate with 
 sharp lines or patterns drawn upon it on which the observer can focus 
 
 5 A chart method of testing photographic lenses, L. E. Jewell, Jour. Opt. Soc. America, II-III, 51, 1919. 
 
LENSES AND LENS ELEMENTS. 267 
 
 sharply.' The image of the test plate, as formed by the lens under 
 test, is brought to sharp focus by racking a high power astronomical 
 eyepiece or an observing microscope in and out, its position is read 
 off directly on a suitable scale, so adjusted that the readings give 
 directly the external focal length (back focus) of the lens. Rotation 
 of the ring mount with the lens about its axis is a test for the center- 
 ing of the lens. If the lens is decentered the image is seen to describe 
 a circle during the rotation, the amount of movement depending on 
 the eccentricity of the lens. After a small amount of practice on an 
 apparatus of this kind, the observer is able not only to measure the 
 external focal length rapidly and accurately and to test for centering, 
 but also to measure the resolving power or definition of the objective 
 and to detect spherical and chromatic aberration, coma, astigmatism, 
 and distortion. For this purpose the test plate should have ruled 
 on it sets of equally spaced vertical lines, also several horizontal lines 
 and small circular holes. In general, however, it is advisable not to 
 attempt, in routine inspection, to measure more than the external 
 focal length and to test for centering on a collimating device of this 
 kind. During the measurement an estimate can be made of the 
 general quality of the image formed and of the degree to which the 
 lens is corrected for the several aberrations; but no actual measure- 
 ments of these factors are made. 
 
 In an optical system the defects of one lens are compensated by 
 equal and opposite defects in other lenses; thus the individual lenses 
 of a system may. show pronounced aberrations whereas the completed 
 system is entirely satisfactory. In order that there be a proper com- 
 pensation of errors between the several components of a system, it is 
 essential that the specifications laid down by the designer be accu- 
 rately followed; if the focal length or external focal length of each 
 element is correct, it is generally safe to assume that the specifications 
 have been so followed. 
 
 In place of the collimator a system of inspection may be adopted 
 whereby the image, formed by the lens of a test plate, is projected 
 on a screen. For the purpose a silvered glass plate on which two 
 sets of equally spaced, mutually perpendicular lines are ruled with a 
 dividing engine is most satisfactory; less satisfactory, but still service- 
 able is a photographic reproduction (negative) of a suitable drawing of 
 coordinate lines. The test plate is illuminated by parallel rays from 
 a strong, point-source of light at the rear focus of a well corrected 
 condenser lens which covers the entire test plate. The rays emerg- 
 ing from the test plate are practically parallel; the rays from any 
 particular point of the plate pass as a narrow pencil through a small 
 part of the lens, with the result that the image points on the screen 
 are formed by the convergence of narrow bundles of rays, thus reduc- 
 ing the effects of spherical aberration and coma. A sharp image is 
 
268 INSPECTION OF FINISHED OPTICAL PAKTS AND SYSTEMS. 
 
 formed and a correspondingly accurate setting can be made for the 
 determination of the focal length. The definition on the screen de- 
 creases from the center outwards because of astigmatism and curva- 
 ture of field. An estimate of the amount of spherical aberration and 
 coma present in the lens is obtained by placing back of the test plate 
 a ground glass or opal glass diffusing screen which destroys the 
 parallelism of the rays emerging from the test plate. Decrease in 
 definition, under these conditions, at the center results from spherical 
 aberration, while in other parts of the image it results from astigma- 
 tism, coma, and other defects. When the test plate is moved a 
 definite small amount in a direction transverse to the axis of the lens, 
 the image is shifted by an amount which depends on the focal length 
 of the lens; and from this the focal length can be computed. The 
 other aberrations are estimated from the appearance of the image on 
 the screen. If the two sets of mutually perpendicular lines can not 
 be simultaneously focussed astigmatism is present; if the lines on the 
 margin of the image are curved, distortion is present. Simple 
 apparatus suitable for the testing of lenses and prisms by this method 
 was devised by the Ordnance Department and found to be satisfac- 
 tory for practical testing at Frankford Arsenal. 
 
 Another method for ascertaining the properties of a lens is to 
 use the metal plate target, described in a foregoing paragraph; the 
 target should be set up at a distance of 100 or more feet from the 
 lens to be tested. The lens is placed in a mounted ring and functions 
 as the objective of a telescope. Its external focal distance, centering, 
 and other properties are then ascertained by the methods described 
 above for the collimator method, except that in this case a correction 
 must be made because the object is not at an infinite distance. For 
 routine inspection, however, where definite tolerances are set this 
 correction is taken into account in the scale used with the eyepiece 
 or the reading microscope. 
 
 The resolving power is ascertained by this method by sighting 
 upon sets of equally spaced lines drawn on a test plate located at 
 a definite distance, 30 feet (10 meters) or more, and illuminated by a 
 strong light. The spacing of the finest set of lines which can still be 
 distinguished as distinct lines determines the angle which is resolved 
 by the objective. An auxiliary reading microscope is commonly 
 used in the examination of the image formed by the objective. 
 Theoretically the angular separation of two fine lines or stars which 
 Can be imaged as distinct elements by a lens of diameter d is approxi- 
 mately equal to the angle subtended by the wave length of light at a 
 distance equal to the diameter of the lens. For two stars which can 
 be just resolved by a telescope objective the formula derived from 
 the theory of diffraction for the angular separation in radians is 
 
 0.61' X 
 
PRISMS. 
 
 269 
 
 wherein X is the wave length of light and r the semidiameter of the 
 lens; to reduce this to seconds of arc divide by 0.0000048; or more 
 directly it is 127000' X/r. For a wave length X = 0.00055 millimeter 
 near the middle of the visible spectrum, this expression reduces to 
 70/r seconds of arc as the theoretical resolving power of a telescope 
 objective of semidiameter, r, expressed in millimeters. This degree 
 of resolving power is rarely achieved by objectives for military optical 
 instruments. Generally the resolving power of a telescope system 
 equal to 1 minute of arc divided by the magnification is considered 
 to be an upper tolerance limit; commonly it ranges from 35 to 50 
 seconds of arc divided by the magnification. 
 
 FIG. 87. Projection method for prism inspection. S is the source of light; D, ground-glass diffusing disk 
 A , pinhole aperture; L\, achromatic doublet; P, prism under test; LI, projection lens; K, projection screen. 
 
 The inspection of prisms. Prisms, like lenses, are inspected for 
 defects in the glass, such as stones, bubbles, striae, strain, and color; 
 for external dimensions; for mechanical defects, such as lack of 
 polish of surfaces, presence of pits and scratches, degree of flatness 
 of surfaces and accuracy of interfacial angles; for optical qualities, 
 such as distortion and deviation of transmitted light rays from the 
 prescribed paths. 
 
 The examination for stones, bubbles, and pressing defects is best 
 made under conditions of illumination from the side (dark-ground 
 illumination) ; striae are detected by any one of the methods described 
 in the foregoing section on lenses. These defects can also be seen to 
 advantage by projecting the prism on a screen by the projection 
 3922921 18 
 
270 INSPECTION OF FINISHED OPTICAL PARTS AND SYSTEMS. 
 
 method described above. This projection method was widely adopted 
 for routine factory inspection during the war and proved to be satis- 
 factory. The optical arrangement for the purpose is shown schemati- 
 cally in figure 87. Prisms are inspected for strain by the standard 
 methods which have already been described. 
 
 The dimensions of the prisms are measured either by means of 
 calipers or of sets of standard gauges. Interfacial angles may be 
 measured mechanically by standard angle gauges. For inspection 
 purposes it is preferable, however, to test the optical performance 
 of the prism rather than to measure interfacial angles. 
 
 The degree of flatness of the prism surfaces is best tested against that 
 of a standard optically flat plate accurate to a tenth or a twentieth 
 of a wave length. For most purposes a prism surface flat to one or 
 two wave lengths is satisfactory; but in prisms, such as the Dove 
 inverting prism (vertical rotating prism in the panoramic sight), in 
 which the light rays are reflected at a large angle, a slight departure 
 from flatness in the reflecting surface may cause an appreciable 
 warping of the reflected light-wave front and thus impair the defini- 
 tion of the image. The reflecting surface of such a prism should be 
 flat to a part of a wave length over its entire area. 
 
 In the manufacture of precision optics, a tolerance of 2' of arc 
 is considered a fair and reasonable specification for most prism 
 angles. In certain prisms, such as the Dove erecting prism and 
 the pentaprism, the angle must be correct within 1' of arc; in 
 the roof-angle prism, the roof angle must be correct within a few 
 seconds of arc. These prism-angle tolerances are commonly meas- 
 ured optically and the specifications are stated in terms of permissible 
 angular deflections of transmitted rays from the prescribed paths. 
 
 In conformity with the practice adopted at Frankford Arsenal, on 
 the recommendation of Col. G. F. Jenks in 1910 for the inspection 
 of the optical performance of prisms, the following conventions may 
 well be followed: A reflecting prism in an optical instrument serves 
 to change the direction of the optical axis of the instrument through 
 a definite, prescribed angle. Let the plane in which this angle is 
 measured be called the " axial plane." Then the "axial-angle error" 
 of a prism placed with its reflecting surface in the correct position 
 is the angular component, in the axial plane, of the deviation of the 
 reflected ray, after emergence from the prism, from the prescribed 
 path. The " side-angle error" is the angular component, in the 
 plane normal to the axial plane, of the deviation of the reflected ray, 
 after emergence from the prism, from the prescribed direction. The 
 Dove erecting prism serves only to invert and not to change the direc- 
 tion of the axial rays after emergence; for this prism which presents 
 a limiting case, let the axial plane be perpendicular to the hypoth- 
 enuse surface and to the first surface entered by the incident ray; 
 
RIGHT ANGLE REFLECTING PRISM. 271 
 
 it is the plane of incidence of the incident rays and contains the nor- 
 mals to the incident refracting surface and to the hypothenuse sur- 
 face. These conventions establish the reflecting surface of a prism 
 as the surface of reference. Any other surface might be used, but 
 for the sake of uniformity in the testing and designation of the differ- 
 ent types of prisms the reflecting surface appears to serve the pur- 
 pose best. In the case of roof-edge prisms and pentaprisms two 
 reflecting surfaces replace the single reflecting surface and serve to 
 invert the image; in these types of prisms the two reflecting prisms 
 function as a unit and may be treated as such. 
 
 Experience has shown that different optical methods of different 
 degrees of sensitiveness may be used to test the optical performance 
 of any given type of reflecting prism. The deviations should be 
 measured of those rays which traverse the prism along the path 
 which they follow in the actual instrument; in other words, a meas- 
 ure of the actual performance of the prism should be taken. 
 
 In view of the fact that the path of a ray through a prism depends 
 on a number of different factors, such as defects in the glass, degree 
 of flatness of prism surfaces and in addition to different interfacial 
 prism angles, the only feasible method for computing the deviations 
 of transmitted axial rays from the prescribed path, which result from 
 slight departures of the several prism angles from the prescribed 
 angles, is to consider the glass to be free from defects and optically 
 homogeneous and the prism surfaces to be optically flat. Under 
 these assumptions it is possible to ascertain the competency of the 
 several different optical methods available for testing and to determine 
 the significance, in terms of actual prism angles, of different optical 
 .tolerances which may be set. The relations between actual prism 
 angles and the resulting ray deflections in the axial and side (normal 
 to axial) planes are indicated for the different types of prisms in 
 figures 88 to 94. In these figures a section of the correct prism is 
 indicated by dotted lines; the incorrectly oriented surfaces are indi- 
 cated by full lines; the paths of the rays are indicated by full lines 
 with arrow marks. In each set of curves the ordinates are the 
 angles of departure, t, of the prism surface from which the ray emerges; 
 the abscissae, the deviations, d, of the emergent ray from the pre- 
 scribed path; the series of oblique lines represent the departure 
 angles, s, of the incident prism surface. 
 
 THE RIGHT ANGLE REFLECTING PRISM. 
 
 Three optical methods may be used to test this type of prism; in 
 two of the methods a telescope and distant target or collimator are 
 employed; in the third an autocollimator serves the purpose. A 
 variation of these methods is obtained by the use of a projection 
 screen in place of the observing telescope. 
 
272 INSPECTION OF FINISHED OPTICAL PARTS AND SYSTEMS. 
 
 FIRST METHOD. 
 
 Axial angle errors. Let it be assumed, for the sake of simplicity, 
 in this and other cases of axial angle error that the faces of the 
 prism are in the same zone i. e., all vertical to the same plane; 
 this is practically the case in all prisms and the slight depar- 
 tures from this condition introduce departures of the second order 
 only, in the axial angle errors. Let s be the error of the angle (posi- 
 tive if the total angle exceeds 45) which the first side face includes 
 with the hypothenuse (fig. 88) and t the error of the second side 
 face. These errors are always small and do not exceed 10 minutes 
 even in very poor work. The deviation of the emergent ray from 
 its prescribed path in the axial plane is d=(n-l) (s t), wherein 
 n = 1.515 is the refractive index of the prism. In figure 88 the devia- 
 tions of the rays are represented by the abscissae, the angles t by the 
 
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 FIG. 88. In this diagram are given the deviations of the emergent ray, from the prescribed path, after trans- 
 mission through a right-angle prism of slightly incorrect interfacial angles as indicated in fig. 880. 
 
 ordinates and the angles s by the series of oblique lines ; thus a prism 
 for which the first interfacial angle is 45 02' (s = + 2') and the second 
 interfacial angle is 45 01' (t= + 1') causes a deviation of the emergent 
 ray in the axial plane of \' from the prescribed path. 
 
 'Side-angle errors. In this and other cases of side-angle errors the 
 angles which the side faces include with the hypothenuse face are con- 
 sidered to be exactly 45. This assumption is made in order to facili- 
 tate the computation; the error introduced by it is of the second 
 order only because the departures in these angles from 45 are very 
 slight. In this method the incident ray enters along the normal to 
 the first side surface, is reflected at the hypothenuse face, and is then 
 refracted at the second side surface whose trace on the hypothenuse 
 face includes a small angle t with the trace of the first side surface on 
 the hypothenuse. Under these conditions the deviation of the 
 
RIGHT ANGLE REFLECTING PRISM. 273 
 
 emergent ray for a prism of refractive index, n = 1.515, is d=l.07t; 
 in other words, the deviation is practically equal to the departure 
 angle, t. 
 
 With this method, therefore, the angular deviation of the emergent 
 ray from the prescribed path, both in the axial plane and the side 
 plane, is of the same order of magnitude as that of the errors in the 
 interfacial angles, and the method is not especially sensitive. The 
 method has the advantage, however, that the prism is tested under 
 conditions resembling those of actual use. 
 
 For routine testing by this method either a spectrometer or a 
 combination of a telescope of adequate resolving power and a colli- 
 mator or telescope and distant target is used. The prism is placed 
 with the hypothenuse surface against a fixture having rounded bearing 
 points for both the hypothenuse and first side surface. The fixture is 
 accurately adjusted so that the plane of incidence contains the normals 
 both to the hypothenuse face and to the first side face. A coordinate 
 scale in the image plane of the telescope indicates the axial and side 
 angle departure from the prescribed path. In place of the observing 
 telescope a projection screen with properly arranged coordinate lines 
 to mark the tolerance limits may also be used. 
 
 SECOND METHOD THE CONSTANT DEVIATION METHOD OF FRANKFORD ARSENAL. 
 
 Axial-angle error.- The paths of the rays through the prism when 
 used as a constant deviation prism are shown in figure 89. The 
 incident ray is considered to be perpendicular to the hypothenuse 
 face. The deviation, d, of the emergent ray from 90 is then 
 d = 2 s+ (i r) (i f r') } wherein i is the angle of incidence, r, the 
 angle of refraction, and sin i = n sin r, (n = 1.515); r' is the angle 
 of incidence of the emergent ray and equal to (r 3s t), and 
 sin i' = n sin r' . The deviations d for different values of t and s are 
 shown by the curves of figure 89. From these curves it is evident 
 that in a prism whose angles differ from the correct angles by 2', 
 the maximal deviation of the emergent ray from the prescribed 
 path may be d=ll f . The curves show, moreover, that if the prism 
 is observed only in one position, this method may not be adequate 
 to determine its quality; thus for s = 0' and t= +10' the deviation 
 is +9'; if now the prism be reversed so that s = 10' and t = Q' the 
 deviation is d= 46.5'. It is thus evident that unless care is taken 
 to measure a prism from both directions erroneous conclusions regard- 
 ing its quality may be drawn from the result obtained. If, more- 
 over, the incident ray enters at a different angle from normal to the 
 hypothenuse face slightly different deviations result. The deviation 
 depends therefore not only on the prism-angle errors but also on the 
 position of the prism during the observation. The last uncertainty 
 
274 INSPECTION OF FINISHED OPTICAL PARTS AND SYSTEMS. 
 
 is eliminated by prescribing that the incident ray shall coincide in 
 direction with the normal to the hypothenuse face. 
 
 Side-angle error. Let the prism side angle be 45; let the direc- 
 tion of the incident ray coincide with the normal to the hypothenuse 
 face; let the plane of incidence include the normals to the first side 
 face and to the hypothenuse face; let t be the angle between the 
 traces of the first side face and of the second side face, respectively, 
 on the hypothenuse face. The deviation d of the emergent ray from 
 the prescribed path is then approximately d= t/2.l for n=1.515; 
 in other words the deviation is about half that of the angle t and the 
 method is not especially sensitive in this respect. 
 
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 FIG. 89. Constant deviation method. In this figure the deflections are shown of the emergent ray from the 
 prescribed path, after transmission through a prism of slightly incorrect interfacial prism angles, as indi- 
 cated in fig. 89a. 
 
 THIRD METHOD AUTOCOLLIMATION METHODS. 
 
 Axial-angle error. In these methods a well corrected autocolli- 
 mator of adequate resolving power is used; the hypothenuse surface 
 is placed at an angle of 45 with the axis of the collimator. (Fig. 
 90a.) The deviation of the emergent ray from the original line of 
 sight is then approximately d = 2nt 2(n- l)s = 3.Q3t- 1.03s for a 
 prism of refractive index n= 1.515. The curves of figure 90 show 
 that for errors in the prism interfacial angles of s= +3' and t= +1' 
 the deviation <Z = 0; for s= 3' and t= +1' the deviation is d= +6'. 
 The method alone is therefore not adequate to test the prism angles. 
 If, however, the prism be rotated and the hypothenuse face be 
 placed normal to and facing the collimator (fig. 91a), the curves of 
 figure 91 show that the deviation, as computed from the formula 
 d = 2 n (* + s), for s= +3' and t= +1' is d= +12'; for s= -3' and 
 
DOVE ERECTING PRISM. 
 
 275 
 
 t= +1' the deviation, d= 6'. The measurement of the deviations 
 of the prisms in these two positions suffices therefore to measure 
 the angles satisfactorily. The autocollimation method is much more 
 sensitive than the first method and is less encumbered with defects 
 than is the second method. 
 
 Side-angle error. With the prism placed in the position of figure 90a 
 the side-angle deviation is practically the same as that obtained in 
 the first method, namely, d= 1.07't or d = t approximately. 
 
 CU 
 
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 FIG. 90. Autocollimation method. In this diagram are shown the deviations of the emergent ray from the 
 prescribed path, which on transmission are produced by slight departures s and t in the interfacial prism 
 angles from the correct angles, as indicated in fig. SOa. 
 
 A projection method may also be used with autocollimation, but 
 this method is simply a modification of the telescope methods and 
 involves nothing new in principle. 
 
 THE DOVE ERECTING PRISM. 
 
 Vertical rotating prism of panoramic sight. This prism is in 
 effect a truncated right-angle prism, but it is used to invert the image 
 and not to change the direction of an axial ray in the telescope. 
 
276 INSPECTION OF FINISHED OPTICAL PARTS AND SYSTEMS. 
 
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 FIG. 91. Autocollimation method. A graphical representation is given in this diagram of -the deviations 
 of the emergent ray from the prescribed path, after transmission through a prism of slightly incorrect 
 interfacial angles, as indicated in fig. 91a. 
 
 +10' 
 
 z_/ 
 
 1 
 
 I ( 
 
 1 1 
 
 I / 
 
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 FIG. 92. In this figure the deviations are shown of the emergent ray from its prescribed path, after trans- 
 mission through a prism of slightly incorrect interfacial angles, as indicated in fig. 92a. 
 
ROOF- EDGE PRISM. 277 
 
 In the method commonly used for testing this prism a collimator or 
 distant target together with an observing telescope with coordinate 
 scale in its image plane is employed. The prism is placed in front 
 of the telescope with the hypothenuse face parallel with the line of 
 sight. 
 
 Axial-angle error. The path of the axial ray through the prism 
 is shown in figure 92a. The deviation d for prism interfacial angle 
 errors s and t is d = 45 t i' wherein sin i' = n sin r'; r' = r + s t', 
 sin (45 - s) = n. sin r and n = 1.515. The curves of figure 92 show that 
 for s= + 1' and t= -1', d= -3.8'; for s= +3' and t= +10', d = Q; 
 but if the prism be reversed so that s = +10' and t = + 3', the deviation 
 d= 26'. In testing a prism by this method, therefore, it should be 
 examined in both positions, otherwise serious errors may arise. 
 This fact should also be noted in the assembly of the prism; as for 
 example, in the assembly of the vertical rotating prism in the pano- 
 ramic sight. 
 
 Side-angle error. On the assumption that the side angles are cor- 
 rect and equal to 45 and that the axial plane is normal to both the 
 hypothenuse and first side faces, the trace of the second side surface 
 on the hypothenuse face includes a small angle t with the trace of the 
 first side face on the hypothenuse face, the resulting side-angle 
 deviation of a transmitted axial ray is approximately d= t/2.1 for 
 n=1.515. 
 
 THE ROOF-EDGE PRISM. 
 
 This prism may be treated as a simple reflecting prism in which the 
 hypothenuse reflecting surface has been replaced by two reflecting 
 surfaces mutually at right angles and truncating, like a roof, the 
 elongated edges of the hypothenuse face. The roof angle must be 
 exactly 90 within a few seconds of arc, otherwise a double image 
 results. The incident light strikes the first reflecting surface at an 
 angle of 60, is reflected across to the second face, and is reflected by 
 it at an angle of 60 , thus inverting the image completely. The 
 tests for axial-angle and side-angle errors may be made by the 
 methods used for ordinary reflecting prisms; of these the autocol- 
 limating methods appear to be the most satisfactory. The correct- 
 ness of the angle is determined by the absence of a double image of the 
 lines on the horizontal line of the target. The following rule is useful 
 in determining, in the case of a double image, whether the roof angle 
 is greater or less than 90. Rack the eyepiece of the observing 
 telescope inward; if the two images approach each other the roof angle 
 is less than 90; if they approach when the eyepiece is racked outward 
 the .roof angle is greater than 90. Another method for testing the 
 roof angle is to grind and polish a small face truncating the edge 
 between the two side faces of the prism; a ray of incident light normal 
 
278 INSPECTION OF FINISHED OPTICAL PARTS AND SYSTEMS. 
 
 to this face should be reflected by the roof faces as a single ray parallel 
 with the original direction. This test is made with an autocolli- 
 mator; the method is, however, hardly to be recommended, as it offers 
 no advantage over the foregoing method and requires the preparation 
 of an additional face on the prism. 
 
 THE PENTA PRISM. 
 
 Methods similar to those described in the foregoing paragraphs are 
 used for testing pentaprisms optically. In all cases the prism should 
 be held against a proper fixture so that the incident ray enters 
 normally to the first surface of the prism as indicated in figures 93 a 
 and 94a. 
 
 -50' -4Q' -3O' 
 
 -2O 1 
 
 -IO 
 
 -HO* 
 
 +2O* 
 
 +3O 
 
 --4O 
 
 +50' 
 
 FIG. 93. In this diagram the deviations are represented of the emergent ray from the prescribed path, aftei 
 transmission through a penta prism of slightly incorrect interfacial prism angles, as indicated in fig. 93a. 
 
 First method. A collimator or distant target with horizontal and 
 vertical lines is used in conjunction with an observing telescope or 
 projection screen. 
 
 Axial-angle errors. For a ray entering the prism normal to the first 
 face as shown in figure 93a, the deviation of the emergent ray from 
 the prescribed path is d = n(2 s t), wherein t and s are the small 
 angular errors in the interfacial angles of the penta prism, and 
 n= 1.515 is its refractive index. The series of curves in figure 93 
 are a graphical solution of the foregoing equation; thus for t= +2', 
 and s=-l', d=-6'; for t= +2', s=+l', d = Q. The equation 
 
PENTA PRISM. 
 
 279 
 
 shows, moreover, that a slight change s in the angle between the 
 two reflecting faces has twice the effect in deflecting a transmitted 
 ray that the same change t has in the angle between the other two faces. 
 The reflecting faces here function as a unit just as do the reflecting 
 faces of a roof edge prism. 
 
 Side-angle error. Let the line of intersection of the two reflecting 
 faces include an angle t with the line of intersection of the two other 
 faces. This gives a twist to the prism. Let the angle between 
 reflecting faces be 45 and that between the other two faces, 90. 
 The angle of deflection is approximately equal to d = n.t, wherein 
 n= 1.515, the refractive index of the glass. 
 
 -K>', 
 
 -5' 
 
 +5' 
 
 -50' 
 
 -3O' 
 
 -2O 
 
 -10' 
 
 -10 
 
 +20 
 
 FIG. 94.Antocollimation method. In this diagram the deviations are shown of the emergent ray, from the 
 prescribed path, after transmission through a penta prism of slightly incorrect interfacial prism angles, 
 as indicated in fig. 94a. 
 
 Autocollimation method Axial-angle error. The deflection in this 
 case (fig. 94) is d = 2n(2 s t) or twice that obtained by the first 
 method; thus for t= +2' and s= -1', d= 12'; for t= +2' and 
 s= +1', d = 0. Similarly the side-angle error is approximately twice 
 that found by the first method. The results prove, however, that 
 in these methods a compensation may take place whereby zero 
 deflection may be obtained from a prism whose angles are incorrect. 
 
 The interfacial angles of these prisms may also be measured directly 
 on a large, specially designed fixture which is essentially a part of a 
 large spectrometer equipped with well-corrected telescope objective 
 lenses, 3 inches in diameter (theoretical resolving power about 2 sec- 
 
280 INSPECTION OF FINISHED OPTICAL PARTS AND SYSTEMS. 
 
 onds of arc) ; as source of illumination in the collimator a fine silk 
 fiber or spider web illuminated by a strong light from the side may be 
 used in place of the slit. 
 
 For the testing of a right angle an optical square is useful. It con- 
 sists of two plates of glass or quartz mounted together by optical 
 contact; the one plate is polished optically flat on one surface; the 
 second thick plate has at least two optically flat, mutually perpendicu- 
 lar surfaces. One of these surfaces is placed in optical contact with 
 the surface of the first plate. The edge of the second plate is trun- 
 cated slightly before mounting in order to facilitate the cleaning of 
 the edge of the finished optical square. The prism to be tested is 
 placed with its side faces in contact with the faces of the optical square. 
 The interference fringes formed between the several faces are a 
 measure, then, not only of the correctness of the right angle, but also 
 of the degree of flatness of the surfaces as compared with that of 
 the standard optical square. The making of a correct optical square 
 is a task which requires a high degree of skill. 
 
 The resolving power of prisms is tested in a manner similar to that 
 described for lens; a distant target, consisting of a plate ruled with 
 sets of fine, equally spaced lines and illuminated by a strong light 
 from the rear, or a collimator with a similar but smaller test plate in 
 its rear focal plane, is examined through the prism with a well- 
 corrected telescope. The resolving power is then the angle subtended 
 by the spacing of the finest set of lines which can be seen as distinct 
 lines when viewed under these conditions. In case a prism is to be 
 used in front of an objective, as in a battery commander's telescope 
 or a range finder, its resolving power should be at least equal to that 
 of the objective. By examining, through a high-power telescope, 
 the effect which the prism exerts on rays from fine lines and holes in 
 a distant metal plate target or from a test plate in a collimator, the 
 inspector can determine the optical quality of the prism as regards 
 astigmatism, coma, distortion, achromatism, and the like. If the 
 prism functions properly under these conditions of severe test, it will 
 do so in the optical system for which it is intended. 
 
 THE INSPECTION OF OPTICAL SYSTEMS. 
 
 In this section a brief outline only is given of the several factors 
 which enter into the inspection of a completed optical system. No 
 consideration is given to the functioning of an optical instrument as 
 a measuring device of precision; and only a selected few methods for 
 determining certain optical characteristics of a complete optical sys- 
 tem are described in a general way. To do more than this would 
 lead too far. Many military optical instruments, especially ordnance 
 instruments, are telescopes, and these only are considered in the 
 
ENTRANCE AND EXIT PUPILS. 281 
 
 following sections. In Chapter II, an outline is given of the func- 
 tions of a telescope and of some of the factors with which the designer 
 and manufacturer have to contend in order to attain the desired re- 
 sults as laid down in the specifications. These cover commonly the 
 following items: Size of entrance pupil or objective aperture; size of 
 exit pupil ; eye distance or distance of exit pupil from rear of eyepiece 
 lens; magnifying power of telescope; angular field of view, apparent 
 and actual; resolving power; quality of image with reference to color 
 corrections, to definition in different parts of the field, to distortion, 
 and to other defects; percentage light-transmission of the optical 
 system; polish of lens and prism surfaces and presence of film. 
 
 Most of these properties can be determined either by the use of 
 auxiliary telescopes and measuring apparatus or by projection 
 methods in which the image is projected on suitable screens by 
 means of suitable auxiliary lenses. 
 
 Diameter of entrance pupil and of exit pupil. In Chapter II the 
 functions of the several diaphragms in an optical instrument are de- 
 scribed briefly. The aperture of- the lens system determines the rela- 
 tive amount of light which can be received from a distant object 
 point; the aperture in a telescope depends not only on the diameter of 
 the objective, but also on the diaphragms in the telescope. The 
 larger the aperture the more light enters the instrument and the 
 finer the detail which is imaged distinctly; also the shorter the depth 
 of focus. The entrance pupil of the telescope is by definition the 
 smallest diaphragm visible from an object point toward which the 
 axis of the telescope is pointed; similarly the smallest diaphragm 
 visible from the corresponding image point is the exit pupil. En- 
 trance pupil and exit pupil are conjugate with reference to the tele- 
 scope and the one is the image of the other by the lens system. More- 
 over the ratio of the diameter of the entrance pupil to that of the exit 
 pupil is directly a measure of the magnifying power of the telescope. 
 
 In the design of a telescope the size of aperture is determined by 
 the normal resolving power and the magnification desired ; in military 
 optical instruments intended for field use the diameter of the exit 
 pupil should be about 5 millimeters. In few military instru- 
 ments, with the exception of range finders, is the full resolving 
 power utilized. For naval and other instruments, such as musket 
 and machine-gun telescopic sights, required for use on a moving 
 platform, an exit pupil of 7-millimeter diameter is desirable; but 
 this can be attained only at the sacrifice of other desirable qualities. 
 In case the exit pupil is much larger than the iris of the eye, the 
 image appears to jump as the eye occupies different positions in the 
 exit pupil; strong color fringes are also present in the field. 
 
 If the telescope is held at a distance from the observer and pointed 
 at the sky, the exit pupil appears as a disk of light, suspended in 
 
282 INSPECTION OF FINISHED OPTICAL PARTS AND SYSTEMS. 
 
 space, back of the eyepiece. Its distance from the eyelens of the 
 eyepiece is the u eye-distance;" when the iris of the observer's eye 
 coincides in position with the exit pupil of the instrument he is able 
 to see the entire field of the telescope. For convenience the " eye- 
 distance" should be at least 12 millimeters; if less than this the 
 eyelashes are liable to touch and brush against the eyelens. 
 
 The diameter of the exit pupil may be conveniently measured by 
 means of an auxiliary positive eyepiece mounted in a draw tube 
 together with a fine micrometer scale at its rear focus. The eyepiece 
 is focused sharply on the exit pupil and the diameter of the pupil is 
 read off directly on the micrometer scale. The eye distance is also 
 determined by a scale which is attached to the draw tube and indi- 
 cates the distance of the micrometer scale from the front of the draw 
 tube or from the rear surface of the eyelens. The diameter of the 
 entrance pupil is ascertained by placing a scale directly in front of 
 the objective and noting the number of divisions included in the 
 image of this scale as seen in the exit pupil. The ratio of these two 
 diameters is the magnification of the telescope. 
 
 In place of the eyepiece drawtube attachment the exit pupil may 
 be projected by a lens on a screen and its diameter measured in terms 
 of a scale drawn on the projection screen; similarly the diameter of 
 the entrance pupil can be determined. In all cases the exit pupil 
 should be circular in shape and appear uniformly illuminated. 
 
 In telescopes of the Galilean type with a dispersive eyepiece the 
 exit pupil is a virtual and not a real image and can not be measured 
 directly. To overcome this difficulty a positive lens is placed at a 
 distance equal to twice its equivalent focal length from the scale of 
 the eyepiece mounted in the drawtube; a real image of the same size 
 as the virtual image is thereby obtained on the micrometer scale. 
 The eye distance in this case is a negative quantity and is located 
 inside the instrument. 
 
 Magnifying power. As noted in the foregoing section the magni- 
 fying power of a telescope is given by the ratio of the diameter of the 
 entrance pupil to that of the exit pupil, and these in turn are deter- 
 mined by the ratio of the focal length of the objective to that of the 
 eyepiece. 
 
 Another method for ascertaining the linear magnifying power is 
 to measure the angular vergency of the initial and final zone pencils. 
 For this purpose the angle included at the observer's station between 
 the lines of sight to distant points is first measured directly with a the- 
 odolite or transit; the telescope under test is then focused on a distant 
 object and placed in front of the transit with its objective pointing 
 toward the transit; the angle between the same two distant points 
 is remeasured as seen through telescope and transit. The ratio of 
 the tangents of half the angles thus measured is the linear magnifica- 
 
RESOLVING POWEK. 283 
 
 tion. The ratio of the angles directly is the angular magnification. 
 In place of the distant objects a collimator with properly ruled test 
 plate may be used. 
 
 The magnification can also be ascertained by a projection method 
 in which the lines on a ruled test plate in a collimator are projected 
 by means of an auxiliary lens on a ruled screen. The telescope with 
 its objective pointing toward the collimator is inserted between the 
 collimator and the auxiliary lens and the change in the spacing of 
 the projected test-plate lines is noted. From the amount of change 
 as measured by the ruled lines on the projection screen the magnifi- 
 cation can be determined directly. 
 
 Still another method for ascertaining the approximate magnifica- 
 tion of a telescope is to view a distant target scale simultaneously 
 with the two eyes, the one eye unaided, the second with the telescope 
 placed before it. The number of scale divisions, as seen by the 
 unaided eye, included between two adjacent scale divisions as seen 
 through the telescope with the second eye, is the magnification. 
 
 The real and apparent fields of view. The actual angular field of 
 the telescope is measured by sighting upon a distant object and 
 observing the angle through which the telescope must be turned in 
 order to bring the image of the distant point from one margin of the 
 field along the horizontal diametral line to the margin on the opposite 
 side. The apparent angular field is obtained by multiplying the 
 angle thus obtained by the magnification. The apparent linear field 
 may be deduced by noting that the magnification is the ratio between 
 the tangent of half the angles of the apparent field and the real field, 
 respectively. 
 
 Resolving power. -The ability of the telescope to resolve fine details 
 of an object is a measure of its resolving power. To test its resolving 
 power a silvered test plate, placed at a definite distance from the 
 telescope (10 or more meters) and ruled with sets of equally spaced 
 fine lines and intensely illuminated from the rear, is viewed by the 
 telescope. In order to relieve eyes train and to facilitate the observa- 
 tions the telescope image is viewed with a low-power auxiliary tele- 
 scope. The angular equivalent of the spacing between the finest 
 lines, which appear as separate distinct lines under these conditions, 
 is a measure of the resolving power. On a test chart of this kind 6 
 small pinhole apertures are generally included as artificial stars. 
 
 P(tYom the changes in appearance of the image of an artificial star as 
 the eyepiece is racked in and out and as the telescope is rotated 
 slightly so that the star appears in different parts of the field, conclu- 
 sions may be drawn regarding the degree of correction of the several 
 aberrations, such as spherical aberration, chromatic aberration, coma, 
 astigmatism, distortion, etc.; also flare and double imagesA A good 
 
 T. T. Smith, Jour. Opt. Soc. America, II-III, 76, 1919. 
 
284 INSPECTION OF FINISHED OPTICAL PARTS AND SYSTEMS. 
 
 telescope should resolve without difficulty lines separated by an angle 
 equal to 60 seconds of arc divided by the magnification. In many 
 telescopes the resolution is 40 seconds divided by the magnification. 
 
 Quality of image. To test the optical performance of a telescope 
 lens system a number of different methods are available. One of 
 the simplest of these is the distant artificial star test, especially if the 
 star be illuminated with different monochromatic lights. An auxil- 
 iary telescope magnifying 4 or 5 diameters may be used in the tests 
 to magnify the eyepiece image. The test for spherical aberration is 
 to rack the eyepiece in and out and to note that in a spherically well 
 corrected lens system the disk of light is circular and evenly illumi- 
 nated on both sides of the image plane; if spherical aberration is pres- 
 ent the center of the disk is brighter on one side of the image plane 
 while the periphery is brighter on the other side. Coma is recog- 
 nized by the appearance of a one-sided flare extending from the star 
 image when it is placed near the margin of the field. Astigmatism 
 may also affect the star image when it is placed near the margin of 
 the field; if astigmatism is present, its effect is seen on racking the 
 eyepiece in and out, whereby the star appears either as a short hori- 
 zontal or a vertical line when the eyepiece is racked in and as a short 
 vertical or horizontal line when it is racked out. In an intermediate 
 position of the eyepiece the star image appears as a disk of light. 
 Distortion is recognized by a change in the apparent size of the star 
 image in case it is observed first in the center of the field and then 
 at the margin. Distortion is more readily detected by the use of a 
 distant target ruled with equally spaced coordinate lines. The lines 
 near the margin of the field appear then curved; pincushion and bar- 
 rel-shaped distortion are the two types of distortion which result from 
 the differences in magnification at the center and at the margin of 
 the field. Lack of adequate color correction gives rise to the appear- 
 ance of a colored fringe around the artificial star, the color differing 
 with different positions of the eyepiece. Curvature of field is recog- 
 nized by the fact that when the star is in sharp focus at the center of 
 the field, it is not in focus when placed at the margin, but can be 
 sharply focused by changing the position of the eyepiece. Decrease 
 in definition toward the margin of the field may be incorrectly ascribed 
 to curvature of field when it is actually due to the presence of other 
 aberrations, such as astignatism, coma, spherical zones, etc. 
 
 Other methods requiring the use of distant targets are in use and 
 are well adapted for inspection purposes but it would lead too far to 
 describe these in detail. 
 
 Film and lack of polish on lens surfaces. These defects are most 
 readily detected by pointing the telescope with eyepiece or objective 
 toward a strong distant source of light and observing the different 
 elements in the telescope not in the direct light, but with the telescope 
 
TRANSMISSION OF LIGHT. 285 
 
 rotated slightly so that the lenses and prisms appear in half-shadow. 
 Film and poor polish then appear as a slight haze over the surfaces, 
 which with practice can be detected at a glance. During the war 
 much trouble was encountered because of the appearance of film 
 on lens surfaces; the source of trouble was found to be different in 
 different cases. In some instruments unstable glass caused the 
 trouble; in others the film was found to be organic in nature and 
 deposited from volatile matter such as oil, grease, poorly baked 
 lacquer, etc., included in the instrument at the time of its assembly 
 and adjustment. The general conclusion, reached as a result of many 
 researches into the causes of film, has been that only weather-resistant 
 glass of good quality should be used in military optical instruments 
 and that the most painstaking care should be taken in the assembly of 
 optical instruments to insure cleanliness and freedom from grease 
 and volatile matter in the interior of telescope tubes; under no con- 
 ditions should the operator's fingers touch the lenses and prisms, 
 after cleaning, during their assembly into the instrument. Failure 
 to observe these simple precautions and to provide proper assembly 
 rooms free from dust, was the cause of many rejections of optical 
 instruments, especially binoculars, during the war. 
 
 The light-transmission of a telescope depends on a number of 
 factors, such as quality and kinds of glass, total glass path, number of 
 reflecting surfaces, quality of polish of the surfaces, condition of Canada 
 balsam layer in cemented lenses. There are available different 
 methods for measuring the light-transmission of telescopes; several 
 of these were described in Chapter IV and need not be repeated here. 
 They are without exception photometric in nature. The essential 
 difference between determining the light-transmission of a piece of 
 glass and a telescope is that in the telescope the rays follow prescribed 
 paths whereas in the glass plate they may be transmitted along any 
 direction. 
 
 For the measurement of the transmission of a telescope it is 
 essential that the light rays follow a telecentric course and that the 
 exit pupil of the telescope be imaged in the field of the photometer. 
 A simple attachment to the Koenig-Martens polarization photom- 
 eter 7 is shown in figure 71, page 214. It consists essentially of an 
 achromatic lens mounted in a brass cylinder which slips over the 
 front tube of the polarization photometer and images the exit pupil 
 or entrance pupil of the telescope in the field of the photometer. 
 The lens is so placed that the front aperture of the photometer 
 coincides with its rear focal plane. Either the objective or eyepiece 
 end of the telescope may face the photometer. The source of illumi- 
 nation is the same as that described in Chapter IV; also the method 
 
 i F. E. Wright, Jour. Opt. Soc. America, II-III, 65, 1919. 
 3922921 19 
 
286 INSPECTION OF FINISHED OPTICAL PARTS AND SYSTEMS. 
 
 of measurement in which the position angles of the analyzer in the 
 photometer are recorded for settings with the telescope in the field 
 and for settings without the telescope. The ratio of the squares of 
 the tangents of the angles thus obtained is a measure of the per- 
 centage light transmission of the telescope. 
 
 A photometric bench may also be used to advantage for measuring 
 the percentage light transmission of telescopes. An instrument of 
 this type was manufactured during the war by Keuffel and Esser 
 (Fig. 69, page 212) for the use of inspectors of optical instruments 
 and proved to be satisfactory in practical work. Another type of 
 bench photometer for the measurement of the light transmission of 
 optical glass and of optical instruments is that described by C. V. 
 Drysdale. 8 
 
 s Trans. Opt. Soc. London, p. 100, 1902; 18, 375, 1917. 
 
Chapter VII. 
 
 THE OPTICAL INSTRUMENT SITUATION DURING THE 
 
 WAR. 
 
 In the foregoing chapters a general description is given of the 
 processes of manufacture of optical glass and of the optical parts of 
 lens systems. Emphasis is placed on those processes which were 
 developed during the war period and proved to be suitable for use in 
 an emergency. No picture is, however, given of the development of 
 the optical situation, as a whole, and of the measures taken to meet 
 the ever-increasing demands of the Army and Navy for military optical 
 instruments. As a matter of record a brief sketch is presented in the 
 present chapter of the progress made and of difficulties overcome. In 
 retrospect and with the facts before us it is now a simple matter to state 
 how this and that should have been done; but at the time decisions 
 had to be made on the fragmentary evidence available. The records 
 show, that, although there was much waste effort and confusion, the 
 Army and Navy were actually supplied with most of the optical in- 
 struments which they needed; also that Army and Navy at maximum 
 strength in 1919 would have been adequately equipped with fire- 
 control and other optical instruments. 
 
 The development of the optical-glass situation is briefly described 
 in Chapter I. Several details may be added here to illustrate the 
 kinds of problems which arose and the manner in which they were 
 solved. These included problems of factory organization, of factory 
 operation, of the procurement and transportation of raw materials 
 and of glass melting pots and of coal; also the more difficult task of 
 obtaining hearty cooperation from certain manufacturers who were 
 vitally interested, but who extended formal cooperation only with a 
 strong undercurrent of passive resistance. Cooperation of this kind 
 leads to innumerable delays and unfilled promises and must be 
 treated both tactfully and firmly to accomplish the desired results. 
 
 In the game of war-time production everything is subordinated to 
 the one object of producing the desired material within a definite 
 period; the military program requires munitions of many different 
 kinds, and it is the task of the manufacturing forces of the country to 
 furnish these on time. Everything else, including expense, is for the 
 moment, subordinated to speed; as the manufacturing program 
 
 287 
 
288 OPTICAL INSTRUMENT SITUATION DURING THE WAR. 
 
 gathers momentum, the half-hearted cooperationists either learn 
 their lesson or they are eliminated. Under the stress of high-speed 
 production suggestions and plans for increasing the rate of production 
 are made; and if, after adequate test, these prove to be satisfactory, 
 they are introduced into the factory routine. Many of these sug- 
 gestions come from outside plants. 
 
 Under the emergency conditions many manufacturers of broad 
 vision are willing to exchange information on factory practice with 
 their peace-time competitors with the result that over the entire 
 country the manufacture of munitions soon attains a state of efficient 
 operations. 
 
 The procurement and the transportation of the raw materials for 
 optical glass required constant attention throughout the war. At 
 first it was necessary to locate satisfactory sources of supply for these 
 materials. Specifications had to be stated with reference both to the 
 optical glass requirements and to the chemical manufacturing pos- 
 sibilities. Many chemical manufacturing plants were visited by 
 members of the Geophysical Laboratory before the details of the supply 
 of raw materials were properly arranged. The problem of trans- 
 portation continued to be a constant source of trouble throughout the 
 war period. Innumerable delays in the shipment of raw materials 
 occurred; in many instances the General Munitions Board, and later 
 the War Industries Board and the Production Division of the Ord- 
 nance Department rendered valuable service in expediting railway 
 traffic. It is inevitable, however, that in a war-time emergency 
 railway traffic is overtaxed. Under these conditions it is the duty of 
 each manufacturer of munitions to stock, as early as possible, adequate 
 quantities of supplies so that the inevitable delays in the transporta- 
 tion of additional supplies do not retard production. At one time 
 during the early months of the war about 40 manufacturers were 
 actively assisting in the manufacture of optical glass, chiefly in the 
 supply of the necessary raw materials and of glass melting pots. 
 
 The question of fuel and gas for the glass-melting furnaces and for 
 other operations became serious during the coal shortage of the winter 
 1917-18. When it is realized that the glass plant at the Bausch & 
 Lomb factory alone consumed 33,000,000 cubic feet of illuminating 
 gas per month, a quantity sufficient to meet the needs of a city of 
 80,000 inhabitants, the scale of its fuel consumption and of the 
 difficulty of meeting the situation adequately is apparent. 
 
 From April to December, 1917, the efforts of the Geophysical 
 Laboratory were concentrated chiefly on the development of the 
 manufacture of optical glass. By December, 1917, the production 
 of pot optical glass had reached 40 tons per month; at one plant 
 (Bausch & Lomb) the processes of manufacture had been mastered 
 for the most part; and subsequent efforts were directed chiefly to an 
 
PRODUCTION OF OPTICAL GLASS. 289 
 
 increase in the manufacturing capacity for optical glass throughout 
 the country. Early in December the Geophysical Laboratory took 
 charge of the Spencer Lens plant and, as a result of hearty coopera- 
 tion on the part of this firm and a modern though small plant, was 
 able to produce satisfactory glass from the first melt on. Late in 
 December the Geophysical Laboratory assumed practical charge of 
 the Charleroi plant of the Pittsburgh Plate Glass Co. This plant 
 was an old plate-glass plant in which the Pittsburgh Plate Glass Co. 
 had installed 16 single-pot glass-melting furnaces of a blast-furnace 
 type and had tried unsuccessfully for several years to produce a 
 satisfactory product. At the time the Geophysical Laboratory arrived 
 at the plant several of the problems involved to place the plant on 
 a running basis were not at first apparent, especially the lack of 
 temperature control in the glass -melting furnaces and the impossi- 
 bility of establishing such control with the system then in operation. 
 The entire battery of furnaces was supplied by air from a single low- 
 pressure line fed by a blower operated on an electric circuit subject 
 to rapid changes in voltage; in this system a change in the flow of air 
 in one furnace affected the rate of flow of air in the remaining furnaces, 
 with the result that violent fluctuations in the temperature of each 
 furnace were the rule. Good optical glass can not be made under 
 these conditions. Each furnace had to be equipped with individual 
 blowers and many fundamental changes effected before satisfactory 
 production could be attained. This required some months for 
 accomplishment and proved to be a task of considerable difficulty, 
 partly because of a lack of appreciation on the part of this company 
 of the significance of the factors underlying the manufacture of 
 optical glass and of the fact that optical glass is not plate glass, nor 
 is a plate-glass maker necessarily a competent maker of optical glass. 
 
 In retrospect it is now evident that better progress would have been 
 made, more glass produced, and much money saved had either a new 
 optical glass plant been built or, for example, the plant of the Spencer 
 Lens Co. been expanded rather than the attempt made to remodel an 
 old plate-glass plant. Experience proved, furthermore, that in the 
 manufacture of optical glass it is better to start with new hands than 
 with plate or other glass makers, who are necessarily prejudiced and 
 do not readily change their attitude of mind toward certain factory 
 operations. The conservatism and inflexibility of the ordinary fac- 
 tory hand can be appreciated only through actual manufacturing 
 experience. 
 
 It would seem to be a wise policy for the Government in time of 
 war to concentrate its efforts on two plants for the manufacture of 
 optical glass rather than on three or more. In two plants properly 
 situated adequate quantities of optical glass can be produced to meet 
 
290 OPTICAL INSTRUMENT SITUATION DURING THE WAR. 
 
 all needs. In this connection, however, the increased destructiveness 
 of airplane bombs and other demolition agencies should be consid- 
 ered. In case an existing glass plant is ofl'ered for conversion to the 
 manufacture of optical glass, its adaptability for the purpose should 
 bo carefully considered before the conversion is sanctioned. 
 
 As a result chiefly of the efforts of the plants cited above, the actual 
 manufacturing capacity for optical glass at the time of the signing 
 of the armistice exceeded the total requirements of the Army and 
 Navy planned for 1919. The quality of the glass was, moreover, 
 steadily improving, and the inspection requirements were gradually 
 made more exacting in order that, because of unsuitable optical glass, 
 rejections in finished optics be reduced to a minimum. 
 
 In the early months of the war the production of optical glass had 
 not reached a sufficiently secure basis to permit the placing of definite 
 Government contracts for optical glass. In October, however, a 
 contract was made by the Army Ordnance Department with the 
 Bausch & Lomb Optical Co. for 3,500 pounds of different types of 
 optical glass; by that time it was realized that the manufacture of 
 optical glass was assured and that, although the consumption of 
 optical glass by this plant itself was large, there would soon be a 
 surplus of glass available for use by other optical instrument makers. 
 The first deliveries on this contract were made in November, 1917. 
 
 At the time this contract was placed there were no Government 
 inspectors available for the inspection of optical glass. Accordingly 
 it was necessary to ascertain first the relative usefulness of the different 
 methods available for the inspection of optical glass and to devise 
 new methods in case the existing methods were found to be inade- 
 quate or otherwise unsuitable. A satisfactory routine for inspection 
 was soon established and the first shipments of optical glass were 
 inspected by the writer. Army Ordnance inspectors were then 
 trained to the task and later Signal Corps and Navy inspectors. 
 In order to expedite production and to establish inspection on a 
 firm basis both Army and Navy inspectors were kept in close touch 
 with the factory operations. They inspected large quantities of glass 
 in process of manufacture before it had reached the stage for final 
 inspection and acceptance. Although this practice was different from 
 the established Government procedure of inspecting only the finished 
 articles specified in the contracts, it was found in every instance to 
 be beneficial both to manufacturer and to inspector because it led 
 to a clear understanding of the significance of the specifications and 
 of the steps essential to meet them satisfactorily. 
 
 In each Government contract for optical glass the tolerance limits 
 for refractive index and y-value were made as large as possible, about 
 0.002 in refractive index, n D , for each type of glass. But in the 
 early months of the war good melting pots were not available, and it 
 
LACK OF TRAINED PERSONNEL. 291 
 
 was difficult for the manufacturer to hold the melts of a given type 
 even to these limits. The difficulty was overcome, in part at least, 
 by shipping to a given manufacturer glass so selected from the differ- 
 ent melts that the range of refractive indices was within certain 
 tolerance limits. Unfortunately this plan was not followed consist- 
 ently because of lack of proper care at the Army and Navy distrib- 
 uting centers; as a result, many manufacturers actualty did receive 
 shipments containing glass with much wider variations in refractive 
 index than should have occurred ; much time was thereby needlessly 
 lost in changing and adapting tools to meet the relatively large 
 fluctuations in refractive index. This situation arose because of a 
 lack of appreciation on the part of certain Army and Navy officials 
 of the optical glass requirements. The same lack of optical sense 
 led one zealous officer to enter a glass plant before adequate inspection 
 had been installed, and to order that a ton of glass, which had not 
 been properly inspected, be sent to a manufacturer who was much in 
 need of optical glass. The manufacturer in turn failed to inspect the 
 plates before putting them into work with the final result that several 
 thousand otherwise high-grade prisms had to be rejected because of 
 faulty glass. This case is cited because it shows that the officer's 
 action, commendable as it may have been in some respects, led only 
 to disaster and waste of effort because the material with which he 
 had to deal was optical glass and not simply glass or copper or zinc 
 for which his order might have been justified. 
 
 War-time experience proved the need and value of trained personnel 
 to organize and coordinate scientific and technical work; it proved 
 the futility, as in the above case, of assigning any man or officer to a 
 technical task for which he has no background of experience at least 
 along similar lines. It demonstrated the necessity of centralizing 
 among a competent few the task of handling the many factors which 
 arise in connection with a technical matter such as optical glass and 
 optical instruments. The records show that this plan was finally 
 adopted with the result that in the latter part of 1918 the optical 
 glass situation was well coordinated and well in hand. 
 
 Too strong emphasis can not be placed on the need for properly 
 trained personnel to handle technical problems of this nature. In the 
 last analysis personnel is the prime factor involved, and the nation 
 which, during peace times, properly organizes and coordinates its 
 scientific and technical forces so that they can be called upon in an 
 emergency, has an immense advantage over a nation whose Army 
 and Navy are out of touch with these forces. Proper cooperation 
 between the civilian and military elements is essential in peace times 
 if there is to be an effective working together during war times. 
 Failure to appreciate this fact before the war cost this country much 
 during the war and after. In highly technical lines, especially those 
 
292 OPTICAL INSTRUMENT SITUATION DURING THE WAR, 
 
 lines which have to do with engineering and scientific problems where 
 special skill and experience are necessary, civilian experts are 
 commonly in close touch with the details and are in a position to 
 give good advice and aid. One of the chief difficulties encountered 
 in the optical instrument situation during the war was the remedying 
 of errors made by officials and officers who had been detailed to the 
 job but who lacked knowledge of the problems and were in many 
 instances a hindrance rather than a help. 
 
 This situation can be met only by a realization of the need for the 
 adequate training of a selected few men in the Army along optical 
 lines, not simply as users of optical instruments, such as range- 
 finder and field battery observers, but also, as inspectors of optical 
 instruments. It is equally essential that a certain number of civilians, 
 who are well grounded in optical theory, be instructed during peace 
 times in ordnance optical instruments, their field use, care, and 
 inspection, so that in case of war these men may be ready, as a 
 nucleus, to bear the burden of inspection and manufacturing super- 
 vision and to train others rapidly to become qualified inspectors. If a 
 peace-time policy of this sort were followed civilian interest in these 
 matters would be maintained with the result that in the event of 
 war a potential organization of high efficiency would be available. 
 War-time experience proved that men with a good working knowledge 
 of optics soon became proficient inspectors and officers competent to 
 handle optical matters; but that men, without this background of 
 optical training, assigned as officers to optical instrument problems, 
 proved incompetent and actually did more harm than good, stopping 
 production here because of the arbitrary setting of tolerance standards 
 which could not be met, and expediting production there which 
 eventually proved to be useless because of the lack of proper toler- 
 ances and competent supervision. In retrospect these occurrences 
 may be passed over with equanimity; but during the war they meant 
 constant trouble and discouragement both for the manufacturer 
 and for the field forces; nor did it increase the respect of the manu- 
 facturer to find the Army officer ignorant of the essentials of the 
 technical subject to which he had been assigned and hence incompe- 
 tent to deal with it intelligently and constructively. 
 
 On the other hand it was an inspiration to witness the zeal and 
 ability with which a certain few officers of thorough grounding 
 attacked and solved the problems; and particularly to see manufac- 
 turers lay aside other work in order to undertake the making of the 
 military optical munitions needed by the field forces. It was this 
 spirit of willingness to undertake a job and see it through, that carried 
 the manufacturing program through to a successful end in spite of the 
 lack of preparation and adequate training; but the cost of so doing 
 and of learning by experience was excessively high. 
 
PRODUCTION OF OPTICAL INSTRUMENTS. 293 
 
 Early in 1918 the requests for optical glass became so insistent 
 from so many manufacturers that in March, 1918, a special section, 
 called the military optical glass and instrument section, was created 
 by the War Industries Board to handle the situation. Mr. George E. 
 Chatillon, of New York, was appointed chairman of the section; 
 Commander W. R. Van Auken and, later, Commander H. A. Orr, 
 Navy representative, and the writer, Army representative. It soon 
 became necessary, because of the very large demands of the Army 
 and Navy for optical instruments for this section to take complete 
 charge of the industry and, by a system of permissions, to supervise 
 the entire output of all manufacturers of optical glass and of optical 
 instruments. 
 
 Commercial orders were restricted to absolute necessities; requests 
 for priority were passed upon. The optical glass, as produced, was 
 controlled and distributed by this section to the several Governments 
 departments and to manufacturers having direct Government orders. 
 It was through this absolute control of the optical glass output that 
 the entire optical industry could be so easily and effectively controlled. 
 
 Responsibility for the manufacture of the required quantities of 
 optical glass was placed by the military optical glass and instrument 
 section with the Director of the Geophysical Laboratory. Statistical 
 reports were prepared weekly at the different plants showing the 
 amounts of different types of optical glass ready for final inspection. 
 
 The requirements of the Army and Navy for optical instruments 
 were carefully tabulated and their relative needs for optical glass 
 were thereby ascertained. Allocations of optical glass were made with 
 reference to these needs. This mode of operation of the section 
 proved successful in every respect. The manufacturers, realizing the 
 necessity for centralized control, entered into the plan whole-heartedly , 
 accepted the rulings and requests of the section, and observed them 
 conscientiously. The chief function of the War Industries Board was 
 not to restrict industry but to direct it most effectively to work on 
 munitions. How this was done is a familiar story in many other 
 lines of industry and need not be repeated here. 
 
 Both the Army and Navy placed orders for optical instruments 
 during the early months of the war, at a time before intimate contact 
 with the armies in Europe had been established. With one or two 
 exceptions these orders were placed with optical-instrument makers 
 of established reputation who were in a position to produce the 
 articles ordered, providing the necessary optical glass were made 
 available. The orders, moreover, were for certain standard instru- 
 ments, such as range finders, aiming circles, battery-commander tele- 
 scopes, naval gun sights, field glasses, camera lenses, etc. These 
 orders effectively covered the entire manufacturing capacity in this 
 country for 1917 and for part of 1918. The policy of placing orders 
 _h firms of recognized standing and, in case an inexperienced firm 
 
294 OPTICAL INSTRUMENT SITUATION DURING THE WAR. 
 
 solicited a contract, of awarding a part only of the required number 
 to the new firm and the remainder to an old firm, proved to be a wise 
 policy. The old firms proceeded to fill the contracts and in time 
 delivered the instruments; many of the new firms required assistance, 
 and in this connection the War Industries Board aided materially. 
 
 By the end of 1917 requests for many new types of instruments, 
 which the European armies had found useful in the field, began 
 to flow in from the American Expeditionary Forces; the optical 
 industry was then confronted with the situation that its available 
 capacity had already been bespoken for some months to come. The 
 result was an energetic appeal from the several departments of the 
 Army and Navy to each manufacturer to accept more orders, each 
 department considering only its own needs. Careful search was 
 made for manufacturers who might be persuaded to undertake optical 
 work. By February, 1918, the general confusion which existed 
 because of the many diverse, noncoordinated agencies at work led 
 to the realiztion of the need for centralized control and in a short 
 time furnished additional reasons for the formation of an optical 
 section on the War Industries Board. Much credit is due Mr. 
 Chatillon, chairman of this section, for his effective treatment of the 
 situation arid the organization and coordination of the many factors 
 involved. 
 
 In April, 1918, an army commodity committee on optical glass 
 and optical instruments was formed with the Army representative 
 on the optical section of the War Industries Board as chairman, for 
 the purpose of coordinating the Army needs for optical instruments. 
 This committee functioned under the Director of Purchases and Sup- 
 plies. It held monthly meetings and served to bring together the 
 Army officers directly concerned with optical munitions. One of 
 the results of the efforts of the Director of Purchases and Supplies 
 was the consolidation, in the Ordnance Department, of the procure- 
 ment of all optical munitions for the Army with the exception of a 
 few special instruments. Strenuous objection to the consolidation 
 was made at first by certain Army departments on the plea that 
 special knowledge was required to purchase the optical instruments 
 required by them; but the results of this consolidation proved that 
 the fears thus expressed were groundless, partly because the officer, 
 Lieut. M. P. Anderson, in the Ordnance Department, directly respon- 
 sible for the procurement of optical munitions, was thoroughly 
 trained to the task and was experienced in optical matters. 
 
 From April to August, 1918, an earnest effort was made by the 
 War Industries Board, the Army, and the Navy to interest manu- 
 facturers who might undertake the production of optical munitions. 
 The result was that many firms, who had never done work of this 
 kind, patriotically accepted contracts for instruments or optical or 
 
OPTICAL TRAINING SCHOOLS. 295 
 
 mechanical parts, and at the time of the signing of the armistice 
 were beginning to produce satisfactorily. In general the policy was 
 followed to let contracts for instruments of high precision to expe- 
 rienced firms and to reserve the simple types of instruments for the 
 less experienced makers. 
 
 As a further incentive to expedite production, the Ordnance 
 Department established a training school for lens and prism oper- 
 atives at Rochester, N. Y. It was felt that, were the opportunity 
 offered, many young women might engage to do work of this kind 
 during the war period. At the Rochester school a number of oper- 
 atives were thus trained, but the results were not altogether satisfac- 
 tory, chiefly because of a lack of real interest and support on the 
 part of -the large optical manufacturers in Rochester. A school of 
 this nature to be successful requires the enthusiasm and interest of 
 good teachers, and these evidently could not be spared by the manu- 
 facturers. 
 
 A second, more successful school was established at Pasadena, 
 Calif., for the training of expert grinders and polishers for precision 
 optics, chiefly roof-angle prisms. This school was operated in con- 
 nection with the optical- shop of the Mount Wilson Observatory of 
 the Carnegie Institution of Washington. Credit is due to Mr. G. W. 
 Ritchey for his efficient operation of this school and shop. 
 
 The optical industry during the war proved equal to the emergency 
 and in November, 1918, was rapidly approaching the peak value of 
 deliveries. Production and deliveries were proceeding on an immense 
 scale. By this time the industry had been well organized and coor- 
 dinated. New additions had been made to old factories, new factories 
 had been erected and a manufacturing capacity for optical ins trumehts 
 adequate to supply the needs of both the Army and Navy for 1919 
 was available. This condition had been practically attained by 
 September, 1918, so that the optical section of the War Industries 
 Board was able to devote part of its time to the question of design of 
 optical instruments with a view toward simplification and standardi- 
 zation. Many of the instruments in ' course of manufacture were 
 direct copies of European instruments, and manufacturers com- 
 plained because the designs had not been made with reference to 
 American methods of quantity production. In many instruments a 
 great saving both in time and expense could have been effected by 
 appropriate changes in design without affecting in the least the field 
 performance of the instrument; but to accomplish this properly 
 requires time and much consultation. Under the circumstances, it was 
 decided best to go ahead with the production of usable designs, even 
 if they were direct copies of European instruments and thus to insure 
 the desired output. As soon as adequate production had been estab- 
 
296 OPTICAL INSTRUMENT SITUATION DURING THE WAR. 
 
 lished, steps could be, and actually were, taken to improve and to 
 standardize the designs of many of the instruments. 
 
 With the signing of the armistice the need for further production 
 ceased. Many contracts were canceled outright and appropriate 
 settlements were made; other contracts were canceled in part, and 
 still others were allowed to continue to completion. Manufacturers, 
 who had engaged in optical work as a patriotic duty only, returned 
 to their normal peace-time activities. The optical instrument manu- 
 facturers themselves were soon deluged with commercial orders, 
 because of the release of all restrictions and control of the industry 
 by the War Industries Board on November 16, 1918. Thus the transi- 
 tion from war-time to peace- time" activities was begun. 
 
 After the Civil War there was no corresponding adjustment of the 
 optical industry. The total value of orders for optical apparatus dur- 
 ing the Civil War was almost negligible. During the recent war, the 
 total value of Army and Navy orders for optical munitions was over 
 $65,000,000, of which about $50,000,000 was for the Army. What it 
 may be in another war a century, or even some decades hence, no one 
 .can tell; but, if the lessons of the recent war have not been entirely 
 forgotten this country will be in a better position to produce the neces- 
 sary optical munitions than it was in April, 1917. The purpose of the 
 present report will have been accomplished if a small part of the 
 experience gained in the recent war in the production of fire-control 
 and other optical apparatus is not entirely lost, but is available for use 
 in an emergency. 
 
 In the manufacture of optical glass it was fortunate for this country 
 that war was not declared in the first decade of this century rather 
 than in the second. In 1905 little was known of the chemistry of 
 silicates at high temperatures and there was then no adequate per- 
 sonnel available for the task. The records show that during the recent 
 war we received no assistance, either direct or indirect, from other 
 countries in the solution of the problems of optical glass manufacture. 
 The details of optical glass manufacture were held secret in all 
 European countries and were not to be divulged, even under the 
 stress of war-time needs. Fortunately, it proved possible for us to 
 make good optical glass without the knowledge of these secrets, 
 which, in point of fact, may prove to be Jouble-edged and serve to 
 impede progress and to be an actual hinderance rather than an aid 
 to a manufacturer if he holds them in too high esteem. This 
 would no doubt have been equally true in 1905. Although large 
 quantities of optical glass are not needed, optical glass is an essential 
 part of all instruments of observation. The optical glass industry is, 
 therefore, a singularly important key industry, the control of which 
 means control over a whole series of industries and of research and 
 technical laboratories and institutions. It was probably for this 
 
IMPORTANCE OF TRAINED PERSONNEL. 297 
 
 reason that we were unable to secure any information regarding its 
 manufacture from other countries. 
 
 It was fortunate for us that before this country entered the war 
 certain manufacturers had realized the need for optical glass and had 
 erected optical glass plants of satisfactory design. They had, more- 
 over, produced some optical glass of fair quality and had accumu- 
 lated a good foundation of experience on which it was possible to build. 
 This would not have been the case in 1905. It was also fortunate that 
 there existed at the beginning of the war a scientific research labora- 
 tory with a personnel experienced, not in the manufacture of optical 
 glass, but in the fundamental principles underlying its manufacture. 
 The resources of this laboratory were offered to the Government in 
 the emergency; the offer was accepted and the problem of optical- 
 glass manufacture was attacked as would any similar research 
 problem have been attacked. The research investigator is constantly 
 facing new and unexpected problems in his own work, and, as a result, 
 develops resourcefulness and adaptability as well as the faculty of 
 proceeding directly to the essentials of a problem. Shortly after the 
 work was entered upon the more important defects of the existing 
 practice were discerned by the scientists and appropriate remedies 
 were devised to overcome them. 
 
 It is only just to admit that in addition to the many other elements 
 essential for the production of optical glass in this country we were 
 favored by good fortune and made the best of it. If a future emer- 
 gency should arise it can hardly be expected that a laboratory of 
 similar experience will be available or that the manufacturers will 
 have builded so wisely beforehand. It is therefore a wise policy for 
 the Government to encourage the manufacture of optical glass in 
 this country by placing orders, during peace times, for optical glass 
 with the manufacturers, thereby building up a reserve stock of good' 
 glass against a possible emergency. This policy is being followed 
 by certain other countries. In the case of optical instruments similar 
 arguments are valid; but the fact that new instruments are being 
 devised constantly makes it inadvisable to store large stocks of these 
 instruments. The maintenance of optical repair and manufacturing 
 plants by the Army and Navy is a wise provision, because these 
 function primarily as training schools for personnel against the 
 hour of need. 
 
 In view of the highly technical nature of optical instruments both 
 in design, production, and repair it is desirable that a single optical 
 plant with competent personnel serve the entire Army. It is not 
 feasible and would be unwise for the different departments of the 
 Army each to maintain separate optical shops or even separate sec- 
 tions for the design only of optical instruments. The argument has 
 been advanced that in any given branch of the service only the 
 
298 OPTICAL INSTRUMENT SITUATION DURING THE WAR. 
 
 members of that service have adequate knowledge of the field con- 
 ditions under which it operates and that, therefore, they alone are 
 competent to design the apparatus needed. This is, no doubt, true 
 for much of the material required, but in the case of optical instru- 
 ments special conditions exist which render this plan unworkable. 
 Optical engineers of experience are scarce and unwilling to spend their 
 entire time on the few problems offered by any single branch of the 
 service. The particular problems, moreover, which any Army 
 department considers peculiar to itself have much in common with 
 those of other departments; and for each department to maintain a 
 separate section for the design of optical instruments would mean 
 serious and inefficient duplication of work with consequent waste of 
 time and of public money. 
 
 The fact that both in war time and in peace time the Ordnance 
 Department is responsible for the bulk of the Army optical work 
 and has built up a satisfactory optical shop at Frankford Arsenal 
 with competent personnel indicates that all Army optical work 
 should be done there; also that Frankford Arsenal should be 
 made responsible for the design and production of all experi- 
 mental optical instruments suggested to meet any new conditions 
 which may arise. The special knowledge of the conditions under 
 which the proposed new instrument is to function can be given 
 by officers or civilians of the special branch of the service which 
 is to use the instrument; but the design of the instrument to sat- 
 isfy these conditions and to be at the same time a manufacturing 
 possibility should be made at Frankford Arsenal as the logical optical 
 organization within the Army. It is imperative that the Army and the 
 Navy each maintain an optical plant not only for the design and test, 
 production, and repair of military instruments, but also for the training 
 of personnel along these lines. It is for this* reason especially that no 
 other department of the Government can function satisfactorily for 
 the Army and Navy in this respect, aiid that the Army and Navy 
 optical shops can not well be consolidated into one large shop. 
 
 WAR DEPARTMENT, 
 
 OFFICE OF THE CHIEF OF ORDNANCE, 
 
 Washington, May, 1921. 
 Form No. 2037. 
 Ed. Mar. 17-21-900. 
 
INDEX. 
 
 Page. 
 
 Abbe, E. : New types of glasses 44, 45, 49 
 
 Aberration, chromatic 23, 24, 26, 283, 284 
 
 Aberration, spherical 24, 25, 265, 268, 283 
 
 Aberration, spherical overcorrected 25 
 
 Aberration, spherical undercorrected 25 
 
 Aberrations, monochromatic 24 
 
 Acetic acid test 222 
 
 Adams, L. H.: Optical glass group, 10, 13; strain birefringence, 39, 40, 182. 
 184, 186; viscosity, 149, 150, 151; temperature distribution, 153; elastic 
 
 constants, 159 ; annealing schedule 187 
 
 Aiming circles 293 
 
 A ir bells : 33 
 
 Alkalinity, natural 221 
 
 Alkalinity, weather : 221 
 
 Allen, E. T.: Optical glass group, 10, 12; chemical analyses, 62, 63, 64, 66, 75; 
 
 oxidation of arsenic, 105: heat effect 152 
 
 Andersen, O. : Optical glass group, 10, 12, 13 ; flint glasses 70 
 
 Anderson, M. P. : Fire-control instruments 294 
 
 Annealing, classes of 187 
 
 Annealing furnace 180 
 
 Annealing of molded or pressed plates 179-189 
 
 Annealing period 148-163 
 
 Annealing schedules 187, 188 
 
 Annealing, state of 38, 41 
 
 Apparatus for inspection of strain 207, 208 
 
 Artificial star test 266 
 
 Astigmatism 26, 268, 283, 284 
 
 Autoclave tests 222 
 
 Autocollimation method 274-276, 279, 280 
 
 Autocollimator 271 
 
 Axial angle error 270 
 
 Axial plane 270 
 
 Baking of pot 116 
 
 Barium carbonate 86 
 
 Barium crown 45, 47, 48, 53, 70, 151, 153, 165, 169 
 
 Barium crowns, chemical analyses 60 
 
 Barium crown, dense 51, 53, 74, 151, 165, 169 
 
 Barium crown, dense, elastic constants 160 
 
 Barium crown, elastic constants 160 
 
 Barium disilicate 36, 148 
 
 Barium flint 45, 70, 74, 151, 153, 169 
 
 Barium flint, chemical analyses 61, 70 
 
 Barium flint, elastic constants 160 
 
 Batch, filling in 117 
 
 Batch mixer, mechanical 113 
 
 Batch, mixing of 111-114 
 
 Batches 99-110 
 
 299 
 
300 
 
 Page, 
 
 Batches, computation of 100-110 
 
 Battery commander telescopes 293 
 
 Bausch & Lomb Optical Co.: Optical glass manufacture, 9, 11, 12; production 
 of optical glass, 15; refractivity, 41; optical glass types, 73, 74; cooling of 
 glass, 163, 175; annealing of glass, 187; gas consumption, 288; contract 
 
 for glass 290 
 
 Bausch, Wm. : Optical glass manufacture 11 
 
 Bichowsky, R. : Optical glass group r 10; autoclave tests 222 
 
 Bleininger, A. V. : Optical glass group, 13; cast procelain-like pots 87, 89 
 
 Blocking 125 
 
 Blocking tool 240 
 
 Boil '. , 33 
 
 Boil, open 125 
 
 Borate flint 45 
 
 Borate glasses, chemical analyses 62 
 
 Borate optical glass 45, 47, 50, 53, 152 
 
 Boric acid 86 
 
 Borosilicate crown 45, 70, 73, 151, 153, 165, 169 
 
 Borosilicate crown, chemical analyses 59 
 
 Borosilicate crown, batch composition of 107, 108 
 
 Borosilicate crown, elastic constants '. 160 
 
 Borosilicate flint, chemical analyses 61, 70 
 
 Bowen, N. L.: Optical glass group, 10, 12, 13; crystallization phenomena, 104, 
 
 132, 142; stones, 135; identification of crystallites 147, 148 
 
 Bowlders 158 
 
 Breaking up pot of glass . . . .' 166 
 
 Brewster, David: Strain-birefringence 40, 150, 182 
 
 Bubbles 29, 33, 34, 38 
 
 Bubbles, inspection for 205, 269 
 
 Buckeye Clay Pot Co. : Melting pots 87 
 
 Bureau of Standards: Optical glass group, 9, 13, 15; optical glass types, 73, 74; 
 
 melting pots, 87, 89; molding glass 177, 236 
 
 Calcium carbonate 86 
 
 Calcium metasilicate 148 
 
 Carbonbisulphide 173, 200 
 
 Carnegie Institution of Washington : Manufacture of optical glass 10, 295 
 
 Carr-Lowrey Glass Co. : Manufacture of optical glass 13 
 
 Casting of optical glass 167-170 
 
 Casting temperatures 167 
 
 Cauchy, A. L. : Dispersion formula, 55, 56. 57 
 
 Cauwood, J. D. : Opalescence in glass 104 
 
 Cementing of lenses 254-256 
 
 Chance Bros. : Transparency of glass, 54, 55; chemical compositions 62, 66, 67 
 
 Characteristics of optical glass 16-80 
 
 Chatillon, G. E.: War Industries Board 293, 294 
 
 Chemical composition 58, 59 
 
 Chemical compositions 66-73 
 
 Cheshire, R. W. : Immersion method 202 
 
 Chromatic differences of magnification 24, 27 
 
 Chromatism, axial 20 
 
 Chromatism, lateral 27 
 
 Civil War 296 
 
 Civil War practice 7 
 
INDEX. 301 
 
 Page. 
 
 Clark, J. R. : Optical glass compositions 110 
 
 Cloudiness 29, 37, 144-148 
 
 Collimator 263 
 
 Color, freedom from 29, 74-76 
 
 Coma 25, 268, 283, 284 
 
 Composition, refractivity relations 58 
 
 Compressibility, modulus of 160 
 
 Compressibility 159, 160 
 
 Concave mirror test 199 
 
 Cones, Seger 94 
 
 Constant deviation method 273 
 
 Constringence, optical 43 
 
 Convection currents 138, 139 
 
 Cooling of glass plates 183 
 
 Cooling of melt 137-148 
 
 Cooling rate, insulation for 165 
 
 Cords 29, GO, 31 
 
 Council of National Defense: Optical glass manufacture 10 
 
 Cragoe, C. H.: Thermal dilatation of glass 152, 159 
 
 Cristobalite H8 
 
 Crown optical glass 28, 153 
 
 Crystallization bodies 37 
 
 Crystallization bodies, inspection for 205, 269 
 
 Crystallization phenomena 139 
 
 Crystallites 29, 36, 37 
 
 Crystallites, identification of 147, 148 
 
 Cullet 113, 174 
 
 Curvature of field 26, 268, 284 
 
 Cutting of glass 229-231 
 
 Czapski, S. : Strain birefringence 39, 40, 41 
 
 Day, A. L. : Optical glass manufacture, 10; heat effect in borate glass 152 
 
 Definition 18, 268 
 
 Devitrification 37 
 
 Diameter of entrance pupil 281 
 
 Diameter of exit pupil 281 
 
 Diamond saws 230 
 
 Diaphragm, field of view 20 
 
 Diffraction pattern 18 
 
 Dimming test 219 
 
 Dispersion formulae 55, 56 
 
 Dispersion ratios 29, 43 
 
 Dispersivity 29, 42, 48 
 
 Distortion 26, 283 
 
 Dodd, L. E. : Inspection for striae 203 
 
 Doelter, C. : Chemical analyses, 59, 66; oxidation of arsenic 105 
 
 Double images 283 
 
 Dove reflecting prism 244, 270, 275-277 
 
 Draper, H. : Grinding and polishing 225 
 
 Drysdale, C. V. : Inspection apparatus 286 
 
 Eastman Kodak Co. : Ray filters 213 
 
 Edging of discs 231 
 
 Elastic constants 159, 160 
 
 Elsden, A. V. : Dimming test 219 
 
 3922921 20 
 
302 INDEX. 
 
 Entrance port 20 
 
 Entrance pupil 20 
 
 Entrance pupil, diameter of 281 
 
 Entrance window 20 
 
 Everett, A. : Chemical analyses, 66; iodoeosin test 220 
 
 Everett, J. D. : Chemical analyses, 66; iodoeosin test 220 
 
 Exfoliation 158 
 
 Exit pupil 20 
 
 Exit pupil, diameter of 281 
 
 Exner, F. : Strain-birefringence 40 
 
 Expansion coefficient 153, 160, 182 
 
 External focal length of lens 263-267 
 
 Eye distance 282 
 
 Eye, far sighted 17 
 
 Eye, functions of 16 
 
 Eye, iris of 19 
 
 Eye, near sighted 17 
 
 Feathers 34, 35, 175, 236 
 
 Feathers, inspection for 205 
 
 Fenner, C. N.: Optical glass group, 10, 12, 13; glass compositions, 62, 63, 64; 
 furnace temperatures, 94, 95; opalescence in glass, 104; batch composi- 
 tions, 110: glass stirring schedule, 128, 129; forms of silica, 148; pot arch 
 
 cooling 1 fi3 
 
 Ferguson, J. B.; Optical glass group, 10, 13; reflection of light, 77; determina- 
 tion of iron, 79; bleaching of pots 91 
 
 Meld glasses 293 
 
 Field of view 17, 19 
 
 Field of view, real and apparent 283 
 
 Film on inclosed glass surfaces 223 
 
 Film on lens surfaces 284 
 
 Film on optical glass surfaces 80 
 
 Fining period 117 
 
 Fire control instruments 6 
 
 Flare 283 
 
 Flint, dense 48, 53, 74, 151, 165, 169 
 
 Flint, dense, batch composition of 106 
 
 Flint, densest 48 
 
 Flint, elastic constants 160 
 
 Flint, extra dense.- 48, 52, 53, 54, 74, 151 
 
 Flint, light 48, 52, 53, 74, 151, 153, 165, 169 
 
 Flint optical glass 28 
 
 Flint series 68, 70 
 
 Flint series, batch compositions 101-102 
 
 Flints, chemical analyses 60 
 
 Focus of lens , 23 
 
 Foerster, F. : Weather stability , 219 
 
 Focal length of lens -. 263-267 
 
 Folds 175 
 
 Folds, inspection for 205 
 
 Foucault test 265 
 
 Fracturing of optical glass 161 
 
 Frankford Arsenal: Inspection of prisms, 268, 270, 273, 274; optical shop 298 
 
 Fraunhofer, J. : Optical glass 121 
 
INDEX. 303 
 
 Page. 
 
 French, ,T. W. : Grinding and polishing processes 225, 240. 251-254 
 
 Fresnel reflection formula 216 
 
 Frink, R. L.: Arsenious oxide 105 
 
 Fry, H. C. , Jr. : Lens and prism pressing 177, 235 
 
 Fry Co., H. C. : Optical glass manufacture 13 
 
 Furnace, annealing 180 
 
 Furnace, muffle 234 
 
 Furnace operations 114-134 
 
 Furnace operations, schedule of 133 
 
 Furnace, pressing 233 
 
 Furnace schedule, 24-hour 132-134 
 
 Furnace temperatures 93-98 
 
 Furnaces , 92-98 
 
 Gauss points 264, 266 
 
 General Munitions Board : Manufacture of optical glass 288 
 
 Geological Survey: Raw materials 13, 87 
 
 Geophysical Laboratory: Optical glass manufacture, 10, 12, 13, 288, 289, 293; 
 
 chemical analyses, 66, 70; bleaching of pots, 91; batch compositions, 110; 
 
 glass melting schedule 118, 119, ; photometer 212, 213 
 
 Gifford, J. W.: Refractive indices : 56, 71 
 
 Gill Clay Pot Co.: Melting pots 87 
 
 Glazing of pot 116 
 
 Griffiths, C. H.: Flim on glass 223 
 
 Grinding and polishing of pressed plates 189-192 
 
 Grinding of lenses 246 
 
 Grinding of prisms , 236-241 
 
 Grinding operations ' 228. 229 
 
 Grinding processes 250-254 
 
 Guinand, P. L. : Stirring of optical glass 10, 121 
 
 Hall, R. E.: Optical glass group 10 
 
 Halle, Bernhard: Grinding and polishing 225 
 
 Halowax oil 173 
 
 Ham, W. R. : Precision milling of prisms 245 
 
 Hand stirring 121, 122 
 
 Harcourt, W. V. : New optical glass types 44, 49 
 
 Hardness 29 
 
 Hartmann, J. : Optical constants of a lens 264 
 
 Hartmann-Tillyer test 264 
 
 Hazel Atlas Glass Co. : Manufacture of optical glass 13 
 
 Homogeneity 29 
 
 Hcpkinson, F. : Thermal elastic stresses in glass 153, 182 
 
 Hostetter, J. C.: Optical glass group, 10, 13, 16; bleaching of pots, 91; rolled 
 
 optical glass 169 
 
 Hovestadt, H.: Dispersion, 55; chemical analyses, 59, 66; effect of boron, 
 
 103; weather stability 220 
 
 Howe, H. E. : Manufacture of optical glass 10 
 
 Ihmori, T.: Weather stability 220 
 
 Illumination, intensity of 17, 18, 19 
 
 Image, quality of 22, 284 
 
 Immersion method for inspection 172-174, 199-202 
 
 Inclusions 29 
 
 Inspection of finished optical parts 257-280 
 
 I nspection of lenses 260-269 
 
304 INDEX. 
 
 Page. 
 
 Inspection of optical glass 194-223 
 
 Inspection of optical systems 280-286 
 
 Inspection of prisms 269-280 
 
 Inspection of raw glass 172 
 
 Insulation, rate of cooling . 165 
 
 lodoeosin test 220 
 
 Jenks, G. F. : Inspection of prisms 270 
 
 Jewell, L. E.: Testing of lenses 266 
 
 Joint cracks 162 
 
 Jointing 162 
 
 Jones, H. S.: Dimming test 219 
 
 Kellner, H. : Reflection of light 77 
 
 Kerr, J. D. : Strain-birefringence 39 
 
 Keuffel & Esser Co.: Manufacture of optical glass, 9.13, 286; optical glass types 
 
 73, 74;photometer 212 
 
 Keuffel, Carl : Manufacture of optical glass 13 
 
 Key industry 296 
 
 Kiess, C. C. : Optical glass group t 13 
 
 Knife-edge test ' 265 
 
 Koenig-Martens photometer 213, 214, 215, 217, 218, 285 
 
 Koenig polarization photometer .. . 213 
 
 Kollmorgen, F. : Treatment of glass surfaces . 77 
 
 Laclede Christy Co. : Melting pots 87 
 
 Laps 34, 175 
 
 Laps, inspection for 205 
 
 Lead oxide 86 
 
 Lehr, annealing in. . . 188 
 
 Lenses, defects in 267 
 
 Lens, external focal length 263-267 
 
 Lens, focal length of 263-267 
 
 Lens, mechanical defects in 262 
 
 Lens, optical qualities 262, 263 
 
 Lens, physical defects in . . 260-262 
 
 Lenses, cementing , 254-256 
 
 Lenses, flat 246 
 
 Lenses, grinding 246 
 
 Lenses, of steep curvature 248 
 
 Lenses, photographic 31 
 
 Lenses, polishing 246 
 
 Lenses, pressing '. 232-236 
 
 Light transmission 209 
 
 Light transmission of telescope 285 
 
 Linear expansion 160 
 
 Liquid inspection 174 
 
 Littleton, J. T.: Viscosity of glass 149, 150 
 
 Lombard, R. H. : Optical glass group 10, 13 
 
 Losses, percentage of glass 191 
 
 Magnetic separator Ill 
 
 Magnification - . - : 19, 21 
 
 Magnifying power 282 
 
 Manufacture of lenses and prisms 224-256 
 
 Manufacture of optical glass 81-193 
 
 Manufacturing problem 8 
 
INDEX. 305 
 
 Page. 
 
 Marbles 157, 158, 162 
 
 Martin, L. C.: Film on glass - 223 
 
 Martin, Victor: Manufacture of optical glass 11 
 
 Measurement of light transmission 209-21 7 
 
 Mechanical defects in a lens 262 
 
 Melt, cooling of 137-148 
 
 Melting and fining 117 
 
 Melting pots " 87 
 
 Menvin, H. E. : Optical glass group 10 
 
 Michelson, A. A. : Striae in optical glass 31 
 
 Milky glass 145-148 
 
 Milling of prisms 245 
 
 Milling operations 228, 229 
 
 Molding process 174, 177-179 
 
 Molds, pressing 234 
 
 Molecular ratios 106 
 
 Molecular weights, table of 106 
 
 Monobromnaphthaline 173, 200 
 
 Monochlornaphthaline 173-201 
 
 Morey, G. W.: Optical glass group, 10, 12, 13: 24-hour melting schedule, 118, 
 
 132, 134; annealing of glass 187 
 
 Mount Wilson Observatory: Optical training school 295 
 
 Muffle furnace 234 
 
 Mylius, F. : Weather stability, iodoeosin test 219, 220 
 
 National Optical Co. : Chemical analysis 62 
 
 Natural alkalinity 221 
 
 Naval gun sights 293 
 
 Neumann, C. : Thermal elastic equations 153 
 
 Newton interference colors 242, 243 
 
 Nodal points of lens 23 
 
 Xutting, P. G. : Dispersion formula : 56 
 
 Objective telescope 21 
 
 Onions 157, 162 
 
 Opalescence 29, 144-148 
 
 Operations, grinding and milling 228, 229 
 
 Optical glass, annealing schedules for 188 
 
 Optical glass, characteristics of 28 
 
 Optical glass, chemical analyses 59-65 
 
 Optical glass, coefficient of linear expansion 160 
 
 Optical glass, composition of 44 
 
 Optical glass, cooling of 154-158 
 
 Optical glass, expansion coefficient of 153 
 
 Optical glass, modulus of compressibility 160 
 
 Optical glass, modulus of rigidity : 160 
 
 Optical glass, physical and chemical constants 160 
 
 Optical glass, strain-birefringence 160 
 
 Optical glass, thermal diffusivity 160 
 
 Optical glass plant, organization of 81, 82, 83 
 
 Optical glass, rate of cooling 165 
 
 Optical instrument situation during the war 287-298 
 
 Optical munitions, orders for 296 
 
 Optical parts, inspection of 257-280 
 
 Optical qualities of a lens .- 262, 263 
 
306 INDEX. 
 
 Page. 
 
 Optical systems, inspection of 280-286 
 
 Ordinary crown 73, 151, 153, 165, 169 
 
 Ordinary crown, elastic constants 160 
 
 Ordinary crowns, chemical analyses 59 
 
 Ordnance Department 13, 288, 290, 294, 295, 298 
 
 Orr, H. A. : Navy representative 293 
 
 Panoramic sight 31, 244 
 
 Parra-Mantois: Optical glass types 39, 49, 51, 71, 72, 185 
 
 Partial dispersion ratio 43, 44, 49 
 
 Partial dispersions 50, 51, 52, 53 
 
 Peddle, C. J. : Optical glass compositions 110 
 
 Pentaprism 270, 271, 278-280 
 
 Personnel 292 
 
 Personnel, importance of 81-83 
 
 Peters, C. G. : Thermal dilatation of glass 152, 159 
 
 Phosphate glasses, chemical analyses 62 
 
 Phosphate optical glass 45, 50 
 
 Photographic lenses 293 
 
 Physical defects in a lens 260-262 
 
 Pittsburgh Plate Glass Co.: Manufacture of optical glass, 9, 12, 13, 289; pro- 
 duction of optical glass, 15; dips of glass, 41 ; chemical analyses, 62 ; optical 
 glass types, 73, 74; melting pots, 89; cooling of glass, 164, 165; liquid 
 
 inspection 173 
 
 Photometer, bench 212,286 
 
 Photometer, polarization 212, 285 
 
 Photometer with neutral tint wedges 212 
 
 Pockels, F. : Strain-birefringence 39, 40, 182 
 
 Poisson's ratio 159 
 
 Polarization photometer 212,285 
 
 Polish, lack of 284 
 
 Polished surfaces, treatment of 76-79 
 
 Polishing of lenses 246 
 
 Polishing of prisms 241-245 
 
 Polishing process 242 
 
 Polishing processes 251-254 
 
 Polishing tool 241 
 
 Posnjak, E. : Optical glass group, 10; chemical analyses 63, 64, 66 
 
 Pot, baking of 116 
 
 Pot, cooling of 130 
 
 Pot, glazing of 116 
 
 Pot of glass, breaking up 166 
 
 Pot, preheating of 114 
 
 Pot, removal from furnace 130 
 
 Pot, setting of : 115 
 
 Pot wagon 115 
 
 Pots, bleaching of 90 
 
 Pots, chlorination of 91 
 
 Pots of optical glass, stirred and unstirred 134-137 
 
 Pots, open versus closed 89 
 
 Pots solution 137 
 
 Pot-arch cooling 163, 164 
 
 Potasium carbonate ......,.., 85 
 
 Preheating of pot 114 
 
 Preparation of glass for lenses and prisms 225 
 
INDEX. 307 
 
 Page. 
 
 Preparation of raw pot glass 170 
 
 Pressing defect 30, 34, 35, 175 
 
 Pressing defects, inspection for 205, 269 
 
 Pressing furnaces 233 
 
 Pressing of lenses and prisms 232 
 
 Pressing process 174-177 
 
 Principal points of lens 23 
 
 Prism, Dove reflecting 244 
 
 Prism, resolving power 280 
 
 Prism, roof-edge 244 
 
 Prisms, grinding of 236-241 
 
 Prisms, inspection of 269-280 
 
 Prisms, polishing of 241-245 
 
 Prisms, pressing 232-236 
 
 Prism, right angle 244, 271-275 
 
 Prisms, striae in 31, 32 
 
 Prism surfaces 270 
 
 Production of optical glass 14 
 
 Projection methods 202 
 
 Pulfrich, C. : Dispersion formula 55 
 
 Pyrometer, Fery radiation 95 
 
 Pyrometer, optical 94-98 
 
 Quality of image 284 
 
 Quartz 148 
 
 Rand, C. C. : Chemical analyses 58 
 
 Range finders 31, 293 
 
 Raw materials 12, 83-87 
 
 Raw materials, specifications for 86, 87 
 
 Rayleigh, Lord: Thermal-stress relations, 153; grinding and polishing proc- 
 esses 225, 250, 252 
 
 Ream 29, 30, 32, 169 
 
 Reflecting power 76-79 
 
 Reflectivity 76 
 
 Refractivity 29, 41, 46, 47 
 
 Refractive index, variations in 72 
 
 Repair shops 297 
 
 Resolving power 17, 20, 21, 268, 269 
 
 Resolving power of prisms 280 
 
 Resolving power of telescope 283 
 
 Reticules 32 
 
 Ribbons 29 
 
 Rigidity, modulus of 160 
 
 Ritchey, G. W.: Grinding and polishing process, 225, 241; optical training 
 
 school 295 
 
 Roberts, H. S.: Optical glass group, 10,13; bleaching of pots, 91; annealing 
 
 period, 153-158; cooling of optical glass melts 164, 165, 169 
 
 Roberts, E. H. : Viscosity of glass 149, 150 
 
 Roberts, O. : Dimming test 219 
 
 Rolled optical glass '. 32, 167-170 
 
 Roof-angle prism 270, 271, 277, 278, 295 
 
 Roof-edge prism . 244 
 
 Rosenhain, W. : Grinding and polishing processes 250 
 
 Rough grinding 191 
 
 Rubens, H. : Dispersion in optical glasses 45, 46, 52 
 
308 INDEX. 
 
 Page. 
 
 Ryland, H. S.: Film on optical glass 223 
 
 Sand 84 
 
 School for training operatives _ _ 295 
 
 Schott, O.: Strained glass 41; new types optical glass 44, 45, 49; strain bire- 
 fringence. 153 
 
 Schott & Genossen: Types of optical glass 39, 51, 56, 57, 66, 67, 71, 72, 163, 185 
 
 Schulz, H. : Thermal-stress relations 153, 182 
 
 Seeds 33 
 
 Seeds, inspection for 205, 269 
 
 Sellmeier : Dispersion formula 55 
 
 Setting of pot 115 
 
 Side angle error 270 
 
 Signal Corps x 290 
 
 Silica 147 
 
 Silfcmanite 141, 148 
 
 Simon, H. T. : Dispersion in optical glasses 45, 46, 52, 54 
 
 Sine condition 24, 25 
 
 Sizes of blanks , . . 226 
 
 Skimming '. 120 
 
 Smith, T. T. : Inspection for striae, 203; resolving power 283 
 
 So, M.: Viscosity of glass.. 149 
 
 Sodium carbonate 86 
 
 Spectrum, secondary 26, 108 
 
 Spencer Lens Co.: Manufacture of optical glass, 9, 12, 289; production of opti- 
 cal glass, 215; chemical analyses, 62; types of optical glass, 73, 74; pot arch 
 
 cooling 163 
 
 Spherical zones 25 
 
 Spherulites 36 
 
 Spherulites, radial 37 
 
 Stability, chemical 29 
 
 Stability of optical glasses 218 
 
 Stability, physical 29 
 
 Stirring machine 122,123,126 
 
 Stirring process 120, 123-129 
 
 Stirring rod % 122 
 
 Stokes, G. G. : New types of optical glass 44 
 
 Stones. . 29, 34, 35, 36 
 
 Stones, inspection for 205, 269 
 
 Strain 29, 37 
 
 Strain, inspection for. . . '. 206 
 
 Strain birefringence 39 
 
 Strain birefringence 160 
 
 Striae 29, 30, 31, 38 
 
 Striae, inspection for 195-205, 269 
 
 Taylor, H. D. : Treatment of glass surfaces 76 
 
 Taylor, W. H.: Optical glass group, 13; liquid inspection 173, 199, 200 
 
 Telescope, lens system of 16, 19, 22 
 
 Telescope, light transmission 285 
 
 Test plate '. 267 
 
 Therelfall, R. : Grinding and polishing operations 225 
 
 Thermal diffusivity r . - 160 
 
 Thermo-element 94, 186 
 
 Threads.. 29,31 
 
INDEX. 309 
 
 Tillyer, E. I>. : Optical constants of a lens 264 
 
 Toepler method 197 
 
 Tolerances 257-259, 290 
 
 Tolerances, prism angle 270 
 
 Tool, A. Q.: Viscosity 'of glass, 149, 150; heat effects 151,152 
 
 Toughness 29 
 
 Training school. 295 
 
 Transmission of optical instruments 218 
 
 Transparency 29 
 
 Transparency of glass 209 
 
 Tridymite 148 
 
 Trimming raw optical glass 171 
 
 Turner, W. E. S. : Opalescent glass, 1Q4; optical glass compositions 110 
 
 Twyman, F. : Viscosity of glass, 149-151; annealing of glass 184, 185 
 
 Types of optical glass 73, 74 
 
 Vacuum bubbles 33, 139 
 
 Valasek, J. : Viscosity of glass, 149, 150; heat effects 151, 152 
 
 Van Auken, W. R. : Navy representative 293 
 
 Veins 29 
 
 Viscosity 180-182 
 
 Volumes of lenses 226-228 
 
 War Industries Board: Production of optical glass and of optical instruments.. 13, 
 
 288, 293, 294, 295, 296 
 
 Warburg, P. : Weather stability 220 
 
 Washington, H. S. : Optical glass group 10 
 
 Weather alkalinity 221 
 
 Weather stability .* 29, 79, 80 
 
 Weather stability of optical glasses 218 
 
 Weights of lenses 226-228 
 
 White, W. P. : Optical glass group 10 
 
 Williams, W. S. : Chemical analyses , 58 
 
 Williamson, E. W.: Optical glass group, 10, 13; strain birefringence, 39, 40; 
 viscosity, 149-151; thermal stress relations, 153; elastic constants, 159; cool- 
 ing of glass, 182, 184, 18P; a mealing schedules 187 
 
 Willetts Clay Pott Co.: Melting pots 87 
 
 Winkelmann, A. : Chemical analyses, 59, 66; thermal-stress relations 153 
 
 Wollaston prism 214 
 
 Wollastonite '. 148 
 
 Wright, F. E.: Optical glass group, 10, 13; strain birefringence, 39, 208; partial 
 dispersion relations, 51, 52; dispersion formulae, 56,' chemical compositions, 
 67-71; treatment of glass surfaces, 77-79; portable thermoelement, 94; 
 statistical study of analyses, 99; batch compositions, 101; differential 
 melting and settling of batch, 135; identification of crystallites, 147; im- 
 mersion method, 172; annealing of plates, 180; inspection for striae, 188; 
 photometer, 213, 214, 285; inspection of prisms, 270-280; inspection of 
 
 optical glass, 290; Army representative on War Industries Board 293 
 
 Young's modulus 159 
 
 Zies, E. G.: Optical group, 10, 12; chemical analyses, 63, 64, 66; oxidation of 
 
 arsenic, 105; time-temperature-gas evolution relations 119 
 
 Zinc silicate crown, chemical analysis 59 
 
 Zschimmer, E.: Chemical analyses, 59, 66; formation of borates, 103; thermal- 
 stress relations, 182; dimming test, 219., 220; acetic acid test. 222 
 
 3922921 21 Q 
 
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