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L'axampiaira film* fut raproduit grica * la gAnirositi da: Stauffer Library Queen's University Las imagas suivantas ont ttt raproduitas avac lo plus grand soin. compta tanu da la condition at da la nattat* da I'axamplaira filmi. at en conformity avac las conditions du contrat da filmaga. Original copias in printad papar covars ara fllmad baginning with ttia front covar and anding on tha last paga with a printad or illuatratad impraa- sion, or tha back covar whan appropriata. All othar original copiaa ara filmad baginning on tha first paga with a printad or llluatra*ad impraa- aion. and anding on tha laat paga with a printad or illuatratad impraaaion. Laa axamplairaa originaux dont la couvartura an papiar aat imprimAa sont filmis an commandant par la pramiar plat at tn tarminant soit par la darniira paga qui comporta una amprainta d'imprassion ou d'illustration, soit par la second plat, salon la cas. 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Lorsqua le document est trop grand pour itre reproduit en un saul clicha, il est film* A partir da Tangle supirieur gauche, de gauche i droite. et da haut an baa, an pranant la nombre d'imeges nicessaira. Las diagrammas suivants illustrant la mAthoda. 1 2 3 1 2 3 4 5 6 mffum MIOtOCOfY RESOIUTKM TEST CHART (ANSI and ISO TEST CHART No. 2) 1.0 I.I ■ 50 l£ 12.5 Ui tSi 1^ tii Hi jnA Its IK 1^ II 20 1.8 1.25 1.6 ^ APPLIED IM/IGE Inc ^P. 1653 East Main Street rjS Rochester. Ne» York 1*609 USA '^S (716) 482 - OK - Phone ^B (716) 288 - S9B9 -Fa« THE SCIENCE AND PRACTICE OF PHOTOGRAPHY 0£ X THE SCIENCE AND PRACTICE OF PHOTOGRAPHY AN ELEMENTARY TEXTBOOK ON THE SCIENTIFIC THEORY AND A LABORATORY MANUAL BY JOHN R. ROEBUCK, Ph.D. AWMTANT PBoramOH or rBTHIC* IN TM C.NIVUWITT or WIKONIIN ILLUSTRATED D. APPLETON AND COMPANY NEW YORK LONDON 1918 ■■■tl r H-- COFTRtOHT, 1918, BT O. APPLETON ANU COMPANY Printed in the United States of America PREFACE Many students are attracted by a course in photography who have had little acquaintance with science and less understanding of its spirit. There is no reason why the methods of modern science, as well as its attitude, cannot be taught by a course in photography as well as by a course in quantitative chemistry or in the theory of electricity. It has the enormous advantage over many other subjects of starting with a vigorous curiosity on the part of the student, and of maintaining his interest by the strong desire to master the subject for his own use. In all the author's experience there has been no opening for criticism of the amount of work his students were willing to do ; the diffi- culty has been to keep the laboratory closed at unreasonable hours. The methods of science arc necessarily quantitative where such methods can be applied. The quantitative study of the dry plate offers a very attractive example to induce the student to chew with some relish the tough grist of close thinking. For the student who has already developed some powers of mastication, the case is not particularly difficult, and its understanding is absolutely necessary for an intelli- gent use of any of the photographic processes. No collec- tion of apparently empirical rules can take its place. It was this attitude, determining the treatment, which persuaded the author to undertake this text. Also the available books offered nothing at all suitable for a laboratory manual where even a small number of students were to be cared for. In consequence, the broad treatment as well as the 1115U:? , I « PREFACE details are the product of experimental selection in this laboratory. The course has now been given for seven years, during which time both text and laboratory manual have been rewritten twice in mimeograph form, besides receiving innumerable additions and corrections. The author hopes that this summary of his experience will be found of value to other instructors and students, as well as to many not in university work yei who are interested in the real under- standing of their subject. While the primary purpose of the book is a class-room text, the author has aimed to make it useful to many earnest amateurs by collecting and systemat'zing for them material now widely scattered through the literature of photography. For the aiu of such workers and for the instructor m -y references are given to the literature which will draw atten- tion to and aid in the study cf the original work. In the British Journal Almanac for 1916 (at page 503) will be found a list of the journals of the world which print photographic information. For an English-speaking worker the most generally useful are: the British Journal of Photography (U. S. agent, George Murphy, 59 E. 9th Street, New York), the Journal of the Royal Photographic Sodety (Harrison and Sons, 45 Pall Mall, London, S. W.), the Photo Miniature (Tennant and Ward, New York), American Photography (221 Columbus Avenue, Boston, Mass.), and possibly still moie popular ones, as the Photo Era and the Camera. Any teaching in science should keep before the student that he is just beginning and that the most experienced worker carries in his head but a small proportion of the knoA^n facts and even of the accepted generalizations relat- ing to his subject. The literature is the record of all this information, and the sooner a student can be brought to turn to it for chance reading as well as for information on specific problems, the more satisfactory will his progress -aaw'j^Tflfe^i'M PREFACE vii be. Tt is not easy to get him interested to the extent that he will read voluntarily, and it will aid materially if a few current numbers of the scientific as well as the more pop- ular journals are kept in the laboratory, where they will be seen thus announcing their existence and be picked up dur- ing waiting moments. For the library it hardly needs say- ing that the more current journals in all languages that are kept, and the more complete the files of back numbers, the better for the real worker. To some not directly in touch with research it may be advisable to point out some justification for burdening the practical photographer with so much work in mathematics, in physics, and in chemistry. There is no doubt that many photographers doing excellent technical work know very little of these subjects whose application is the basis of the methods they use. This last statement contains the justifi- cation, and if an experimental proof is desired, it is only necessary to point out the painfully slow progress of the methods of time and factorial development in ousting the old-time guess work with the tray. Further, if there were an intelligent demand from the user, manufacturers would soon mark their plates with the actual development speeds and the actual sensitiveness. Hardly anyone doubts that the empirical worker could have acquired his skill with less experience, and could probably improve it now, by under- standing the relationships among the variables with which he works. The course as outlined in this book requires an ele- mentary knowledge of mathematics, of physics, and of chemistry. Good high-school algebra as well as good high- school courses in physics and in chemistry will be found sufficient to enable the student to take the further steps required. No extended description of the common photographic apparatus is included in the text since it can be given much ...j imKES^i-sumsaoft- sSf IRir^aHJ^T ftTiVf*^* »'5(a!Ki^^fl2*!aBT»flSWT»!^:3???=2r-»V« viii PREFACE better by the instructor with the apparatus beside him. The student finally becomes familiar with it by use. Any dis- cussion of the variations, with uses and advantages, would extend the text unprofitably. The discussion of lenses has to stop short of any real mathematical treatment for which the average undergrad- uate has no preparation. There are many excellent special treatises now to be had. The careful testing of lenses requires an expensive optical outfit, some advanced mathe- matics, and considerable practice. In any case the main problems of the young photographer are not with his lens performance, and very few expert photographers are com- petent to pass upon the optical questions involved ; hence it has seemed profitable to limit the discussion of lenses to practically that contained in a good book on college physics. The number of photographic reproductions in the text has been limited by consideration of the fact that they are reproductions varying necessarily in character from the originals, and by the fact that negatives and positives them- selves should be displayed in the laboratory and the instruc- tor should call attention to the points in the student's own work. My thanks are due to Mr. R. C. Williamson, who so ably assisted me for three years in giving this course, and to whose criticism and suggestion many improvements in both laboratory course and text are due. John R. Roebuck. Physics Laboratory University of Wisconsin jgtr •it3k'j:^iir\ . JatU1M'>C'.>''il CONTENTS PART I GENERAL THEORY CHAPTER I. HISTORICAL DEVELOPMENT PAGE Art. 1. The Beauty and Utility of an Image . . 3 2. Niepce 4 3. Daguerre 5 4. Talbot .6 5. Wet Collodion 7 6. Collodion Emulsions 9 7. The Development of the Gelatine Dry Plate 10 8. Gelatine 11 9. Silver Haloids 12 10. Dry Plate Making 13 11. Ammonia Modification .... 16 12. Centrifuge 16 13. Processes in General 17 14. Summary 18 IL PROPERTIES OF THE GELATINE DRY PLATE— EXPOSURE AND DEVEL- OPMENT 15. The Problem 16. Hurter and Driffeld . 17. Law of Absorption of Light 18. Photometer .... 19- Fog 20. Ferrous Oxalate . 21. Growth of Density 22. Constant Density Ratios . 23. Application in Practice 24. Intensification 25. Characteristic Curve . 26. Period of Correct Exposure ix 19 20 21 23 25 27 28 29 31 32 33 37 \ TES^B.mnK^'^Titwu' v^ficMiV'f^^mBs^nsxjt-.aii^ .■ ■:'/K^m/' iuiitl^VXTWit^'SKS <£&,' X CHAPTEK 27. 28. 29. 30. 33- 33a. 33b. 33c. 34. 35- 36. 37- 38. 39. 40. 41. 42. 43- 44. 45- CONTENTS Perfect Negative Period of Underexposure . Period of Overexposure . Period of Reversal . . . ! Dependence of Slope on Development Development Factor . . , , Practical Control of Amount of Develop ment (Contrast) .... Constant Time at Const. Temperature Time Adjusted to Temperature . The Watkins Factorial System . Contrast Thickness of Film . . .' . . Like Negatives from Different Exposures V ^ ititude) ••-r nic Dry Plate Developers The Choice of a Developer Estimation of Exposure Shutters The Stop . Dry Plate Speeds Preliminary Exposure Character of Subject . Lighting m. PROPERTIES OF THE GELATINE DRY PLATE— COLOR SENSITIVENESS 46. Colored Object with Ordinary Plate 47- Color Sensitiveness Curves 48. Dyes 49- Absorption Curves 50. Absorption and Sensitizing 51. Practical Application . 52. Screening Action 53. Color Screens . 54- Rendering Color Contrast . IV. LAT. 55. 56. 57- 58. 59- 60. 61. iMAGE THEORIES Basic Facts . Theories Physical Modification Silver Grain Silver Sub-bromide Solid Solution . Conclusion . PAGE 39 40 42 43 43 45 47 47 48 49 50 52 53 55 56 57 58 61 64 67 68 69 72 73 74 77 78 80 83 84 87 89 yi 91 92 93 94 97 taA»iai.- CONTENTS CHAPTEB V. NEGATIVE DEFECTS 62. Classification 63. Thin Negatives . 64. Intensification 65. Overexposure 66. Overdevelopment 67. Fog .... 68. Reduction . 69. Local Reduction . 70. Conclusions 71. Frilling 72. Dust .... 73. Air Bells . 74. Black Spots 75. Finger Marks 76. Pressure Marks . yj. Halation 78. "Oyster Shell Marking" J^). Stain .... 80. Bubbles 81. Drying Troubles . VI. POSITIVE PROCESSES 82. Transparencies . 83. Lantern Slides . 84. Blue Prints . 85. Printing Out Paper (P. O, 86. Developing Paper 87. Sulphide Toning 88. Platinotype . 89. Carbon Printing . 90. Photo-engraving . P.) VII. LENSES 91. Pinhole Images 92. A Lens 93. Images 94. Size of Image .....* 95. Defects in Images Formed by Lenses- Aberrations 96. Spherical Aberration .... 97. Coma 98. Chromatic Aberration ... 99. Achromatism 100. Curvature of Field .... xi PAGE 99 99 101 103 104 104 107 109 109 no III 112 112 "3 114 118 118 118 119 121 121 124 127 132 133 135 139 142 146 149 153 159 -M .\sisai ■^am^'- xn CHAPTER CONTENTS 101. Astigmatism 102. Distortion . 103. Flare . 104. Unequal Illumination 105. Depth of Focus . 106. Speed of Lenses . 107. Lens Types . 108. Telephoto Lens . VIIL COLOR PHOTOGRAPHY 109. Classification of Methods no. Lipmann Process 111. Pigment Processes 112. Three Negatives . 113- The Additive Method . 114. The Subtractive Method 115. Carbon Process . 116. Pinatype Process 117. Traube 118. Photo-engraving . 119. Mosaic Screen Process 120. AutocLrome 121. Mosaic Screen Making 122. Conclusion . 123. Bleach-out Process . IX. GOOD PICTURES 124. Definition 125. Pictures 126. Picture Composition . 127. Unity 128. Simplicity 129. Foreground 130. Sky-line 131- Sky 132 Aerial Perspective 133. color m Subject .... 134. Lighting 135. Notan 136. Balance 137. Focal Length of Lens . 138. Distortion from Short Focal Length 139. Angle of View .... 140. Focusing 141. Distortion from Inclined Plate . 142. Lens Axis PAGE 159 160 162 163 165 167 168 170 173 ^74 178 179 179 180 183 185 186 186 187 190 193 193 195 196 198 199 200 201 201 202 203 204 206 207 208 208 210 210 211 213 213 -tr^^ CONTENTS xiii CHAPTEK PAGE 143. Exposure and Development . . .214 144- Printing 214 145. Mounting ....... 216 146. Conclusion 217 APPENDIX DEVELOPMENT 219 WATKINS SPEED LIST 220 TABLE OF PLATE SPEED NUMBERS . . .225 PART II LABORATORY MANUAL PREFACE 229 GENERAL DIRECTIONS 231 EXPERIMENTS 1. Negatives 234 2. Negatives Repeated 235 3. Solution Making 236 4. Prints on Developing Paper . . . 237 5. To Test Systematically for Correct Ex- posure 230 6. Factorial Development . . . .241 7. Contact Lantern Slides .... 242 8. Lantern Slides by Reduction . . .245 9. Halation 246 10. Intensification 247 11. Reduction 251 12. Color-sensitiveness 252 13. Spectrum Photography for Plate Color- Sensitiveness 253 14. Dying and Testing of Plates . . .256 15. Microphotography 257 16. Blue Print Paper 258 17. Gelatino-chloride Paper Also Called Printing Out Papei (P. O. P.) . , 260 18. Enlarging 262 19- Sulphide Toning (sepia) . . . ,264 20. Wet Collodion Plates 265 21. Pictures Showing Clear Distance and Clouds ....... 269 m^^i'i^^''fm^. T^^i^^^^m^^w^^mm^ XIV KXPEUMENTS 23 CONTENTS INDEX Autochromes 23- Carbon Printing . 24- Characteristic Curve Finishing Up Work . APPENDICES Laboratory Apparatus Special Apparatus Supplies per Student . Chemicals per Student Photometers Order Slip ..'.'. Photography Laboratory Record i^ACE 269 271 273 274 276 V7 j 278 279 280 288 288 285 I :l PARTI GENERAL THEORY "s. f t' I. -i r-f-iP^ris^ m i i mjm m^^^,^^ a] tb fo th. as bej of obi ish sih sut :i ^■kt^'-. 14 PHOTOGRAPHY i^ ii The use of the two kinds of gelatine is for the pur- pose of making the fihn hard enough when wet to stick sufficiently strongly to the glass, and yet of low enough melting point to absorb the solutions readily. Both lots of gelatine are washed with water to free them from dust and then allowed to swell in water. To the silver nitrate a drop of hydrochloric acid is added to make the emulsion acid. No. 3 Gelatine is melted by standing the drained swelled gelatine in a water bath at 50° C. and the silver nitrate added and well mixed. Then No. 2 Pot. Bromide is added slowly drop by drop, with constant agitation, of course in the dark room, till ^ is added, when No. i Pot. Io- dide is mixed with the remaining Pot. Bromide and the mixture added to the gelatine as before, drop by drop. This makes the precipitated haloid very finely divided, and the liquid with suspended precipitate is called an emulsion. It will be noted that the bromide and iodide are present in more than suflficient quantity to combine with all the silver nitrate, so that there is no free silver nitrate left to react with the gelatine. U a little of this emul- sion is now examined by looking through it, it will be found to be ruby red. Place the flask in boiling water in the dark for 40 minutes. If plates )e made from it at intervals, it will be found that the sensitiveness will be increasing steadily and the sil- t=T*<,*vfr '?i:&iM&^'-fif^ ^**s^^4^5^'';^' .>?*^^*»fc;*y$i^;L^^;;'\V?^^r^r"*^?^ HISTORICAL DEVELOPMENT 15 Iver bromide particles increasing in size. At some Itime the sensitiveness will begin to decrease and the [boiling should be stopped short of this point. It jvvill be noted that part of the gelatine was reserved so that it would not be affected by the boiling. It lis now warmed to melting and mixed with the emul- [sion, and the whole allowed to set. In the emulsion besides the water, gelatine and silver haloids, there will be the following substances, KBr, KI, KNO3, HCI (see reactions in Art. 5), and if these latter arc removed the plate wil' be much more sensitive. To this end the jelly is now well broken up by squeezing through strong cloth, and washed and soaked and washed for hours. It I is then well drained and is ready to coat the plates I as soon as it is melted by warming. If a small quan- tity of alcohol is added before coating the plates it [will increase the sensitiveness somewhat. To make the layer of gelatine adhere to the glass surface through all the later operations it is neces- sary that the glass surface be absolutely clean. No lone who has not actually attempted to thoroughly clean a surface realizes how difficult it is. All grease can be removed by a hot alkali solution, par- ticularly if assisted by friction. Hot chromic acid I or nitric acid will destroy all other organic sub- stances and dissolve a good many inorganic. Wash thoroughly, finishing with distilled water. Many .1 ^^MWrW'- -^-tV'-^^:^-::^V-^-<- 16 PHOTOGRAPHY ij.'-'i workers finish by polishing with French chalk (fine- ly divided calcium carbonate) on clean chamois | leather or cloth, but the latter must be clean and must not be touched by the fingers on the part used for polishing. A layer of hardened gelatine or of| albumen adheres to the glass very strongly, and the emulsion adheres to this layer strongly also so that most plates on the market are made this way when the cleaning does not need to be quite so thorough. The plates are placed on a level surface and a measured quantity of the warm liquid emulsion poured on and spread to the corners with a glass rod in a very dim ruby light. The emulsion soon sets when it is best placed in a current of dry air in the dark till quite dry. II. Ammonia Modification.— The above method is known technically as the boiling method, where the great sensitiveness is largely due to the pro- longed heating during which the silver bromide goes into the sensitive form. Another method reaching about the same sensitiveness, is to add some ammonia at the point above when the boiling commenced, and to then let it stand for a day or so at room temperature. Some workers use one meth- od, some the other. Repeated melting and solidify- ing of the finished emulsion also increases the sensi- tiveness. 12. Centrifuge.-The boiling of the emulsion as ■-.v^s ts&^gagjjiiif awtfcteyji ii^'iiiwunn iimat . ■ .■i«:i««fA?;j A'aisfi!ica»-.L-5v^ HISTORICAL DEVELOPMENT 17 above, deteriorates the gelatine used a great deal, ! and it is of advantage to get rid of this spoiled gela- tine. This can be done following the boiling by whirling the liquid in a centrifugal machine, which is a cylindrical vessel arranged to be rotated rapidly about its axis. The heavy particles of silver bro- mide are by this means driven to the walls of the vessel and the fluid part may be poured out, carry- ing with it practically all the soluble salts and most of the old gelatine. The silver bromide is then added to fresh melted gelatine, and when thor- oughly mixed is ready for coating the plates. Boil- ing the emulsion without any gelatine present gives but a poorly sensitive plate. 13. So far we have been concerned with the im- portant outstanding photographic processes only. Thei are a great many others of more or less im- portance, with other light sensitive substances and supports. There are books filled with short terse descriptions of such processes literally by the hun- dred. We will need to consider a few of them later \yhen we come to discuss methods of making posi- tives. It should also be pointed out that except for some other difficulties in the handling of the flexible support, the films for film cameras are made like the dry plates are made. For some reason films have not been nearly as fast as the fastest plates, but they ^ I I III I iwiiiiii^iiii m ■ I in ■•':f_ ■ ii^w:-'^3»ii:„:iii''::4 i8 PHOTOGRAPHY have been fast enough for most work. Lately a faster film has been put on the market. 14. Summary. — To sum up again the reactions involved in the case of the making of a negative from a gelatine dry plate, thus in the light AgBr -Ag+Br or aAgBr « AgiBr+Br and we will leave the discussion of whi^' occurs and of the theories generally till more of the facts are in hand. Then in the developer aAgjBr+HjO+R = 2Ag2-|-2HBr-f RO or where there is a particle of the metallic silver due to the action of light, in the silver bromide particle, sAgBr+HjO+R = 2Ag+2HBr+RO where "R" is the developer which is in all cases a material which will take -o oxygen, liie func- tion of the carbonate in the developer is partly to keep the solution from getting acid from the forma- tion of the HBr. Then in the strong sodium hypo- sulphite (aiso called thiosulphate) solution 2AgBr-f sNajSjOj = aNaBr+Na^AgaCSjOa), and the complicated compound formed is soluble in water. CHAPTER II PROPERTIES OF THE GELATINE DRY PLATE- EXPOSURE AND DEVELOPMENT 15. Now that we have the description of the manufacture of gelatine dry plates, and have used them somewhat in the laboratory, the next step logically is the investigation of the properties of the plate. Here a qualitative investigation is not of much use, but the properties must be measured and the relation between the different experimental quantities expressed in mathematical form For example we must answer, from the standpoint of expermient, such questions as, What is the relation between the amount of exposure and the density of the negative? How does the kind of developer and the time of development affect the density of the neg- ative, and affect the relation between the densities of different parts of the negative? How should we measure and how express the speed of different plates? and many more such questions. The answers are very valuable both in the experimental use of the 19 .•». 'amA' '•*.'jaKbe^'-i yjomatifr'.'sam. •X^s. 20 PHOTOGRAPHY I, II plates and the interpretation of what we get in the negative. In 1890 two Englishmen, Hurter and Driffield publishea ' a description of some quantita- tive experimental work on the gelatine dry plate. which was the first of many papers by many work ers. It was a splendid piece of work, and stirred up a vigorous controversy which has hardly died out yet. Their results have been checked many times and in different ways, and are now accepted as correct. Since they answer some of the ques- tions which it is worth while to ask, a litlie time can be spent profitably over their paper. 16. Hurter and Driffield begin with a definition of a perfect negative — "when the amount of light transmitted through its diff'erent gradations is in inverse ratio to that which the corresponding parts of the original subject sent out." It i'^- to be care- fully noted that this does not say the same actual differences bui only the same proportions; the actual dift'erences may be either greater or less. That is if one subject sends out twice the light to the cam- era which another does, then it should be rejjre- sciited in the image by a place of twice the opacity. ' Hurter & Dnffeld, 'Thotocheiiiical Investigations," Jour, oj Soc. Chem. Ind. g. 455 (1M90) ; Photo Miniature, No. 56, wliore it is rewritten hv Driffieki; Photo Miniature, No. 66, applica- tion to the general problem of dcvfiopmcnt. For a very com- plete treatment and many references sec .Sheppard & Mtcs, ■'Investigations on the Theory of the Photogranhic Prociss,' Longmans, Green & Co., 1907. III; i ji i I GELATINE DRY PLATE 21 The proportion and not the actual difference is the important matter. 17. LawofAbsorptionofLight.-The next step IS to investigate how ^\ hght absorbed by a trans- lucent layer depends on the incident light and on the layer This is iinpo,-; ni as it is the absorption of light by the sensitive film which makes possible its development into a negative, and the absorption of light by the negative which makes printing possible Suppose we find by measurement that a certain film u.ll absorb 14 the light coming up to it, that is it transmits ^. If we double the light coming up we will find that the fraction transmitted will remain the same, although the film is both absorbing and transmitting more than it did before. More meas- urements would show this to be the general rule that the fraction transmitted is a constant inde- pendent of the illumination. In algebraic symbols ' = ^' (X) where I is the incident and i the transmitted light '^nd i/m is the fraction transmitted, where m is a' constant independent of I but dependent on the film ii^ed. Suppose now that two such layers be used each layer will transmit 34 of the light which gets to 1 but the second layer only gets 34 of the inci- dent hghi, so that the fraction transmitted by the two films together will be ^ X ^ of the originally ^^'"■■ --■i'l---; 22 PHOTOGRAPHY incident light. For 3 layers it would be similarly (HV- Evidently if the fraction absorbed was dif- ferent from }i, the same expression would still hold, that is if i/m was transmitted by the first layer, then (i/m)° would be transmitted by n lay- ers. That is, the more general form of equation i above is i = (i/m)» or l--(m)- (2) and for convenience let us write m = (10)* which gives I = ((10)"^ = (10)*" = (10)- (3) The ratio 4- is called the opacity because it is the amount of light which must fall upon the film to have unit quantity ge through. The power kn, which is usually written d, depends only on the absorbing body and is called its density. In these terms the equation 3 may be written 0=io'» (4)' and this equation is the relation we set out to deter- mine. ' For the student who has studied logarithms this may be written more conveniently thus: 1 T (i/m)' jn-n ^ j-km whence logt -r = kin and logic - 1 2.303 or logio = kn = d (kin) = kn GELATINE DRY PLATE 23 18. Photometer.— To measure the density of a photographic pi ite it is possible to separate the sil- ver from a measured area of a negative, and from this get the density. But this method is very tedious and on account of the very small quantities of silver, is not accurate. It is better to measure the opacity of the plate and from it get the density by use Fig. I. of equation 4 above. For this purpose Hurter and Driffield modified the ordinary bench photometer to make the apparatus pictured in the diagram. Fig. i. Outside of the box at each end is placed a lamp, and the central box can be moved toward either end, thus getting more light from one lamp and less from the other. In addition to this the board with the wedge- shaped opening can be moved in and out, so chang- ing the vertical length of the opening in that end of tlie box, and so varying the amount of light admitted !i« ^^y&i .:> ,,;^ 24 PHOTOGRAPHY from that lamp. The position of each can 1 cad on the scale attached. The arrangements in le the inner box is shown in Fig. 2. The vertical sep- tum in the center of this inner box has a thin paper middle, in the center of which again is a spot which has been made more transparent with grease. The two inclined mirrors in the back of this box enable F Sliding Scalc 3E Fig. 2. the observer to see both sides of this greased paper at once. H the illumination on the two sides of the grease spot is different, the spot looks lighter or darker than the surrounding paper. When the il- lumination on the two sides is equal the spot be- comes invisible, and then the strength of the two sources will be directly as the squares of their dis- tances from the grease spot. Then one light aper- ture is covered with the plate to be measured, the mm^jM^v '^}^..:kip M;ISk;.^^: GELATINE DRY PLATE 25 small box is shifted till the grease spot becomes in- visible again ; from these two settings I/i is easily calculated,^ and from that the density. To deter- mine how much dependence could be placed on the instrument, they determined the density of known solutions of India ink, and the determin d and known values agree to about one per cent. Table i India Ink added to Density 100 cc Water calculated sec 0.240 10 0.480 IS 0.720 20 0.960 25 I. 20 30 1.44 35 1.68 40 1.92 Density measured 0.240 0.500 0.750 0.950 1-245 1.44 1.665 1.885 This is consequently all that should be expected in its regular use. The table showing the measure- ments is given above. 19. Fog. —Obviously if one wants the den- sity of the silver forming the negative image it will not do to include with this the density of the gela- tine and of the glass plate, for both these absorb some light. Also there is always some chemical fog, that is silver reduced which is not due to the action of the light which formed the negative. To get the ' See appendix on photometer, p 280. £^^i<«^ 26 ' PHOTOGRAPHY density due entirely to the action of the light it is necessary to allow for these other densities, and they made a practice of leaving a small part of the plate unexposed, measuring the light absorbed by it, and subtracting this amount from the density of the rest of the plate to get the density due to light. For example to test the effect of time of develop- ment, they exposed a plate except a narrow strip along one edge, then cut up the plate at right angles to this unexposed strip, and developed each of these strips a different length of time. The following table gives the actual measurements. Table 2 Time of development, minutes Density exposed plate . . . Density unexposed plate . Density due to light Percent developed 25 .670 .200 .470 38. 50 965 345 620 2 7-5 I 245 •415 • 830 67.2 10. 1.420 ■505 915 74 I 15 1-755 •575 1. 180 95-5 20. 1-945 .710 1-235 100. Note that the density of the negative grows with time of development, but the density due to light reaches a maximum in this case in about 15 min- utes ; also note what a large proportion the fog is of the total density, and that it continues to grow steadily even after the light image has reached its maximum. The first fog figure (.2t>o) is the den- sity almost exactly of the glass plate and the gela- tine, and this should be subtracted from the later fog &^^S(73^^f^^i£#l»^i^l^M^Ssj;^^l»HP^ GELATINE DRY PLATE 27 readings to get the true fog. At small values the fog is approximately uniform all over the plate and consequently will not affect the printing qualities except to require longer to make the printing ex- posure. 20. Ferrous Oxalate.—All the alkaline develop- ers have this undesirable property of producing a lot of fog, which is especially undesirable in this kind of work. The ferrous oxalate developer which works in acid solution produces very little fog, even an hour's development of an unexposed plate giving hardly enough fog to measure. The developer is made by mixing strong solutions of ferrous sulphate and potassium oxalate. These react to give ferrous oxalate, thus : FeS04-f-K2C204 =FeC204-hK2S04 which is not soluble in water but is soluble in an excess of the potassium oxalate solution. When in contact with exposed silver bromide, they react thus: K2C2O4+ 2FeC204+ 2AgBr = Fe2(C204)3+ 2KBr-h sAg that is the ferrous salt changes over to ferric, me- tallic silver is formed, while the amount of acid pres- ent does not change. Unfortunately the solution absorbs oxygen from the air fairly rapidly, and the ferrous salt not being very soluble, soon precipitates, but does little harm when it does p-ecipitate and 28 PHOTOGRAPHY .>5 11 m merely calls for a little care in not using the devel- oper too long. On account of its non- fogging prop- erty they used it for most of the quantitative work. 21. Growth of Density. — They exposed one-hait the plate to light and then cut it up so as to include a part unexposed and a part exposed on each piece, developed in the oxalate developer, fixed and washed it. After drying the density was measured in the photometer, subtracting the fog to get the density due to the action of the light. The first step was to determine the way in which the density grows in the developer. For this purpose different pieces were developed for different lengths of time,' and with developer differing in the amount of the two constituents, of water or of potassium bromide. The resulting densities are given in the following table : Table 3 Time Mins. 5 10 15 20 25 30 45 60 I 365 525 615 61S 700 700 Density exclusive of fog. II III 350 460 SSo •795 575 650 660 .860 1. 000 IV V 215 30s •570 410 • • • ■ 420 .670 450 •715 515 .740 . . . Mean Calculated •332 . 290 .462 .464 •569 .568 .582 .628 .660 •66s •64s .687 •715 •713 .740 .719 This table shows that, as in the previous table, the densities grow rapidly at first, then more slowly, finally reaching a maximum value. The rate of ni'iii iiiiM iiiwi Ilium " I'lHyi 4ihiiw III I It in iiiiiiii«iiiiiiniw || Ei ) 3( 5 •4 3 »1lN UTC 60 Fig. 3. This equation enabled t'^em to calculate the density I at any time during the progress of development. I This is well shown by the agreement of the column I mean and calculated of Table 3, this data is plotted I in Fig. 3, where the curve is the cai :ulated points I and the experimental points are the dots. 22. Constant Density Ratios.- This formula has jsome very interesting consequences, for example when t is large, a being a fraction, a* becomes small, 30 PHOTOGRAIMIY and when t is sufficiently larjj^c, a' becomes small enough in comparison with i to be nej^lected, that is d becomes the same as I). That is to say the for- mula shows that the density will reach a maximum which is in agreement with experiment. Or sup- pose we had two places of difTcrent exposure on the same plate where the final densities will be Di and Da, then the densities at any time t will be given In di = Di (i-a') and dj = Dj (i-a*) where the time from the start of development is the same in both cases, namely t, and a is the same a!! over the same plate. If we divide one of these equations by the other, the bracketed part divider out, and we have, da D, ^- That is, the ratio of the densities at any time t dur- ing development is the ratio of the final densities, and since this is true for any time it is true for 2.. time during development. That is, the ratio of ihe densities at the two places is always the same n matter when you stop development. This does n:: say that the actual densities do not change, they grow greater and their difference grows greater that is, contrast grows greater during developmer.: But nevertheless one place is always the same pr - pori.on denser than the other. They confirmed th- result, as it is very important, by a lot of exper- •mXZ M^^IKJTmB^:^^ •.i^asr^^^^fMot^'s^ ky^iLL^ GELATINE DRY PLATE 31 ments, using plates from different makers and many other developers than the oxalate. Nor did chang- ing the developer during the progress of develop- icnt affect this result. 23. Application in Practice— It follows that the ratios are independent of any ordinary treatment luring development. Of course covering only part )f the plate with the developer will upset this rule, Dr warming it locally, for example, with the fingers. It is of course a series of densities which make negative and it is the ratios between them and not their absolute values which are most important In the negative and in the print. It is fortunate pat it is not a matter of special skill and manipu- lation to keep the ratios true to the light values laking the negative and therefore true to the sub- jje'^t. A correctly exposed negative fills their defi- nition of a perfect negative almost entirely inde- pendent of the process of development. All that can be accomplished by ordinary manipulation dur- ing development is to fix the density at one point in |the negative and hence to fix it at all other points. Any attempt by manipulation during develop- |ment to bring out one set of detail and to subordi- nate another is useless. Moreover if the plate was Inot correctly exposed it can only be very imper- Jfectly remedied during development. All there is lany need to do is to interrupt the progress of devel- ■niiasias^a^&:3i# 'Jtrtn'm Ttr iTi rii>fiiniiir:rir,ir •"•»!' sa3HiK'a'^Lrii-«» 32 PHOTOGRAPHY opment when the hij^hest and lowest densities are] not too far apart in actual value for the i rint h show their difference, nor so close to,Q^ether that' they do not use a good deal of the range of dctisi ties of the print. It is for this reason that you havej been urged to control the development of your platci by following the time for which they are in the de- veloper. By choosing the time of development to I suit the developer used and the brand of plate, and] then making the exposure such as will give a suit able density for printing, one will get the best i)ic tures. But where one guesses at the exposure and] then tries to adjust the treatment in the developer so as to correct errors of exposure, there will be| two things to blame the failures on, and no real guide as to which one was actually wrong. Hut by choosing and adhering to a suitable time of de- velopment, one will be sure that the errors are in] the exposure, and so by watching the successes and I failures learn to make correct exposures. In fact] tank development is the logical thing. 24. Intensification — The next experiment \va> to test the effect of intensification, and the same rule] of constant density ratios was found here. The increase is proportional to the density before in- tensification. An example is given in Table 4. The plate was exposed, developed with fer rous oxalate, the densities measured, and ainr !:::.'i GELATINE DRY PLATE 37 overexposure. And fourth, the final bending down part IS called the period of reversal. We will con- sider the second part in detail first. CiPOSuee , CANOLe-»«TKe -secoNa» Fig. s. 26. Period -of Correct Exposure.^In the dia- gram, ab is a straight line within the error of ex- periment. Let X be any point on it, and draw xy at right angles to cy. Then wherever x may be on ab, the triangle xyz remains similar, that is yz = 7 (xy) where y (gamma) keeps the same value, being the number of times that one side is greater than the •f-r 'Jlfli'fBftfflv'S??:dBa«ffi» :.TS <^*^m ■ m I E r^ i. i ■ J 38 PHOTOGRAPHY Let us examine further the way we have plotted the exposure. If E is the exposure, we may put E - 10' Ml t ^1 9 •I jM y * eXFDSURC (Ug.^ Fig. 6. where v is a suitable number and then multiplying E by a factor means making an addition to V. For example if we multiply by 10, then loE = lo'Xio = io*+* Or if we multiply by any number X, we can always write X = irf* where n is a suitable number, and therefore XE = lo'Xio" - io'+* So that we really plot the index v of some suitable number such as 10. In the actual case above plotted ':^^f;^ ■ B>!sSS%QI-~SBL ''«MWIBUU»P^ GELATINE DRY PLATE 39 the suitable number was 2 and when we had chosen a distance to represent unit exposure, we plotted the index in the series i, 2», 2^ 2», 2*, etc. If we have been careful to make our plotting of the above diagram suitably, we may write E = io«» and let us choose "i" a number such that i = lo** Dividing one by the other, we get E i 10' ,ey 10' .«• But in the discussion above it is shown that Whence zy = T (xy) = 7d E/i = io>« 27. Perfect Negativc-^Experiment shows that as development proceeds the curve ab swings up about the point z as center, that is, becoming stead- ily steeper. Let the amount of development be so chosen that thev in our above expressions is unity, that is, the slope of the line ab will have to be 45^ For this special case, then, E/i = 10' h was shown above in Art. 17 that lo" ^ II ffi I 40 and therefore PHOTOGRAPHY E/i -O (6)' For any particular curve the value of i is a con- stant, and therefore the opacity (O) is directly proportional to the exposure (E). The amount of light transmitted is inversely proportional to the opacity so that this statement becomes, the amount of light transmitted is inversely proportional to the exposure, which is the definition, Art. 16, that Hur- ter and Driffield give of a perfect negative. Hence it is possible to make such a perfect nega- tive by filling two conditions : ( i ) the exposure of every portion of the plate must be such as to brine; it onto the straight line portion of the characteristic curve, and (2) the time of development must be such that V will be unity. 28. Period of Underexposure. — This includes the initial bend of the characteristic curve lying be- •For the more advanced student this may be more conven- iently written thus: d tan a = =7 log E — log i where a is the angle between the straight part and the E axis- For the case of 7 = i, d = log— r But d = log O (Art. iq) .-. O =~ and since i is a constant for all the straight part O :E or the opacity is proportional to the exposure. GELATINE DRY PLATE 41 tween the letters c and a in Fig. 6. The slope of any part here is always less than that of the part ab, so that for the san,e tin>e of development the densities will be much less, and also the density dif- ferences between any two points, that is, the con- trast will be less, and the negative will in general be th,n and flat. Not only that but the contrasts w, I not be graded as they are in the subject, but will be relatively greater than they should be in the longer exposed parts, and detail will be lacking in I the shadows. The print will not be true to the sub- Note carefully that the above is true for the same time of development or in better words for the same development factor (gamma). Many texts and workers say that "underexposure gives con- trasty negatives," which is not strictly the case It sWd read that "the longer development usually fr ""''^^^•''P"^'"-^^ l^^ids to contrasty negatives " \ hen the progress of development of a negative is n:t^ '""""^ I' " "^ '"^-S" ". 'he'vatch "PS development when the negative has reached C , „ ' ' '''°" ""P"'"'' "^^'"'ve is devel- fcpria long time and as contrast-density differ- on^ "■•" "^-'"P™™*. this results in a exposed negative grows black quickly all over in ■it Mi 42 PHOTOGRAPHY in I '1 the developer and is removed before the contrast has had time to grow great, resulting in a soft or even fiat negative. For the same subject and light- ing the difference in the contrasts in the two cases is produced in the developer, not in the exposure. Exposures falling on the part of the curve ca are so small as hardly to give printing density even on prolonged development. Such a negative may be intensifi d, so continuing the process of develop- ment, and thus be built up to good printing density. The contrast will however become very great, giv- ing hard, black and white prints which are quite untrue to the subject. There are many negatives which for the greater part of the subject are satisfactorily exposed but for some dark objects or weakly lighted ones are given such exposures as to fall on this initial part of the curve. In the print these objects do not appear as they should but are weak or even lacking in detail. The old maxim of "expose for the shadows and the high-lights will take care of themselves" is worth remembering. 29. Period of Overexposure. — This includes the upper curved portion as far as the highest point, that is from b to f in Fig. 6. As in the part -.a, the slope here is always less than that of ab, so that, though the densities may be much greater, the con- trasts are less and the negative is flat. The nega- ■h^^:-'^' GELATINE DRY PLATE 43 tive is so dense as to require a lonfr exposure for printing, so that it is hardly feasible to prolong development or to intensify to increase contrast. In skillful hands reduction offers a means of im- ' proving the printing qualities. Plates the main part of whose exposures fall on this part of the curve, that is true overexposure, occur less often than true underexposure, since the regular practice aims to give an exposure lying close to the lower limits of correct exposure. 30. Period of Reversal.— This includes the curve from f to the right in Fig. 6. Here increase of exposure actually decreases the density obtained on development. It is possible with care to get a positive, a flat poor one, but the exposures have to be very great indeed, so great that one never meets with such reversal in ordinary work. It is of in- terest largely to the investigator who is working to develop a satisfactory theory of the latent image. It is difficult to overestimate the importance of the characteristic curve for a correct understand- ing of photographic work. It summarizes in a handy, usable way the most important properties of the dry plate. The previ- ous paragraphs have indicated some of these and there are a number yet to be discussed. 31. Dependence of Slope on Development. The method of obtaining the characteristic curve, see M 44 PHOTOGRAPHY Art. 25, was to expose a plate for known different times on different spots, and then develop the whole plate alike. Curves can be obtained in this way where the only difference in the treatment of the plate is in different times of development. Fig. 7 is a typical case of two such curves where the developer used contained no soluble bromide. The EXPOSURE Fig. 7. Upper and steeper curve is for the longer develop- ment. The straight portions when extended meet in the point z which lies on or near the horizontal axis. Hence as development proceeds the straight part really rotates about this point. If the devel- opment be very prolonged the curve reaches a maxi- mum slope since development finally ceases, see Art. 19, and this maximum slope or value of 7 (called 7oo) varies with different kinds of plates. These observations are in agreement with the GELATINE DRY PLATE 45 i-esults given in Art. 21 and could have been pre- dicted from them. Equation 5 was d, Di d, "d. That is, as development proceeds the different densi- ties always keep the same proportion. In Fig. 7 from similar triangles af _ xf hg "yg thich is the same statement as Equation 5. The presence of soluble bromide, potassium bro- lide usually, in the developer shifts the point z below the axis. This means suppressing the lower densities, and in the case of piates there appears no bompensating advantage, so that plate developers jfree of bromide are to be preferred. 32. Development Factor.— In Art. 26 it was ^hown that xy » 7.yz xy 7 = — yz bd therefore the value of y (gamma) depends on khe angle between these two lines, or in other words bn the slope of ab (Fig. 6). From the preced- ing discussion, Art. 31, it will be evident that the pue of y is a measure of the amount of develop- ment, and it is hence called the development factor. ^or a true picture the lights and shades must be 4I ^a^^'^'iflBBW 46 PHOTOGRAPHY r i "■j i , I r 1 if ; proportional to those in the subject, which requires | a "perfect negative," see Art. 27, for which y must I be unity, and the slope of the line ab there fore j 45". H the development is carried beyond this, y becomes greater than unity and the contrasts wi be exaggerated. If develi»pment is stopped short of this, y will be less than unity, and the contrasts! will be softened. In Art. 86 are discussed the real sons which may lead to the choice of a development | factor different from unity. The amount of development, that is the develop- ment factor, y, depends on a number of things, (a) The developer, meaning by this the kind and amount I of every constituent. By ordinary care in the mak- ing up of the solutions this may be easily kept suffi- ciently constant to be relied upon. The choice of ai developer is discussed in Art. 38. (b) The time oi| development, which is readily controlled, (c) The temperature, which may be controlled or different temperatures may be readily allowed for by chani:- ing the time of development. 'J'his will require I reading the temperature and knowing the effect of j temperature on the rate of development. See Art. 33- (d) The dry plate. In order that a particular brand of plates may give uniform results, the tirae of development for y equal one must not vary ma- terially, and the good plate makers' efforts are di- rected to maintaining this constant in different PI •rn^-jCiiujg mm 'J GELATINE DRY PLATE 47 Ibatches. With different brands of plates this time lof development is very different. The careful user lof plates will welcome the time when each box of Iplates w'll contain a statement of the plate speed, la formula for a good developer, and a table of the jteniperatures and the times of development for lobtaining y equal one with this developer, (e) The jage of the plate. In moderately new plates which jhave been kept dry and cool, and away from active (gases, the age is not a material factor. But under lother circumstances the age of the plate may intro- jduce as much as a factor of three in the time of jdevelopment for y equal one.' I 33. Practical Control of the Amount of Develop- Imcnt (Contrast).-In the dark room, two of the jfactors in the above list offer difficulty, that is the jtemperature and the plate. To manage all of them land so control the contrast, there are several dis- |tinct procedures possible. (a) Use always the same brand of plate so that it lay be assumed that its development speed remains jconstant. This will hold quite satisfactorily for Iplates of the same batch of emulsion but the varia- jtion of different batches may have to be compen- jsated for by changing the tune of development; the jprobabilities are, however, that in ordinary picture jtakmg it will require careful attention to notice the •Sheppard & Mees. "Investigations," p. 56. 48 PHOTOGRAPHY 11 U If If difference. Develop all exposures for a fixed tinie| at a fixed temperature. Determine the time of de velopment by exposing several plates for the samel time to the same subject, and develop them for different times at the fixed temperature. From the print select the development time for the plate whicJ has the contrast you like. The temperature may be readily controlled by using a large tank of water and adjusting its temperature as necessary by the addition of hot or cold water. The size of the tank will prevent its temperature changing quickly. The plates may be developed in metal trays floated inj the tank, or in a closed box immersed in the tank. This general method has been used in considerable] experimental work. (b) Again assume the plate a constant factor by I using one brand. Make up the developer from water and concentrated solutions, all of which have stood in the room long enough to have acquired its temperature. Read the temperature of the mixed I developer and by means of a development time- temperature table determine the time. In thisj Art. a list of temperature coefficients for some oi| the ordinary developing agents is given. The tem- perature coefficient is the factor by which the speed I of development changes for a rise of lo" C/ In 'For i" C. the coefficient would be the antilog of i/io the lof of the coefficient for lo" C. ' If GELATINE DRY PLATE 49 another column is given the coefficient for T C. If a suitable time giving the contrast desired has been determined, as in method (a) above, the time for each degree above (or below) the temperature used in the test may be found by successive multi- ' plication (or division) by the one degree coefficient. In this way can be built up a table giving the times I of development for the various temperatures likely I to occur. This is probably the most usable method for tank development and for plates which are sensitive to the dark room light and must be de- veloped entirely in the dark.« It sijould be observed however thai these coefficients are dependent some- what on the plate usca « and are not constant over a wide range of temperature, bui when used as above the errors will not ordinarily be troublesome. (c) The Watkins Factorial System.^"— Watkins found by experiment that the time taken for the first detail of the high light of a negative to appear when immersed in the developer, was not dependent on the exposure if it was within the usual limits but dependent only on the speed of development.' I'v^stfandCo ^I^'y^'o;? ^^t"-^"^' f T k"^' ^^ ^^" I published. ^- ^- ^ '911). This IS one of the best texts now -w^t''°.^P.P*'"'^'''' 1'^^^ ^'9. on development. iPhoto^raZ n""v' \'^' ^".^ i?"""' Chicago; Watkins' rnowgraphy, D. Van Nostrand Co.. New York r mm • I M«s .„d Wra...„, BrU. Jour, of pU.. v„r54 p 560 Vw)'. -H ■.llS3i.v?-'*5«fflW> - 1^ 50 PHOTOGRAPHY In consequence this time is a measure of the speed of development, and one only needs to continue de- velopment for a definite multiple of this time to reach the same contrast in all cases. This multiple, called the "Watkins Factor," is not dependent on the plate used, nor on the temperature of develop- ment, nor within moderate limits on the concentra- tion of the developer, but is dependent on the de- veloping agent used and on the contrast desired in the negative. See the Manual, Exp. 6, for detailed directions. Below is a list of factors for some of the common developing agents where the contrast reached is 0.9 gamma. Table 7 Developing Watkins Factor Temp, coeff. Temp, coeff. agent for = 0.9 for 10° C for 1° C Pyro, 8g/L, no bromide 9 1-5 1. 041 Metol Hydro as in Manual . . . 10 I.Q 1.066 Hydrochinon with Bromide 5 2.2 1.082 Ferrous oxalate 1-7 105s Rodinal 40 1.9 1.066 •s, ,- J ! 34. Contrast. — Contrast is defined as the actml difference in density between the two points being compared on the negative, or dj — di = contrast Mi' GELATINE DRY PLATE 51 lit is not concerned with the magnitude of the den- Isity, so that a negative in general very dense may Ihave very small contrast if it has small differences jin densities between the various points. In a nega- Itive which has been exposed so as to fall within the jperiod of correct exposure, contrast depends on [three things: (a) Brightness differences in the image al- lowed to fall on the plate. In studio work Ithis is under ready control, but in outdoor work the lonly control is by choice of time of day or of char- latter of day. (b) Development factor. As devel- jopment proceeds the densities increase proportion- lally so that the differences increase, or contrast Igrows with the progress of development. When Iproionged, development slows up or stops and maxi- jmum contiast from this factor has been obtained. 1(c) Fog. It is the result of the reduction by the jdeveloper of the silver bromide which has not been jaffected by the light. Obviously the most fog will jbe produced where there is the most unacted on jsilver bromide, that is, in the thin parts of the nega- Itive. Its effect will always be to reduce the contrast land it may be a very material factor in moderately [loncf development. Maximum contrast is obtained by choosing a iplate having a large possible development factor (process plates), exposing it to a brilliantly lighted M J^'iJK'.KiA 52 PHOTOGRAPHY subject, and developing it for a long time in a de- veloper which produces as little fog as possible. This contrast may be increased greatly by intensifi cation. 35. Thickness of Film.— The effect of the thick- ness of the film on the glass plate is shown by the curves of Fig. 8. For a given exposure and i ■ I ih ■$ i '! amount of development, the density depends on the | thickness of th-^ film ; in general the thicker the film the greater the density. The thinner coated plates i have a much shorter straight part to the curve, that | is, the period of correct exposure (called latitude) is much shorter, the exposures have to be more ac- curately timed to fall within it, and it may be even GELATINE DRY PLATE 53 incapable of rendering strong contrasts. The thin- ner coated plate is however a trifle faster but the greater speed is obtained at too great a cost, much the better all round plate is the one with a generous amount of the silver salt per unit of area riititud';'!'' J''^'"™' '"" ^'f'""* Exposures (Lat.tude)._Suppose a number of negatives be made giving each a difl'erent exposure but yet such a one that It falls completely within the period of correct exposure All are developed for the same length of «me m the same developer. This will make all the plates fall on the same characteristic curve The longer exposure given a plate merely multiplies the exposure everywhere on the plate without affecting he proportion between the various spots on th: pate. Ihat is, the amount of light coming from he brightest object will be the same multiple of that ^rom ti.e weakest object for each of the different Mates. In the regular characteristic curve diagram qual multiples of exposure are plotted as equal 'oriental shifts. Hence in the diagram Fig o f xy are the limits of exposure for one plate ' .v_ wil oe for the other, and the distances xy and J will be alike. Then the distances be and 'c will also be equal. That is, the differences in 'nd they w.ll g,ve identical prints. One plate will « denser than the other, ax' being greater than ax 4 54 PHOTOGRAPHY 1 1! and will require a longer printing exposure, but if the printing exposures are made such that the same amount of light gets through to the paper in the two cases and the papers are developed alike, the resulting prints will be identical." ij-' ft --i hi 1^ (, I J Y^ >- d z UJ o ^' 1 X Y x- Y- if UfcE Fig. 9, This result is very important, as it means that it mahes no diflference in the print where the plate exposure lies within the period of correct exposure, The latitude of good plates is large, about a factor of 60. Outdoors, in sunlight, the sky will be about 30 times as bright as a piece of black velvet.*' An interior picture including a window will exceed this considerably. On the other hand, subjects whicii do not include any brightly lighted white articles " See series of prints published by F. D. Todd in Photo Bea- con 16,300, Nov., 1904, where the plate exposure was varied from I to 16 and the prints are indistinguishable. " C. E. K. Mees, Wilson's Photographic Magazine, Dec, 1913. m m GELATINE DRY PLATE 55 or very dark ones will be considerably less than this which gives considerable margin, a factor of at least 4, often more than 10, in estimating exposures within which the prints will be the same. 37- Organic dry plate developers are very much alike. The active material is some good reducing substance like pyrogallic acid, metol, hydrochinon, fjlycin, amidol, and so on for several hundred. Too strong a reducing substance will attack the whole of the silver salt, so that the substance used has to be of a reducing power such as to distinguish be- tween the two states of the silver salt. Experiment has shown that there is nothing gained in having It present in greater proportion than a few grams per liter, for example about 4g per L is the maxi- mum useful amount of pyro. A few of the devel- oping agents work without the simultaneous pres- ence of an alkali (amidol, dianol). Most organic developing agents require the presence of some al- kalme substance to cause it to become active. It is possible that the hydrobroriic acid liberated by the reaction prevents further reduction unless it is neu- tralized. Almost any alkaline material will serve, the usual ones being the carbonates or hydrates of potassium or sodium. The carbonates are to be preferred, as they are not as destructive of every- thing they get on including the skin. The speed with which the developer acts depends more on the r M i 5«5 PHOTOGRAPHY 3-1 If i' r I concentration of the alkaline substance than on that of the reducing agent itself. As a rule most developing agents stain a little, some stain vigorously, and the sulphite has the prop- erty of holding up this staining habit, but most de velopers will work well without it. Unfortunately the sulphite adds strongly to the fogging properties, but this fogging may be prevented for the ordinary times of development by the presence of a definite amount of potassium bromide. This latter is called the restrainer, as it also slows up the action of the developer on the latent image. In a general way the velocity of development depends on the concentra tion of the developer, and halving the concentra- tion, that is, doubling the volume by adding water, will double the time necessary for the same contrast, that is for the same development factor. In the use of the tank one can calculate the time required as inversely as the dilution, though it is only approxi- mately true and will not hold over wide ranges. 38. The choice of a developer will turn upon several factors. It should be cheap, easily com- pounded, keep well when inade up, produce ver} little fog, and not stain the gelatine, paper, or fin- gers. There should be sufficient of the active agent present to complete the development of a dense plate without the solution becoming exhausted. The alkali should be adjusted so that the time required iifi ^JiSWKH-if GELATINE DRY PLATE 57 is a convenient one, three to seven minutes,— so that the time required for immersing the 1 'ate in the developer, or for changing it to the hypo will not be a material fraction of the time of develop- ment. If sulphite is present it should be there in sufficient quantity to hold up the staining for the time required for the development. In paper de- velopers the presence of the sulphite will require the [presence of bromide to hold up the sulphite fog I which would gray the whites.*' The effect of the bromide on the lower gradations makes it advisable to omit it from the plate developers. Developers for use in tanks must be chosen with special refer- ence to their resistance to air oxidation and to free- dom from staining habits. In all cases the smaller the fogging property the better. The developer advised by the maker of the plates or paper, while hot necessarily the best in each case, may be de- |pended on to give good results. 39. Estimation of Exposure- Exposure is de- fined as the ratio between the amount of light and the area on which it falls, or in other words the amount of light per square centimeter. It cannot be too strongly emphasized that to make a satisfac- tory negative the plate must be correctly exposed, |that IS all the different amounts of light from the /--"sS^Q^^^ ^°"" °^ Developers." BrUisH -ii 58 PHOTOGRAPHY wr •S # '■9. V. different parts of the subject must expose the cor- responding part of. the plate so as to fall within the latitude. The latitude includes the exposures falling on the straight part of the characteristic curve, and a limited amount of the curved parts may be included without noticeable loss. This leaves a good margin with almost all subjects and irost plates. See Art. 36. The different factors affect- ing the exposures are: (a) the time for which the camera remains open, (b) the stop used, (c) the i speed of the plate, (d) the character of the sub- ject, and (e) the lighting. The first three, time, stop, and plate speed, practically are easily man- aged and fairly constant factors, requiring only a little experience to make their management certain. The subject is often deceptive and uncertain, but judgment of the lighting is of all the most continu- ous and persistent difficulty. All these factors are discussed in detail below. 40. Shutters. — The time for which the camera remains open depends usually on the mechanism called the shutter. When used merely as a means of opening and closing the lens ("time" and "bulb" settings) they are usually reliable, though they will occasionally fail to open or close as expected, so that some experienced workers refuse to use any save the very simplest, most reliable types. There are a number of different types of shut- GELATINE DRY PLATE 59 ters, drop-curtain, revolving sector, roller-blind, diaphragm, all of which are placed close to the ler j either in front, behind or between the compo- nents. Undoubtedly the most consistent, accurate, and efficient are the roller-blind, focal-plane shut- ters. They can be fitted to almost any camera, but are expensive, somewhat bulky, and less simple to use. In these a roller blind, with a slit of variable width across it, passes just in front of the plate at an adjustable speed. They are the only fully depend- able shutters for speeds above 1/50 second. The great majority of cameras are fitted with be- tween-the-Iens shutters, in which the iris diaphragm opens and shuts, or separate blades move to open and close the lens. There are several ways in which shutters give trouble. The most difficult to deal with is lack of consistency. They do not always give the same duration of opening when set in the same way. Most shutters now use the escape of air from a partly closed cylinder as the means to control the time. To obtain rapid motion without jarring the camera, the moving parts have to be very light and the springs weak, under which circumstances fric- tion becomes a very difficult thing to keep constant. The time markings are usually intended to mean the time between the first and last appearance of opening. With a moving subject the duration of i- -h H 6o PHOTOGRAPHY :• %■ exposure must be short enough that the blnrritifj produced in the picture will be barely visible. Very few shutters are reasonably true to the tiims marked. The highest speeds seldom exceed 135 second, that is the settings marked 1/25, 1/50, i/ioo, all give 1/20 to 1/35 second. The slower speeds can Ix.' adjusted to offset the aging of the spring and the wear on the hearings but wit'iont any great effect on the higher speeds. With this way of marking the times, no account is taken of the fact that the shutter requires tiir" to open and time to close. For speeds of one sec- ond the proportion of the time spent in opening and closing is small, so that the amount of light getting to the plate is almost as great as if the time spent in opening and closing was zero, and the efficiency is therefore nearly unity. But as one goes to the higher speeds the proportion of the time spent in opening and closing becomes greater, so that the efficiency may drop as low as 25%. In many shut- ters the higher speeds are limited by the time re- quired for opening and closing, and the lens is at full aperture only a very small fraction of the total time. One can conclude that the reliability, the accu- racy and the efficiency are none too good and that they are often unsuspected but prolific sources of trouble. The complete testing of a shutter for ac- w^isu:^.^m GELATINE DRY PLATE 6i curacy and efficiency is a complicateci matter. Suf- ficient information for ordinary use may be obtained by exposing a plate in sections by each time covering tr.- rest of the plate with black paper. Use a uniform, evenly lighted subject— a white brick wall for instance— and change the shutter setting for each section, at the same f' n. changing the stop so as to offset the chang m tim- ^-^ development all the sections shoul.i k u.if >-t>. ui density. If they are not the troc" i . V ,:, . c r- tainly with the shutter, as the di-.;): ,.... ,.n\, du- ally be depended on. By meas , n^ .^ 'u,min i . the photometer one can determir.t )- . >•; ,'thj shutter is wrong in the matter at lea,t - '.-h ror- earns the user most, namely the total ....uuc of light admitted. Fig. 10. 41. The Stop—The system of numbering ap- PW to stops requires a very detailed explana- In the figure let ab represent the diaphragm in the lens through which the light passes to the plate 62 PHOTOGRAPHY IP CE. Consider a single point in the opening ab; from it the light will spread out in a cone to cover CE. The amount of light falling on a square centi- meter of CE will evidently depend on the distance of CE from ab; the greater the distance the greater the area over which the light must be sprep.d. The area increases as the square of the distance x in- creases, so that B : i/x« where B is the brightness of the image. This will evidently be true for every point and consequently for the whole area ab. The amount of light entering ab will depend on the area of the opening (usually circular) increas- ing as this area increases, that is where d is the diameter of the circular stop ab. These two proportions may be combined, thus B : dVx» For the ordinary use of the lens in taking pictures (not in enlarging) x is so close to the focal length of the lens that for this work we may replace x by F, and write B : dVF» Hence if we make the diaphragm diameter the same proportion of the focal length of the lens it is to GELATINE DRY PLATE 63 be used with, we ma^ ^hift from one lens to another without changing the brightness of the image and ! consequently without changing the time required for exposure. For this reason the diaphragm is always referred to as a proportion of the focal length. These proportions are whai are called the "F numbers," thus "F16" or more strictly "F/16," means that the diameter of the diaphragm referred to is 1/16 of the focal length. A factor of 2 in the F number will mean a factor of 4 in the image ! brightness ; see the above proportion. For ordinary use a factor of 2 is very convenient, so that a series of F numbers are arranged such that the image brightness changes by a factor of 2 and there- fore the F number by a factor of the sp'iare root of 2. These F numbers are given in the following table in the left hand column. Table 8 Faumbers U. S. numbers 4 I S-6S 2 8 4 "•3 8 16 16 22.6 3» 32 64 45-2 138 64 256 The series in the right hand column is called the Inivcrsal System numbers and is a little simpler -ri ^i 64 PHOTOGRAPHY [*-i- i-^i^^jy"^ in use. The number 16 is chosen to agree with the F number, and from this point the numbers run both up and down, always changing by factors of 2 in the number and in the image brightness, but an increase in the number corresponds to a decrease in the brightness, and therefore also to an increase in the time of exposure to keep the exposure the same. Shutters may be obtained marked with either series of numbers, the U. S. probably being the more usual in this country. When referring to the speed of a lens the F number correspijnding to the largest stop which can be used with the lens is usually given. 42. Dry Plate Speeds."— By the speed of a dry plate is meant the ratio between the opacity in the negative and the exposure required to produce that opacity. A fast plate is one which for a small ex- posure will give a dense negative, that is the ratio O/K is large. Plate speeds vary a great deal. Not only do they vary when they are meant to, as in different makers' plates or in different brands fruin the same maker, but different plates of the same brand or even from the same batch will vary vme- what. For speed determination it is necessary t find a reproducible light which shall bear some dcil nite relation to daylight, to fix the conditi..ns oi ^* Watkins' Photography, page M^y , Slicppard & Mccs, !n:::- ttgatwns, p. 278. GELATINE DRY PLATE 65 exposure and of development, and to test the result- ing negative. (a) Hurter and Driffield's work has led to the most satisfactory method of measuring and ex- pressing the speed of plates, but it still leaves the difficulty over the light. In Art. 27 Equation 6 gives the relation between opacity and exposure as E/i -o j where i is the distance of the point z from the origin [ and IS best expressed in exposure units. This equa- I tion is true only for a development factor of unitv lAIso ^' 1/i = 0/E [from which it appears that 1/i is a measure of the speed of the plate. A somewhat different argu- ment will make this even more evident. In Art 31 IS described the way the characteristic curve jshitts with time of development, which for devel- l<'pers not having any potassium bromide, makes jthe common point z lie on the axis. Fig 1 1 gives lanother example taken from the later work of pppard and Mees. The distance of z from the Wm IS the quantity above called i; i is therefore he exposure which is necessarv to bring the densi- les onto the straight part of the curve and is called he inertia" of the plate. Obviously if the inertia fs lar-e the plate will be a slow one as it will take t ! m "t ' i^i. 66 PHOTOGRAPHY a large exposure to bring the plate to the useful part of the curve. Conversely for small inertias. From all which it appears that the inertia, i, is ac- curately inversely proportional to the speed of the plate. The "H and D number" often given with ta y / / / f / y ^ > / _rl y LU > / \y ^ ^ y ^-^ D y L/ -^ ^ ^ -^ A X ,»<• r' r*^ /" ^ ^ fl r-^ ^ i^ .^ Fig. II. Loe e English-made plates is the constant 34 divided by the inertia."* American plate manufacturers and photographers have hardly used this system at all (b) In this country the usual number one sees referring to the speed of the plates is the Watkin: number for use with his meter. This is based on calling a plate unit speed when it takes two second? in bright sunlight, in midday in June, with stop 8 to make a minimum correct exposure. This is a very "See appendix on Plate Speed Systems. GELATINE DRY PLATE 67 slow plate, and the numbers run from about 20 for a slow process plate to about 500 for the fastest The numbers increase with the speed and hence the time required for an exposure is in the inverse ratio the exposure decreasing as the number increases.' These numbers are mostly useful for comparison and one has to remember some exposure to start with as correct, and then modify it according to the plate, time of day, etc. Besides these systems of referring to the speed of plates there are two others one sees in English works, called the Wernerck number and the Wynne number. 43. In connection with the speed of plates, there IS an interesting deduction from the characteristic curve as to a method of increasing the speed some- what. Every part of the plate should receive enough exposure to carry it to the straight part of the curve, that is, an exposure corresponding to the inertia. After that the light from the subject be- comes really effective in building up the desired atcnt image. Evidently one could give a uniform exposure all over the plate, for example bv opening e hokier to weak, artificial light for a short time! then the picture one wants can be taken with less exp-"re to the subject-that is the speed of the pen increased. The increase however is small and hardly worth the trouble, except in unusual I can., as m astronomy, where the observer has to 68 PHOTOGRAPHY hold the object to be photographed on the cross hairs of the telescope so that the image will stay at the same place on the plate, often for hours or all night, so that an increase of a few per cent in the speed of the plate will be welcome. The speed of dry plates are also connected some- what with what is called the "grain." It will be recalled from the way in which the emulsion was made that the silver bromide will be in the form of little discreet particles, and when the bromide is reduced to silver in development, the silver parti- cles will correspond somewhat with the brcjiiiide ^articles. These silver particles are what is called the grain of the plate. The size varies greatly, de- pending on the method of making the emulsion, the rime of boiling, etc., the fast plates having almost nvariably the larger grain. In the slower i)lates a microscope is required to make them visible Imt in ame of the fast plates they are almost visible to he V ided eye, and under these circumstances will : ff e. he geometry of the picture somewhat. A hw* grained plate is much to be preferred." 44. Character of Subject.— The subject may af- fect the exposure in several ways, by its color, in- cluding white, gray and black, by its shadows, and "Microscopic studies of the dry plate are of great iiUcrest. See series of articles by Dr. VV. Schafer, British Journal. -J. 540 (1907) and 55. 472 (190?;. GELATINE DRY PLATE 69 by its distance. As will be discussed in detail later, Art. 47, the ordinary silver bromide emulsion is most sensitive to the blue light and least sensitive to the deep red. With the ordinary plate, photo- graphs of red objects, a red brick building for ex- ample, have to be taken by the scattered white light which is always mixed with the colored light from any object. In consequence red objects need a ^ markedly long exposure. This is particularly true I when there is detail in the red which it is desired to have rendered. The best remedy is discussed later under color sensitiveness, Art. 51. Gray or black objects, or any dark-colored objects except blue, shadows where the illumination is weak as in deep wmdows or doors, under trees or under the bnni of a hat, all send a relatively small amount of light to the camera, and the general exposure of the hvhole subject has to be increased so that these places Ijvill get sufficient exposure to record their detail j If the picture includes any near foreground it is apt to be less exposed than the rest of the subject and a ^j^^^„^ ^^^^.^ .^ ^^^ ^^ ^^ ^^^^^^,^> ^^ • h a fog due to the blue haze always present more or Jess m the atmosphere probably due to fine dust 45. L,ghting.-For some typical subjects with e ordmary fast plate (200 Watkins) the actual exposure m June, between 9 a.m. and 3 p.m., with 70 PHOTOGRAPHY the sun shining, and using stop i6, are about as fol- lows: Average subject outdoors, street scenes, figures., i/isseti Light open landscape perhaps including some water 1/30 " Sea, clouds and sky, snow 1/150 " Dark outdoor subjects, such as trees H to i " Indoors in well lighted room i to 10 " Such a table can only be a general guide. In cloudy i weather these times must be multiplied by a factor j up to 4 or 6, depending on the amount of cloudiness. The following table will serve as a guide to the I effect of the time of day and the time of year. Table 9 a.m. p.m. June May April March Feb. Jan. Dec July Aug. Sept. Oct. Nov. 12 or 12 I I 25 15 2 3 5 4 II " 1 I 125 IS 2-5 4 5 10 " 2 I I 25 1-75 3 5 6 9 " 3 1-25 i-S 2 4 12 8 " 4 15 i-S 2 3 10 7 " 5 2 2-5 3 6 6 " 6 2 5 3 6 5 " 7 S 6 To keep such data as this in portable form and , to help in the calculations there are a great numtej of exposure calculators and exposure meters on the market. They are mostly applications of the slide rule principle, one scale taking account of the dill ference in subjects, others of the time of day, ofi GELATINE DRY PLATE 71 he time of year, of the speed of plates, and of the Itop. Another class of these exposure meters in- cludes a device for measuring the photographic blue of the light. This is accomplished in two dif- rerent ways. In one class, which includes the Wat- kins, the Wynne, and the Johnson meters, the light is |liowed to fall upon a piece of light sensitive paper hd the time noted which it requires to darken to ^atch a neighboring gray patch. This time is a tieasure of the photographic value of the light and s used to calculate the exposure. In the other class ^hich includes Heydes Actino-Photometer, a wedge \i blue-gray glass is placed in front of one's eye hd by shifting to the darker end of the wedge the' leta,! observed begins to become invisible. The ^)S!tion of the wedge at which this occurs gives a [leasure of the amount of light and is readily used calculate the exposure. The second class has ome po,nts of superiority-greater rapidity in ^eak light, measurement of the light coming from he subject, not of that falling on it, freedom from anation ,n the sensitive paper, but on the other and ,t depends on the constancy of the eve. Anv these ight meters are very satisfactory helps and 'II qiHckly pay for themselves in plates saved, but pey have to be used with judgment, as they cannot f e care of the great variety of actual circum- CHAPTER III PROPERTIES OF THE GELATINK DRY PLATE- COLOR SENSITIVENESS 46* Anyone using the ordinary silver bromide plates will soon notice certain peculiarities when they are used for making the usual nionachronie (black and white) prints of colored objects. Wht objects, even dark blue ones, will appear almost white in the print, while red, yellow, and green will appear in this order, frctm black to gray. The brightness of the image for different colors is not at all as the eye sees them. There is also a sciund difficulty in many colored subjects. If the c.\[)o- sures are made right to show the different shades of blue, then the reds are uniformly black, showin;; no difference between the different shades of red that is giving no detail in the red. P»y making very much longer exposures a certain amount of detail can be obtained in the red, but then all the (ktail in the blue objects will be lost ; they will appear uni- formly white. The order of decreasing action of the different colors in making their record on the| plate is, violet, blue, green, yellow, orani;o, red. 72 nm^i^ii -t'S7 m^SmmJmk v^^-^^'.'- - '?S^^ COLOR SENSITIVENESS 73 that is practically the short ,0 the long wave lengths m the visible spectrum. This is not the order at all for visual color briRhtness, yellow being the brieht- est color. As a matter of fact, the ordinary plate docs not render visual color brightness at all. We have become so used to black and white pictures showmg the shadows and the geometric form, that the lack of this less important detail is not felt an less the object be very brightly colored It will hardly be necessary to point out that color IS an attribute of light, that the color of an object IS merely the result of its power of reflect- W some of the colors of light and absorbing others Most colored objects reflect light which is a mixture « *red lights, and we call it an impure color as If, • 'T''"""'' "^'^ ~'°^ rendering of I he Plau. ,t „ advisable to use pure colors and ge their effect on the plate, and then the effect of fhl ..Ptno colors will be ,he sum of the effects o the different colors composing them. V- Color Sensitiveness Curves.- There are va nous means of separating the different pure colors -.r^-g white light into a band of p'ure colo ,1 ' ■'""'"'";• ^"-^ *«»' «f 'hese means for ,rp„se IS probably the one we use in the lab- V V ,11' f '""!• ''"'' " ^P""''' 'he colors out "'* "" '^'""e 'he spectrum. Reference must be &;" MICROCOPV RESOLUTION TEST CHART (ANSI and ISO TEST CHART No. 2) 1.0 I.I lii IS u 1^ I I 25 1.4 2.2 |3^ m 1.6 ^ /1PPLIED IIVMGE Inc ^^ 1653 East Main Street ^S Rochester. Ne« York U609 USA .as (716) 482 - 0300 - Phone ^S (716) 288- 5969 -Fox ■^^?S. 74 PHOTOGRAPHY made to any text book on physics for an explana- tion of its action. Accepting this grating spectrum then as giving up the sum total of the different colors of white light distributed in strict proportion to the wave length, it will only be necessary to let this band of color fall upon the sensitive plate and then develop it as usual, in order to test the effects of the different colors. To get the greatest amount of information it is best to make a series of increas- ing exposures so that the part of the light which has only a small action will be able to record that which it has. For the ordinary silver bromide plate the action is greatest in the blue-violet, decreasing rap- idly to almost nothing in the yellow and red and only a little less rapidly in the violet and the ultra violet. H we plot density vertically, and color (bet- ter wave length) horizontally, we will have a curve which Yviii represent the color sensitiveness of the plate, since the sensitiveness is approximately pro- portional to the density produced. In the same way we could measure and represent the color sensitive- ness of plates made of the other silver haloids, sil- ver chloride anc' silver iodide; see Fig. 12. In the visible part of the spectrum they do not differ very greatly from each other. 48. Dyes.— Some time before gelatine plates were known, a German worker. Professor \'o,2:el, discovered that treating the collodion plates before COLOR SENSITIVENESS 75 exposing with certain dyes changed their color sensi- tiveness very markedly. Since his work the number of dyes known to have such action has increased very greatly.* It was a very direct experiment to ■ 4 '■ I »— — 1 1^— J L— _l L_J Fig. 12. Curve A is the color sensitiveness of silver bromide gelatine emulsion. Curve B that of silver chloride gelatine emulsion. Curve C that of silver iodide gelatine emulsion. try the same thing with gelatine dry plates, where they were found to have the same kind of action. It has been found that the dye has to actually color the silver bromide particles themselves; merely col- j2' t ^^"V "^'f "* "^""'^ '" ^^'^'^ Sensitizing." British •"""^•^ 54. 305 (1907). r.'ST-S.'fff.^^.M^S^ 76 PHOTOGRAPHY oring the gelatine does not give the action referred to. That is only those dyes which are actually ab- sorbed by the silver haloid change the color sensi- tiveness. The amount of the dye required is very small; the usual bathing solution for all the dyes is about i part in 100,000 water. As a matter of fact, the amount of staining of the silver bro- mide depends very little on the amount of dye in the solution ; the particles absorb so much and then will absorb no more. From this, as one would ex- pect, it is very difficult to wash the dye out, prac- tically impossible in fact. Enough of it to give a very noticeable effect on the color sensitiveness re- mains there after prolonged washing. The dye so- lution is composed of very fine particles of the solid dye suspended in water (colloidal solution) and the smaller these particles are, the better they are absorbed by the bromide. Dilution of these solu- tions divides the particles, which is one reason that a dilute solution works better than a more concen- trated one. Only a few of the very great number of known dyes have this property, and the dyes have been very extensively tested, two workers recording the test- ing of 600 dyes, for example. There is not any apparent rule as to what dyes will prove useful for this purpose, and dyes which are very cloi ely re- lated in composition are often very different in this m^^'^/sw^w-^T COLOR SENSITIVENESS 77 property. It is worthy of note also that the par- ticular haloid of silver used influences the action of the dye quite materially, some dyes serving for one when they will not act with the others. The vehicle in which the haloid is carried has also a marked action; dyes serviceable for gelatine plates will often prove quite useless for collodion. Why certain dyes should have this sensitizing action on the silver bromide is not at all evident. Some of these dyes are bleached by light, and one theory proposed assumes that the compounds formed by this bleaching cause or help the reduc- tion of the silver bromide in their neighborhood. Against this explanation is the fact that the ease of bleaching and the sensitizing action are not appar- ently related ; dyes which bleach easily do not give the best or greatest color sensitiveness, and some dyes which hardly bleach at all give good color sensitiveness. Apparently the best one can say is that the silver bromide dye compound, or complex as some writers call it, acts toward the other col- ored lights as the simple silver bromide does toward blue light. Later we will consider some of the theories for this action. 49. Absorption Curves.- Draper's law for the action of light on substances only needs under- standing to make clear its probable truth, and it has also been proved experimentally. It is to the i 78 PHOTOGRAPHY effect that only the part of the light which is al)- sorbed can produce chemical — rather photochemi- cal — action. In other words, light which passes right through a substance will not produce any change in it. That is to say, if wc determine what colors of light the dye absorbs when it is used to dye the silver bromide, these will be the colors for which it will sensitize the plate. The colors the dye absorbs when in solution in water or in alcohol are generally much the same as when in the silver bromide, so that if one lets the spectrum pass through a solution of the dye, the colors which are missing will be the colors for which the dye will sensitize the plate if it changes the sensitiveness at all. One places a weak solution of the dye in the path of the beam of light forming the spectrum. and then measures the amount of each color which has disappeared, that is measures the absorption. These values are then plotted vertically against color horizontally, giving an "absorption curve" = which for a yellow-green absorbing dye, for exam- ple, will resemble Fig. 13-B. To the eye the dye solution will appear the complementary color to the ones absorbed, that is in this case red. 50. Absorption and Sensitizing.— Now let us dve an ordinary dry plate with this yellow-green absorb- ' See also Mees, "Atlas of Absorption Spectra." 'W^mm COLOR SENSITIVENESS 79 ing dye, expose it to the spectrum, develop it and measure the resulting densities. By plotting these densities against color we will get a "density-color" curve (Fig. 13-C). As well as the natural color I J J II Fig. 13. Curve A is the color sensitiveness of silver bromide gelatine emulsion. Curve B the absorption curve of eosine in water. Curve C the color sensitiveness of silver bromide gela- tine emulsion dyed with eosine. sensitiveness maximum in the blue- violet (Fig. 13- A), which is always present in dry plates, we have a secondly maximum in the yellow-green due to the dye and corresponding in position to the maxi- ...L; lOmT^SiE' 8o PHOTOGRAPHY mum in the water-solution absorption curve above except that it is shifted slightly towards the red end of the spectrum. These color density curves are also color sensitiveness curves, as the sensitive- ness is approximately proportional to the density under these circumstances. 51. Practical Application. — The curves we get by the use of one of these dyes are always irregular 5 S S S" _ Fig. 14. Luminosity curve for the average eye. with peaks and hollows and not of the kind we would choose for ideal photography.' If we wish to represent things exactly as they appear to the eye, then the plate should have a sensitiveness curve like the curve for the action of the different colors of light on the eye, that is what is called the lumi- nosity curve; see Fig. 14. While visually correct this would sacrifiL . detail, particularly in the violets and blues. For the general case it is better to ask that the photograph show all the detail possible in ' For an excellent discussion of this whole subject, see Th{ Photography of Colored Objects, by C. E. K. Mees, or Dis Arbeiten tnit Farbenetnpfindleckten Flatten, von Dr. Ernst Konig, G. Schmidt, Berlin, 1909. COLOR SENSITIVENESS 8i sen- all the different colors. This requires that the sitiveness in all the colors should be as nearly alike as possible, that is the color sensitiveness curve Plnacyanol Fig. 15. ii J's.^JUL'^ ^. -XHCn St.'Ul^W^^iiSVILWSitl'W. 82 PHOTOGRAPHY should be a straight line running horizontally through all the colors. This ideal is approached fairly closely by a number of panchromatic plates now on the market, with however some sacrifice of plate speed. The sensitiveness curves obtained bv the use of certain good dyes * are given in Fig. 15. It will be noted that the curves for the different dyes are very far from the ideal curve desired, al- though these dyes are among the best known for this purpose. The natural sensitiveness in the blue- violet exceeds all the rest and there are usually quite pronounced dips in different colors. These dips or minima occur at different colors with the different dyes so that it would look likely that one could avoid them by the use of two of the dyes at once, that is dye the plate with a mixture of two or more dyes, or successively with different dyes. When one tries the experiment this way it is soon evident that the effect obtained is not at all the sum of the effects of the two dyes when used separately. It becomes necessary to try the effect of the dyes when used together by actually making the experiment. Such experiments have shown that it is possible to make a fairly good plate by a mixture of certain dyes in certain proportion ° and some of the com- * Some of the best dyes may be obtained from The Bayer Co, 117 Hudson St., New York; Farbwerke Hoechst Co., 122 Hud- son St., New York. *See R. J. Wallace, Astrophysical Journal, 26, 317 (1907). COLOR SENSITIVENESS 83 mercial plates may be made this way, as it is prob- ably the simplest method. But the interference of one dye with the other in the silver bromide can be avoided by adding one dye to part of the emul- sion and another dye to another part and so on as far as necessary to build up the curve desired Then after the different parts of the emulsion have been washed ready for coating- the plates, they are mixed thoroughly with each other ; and the color sensitiveness of the resulting plates will be that expected from the way the different dyes act when alone. In this way it is possible to build up a sensi- tiveness curve which is almost straight, the remain- mg difficulty being the too great sensitiveness in the blue-violet and usually a small dip in the green and in the deep red. There are a variety of names used for such plates by the manufacturers, but the term Panchromatic is probably as good as any 52. Screening Action—Besides the direct s, nsi- tizmg action of the dye on the silver bromid. ther IS another action possible which has to be takei account of; if the dye stains gelatine stn,ngly or • It IS not well washed out, then the surface layc of the emulsion will absorb the light the dye absorb and this absorbed light will not affect the silver bromide. Usually it is an undesirable action but "can be used to cut down the blue-violet maximum w 84 PHOTOGRAPHY by staining the gelatine with a do * which will absorb some of the !)lue-violet light and so prevent this part from reaching the silver bromide, and so reducing this maximum to any desired jxtent. 53. Color Screens.— Attempts to build up the color sensitiveness to eciuality all through the s[)cc- trum all meet finally the difficulty of the invariably greater sensitiveness of the silver bromide itself in the blue-violet. To get over this difficulty there remains finally only the objectionable way of cut- ting down the amount of the light in those colors for which there is too great sensitiveness, so as to leave them on a footing of etjuality with all the other colors. This is objectionable because it nec- essarily increases the exposure recjuired, that is re- duces the apparent speed of the plate. This cutiint;: down can be accomplished by passing the light through a transparent, colored material before it is allowed to fall upon the plate. The more conven- ient of these "color screens" are made by a layer of dyed gelatine cemented between two pieces of glass and fastened in a tube which can be slipped over the front of the lens of the camera. To accomplish the desired result it v/ill be cvi- *Meister, Lucius & Briiniug, Hoeclist, o. M., advertise I'.ryih- rosine — Filter Yellow for subduii.g the blue sensitiveness, and the same dipping bath may contain pinachrom or pinacyanol for sensitizing to the other colors. American agents are II. A. Metz & Co., P. O. Box 753, New York. ..^MmkJ^M COLOR SHXSITI\'E^ 85 (lent from what has preceded that the color screen must be fitted to the particular plate with which it is to be used, and in order to cut down only those colors desired and to the amount desired, the color of the screen must be very carefully adjusted. A study of their absorption curves will show what dye to use, or often a mixture of dyes is needed, and in the ordinary case it will absorb principally the blue and violet light so that the screen will appear yellow or red. The adjustment of the amounts of the dye to use can be calculated approxi- mately from the absorption curve, but the careful final adjustment of the amount or amounts, if more than one dye be used, can only be accomplished by makmg a screen and trying it on the plate.^ A very small change in the amounts of the dyes produce considerable changes in the desired effects. To consider a particular case suppose we wish to make a color screen for use with a plate dyed with only one dye, pinachrom; refer to its color sensi- tiveness curve in Fig. 15. Jf we wish to make the ettect of the green and red light from the object have ar, great an effect on the plate as the rest of the colors, it will be necessary to absorb a large proportion of the blue and violet and some of the orange. The plate is very little sensitive to deep ^esXioeT'"'"' "^°'" ""•''"'" ^^'-^^^--' Journal, .4. - i. .J 1.1 !. 86 PHOTOGRAPHY red, and if we wish to hoM the other colors down sufficiently to make thern all equal, it will make the exposure extremely long, possibly hundreds of times the unscreened plate. This makes the eicposures un- reasonably long, and a compromise is usually made by neglecting the deep red so that the increase in the exposure will not be too great. Such a plate as this is usually called isochromatic where it has very pronounced gaps in the color sensitiveness curve, as distinguished from panchromatic where the gaps (or dips) are small or absent. A screen for a good panchromatic plate may hardly double the exposure. It will be at once apparent that any screen will not usually do for any plate but they should be made to fit each other. Since all plates need screening, mostly in ^he blue-violet, any screen made for an isochromatic plate will help most other isochromatic , plates. There are a lot of diflferent color screens on the market with which the exposure will be in- creased anywhere from twice to fifteen times, and it is a matter of pure guess how they will afifect the picture of a multi-colored object taken on any chance plate. It is much better to buy a screen made for a particular plate and use it with that plate.* "Eastman Kodak Co. & Cramer Dry Plate Co., St. Louis, both make screens adjusted to their own makes of plates. They both issue excellent booklets on the general subject. "JStafSUKJR XkSi. zsifsiw3t^kS!o.'. . ^«.X7r> Tn-rrm^^w.-~ 90 PHOTOGRAPHY the image will gradually fade out, the time required varying very much for the different kinds of sensi- tive films. A great many things retard or hasten the process ; some papers which have a lot of chro- mate in the film to harden it will fade out in a few da>s. Most good plates keep the image a very lonj^ time, and instances are on record where good nega- tives have been developed years after exposure. The action of the light on the film does not begin with the first light striking it, but the action gradu- ally builds up as more light is added. This starting action quickly fades out if only a small quantity of light is allowed to act, and this, light producing small action has to be added again in the next ex- posure. After a normal exposure the formation of the latent image does not cease with the stopping of the light addition but continues for some time, min- utes perhaps, and then the gradual fading out begins. It is possible to make an artificial latent image by allowing a dilute developer to act on a plate, and also by heating the plate somewhere above loo" C. An enormous amount of experimental work ^ has been do/e on the latent image and its properties, and some ">f these results will be referred to later on, but till the theories have been described the 'Sheppard & Mees, "Investigations," Chap. VI.; Bancroft, "The Latent Image," Jour, of Phys. Ghent., ly, 93 (1913). '^Bn>2nu«>'.L. LATENT IMAGE THEORIES 91 above will be the main facts \/hich the theory must account for. 56. Theories.— There are a considerable number of such theories, but for our purpose we will refer only to four. They are that the latent image con- sists of: 1 A physical modification of the silver salt. 2 Small discrete particles of silver (silver grain hy- pothesis). 3 Silver sub-bromide in the silver bromide particles. 4 A solid solution of silver in silver bromide particles. 57. The first theory is that the action of the light is to produce some physical change in the sil- ver bromide, such as for example shattering the particles and the fragments would probably be more easily developed than the unaffected particles (com- pare pressure marks). Or it may be to polymerize the silver bromide or to unpolymerize (that is to divide the molecule into smaller ones, unpolymerize, or to gather several molecules into a larger one,' polymerize), or some writers do not attempt to say what the nature of the physical change is. Then assuming that the change makes the particle more easily developed, the theory will account for the latent image as far as its developing properties are concerned. But this theory fails completely to ac- count for the destruction of the latent image by all the chemical reagents given above ; there is no sim- T I 92 PHOTOGRAPHY pie w?y in which the reagent can be made to gather the shattered particles together again. The theory is not very generally held but deserves mention for its place in history and for its simplicity. 58. The silfcr grain hypothesis begins with the experimental fact that light separates some of the bromine from the silver, so that there are left small particles of free silver scattered through the gela- tine but not necessarily u. contact with the remain- ing silver bromide. When the developer comes along it is assumed to carry silver from the silver bromide grains in the neighborhood and deposit it on the silver grain. This assumption had as justifi- cation Eder's experiment, where he touched a plate in the developer with a silver wire and there was a deposit of silver at the point of contact with the film. It was soon found that the silver wire had to press on the film — compare pressure marks— finely divided silver lying on the film would not produce the deposit. But nevertheless it is an ex- perimental fact, silver particles will grow in size in the film in the presence of the developer and more quickly if there is some silver nitrate added to the developer. But without the presence of the silver nitrate the growth is too small to be of any use practically and will not account for the growth of the silver image. Even if finely divided silver is actually incorporated in the film itself by being i.r LATENT IMAGE THEORIES 93 mixed with the molten emulsion, it will not produce appreciable development unless the silver particles are actually in contact with the silver bromide par- ticles or actually incorporated in them; in which latter case the film develops vigorously. But this case is the theory 4, page 91. Other things also tell against the silver grain theory, as for example the fact that the latent image is not destroyed by dilute nitric acid whereas finely divided silver is very read- ily dissolved by it. Other silver solvents act simi- larly. So that this hypothesis has had to be modified to give the fourth theory above. 59- The silver sub-bromide hypothesis (theory 3, page 91 ) is one which has held the field for a long time and possibly is the most popular theory yet. It is also based on the experimental fact of the separation by light of some bromine from the silver bromide, but it assumes that this takes place right in or on the silver bromide particle and that it leaves a compound of silver with bromine which therefore is richer in silver than the ordinary silver bromide. The composition of this compound is quite unknown, but it is often spoken of as if it were AgoBr, for which there is no real justification. Gary Lee made a lot of substances resembling the latent image in properties and which contain less bromine than the ordinary bromide. They were made usu- ally by taking some of the bromine from silver 94 PHOTOGRAPHY bromide, that is reducing it by substances like de- velopers. The compounds are variously and often beautifully colored; dilute nitric acid will not take the extra silver out of them nor will hypo dissolve them completely, but the latter always leaves a resi- due of metallic silver. Other experiments have shown that some of the metals are soluble to some extent in their fused chlorides. Addition of a free halogen to the latent image destroys it if suitable precautions are taken to prevent the halogen being all taken up by the gelatine. When a plate is black- ened by light free halogen can be detected by the odor; even a short exposure to light produces a visible change in the color of the plate even though no bromine seems to be given off. Probably the bromine is taken up first by the gelatine till pres- ently it becomes saturated. All of these facts can only mean that there will be an excess of silver in the light affected haloid, and experiment has shown that this sort of a haloid is easily reduced. Whether this excess of silver is present in the form of sub- bromide or in the form of very finely divided metal- lic silver suspended in the rest of the silver bromide (that is in colloidal solution), is a point very hard to determine directly, but the indirect evidence favors the free silver hypothesis. 60. The fourth hypothesis is the solid solution theory, which is referred to in the last line of the immm LATENT IMAGE THEORIES 95 preceding paragraph. To repeat, the latent image I consists of very finely divided metallic silver sus- pended in the solid silver bromide much as fine ' particles would be suspended in water. Such sus- I pensions are now called "colloidal solutions." More- over, solutions do not need to be liquid ; many cases are known where a solid dissolves other substances, and these cases are usually referred to as "solid solutions," so that in the case of the. silver in the silver bromide it will be a "colloidal solid solution." In this connection it has been shown experimentally that silver bromide precipitated from solution con- I taining also suspended materials carries down these suspended materials imbedded in the precipitate. This is not limited to suspended solids but takes I place even with substances which are in true solution. These carried-down materials may be the dyes that we have been studying a few pages back, or other metallic chlorides, or gelatine, or albumen, or a great many other substances. They i are carried down even though they are present in very small amount and it is almost impossible to wash them out completely from the precipitate. The presence of these small quantities of inclu- sions may afifect the sensitiveness of the plate very materially, as in the case of the dyes, or in the case of mercuric chloride or of ferric chloride, in which latter cases the sensitiveness will be greatly reduced. 11 96 PHOTOGRAPHY The presence of o.ooi gram of gelatine in 10,000 cc of solution from wh'ch the silver salt was pre- cipitated distinctly increased the sensitivity. The amount of silver actually set free by the light and thus forming the latent image must be very minute. but all these facts go to show that it may be quite sufficient to accomplish the result of makint,^ the silver haloid more easily reduced. Finely divided silver may be carried down in the same way by the precipitated silver haloid. The color of such finely divided silver varies with the state of division of I the silver from \vhite to pink, rose colored, red, brown, blue, to black. The color of the silver haloid precipitate carrying the finely divided silver agrees exactly with it. Treatment with strong nitric acid does not take out the extra silver unless the treat- ment is very prolonged — 25 hours at 100° C in one case. Almost any means of producing the haloid, and at the same time reducing some of it to metallic silver, will give these colored precipitates. The col- ored chloride precipitate dissolves very slowly in ammonia solution, leaving a residue of metallic sil- ver which may amount in extreme cases to "j^c of the haloid but is usually around 1%. Hypo or ammonium chloride solutions will do the same. On exposure to light all these colored haloids blacken very rapidly, more rapidly than the uncolored. They are all very readily reduced to metallic sil- LATENT IMAGE THEORIES 97 ver by developers. In fact, the behavior of these compounds is exactly like that of the latent image. If an exposed plate be fixed in plain hypo it is still possible to get a negative from it by developing it in a solution which will deposit metallic silver. Plain hypo solution does not dissolve metallic silver even when the latter is finely divided, but if acid bisul- phite' be added to the plain hypo solution, the latent image will be destroyed, that is it can no longer be •Jeveloped by a solution which deposits silver, be- cause the acid bisulphite dissolves the silver form- ing the latent image at the same time that the hypo is dissolving the rest of the silver haloid. The silver particles forming the latent image, being bedded in the s. • haloid, are protected by it from such reagents as nitric acid, and the i nee of the silver particles makes the particle of ha.. ess soluble in its solvents but m ich more readily re- duced by developers. 61. Conclusion—From all this work it will ap- pear that the hypothesis of the formation of silver sub-bromide is quite unnecessary to explain the ex- perimental facts, the colloidal solution theory aeing simpler, is to be preferred. Also in a number of cases the latter hypothesis is in better agreement with the experimental facts, as for example in the colors of Gary Lee's photohaloids. It seems best to consider the latent image as consisting of .ilver 98 PHOTOGRAPHY particles suspended in silver bromide in colloidal solution— that is in a very fine state of division. The action of the light has been to set free some bromine which combines with the gelatine, leaving a minute portion of metallic silver incorporated in the particle of silver haloid, where its mere pres- ence greatly increases the ease of reduction. -■^ CHAPTER V NEGATIVE DEFECTS 6a. The following series of defects in negatives are of frequent enough occurrence to require a somewhat extended discussion. I. Thin from (a) Lack of sufficient silver bromide on the plate (b) Underexposure . Dense fn^rnVStex^SlI?""-'"'"''"'-"™ (b) Overdevelopment— Reduction (c) Fog, from light, from age, or from defective emulsion. 3. FnUmg, 4- Dust marks, 5- Air bells, 6. Black spots, 7- Fingermarks, 8. Pressure marks, 9- Halation, 10. Oyster shell markings, 11. Stain, 12. Bubbles in the gelatine, 13- Drying troubles. 63. Thin Negatives.--The negative may lack sufficient printing density so thit detail visible in the negative will not show in t\ . print. This may be due to a number of causes. In the first place 99 > ■■ 100 PHOTOGRAPHY the total silver bromide put on the plate in the emul- sion by the maker may not have been sufficient; that is there may not have been sufficient of the emulsion or the emulsion may have been too poor in silver bromide. The remedy is to make another negative on another make of plate. But it rarely happens with reliable makers of plates and usually requires direct experiment to show that it is the cause of the thinness. Nevertheless if one has a series of thin negatives of which the exposure and development can be relied upon, then it will be worth while to try another make of plate to see whether il will help the trouble. Or it may be underexposed, in which case the shadows will lack detail and the print will be ilat. It may even have what resembles a white veil a" over the print, which one sees very frequently in "snap shots." Occasionally one can build up suffi- cient printing density, so that the print becomes clear, but the intensification will not find out any more detail than there is already there. As well as lacking detail the resulting print, after the intensifi- cation, is sure to be harsh and without half-tones. in general unsatisfactory. If the underexposure is at all serious the only real remedy is another nega- tive giving more exposure, 2, 4, or 8 times as much: there is seldom any use in changing the exposure m less than factors of two. I NEGATIVE DEFECTS loi Or the negative may be underdeveloped, due to too short a time in the developer, to too weak, or too cold a developer. In this case there will' be more detail visible in the negative generally, and particularly in the shadows. Intensification will set the case right exactly as the process simply sup- plements, or rather continues, the incomplete proc- ess of development. In this case, a satisfactory in- tensification is a complete remedy, but in the other cases it is only a palliative, though it will often im- prove matters. That is to say, in the great majority of cases both intensification and reduction are attempts to patch up previous faulty work. In most cases the best remedy is to repeat the work, correcting the fault. In case it is not possible to repeat the ex- posure or in case the subject is a difficult one, these methods may be the only available resource 64. Intensification—A very great many proc- esses of intensification ' have beer: described, any one of which will undoubtedly add density to the Sliver image. With many of them staining is diffi- cult to avoid. With most of them the intensifica- tion is very irregular both on diflferent parts of the same plate and on diflferent plates. The added density is not proportional to the original density .IrrNo^'U" ^- ^- ^''"''"^ f^' '''^' P- 583. or Photo. 102 PHOTOGRAPHY nor does it vary from the proportionality in any definite way, so that the gradation in the resulting negative is largely a matter of chance. In some cases this is due to the intensifier actually reducing part of the image while intensifying the rest. Mam workers recommend the use of such intensifiers ■ (and reducers) as a means of controlling the grada- tion in the negative. For all record photography such manipulation is to be condemned, but in pic- torial photography it has a place as giving the worker control, even if uncertain, over the charac- ter of the picture which in such work has no neces- sary relation to the original subject. A method which may be relied upon to give pro- portional intensification is the one advocated by Chapman Jones ' and described in detail in the Manual, Exp. lo; see also Art. 24. The density is almost exactly doubled by one application and the gradation is not altered. After the final washing the process may be repeated with similar resuiis. In negatives where gradation is not important, as in line drawings and where great intensification is desired, the bleached, well-washed negative may be immersed in a dilute solution of sodium sulphide, " See, for example, W. F. Ellis, American Photography. ?. 293 ';i9i4). ' See Photoriiniature, No. 74, page 67, or also British Joimul, 57.495 (19^0). NEGATIVE DEFECTS 103 by which both the silver and the mercury chlorides will be changed to brown sulphides, thus : 2Aga + NajS = Ag,S + 2NaCl 2HgCH-Na,S = Hg,S+ 2NaCl and the density of the negative will be very greatly increased, but the process cannot now be repeated 65. Overexposure.— Too dense a negative may also be due to several causes. The negative may have been exposed much longer than was necessary and yet hardly be what should be called overex- posed. That is the densities may still lie on the straight part of the characteristic curve. The prints made from it will be correct in rendering of the light and shade, ' nt the time of printing will be long. The safest remcuy is to spend the time required for the printing. If it can be arranged to use daylight, or in extreme cases sunlight, the exposure can readily be made as short as will be manageable. If, however, the exposure has been so great that the densities have got beyond the straight part of the curve to any ex! r the plate will be lacking in contrast and look fogged all over. The printing qualities may be im- proved by a complicated process of reduction and intensification, requiring considerable skill and familiarity with these processes, and it is only in rare cases that it is profitable. Ordinarily the Prmt will be flat, that is lacking in contrast. In 104 PHOTOGRAPHY such cases, the simplest, quickest, and best course is to take another negative with a very much shorter exposure, J4 or l^ or even less, depending on the degree of overexposure. 66. Overdevelopment. — The great density may have been due to too long in the developer, to ton concentrated a developer, or to too warm s -lution, any one of which will carry the development h^o far, that is give too large a development factor. which will mean the exaggeration of the contrasts and the increase of any halation which may have been presen*^ The halation deposit being mostly on the under side of the film, it is practically impos- sible to remove it without at the same time destroy- ing the negative detail. H the plate is important and another exposure too difficult to get, the bes: plan is to spend the time necessary to make the exposure, shading the plrte locally so as to force the exposure at the dense places. A few trials wili often enable one to make quite satisfactory printv Lacking any great amount of halation, the best rem- edy is one of the reducers. 67. Fog. — The large total density may be at least partly due to fog. This has been referred t already in Arts. 19, 34 and ^y. It is a deposit ■ ;' silver in the film of the negative which is \v<: the result of the action of the light used to make the negative exposure. Overdevelopment is liable i' NEGATIVE DEFECTS 105 include as a result a lot of chemical fog If this is even over the whole plate it will merely mean longer prmtmg exposure, but it will not be even as can readily be seen.* The amount <-f fog formed at any given place in a given time depends on the amount of unreduced silver haloid at that place From which it follows that the fog will be greatest at the places of least density in the negative Car ried to the extreme, this will mean that the nega- tive will get black all over, which actually happens 't It IS kept long enough in the developer The effect of the greater fr,^ in the thin places will be to reduce the contrast in the negative, so that while nnrnially the effect of long development is to in- crease contrasts, this increase of contrasts may be partially or completely offset by the formation of tog Reduction will help materially if the right reducer be used, that is one which will dig out the thin parts more proportionately than the thicker |vh.ch will tend to undo the fogging. The fog is' ■able to be a good deal on the surface, which will elp ,ts removal by the reducer. Besides chemical f'^S- reterred to above another common cause is )>?f't struck," due to some unpremeditated expo- sure of the plate to light. The camera or plate Holders may not be tight (if possible never let the 'I.uniicre & Seveuet7 n^;, r io6 PHOTOGRAPHY sun shine directly on the plate holders), the dark room light may not be safe, or the dark room may not be really dark. In case it is even over the whole plate, the safest procedure is to spend the re- quired longer time over the printing, as besides the risk of mishap reduction will often change the con- trasts so as to impair the printing quality. In case the light struck parts are not evenly distributed, as is usually the case, attempts at reduction usually do not improve matters ; the safest procedure is to manipulate the printing exposure by shading part cf the print with the hands or cut-out cardboard so that the denser parts will receive the required extra exposure. It will take a number of trials to get the right proportion between the various expo- sures when satisfactory prints can often be made. In case a large number of prints are desired it will pay to make a new negative from one of the best obtainable prints and from which good p'-ints can be readily made. Fog may be due to the plates being old, or having been kept in damp or hot places. The edges of such plates usually show it more than the middle ; some- times the edges are quite thin, due to the falling on in sensitiveness of the film. Plates go bad very quickly when kept in damp places. From the discussion of the manufacture of dry plates you will understand that the plates can k NEGATIVE DEFECTS 107 badly made so that they foj^ heavily in the devel- oper. But with reliable makers this rarely happens. 68. Reduction — There are a great many re- ducers which one might use, just as th.re are a great many intensifiers. They can be divided into three general classes. (a) Those which increase the density ratios by removing more silver proportionally from the thin parts of the image than from the denser parts. An example of this class is the ferricyanide-hypo re- ducer (also called Howard Farmer reducer). See Manual, Exp. 11. (b) Those which do not alter the density ratios, that is, remove proportionate amounts of silver from all parts of the image. The potassium per- manganate reducer comes near to this though really belonging in class (a).'^ (c) Those which reduce the density ratios by re- moving more proportionately from the denser parts of the image. The best example is the ammonium persulphate reducer. The microscope " shows that class (a) sinks into the film, dissolving practically all the silver pa/ticles as it goes in. The microscope also shows that the silver is distributed deeper into the film in the Huse & Nietz, Scientific American Supplement Dec 2^ p 405 (1916). 'Dr W. Scheefer, Brit. Jour. 54, ^40 (1907), and 55, 472 (1900). II io8 PHOTOGRAPHY denser parts, less deep in the thinner parts. The reducer mentioned then acts just as if one took a shaving off the surface of the fihn, which might easily remove all the deposit from the thin places while leaving untouched a good share of that in the denser places, that is, of course, changing the proportions among the densities. The ammonium persulphate has a very peculiar action. The microscope shows that it attacks the denser parts of the image strongly, while the thin parts are almost proof against it, with the results that it cuts down the contrasts as much as desired. Both the solid ammonium persulphate and the solu- tion in water (37o) used as the reducer arc very unstable, decomposing steadily so that the more stable one, the solid, will keep only a few months. The chemical action of all these reducers is a good deal alike. They change the silver from metal to a metallic salt which is soluble in water oi in some other reagent and is thus removed from the film. Thus for the ferricyanide 2Ag+2K3Fe(CN), = K4Fe(CN)6+K2Ag2Fe(CN)« The resulting potassium ferrocyanide is soluble in water and the potassium silver ferrocyanide dis- solves in the hypo thus K2Ag2Fe(CN)6+3Na2S203 = Na2K2Fe(CN)«+Na4Ag2(S20,)j NEGATIVE DEFECTS 109 And for the persulphate reducer thus, (NH4),S,0,+ 2Ag - 2AgNH4S04 - AgjSO*^ (NH«),SO« and both silver and ammonium sulphates formed are soluble in water. 69. Local Reduction.— At times it may be ad- vantageous to reduce part of a plate and leave the rest unchanged. This can be done by applying one of the above reducers with a brush where it is de- sired to reduce the deposit, and rinsing the plate frequently in water to stop the reducer spreading and to av^oid marks. It is not possible to limit the reduction at a sharp line but it can be made to give nicely graded effects. Fairly sharp line limits to the reduction may be obtained by rubbing the dry film with a cloth wet with absolute alcohol, which does not soften gelatine. The surface of the film can be rubbed off, carrying with it the silver deposit and so reducing the density locally. This may also be accomplished by grinding off the sur- face of the dry film with an abrasive powder or by scraping it off with a sharp knife. It is very difficult to do any of these things so that they will not be seen easily in the print. 70. In normal exposures developed without the presence of added bromide, the proportions between the densities, that is the gradation, was not under any control, and the negative remained always true '?MM no PHOTOGRAPHY ''mm to the subject. The magfnitudc of the density and with it the contrast, was all that could be altered. But by the use of these intensi tiers (except tlio Chapman Jones) and reducers without excej)ti<)n, these proportions, the gradation, may be manipulat- ed very greatly, and the picture will be no longer a true record of the light and shade. One used to studying photographs will frequently detect it. Their use is in helping already very defective ne,t,M- tives, which however they do not make true, and in pictorial work. They have always t( be used with great judgment or they will make matters worse. Their indiscriminate use is to be heartily con- denmed. 71. Frilling. — When dry the gelatine layer ad- heres to clean glass with very great tenacity. As the film dries it shrinks so that there is usually tension between thv film and the glass. If the tilii! becomes exceedingly dry, as it does occasionally in our steam-heated rooms in the winter, the tension • becomes so great that something breaks. Quite fre- quently the film pulls the glass off in small chips which will be found adhering to the gelatine film. When water soaked, however, the union between the glass and the film is weak enough to at times give trouble from the two parting company. It is called frilliny and commonly takes place around the edijes of the plate, but in serious cases may result in luos- 11 NEGATIVE DEFECTS m ening the entire filni from the fi:lass. It is much more troublesome in summer than in winter, and much more in warm ch'niatcs than in the cooler ones. It tan usually be controlled by using cool solutions and wash water, below 22"^ C. (75° R) if possible. Some of the manufacturers have special plates for hot climates in which the ^^elatine used is harder, that is has a hi.i^her meltin^r point and consequently does not soften so much in the warm water. The purpose of the alum which we use regularly in the hypo solution is to harden the gelatine, so avoiding frilling in the later washing and also in the hypo itself. In case one uses plain hypo it will often be necessary to put the plate into a bath of alum after the developer, so that it will not frill too badly in the fixing and washing. 72. Dust — Sometimes on looking over a nega- tive you will find a lot of small clear places of irregular shape and sharp angles which will come out as black spots in the print. These are due to dust on the plate during exposure. In the original box the plates are apt to be free of dust, and if the plate holder be kept thoroughly clean there is not likely to be trouble. Besides seeing that the plate holder is clean, it is a wise precaution to dust the plate very lightly with a lintless duster or cam- el's-hair brush. If the plate be rubbed vigorously the film surface will become electrified and attract ^f t ■ t iia PHOTOGRAPH Y dust strongly — it will be worse than if the plate was left alone. An alternative plan is to blow gently along the film, being very particular not to blow any saliva onto the film, as it will leave a spot. 73. Air Bells. — Sometimes one will come on round clear spots, larger than dust marks, or the spelts may be irregular with gently curved edi,^;;, '1 .. se are usually due to air bubbles stickinij^ tu the film during all or a part of development. Run ning the tips of the fingers or a soft brush over the film right at the commencement of development. before it has had time to soften, will effectually do away with this trouble, or indeed a little vigorous swishing of the developer back and forth over the plate at the start of development will also serve. 74. Black Spots. — Occasionally the film will have small black marks which will print white. Tnese may be foreign particles sticking ^o the ulni and they may or may not have caused an extra deposit of silver close to them. It is well to move the developer over the film a few times during de- velopment to aid in avoiding such marks, but the most important precaution is to keep all trays clean and solutions clear. Always pour out the solutions gently so as not to disturb any deposit there may be at the bottom of the bottle. Care in keeping everything clean, trays, desk, shelf, plate holders, solutions, will banish such trouble. If the plates '^ NEGATIVE DEFECTS 113 frill during development pieces of the film often become detached and Moat around in the solution; in which case it is well to keep the (!cveloi)er mov- inij almost continuously. In /.general it is better to rock the tray only occasionally. In rcK-king the tray do not lift one end suddenly and then drop it right away again— watch the liquid and you will set that it has not moved. But raise one end of the tray and hold it up for a few seconds, then lower it for a few seconds, and the li(|uid will have time to flow over the plate. Moving the licpiid helps to keep the developer alike all over the plate and so helps to secure even action. 75- Finger marks identify themselves readily, and it is best to touch the surface of the film as little as possible, particularly if the skin is moist. When necessary to tell the film side touch only the cd^es or corners. 7t). Pressure Marks. —If the sensitive film be ' subjected to pressure at any time it will deposit Sliver at these places during development, in other words it will develop. The plate can be marked with a hard metal point just before placing it in the developer, and the marks will develop along with I the negative, giving a very convenient way of keep- ing track of plates. For this reason plates are packed so that the films are held apart by paper I along the edges; and T the same reason it is wise iib;i2n^-:> generally and by limiting the development still iiiorc of the halation exposure. When possible ch ( >: your view point so as to avoid such contrasts. ;: they are unavoidable, then cover the back oi t: glass with something which will absorb the lii:':.; which gets through. Black paper wet with g y. erine and rubbed into intimate contact with tlie g'.:; .- will do well if the plate is to be used imniedia:. If not for immediate use a paint of lampblack a: mucilage is better but is a messy job to apply ; [ :.:.- can be bought already backed thus. Probably best arrangement of all is to put the absorbing :. in between the gelatine and the glass, so tha: :. light is absorbed before it gets to the glass. .:> absorbing film can be made of a very slow emu'-; (double-coated plates) or of a gelatine film earr- ing a brown or black dye, which will be bleached :: the acid hypo, leaving the plate clear for printing' See Fig. 17, page 144. si pitei • Such for example as the "Non-halation Simplex" the Lumiere N. A. Co, Ltd., ii VV. 27th St., New YcrK 'W^^^- ttMTv.'jnauiitit*.-',. •tik'nimuwi .''aqi^MnHHHSMiBB^^^i^f^'^^^nuafF'^rTv^aj'wia^fara'n NEGATIVE DEFECTS u; The second cause of the spreading of th- lic>-ht irradiation, is the scattered reflection from the par- ticles of silver bromide in the emulsion, as in the diagram, thus spreading: the light sideways to af- fect particles which should not be affected if the Fig. 1 8. P.c ure IS to be true geometrically to the subject Th,s of course w,ll show most distinctly at the efes 0. a br,ghtly lighted area, for while it^s p esemd fb r ed. There ,s no remedy for it. Fortunately Cue V"=^y^^'-'°"=^ -matter in its effect on the IPWuic. T.,e glass reflection part is a good deal "ore ser,o„s. Halation shows very little in nega m '* ii8 PHOTOGRAPHY t" s where the sensitive film is carried on the thin c _iluloid sheet, as in roll-fihns and film-packs. 78. "Oyster Shell Markings." — ^In the laboratory a number of times now we have run across very peculiar markings on some plates and paper. It consists of wavy somewhat parallel groups of lines often spread over the whole plate. They resemble flow lines in a viscous fluid or the "oyster shell" markings in collodion plates. So far I have not been able to find any satisfactory explanation of their cause or cure. 79. Stain. — With most organic developers, and especially with pyro, the gelatine film is stained somewhat during development. With prolonged development in pyro it may be quite pronounced. Sodium sulphite in the developer helps to check it, and the acid along with sodium sulphite in the hypo bath removes a large part but not all. It ii of no particular harm if not so dense as to unduly prolong the time of printing. 80. Bubbles. — Mention has already been made of the formation of small bubbles right in the gela tine film if the hypo bath is too warm, particularly if the bath is old. The bubbles do not disappear on drying, and if in a negative will show in the print made from it. The same sort of thing shows occa- sionally in prints, particularly in bromide enlarge- ments, caused apparently by too great changes ic ^i^r-^m^ m' 1^. NEGATIVE DEFECTS 119 temperature in the different baths, or by too strong hypo bath. 81. Drying Troubles.—In addition to the care [necessary to remove the deposit which settles from I the wash water on the film before putting it up to dry, there are several other things to attend to in the drying of the film. Prevent the settling of dust on the wet surface of the gelatine by setting it on edge to dry, or even set sloping film side down. The film sags slightly during drying so that it is best if it dries at a uniform rate and in one position, which will give the least distortion of the picture and least chance for variations in thickness. The fibi may be dried somewhat quickly by leaning it film side toward a cold window with a warm room behind. The water will distill rapidly from the I film and condense on the window. Of course, if one tries to hasten the drying by warming, the film will melt and the negative be ruined. If the wet plate be immersed in alcohol containing very little water, the water in the film will pass out quickly and almost completely into the alcohol, the film los- ing its gummy feel and becoming tough and hard. When the alcohol is dried off the film will stand gentle heating to finish the drying, but is still quite easily melted. If the wet film be immersed in a [solution of formaldehyde it will harden the gelatine I20 PHOTOGRAPHY and render it insoluble in water so that the film may be dried by quite vigorous heating. For further reading an excellent list and discus- sion of plate troubles is given by Olaf Bloch in the Photographic Journal, 55, 219 (1915). i it. w-^^^m&m .^^■■::m^^:^M'^^^^^ CHAPTER VI POSITIVE PROCESSES In connection with the work in the laboratory it will he advisable to describe a few of the very great [many methods of making positives. 82. Transparencies—The most obvious way to imake a positive is by exposing another plate either through a lens or by contact, to the light transmit- jted by a negative. The larger sizes made this way Igo by the name of transparencies, and when the glass support of the film is translucent milk glass, jthe effect is in some respects far superior to prints 'on paper. The total scale, that is the diflference be- Itween the greatest blackness and the greatest jbn-htness, is many times greater for the plate, l^^;^ch gives them a luminosity and detail in the ^shadows unapproachable in a print. On the other hand they are more clumsy to view and more bulkv ^0 store. 83. Lantern Slides—The small size used in the Pantern are the most important transparencies. jAmcncan slides are made 3^ inches by 4 inches, I 'hile English ones are 3% by 3%, and French are 8 121 f ( 122 PHOTOGRAPHY by lo centimeters — 3.54 by 472 inches — and gen- eral purpose lanterns should be provided with slide carriers to take all these sizes. All lanterns are made to take the slides loni; way horizontal and it is hence necessary to make the horizontal lines of the subject run the long way of the slide. For a good slide the grain of the reduced silver should be very fine, which requires a fine- grained emulsion, and these are slow ones. In con- sequence lantern slide plates are usually especially made slow plates of fine grain, having a thin layer of emulsion on a good grade of glass. The thinness of the layer makes them cheaper and is better since such dense deposits are not required, as in negatives, nor is such latitude in the exposure necessary Lantern slides are better when they have consider- able contrast, and therefore the negatives from which lantern slides are to be made can usually be developed for longer than usual with advantage, particularly where the subject is blark and white, as in line drawings or ordinary printing. It must be observed that the general properties of the gelatine plate, as already described in some de- tail, are no different in the case of lantern slide plates. Contrast is determined by the subject and the amount of development, so that the maximum density must be controlled by the exposure. The maximum density is always much less than the if^s^^l i#*.v. POSITIVE PROCESSES 123 thinnest usable negative. The one distinctly new denir.nd is that the clear places must be free from deposit, and hence these places must receive an exposure of about half the inertia. Practically in the laboratory this means the least exposure, which, on a generous amount of development, will give sufficient maximum density. Too long an ex- t posure can not be corrected by (|uick removal from the developer, but this will always result in a gen- eral deposit and in lack of contrast. If the nega- tive has sufficient contrast and the development has not been excessive, a deposit in the clear places means too long an exposure. To avoid a deposit in the clear places a developer giving very little fog is desirable, and on this account the ferrous oxalate developer ' is without equal. It has the disadvantage that it spoils quickly when exposed to the air and it is best mixed fresh for each plate. It is also necessary to wash out the developer or rather the iron salts in it, by following a rinse under the tap with a soaking for a few minutes in a solution of oxalic acid, as otherwise the clear places may show a deposit of an iron compound which will not wash out in water. The acid in the fixing bath also helps to remove any such deposit. Most other lantern slide developers are loaded with bromide, which ' See Manual, Exp. 7. 124 FMIOTOGRAPHY while supprcssinjjf fojj^ also suppresses the lower densities, and so interferes with the j;radation. Hm with the ferrous oxalate the advantage of clear places is accompanied hy heautifu! full scale grada- tion so that the extra trouble if any is well repaid. Lantern slides made hy the wet collodion pntc ess a1»e still ranked as perha[>.s the best, as they have very clear h'vj^h lights and abundant detail. Thev require however a great deal of care and skill for first class work, and are not often seen today. 84. Blue Prints.— Probably the simplest of al! the photographic processes for making prints on paper is the "blue print." In it the light-sensitive substance is a salt of iron in the ferric condition Ferric ammonium citrate is the one frequently used, and the action of light on it is to change the iron over to the ferrous form. Both ferric and ferrous salts are usually only slightly colored so that the image has to be "developed," that is the compounii formed by the light has to be changed into an in- soluble colored compound. The substance gener- ally used for this purpose is potassium ferricyanide. which forms a deep blue, insoluble compound called Turnbull's Blue with the ferrous salt, while it form« no insoluble compound with the ferric salt. The paper having a suitable surface to carry the colored image is brushed over with a solution of the two salts dissolved in water and dried quickly and thor- 'j * T^,b&/j^^m POSITIVE PROCKSSES 125 oughly.' Exposed to sunlijrht under a nej^ative, it turns blue showing a vsihlc image; the depth of printing has to be found by trial. If the paper be nf)vv dipped in water the ferrous salt formed by the action of the light will react with the potassium fer- ricyanide, whore it has not already done so, chang- ing' the ferrous image into Turnbuirs Blue, and the extra unused materials will be washed out, thus ."fixing" the image. The paper does not keep very well after it is coated, and when it begins to go bad the whites will be tinged with blue. Keeping the paper thoroughly dry retards the deterioration very much, and, conversely, dampness ruins it quick- ly. It is advisable but not at all necessary that the coating should be uniform, as quite streaky papers will jive good prints if there is suffici-nt supply of the salts at all places, so that the light will have plenty to act upon, and the excess disappears in the washing. Not all papers are suitable for coating; a tough paper with a good surface is required, and often it will pay to size it by coating with albumen or gelatine. 85. Printing Out Paper (P. O. P.)._lt is also called gelatino-chloride paper. This is made by coating paper with a layer of gelatine in which is present some soluble chloride like sodium chloride. When dry this is floated on a solution of silver ni- 'See Manual, Exp. i6. 126 PHOTOGRAPHY tratc, when tlie silver salt reacts with the chloride to give tne insoluble silver chloride, and the other compounds present at first or formed are removed by washing. The gelatine may be partially or en- tirely replaced by albume:., giving the albumciio- chloride papers. The treatment in all cases is very much alikc.^ They all re(|uire sunlight for print ing and give a red image which fades out materi- ally in the fixing; if the hypo is used too strong the bleaching is serious. The red color is usually not considered a pleasing tone, and there is some doubt as to the permanence of the red image, but if it is made with care, especially by the use of fresh hvpo and thorough washing, it is probably permanent. To change the color and to add permanence it is customary to exchange the silver forming the iniatje for gold or less frequently for platinum. Simple contact between the silver in the image and the gold or platinum salt in solution is sufficient to cause them to exchange places, the silver going into solution and the gold changing from the compound to metallic gold, or metallic platinum in case a platinum salt is used. The image in either case is very permanent. With gold the color or tone niav be varied through red to purple to a good black. Most photographers' proofs used to be made on this kind of paper, but of a very poor grade and not very 'See Manual, Exp. 17. :£^aSSJbk^\t^Jl^islJ^ li^^lJis^m^^^- POSITIVE PROCESSES 127 suite ,le for toning. "Solio" is one of the best known makes of this paper. In the past photo-eng^ravers much preferred prints on this paper from which to make cuts. They liked the color, the glossy surface, and the wealth of detail. Opinion is changing somewhat, and prints on glossy developing paper, where the image has a green tone, are now often asked for. Photogra- phers generally still rank prints on printing out paper ahead of prints on developing paper, but the fonner re(|uires more time and labor, so that they have gone largely out of general use. 86. Developing Paper.— At the present time this is by all means the most popular printing method both among amateurs and professionals. Its popu- larity is justified by its ease and certainty of work- ing, by the permanence of the prints, and by the great variety of surface and of ability to render contrast which may be obtained. The paper is coated with an emulsion which is very similar to the plate emulsions except that it is much finer grained and slower. Besides this it usually has pre- servatives added to improve its keeping qualities, and good prints can often be made on paper years old. Two distinct grades are on the market, one called "bromide paper," designed for enlarging work and therefore having a fast emulsion, the other slower, designed for contact printing. By 128 PHOTOGRAPHY ft the choice of the paper on which either is coated §i and by manipulation of the emulsions, the surface of the dry print may be varied from a very brilliant gloss gradually to a very coarse pebble or matt. The choice of the different surfaces is determined by the taste of the worker and the character of the picture; where great detail is desired the glossy surfaces are best, and where less detail is desired, as in portraits, some one of the matt surfaces is to be preferred. It is worthy of note also that the amount of contrast which the paper can render is greater in the case of the glossy surface. Besides this range of surfaces, most makers turn out sev- eral different grades of paper, which differ in their ability to render contrast, some designed to give strong contrast for use with flat negatives or where harsh prints are desired, the others grading over to a paper intended to lessen the contrasts in too harsh negatives. Skillful use of these varieties gives the worker great command over the character of the print. Ever since Hurter and Driff.eld's work with plates efforts have been made to systematize the prop- erties of developing paper. This has finally been accomplished * and the results are of great interest to the user of the paper as well as to the manufac- turer. Since prints are observed by reflected \'v;k * Jones, ""Tutting & Mees, British Journal, 62, 9 (1915). 'f ■^ ^um^m. mf^'-^j^r-.. POSITIVE PROCESSES 129 while negatives are used always with transmitted light, the "density" of the image will have a some- what different meaning in the two cases The curves are very similar to the plate curves and the conclusions with regard to exposure and develop- ment are also very similar. As a rule the papers develop very rapidly, so that the value of gamma quickly reaches a maximum; with the usual bro- mided developers the further progress of develop- ment simply shifts the curve parallel to itself The contrast of the paper is fixed by the slope of this line and its projection on the exposure axis. Usu- ally as this slope increases with different papers the latitude of the paper decreases. As with plates a portion of the ben^ '^nds of the characteristic curve IS used in practice, and the projection of these straight and curved parts on the exoosure axis is called the "total scale." As would be expected the total scale of hard papers is much smaller than that of soft papers. On account of the rapidity of devel- opment it is not possible to control the contrast materially by varying the development. The only control is by choice of the grade of paper. In most cases the printing method is at its best only with a particular amount of contrast in the negative. In fact such an amount that t i. differ- ence m the printing exposures between the densest and the thinnest parts of the negative should be r>f ^s^^v^m^i^m^'^- 130 PHOTOGRAPHY such that when the thinnest part exposes the paper for its maximum density, the exposure under the dense part will be just not sufficient to show gray- ness. That is the amount of contrast in the nega- tive should be such as to give a series of exposures to the print as nearly equal as possible to the full scale of the paper. Otherwise part of the detail in the negative will be lost — a very common case— or use will not be made of the full capabilities of the paper. The latitude of a plate always exceeds the variation in the light intensities in the ordinary subject so that the plate can render truthfully the full scale in the subject. But with printing papers this is usually not so, the total scale of the paper being too short. In such cases one can reduce the gamma of the negative to less than unity by lessen- ing its development, or sacrifice detail at one or other end of the scale. To illustrate, by lengthen- ing the printing exposure with an ordinary nega- tive the clouds may usually be shown on an ordi- nary print, but then the whole foreground will be black; while if the printing exposure be short enough to show the detail in the foreground, then the sky will print white. But if the negative had been developed for a shorter time, thus lessening the amount of contrast, it may be reduced to the total scale of the paper, so that the sky and foreground detail will both appear in the same print. It may LlM^' hi rm' ^^"^^.^^r^-m^^KM POSITIVE PROCESSES 131 also be possible to select a paper with a greater total scale, and so accomplish the same end. A developer such as the ferrous oxalate, or metoi or hydrorhinon, etc., which will not stain the paper is required. An acid hypo will aid in avoid- ing any stain from the developer, and the harden- ing agent, alum, will aid in avoiding trouble in any subsequent treatment of the print. Care is required in the use of the hypo bath, that it be kept clean and reasonably fresh, and that none of it be allowed to get mto the developer. Yellow stains in the print will be traced most often to the presence of a I small quantity of hypo in the developer; these stains 10 ten do not show or are not noticed till the batch of prints IS dry, so that the trouble escapes notice at the time when it could be remedied by the use of fresh developer. It is one of the cases where care and cleanliness bring an ample reward. Refer to the Manual, Exp. 4, for further laboratory details. The color of the image is ordinarily a shade of gray or black. If the developer contains a large proportion of soluble bromide, say potassium bro- mide the image will have a marked green shade if he ordinary metol-hydrochinon developer is ma ewuh little or no carbonate, the image will be shade of brown with lessened contrasts. Besides pse shades the color of the image in the finished ipicture may be changed to one of many other colors III 132 PHOTOGRAPHY by the use of suitable reagents, and of these meth- ods we will consider only one. 87. This process called "ynlphidc tonuup changes the silver in the image into silver sulphide, which has a brown color when divided, as it is in the print. It is not feasible to change the metallic silver directly into the sulphide, so that the proce^ has to be carried on in two steps, first the change of the metallic silver into some insoluble silver sal:, usually the bromide, and second the change of this salt into the sulphide. The well-washed print i> first immersed in a solution of potassium ferricya- nide and potassium bromide. The ferricyanide at- tacks the silver forming the potassium silver ferrc- cyanide, thus, Ag,+ 2K,FevCN), = KiFe(CN)6+K2Ag2Fe(CN)6 and the potassium bromide changes this over :: silver bromide on account of the much greater h- solubility of the latter, thus: K*Ag,Fe(CN),+ 2KBr = K4Fe(CN)6+ 2AgBr The silver bromide is a pale yellow so that the resu: of this is to change the black silver image :: an almost invisible one. The bleached print has :': r: to be thoroughly washed to free it from all > f :hot substances except the silver bromide, whkh :.- : ' Manual, Exp. 19. POSITIVE PROCESSES 133 I insoluble to wash out. The second step in the proc- I ess is to immerse the washed bleached print in a solution of some soluble sulphide, such as sodium sulphide, Na^S, when the silver bromide will be changed into the brown silver sulphide, thus: Na2S+2AgBr =Ag2S+2NaBr I this change also being induced by the greater in- solubility of silver sulphide than of silver bromide Shades between brown and black may be obtained by partially developing the bleached image in any paper developer, and then changing the remaining silver bromide into silver sulphide, as above 88 Platinotype_This is a process which is closely allied to the Blue Print Process, and is often reierred to as platinum paper. In this connection It IS well to note that a number of the commercial toers described as platinum papers are only inii- Mions of them, the paper being really a slightly fnodihed developing paper. Platinum is an exceed- Pngly expensive metal, three or four times the cost !jf gold, and It takes enough of it to make a satis- ^c 00. paper that it is a very important item, so |hat there is a great incentive to substitute some less expensive material. The modified developing I per IS a very good imitation. The light sensitive nee m real platinum paper is a ferric salt, buallv ,rnc oxalate, Fe.(C.O.)3, which by the f-i 1 . '«uiiMmjiu.>'r. '•m^^j^^'rimi^x 134 PHOTOGRAPHY IP- action of the light changes over to ferrous oxalate. The ferric salt is mixed with a salt of platinum, usually platinic chloride PtCls, and the solution coated on paper exactly as with blue print paper. The surface of the paper is often not suitable, in which case it has to be coated with something whicli will hold the particles of platinum forming the fin- ished image. The coating may be done with vari- ous substances, as starch, or gelatine, by floating the paper on the solution of one of these and then drying it. After sensitizing the paper has to be dried as quickly as possible, and to have it keep good for even a few days it must be kept exceed- ingly dry. The commercial makers have some tricks for aid ing the keeping qualities, but they all have to be kept dry also. For this purpose the paper is usually kept in closed boxes, where there is also placed some water-absorbing agent, such as calcium chloridt In spite of all care the paper will only keep a few weeks, after which it will have a deposit in the clear parts of the image, that is, show fog. It has to be printed in direct sunlight, and the image shows only slightly, so that the progress of printing has to be followed by some kind of exposure meter, such tor example, as a piece of solio under another negative but exposed to the same sunlight. It is much betterj to use a constant light source, as the arc or a high aa*T«AHI»»»Tr" POSITIVE PROCESSES 135 ower tungsten lamp, when the time of exposure :ives a direct measure of the progress of printing ^fter exposure the paper is developed by floating race downward on a solution of potassium oxalate h which the iron salts are soluble. In solution fer- fous salts quickly reduce the platinum salt to metal- Kc platmum with the simultaneous formation of rerric salt again. This reaction takes place only khere the ferrous salt has been formed, so that the ^rrous image is changed into a metallic platinum ^age. The prmt must be washed carefully to rid : of all iron salts. Such prints are very permanent lut they can hardly be more permanent than the »aper support, and a good developing paper is as ernianent as that. This paper lends itself readily ) a lot of different surfaces and grains, and par- Icularly readily to the production of pictures in fhich the fine details are run together to give large "ght and shade effects-what are often referred to ' artistic" prints. In special cases there are ad- antages to be gained by this sacrifice of detail but 1 a great many of the pictures one sees the gain I very doubt ful.« ^ ^89. Carbon Printing.-Jn this process we come fck to the use of gelatine, but this time it is the ^ht sensitive material itself. It is based upon the I' * 136 PHOTOGRAPHY property that if it be bathed in a solution of a salt of chromic acid, like potassium chromate or ammo- nium chromate, and then dried, the gelatine remains soluble in warm water, but if while still dry it be exposed to light, the gelatine becomes insoluble in warm water. In consequence a film of gelatine sen- sitized with the chromate may be exposed to sun- light under a negative, and the parts left soluble can be washed away with warm water, leaving a positive. The gelatine film may be colored with dyes or carbon (lamp black) so that the resulting print can be any tone desired and the same tone may be duplicated any number of times whenever | desired. It will be evident from the way the ex- posure is made that the insoluble image will be on I the surface of the gelatine film and extending down into the unaffected gelatine and probably in no places extending right through to the paper sup- port. So that when treated with warm water the | whole image will float off. To avoid this it is nec- essary to support the image on the exposed side by transferring it to a support in contact with this| surface. This is very readily accomplished by bringing the wet image surface in contact withal wet, bar dened, gelatine surface, and when the excess of water is pressed out they will adhere quite suffi- ciently to preserve the image while the original sup- port is soaked off with warm water and then the %^*. POSITIVE PROCESSES i^y remaining soluble gelatine washed away This transfer reverses the image, making it right lef handed and ,„ bring it back to agreement lith the subjec ,t requires another „ ansfer. I„ some cases »s portraits for example, this reversal does no harm but in case .t ,s necessary it is readily done If he surface to which the print is first transferred be previously rubbed over with a small quantity of beeswax dissolved in turpentine, the' developed usly the final support, usually a soft gelatine sur- face, has to be rubbed into contact with the devel- oped print while all are still wet. and all allowed to £:" fftS: ""^ '^ ''' ''""^--y ^™ The tissue as supplied by the dealers ' is simolv a hin layer of colored gelatine spread over a "tig in I IT" '?'' '° "^ ^^"^'"^^ by Lth- ng m the solution of the chromic salt, or by rushing the solution over the surface of the gela- ne, or the most convenient way is to brush over e surface an alcoholic solution of ammonium iToniate, which dries off quickly, and the tTssue U Po* who al,„ „„ . -^ "rphy. Inc., 57 Ea^, g,^ g fj hsparcn. celS rporrJr"!, "^"t" ."•""<- Tissu^ „„": 138 PHOTOGRAPHY ■M ft ready for exposing in an hour or two. The film must be thoroughly dried and then kept as dry as possible or it will fog in a few days or even hours. It is best kept in a closed box in the presence of calcium chloride. Even with all precautions it only keeps in good condition for a week or two, so that it has to be sensitized when needed. As with platinotype paper, the progress of print- ing cannot be seen, as the image is almost invisible, and it has therefore to be followed with some kind of an exposure meter when the source of light is a variable one like sunlight. If a steady source, like a good arc lamp, be used a few trials will determine the time required. The tissue can also be bought where the gelatine layer is carried on a thin, trans- parent celluloid support and the printing can be done through the support, thus saving the trouble of the first transfer. The process requires more labor and skill than the ordinary developing paper process, but is not difficult enough to be beyond the reach of any ordi- nary worker. The method is exceedingly flexible as to the rendering of fine detail or large fuzzy eflfects depending on the treatment in the ^^a^nl water, and as to the tone of the finished print. It can be used to advantage in Jantern-slide-niaking for the same reasons, and has a very important ap- ..X^lST} - -J POSITIVE PROCESSES 139 plication in the making of pictures in their natural colors. 90. Photo-engraving — There are many printing press methods which come naturally under this head. But the great majority of newspaper, mag- azine, and book illustrations are made by methods of which the half-tone zinc etchmg may be taken as a type. The first step is to make a negative of the subject to be copied. In this, one detail is diflFerent from the ordinary process. At a short but definite distance in front of the plate in the camera is placed a line screen made of two sets of parallel lines, the sets being at right angles to each other, and in each set the width of the opaque lines and the clear places are about equal. This breaks up the light image into a series of dots, but on account of the separa- tion of the line screen and plate, the dot on the plate is brightest at the center and falls off gradu- ally instead of ending sharply. The result of this in the negative is to make the dots large in the brightly illuminated parts and small in the dark places. For the best effect these dots have to be very opaque and clearly marked so that the nega- tive has usually been made by the wet collodion process, which gives very clear shadows and allows readily of very great intensification. A print is made from this negative as in the carbon process, only using a very thin layer of bi- I40 PHcyrOGRAPHY k I 'i chromated fish-glue supported on a polished zinc plate, and the exposure is made long enough that the insolubility extends right through to the metal. This is developed in warm water, dried, and the plate heated to almf)st char the fish-glue, which makes it very hard and also insoluble in the acid bath into which the plate is next dinped. Where exposed the zinc is eaten away and special pre- cautions have to be taken to prevent the acid eat- ing under the edges of the glue. The hardened glue is left on as it forms a good wearing surface, the plate dried, inked by passing an inky roller over it, and the inked plate pressed against paper. The unetched parts of the surface will transfer ink to the paper, while the etched parts will not touch it. The unetched parts correspond to insoluble glue, these to clear places in the negative, these to dark places in the original subject, so that dark places in the original subject are represented by inky places in the print. Where the light is strong in the image, the silver dots in the negative spread out and overlap, leaving, therefore, clear dots on a black ground ; and where the light in the image is weak, the silver dots in the negative stay small, appearins^. therefore, as black dots on a clear ground. In the positive image in glue and in ink, the clear and black places have been reversed, but the same general character of dots and background remains, the IS POSITIVE PROCESSES 141 bright parts of ti.e subject being represented in the ink print by small black dots on a white ground, and the dark parts by small white spots on a black ground. The grays are represented by intermedi- ate conditions of dots and ground, and the ability to thus render the half-tones is responsible for the name and larjr -ly responsible also for the beauty of the process. The number of these dots is deter- mined by the spacing in the line screen used in front of the negative, and the screens in use vary from 50 to 400 lines per inch. The best screen to use depends upon the character of the subject, the surface of the paper used for the printing, and the mechanical method of making the ink impression. CHAPTER VII :^ 4^& .-an! H •t LENSES 91. Pinhole Images. — It is not necessary to have a lense in order to take a picture on a sensitive plate. If the ordinary lens of a camera be replaced by a very fine hole, like that made by a very fine needle Fig. 19. in a piece of tinfoil, there will be an imag^e formed on the plate. The way in which it is formed is shown in Fip. 19. Light from one point in the field of view can get at only one point on the plate, so that the light forms an image, inverted and riiiht left-handed, as in the case of the lens image. The sharpness of this image depends first of all on the size of the pinhole and then on the distance of the 142 'j^My^i!- '^'j&*^-^<\ LENSES 143 plate from the pinhole. At first sight it would look as if reducing the size of this hole would sharpen the image no matter how small the hole already was, but another property of light which we do not ordinarily notice enters to upset this predic- tion. Along the edge of a sharp shadow light does not travel in perfectly straight lines but bends into the shadow very slightly, so slightly in fact that it is very difficult to detect. Moreover, as it spreads, light from the two sides of the pinhole have differ- ent distances to travel to reach a point in the image out of the direct line with the pinhole and source of the light, so that the two waves as they reach this point may be out of phase, that is one wave may be a crest and the other a trough, and the addition of the two will result in no wave. This takes place in such a way around the central bright spot that there will be formed a series of bright and dark rings, Fig. 20, which is a photograph of a small bright source of light taken through a pinhole. The central bright spot is much the brighter, and, going outward, the rings decrease rapidly in brightness till they disappear.' As the pinhole is made smaller than a cer- tain size the central image becomes larger and 'See, for example, M. E. Hufiford, Physical Rev., second se- ries, 3, 242 (1914). t- 144 PHOTOGRAPHY also the proportion of the total light which is in the central ima^e becomes smaller, so that the image of the point does not become smaller but really larger when the pinhole has passed a certain size at which the definition is best. The size of the image is directly proportional to the distance of the plate from the pinhole, as is easily seen from the 1-. i^ txHaje ^\^ lins c/' Fig. 22. geometry in Fig. 19. Compared in brightness with the image formed by a lens, this image is exceedingly faint, and the exposure required may be hundreds or thousands of times that with the lens. But it has some marked advantages over an image formed by a lens; the whole picture is in focus at once, foreground and background, Fig. 21; page 146; it cannot be got out of focus, and the Fig. 17. Photographs of a narrow slit illuminated from be- hind with an arc light. Each image from left to right received 5 times the exposure of the preceding one. Series b was taken on an ordinary plate, and series a on a douide coated plate talfen"th?o,JvI'^ ""T °^ ^ 'I"^" ^^""^ ^"^^^ source of light ak 1 hrough an almost perfectly round pinhole on a plate « . I 4 ^:i imti-^ LENSES 145 geometric correspondence between image and object is perfect.' 93. A lens may be described as a transparent medium bounded by regularly curved surfaces, or some of the surfaces may be planes. The curved surfaces are almost invariably spherical; some few exceedingly expensive telescopic lenses are some- times varied from spherical by tedious hand polish- ing, taking years to do. All photographic lens sur- faces are plane or spherical, see Fig. 22. This allows of several different types of lenses, Fig. 27,, falling into two classes, (a) one spoken 'of as convergmg lenses because they will bring a parallel ^^ C0Nve«*eNT Fig. 23. Oiveit««NT beam of light, like sunlight, to a focus, and (b) the other spoken of as diverging lenses because a beam of light like sunlight will be spread out by passage through the lens so that it will appear as if it spread from a point on the side of the lens on which the light came. These two points are both called 'See "Pinhole Photography," Photominiature. No. 27. \l :^ 146 PHOTOGRAPHY m I «l *4^ foci, the first a real focus and the second an imagi- nary or virtual focus.. The straight line through the centers of the spheres of which the lens surfaces are a part, is called the axis of the lens. If one of the lens surfaces is a plane, the axis meets it per- pendicularly while it also passes through the center of the sphere of which the other surface is a part. ' or K--/---J L f...^y^ a ' X ■b b y <** ^ "P- \ -/»'-- H Fig. 24. When a lens is formed of more than one piece of glass, as with the majority of photographic lenses, the centers of all the spheres should lie on the one line; a lens otherwise perfect may be very much impaired in definition of image if the makers of the mount have not been careful to have this condi- tion filled. 93. Images. — The method of formation of an image by a converging lens is shown in diagram in Fig. 24. H the object is very distant and on the principal axis the light from it will be practi- cally parallel as it strikes the lens and it will be xaciTvis^Sii .. »i„u.e. Reproduced ,Vl^l"t:. See' Ar."; "'""™"' °' " .n^,l,rc™,er of fh 1 f '"^'^ °' ""■ '™' "^^ 5 inches center of ih^- Look «a., 15 inches from the lens. : ua!Qiz^sc^Ei.'r:^=j0^3iL/i;AxndiT'%* '!'*« " Hi gam- ^ LENSES 147 brought to a focus at a point which is called the principal focus, see Fig. 25. The distance of this point .rom the lens is called the focal length of the lens. In this connection the two points N and N' are important, see Fig. 24. They are called nodal points and are so placed that rays of light entering Fig. 26. one from its side of the lens will appear to have come from the other as it emerges from the lens. 1*1 H 148 PHOTOGRAPHY They are more useful and accurate than the older treatment using the optical center. They have anoLier property, shown in Figs. 24 and 26; it is that planes cutting the axis perpcn- ':i ■Jf Fig. 27. dicularly at the points N and N' are so placed that rays appearing to strike a point in the forward plane, leave the lens as if they came from a point in the rear plane immediately back of the first point. This makes it very simple to follow the actual path of a ray through the lens and therefore LENSES 149 also simplifies the construction of lens diagrams showmg the formation of images. In measuring the focal length of a lens, the distance is measured from the nearest nodal point to the image of a very distant object, the sun for example. These points are of the greatest value in the calculating and de- signmg of lenses. One other property deserves mention; that is if the lens be rotated about either of these points the image will not move. It is about one of these points that the panoramic camera lens is made to rotate in order to get the picture covering at least 180'. These points apply not only to lenses formed of one piece of glass but also to lenses formed of a number of pieces and whether they are in contact or not. 94. Size of Image—From the diagram in Fig. -'4. It IS very simple geometry to show that the size of the image is to the size of the object as their respective distances from the lens, or more strictly from the nodal points. The distance of the image from the lens, for a given object distance, is deter- mined by the focal length of the lens. Thus if one has to take a picture of an object from a fixed dis- tance, the size of the picture, that is of the image, can be varied by choosing lenses of diflferent focal lengths, the longer the focal length the larger the image, see Fig. 27, the size of the image and its. 4. •- % M^ i A' 150 PHOTOGRAPHY distance from the lens being in all cases propor- tional. For the same reasons the focal lengths of Vnses made for use with large plates are in general longer than those for use w-ta small plates. A choice of focal length is an advantage in another case, namely where to get the desired size of pic- ture, the object has to be brought close to the lens, which iCsults in bad perspective and in distortion the near parts of the object looking too large; A^ 139 and Fig. 28. A hjnger focus lens enables o ' move the camera farther away from the r'i|., with a corresponding improvement in the cha >.\c of picture. 95. Defects in Imaires Formed by Lenses.- Aberrations. — If one e:.amlnes the image fo.nied by a lens consisting of one piece of glass with spheri- cal surfaces, one will hnd some serious defects, that is to say a serious lack of correspondence between •the geometrical form of the object and of the image. For instance fine lines in the object wi-'. be r'-pre- sented by blurred lines hi the image, that is ilie image is not sharp, no matter how carefully one focuses. The middle of the image will be in better focus than the edges, and if one examines carefully it will be found that every bright object is fringed with color in the image. It will be necessary to dis- cuss at least seven different things which go to make i 1 : 'iY '^a. ' he focal length of the lens was 13 inches and " s muvcd away so as to make the central vvitltli lOi. 1) I iff. -^8. Fio. 28b. The telephoto lens had about 30 inches equivalent focal length. Z-^E£^7'.I ■-> ■_ :'.'•% Ji-iT-^.'^'f*."! rtMi LENSES 151 a lens image an imperfect copy of the original. They are Sphencal aberration and coma, Chromatic aberration, Curvature of field, Astigmatism, Distortion, Flare, Unequal illumination. Fig. 29. 96. The most serious, since the most difficult to correct, is spherical aberration. It causes a blur- ring of the image and the explanation is best made in terms of the diagram Fig. 29. The rays of light near the edge of the lens are brought to a focu5 usually nearer to the lens than the rays pass- ing near the center, the result being a blurred focal point; of courie, the same thing will happen with the image of every point in the object. If one puts a diaphragm in front of the lens so as to cut off the marginal rays the image will become clearer the mes snarper and the image will seem to move farther from the lens, at least the clearest place tf a rti!-:i If 152 PHOTOGRAPHY will be farther from the lens. But it helps the defi- nition, and that is the reason one uses a small dia- phragm when possible. The diaphragm is best placed some distance from the lens, as will be dis- cussed more in detail under distortion. A form of lens in which spherical aberration is small is one Fig. 30. with concentric spherical surfaces, and a number of fine lenses are on this plan. A deep meniscus lens, see Fig. 23 third lens, used with the hollow surface toward the object has also a relatively small amount of spherical aberration. 97. Coma. — If rays coming from a distant ob- ject placed on the axis of the lens, form such a blurred image, it will be evident also that moving the point along the axis toward the lens will not improve the definition or indeed change it niateri- 1 LENSES 153 ally. If the point from which the rays come is moved off the axis, the definition is made even worse, see Fig. 30, and in this case it has received the special name of "coma." In the case of lenses for use in telescopes coma will not matter since the object is always on the axis or very nearly so, and in this case the spherical aberration correction be- comes a much simpler matter and can be made with a high degree of perfection. But in photographic lenses, where both should be corrected, approximate corrections have to suffice. The amount of spheri- cal aberration in a lens depends upon the particular curves used to form the lens surfaces and upon which of the two curves faces the incident light. Turning a lens with the other face to the incident light often changes the amount of spherical aber- ration quite materially. The amount does not de- pend, except as it affects the curves, upon the dif- ferences in thickness at the center and at the edges of the lens. The correction for chromatic aberra- tion does depend upon this so that one correction need not interfere with the other. 98. Chromatic aberration is a very serious fault with simple lenses, that is lenses formed of one piece of glass, and all photographic lenses worthy of the name are formed of at least two pieces of glass designed to correct this fault, and are called "achromatic lenses." The difficulty arises from the B" ' i 154 PHOTOGRAPHY fact that the diflferent colors of light are not brought to the same focus by the simple lens, but the violet rays which are refracted most are brought to a focus nearer to the lens than the red rays, V\g. 31. The foci for the other colors fall in between. With a simple lens the difference may amount to Fig. 31. 1/30 of the focal length of the lens, and is particu- larly bad in photography because the focusing is done largely with the yellow light, while the photo- gr?ph is taken largely by the blue, so that in the photograph the object will very generally be out of focus. It affects not only the sharpness but since the size of the image depends on the distance from the lens which is fixed by the focal length, the images formed by the different colors of light will be of different sizes. 99. Achromatism — The approximate correction is not a very difficult matter. Glasses differ very markedly in the amount of separation of the focus J LENSES 155 for the different colors, that is in what is called their dispersion. A flint glass lens has a much greater disperse n than a crown glass lens of the same focal Im^ih see Fig. 32. It will be a sim- ple matter to muk^ a diverging flint glass lens whose dispersion will he ecjual in amount but oppo- FlG. ^2. site in kind to that of a converging crown glass lens. If light is passed through one right after the other the dispersion will be approximately corrected, but the convergency of the crown glass lens will more than ofi'set the divergency of the flint and the combination lens will be a converging one though of less convergency than the crown glass lens alone. Such a lens is called an achromatic lens or simply an achromat, see Fig. 33. This is, however, only an approximate correction. If the series of points at which the different colors l'4\ ■i . t- 1 t-f II n ii M 156 PHOTOGRAPHY are focused in the two lenses, are placed side by side, see Fig. 34, it will be immediately apparent that if any two of these color foci are made to Fig. 33. match, as for example in the violet and red in the figure, then the others will not match exactly but only approximately. Lenses can be made easily so that the dispersion of any pair of colors will be Flint i i * t * [. Crown E ) ( : 1 3 E F 6 HK Mlp Vlotat Fig. 34. equal and opposite to that with the same pair of colors with the other lens, and then the foci for these two colors will be the same in the composite lens and the foci for the other colors will be nearly the same. The pair chosen for exact matching depends on the use to which the lens is to be put In lenses for visual work, like in microscopes, etc., LENSES 157 the green and the orange are usually chosen because these along with the yellow, which will fall very close to them, are the colors which aflfect the retina of the eye most strongly. For photographic work another pair is better, namely the violet and the yellow, the yellow being about midway among the colors used for focusing and the violet being about midway among those which actually take the pic- ture when using the ordinary silver bromide plates. This makes a fairly satisfactory lens for all ordi- nary work, and is the arrangement used in all ex- cept the high-priced lenses. But in case one is doing three color work, this will not be sufficient correction since the pictures, one in each of the three colors, must agree exactly in size and sharp- ness. The use of a third lens of another kind of glass, all three lenses cemented together, will enable one to make three colors fall exactly at the same focus, instead of two as with two glasses; these three colors which match should be those used in three color work, usually red, green, and blue. When three colors agree in foci exactly, the re- maining colors will of necessity focus so close to the same point, that the lens is practically perfectly corrected in this respect and may be used for all kinds of fine work. Another plan, and that used in many or most modern lenses since it requires fewer pieces of glass, applies only to lenses of the ri . k «t K < ! '■! i >ia will give the satisfactory definition with the larger stop will allow of the picture being taken in the I least time, and is hence spoken of as the fastest] lens. Lenses of different focal length may be com- Fig. 43. Convertible Lens. pared if the size of the diaphragm ii measured in] terms of the focal length, that is in the F numbers, So that the speed of the lens is usually spoken of| in terms of the largest stop it will allow for satis- -L _r Fig. 44. Petzval's Portrait Lens. factory definition and be referred to by the numbers | of the "F" or "U. S." scale. 107. Lens Types — The rapid rectilinear, Fig I 38, page 161, has already been mentioned as. the important lens in the medium-priced cameras, anil LENSES 169 of course there are all different grades of rapid rectilinear lenses, depending on the glass used for the different components and the care with which they are finished and mounted. The cheap cameras have single achromatic lenses, or in the cheapest kinds a single glass lens. Of these the deep menis- cus type is the best, having the least spherical aber- ration. Some of the expensive doublet lenses have the different components of different focal lengths and even of different diameter, Fig. 43. Each com- ponent is corrected as far as possible so that either one may be removed from the tube, thus providing the possibility of three different focal lengths for different work, and hence they are called converti- ble lenses. The Petzval portrait lens, Fig. 44, has been such an important lens in the past that it deserves mention, but it is not as good for its own special work as some of the more modern lense= whi it is niore expensive to make and so is being gra- displaced, but the professional photographers use a good many of them. It is of the d. type, but the spherical aberration is corrected meniscus component separated by an air space the next member. By varying the air space, spherical aberration can be varied at will for' or for sharp definition. aiy till et t p 170 PHOTOGRAPHY Some good modern doublets are the Zeiss * "Tes- sar," Goertz" "Dagor," Cooke* Anastigmatis, Voigtliinder,' "Healiar," Euryplan." 108. Telcphoto Lens. — There is yet one lens which requires some mention. It is the telephoto combination. The purpose is to enlarge the size of the picture without a great extension of the bellows, which adds other difficulties, such as clumsy appara- I ■■••■..... ? N Fig. 45. Telephoto Lens. tus and very steady support, which would be needed if the enlarging was done in the ordinary way by increasing the focal length of the lens. The idea of the telephoto lens will be best obtained from the diagram, Fig. 45. It consists of the usual high class lens of the converging type, and at some dis- tance behind it is placed a diverging lens. As shown in the diagram this second lens reduces the convergency of the rays so that they will strike the plate as if they had come through a very long * Bausch & Lomb Optical Co., Rochester, New York. *C. P. Gocrtz American Optical Co., -^17 E. 34th St., New York. 'Taylor Hobson & Co., 1133 Broadway, New York. ' Voigtlander & Son, A. G., 225 Fifth Avenue, New York. * Ralph Harris & Co., 26 Bromfield St., Boston, Mass. LENSES 171 focus lens. By varyinpf the distance between the two lenses the apparent focal lenj^th changes and with it the size of the picture. The greatest sepa- ration is when the diverging lens is at the focus of the converging, when the magnification becomes in- finite, and in practice the separation is always less than this. In the first arrangements of this combi- nation put out by the lens makers the highest pos- sible magnification was sought, and this was not always needed and was frequently the cause of poor results. So that now the magnifications usually run from four to ten or twelve. The camera requires stea('y support and the exposures have to be nmch longer than with the usual lens. If the arrange- ments are good the picture is better in the way of detail than can be obtained by enlarging a pic- lure taken with an ordinary lens." The diverging lens part of the combination is not very expensive, and it is used vv'ith the high grade general purpose lens. There are a number of very readable books on lenses, for example : C. L. Johnson, Photographic Optics and Color- Photography. J. T. Taylor, Optics of Photography and Photo- graphic Lenses. 's, ee "Telephotography," Plwtominiatiirc, Xo. 26. f! 'If I, j-ji 172 PHOTOGRAPHY Beck, Photographic Lenses. While a more advanced treatment may be found m: Otto Lumner, Photographic Optics, translated by S. P. Thompson. m CHAPTER VIII COLOR PHOTOGRAPHY 109. There are a great many reasonably work- able methods of taking photographs of objects where the photograph will show the colors as well as the geometrical shape and the light and shade. This is a diflFerent problem from the photography of colored objects, where the purpose was to render the visual light and shade of the colors in one color, eg.f gray. Now we desire to include the color itself in the finished picture. These processes may be grouped immediately into two classes : I. those in which no pigments are used to produce the colored picture, of which the Lip- mann process is the only one, and II. those usually known as three-color processes which use pigment to give color to the print, which includes a great number of methods. This last class may be subdi- vided again: I. Those using three negatives (A) Addition positives (B) Subtraction positives a. Those using a mosaic three-colored screen 3. One using the bleachinR by light of some dyes. 173 I 174 PHOTOGR.\PHY Ci are not very sharp but are not unpleasantly tuzzy and may be duplicated almost indefinitely. COLOR PHOTOGRAPHY 185 117. (c) All of you will recall that dry plates are Imade sensitive to the spectrum by dipping the dry [plates in a solution of a dye. The particles of silver [bromide absorb a small quantity of the dye. Silver liodicle will absorb large quantities of many dyes Ibut, unlike silver bromide, the dye has but little leffect on the color sensitiveness. Traube has made Ithis property of silver iodide the foundation of a process for printing in one or more colors.* The fiilver in an ordinary positive on paper or other sup- port is changed over to silver iodide by immersing In a solution of potassium ferricyanide and potas- sium iodide (compare the ferricyanide reducer). This bleached positive is then immersed in a solu- |ion of the dye which stains both iodide and gelatine but washes readily out of the gelatine. The next Itep is to dissolve out the silver iodide by a solution J)f hypo, which also has to contain a substance to lender the dye insoluble, a mordant, such as tannic Icid. The silver image is thus replaced by a dye ma^e whose color and density may be regulated llmost at will by the choice of dye and density of V original silver positive. The modification nec- ssary to make three-color prints is slight. The re- |uired three-color prints on transparent supports re made from the red, g' ;en, and blue sensation I Supplies and detailed directions may be obtained from Otto |erutz, Miinchen, Dachauerstrasse =0, Germany. f 1 m ifi.jii J:i iifi i86 PHOTOGRAPHY negatives and superposed. The process offers ad- vantages to fhe amateur in its similarity to the customary process and its ease of control. 1 1 8. ( d ) The printing press using colored print- ing inks occupies first rank when measured by the number of pictures produced. In the usual three- color process, the three negatives described above are used to make three printing blocks, and each of these is used to impress its appropriate color on the print. In skillful hands the results are ex- ceedingly fine. For the making of one copy it would be a very expensive process, but when the cost of the blocks is charged against some thousands of copies, the cost per copy is not large. It is a diffi- cult process requiring extensive equipmf nt and highly skilled workers, so that it is not at all suit- able for general use. zzg. Case a. — Mosaic Screen Process." — At the present time these are the most popular with the ordinary worker. The method was first worked out by Professor Joly of Dublin. A number of makes have been tried commercially — ^Joly, Mac- Donough, Warner-Powrie, Thames, Krayn, Omni- color, Autochrome, Dufay, Leto, Paget,'" the last •Mces, C. E. K., "History of Color Screen Photography," British Jour. 5"m/>., 1908, p. 12. Mccs & Pledge, "A General Dis- cussion," British Jour. Sup. to vol. 57, 45 (iQio)- " Supplies and directions can be obtained from Hc.bert & Huesge.i Co., 456 Fourth Avenue, New York. COLOR PHOTOGRAPHY 187 four of which are still on the market. The theory is the same for all, and the general process of using the plates is also very similar. The Autochrome was the first commercial success and is still probably the best though some of the others are better in some particulars. The theory will be most readily understood from a detailed description oT one plate, the Autochrome, and the others can be then related to it. 120. Autochrome." — The process of making a plate is about as follows: a quantity of wheat starch is sifted to separate all but one size, about 0.015 m.n. diameter. The sifted starch is divided into three portions, and each portion is dyed one of the three primary colors. The three portions are mixed very thoroughly, and if the dyeing has been satisfactory the mixture will be gray, almost black. A glass plate is coated with a sticky varnish, the starch grains dusted onto it, and then shaken off, so coating the plate with one layer of the col- ored grains. The interstices arc filled with an opaque material, probably by dusting a much finer powder on, and all rolled flat and varnished with a waterproof varnish. On holding the plate up to the light it appears gray. H it did not it would "If supplies cannot he obtained from a local dealer, they may he obtained from Lumiere Jougla Co., 75 Fifth Ave., New York. 'If 3 i88 PHOTOGRAPHY I impose its color on the color of the finished picture. The grains are so small that the eye can not dis- tinguish the individuals, and the resulting effect on a point of the retina is the sum of the effects of a group of the grains. H one could cover up part of the grains, then the color the spot would appear, would be that due to the uncovered grains. The three colors used for the grains are the three pri- mary colors, red, green, and blue violet, by a suit- able mixture of wh' n any color may be imitated to the eye. So that by covering up a selected set of these colored grains the rest will imitate any color desired. The problem is to cover up the required set. The next and final step in the preparation of the plate is to coat the waterproof varnish with a very thin layer of panchromatic emulsion. These plates are packed in boxes and shipped like ordinary dry plates. To make the picture the plate is exposed in the camera like an ordinary plate except that the i^lass side of the plate is put toward the lens and allow- ance must be made for this in focusing, either by reversing the ground glass, or by moving the lens toward the plate a distance equal to the thickness of the plate after focusing on the ground glass. Since no emulsion can l)e made equally sensitive to all the colors, it is necessarv to use a color screen COLOR PHOTOGRAPHY 189 Ito lessen the effect of the blue and violet light. Both jthe mosaic screen and the color screen absorb con- [siderable light so that the exposures have to be [about 60 times that for a medium rapid dry plate. Also since the emulsion is so thin the latitude of ex- jposure is very narrow, and exposu'*es have to be [very nearly correct to be at all satisfactory. The exposed plate is developed, using a definite Itime or following the progress with a green light of a particular shade for which the emulsion is not very sensitive. The plate is washed and im- mersed in a solution which will dissolve the finely divided silver forming the negative image, so that there wi^' be clear places in the negative everywhere where tnci*e was negative deposit, and negative de- posit occurs only where red light struck the red grain, green light the green grain, and violet light the violet grain. So that when the deposit is re- moved these grains transmit light like that which I came up to them in the camera. It is only necessary r w to reduce the rest of the silver bromide to I metallic silver by an ordinary developer, in order to block out effectively the grains which did not transmit light in the camera, after which the pic- ; ture stands out clearly. Treatment with hypo may be omitted when all of che undeveloped silver bro- mide has been reduced by the second development. After the film is dry a coating with varnish makes "I 190 PHOTOGRAPHY I* ' the picture more brilliant and less easily damaged. To protect it effectively it should also be covered with a piece of clear glass. 121. Mosaic Screen Making. — It will be evident from the above that the central point of the method is the use of the mosaic screen, and the different makes of plates mentioned above differ mainly in the character, and method of manufacture, of this screen. The first such screen was made by Joly, who ruled parallel lines in the three colors in order to cover the plate, so that the color elements were not dois but extended right across the plate. The MacDonough screen was made in very much the same way, but was too expensive to make, and it was difficult to get the colored strips in contact and yet not overlapping. Another promising method (Krayn) was to make a pile of thin sheets of celluloid of the ♦^hree colors. cement them together, and then take thin shavings off the edge of the sheets. This would give the color elements as lines, but they could be made into squares by cementing a pile of these shavings to- gether so as to mix the lines and shaving again across the liiies. But experiment showed unex- pected difficulty in making thin enough celluloid sheets and also in making the shavings thin enough, The elements must be 150-200 to the inch to beat COLOR PHOTOGRAPHY 191 all satisfactory, while the specimen the writer has I seen had about 60. Many of the other screens depend on the dyeing I of a gelatine layer, for example, Warner-Powrie, Krayn, Dufay, etc. In making the Warner-Powrie screen a layer of gelatine or fish-glue is sensitized with chromate and then exposed under a screen composed of alternate opaque and transparent bands, the former being twice the width of the lat- ter. The soluble part of the gelatine is then washed away and the rest, consisting of the insoluble bands immersed in a green dye which is afterwards made fast or mordanted by immersion in tannic acid solu- tion. This will give a plate covered with green lines. Another layer of gelatine is then spread over these green lines and the whole process repeated but setting the screen so that the new set of lines (dyed red) fall between but next to the blue lines. The plate is coated a third time and exposed through the other lines to blue light, which is, therefore, ab- sorbed by the green and red line?, so that the gela- tine is hardened only in the as yet uncolored areas. The plate is then washed, dyed blue, and mordanted. the j)rocess seems to be a good one. Another method for coloring the film in patches is used in the Dufay '" screen. A soft gelatine film "Made by R. Guilleminot Bocspflup & Cic, 22 Hue de Cha- teaiulun, Paris, agent in U. S. being George M rphy, 57 E. Ninth Street. New York. 192 PHOTOGRAPHY h,-' is printed in a printing press with parallel lines c a greasy ink. This plate is then immersed in water solution of a dye. The solution will not pass into the gelatine through the greasy ink but onlv in the clear places between. The plate c?n now be varnished with a waterproof varnish. When dn the application of a solvent for the greasy ink and a little friction will clear off the greasy ink and its varnish covering, leaving the clear parts of the film exposed, but the dyed strips protected by the varnish. The process is then repeated but with the lines running across the previous ones. This \vi give two colors, the first lines and the second scjuares and the final square left exposed by the two previ- ous processes is dyed the third color. The Diifav screen is an excellent one mechanically, having sharply marked edges to the color dots and being reasonably fine. The latest plate on the market is the Paget Prize Plate, *"' in which the mosaic screen and sensitive plate are separate. They are placed in contact fur exposing but are separated for developing and mak- ing a contact transparency, which is bound up with a different mosaic screen. This allows of duplica- tion, but the screen has to be somewhat coarse, and there is the serious difficulty of securing good rcg- " May be obtained from Herbert & Huesgen Co., 456 Foiirii Ave., New York. COLOR PHOTOGR.^PHY 193 istration all over the plate between the color ele- ments and the positive. A great many other methods have been patented for making these mosaic screens. A good method offers large financial returns. 122. The use of these mosaic screen plates puts in the hands of the careful amateur a means of tak- ing pictures in their natural colors with only one exposure and with comparatively little manipula- tion. The pictures are satisfactory in many ways. But they have certain disadvantages. They are all transparencies, and copies are not very satisfactory, being particularly unsatisfactory with the Auto- chromes. So that each picture requires a new ex- posure. The most satisfactory method of reproduc- tion at present is by means of the three nega- tive-printing press method, where the mosaic screen picture is treated as a colored subject to be copied. A method usable by the ordinary worker is in great demand. The following method offers a good deal of promise. Class 3. Bleach -out Process i?3- This is based on the fact long known that many dyes which bleach readily in the light are bleached by 'le colors of light which they absorb, and which is the complem :ntary color to which they [appear. For example, if a red and blue dye of this Illi ill >*', 194 PHOTOGRAPHY type be mixed and coated on paper, and say, blue light be allowed to fall on it, the red dye will absorb the blue light and be bleached, while the blue dye will reflect the blue light and remain unbleached. The paper will hence turn blue. It will, therefore, only be necessary to mix three such fugitive dye? of the three primary colors — red, green, and blue- violet — coat them on paper and expose the paper under a mosaic screen picture. Where red light gets through it will bleach the other two dyes, leav- ing the spot red ; and similarly for the other colors. The mosaic screen picture will be reproduced in colors. But the process is not perfected yet— one difficulty being to make the colors stop bleaching when the printing is done, that is to **fix" the pic- ture. This is partly attained by using dyes which bleach much more rapidly in the presence of certain reagents like anethol for example, which may be washed out. The other serious difficulty is that the different colors of light bleach the dyes at very different rates — so that for instance when the red is bleached the blue will not be. Two or three dif- ferent makes of such papers have been put on the market but they do not seem to be very successful yet. CHAPTER IX GOOD PICTURES 124. A picture is a representation of a three-di- mensional subject on a two-dimensional plane. It is a representation, not the thing itself, and as such is necessarily a counterfeit or an imitation. Truth or untruth in a picture has therefore a very special meaning, in fact truth is determined by how far the picture fills the purpose of the worker. A photo- I graphic surveyor desires pictures which shall be I accurate geometric projections of the subject show- ing every detail near and far. For him the truth is determined by how far his pictures fill these re- quirements. On the other hand a portrait of a familiar person has to represent for us a variety of changing attitudes and expressions, and a detailed accurate delineation of one momentary condition I may satisfy this desire to some extent, but the prob- ,-bilities are very much against it serving as well as a picture in which the rendering is less complete because the delineation is then not so sharply one rnood and because we do not remember the fine I detail as a rule so well as the general eflfect. There 195 m" ! 1 196 PHOTOGRAPHY is hardly a professional portrait made which is not retouched. That is a portrait is to be judged on the basis of suggestion of the person pictured. 125. In a general way pictures may be divided into two classes: (a) Record Pictures, where the aim is a geomet- ric rendering of the subject. Pictures of this class are of the very greatest importance, particularly in all scientific work. The purpose makes this class include the great majority of amateur photo.^raphs taken as records of persons and places, and also the great majority of portraits. There is no doubt that this purpose is the predominant one in the vast majority of pictures, and the world could much better afford to lose the second class than this one. (b) Pictorial Pictures, where the essential pur pose is to attri t, arouse, and generally please the be- holder, not so much by the particular scene pictured as by ideas suggested. As such they appeal tu the imagination, and to attain their purpose it is not at all necessary that the object forniiti.c: the actual subject should be suggested by the finished picture. In this class are to be placed many paint- ings and a small proportion of photographs. Any liberties with the geometry, with the lightinj^;, ^vi'!l the color, are justified by aiding toward the object sought. It must be noted however that such lib- erties must be handled in a masterly way or they '?^:'^ GOOD PICTURES 197 I have an effect very diflferent from that intended. I While these two purposes should be clearly dis- ! tinguished and acknowledged in their extreme ex- lamples, there are relatively few pictures which come only in one class. The first ideal of the young photographer, as also of the early school of photog^ raphy, is the record photograph, and so well under- stood is this ideal and so useful that only a few unbalanced "Art" workers try to belittle it. But the worker soon observes that while equally good records, some of his photographs are more pleas- ing, are turned to oftener, and are exhibited to his friends. He begins to work for these eflfects as well as for the original record purpose, and he hence begins to include the ideas of the second class. The great majority of workers never give up precedence of their first purpose, and a study of the mass of pictures of those who have makes it seem well it should be so. Very few of us have the temperament or time to be artists, but we can often recognize and appreciate good imaginative Jwork; while botched work of this kind is an even jniore serious ofifense to the ordinary man than to Ithe artist who sympathizes more readily with its laim and who understands its diflficulty. This chap- |er is written for the ordinary worker who is pro- cessing sufficiently with the straight record work m he desires to include as far as possible more iiiiilii 198 PHOTOGRAPHY of the somewhat intangible features which make some pictures more pleasing than others 126. Picture Composition. — The choice and ar- rangement of the detail forming a picture has re- ceived the general name of composition. The sub- ject is a very extensive one, but it lacks the definite- ness and precision characteristic of the sciences. Several centuries of vigorous controversy by both painter and critic has not led to any great unanimity, r.nd any generalizations are opposed by marked ex- ceptions. Most writers admit that there are a group of rules followed in general by all great painters. though at times they may break any of them. From the discussion it also appears that one great pur- pose served by such controversies has been the atten- tion to such detail thereby promoted. The literature * upon the general subject is vol- uminous and varied, but the great majority of sucli books are by painters for the use of painters. On account of the great difference in method be- tween painting and photography, combined with the restrictions inherent in photographic composition and the loss of all color, these books are not very serviceable to the young photographer. There are. however, an increasing number of books ^ and maga- > Joshua Reynolds, Discourses,; Burnett, Art Essays. «R. H. Poore. Pictorial Composition; Sadakichi HartmarJ. Landscape and Figure Composition; H. P. Robinson. Pictom Effect in Photography. GOOD PICTURES 199 Izine' articles written especially for photographers which well repay study. The control by the photographer over his subject las compared, for instance, with that by the painter is very limited, being restricted to choice of subject as a whole, of point of view, and of lighting. The Itreatment following exposure may also modify the picture materially but, only by suppression, seldom satisfactorily by addition. In the following articles Ian attempt is made to review systematically only the Imost elementary aspect of photographic composi- Ition, treating it in the way it is apt to present itself Jto the young photographer and hoping thereby to jawaken an interest in the subject that will lead to [further study elsewhere. 127. Unity.--Every picture needs some definite jfairiy conspicuous object to form the center of in- jterest and to which all the rest of the picture will ■be subordinate. It is the reason for the existence of the picture. There is little limitation to what may be so used except that it must have enough human interest to justify the important position. A human figure or face is preeminently such an object, and its treatment, portraiture, is an important f)ranch of picture making. In landscape pictures an 'R. H. Poore, "Figure Composition," Photominiature, No. k-F w .In' "^ '*''"'^' Principles," Photominiature. No. > .. v.pston, "Compo.,ition," Auurican Photo., p. 325 (1914). « ! 200 PHOTOGRAPHY interesting glimpse of a building, or a lake, or a pretty shore line, or a group of cattle, may readily serve as the basis of a picture and justify its ex- istence. The best position in the pic .ite for the center of interest varies, but no one advice ^ tbit ir be place'J at the geometric center. Somewhere near but not at the center is the favorite place. When placed very far from the center it requires very special arrangements to be pleasing. 128. Simplicity. — To be satisfactory a picture must not include too nuich lest it divide and thereby weaken the interest. The com. i.onest mistake with the camera is to seek for a poim of view which will show as much as possible. While this serves a rec- ord pur^jose it weakens decidedly the appeal as picture. A compromise between the two purposes i^ often on the whole a gain. If unnecessary or dis- tracting detail cannot be omitted by choice of point of view, it can often be e'iminated later or at lea^t be rendered less conspicuous by loss of its tine detail or by subdued lighting. To be elTectivc all the subordinate detail should be naturally associated with the main subject and should be so placed if p- sible as to direct attention to it. lM)r the landscape photographer the amount of such choice is dccidedlv limited, but in portrait work the background is iior- uiallv entirely controllable ; directly around the head 1 GOOD P; -TURKS 201 it is preferably of different brightness from the I head outHne so as to show this where desired and to l^ive rehef, while away from the head, detail should j merely soften the blank area but not be sufficient to [distract attention from the face. 129. In landscape work the foreground offers I special difficulties. On account of the nearness to the camera, the area occupied on the picture is relatively large, and thereby small, unimportant detail be- comes unfortunately conspicuous. Slight sideways shift of the camera will move a conspicuous bit of detail to the corner or partly off the plate, in which position it may possibly help the composition, or it may be moved off the plate altogether. Also the jforeground may occupy too large a fraction of the jarea of the print. The first correction usually tried lis to raise the lens, but if feasible it is usually much ^better to raise the camera itself. Failing these, it comes necessary to trim the print. 130. The pi.sition in the picture occupied by the \ky-lm is important. Also often when it is absen* [here will be some other line which will take its place. |Hir habit of looking horizontally places the horizon ^miewhcre ir the middle of the view, and conse- huemly the observer recognizes the view point most readily when the sky-line is somewhere near the hter of the picture. If it deviates widely from that jhe observer has to recognize a somewhat unusual wm 1 fli. 202 PHOTOGRAPHY view point — which of con se for special reasons may be a distinct gain in case the rest of the picture en- forces it, but only in such cases is it an advantage, 131. A print where the sky is pure white paper is at a great disadvantage when compared with one showing clouds. Cloud- forms in themselves are often very beautiful, and they have such significance for us in our daily life, that a picture using thetn to fill in an otherwise empty area gains greatly. There are several ways in which cloud-forms may be ob- tained in the print. They may be put in, using a wad of cotton, or a brush, or a pencil, but it takes j^reat skill to make them look natural in shading and form. Or with a little care they may be printed in from one negative and the rest of the picture from an- other. Outside of the difficulty of matching them on the sky-line, is the greater difficulty of makinjj: the cloud shapes fit the horizon, and the shadows they cast agree with those in the landscape. Without being always able to specify, one often feels that there is something incongruous. By all means the best procedure is to take both clouds and landscape on the same plate at the same time, by using a suit- able color sensitive plate and a color screen. The temptation to make the clouds too prominent b strong, and one should never forget that both clouds and sky are very much brighter than tlie earth This usually reiiulics a rather transparcii' .If GOOD PICTURES 203 screen, factor of two or four, or possibiy eight, and the whole proceeding requires patient watching for a day when clouds and light are suitable. 132. Aerial Perspective.— The geometric per- spective will be considered later, but in landscape work there is an effect due to the varying density of the air — a great exaggeration of which one gets by looking at something across a hot stove — leading to Slight movements of the image and therefore blur- ring, whose amount depends on the distance. There is sometimes an actually visible tremor which like all motion can only be imperfectly suggested in a pic- ture. Added to this is the effect of dust always present in the air but in greatly varying quantity. Its effect is to scatter the light coming from a dis- tant object, thereby reducing its brilliancy but at the same time to add to Us brilliancy by light scattered from other images. The result is a tendency to bring all distant objects toward the same brightness by dulling the bright ones and brightening the dark (mes. Also since the added light is a bluish white, the result is to dull all bright colors by an admixture oi white light. Unfortunately for the photographer the amount of the scattering by the atmospheric dust depends on the color of the light being mark- edly greater for the blue end of the spectrum. For not very luminous objects the loss of blue is more thp.n made up by the added light which has been f f 204 rnOTOGRAPHY ^Csic'^ scattered from other images, the result being the overlaying of all distant imrj^es with a marked blue haze, conversely a reddening of all relatively bright objects, as in sunset and sunrise. The photograph with the ordinary emulsion is practically taken with the blue and violet light, so that this blue haze, be- sides leveling light contrast, produces so much de- posit in the negative as to obscure in the print dis- tant detail ordinarily visible. Hence practically dis- tant objects require less exposure than near ones. The whole objectionable effect in the plate can be corrected by using a plate whose sensitiveness curve is approximately that of the visibility curve for the eye, see Art. 51, Or the same result may be obtained by using a color sensitive plate and a matched color screen whose combined effect is to match the visibility curve of the eye. With suitable screens the effect on the eye can be imitated as ex- actly as desired or the rendering of distant detail can be accentuated or obscured. Except for special reasons it is best to imitate the amount of detail the eye sees. This will usually mean using a good panchromatic plate and an adjusted screen and giv- ing a time exposure with a relatively small stop. The time exposure also allows the atmos- pheric tremor to produce its visible amount of blur- ring. 133. Color in Subject. — The necessitv "f trans- GOOD PICTURES 205 lating the color of the ordinary subject into mono- tone leads to two separate difficulties, one purely- photographic, the other pictorial. The first, that of making the dry plate translate the color according to its visual brightness, has been discussed previous- ly in some detail. Art. 51. It is only necessary to add here a few words on the importance of doing it correctly by the use of good color sensitive plates and matched screens. The more pronounced the color the more obvious the improvement ; the browns and particularly the greens in landscape, and freckles, skin-tone, and blue eye in portraiture, are very untrue with the ordinary plate, and repay gen- erously in truthful rendering and personal satisfac- tion any added difficulty in the use of special plates and screens. In fact one can hardly urge too strongly their uniform use for all subjects showing definite color, distance, or clouds. The second difficulty referred to abov:, the pic- torial one, is that one must estimate the effect of the picture, minus color, from the beautifully colored nm^e on the ground glass. Until hardened by repe- tition one is always disappointed by the print, and one of the major factors in producing this feeling is the loss of color. Practically one can get some assistance by using a screen, yellow for example, transmitting a moderately narrow band of the spec- trum. This will remove the color from the image '■fi pi h M i • J 5 206 PHOTOGRAPHY but v/ill not give the correct brightness to the colored objects. One has to be wary of brilliantly colored subjects, such as flower beds, whose color may be their predominant attraction. The effort necessary to observe form, light and shade, and to neglect color, leads directly to greater command over the factors themselves. 134. Lighting. — Next in importance to subject and point of view is the lighting. It should also be made to serve the purpose of the picture in exhibit- ing the center of interest. The position of the sun, that is the time of day, by the spots lighted and the shadows cast, has an immense effect on the resulting picture. No one can advise what lighting is best; it has to be examined and tried in each case by the worker. One can say however that as a rule harsh lighting, that is direct sunlight, with its deep shad- ows, is not as desirable as a softer more uniform lighting, where the shadows, being somewhat light- ed, show some detail. Of course there are li- ;. when glaring sunlight exhibits the aspect one wishes to show — a camel on the Sahara desert for example — but these subjects are exceptional. A partly cloudy or even a heavily cloudy day is in general to be preferred, provided of course there is not motion enough to interfere with the required time of ex- posure. On the other hand the lighting can 1"* t(«' uniform, so that the sliadov.s are hardlv noliocab!? GOOD PICTURES 207 and one of the important features by which we rec- ognize shape and perspective will be lost. This is particularly noticeable in portraiture, where the de- sired roundness and relief — representation of three dimensions — h sought after regularly by a stronger lighting to one side and over the head. The other side and below must not be neglected entirely but be lighted sufficiently to show some detail. Consider- able variety is obtainable by varying the relative strengths of these lightings, thereby varying the amount of detail in the shadows and the amount of suggestion of the third dimension. 135. Notan (Chiaroscuro, or mass eflfect). — There is another aspect of composition to be con- sidered which is of great importance in painting and engraving, so much so that whole schools have been founded on it. This is the view which looks upon die whole picture as groupings of masses of light and dark. The detail in each mass is temporarily neglected or blurred over, and the resultant group of areas studied as to whether they form a beautiful pattern. This pattern forms a more or less satis- factory basis for many pictures, so that the artist may neglect entirely to fill in detail, or only a meager amount, thus making this pattern the whole picture. One does not need to go this far to make use of the ''k:\, and every composition should be considered as I' \ 208 PHOTOGRAPHY to whether the grouping of the large masses is pleas- ing.* 136. Balance. — The most important considera- tion as to whether this pattern will be pleasing or not is what is called balance. In a general way this means that if the picture be divided vertically (most important) or horizontally, approximately in half, the interesting features will be disposed about equally as regards general interest or attractiveness on each side of the dividing line. Also it is to be noted that an interesting detail gains in its effect on the balance as it is moved away from the center, and that objects showing little detail do not hold the attention for long, so that their importance in the balance can therefore be adjusted by changing the sharpness of focusing. When one first looks at a picture the impulse is to run the eye swiftly over the whole area, and the first impression is then strongly influenced by the balance. Af ■ t the first quick sur- vey one settles down to examine the interesting de- tail. The importance of unity and balance in the final judgment is very great. 137. Focal Length of Lens. — There ai .' some points about the camera which by their effect on the satisfaction from the picture need some further con- sideration. If the picture is to be a representation * See Poore, Pictorial Compcsition, for an excellent discus- sion of the various possible groupings. GOOD PICTURES 209 of a natural scene it should resemble the view through a window frame with the observer using only one eye. If the eye is kept in one position— for example by observing through a hole in a fixed screen — then the scene could be copied with a pencil faithfully on the glass. On account of the actual construction of the human eye the glass to be dis- tinguished at all must be five or six inches from the eye, and to be seen with the greatest comfort and clearness it has to be ten or twelve inches. Anyone observing a small picture instinctively adjusts it to this distance of distinct vision. If the focal length of the lens used in making the picture is so short that to get the view through the window effect the ob- server has to put the print four or five inches from his eye, then when put at ten or twelve inches it will not look natural. The perspective will not look right, the foreground will be exaggerated in com- parison with the distance, and parallel lines will not converge as they do in the window drawing. This becomes very marked in some of the beautifully compact, convenient vest pocket cameras, where the focal length of the lens runs around one and a half inches. On this basis alone the ideal focal length would be ten to twelve inches, but the variation al- lowable before the ordinary eye observes it is large, so that lenses of five inches focal length are moder- j ately satisfactory. Difficulty on account of too great iS-3 Ui ■%kt V! isr" ■ I- 210 PHOTOGRAPHY I focal length is rarely met with, as such focal lengths are too large for convenient use, Fig. 28, p. 146. 138. Distortion from Short Focal Length. — The actual size of the image on the plane of the picture is approximately inversely proportional to the object distance from the lens That is, the tip of the nose being nearer to the lens than the eyes will be drawn to a greater scale. So also if the hands are hold in front they will be proportionately too large. This is inevitably the case even in the window frame drawing described above. The difficulty is that a picture in which the perspective including distortion, would be correct if viewed at, say, five inches, is ac- tually \ iewed at ten inches, so that neither perspec- tive nor distortion match with one's experience, that is with the window drawing. If, however, the sub- ject is kept at a greater distance by the use of a longer focus lens, the perspective and distortion be- come more nearly that with which we are familiar, that is it approaches more nearly to the window- frame drawing, see Fig. 28, page 146. This effect is particularly troublesome where in portraits and in pictures of small objects, the camera is moved close up so that the area occupied by the image will be large enough to show the desired detail. The only solution is a longer focus lens. 139. Angle of View.— Using both eyes, one can readily distinguish moving objects where the .'inLrle GOOD PICTURES 211 of view between them is i8o°, which is greater than any photographic lens. But the detail which the eyes can observe when so used is very small and the effort required way beyond the ordinary use. The user of spectacles is hardly troubled by the limita- tion of his angle of view. It seems that the angle of view of the eyes used freely and without direct effort is hardly 60°. In the camera an angle of 60 to 70° is to be preferred, and one can make a practice of trimming the print freely. Angles of view of more than 70° become progressively more unpleas- ant because they demand an unusual effort on the part of the observer to interpret the picture. 140. Focusing. — On account of the narrow depth of focus of all lenses used with the ordinary apertures, it becomes necessary t select a plane in the three-dimensional subject where the focusing shall be sharpest. On each side of this plane the definition will fall off, the most rapidly with the largest apertures. Ordinarily the center of interest in the picture is the place to focus most sharply, and where light and motion conditions permit, the choice of aperture, and therefore the depth of focus can be profitably considered in relation to how strongly one I wishes to concentrate attention on the center of interest by causing the neighboring objects to go rapidly out of focus. For example the practice in I portraiture is to use large apertures and to focus 212 PHOTOGRAPHY sharply on the eyes. This has the effect of throwing the ears somewhat out of focus and the background completely so, and serves to concentrate attention on the part of the ;ace normally observed closely. One can imagine without trying it the effect o: focusing sharply on the ears. In cases where detail is a very subordinate part of the picture, the rendering may be made more ef- fective by sacrificing more or less of the fine detail. There are many ways of doing this, for example deliberately throwing a fine lens out of focus, using a poorly corrected lens, or, better, a lens havinj,^ ad- justable spherical aberration, printing with a slight distance between negative and paper, or developing to destroy detail in a process like carbon printing. Each method has a somewhat different eft'ect in the print, and the method used should be chosen with special regard to its particular character. A hlgb class modern lens gives more detail than is effective in the average picture ; it gives so much more than the eye can see when viewing the picture as a whole, that the observer is tempted to examine parts, and thereby the general effect is weakened. This is not to say that every picture should have all the detail blotted out, an effect of which one sees far too much in any group of pictures at the present time, as i; all that was necessary to make a picture artistic was to blur it. Ail such devices \;sed without any con- e/rt Ti.-'.*- GOOD PICTURES 213 sideration of their appropriateness serve merely to show that their user lacked judgment. 141. Distortion from Inclined Plate— \\'e are so accustomed to looking horizontally that we in- terpret every picture as made in that direction. If one takes a picture of a tall building by tilting the camera so as to look up at it, and then holds the print vertically for observation, the building will appear to be falling over. But if the print be tilted, upper part toward the observer, in agreement with the tilt of the camera, a position can easily be found where the re])resentation will be correct, and the building look vertical. But since the habit of look- ing perpendicularly at the print is so fixed, a picture taken for any other viewing is misleading or escapes interpretation unless there are perfectly obvious guides to the required orientation. Also this re- quires effort on the observer's part which is not ap- preciated, so that unless physically impossible pic- uues should be made with the plate in the camera placed carefully vertical. The lens may be tilted, or shifted up or down, without difficulty in this respect. 142. Lens Axis.-If a picture contains a few straight lines it is easy to determine fairly closely where the axis of the lens intersected the plate. Even if there are no straight lines the effect can not bejntirely ignored. We ordinarily look . aight at what we desire to see, and a picture \\hich repre- p i •' .1 214 PHOTOGRAPHY sents something as seen above or below while look- ing straight ahead will not look natural, that is will require special effort by the observer to interpret it. Hence, one can say that the axis of the lens should fall somewhere near the central part of the picture. Pictures where the lens has to be shifted way up or down to get the subject on the plate, for example tall buildings, are not the most pleasing composi- tiohs. 143. Exposure and Development. — The expo- sure should always be within the latitude of the plate except for definite, well- formulated reasons. By underexposure of various degrees it is possible to subdue the shadow detail or block it out entirely, which enables one to imitate moonlight conditions and to emphasize the notan. Actual overexposure offers hardly any results which can not be better ob- tained by other means. The ideal development is for gamma unity, but it may be varied from (a) when it is desired to over or under exaggerate the actual contrast in the subject, or (b) whi .1 the full scale of the printing method is not as great as the range of illumination in the subject so that it b preferred to lessen contrast rather than sacrifice detail at one or other end of the scale, see Art. cS6. 144. Printing.— The pictorial element is impor- tant in the printing. One's choice of the various methods is based on many considerations,— total GOOD PICTURES 215 scale needed, rendering of detail, tone, surface, ease and certainty of control, labor required, flexibility, permanency. In regard to tone and surface there is one consideration which should never be forgot- ten; anything which calls attention to the method and away from the picture is thereby condemned. The ideal arrangement is where these details har- monize so well with the picture that to be noticed they must be looked for directly. They should con- tribute to the pictorial effect, not distract attention from the thing itself. Very brilliant tones are for this reason almost always unsuitable, as also are ex- ceedingly coarse, visible grain and imitation of other fabrics The control over the detail rendered and the trimming offer however the greater opportunity for pictorial improvement. By the use of a glossy developing paper one can render almost all the de- tail in the negative, and by successive steps to a coarse matt the fine detail may be more and more subdued. This control however extends to the pic- ture as a whole ; it is much more difficult with de- veloping and printing out papers to control locally. It is in these features that platinum, carbon, and gum methods have the advantage, as ..1 development one can make marked local variations. Also with all methods local shading during printing enables p ' !»■ , « f ,5 J f 1 I 2l6 PHOTOGRAPHY one to control definite areas, and by over- or under- exposing locally reduce the detail. The natural desire of the beginner is to make his print cover the whole negative ; and it is only w hen the advantages of liberal trimming are felt that this attitude is modified. In case one desires a picture of a certain size it is advantageous to enlarge the part ot the negative finally selected. To determine this part, experiment by covering the four edges of the print with four pieces of paper and move them to various positions on the print and observe the effect on the picture as a whole. Keep in mind the pre- dominant position the center of interest should oc- cupy, and also the requirements of notan and bal- ance. A careful study of one print you are inter- ested in will teach a great deal. When satisfied mark the print and trim to the marks. In case a white margin is desired, surround the selected part of the negative with black paper pasted to the gel- atine, and print through this mask. 145. In mounting try to make the mount serve the purpose of the print and not distract from it. When several pictures are mounted together on one page, choose pictures which form a harmonious group by having some common features — similar printing method; similar treatment, similar subjects. Choose the most interesting and conspicuous one as GOOD PICTURES 217 Ithe center of interest and group the others around it I in balance and to help the central interest. 146. Conclusion.— In games of chance and skill llike whist, bridge, checkers, chess, or even billiards land pool, the pleasure to be derived from the routine lof the game— the mechanical arranging and holding lof the cards and dropping one as desired— is largely Igone as soon as it is mastered reasonably well. For jthose who go on, the interest must shift to the intel- lectual problem of the management,— order of play -of the complex interrelated group of factors,— kbe cards,— and the checking of one's deductions land inferences by the results achieved— tricks Itaken. The mechanical part becomes relatively very Subordinate except as it retains some of the intel- lectual problem. So also in picture making, the pleasure in the nere routine of exposing, developing, fixing, print- ing, continues only so long as it presents difficulties lusceptible of master^'— that is essentially intellec- lual. When one has mastered the routine suffi- pently well that there are no obvious improvements wsible, then the work will be laid aside to be called [nto play when needed for other reasons, unless new nethods are experimented with or unless the intel- ctual problem of the building of the picture is real- "d. This new problem makes use of the mechani- cal routine as a means, tlie facility with which con- +-! rn' '' i it 't'l -I 2i8 PHOTOGRAPHY tributes to the solution of the new problem. For this reason the pictorial element dominates photo- graphic exhibitions, relegating methods to the posi- tion of means to this end. And the successful ex- hibitor is he who can use these methods freely and skillfully in the solution of this more complex prob- lem. The next and perhaps final step in the series is when pictures are attempted which tell a story or represent (and thereby transfer) abstract ideas and emotions. 4 !l 4 » Lm APPENDIX Development Through the courtesy of Mr. Alfred Watkins of the Watkins Meter Co., Hereford, England, is print- ed below his list of plate speeds and development speeds. The numbers apply to the speed of the plate, being directly proportional to the speeds, and there- fore inversely proportional to the time of exposure. The letters apply to the development speeds, thus placing all the makes of plates in eight classes. The strength of developer for the same time of develop- ment is indicated by the drams of concentrated de- veloper to be diluted to the three ornces. The same company makes the concentrated developer ready for use, and the formula is given in the Watkins Manual. 319 I s" ^A 220 PHOTOGRAPHY Watkins Speed List Compile solely from actual tests for use with Watkins Meters (any developer), not to compare speed or quality. Exposure speeds are indicated as numbers and grouf)ed ; thus 1 80 means a speed somewhere between 152 and 215. Development speeds are indicated by code letters, the fig- ures below being drams of two concentrated solutions in 3 ounces of complete Watkins Thermo Pyro-Soda Developer, for 6^ minutes at 60 degrees. This applies to the Time Thermometer. Where two brands of the same maker are quoted alike they may differ slig'- cly, but samples tested came in the same group. VS Development Speeds WQ VQ Q MQ M MS S Drams i iM i^ 214 3 45 Note separate heading for films and color plates. Development Speeds have nothing to do with Exposure Meters. The words Simplex, Anti-Halo, or Backed, do not indicate any difference in speed. Agfa, Ex. Rap " Isolar " Chromo " Isolar «< « <( Isorapid Iso. Rap. Central Ortho " Spec. Non. Hal. (< << " Comet Cramer Anchor Banner Crown Isonon Port. " Com. « « 180 MQ 130 MS 250 Q 130 MS 250 MQ 250 M 290 MQ 130 MS 180 M 130 M 130 MQ 180 S 250 S 350 M 350 MQ APPENDIX 221 Cramer Polychrome " Spectrum " Trichrorae " Iso. Slow " Med. " Inst. Defender Slow Eastman Rapid Extra Rapid Ortho " Spec. Sens " Ultra Rap. Forbes Challenge Hammer, Record " Ex. Fast " Aurora Ex. Fast " Ortho Slow " " Extra Fast " Non. Hal. Jougla Intensive " Violet " Green Seed, 23 " 26 " 30 Gold Edge " L Ortho Non. Hal " Non. Hal " Color Value Standard, Thermic " Orthochrome Stanley Commercial Roebuck Blue Ortho Bamet Ordinary Meditun Ex. Rap. Studio Press Spec. Rap Super Speed Ortho S. R, Red Diamond 180 MQ iSoMQ 180 MQ 180 MQ 180 MQ 250 MQ II WQ 130 M 180 M 180 Q 250 M 250 MS 90 VQ 180 M 250 MS 180 MS 45 VQ 180 M 180 M 13c 3 180 130 90 MQ 180 MS 350 MS 180 MQ 250 MQ 250 M 180 MQ 180 MQ 180 MQ 180 S 130 M 45 M 90 M 250 S 250 S 500 VS 350 MS 130 MS 250 S 350 MS 1 ,* 1 J ^ if) i ruit.' I,' 222 1 ' <( It tt << (< (( (( « « PHOTOGRAPHY Bamet Red Seal " Med. Ortho " Ex. Rap. Ortho. " Self Screen Ortho Ilf ord Ordinary Versatile Rapid " Most Rapid " Ortho Screened Chromatic King's Own Chromatic Empress Rap. Chrom. Spec. Rap. Monarch Zenith " Panchromatic Imperial Ordinary " Fine Grain Ord. Sovereign Spec. Sensitive Spec. Rapid Flash Light Ortho Spec. Rap. Ortho Spec. Sens. N. F. Ortho Lumiere Ex. Rapid (Blue Label) " Ortho A « Ortho B " Ortho C " Sigma Ordinary Violet Label Rogers Regular " Ortho " " Non. Hal. Warwick, Gold Label " Silver Label " Bronze Label " Rainbow, Fast " " Slow " Warpress it « « <( « 250 S 90 MQ 180 MS 250 MS 45 Q 130 MS 250 MS 180 M 180 MQ 180 S 130 Q 90 MS 180 M 130 VS 350 vs 250 VS 250 M 45 Q 16 WQ 130 MS 250 S 180 MQ 350 S 130 MS 180 MQ iSoMQ 90 MQ 180 M 130 MQ 90 MS 250 MS 6M 500 S 180 M 180 MQ 180 MQ 180 M 90 M 65 M 350 M 180 Q 180 M APPENDIX Wellingtoi a Speedy 180M << Ex. Speedy 3SoMS (< Speedy Iso. 180 M « Speedy Portrait 180 M « Landscape 65 Q t( Ortho, Process 32 « Anti-Screen 180 MS « Spec. Ex. Speedy 350 MS << Extra Speedy Press 250 MQ (( Xtreme 500 S 223 Color Screen Plates Speed when used with maker's filter. For Color Plate Meter, also See Meter with bright light nd large stop only. Autochrome 4 Paget II Ehifay 5 Omnicolor 6 Films "Roll," "Pack," "Autographic," eg.," same speed. Ansco Extra Fast and "Kinematograph 180 S Austin Edwards 180 S Ensign Goerz Tenax 180 MS 130 vs Kodak N. C. 180 MS " Ex. Rapid Lumiere Plavik (and Block) " Cinemat 2508 130 MQ 130 M " Sigma Ortho Premo Film Pack 180 S 250 s " Ex. Rap. Vulcan Ortho 250 s 180 s Pathe Cine 180 MS Criterion Cine 130 MQ Just as convenient are the directions issued with fie "Agfa" Rodinal (Berlin Aniline Works, 213 i.^ i«'jpM 224 PHOTOGRAPHY Water St., New York), which gives detailed diret tions, list of development times, temperatures, dilu tions, and list of plate development speeds; all b Mr. Watkins. It is one of the best guides availabl at the present time for the occasional worker. ] all applies, of course, only to Rodinal, which is a excellent developer. • Burroughs Wellcome & Co. (35 W. 33d St., Ne\ York) publish the "Wellcome Photographic Expo sure Record and Diary" yearly. It clas.sifies tb plates on the market as to development speeds, an( gives also development times, dilutions, and teni perature coefficients for use with their ready pre pared developers. The Eastman Kodak Company's book "How t( Make Good Pictures" gives full directions as t( time, temperature and developer for use with theii films. Similar information is given by the Ilforc Company for their "King's Own Plate," and b) Wratten & Wainwright for some of their plates, ! 1 Char- acter of plate APPENDIX Table of Plate Speed Numbers 22^ Where E is approximately the minimum exposure in sec- onds for an open landscape at noon in June at F xl And, Wynne F number = 135^ Vr=7.7v/H&^; ^^ ' H&D=34/i;p= 5o/i = H&D so/34, • i txX:^2:^.^ttr ^^^^- -d "A" has the value in ! (i^) •'■ • ^'^^'' P^^^ograpxschc Correspondent,. 37, 249 t- \ PART II LABORATORY MANUAL I !!'' ■3 ih % t, PREFACE The course in this department has called for one lecture and three laboratory hours per week over one semester, and confers two credits. No student comes to the laboratory for a smaller interval than two hours. The work has been offered always in the second semester because for the latter half, when the student has had some experience, the weather, the lighting, and the foliage are good. The student opinion has been that the work required fully earned the credits— which feeling is to be desired, as the tendency for the outsider is to look upon such a course as a "play" course. The student attitude toward it is well described by one student's imme- diate objection to omitting anything from the list "but increase the credits." It has been the practice here to furnish all the supplies and chemicals as well as apparatus, and to charge a laboratory fee. There has been no diffi- culty about pilfering or extravagance since a defi- nite statement was made of the material to which the fee entitled the student, excess to be separately paid for and all supplies to be delivered only on ^ ^ign^d order. This plan has the great advantage aa9 230 PHOTOGRAPHY that the student is not worried about the cost of each experiment and hence skimping on materials. The laboratory can buy to better advantage than the student, and uniformity and reliability in the supplies can be assured. Physics Laboratory, University of Wisconsin, 1917. LABORATORY MANUAL General, Directions Each student is provided with a small locker for his own special material and apparatus. When as- signed this locker should contain : 1 small negative drying rack 2 pads report blanks I print albiun i binder for reports I empty box, 4 x 5, for negatives x box, 4 x 5, diy plates In the allotment of laboratory places the student will be assigned a partner as a rule. It may not always be possible that the student shall work all his time with one and the same partner. The two will have to compare wo»k done and take up what will be for both the most convenient work next in order in the manual. On account of the time re- quired for the drying of plates and prints as well as long continued washings, it is necessary that the student carry on more than one experiment at once, so that the time will not be spent in idle waiting. \\ hen a stage in an experiment is reached requiring a long wait, look back to see that printing and mounting is up to date, and if so start the next ex- »3« t1 232 PHOTOGRAPHY periment. Very much time can be saved by intelli- gent planning of the work. Beside the apparatus in the individual locker each dark room should have on its shelves at the begin- ning of the work the following: 4 printing frames 1 centigrade thermometer I camel's-hair brush I print washer I measuring glass 1 tube paste 2 hard rubber trays, 4x5 2 hard rubber trays, 10x12 1 print stick 2 wooden tray covers I negative washer I glass funnel I bottle "soda" I bottle "Pyro" I bottle acid "Hypo." As far as possible each article is marked for the locker to which it belongs— and if for any reason any article or bottle is torrovved from another room it must be returned where it belongs. When the experiments require apparatus not listed above it is to be obtained from the instructor and returned as soon as out of use, or on finishing the day's work. At the close of the period all apparatus and sup- plies are to be returned where they belong and the desk left clean and dry for the next group. In the case of spilled solutions, particularly hypo, it must be washed carefully into the sink. The sink must not be made a receptacle for waste paper, etc., a> blocking of the drain may flood the whole labora- tory. The work for the period will have to be s so LABORATORY MANUAL 233 I arranged as to allow of vacating the bench promptly at the close of the period so as not to interfere with the succeeding group. The staff reserves the right to impose fines for gross carelessness in the care of the laboratory and apparatus. In general, small routine breakages are covered by the laboratory fee, but in case of serious breakage or injury resulting from gross carelessness, special assessments will have to be I made. As each experiment is finished a report coverin^^ I the following items is to be handed the instructor 1. Negative and print data sheet filled in. 2. Negatives and si)ecimen prints 3. A statement of the object of the experiment with brief theoretical discussion 4. Statement of the results obtained with a criticism of them. There are two forms of report blanks, one for butine reports on negatives and prints, the other Nank for reports on other work. Every plate should -^ reported on whether spoiled or not, and the spe- N directions for reports on the particular work Hlovved. In every case with the report is to be sub- nitted the material (negatives, prints, etc.) reported pn. These reports with comments will be returned If possible the following day, and the first item of Ihe day's work should be their review and correc- W They shnuld be carefully preserved in the iii 234 PHOTOGRAPHY binder supplied for the purpose, and at the end of the semester form the laboratory report, which, to- gether with other submitted material, determine the laboratory standing. These reports, negatives, al- bum, prints, etc., become the property of the stu- dent after examination at the close of the semester when the laboratory keys are returned. tfe^fe Experiment i The work for the first day consists in making two negatives. Load one plate holder on each side, film side outward, using only the desk ruby lantern in the dark room. The film side may be distin- guished by the feel— the finger tips slip more easily on it while they stick more to the glass which feels smoother. Or in the ruby light the film side gives a less sharp reflected image and looks brighter gen- erally than the glass side. Avoid as much as pos- sible touching the film with the fingers. Take the camera, loaded plate holder, tripod, and focusing cloth, and go out with the instructor to mak^ one exposure. Return to the dark room to develop the exposed plate. Mix together equal quantities (total 2 or 3 oz.) from the two bottles marked "Soda" and "Pyro," using a fresh mixture for each plate. Determine the temperature of the mixture with the thermometer and refer to the temperature-time oi LABORATORY MANUAL 235 development table posted by the sink, for the time for which to keep the plate in the developer. Im- merse the plate film side up, remove all air bells, cover the tray, and leave it there this predetermined time, with an occasional rocking. Rinse under the tap and place in the "hypo" solu- tion in large tray till all the white in the film disap- pears (5-15 min.). Wash in changing water in the washing box for 15 to 25 min. Discard the used developer but return the hypo solution to the bottle. While waiting for the plate to "fix" in the hypo, reload the plate holder and repeat the above process, making a second negative. When washed see that there is no deposit on the film, and dry the plate on edge in the rack. On request the instructor will put out to dry any plate not finished washing at the end -f the period, and the diying rack (which is numbered) should be left out ready to receive them. Submit the negatives when dry with the report on each. Experiment 2 Repeat Exp. i making negatives two at a time. Do not take more than two exposures on one trip out; later when you are more sure of the routine and will not waste plates uselessly use both plate holders. The choice of subjects to photograph is in i- 1 1 1 ifw i "i-'lf \ 1 • • 236 PHOTOGRAPHY your own hands, except for the request not to choose subjects far away or to spend too much time over the choice. Take a look at the light outdoors today and, con- sidering the subject to be photographed, estimate the exposure. Consult the instructor for confirmation of the exposure chosen. Experiment 3 At some time in the early part of the semester make up the following solutions used in this work. Bring the three bottles from the shelves in the de- veloping room, return whatever solution they con- tain to the stock bottles, and make up sufficient of each solution to fill the corresponding bottles. The compositions are as follows : Stock Pyro Stock Soda Water up to 1000 cc Water up to 1000 cc Pot. Metabisulphite 1.4 g Sod. carbonate (dry) 40 g Pyrogallic acid 8.0 g Sod. sulphite (dry) 50 g Dissolve in the order given. Attacked by the oxygen of the air so keep stoppered. Acid Hypo A B Water 2000 cc Water Hypo SCO g Sod. Sulphite (dry) Acetic acid (glac.) Alum (dry) Dissolve A and B separately and in the order named, then add B to A with constant stirring and add sufficient water to make 2500 cc. 200 cc 35 g 30 cc 35 K LABORATORY MANUAL 237 Paper Developer Water up to 1000 cc Metol o-7Sg Hydrochinon 30 g Soda Sulphite (dry) 12 g Sod. Carbonate (dry) 21 g Pot. Bromide 0.4 g Dissolve in the order named in somewhat less Ithan 1000 cc and after all are dissolved add water lenough to make up to icxx) cc. Use about 4 oz. at la time and this will develop 3-4 doz. prints ; never Ireturn any to the bottle after use. It does not spoil ISO quickly in the air as Pyro-Soda does. Make out report on plain report blank of the [actual quantities as calculated for the size of bottles lused. Did you observe anything which will be of lvalue the next time you make up these solutions? Experiment 4 Prints on Developing Paper (Gas Light Paper). I -Print the negatives from Exp. i and 2 on the de- veloping paper. Lay the negative film side up in I the printing frame, lay a piece of the paper film side down on the film side of the negative, fasten the back in the frame, and expose the glass side to the Ii?ht either at the room door or before an incandes I cent light. As a first trial give 15 seconds at 10 inches from a 40 watt tungsten lamp. Develop in the special paper developer (Motol-Hydro) as the 238 PHOTOGRAPHY m Pyro-Soda stains the paper. The print should take one to three n^^iutes to develop. H it is dark enough in less than one minute the exposure has been too long. In making a second trial change the expo- sure by a factor of two or four. The print will nev'_r be very satisfactory if the develop nent is cut short in order to avoid too great general blackness. For the best results the exposure must be so ad- justed that on a full development the dark parts will be black, not just gray, and the light parts will be as white as the unexposed margin. If the unex- posed margin becomes gray, the development has been too long, or the developing light is not safe, or the paper has been unintentionally exposed to white light. If with an exposure just short enough to keep the white of the print clear the dark parts do not become a strong black, then the negative lacks cmi- trast and will either have to be intensified or a more contrasty grade of paper used. Conversely it with clear whites the dark parts become a dense black in which all the detail has disappeared, then the nega- tive is too contrasty and will either have to be re- duced or a less contrasty grade of paper used Refer to Text, Art. 87. Rinse the developed print in water and put in the hypo for 15 minutes, then in changing water in the circular washing box for 15-25 minutes. Put out to LABORATORY MANUAL 239 dry face up on a clean paper towel. After the excess of water has been removed either by drying or by the towel, the print may be turned film side down and will then dry with very little tendency to curl. Exercise great care not to get any hypo into the print developer as, if the slightest trace gets in, it is liable to stain all the prints with brown patches which may not show till the prints dry. For this reason it is advisable not to put the hands in the hypo at all when printing but to handle the prints in the hypo with the stick provided. The paper is not nearly as sensitive as ordinary plates and may be handled in much stronger light; the hanging red lamp is safe only for short exposures, and weak diffused light is not likely to bother. But if the prints at any time get gray all over, the light may I be too strong or the development too prolonged. Fill in print record of negatives printed, and I hand in with the negatives and prints. When [returned to you mount in your album a print from each of the negatives. With the tip of the finger rub on a narrow band of paste along the upper and lower edges of the print, and fasten in j the book. Nui iber to correspond with the negative. Experiment 5 I To Test Systematically for Correct Exposure imakc a negative exposed in strips where the expo- l 240 PHOTOGRAPHY r '-i i I sure of the successive strips increases by a factor of two. In order to make the exposure, choose the plate holder with the s'' -e marked oflF in half inches, draw the cover slide out till the first mark shows and give the exposure No. i in the table (i. e., i sec at stop 8). Then draw out to the second mark •'nc give the second exposure, and so on for th^ o ex posures. Note that the last two expo*, rt r alike. The first strip uncovered will get .11 U .n: ceeding exposures, the second strip all j u i' v I , exposure and so on. The fourth coluPi ;n * ) - 1 '', gives the time required at stop 1 6 to i,ivr t> ■. exposure as that actually given by the tinie anu i j of the second and third columns. Then if tb > -s i group of exposures given each strip be added i will give the fifth column, if the times are thosi required for stop i6. These increase very nearly b; a factor of 2. The changes are made almost entirel with the stop as the shutter speeds are very unreli able. For an ordinary bright day the series of expc sures in the table will be sure to have some in th good-exposure range. Choose a subject as uniform as possible a^ms the plate as, for example, the uniform front o.' building or a uniform landscape. Consult the bul letin board for suggestions. Expose one plate, df velop and fix as usual. Note the change in chai LABORATORY MANUAL 241 acter of the various strips, and make your choice of the best. Print the negative making good prints from as many of the strips as possible, keeping a record of Table io r.u'.-^ No. Exposure Equivalent Time, Stop Total Exposure Time Stop 16 of Strip I I 8 2 4 -1 I 16 I 2 3 I 32 1/2 I 4 I 04 1/4 1/2 J I 128 1/8 1/4 6 1/25 8 2/25 I/O. 25 7 1/25 16 I/2S 1/12.5 8 1/25 32 I/SO 1/25 9 1/25 64 i/ioo 1/50 10 1/25 64 i/ioo l/iOO the exposures required in each case. What is your estimate in regard to this plate's latitude of expo- sure, that is, the factor between the least and great- est exposures both of which give fair prints? EXPIJRIMENT 6 Factorial Development. — Go out and make two good exposures, such as suggested by \our strip negative. Flood the plate with the usual pyro-soda mixture, noting the exact time with the second hand of your watch. Observe the jjiate carefully in as 242 PHOTOGRA'-HY good a red-light as possible for the first appearance of image and note the time required from the flood- ing of the plate. Cover the tray. Multiply this time required for the first appearance of image by 9 (see text, Art. 33c) to get the total time fc which the plate is to be m the developer. Rock the tray only a few times during development. Finish the plate as usual. Repeat the process with other plates. Submit prints and negatives with report. Experiment 7 Contact Lantern Slides. — Go out with the camera and make two exposures suitable for lantern slides. Take a lantern slide matt along to tell what size to make the subject on the plate. Develop by factor as in Exp. 6. When dry pri.it these negatives on lan- tern slides in the regular printing frame, using the exposure time for the first trial as suggested on the bulletin board. As in the case of plates and paper prints, the con- trol of the character of the lantern slide is best un- derstood by reference to the discussion in Chap. II, and especially Arts. 34, 83, 86 and Exp. 4. It is even more important than in prints that there should be no silver deposit in the clear places (high lii^^hts). I f the slide is gray all over the exposure has ken too long, and if at the same time the dense places are •% 'wmmr^ !.^' .">';jfj^ LABORATORY MANUAL 243 not dense enough either the development has not jbeen long enough or the negative lacks contrast. Hence in general it is best, as with prints on paper, I to give the least exposure which will yield the de- ! sired maximum density if the development is pro- longed as far as may be without noticeable fogging. Lantern slide plates are very fine grained and I much slower than ordinary plates. A lantern slide should be very much thinner than any negative to be satisfactory; when the picture looks clear and strong in the developing tray it is about right, long I before it gets black all over as a negative does. On account of the small amount of fog produced, I the ferrous oxalate developer is an excellent one for lantern slides. The two solutions re(|uired are (a) a saturated solution of potassium oxalate and (b) a mixture of 4 parts of water and one part of a sat- [urated solution of ferrous sulphate containing about 2% sulphuric acid. The latter is supplied already di- lluted. Mix one ounce of each and use for devel()i)ing only one slide, as it oxidizes rapidly in contact with the air and a precipitate appears. The development starts slowly but when the image begins to appear it conies up rapidly. Since there is no need to fear fog the u^velopment may be prolonged for 15 minutes or more. Rinse unde. the tap and soak for a few min- lutes in a i'/' solution of oxalic acid to remove any ^^^^'■'T!^^ F i 244 PHOTOGRAPHY iron which might be deposited in the film. Fix as with the ordinary plates, wash and clean carefully when putting out to dry. Save the spoiled lantern slides so that they may be cleaned for use as CDvcr glasses. Mount the slide by covering the ed^cs ot the film with a paper matt and covering this with a clean piece of glass of the same size as the lantern slide and binding around the edges with strips of gummed paper. Consult the instructor for hints a< to the easy way to manage this. If the picture re- quires a different shaped matt, change the shape 01 the one supplied by sticking strips of gummed paper — the gummed binding strip will usually do — around the sides. With the report submit the negatives used as weii as the finished slides.* •The saturated ferrous sulphate solution offers certain difii culties which are, however. rca«lily surniountc...v removed with a pipette. 245 LABORATORY MANUAL Experiment 8 Lantern Slides by Reduction. —Set up the con- centrated filament tungsten lamp in its box and place the condensing lenses and water cell before the aper- ture. In the aperture place the frame with the 4 x 5 opening for carrying your negative. See that there is a ground glass between the negative and the con- densing lens. This should illuminate the negative uniformly. Put the 4 X 5 back in the view camera, and set I up on the table to make a picture, lantern slide size, I of the illuminated negative. L'se a kit in the 4 x 5 plate holder to carry the lantern slide plate. Con- sult the bulletin board for the tirst trial exposure. The same considerations govern the exposure as in I the contact slides. To become accustomed to the developer usually Used for lantern slides develop these in the special liydrochinon developer in which cont ast grows lairly rapidly, while it does not stain the fiim nor give much fog. The best slides are usually ob- tained by exposures recjuiring 2-4 niin. develo,j- Iment. HVDROCHINON DEVELOPER [Water up to looocc IHydr(Khinon 45 ^ |S*)d. Sulphite, dry 30 j; |Sulphuric Add 5 cc B Water up to SckI. (\irlwnate, dry Pot. CarlxMiale Pot. Bromide Sod. SulpinU', dry 1000 cc 30 g 9og iie m 246 PHOTOGRAPHY For use mix A and B in equal quantities. Fix as with the ordinary plates, wash and clean carefully when putting out to dry. Mount as with the contact slides. If you have time for more than your own negatives exchange negatives with some of the other students. Report any difficulties encountered. How do you find this developer compares with the ferrous oxa- late? Report any difference whatever which you observe. Submit the finished slides with the negatives used along with the report. Experiment 9 Halation. — ^Take three negatives of the same sub- ject using (a) an ordinary plate, (b) ordinary plate backed with black paper. Smear over a smooth 4x5 piece of black paper with sufticient glycerine to leave the surface well wet, but not sufficient to run off when against the glass. Lay the i)late (glass slide down) on the glycerined paper. Rub gently into contact all over, being very careful not to get glycerine on the sensitive film. Load as usual and expose without unnecessary delay. Wet the plate all over and wash off the glycerine before de- veloping, (c) A double-coated plate whose speed is given on the bulletin board. LABORATORY MANUAL 247 Choose a subject liable to show bad halation, such as a lights'' lamp in a room, or an interior with sunlight coming in at a window or the fine branches of the trees a^inst a bright background of sky or lake, in general wherever very great sharply marked differences of illumination occur. In all cases the effect is more obvioos in the print than in the nega- tive. The test is made particularly severe if the development is prolonged to give strong contrast — conversely for avoiding it. P'inish, print and report as usual, paying particular attention to a comparison of the results with the different ar- rangements a, b and c above. Experiment 10 Intensification. — Make two underexposed nega- tives by exposing a plate % or '/„ that judged cor- rect. Lessen the exposure by manipulating the stop rather than the time on account of the unreliability of the hi'rher .speeds of the shutter. Finish as usual, being particularly careful to wash well (i hour). The negative should be very thin especially in the shadows. On the same trip out expose 2 plates as correctly as possible. Better choose different subjects to keep the negatives distinct. In the development cut the 248 PHOTOGRAPHY Km I i fi t time to about J4 o^ the usual time so that after fixing it will be very thin. Look for any difference observable in the character of these negatives, one pair underexposed, the other underdeveloped. When they are dry make prints from them. Note any difference in the character of the prints. vSuch prints are usually fiat, that is, lacking in contrast Refer to the discussion in Chap. II and especially Art. 34; also Arts. 83, 86, Exps. 4 and 7. Con- trast grows with the progress of development until fog intervenes. To keep the print from be- coming too dark all over lessen the time of ex- posure as much as possible, and to get the maximum contrast leave in the developer till all development appears to have stopped but not till the unexposed edges of the print become gray. It is possible by varying the method of manufacture to make devel- oping papers which vary greatly in their ability to render contrast. Most makers offer several grades from hard contrast for flat negatives to very soft for hard contrasty negatives and for portraits. The best grade to use depends on the character of the negative and of the subject. An intelligent choice of these papers can hardly be made till the capa- bilities of one grade arc known from experience, and for that reason we will use only the one i^rade made for correctly exposed and developed nega- tives. a LABORATORY MANUAL 249 With your report submit your negatives and the best print from each. Proceed to intensify ' these thin well-washed neg- atives by immersing them first in a solution of mer- curic chloride made up as follows. The solution is exceedingly poisonous. Mercuric chloride Hydrochloric acid, cone. Water 130 g 3 cc 1000 cc The mercury salt dissolves slowly and there is present more than sufficient to saturate the solution. Leave the residue in the bottom of the bottle and use the clear supernatant liquid. Keep the used solution clean and after use return it to the bottle. Allow the plate to lie in this solution till it just shows white through the glass, then remove it to the washing box and wash for an hour. If necessary the negative may then be dried for later finishing. At times the acid in the mercury solution causes the gelatine surface to break up into a mass of lines re- j sembling the cross section of the cellular structure of a plant. The alum in the hypo bath helps to avoid this reticulation but if persistent with the brand of plates used the acid should be cut down to half or less than that ^nven. The mercury chlo- |ri(le reacts with the metallic silver thus HRCli+AK = HgCl-hARCl 'Stf text, Art. 64. 250 PHOTOGRAPHY Both resulting chlorides are white and insoluble, so that the washing removes only the excess of the mercury chloride and the acid. Develop these chlorides in the daylight with a developer made by mixing 3 volumes of a saturated solution of potassium oxalate with one volume of saturated solution of ferrous sulphate.* Allow the plate to remain in this till thoroughly black, then wash it moderately (15-20 minutes) and put up to dry. Before drying see that the surface of the film is clean by rubbing it gently with the tips of the fingers while flushing with water. If the washing water contams calcium there is apt to be a deposit of calcium oxalate on the surface of the gelatine. A very great increase of density may be obtained by blackening the plate in a dilute solution of sodium sulphide ° instead of in the ferrous oxalate. Try one plate. The increase is apt to be larger than is useful except in special cases. Also the process can not be repeated, while when blackened in ferrous oxalate it may be repeated many times, each time about doubling the density. When dry, print the negatives again and com- pare these prints with the previous ones. Make oul reports by adding the new treatment to the prcvioui history of the negative, and submit prints and nega * See Exp. 7. •See Exp. 19. I ■^■i^'-A LABORATORY MANUAL 251 tives again. Finally mount a set of prints in the album. Experiment ii Reduction. — Go out with the camera and make two distiiict overexposures, about 4-8 times esti- mated correct exposure. Finish as usual, and when dry make prints from them. If the incandescent lamp requires too long an exposure go out into the daylight. To reduce the negatives soak them for a few minutes in acid hypo diluted with its own volume of water. Then remove the negative and add to the solution a few cc of a io7^ solution of pot. ferri- cyanide, say 10 cc to icx) cc of the diluted hypo, stir and immerse the negative again. Watch for appearance of thinning. If none shows in 1-2 min- utes repeat the addition of the ferricyanide. The mixture is attacked by the air so that it does not stay active very long. Proceed the same way till nearly thin enough, when the negative should be put quickly into chang- ing wash water, where the reduction will proceed a little farther during the first part of the washing. Dry as usual. Record the potassium ferricyanide and hypo used for future assistance. Practice this on some others of your dense nega- tives. Print these reduced negatives and compare 252 PHOTOGRAPHY a with the previous prints. Report by adding the account of the treatment to the previous history of the negative and submit negatives and prints with the report. Finally mount the prints in your album. Experiment 12 Color-Sensitiveness. — Make three negatives of a colored subject — an ordinary colored print — with the view camera in daylight. Use the special 4x5 back. Make the negatives on (a) an ordinary plate, (b) an isochromatic plate, and (c) ^n isochromatic plate with a color screen. The color screen is to be placed in front of the lens and as it absorbs the most effective part of the light the time of exposure with it has to be lengthened. Consult the bulletin board for the necessarv increase and also for the relative speeds of the ordinary and isochromatic plates. The latter is sensitive to the red light of the dark room and is better handled entirely in the dark. Keep a record of the plate used by placing a marked piece of paper beside the print being copied. Develop the negatives all together in the tank fnr 20 minutes using the pyro-soda diluted in propor- tion as the time of development has been increased from the usual time. Measure the tank and make up a volume of solution sufficient to completely cover the plates. P>eware of using the tap water for the LABORATORY MANUAL 253 dilution as it may be very cold; the diluting had better be done with distilled water which has stood in the room, and the temperature of the diluted developer should be near 21° C. During develop- ment reverse the plates three or four times to pre- vent them developing more at one end than at the other. Fix in the tray or in the tank, but before returning the tank to the instructor wash it so thor- oughly that you can guarantee that the next user will not have hypo in his developer. Compare the prints of these negatives with the original subject and in your report discuss the ef- fectiveness of the various colors in recording them- selves on the plates under the three different cir- cumstances used. Submit negatives and prints with your reports. Experiment 13 Spectrum Photography for Plate Color-Sensi- tiveness. — As light source use the Xcrnst lamp set up in its box with filament horizontal. Remove the water cell and the condensing lenses and replace the nejj^ative carrying board by the board with slit, plac- ing the slit horizontal. With the long focus achro- matic lens focus an image of the slit about four fet't away and set up the view camera (see dia- gram) so that the ground glass will be in the same i^SttSnSi frv ^^mMi^KJrss;/:? :s3B; MHdOCOPV NSOUITION TBT CHART (ANSI and ISO TEST CHART No 2) A /1PPLIED IIVA^OE Inc '»^> fo«l MO'K SleMI llVfhWt*'. NM Votk <4«0« USA [MB) ftAj 0VX3 Phort« 254 PHOTOGRAPHY vertical plane as the image. Remove the lens b(iard and raise the front of the camera. Interpose the replica grating® with its rulings horizontal in the path of the light and in such a position that the lower spectrum of the first order will enter the ojjen front of the camera and fall on the ground glass. Fig. 50, Exp. 13. Diagrammatic arrangement of apparatus for photographing the spectrum. A is a rihbon filament tun^'s- ten lamp or a Xernsl lamp ; S an adjustable slit set horizontally in the wall of the light tight box enclosing A ; B a long focus achromatic lens; C a replica grating; and D the sensitive plate at the back of a view camera without a lens. Adjust the slope of the beam of light and the posi- tion of the grating so that the spectrum will be a convenient length and all of it fall on the ground glass. In the view ramera use the 4 x 5 back w ith long edge vertical. Just in front of the ground glass is a piece of cardboard made to shift side- ways to four numbered positions and having a lit through which the spectrum strikes the plate while at the same time the rest of the plate is protected •R. J. Wallace, "Use of Replicas," Astrophysical Journal 2^. 121 (!9Cv). LABORATORY MANUAL ^50 from stray li^ht. Along the side of this slit is a transparent film with a scries of numbers which throw a shadow on the ground glass or sensitive film. Make a careful record of the colors at the different numbers so that these numbers on your photograph will enable you to tell the colors which produced the deposit, and make this information part of your report. Make a series of three exposures on each plate by setting the cardboard screen successively at three of the four positions and at each shift of the screen move the camera sideways so that the spectrum will continue to fall on the opening in the cardboard screen. Make each exposure four times the pre- ceding one and consult the bulletin board for the lowest exposure for each i)late. Do this for three plates, (a) an ordinary, (b) an isochromatic, and (c) a polychromatic plate. To keep track of the different plates used, mark them in the corner with a steel point and the pressure mark will develop so as to be visible in the negative. The two latter plates are sensitive to the red light of the dark rootn and must be handled entirely in the dark. Develop in the ordinary way in the pyro-soda. Make a print of each negative on a single sheet of paper to show the effect of longer exposure to the diturent colors. Mark the colors on the print and iht; exposures on the different strips. Estimate the 256 PHOTOGRAPHY #1 ^v different densities along the strip and draw a curve showing the relation of color to density. Submit negatives, prints and curv , with your reports. Experiment 14 Dyeing and Testing of Plates — The stock dye solution is made up by dissolving o.i g of the dye (pinacyanol) in 100 cc of alcohol and this solution keeps well if kept in ihe dark. For use 15 cc of this solution is made up to a liter with distilled water. This diluted solution is on the laboratory shelves. Soak two of the ordinary unexposed plates in it for 2.5 minutes, shake off all the loose water possible, and place the plates on edge on the rack in the drying cabinet till they are dry. Leave an empty plate box numbered with your laboratory number beside the cabinet to receive your plates when they are dry. As soon as the plates are bathed avoid all light as much as possible as the dyeing makes the plates sensitive to the red light of the dark room. When dry test these piates for color sensitiveness with the same arrangement used in Exp. 12. To test the color screen at the same time make two exposures without the color screen in the path of the incident light and two with. Make the second exposure of each pair LABORATORY .MANUAL 257 of exposures four times as long as the first, and for the first exposures consult the bulletin board. As something may possibly have happened to make your dyed plates fail, make two exposures using the color screen on a polychromatic plate. Develop, fix, and print each plate as usual. As in Exp. 12 draw curves representing the color sen- sitiveness of the plate — one curve for the ordinary light and the other for the light as modified by the color screen. Submit negatives, prints, and curves, with your reports. Experiment T5 Microphotography.— In the discussion on lenses, in the text it is pointed out that the relative sizes of object and image depend on their respective dis- tances from the lens. As the object is brought nearer the lens the image moves away and grows larger. For great enlargements this latter distance becomes unmanageable with the ordinary lens and so a short focus lens has to be substituted. The object glass of a microscope is essentially only an excessively short focus lens. Remove the lens and lens board from the view camera and replace with the microscope with the tube set horizontal. Close the opening around the microscope tube with a black cloth. Set the tungsten lamp and condenser so as to 258 PHOTOGRAPHY illuminate the object brightly in line with the tube and focus the image on the ground glass. This may be done with the eyepiece either in or out, with somewhat greater magnification in the former case. Good illumination depends very greatly on good adjustment of the light. The focusing may be done either by the rack and pinion of the microscope or by shifting the ground glass. Very few microscopes will give a picture 3/4 x 4, all of which will be in good focus at once ; either the center or edges will be indistinct (curved image). Also the visual focus will not usually be the photographic focus. The photographic focal length will usually be shorter than the visual, so one errs by setting the ground glass a little too near the microscope. Use a lantern slide plate in a kit in the ordinary 4x5 holder. Consult the bulletin board for details as to exposure. Make two good negatives of differ- ent subjects, finish and print as usual. Submit negatives and prints with report. Experiment 16 Blue Print Paper.— Put your name on the back of two 8x10 sheets of paper ^ to be obtained from ^ The adherence of both the sensitive materials and the im- age may be greatly helped by tiie coating on the surface of the paper. The paper is floated on or i)asscd through a dilute solu- tion of boiled starch or a dilute gelatine solution and dried. LABORATORY MANUAL 259 the instructor. Wash the camel's-hair brush thor- oughly to be sure that it is free from all remains of developer and with it paint the solution of ferric ammonium citrate and potassium ferricyanide over the paper. This may be done in daylight and it is advisable to spread it as evenly as possible taking particular care that no place is missed. If there is plenty everywhere the unevenness will disappear in the printing and washing. After it is surface dry take it to the instructor to put in the box over the calcium chloride till dry. The composition of the solution is given below. Print in the sunlight under a good negative. From time to time open one half of the back of the printing frame and examine the print. The image will show clearly and must be overprinted as it bleaches decidedly in the subsequent treatment. A thorough washing in water and then drying com- pletes the print. If the highlights are blue the ex- posure has been too long, the negative is too thin, or the paper is defective. Test for defective paper by washing it without having exposed it to light when it should wash white. Foggy prints can be improved by a quick bath in a very dilute ammonia solution followed by a very dilute hydrochloric acid solution. For comparison with your own paper, make some 260 PHOTOGRAPHY prints on the paper made by a regular manufac- turer. B Potassitun ferricyan- Ferric ammoniuni a- ide 15 g trate 19 g Water 140 cc Water 140 cc Mix equal parts before use and filter if there is any deposit. Discard the mixed solution left over. Experiment 17 Gelatino-Chloride Paper, Also Called Printing Out Paper (P. O. P.).— Pick out several of yom good negatives and expose the P. O. P. glossy sur- face under them in direct sunlight for 3 to 30 iiiin- utes, depending on the negative and the light. The progress of the printing may be judged by lifting half of the back of the printing frame and observ- ing the paper. The printing should be carried dis- tinctly beyond the desired final density, as it bleaches markedly in the toning and fixing. Wash the print for about 10 minutes in chanf]^in£ water or, better, till the water leaving the priir shows no cloudiness. Then tone in a gold chlorld. bath made by adding 90 cc of water to 10 cc of c stock gold chloride solution from the laborator; shelf. Test this bath with litmus paper and nuiki it neutral or slightly alkaline by adding some the saturated solution of borax. Immerse the prin LABORATORY MANUAL 261 in this bath and the color should change slowly from shades of red through purple to black in from four to eight minutes, i f the print shows no change in one or two minutes and the bath is neutral or slightly alkaline, remove the print and add a few cc more of the stock gold solution, stir, test for acidity, and replace the print. When toned to the desired I color, wash in water for a few minutes, and fix for ID minutes in a hypo bath whose composition is I given below. The ordinary acid hypo will usually ruin the print. Finally wash for about an hour in changing water, keeping the prints well sepa- rated. Dry face upward on a towel as they stick strongly if face down. These prints still have the I reputation of yielding the best and finest detail. Report the work done and any observations which I would help you to do the work better next time. Prepare at least four prints for xiiounting finally in your album but first submit them for examina- |tion along with the negatives and the reports. Stock Gold Chloride Hypo Solution I Gold chloride 15 grains Water 2000 cc l^'ater 45° cc Hypo 180 g Alum, dry 60 g Sodium sulphite, dry 6 g and when all dissolved mix with Borax Water, hot 25 g 300 cc 262 PHOTOGRAPHY J'"^*! and let stand over night. A heavy precipitate will form and settle. Use the clear supernatant liquid, Experiment i8 Enlarging.— Set up the lamp, water cell, con densing lenses, and negative carrying board as foi reduction lantern slides (Exp. 7). Select one 0] your best negatives and mount it in the window filn side away from the light. Set the open back of thi view camera up against the negative carrier so tha the only light getting into the room has to pas through the camera lens. Open up the stop in the lens and focus the pictiir carefully on the vertical board. You will find b; shifting the lens vertically and horizontally tha there is one position for best illumination. Adjus the size of the picture to suit the sheet of sensitiv paper by shifting this board backward or forwar and re-focusing. Paste- the paper on the board 1) >ms each on bottom and aper in behind the pins b using two glass-headt each side, and slip thr leaving them just lo(»- and focus it on a pit of red glass be place the resulting red light i direcdy on the sensiti adjusting it in position 'ough. Adjust the pictui hite paper. If a piec I the front of the ler be thrown without han ipcr which will aid i hen all adjusted, cloi LABORATORY MANUAL 263 the shutter, slip the sensitive paper into position, set the diaphragm, and expose with the shutter. The exposure depends so much on conditions of lighting, size of enlargement, density of negative, and speed of paper that it is a matter of guess and I try. Consult the bulletin board for a suggestion as to the first trial exposure. With regard to contrast the same remarks apply in this case as in the case of contact prints (Exp. 4) and lantern slides (Exp. It will be economical of paper if you make the first trials on 34 or 3^ sheets. If one ground glass I does not make the lighting uniform put a second in contact with, but ground glass away from, the nega- tive. Note that this paper, usually called Bromide I Paper, is very sensitive, the emulsion is much the same as that used on plates, and hence should be exposed only to the safe red light. Develop in the metol-hydro developer in the large tray. Dilute this developer with at least an equal volume of water to slow development. Immerse the paper in the devel- oper by sliding it in face down over the edge of the tray, quickly and steadily. When wet all over the surface, lift out and lay back face up, and keep developer always over the surface. A rinse before the hypo-bath helps to avoid stain, and so also will I laying it face down in the hypo and then turning ' See also Text, Art. 87. 264 PHOTOGRAPHY over. If any part of the print stands up out of the hypo even for a few minutes it is very liable to be stained. This paper is too large for the piint washer and will have to be washed by changing it a sheet at a time from one tray to another with con- stant change of water, say 12-15 changes. Dry as usual on towels. Submit the negatives used and the enlargements with your reports. State any trouble encountered, the best exposures, and your own opinior of your results. Experiment 19 Sulphide Toning (Sepia). — May be applied tc the contact prints or the bromide paper enlarge ments, but the bromide paper is much the inor( liable to spot and blister. Prints which have beer thoroughly dried once since making are less liabh to spot. The prints fade somewhat in the procesi and hence require to be printed a little darker thar your taste calls for. U you have no spare print: suitable, make some but let them dry thorough!) before proceeding. It is advisable in all cases tha the hypo should contain an acid hardening solution and must have been well washed out of the print Bleach the print in the solution (formula below •till only a faint image remains. Wash till all thi yellowness from the ferricyanide solution has gone LABORATORY MANUAL 265 Immerse in the sodium sulphide solution (formula below) till of an even tint. Wash moderately. Dry. Bleacher Potassium Fcrr.j- cyanide 40 g Potassium Bromide 10 g Water 1000 cc Sodium Sulphide Solution 'indium Sulijhidc lo g ^\ icr 100 cc Boil the sulphide solution to precipitate the iron, and filter. Dilute i-io for use. That supplied is diluted ready for use. Submit toned prints with report. Experiment 20 WET COLIvODION PLATES* Preparing the Glass.— Lantern slide size. Wash first under the tap to remove all coarse dirt, then by immersion for 3 or 4 minutes in boiling hot alkali and soap solution; wash well under the tap, handling the glass entirely by the edges, and rinse in distilled water. Dry thoroughly by wa-ming over a Bunsen flame, holding the plate by the fingers on the edge and keep the plate moving. If you have lime to wash several plates and let them dry by themselves on the rack they will be much clearer. Forming the Film. — As soon as the glass plate "See Foxlee, "Wet Collodion Process," British Journal, S4, 483 (1907). 266 PHOTOGRAPHY has cooled to room temperature, use the brush to run a streak of the rubber solution about Ys inch wide all around the ed^e of the plate to help keep the film attached to the glass plate. Then hold the plate horizontal by one corner with the left hand, and pour a generous pool of iodized collodion near but not touching the thumb. Spread the pool by gentle tipping so it tlows along the far side of the plate and then toward you along the edge opposite the hand, which usually completes the covering of the plate. The corners are not important but avoid getting any colhnlion on the under side of the plate nr on the holding lingers. To make a thick fihn allow the collodion to rest horizontally on the plate for i-J minutes, then pour ofT the excess into the Ixittle again from the near right hand corner, keep- ing the plate nK'king to avoid streaks. The plate nmst Ih- uniformly coated and free from dust to ^a-t g«M>d results /\11 the aln.ve can be done in any light but the following must be done in the hanging red lamp light, avoiding long exposures. Sensitiiing.— .\s soon as the film has dried rnongh to ft-fl firm on the C(»rners. J-4 minutes, it i% to W placed on the silver wire frame and lowered grnily atul steadily into the silver nitrate solution in the glass Ixtx ijisido the sloping wtKxlen 1m)x. If the edge of the solution stops at any place durini: lowering, it will leave a nuirk. Leave in the silver LABORATORY MANUAL 267 bath 3-4 minutes. On removing the plate handle it by the edges and if the fingers become stained by the silver nitrate wash thoroughly in water and remove the stain by soaking in the ferricyanide hypo reducer. If any of the developer remains on the fingers or under the nails they will be stained blue by the ferricyanide. The film may become loose from the glass (a) if the glass is not clean, (b) if the glass was not dry, (c) if the film was not al- lowed to dry sufficiently before placing in the silver solution. On the other hand, if allowed to dry too much the negative is apt to be thin. Exposing. — Do the exposing in the special plate holder which has the middle partition removed. Put the plate in from the back into the silver plated kit, film side down, cover with two small and one large piece of blotting paper, and then slip the brass plate on top of these and into the spring catch which ordinarily holds the plate. Replace the back slide and expose from the other side. Take care not to handle the plate holder in such a way as t(^ press the hard rubber slide against the sensitive i.lni. Expose in the view camera to a negative illuminated as for reduction latuern slides ( {'"xp. S), Consult the bulletin board for the first trial exjx)- sure. Development. — Hold the plate as for film making and pour onto the exposed tiim enough of the iron 268 PHOTOGRAPHY developer to cover the film but not enough that any runs off (the adhering silver nitrate and developer intensify the film during development). Make the developer flow gently around the film and the image should appear in about a minute. When develop- ment has apparently stopped, rinse the plate under the tap, and fix by pouring on the film some of the saturated hypo solution. Rinse for a minute under the tap which will wash the exceedingly thin film. Thin positives are quite as apt to be from insuffi- cient development as from underexposure. As with gelatine plates overexposure gives a flat pic- ture with deposit all over the film. They are easily intensified either before or after fixing. Plain Collodion Pyroxline 9 g Alcohol (pure) 1 50 cc Ether (pure) 300 cc lodizer Ammoniuin iodide 3 k Cadmium iodide 3 g Ammonivmi bromide 1.3 r Alcohol 1 50 cc Mix three volumes of the plain collodion with one volume of the iodizer a few days before use. Iron Developer Ferrous sulphate 30 g Acetic acid 20 cc Water 500 cc Silver Bath ' Silver nitrate 90 r Water 600 cc Nitric acid to acid reaction. Mount two of the best of your collodion plates as lantern slides, and submit them with an account of the day's work including description of any special difficulties and successes. LABORATORY MANUAL 269 Experiment 21 Pictures Showing Clear Distance and Clouds. — Make as many negatives as the time will allow, choosing views showing clouds, blue sky, and dis- tances. Use the color screen and the isochromatic or polychromatic plates. Consult the bulletin board for screen factor and the table in the text (p. 220) for the speed of the plates. Also consult the bul- letin board for suggestions for good views. It will save time to develop in the tank. Report on each plate used and submit negatives and prints. Experiment 22 Autochromes. — Each box holds two packages containing two plates each. In the package these two plates are placed film sides toward each other with two pieces of thin cardboard in between. One piece of cardboard should be kept against the film (which will help to remen ^er which is the film side) aiiu the plate is to be placed in the kit with the glass side toward the lens and the black cardboard still against the film. This is all to be done in the dark. In focusing allowance must be made for the reversal of the plate by reversing the ground glass or allow- inj^ for the thickness of the .-Xutochrome glass plate. 270 PHOTOGRAPHY The exposure is to be made through a special color screen. As a guide, four seconds through the screen at 16 in the bright summer sunlight will give a good exposure, and it is to be noted that the latitude of exposure is narrow. Develop in the special developer (formula below) for 2.5 minutes between 16° and 20° C. The whole development is to be carried out in the dark with the time followed by the light of the red lamp facing away from the tray and the tray covered. Rinse off the developer by a gentle stream of water from the tap or better still in clean water in a tray, as the tap water is apt to spot the film with solid particles. Then immerse in the potassium per- manganate solution and go out into the light. In 3-4 minutes all the negative image will have been dissolved out as shown by the transparency, but the plate will still appear a negative by reflected light on account of the black film under the sensitive fihn. Rinse thoroughly and immerse again in the same developer in which the plate was first developed till it all goes black, 3-4 minutes. Rinse again for 3-4 minutes and dry. After the film is thoroughly dry varnish it by pouring a film of the varnish solution on the plate like a collodion film is made. Better practice it on a glass plate first ; mount the plate after the varnish LABORATORY MANUAL 271 is thoroughly dry as for a lantern slide so that the film may not get injured. The film, is very thin and very frail, so that it must not be touched through the whole process by any- thing but the solutions. Breaks through to the color film usually show a green patch around the break. If specks get on the film and they do not wash off under a gentle stream of water, leave them alone, as attempts to remove them will almost inevitably break the film. In case the finished picture after final develop- ment lacks vigor, it may be intensified with advan- tage in the special intensifier. The composition of the sfjlutions used is given on the printed slip in each box. The Lumiere Com- pany also issue a booklet giving detailed directions for the use of these plates. Report and submit each plate. Experiment 23 CARBON PRINTING Sensitizing. — Brush the colored gelatine film over with the Autotype Spirit Sensitizer solution, usuvj; some of the solution poured out in a dish and the piece of flannel tied over the end of the strip of glass. Brush the film both ways lightly, avoiding finger marks, and when the solutif^i has soaked in 272 PHOTOGRAPHY (1-3 min.) the brushing may be repeated. Let dry in the room out of strong light for half an hour and then keep in the dry box. The film is ready to print as soon as dry enough not to stick to the negative. Printing. — The dry sensitized film will usually be rolled up and quite hard and must be unrolled with care and slowly to avoid cracking. The printing is to be done in the direct sunlight or in the light from the electric arc. Since the image is not visible, the progress of the printing has to be followed by the use of a strip of solio paper in the actinometer — a long narrow printing frame with a series of num- bers increasing in density. The time of printing is about that required to make a solio print showing its best detail, that is, perhaps half printed for a solio picture. As an approximation try 5 minutes at 18 inches from the arc light. The printing goes on after the exposure has been stopped, so if the exposure is full time, the development should take place within the next hour. Transfer. — Soak the print and a sheet of single transfer paper in water at room temperature till the print is pliable. Then bring the two faces to- gether, under water, avoiding air bells; lift out, and lay on a pad of blotters. Squeeze them together to get rid of the excess of water. Put another pad LABORATORY MANUAL 273 of blotters on top and a weight on that and let stand 5-10 minutes. Develop by immersing in water at 38-40° C, using the enameled iron tray on th - iron tripod, and keep the water at this temperature by regu- lating the height of the Bunsen flame underneath. After a few minutes' soaking, the soft gelatine will be found oozing out between the edges of the print and transfer paper. They are now to be separated gently leaving the picture on the trans- fer paper. Soaking longer in the hot water will remove the rest of the soluble gelatine or the proc- ess may be hastened by stirring or by laving the print; the most detail is preserved by disturbing only a little. Clearing After development place the print in a S7o alum solution for 10-15 minutes to remove any remaining chromate. Rinse in cold water and allow to dry. The print will be right-left handed. To make them correct requires another transfer which modi- fies the above procedure somewhat. Experiment 24 Characteristic Curve. — Cut down one of the or- dinary plates so as to fit the 33/4 x 4j4 holder for the Chapman Jones Plate Tester. Scratch the glass 274 PHOTOGRAPHY with a wheel cutter and the film will tear easily. Expose the plate behind the exposing plate for 30 seconds to the candle after the candle has burned for several minutes. Finish as usual. When dry measure the densities of the spots from 1-20 in the photometer. Read over the description of the photometer, page 280, and get from the instructor a negative similar to yours to use in the right hand window. The densities of the numbered spots on the exposing plate are supposed to increase by equal additions, each addition absorbing -^ the light get- V 2 ting to it so that the exposures decrease by a factor of V2. Hence plot the numbered spots at equal dis- placements on a horizontal axis and the densities of each spot vertically and draw the curve through the resulting points. Submit negative, numerical data, and curve with your report. Finishing Up Work In connection with the grade for the semester, will you please leave in your locker for inspection the following minimum requirement : Complete reports to date Album with the full list of prints, including also four P. 0. P. prints and two carbon prints. 4 Lantern slides 2 Collodion lantern slides Autochromes Two best enlargements LABORATORY MANUAL 275 The negatives are not needed. After inspection all these are your personal property. The labora- tory keys must be surrendered when the above re- sults of the semester's work are taken into private possession. The laboratory will be open for this at the times posted on the bulletin board. The written examination is supposed to cover all the work of the laboratory and of the lectures. APPENDIX I Laboratory apparatus Required for each dark room which accomiiK dates two students at one time. I camera, 4 x 5, in case 4 plate holders, 4x5, with hard rubber s. I focusing cloth 6 printing frames, 4x5 I Centigrade thermometer I camel's-hair brush, flat, i inch I circular print washer I plate washer, for 4 x 5 and 3^^ x 4 plii.«a I measuring glass, 4 oz. 1 tripod 4 deep hard rulj. ^ays, 4H x 5V2 2 deep hard rubber nays, 10 x 12 1 print stick 2 tray covers I glass funnel I ruby safe light for plates I hanging tipless ruby incandescent lamp I large sink, 3 faucets Group of lockers, 10 or 15 per room Desk, top preferably covered with sheet lead, t . atf tight. , . I drying cabinet The simplest form is a U)x with light tight door and large enough to hold a plate dryin rack with a dozen plates. It has to be provided with toi tuous passages for inlet and outlet of air and some arrangt ment for forcing the air through. In addition to the above each locker needs a oollapsibi dryin*' rack to hold one dozen negatives. 276 APPENDIX Special Apparatus 277 Required Set for Each Two Dark Rooms • ' 1!!^^ "^T""^' ^^ "" ^^ standard type with long exten- sion beUows large removable lens board, and vertical ad- justment of lens. The plate holder carrier must be^ulre and removable so that It may be placed either edge^! tical, or replaced by other parts. I special back for the view camera having ground glass and opemng for 4 x 5^ plate holder. It requires alsothl shdmg screen described in Exp 12. I high grade modem lens, to cover sharply a 4 x ? olate I incandescent tungsten nitrogen-fiUed lamp, 100 watt locomotive headlight type with concentrated filament .nicTfaL ^"^""^^^ ^° "^"^ ^ ^'^""^ condensing lens I light tight box for holding the above lamp and water A ^S^^^^l^l^ ^ dopr for adjustment and an opening, the n ide of which is shaped to fit the backs for the '^dew cameras, and the outside of which is fitted to receive the optnmg in the back of the view camera itself I back for view camera fitted with simple adjustable slit I back for the view camera made with 4 x 5 opening for =arr>ing a ground glass and 4 x 5 negative ^ ^ ^ '^ '""^ I low grade achromatic lens, 30 or 40 cm focal length and 3 or 4 cm diameter. ^ I replica grating, second quality, 3 or 4 cm square I color screen, fairly dark, to give unmistakably clear distances and strong clouds. I drying box. An ordinary bread box fitted with a glass tray of calcium chlonde covered with brass wire gauze 1 Watkins Bee meter. 8 kits, 4 X 5 to 31^ X 4. 2 doz. push pins. sUdes^'"*" '^"^"^ ^°^ ^°^ cleaning the glass in making coUodion I developing tank, 4x5, reversible. ^2 enamel trays for holding hot water for carbon print- Trimming board one or two for the whole laboratory, thapman Jones Plate Tester, made by Sanger Shepherd 278 PHOTOGRAPHY and Co., Holbom, London, W. C, England. One or two for the laboratory. Scales and graduates for making up solutions. I plate holder for collodion plates, made by ^emo^nng the central septum from an ordinary 4x5 P^^ "older. The ^i^ X 4 plate is held in a brass kit which is sUver plated and is arranged to be filled from the back and needs no fas- teners, being held by some blotter and a brass plate, 4x5, with a finger hole. See Exp 19. 1 dipping bath for the collodion plates. Anatomy mu- seum jars of suitable size are fitted into a wooden box with a hinged cover and with feet arranged so that the box stands with a slight tilt. The plates are carried on a sUver wire frame fastened together with silver solder. I small single filament Nemst glower. Supplies per Student These quantities are necessarily very approximate as thej vary materially from year to year. 1 album for 4 X 5 prints 2 pads, report blanks (50 pages) Dry plates, 3 doz. ordinjuy I doz. isochromatic 14 doz. polychromatic 12 doz. developing paper, 4x5, medium contrast, semi matt, thin paper. I manilla paper report binder I manilla paper envelope in which to hand in reports, etc Paper toweling. 1 doz. enlarging paper, 10 x 12 1 doz. printing out paper 2 doz. lantern slide plates 1 doz. lantern slide cover glasses 1 doz. lantern slide matts. 14 doz. carbon tissue, 4x5, assorted colors. J^ doz. single transfer mount, 4x5. These last two iten it pays well to buy in the roll and cut up. Autotype tissi and ?npplies may be bought through George Murphy, ; East ^inth Street, New York. j/2 pkg. lantern slide binding strips. yi blotter, photographic, 19 x 24. APPENDIX 279 J^ doz. blue print paper, 4x5. 2 sheets 8 x 10, good V n paper for blue print sensi- tizing, bize with starch o tter gelatine. 2 Autochromes, 3^^ x 4 Chemicals per Student Acetic acid, 2 oz. Alcohol, absolute, i oz. Alum, powdered, 2 oz. Borax, }4 oz. Ether, i oz. Ferrous sulphate, 2 oz. Glycerine, }4 oz. Gold chloride, 2 grains Hydrochinon, i oz. Sodium hyposulphite, 4 lbs. Mercuric chloride r oz. and small quantities of the Ammonia liq. Ammonium bromide Ammonium iodide Benzol Cadmium iodide Ferric ammonium citrate Litmus paper Metol, }4 oz. Potass, bromide, i oz. Potass, carbonate, dry, 2 oz. Potass, ferricyanide, i oz. Potass, oxalate, 4 oz. Pyrogallic acid, i oz. Silver nitrate, ^ oz. Sodium carbonate, dry, i lb. Sodium sulphite, dry, i lb. Spirit sensitizer for carbon printing, i oz. following: Negative cotton Oxalic acid Potass, metabisulphite Potass, permanganate Rubber cement Sodium sulphide ^gb^'^jfibii^^Sii^'.' APPENDIX II Photometers There are several types of photometers on thi market but they are expensive, often elaborate in striiments and hardly suited to this work. A mod ification of Hurter and Driffield's original arrange ment, making use of the inverse square law, can b made sufficiently accurate, simple, and inexpensiv( Such an arrangement is described by W. B. Fergu son.* His comparison arrangement, however, is nc satisfactory and a much better arrangement is sul stituted here. Two ways, a Yi inch brass tube and a 94 by J A 1 A ■■BBBs==ai* Fic. 51. Rcnch F'hotometer. brass strip, are screwed parallel to each other ar four inches apart on a piece of seasoned o; 7 X I X 60 inches. Mg. 5» 's a diagram (^f the -c ' pkoto"ratmm^fimjsL^^^&jM^^ INDEX 291 Contact lantern slides, 242 Contrast, 30, 41, 47, 50 color, 85, 87 Contrast papers, 248 Control of development, 47 Convertible lens, 169 Correct exposure, 239 period of, 37 Crown glass, 155 Curvature of field, 159 Curves, color sensitiveness, 73 Daguerre, S Dark room apparatus, 232 Davy, 4 Daylight table, 70 Dense negatives, 103 Density, by photometer, 284 definition of, 22 of gelatine, 25 of glassplate, 25 growth with time, 28 Density measurements, 23 Density ratios, 29 Density-color Ce, 79 Density -development, relation to, 29 Density-opacity relation, 22 Depth of focus, 165, 211 I Detail, 215 Developer, ferrous oxalate, 27 Devdoper for paper, formula, 237 Developers, 55 Developing collodion plates, 267 Developing factor, 44, 45. 51 condition affecting, 45, 46 Developing paper, 127, 237 effect on Ch Ce, 43 effect of time of, 26 Development -density relation, 29 manipulation during, 31 methods of control of, 47 pictorial considerations, 214 bv tank, 32 r> elopment speed list, 220 Development -temperature co- efficient, 48 Development time by photom- eter, 286 Diaphragm, 151 Diffraction, 144 Dipping bath, 278 formula, 268 Directions, general, for labo- ratory, 231 Dispersion. 155 secondary, 156 Distance of distinct vision, 209 Distance pictures, 269 Distinct vision, distance of, 209 Distortion, 160 barrel, 161 from inclined plate, 213 from short focus, 210 pin-cushion, 161 Dodging, 104, 106 Double-coated plates, 116, 246 Doublet lens, 160 Draper's Law, yj Driffield, 19 Drying cabinet, 256 Drying film by alcohol, 119 t>:^i^^i^^M^^&k:2aeS^iS^iM^'^ 292 INDEX Drying negatives, 119 Dry plate, 10 age of, 47 Dry plate making, 13 boiling method for, 13 Dry plate speed, 64 Dufay, 191 Dust in the air, 203 Dust marks, 111 Dyeing plates, 256 Dyes, 75 Eastman Kodak, 224 Eder, 92 Eder Handbuch, 225 Effect of time of development, 26 Efficiency of shutters, 60 Ellis, 102 Emulsion, ammonia method, 16 coating, 16 centrifuge method, 16 washing, 15 Emulsions, collodion, 9 Enlarging, 262 Enlargement, contrast, 262 exposure, 263 Examination, 275 Exposing collodion plates, 267 Exposing paper, 238 Exposure, definition of, 57 pictorial considerations, 214 Exposure calculators. 70 Exposure-density relation, 33 Exposure meters, 70 Factor development. 44, 45 1 actorial development, -'41 Factorial system, Watkins, 50 Fading of latent image, 90 Farmer reducer, 251 Fast lens, 168 Ferguson, 280 Ferric ammonium citrate, 124 Ferricyanide hypo reducer, 251 Ferric oxalate, 133 Ferrous oxalate, 123 Ferrous oxalate developer, 27, 244, 250 Ferrous sulphate solution, 245 Film, microscope study, 107 Film cameras, 17 Film side, to tell, 234 Film thickness, 52 Films, 17, 223 Finger marks, 113 Fish glue, 140 Flare, 162 Flint glass, 155 F numbers, 63 Focal length, definition of, 147 of lens, 208 Focus, 146 virtual, 146 Focusing, 211 I'og, 25. 51 chemical, 25 definition, etc., of, 104 Fog due to age, 106 l"og strip. 26 logging exposure, 67 I'oreground, 201 l-ormitig collodion film, 2<)6 Foxlec, 265 Frilling, no INDEX -93 Full scale of paper, 214 Fuzzy effects, 215 Gallic acid, 6 Gamma, 44 by photometer, 286 Gamma infinity Too , 44 Gas light paper, 127, 237 Gelatine, 11 absorption of dyes, 184 hard, 11 Gelatine chloride paper, 125, 260 Gelatine dryplate, 10 Gelatine solubility in alum, 12 Gelatine substratum, 16 General directions for labora- tory, 231 George Murphy, 278 Ghost, 162 I Glass, cleaning, 15 j Ghitin. II t Glycerine and paper backing, 246 [ liold chloride solution, for- mula, 261 lood pictures, 195 I -rain of plate, 68 irating. 7^ replica, 254 ■reasy ink, 193 if.isc spot photometfr, 23 ■rowth of density, 28 jialation, 104, 114, j^jj ■Salffonc. 1^9 [■\h I). 225' nuwlitr, 66 Hartmann, 198 Haze, blue, 204 Herschel, 6 Heydcs action photometer, 71 High-lights, 42 Howard Farmer reducer, 107, Hurter and Driffield, 19, 23, 65, 128, 280 Huse and Nietz, 107 Hwlrochinon developer, for- mula for, 245 Hypo for p. o. p., 261 Hypo acid, formula for, 236 I! ford, 224 iimination for microphotos, Image, size of, 149 Image formation, 146 Impressionist effects, 212, 215 Impure color, 73 Incident light, 21 Inclined plate, distortion of, 213 India ink. 25 Inertia "i," 22^ Incrlia of plate. 65 Intfilecfual problem. 217 Intensification. 33. 101, 247 Intensifirr, Chapman Jone». t02 me r. ur>, loj Interfrrriu-f of light, 176 l<»«li/rr. f..tMiii?« for. 26(1 Iron druiaptr, formula lor. Irf-^diation, 117 294 INDEX Isochromatic plate, 252, 255 Joly, 186, 190 Jones, Mutting and Mees, 128 Krayn, 190 Laboratory, general directions for, 231 Laboratory apparatus, test of, 276 Lambert, 199 Lamp for photometer, 281 Lantern slide, developer, 244, 245 plates, 243 printing, 243 Lantern slides, 121 contact, 242 reduction, 245 Latent image, 43, 89 Latent image fading. 90 Intent image theories, 91 Latitude. 52, 53. 58 of autochromcs, 189 of paper, 130 Law of absorption of light, 21 Uns, 145 angle of view, an doublet, 160 focal length of, 3o8 wide angle, 165 Lens axis, 213 Lenses, apochromattc, 158 types of, 145 Lieht absorption, law of, 21 Lighting. 69. ao6 Light struck, 105 Line screen, 139 Lipmann process, 173, 174 Local reduction, 109 Locker apparatus, 231 Long focus achromat, 253 Lumiere and Seyewitz, 105 Luminosity Ce, 80 MacDonough, 190 Manipulation during develop- ment, 31 Mask, 216 Mass effect, 207 Matt surfaces, 128 Maximum contrast, 51 Maximum contrast in prints, 248 Maxwell, 178 Mees, 78 Mees. C. E. K., 54 Mees and Piper, 56 Melting. 162 Meniscus lens. 152, 169 Mercury chloride intensifica- tion. 102 Mercury intensifier, 249 V ^. cxi>osHre, 70 : hydro, see paper devel- oper. 237 Microphotography. 257 Microscopic study of film, 107 Mordant, 185, 191 Mosaic screen making, 19^" Mosaic screen plates, sum- mary, 192 Mosaic screen processes. 186 Mounting, 216 Moving picture films, i'i INDEX 295 Nernst lamp, 253, 254 Niepa, 4 Nodal points, 147 Notan, 207 Numbering of stops, 63 Opacity, definition of, 22 Opacity-density relation, 22 Order slip, 288 Overdevelopment, 104 Overexposed negative, 251 overexposure, 103 period of, 42 Oyster shell markings, 118 Paget Prize Plate, 192 Panchromatic plate, 83 Paper developer, fornnil.i '' >r, Period, of correct exposure, 37 of overexposure, 42 of reversal, 43 of underexposure, 40 Perfect negative, 20, 39 conditions for, 40 Perspective, 165, 210 aerial, 203 Petzval portrait lens, 169 Photo-engraving, 139 Photohelids, 93 Photometer, 23, 280 Photometer algebra, 285 Photometer, bench. 23 Physical modification theory. 9« Pictorial pictures, 196 Picture, definition of, 195 Picture composition, 197 Pictures, pictorial, 196 record, 196 Pigment color processes, 173 Pinachrome, 81, 85 Pinacyanol, 256 Pinotype, 181, 183 Pin-cushion distortion, 161 Pinhole image, 142 Plain collodion, formula, 268 Plate, dry, 10 Plate developer, formula for, 236 Plate grain, 68 Plate inertia, 65 Plate speed, 64 Plate speed by photometer, 287 Plate speed numbers, table of, 225 Plate speeds, list of, 220 Platinotype, 133 Polychromatic plate, 255, 257 Poore. 198, 208 P. o. p. hypo, formula for, 261 Portrait. 200 Potassium bromide, effect on Ch Ce. 45 Potassium cyanide, 8 Potassium iodide, in developer, H4 Potassium oxalate, 135 Potassium fcrricyanide re- ducer, 25! Preface to manual, 229 Preliminary exposure, 67 Pressure marks, 92, 113 2g6 INDEX Primary spectrum colors, 179 Printing, 238 pictorial considerations, 214 Printing colors, 181 Printing out paper, 125, 260 Printing press, 186 Prints, maximum contrast, 248 Properties of developing pa- pers, 128 Pure color, 73 Pyro, 234 stock formula, 236 Pyroxylene, 7 Rapid rectilinear lens, 161, 168 Reactions with dry plates, 18 Record pictures, 196 Rectilinear lens, 161 Reducer, ammonium persul- phate, 107, 108 ferri-cyanide, 107 Howard Farmer, 107, 251 potassium permanganate, 107 Reducers, classification of, 107 Reduction, 251 local, 109 Reduction lantern slides, 245 Reflex camera, 167 Replica grating. 73, 254 Report, general directions, 233 Report blanks. 233, 288 Restrainer, 56 Reticulation, 249 Reversal, 37, 43 Robinson, 198 Rodinal, 224 Rotary, 180 Sagging of film, 119 Sanger-Shepherd, 180 Scattering by dust, 204 Scheeper, 68, 107, 115 Scheiner, 225 Schott, 158 Schule, 3 Screening action of dyes, 83 Screens, color, 84 Secondary dispersion, 155 Sensitiveness, 14 Sensitizing collodion plates, 266 Sepia toning, 264 Shadow detail, 42 Shadows, 42 Sheppard and Mecs, 20, 90, 105 Shutter, consistent, 59 Shutter efficiency, 60 Shutter markings, 60 Shutter reliability, 60 Shutter testing, 60 Shutters, types of, 58 Silver bath, formula, 268 Silver bromide, 4 Silver grain hypothesis, 92 Silver grain theory, 92 Silver haloids, 12 Silver sub-bromide theory, 93 Simplicity, 200 Size of image, 149 Sky, 202 Sky-line, 201 Slopes of Ch Ce. 43 Soda, 234 stock formula for, 236 Sodium hyposulphite, 5. 18 Sodium sulphide, 103, 132, 250 INDEX 297 Sodium sulphide solution, for- mula for, 265 Solid solution theory, 94 Solio, 126 Solution, colloidal, 95 Spectrum photography, 253 Speed, development of, 221, 222, 223 plate, 221, 222, 223 of color screen plates, 223 of lenses, 167 of plates, 64 of plate by photometer, 287 Spherical aberration, 151 Stain, 56, 118 Staining in intensification, loi Star, 12 Starch grains, 187 Stock pyro, formula for, 236 Stock soda, formula for, 236 Stops, 61 system of numbering, 61 Strip negative, 241 Sub-bromide theory, 93 Subject character, 68 Subjects, classification of, 70 Subtraction method. 180 Sulphide tones, formula for, 265 Sulphide toning, 132, 264 Supplies per student, 278 System of numbering stops, 63 Systems for plate speeds, 225 Tabl- of plate speed numbers, 225 Talbot. 6 Tank development, 32, 57, 252 Tannic acid, 185, 191 Telephoto lens, 170 Temperature, coefficient of de- velopers, 48, 50 Test for correct exposure, 239 Tilting camera effect, 213 Tissue, carbon, 137 Thickness of film, 52 Thin negatives, 99 Thiosulphate, 18 Todd, F. D., 54 Toning developer for paper prints, 132 Toning p. o. p., 126, 260 Total scale of papers, 129 Transfer, carbon printing, 272 Transmitted light, 21 Transmission, 21 Transparencies, 121 Traube iodide, 181, 185 Trimming, 216 Tripack, 180 Tungsten lamp, 237 Turnbull's blue, 124 Underdeveloped negative, 248 Underdevelopment, loi Underexposed negative; 248 Underexposure, 100 period of, 40 Unequal illumination, 163 Unity, 199 U. S. numbers, 63 Vertical focus 146 Vogel, 74 Wall. 75 Wallace, 82. 85. 254 it 298 INDEX Warner-Powrie, 191 Watkins, 50, 64, 66, 71, 225 Watkins factor, 50 Watkins numbers, 66 Watkins speed list, 220 Wedgewood, 4 Wellcome exposure record, 224 Weston, 199 Wet collodion, 7, 139 Wet collodion plates, 265 Wide angle lens, 165 Window frame picture, 209 Wratten and Wainwright, 224 Wynne, 225 Zinc etching, 139 (2) J 15-1 45bU- Sip*!* -4f*'' .-?*» ''■jSfi^.H.'...i.^h li 'it-u"" - I r* f. -riA-' tVmvM^^T'mTr^t ..•4 ^'.i, > ■kS» ^ /L.'ji-' "'■.i«'r«*',