FAMILIAR INTRODUCTION 
 
 TO THE STUDY OP 
 
 POLARIZED LIGHT; 
 
 A DESCRIPTION OF, AND INSTRUCTIONS FOR USING, 
 
 THE 
 
 TABLE AND HYDRO-OXYGEN 
 
 POLARISCOPE AND MICROSCOPE. 
 
 BY 
 
 CHARLES WOODWARD, F.B.8. 
 
 PRESIDENT OP THE ISLINGTON LITERARY AND SCIENTIFIC SOCIETY. 
 
 ILLUSTRATED BY NUMEROUS WOOD ENGRAVINGS. 
 
 SECOND EDITION. 
 
 LONDON : 
 
 PRINTED FOR SMITH & BECK, 
 
 ACHROMATIC MICROSCOPE MAKERS AND OPTICIANS, 
 6, COLEMAN STREET ; 
 
 AND PUBLISHED FOR THEM BY 
 
 JOHN VAN VOORST, PATERNOSTER ROW, 
 
 MDCCCLI. 
 
PRINTED FOR SMITH & BECK, 6, COLEMAN STREET, 
 BY COMPTON & RITCHIE, CLOTH FAIR. 
 
TABLE OF CONTENTS. 
 
 PART I. 
 
 Section 
 
 Introduction 1 
 
 Hypothetical opinions regarding the nature of light 2 to 7 
 
 Interference of ethereal waves 8 10 
 
 Undulatory hypothesis illustrated 11 18 
 
 Velocity of light 19 20 
 
 Analysis of light and theory of colors 21 33 
 
 Refraction, single and double 34 37 
 
 Polarization of light 38 
 
 What is it? 42 
 
 - by reflection 39 46 
 
 by transmission 47 49 
 
 by double refraction 50 52 
 
 Summary of the hypothesis 53 55 
 
 Illustration of the polarization, analyzation, and interference 
 
 of light 56 
 
 Application of polarized light 57 60 
 
 Phenomena exhibited by sections of crystals 61 63 
 
 Practical observations 64 67 
 
 Conclusion 68 69 
 
 PART II. 
 
 Table polariscope 70 
 
 Description and uses of ditto 71 74 
 
 Table microscope 75 
 
 Gas polariscope 76 
 
 Description of and instructions for using ditto 77 79 
 
 Gas microscope 80 82 
 
 Practical observations 82 83 
 
 Advantages of the arrangement 84 90 
 
 Supposed disadvantages 91 and 93 
 
 answered 92 94 
 
PREFACE 
 TO THE FIRST EDITION. 
 
 Messrs. SMITH & BECK having requested me "to oblige 
 them with a description of, and instructions for using, my 
 Table and Hydro- oxygen Polariscope and Microscope," 
 and several friends possessing achromatic microscopes 
 titted with the polarizing apparatus having lamented the 
 difficulty of comprehending the laws of the phenomena 
 thereby exhibited, I have endeavoured to render the use 
 of these instruments more interesting, by presenting, in a 
 concise and familiar manner, such introductory information 
 as may smooth the threshold of a subject confessedly ab- 
 struse, and lead to the study of works of far higher pre- 
 tensions and deeper research. 
 
 I have briefly noticed those facts which have gradually 
 led to the very general adoption of a theory concerning the 
 nature of light differing in some important respects from 
 that advanced by Sir Isaac Newton ; and I have endea- 
 voured to shew that his successors, whilst they have adopted 
 another theory, have yet done little more than given new 
 names to old facts, leaving his discoveries and his calcu- 
 lations still to form the firm basis of the science. 
 
 During my earlier studies I read many works connected 
 with the subject of light, and have, no doubt, almost im- 
 perceptibly adopted the ideas of others, when they were 
 
VI PREFACE. 
 
 subsequently confirmed by my own observations or expe- 
 riments. As, however, the following pages have been 
 written without referring to any but a few notes forming 
 the outlines of lectures on the polarization of light, which, 
 as an amateur, I have occasionally delivered in a popular 
 style, I am now only able to acknowledge my obligations 
 generally to those who may have assisted my investigations, 
 and to evince my gratitude by attempting to facilitate the 
 inquiries of others. 
 
 C. W. 
 
 (JOMI'TON TEKKACE, ISLINGTON, 
 APRIL 1848. 
 
PREFACE 
 
 TO THE SECOND EDITION. 
 
 HAVING undertaken to prepare for Messrs. SMITH & 
 BECK another edition of my "Familiar Introduction to 
 the Study of Polarized Light/' I readily embrace the 
 opportunity thus afforded me of tendering my thanks to 
 the many scientific friends who have expressed their ap- 
 probation of the attempt " to facilitate the inquiries" of 
 those Mho desire to study this highly interesting but 
 "confessedly abstruse" branch of science. 
 
 Some few, indeed, have intimated that I should have 
 prosecuted the subject still further; but this could scarcely 
 have been accomplished without defeating the professed 
 object of " presenting, in a concise and familiar mariner, 
 such introductory information" as might prepare the in- 
 quirer for " deeper research." 
 
 I have, however, carefully revised the whole ; and by 
 altering Figures 9 and 10, and adding two others illustra- 
 tive of the polarization of light by reflection, I trust it will 
 be found that I have more clearly explained that part of 
 the subject. 
 
 C. W. 
 
 COMPTON TERRACE. ISLINGTON 
 JVXE 1651. 
 
PART I. 
 
 A 
 
 FAMILIAR INTRODUCTION 
 
 TO THE STUDY OF 
 
 POLARIZED LIGHT. 
 
 (1.) PHILOSOPHERS, at a very early period, attempted 
 to investigate the nature of light, and, until the time of 
 Newton, they generally concurred in opinion that it is pro- 
 duced by some affection of the particles of matter, and is 
 not itself a material agent. 
 
 (2.) Aristotle observed that light is not fire, nor is it any 
 thing bodily radiating from the luminous body, and trans- 
 mitted through a transparent one ; but the mere presence 
 of fire, or some other luminous body, at a transparent 
 surface. 
 
 (3.) The followers of Descartes considered that light as 
 it exists in the luminous body is nothing but a power or 
 faculty of exciting in us a very clear and vivid sensation ; 
 that in the hidden pores of transparent bodies there is a 
 certain subtile matter, which, by means of its exceeding 
 smallness, can penetrate even glass ; and that this matter 
 is impelled by the luminous body so as to affect the organ 
 of sight. 
 
 (4.) Malebranche explained the nature of light from a 
 supposed analogy between it and sound. He considered 
 that all the parts of a luminous body are in a rapid motion, 
 which, by very quick pulses, is constantly compressing the 
 
 B 
 
10 ON POLARIZED LIGHT. 
 
 subtile matter between the luminous body and the eye, and 
 exciting vibrations: as these vibrations are greater, the 
 body appears more luminous ; and as they are quicker or 
 slower, the body is of a different color. 
 
 (5.) Huyghens supposed that an exceedingly thin and 
 highly elastic medium, called ether, fills all space, and occu- 
 pies the intervals between the particles of all substances ; 
 that luminous bodies excite vibrations in this ether, which 
 spread like waves formed by dropping a stone into still 
 water ; and that the ethereal vibrations, impinging on the 
 retina of the eye, produce the sensation of light in the same 
 way as the undulations called sonorous, affecting the audi- 
 tory nerve, produce the sensation of sound. 
 
 (6.) Newton objected, that if light, like sound, be pro- 
 pagated by undulations, like sound, it would pass through 
 bent tubes, which he considered contrary to fact ; hence he 
 supposed light to consist of material particles emitted by 
 luminous bodies, and moving through space with an im- 
 mense velocity. 
 
 (7.) To this it was objected, that as light is ascertained, 
 by repeated observations on the eclipses of Jupiter's satel- 
 lites and the aberrations of the fixed stars, to be propa- 
 gated at the rate of 192,000 miles in a second, its particles, 
 if material, ought to acquire a momentum ; but that no 
 indication of such an effect had ever been experienced even 
 by that delicate organ the eye. Newton's objections, how- 
 ever, prevailed; his views were generally adopted under 
 the name of the corpuscular theory, and the question as to 
 the propagation of light was supposed to be decided. 
 
 (3.) But the attention of philosophers had been for 
 some time directed to the dark bands seen when light 
 passes through very narrow apertures. If, for instance, 
 we hold the hand between the eye and a bright cloud, or 
 the ground glass of a lighted lamp, and open the fingers 
 so as to admit the smallest portion of light, we shall per- 
 ceive dark bands intersecting the luminous space at regular 
 intervals. 
 
ON POLARIZED LIGHT. 11 
 
 (9.) Little progress was, however, made towards the ex- 
 planation of this phenomenon until Dr. Young pointed 
 out a principle upon which it could be accounted for, and 
 which he termed the interference of the waves of ether. 
 
 (10.) Dr. Young admitted a sunbeam through a hole 
 made with a fine needle in thick paper, and brought into 
 the diverging beams a slip of card one-thirtieth of an inch 
 in breadth, and observed its shadow on a white screen at 
 different distances. The shadow was divided by light and 
 dark parallel bands alternately arranged, but the central 
 line was always white. He then intercepted the light on 
 one side by a second card interposed between the first card 
 and its shadow, and allowed the rays to pass freely on the 
 other side, when all the bands immediately disappeared ; 
 and he thus proved, beyond all doubt, that they were oc- 
 casioned by the interference of the light passing on both 
 sides of the first card. 
 
 (11.) From this experiment it is evident that, as the 
 central line in the shadow was always w r hite, light can turn 
 a corner, though, from the extreme minuteness of the un- 
 dulations, not to the same extent as sound does ; and thus 
 one of the greatest objections to the undulatory theory is 
 removed (6) : and it is also evident that two rays of light 
 may be so superposed as to produce darkness, a result 
 which cannot be explained on the corpuscular theory, al- 
 though it can be easily accounted for on its rival, which is 
 called the undulatory, being perfectly analogous to the 
 effects produced by the interference of the waves of both 
 water and air. 
 
 (12.) For if we stand at the confluence of two rivers, 
 when the streams or tides are running down against a 
 strong wind, equally opposed to each, we shall perceive 
 that, when the waves from the two rivers meet in the same 
 state of vibration, they will form large waves ; but when 
 they differ half a wave, the high part of one will fill up 
 the hollow of the other, and the water will be compara- 
 tively smooth. 
 
12 
 
 ON POLARIZED LIGHT. 
 
 (13.) Again; if we put the arms of a tuning fork into a 
 state of vibration, and hold it over a glass vessel of such a 
 capacity as that the air in the glass will vibrate in unison 
 with the arms of the tuning fork, a continuous sound will 
 be heard so long as the arms continue to vibrate. But if 
 we take a similar glass vessel, and hold it at right angles to 
 the first, somewhat as if pouring the contents of one into 
 the other, the sound will cease, as completely as the sound 
 of a piano-forte ceases when the vibrations of the strings 
 are stopped by the damper, because the undulations in the 
 one will interfere with and neutralize those in the other. 
 If we remove the upper glass vessel, the sound is again 
 heard, and so on alternately ; and thus the alternations of 
 sound and silence are analogous to the alternate bands of 
 light and darkness. 
 
 (14.) The nature and effects of interference may, how- 
 ever, be more readily explained by a diagram. 
 
 Fig. 1. 
 
 Fig. 1. A model of waves with moveable rods. 
 Fig. 2. A solid model with fixed waves. 
 
 A B, Fig. 1, represents a model with square bars or rods 
 freely moving in a perpendicular direction through the 
 frame A B, and furnished with pins resting upon the upper 
 part of the frame, so that, when at rest, the whole may 
 assume the appearance of waves, as in the diagram. 
 
 C D, Fig. 2, represents a solid model with fixed waves of 
 similar size or intensity, and numbered so as to distinguish 
 each half undulation. 
 
ON POLARIZED LIGHT. 
 
 13 
 
 (15.) Now it will be seen, that, when the stars indicating 
 the highest part of the waves on A B correspond with the 
 odd numbers of half undulations on C D, each system of 
 waves will be in the same state of vibration ; and if so 
 superposed, a series of \vaves of doubled intensity will be 
 the result, as in Fig. 3. 
 
 Fig. 3. 
 
 1234567? 
 
 Fig. 4. 
 
 JB 
 
 Fig. 3. Intensity of waves doubled by the superposition and coincidence of two 
 
 equal systems. 
 Fig. 4. Waves neutralized by the superposition and interference of two equal 
 
 systems. 
 
 (16.) If, on the other hand, the two systems be so su- 
 perposed as that the stars on A B shall coincide with the 
 even numbers on C D, as in Fig. 4, there will be a differ- 
 ence of half an undulation in the two systems : the one 
 will neutralize the other by interference, and darkness will 
 be the result. 
 
 (17.) If C D be constructed so that A B may be moved 
 forward indefinitely, it will be obvious that the waves will 
 be equally increased in intensity by a difference in the two 
 systems of any even number, and neutralized by a difference 
 of any odd number of half undulations. 
 
14 O1S POLARIZED LIGHT. 
 
 (18.) Thus, in Dr. Young's experiment, the systems of 
 waves passing on each side of the card would meet in the 
 centre of the screen in the same state of vibration, this 
 line being equidistant from the two edges of the card, and 
 hence would be illuminated by the sum of the lights pass- 
 ing by them. But on either side of this line there would 
 be a difference in the length of the paths of the two sys- 
 tems, increasing with the distance from the central line. 
 When this difference amounted to any odd number of half 
 undulations, interference would take place and produce 
 darkness, as in Fig. 4 ; but when the difference amounted 
 to any even number of half undulations, the two systems 
 would again coincide, as in Fig. 3. The central line would 
 then be necessarily white, and those on either side would 
 be alternately dark and light, as the increasing difference 
 in the length of the paths amounted to an odd or even 
 number of half undulations. (See 17.) 
 
 (19.) When speaking of the velocity of the propagation 
 of light, we are, however, too apt to adopt the language of 
 the corpuscular theory, and conclude that each particle tra- 
 vels from the sun to the earth at the rate of 192,000 miles 
 in a second of time ; whereas, on the undulatory theory, 
 we merely suppose the particles of ether to be successively 
 excited into waves of vibrations by the illuminating body, 
 as the waving corn by the wind, when, although the ears 
 themselves remain attached to the same part of the earth 
 by their roots, yet they produce by their vibrations those 
 beautiful undulations which appear to flow from one end 
 of the field to the other, the progression of the waves being 
 at right angles to the plane of vibration. 
 
 (20.) A familiar illustration may afford us more correct 
 views respecting the velocity with which motion may be 
 propagated and matter affected without the transmission 
 of matter itself. If we fill a boy's pea-shooter with peas, 
 and hold it in a horizontal position, it will be obvious that, 
 as soon as we force in an additional pea at one end of the 
 tube, a pea will drop out at the other ; and if we suppose 
 
OX POLARIZED LIGHT. 15 
 
 a similar tube extending from London to York, and that 
 we could move the whole line of peas in such a long tube 
 with as much facility as that in the short one, it would be 
 evident that, almost as soon as an additional pea was put 
 into the tube at London, a pea would drop out at York. 
 Now, had the means whereby such motion was commu- 
 nicated been as imperceptible as those by which light is 
 propagated or electricity transmitted through good con- 
 ductors, we might have concluded that the same pea had 
 travelled from London to York in an inconceivably short 
 space of time ; we should have been astonished at the ap- 
 parent velocity of its transmission, and surprised that it 
 had not acquired a momentum : but, knowing the condi- 
 tions of the proposition, we perceive that the particles of 
 matter may be affected, and motion propagated almost in- 
 stantaneously throughout a very long line, although each 
 individual particle may not be far removed from its original 
 position. This illustration may also convey some idea 
 respecting the instantaneous affection of matter, and, con- 
 sequently, instantaneous propagation of motion from one 
 station to another by the electrical telegraph. 
 
 (21.) Newton was the first who attempted to analyze 
 light, and found that of the sun to be composed of seven 
 differently colored rays, viz. red, orange, yellow, green, 
 blue, indigo, and violet, each varying in refrangibility, from 
 red, the least, to violet, the most refrangible : of these, the 
 red, yellow, and blue, are considered primary colors, the 
 others being merely compounds of two of these three. 
 
 (22.) Before Newton investigated the theory of light, 
 Dr. Hooke had succeeded in splitting mica into films of 
 such extreme thinness as to shew brilliant colors: one 
 gave a yellow, another a blue, and the two together a deep 
 purple ; but as the thickness of the plates was less than 
 the twelve-thousandth part of an inch, it seemed impos- 
 sible by actual admeasurement to determine the law ac- 
 cording to which the color varied with the thickness of the 
 film. Newton, however, surmounted this difficulty by 
 
16 ON POLARIZED LIGHT. 
 
 laying a double convex lens, the radius of whose convexity 
 was fifty feet, upon the flat surface of another lens, so 
 that the two surfaces were in contact at the centre, the 
 distance between them increasing with the increased dis- 
 tance from that centre. A plate of air was thus inclosed 
 between the two lenses varying in thickness in a certain 
 known ratio as the surfaces of the lenses receded from 
 each other. 
 
 (23.) When white or undecomposed light was allowed 
 to fall upon these lenses, a very minute black spot was 
 observed at the centre, while every different thickness of 
 the plate of air gave different colors; and as the same thick- 
 nesses occurred all round at the same distances from the 
 centre, the point where the two lenses touched was the 
 centre of a number of concentric and differently colored 
 rings. 
 
 (24.) He then by a prism decomposed a beam of white 
 light, and allowed the different colors of the spectrum to 
 fall successively upon the lenses. In each case the rings 
 appeared, but no longer exhibited any variety of tint, the 
 central spot being surrounded by rings of the same color 
 as the light incident upon the lenses, separated by dark 
 rings ; but yet with this difference, that the rings formed 
 by the less refrangible rays were broader than those formed 
 by the more refrangible, so that the rings were broadest in 
 red light and narrowest in violet. 
 
 (25.) In prosecuting his inquiries, he ascertained that, at 
 whatever thickness of the plate of air the colored ring first 
 appeared, there would be found at twice that thickness the 
 dark ring, at three times the colored, at four times the 
 dark, and so on, the colored rings regularly occurring at 
 the odd numbers, and the dark rings at the even ones. 
 But, upon looking through the lenses towards the light, 
 the order was found to be reversed ; for the colored rings 
 were then seen at the even and the dark rings at the odd 
 numbers ; the colored light appearing to be reflected and 
 not transmitted at the odd numbers, and to be transmitted 
 
OX POLARIZED LIGHT. 17 
 
 and not reflected at the even ones. Hence a ray of light 
 appeared at certain intervals to be in alternate states of 
 easy transmission and easy reflection ; in order, therefore, 
 to give names to facts which he had proved to exist, and 
 not to introduce a theory, Newton called these changes 
 " fits of easy transmission and easy reflection." 
 
 (26.) If we now refer to the diagrams, we shall see how 
 beautifully these phenomena are explained by the . wave 
 theory. For this purpose the reader's attention is directed 
 to the following figure, which shews the "rings," usually 
 known as Newton's, when seen by the reflection and 
 transmission of red light. 
 
 Fig. 5. 
 
 Fig. 5 represents Newton's rings as seen with red light. 
 The central spot, where the two lenses are in contact, will 
 necessarily be dark, the incident light being all transmitted ; 
 but a colored ring will appear around the centre, where a 
 thickness of air capable of reflecting red light first obtains. 
 If this thickness be considered equal to 1, and we add 
 thereto the numbers of half undulations forming the dif- 
 ference between the two systems of waves reflected to the 
 eye by the upper and under surfaces of the various thick- 
 nesses, we shall perceive that the colored rings will regu- 
 larly occur at the odd numbers, and the dark rings at the 
 even ones. (25.) 
 
 c 
 
18 
 
 ON POLARIZED LIGHT. 
 
 (27.) Let Figs. 1 and 2 represent two equal systems of 
 waves for red light, the one reflected to the eye from the 
 upper and the other from the under surface of Newton's 
 thin plates of air. If they be superposed, as in Fig. 3, the 
 waves will coincide, and there will be red light, as in the 
 first colored ring. On moving A B a distance equal to 
 one half undulation, as in Fig. 4, the waves will be neu- 
 tralized by interference, and there will be darkness ; on 
 moving A B a second half undulation, there will be light ; 
 on moving it a third, there will be darkness, a fourth light, 
 and so on ; for when the stars indicating the highest part 
 of the waves of A B coincide with the odd numbers of 
 half undulations of C D, there will be light, as in Fig. 3 ; 
 and when they coincide with the even numbers, darkness 
 will be occasioned by interference, as in Fig. 4. . 
 
 (28.) Thus, then, we perceive that half an undulation of 
 a wave for each color is equal to one of Newton's " fits" 
 for that color, and is equal in length to the thickness of 
 the plate of air at which that color is first reflected. This 
 he has ascertained by calculation, and hence we have before 
 us the data by which we can determine the length of an 
 undulation for each colored light. 
 
 Fig. 6. 
 
 Relative thicknesses of the plates of air by which each color is first reflected, 
 
ON POLARIZED LIGHT. 19 
 
 (29.) Fig. 6 represents the relative thicknesses of the 
 plates of air at which each colored light was first reflected, 
 and they are found to be as about 8 for red light to 14 
 for violet in a given space, or, more accurately, as 1 to 
 1*58 ; but the most astounding fact is, that the unit of the 
 numbers 133, &c. is but the ten-millionth part of an inch, 
 and which numbers Newton supposed to indicate the dia- 
 meters of the molecules of each differently colored light. 
 Taking, then, the half undulation as equal to one of 
 Newton's spaces or fits, and the whole undulation as equal 
 to two spaces, or what he termed the length of an in- 
 terval between the fits of easy reflection, it will follow, 
 that, if we draw waves so as to occupy each one interval or 
 two spaces, we shall ascertain not only the relative propor- 
 tions of the waves, but that the length of an undulation 
 for each colored light is as follows : 
 
 For Red light, 266 ten-millionths of an inch ; 
 
 Orange 240 
 
 Yellow 227 
 
 Green 211 
 
 Blue 196 
 
 Indigo 185 
 
 Violet 167 
 
 And so, as the various notes in music are determined by 
 differences in the frequency of the aerial pulses, in like 
 manner differences of color are determined by differences 
 in the frequency of the ethereal undulations ; and as the 
 deeper the arc of vibrations producing sound the stronger 
 the tone, so the more ample the undulations the more in- 
 tense the color. 
 
 (30.) The interference of the waves of simple or homo- 
 geneous light will, as we have observed (16), produce dark- 
 ness ; and we have now to explain w 7 hy the interference of 
 the waves of compound or white light will produce colors, 
 as seen in mother-of-pearl, Barton's buttons, &c., and 
 which are evidently caused by inequalities in their surfaces, 
 
20 ON POLARIZED LIGHT. 
 
 as impressions taken on black wax will reflect the same 
 colors. 
 
 (31.) In order, however, to simplify this subject, we will 
 notice only the three primary colors, red, yellow, and blue, 
 which by their combination will form white light. 
 
 (32.) When white light is reflected from the upper and 
 under surfaces of very minute grooves, as those which 
 reaUy exist in mother-of-pearl and Barton's buttons, al- 
 though impalpable to the touch and invisible to the un- 
 assisted eye, there will be a difference in the length of the 
 paths of the waves reflected from the two surfaces equal 
 to the depth of the minute groove ; while the distance of 
 the paths from each other being only equal to the width 
 of such groove, the two systems of waves will not be suffi- 
 ciently separated to prevent interference. If the difference 
 in the length of the paths amount to any even number of 
 half undulations, the two systems of waves will coincide, 
 and produce a color equal to the intensities of the two 
 combined; but if this difference should amount to any 
 odd number of half undulations, the two systems would 
 neutralize each other, and produce darkness (17). Thus, 
 suppose the two systems of waves for red light to meet in 
 the same state of vibration, the intensity of the red light 
 would be doubled ; and if at the same time the respective 
 systems for yellow and blue were to meet in similar states, 
 the intensities of the yellow and blue being also doubled, 
 white light would be the result ; for the red, yellow, and 
 blue, would be combined in the same relative proportions : 
 but if the respective systems for red and yellow should 
 coincide, and those for .blue should interfere, the combi- 
 nation of red and yellow would produce an orange, which 
 could only form white light by the addition of the blue 
 neutralized by interference, and which is called its com- 
 plementary color, or that color which is required to make 
 up the full complement of colors necessary for the pro- 
 duction of white light ; and so, if the systems of waves for 
 red light should interfere, and those for yellow and blue 
 
ON POLARIZED LIGHT. 21 
 
 should coincide, the result would be green light, which 
 would require its complementary color, red, to form white 
 light. 
 
 (33.) A minute difference in the distances through which 
 the ethereal waves are propagated is thus necessary for the 
 production of colors by interference ; and this difference 
 may be obtained not only by reflection from the two sur- 
 faces of minute grooves, but also by transmission through 
 any doubly refracting body. 
 
 (34.) It is well known that refraction depends not merely 
 on the obliquity with which a ray falls on the refracting 
 medium, but also on the density of the medium itself; 
 hence, if the particles of a body be operated upon, either 
 naturally or artificially, so as to produce unequal elasticities 
 in different directions, the refraction of light through that 
 body will be unequal in such directions; and if the surfaces 
 be parallel, the paths pursued by the differently refracted 
 rays will necessarily be unequal also in their lengths ; and 
 thus, while double refraction is the result, there will be such 
 a minute difference in the length of the paths pursued as 
 may produce interference. 
 
 (35.) A rhomb of Iceland spar is an example of the natu- 
 rally unequal elasticities or densities in a body, and, if we 
 transmit a ray of light through it, the ray will suffer bifur- 
 cation : one part will be refracted according to the ordinary 
 laws of refraction, and is called the ordinary ray; the other 
 part will be refracted according to some extraordinary law, 
 and is called the extraordinary ray ; and two images of the 
 object from which the ray emanated will be produced. 
 
 (36.) If we take a cube of regularly annealed glass, whose 
 elasticity is the same in every direction, and submit it to 
 the pressure of a screw, or to the action of heat, unequal 
 elasticities will be produced by what may be termed arti- 
 ficial means, and double refraction will be the result. In 
 this case there will be a sufficient difference in the length 
 of the courses pursued by the ordinary and extraordinary 
 
22 ON POLARIZED LIGHT. 
 
 rays to produce color by interference, though not a suffi- 
 cient separation to cause two images. 
 
 (37.) Double refraction, then, may produce color by in- 
 terference, and color will be the test of double refraction 
 when the rays are not sufficiently separated to form two 
 distinct images. 
 
 (38.) The interesting phenomenon of the polarization of 
 light was accidentally discovered by Professor Malus in 
 1808, while viewing through a doubly refracting prism the 
 light of the setting sun reflected from a French glass win- 
 dow, which happened to stand open, " like a door on its 
 hinges/ 5 at an angle which must have closely approximated 
 that of 56 45', and which has since been ascertained to 
 be the polarizing angle for glass. 
 
 (39.) If we allow a beam of the sun's light to fall upon a 
 plate of glass at any other angle of incidence than that of 
 from 56 to 57 degrees, one portion of the light will be 
 transmitted and another part reflected ; and if we hold a 
 second plate of glass over the reflected ray, a second re- 
 flection will take place, on whatever side of the reflected 
 ray the second glass be held ; but if the two glasses be so 
 placed that the sun's light shall fall upon the first and the 
 reflected light upon the second glass at an angle of inci- 
 dence of 56 45' from the perpendicular, the light reflected 
 by the first will be again reflected by the second glass when 
 the planes of the two glasses are parallel to each other ; 
 but if, without altering its angle to the horizon, we turn 
 the second glass a quarter of a circle, the light will be 
 transmitted and not reflected : if we turn it another quarter, 
 the light will be reflected and not transmitted; and so on 
 alternately for every quarter of a revolution that the second 
 glass is made to perform round the ray reflected by the 
 first glass. 
 
 (40.) A ray of light, then, when reflected at an angle of 
 incidence of 56 45' from the perpendicular, has undergone 
 some modification : it has acquired the properties of sides ; 
 
ON POLARIZED LIGHT. 23 
 
 it has two sides on which it can and two on which it can- 
 not be again reflected ; and as these opposite properties of 
 the different sides were supposed to bear some analogy to 
 the opposite properties of the different poles of a magnet, 
 a ray of light so modified was said to be polarized. It is, 
 however, universally admitted that the term is unfortunate, 
 as it affords no indication of the phenomena it professes 
 to describe. 
 
 (41.) In order to develop by degrees a subject confess- 
 edly abstruse, we have hitherto noticed only one plane of 
 vibration of the ethereal particles; but Fresnel assumes 
 that the vibrations are performed in two planes at right 
 angles to the direction of the progress of the wave, which 
 is analogous to the motions of the waves of the sea, as ex- 
 perienced by those who have crossed the channel in a 
 steam-boat during a brisk gale, when the rectangular 
 vibrations occasioned by the alternate pitchings and rock- 
 ings of the vessel have caused the mast-head to describe a 
 circle or an oval, as,*the case might be, and afforded those 
 who could enjoy thff-^icene a fine opportunity of studying 
 the resolution of forces. 
 
 (42.) Before, however, we attempt to explain the nature 
 of polarized light, it may be expected that we should define 
 the term, and thus answer the oft repeated question, What 
 is it? We may say, It is light reflected from or trans- 
 mitted through glass at an angle of incidence of 56 45' ; 
 but this only describes one of the modes of producing it, 
 and is no reply to the question, What is it ? As this can 
 be most readily answered by hypothesis, we shall endeavour 
 to shew, that, in the language of the undulatory theory, it 
 is light propagated by only one plane of vibrations ; where- 
 as common light and this term is merely used in contra- 
 distinction to that of polarized light is propagated by 
 several or at least two planes at right angles to each other; 
 and that the act of polarizing light is, in the same language, 
 the act of separating or modifying these rectangular planes, 
 
24 ON POLARIZED LIGHT. 
 
 so that the vibrations may be reflected or transmitted in 
 one plane only. 
 
 (43.) Let A B, Fig. 7, represent the perpendicular and 
 C D the horizontal vibrations of the ethereal particles, by 
 which a ray of common light is supposed to be propagated ; 
 and let E F and G H represent the two plates of glass used 
 as reflectors. 
 
 Fig. 7. 
 
 f\. 
 
 +' 
 
 . A B, C D, rectangular vibrations for common light. 
 E F, G H, I K, plates of glass used as reflectors. 
 
 Fig. 8. A B, the perpendicular vibrations transmitted ; C P, the horizontal vibrations 
 reflected by E F. 
 
 (44.) Now we can readily conceive that the ethereal 
 particles which vibrate perpendicularly to the plane surface 
 of a plate of glass will be more likely to communicate 
 motion to the ethereal particles within the glass, than those 
 which vibrate horizontally upon it ; and, if motion be com- 
 municated to the ethereal particles within the glass, the 
 vibrations will be continued, and transmitted through the 
 glass ; we can also as easily conceive that the ethereal par- 
 ticles which vibrate horizontally upon the glass will be 
 more likely to rebound, or, in optical language, to be 
 reflected, than those which vibrate perpendicularly to it. 
 
 (45.) When, then, a ray of common light falls on a plate 
 of glass, as E F, Fig. 7? at the polarizing angle, the per- 
 pendicular vibrations A B are transmitted and the hori- 
 zontal vibrations C D are reflected. A B, Fig. 8, will then 
 represent the vibrations of the transmitted and C D those 
 of the reflected light. C D after reflection from E F would 
 be again reflected by G H, but if G H were turned a 
 quarter of a circle as I K, C D would be transmitted and 
 not reflected. 
 
ON POLARIZED LIGHT. 
 
 25 
 
 The plate E F polarizes the 
 light in two planes at right 
 angles to one another, by trans- 
 mitting the perpendicular vi- 
 brations A B, and reflecting 
 the horizontal vibrations C D, 
 as in Fig. 9. 
 
 If now G H be held over 
 E F, and parallel to it, the 
 horizontal vibrations C D re- 
 flected by E F will be again 
 reflected by G H, because both 
 plates will be in the same re- 
 lative position with respect to 
 CD. 
 
 fig. 9. 
 
 ll 
 
 ll 
 
 Fig. 9. The perpendicular vibrations 
 A B transmitted by E F. The horizon- 
 tal vibrations C D reflected first by EF, 
 and again by G H. 
 
 Fig. 10. 
 C D 
 
 X 
 
 Fig. 11. 
 
 Fig. 12. 
 
 1 
 
 Fig. 10. The horizontal vibrations C D, which had been reflected by E F, trans- 
 mitted by G H turned one quarter of a circle. 
 Fig. 11. C D reflected by G H turned a second quarter. 
 Fig. 12. C D transmitted by G H turned a third quarter. 
 
 But if G H be turned one quarter of a circle round C D, 
 as in Fig. 10, C D will then vibrate perpendicularly to the 
 plane surface of G H, and will be transmitted. 
 
 If G H be turned round a second quarter of a circle, as 
 in Fig. 1 1, C D wall again vibrate horizontally upon it, and 
 be reflected. 
 
 If G H be turned a third quarter, as in Fig. 12, C D 
 will vibrate perpendicularly to it, and be transmitted ; and 
 so on alternately for every quarter of a circle that G H is 
 turned round C D : and thus light is said to be polarized, 
 
26 ON POLARIZED LIGHT. 
 
 or to have acquired {he property of sides, there being two 
 sides, as in Figs. 9 and 1 1, on which it is, and two, as in 
 Figs. 10 and 12, on which it is not, again reflected (40). 
 
 (46.) But whether the theory be correct or not, the facts 
 are as above stated ; for the ray reflected from glass at an 
 angle of 56 45', and the ray transmitted through it, are 
 both polarized, and they are polarized in planes at right 
 angles to one another, as A B and C D, Figs. 8 and 9. 
 
 (47.) Light may also be polarized by transmission through 
 a variety of bodies, as through a bundle consisting of from 
 16 to 18 plates of thin glass, or through a plate of tourma- 
 line cut parallel to its crystallographical axis. Of these the 
 tourmaline is the most perfect, and, therefore, the most 
 useful polarizer: it is a doubly refracting crystal, and 
 occurs in long prisms, which may be cut into parallel 
 plates sufficiently transparent to allow light to pass through 
 them. 
 
 (48.) Let E F and G H, Fig. 13, represent the surfaces 
 of two such plates of tourmaline, and the lines the direc- 
 tion of their crystallographical axes. If we look through 
 them separately the light will be transmitted, however we 
 may turn them round ; and if we superpose them, the 
 
 E F, G H, IK, plates of tourmaline. The horizontal vibrations C D stopped by 
 E F. The perpendicular vibrations A B transmitted by E F and G H, 
 but stopped by I K. 
 
 light will likewise be transmitted so long as their axes are 
 in the same direction ; but if we fix E F and turn G H 
 one quarter of a circle, as I K, the light will be stopped ; 
 if we turn it a second quarter, the light will be transmitted; 
 and so on alternately, the light being transmitted when 
 their axes are in the same direction, and stopped when at 
 right angles to each other. 
 
ON POLARIZED LIGHT. 27 
 
 (49.) As in the former experiment, we can easily con- 
 ceive that the particles A B vibrating in the direction of 
 the axis of the crystal E F will be more likely to produce 
 a continuation of the undulation than those of C D vi- 
 brating at right angles to the axis, and hence that A B 
 will be transmitted and C D stopped. 
 
 The tourmaline E F polarizes the light in the plane A B 
 by stopping C D, which would otherwise form common 
 light; and it is obvious that A B, after transmission through 
 E F, would also be transmitted through G H 1 , when its 
 axis was in the same direction ; but \vhen G H is turned 
 one quarter of a circle, as I K, its axis will be at right 
 angles to A B, and will stop A B as E F had previously 
 stopped C D. If we turn G H another quarter, A B will 
 be again transmitted; and so on, the light polarized by 
 E F being alternately stopped or transmitted by G H at 
 every quarter of a revolution. A large portion of light is, 
 however, lost, for one-half must be stopped to produce 
 perfect polarization, besides what is lost by absorption, &c. 
 
 (50.) Light may also be polarized by transmission through 
 other crystals possessed of the property of double refraction. 
 
 (51.) We have already shewn (35) that a ray of light 
 transmitted through a doubly refracting medium suffers 
 bifurcation, producing what are termed the ordinary and 
 extraordinary rays. If these be examined, the rectangular 
 planes of vibrations of the incident common-light will appear 
 to have been separated in their passage through the crystal, 
 the one plane forming the ordinary and the other the extra- 
 ordinary ray ; hence, both rays will be polarized, and in 
 planes at right angles to one another. 
 
 A B, Fig. 14, represents the ordi- Fi 9- u - 
 
 nary and C D the extraordinary rays, 
 by which the two images are pro- 
 duced. If these be viewed through 
 the tourmaline E F, Fig. 13, with its 
 axis in the same direction as A B, the 
 
 -, i i A T MI i_ AB the perpendicular, 
 
 image produced by A B will be seen, CD the horizontal vibrations, 
 
 j ,, , ~ i i /i T* MI i M ^Parated by a doubly re- 
 
 and that formed by C D will be cut fracting crystal. 
 
28 ON POLARIZED LIGHT. 
 
 off. On turning E F 90 degrees, as I K, the image pro- 
 duced by A B will disappear, and that formed by C D will 
 appear ; and so the images will alternately appear and dis- 
 appear at every quarter of a revolution of the tourmaline. 
 
 (52.) If, however, the tourmaline be turned only 45 de- 
 grees, two faint images will be observed; hence it has 
 been assumed that polarized light is transmitted, when the 
 plane of vibration does not vary more than 45 degrees 
 from the axis of the tourmaline, or other polarizing plate, 
 but is effectually stopped by a difference of 90 degrees. 
 
 (53.) In the preceding experiments we have seen that 
 light polarized by one plate is alternately stopped and 
 transmitted at every quarter of a revolution of a second 
 polarizing plate ; but if we interpose another doubly re- 
 fracting crystal, a new and interesting series of phenomena 
 will occur. 
 
 (54.) We have shewn that colors are produced by the 
 interference of the ethereal waves (32) ; that a minute dif- 
 ference in the distances through which such waves are 
 propagated is necessary to cause interference, otherwise 
 they would always meet in the same state of vibration (33) ; 
 and that a difference is occasioned by the bifurcation of a 
 ray when transmitted through a doubly refracting medium, 
 one part being always more refracted than the other, and 
 hence pursuing a different course, and being of a different 
 length (34). 
 
 (55.) If, then, we interpose between the tourmalines, or 
 other polarizing plates, a doubly refracting medium, such 
 as a plate of selenite, color will be produced by interference 
 when the light polarized by the first plate is analyzed by 
 the second, and the color will change to its complementary 
 tint at every quarter of a revolution of the analyzer, instead 
 of being alternately stopped and transmitted, as in the 
 case of light merely polarized. 
 
 (56.) But this subject will be better understood by re- 
 ferring to a diagram exhibiting at one view the effects pro- 
 duced by the polarization, analyzation, and interference of 
 light. 
 
29 
 
 " Y I B I 
 
 ' J) Ye I B e I 
 
 K 
 
 M 
 
 Fig. 1G. 
 
 Fig. 15. A B, C D, common light. E, a plate of tourmaline termed the polarizer. F, po- 
 larized light. G, a plate of selenite. H, dipolarized light. I, a plate of 
 tourmaline termed the analyzer. K, coincidence of waves for red light. 
 L, interference of waves for yellow, and M of those for blue light. N, the 
 result red light. 
 
 Fig. 16. I, the analyzer turned round 90. K, interference of waves for red light. 
 L, coincidence of waves for yellow, and M of those for blue b'ght. N, the 
 result green light. 
 
 N 
 
 X 
 
 A B, C D, represent the rectangular vibrations by which 
 a ray of common light is supposed to be propagated. 
 
 E, a plate of tourmaline, called in this situation the po- 
 larizer, and so turned that A B may vibrate in the plane 
 of its crystallographical axis. 
 
 F, light polarized by E, by stopping the vibrations C D, 
 and transmitting those of A B. 
 
 G, a piece of selenite of such a thickness as to produce 
 red light, and its complementary color green. 
 
 H, the polarized light F bifurcated, or divided into ordi- 
 nary and extraordinary rays, and thus said to be dipolarized 
 by the double refractor G, and forming two planes of 
 polarized light o and e, vibrating at right angles to each 
 other (51). 
 
 I, a second plate of tourmaline, here called the analyzer, 
 with its axis in the same direction as that of E, through 
 which the several systems of waves of the ordinary and 
 extraordinary rays H, not being inclined at a greater angle 
 to the axis of the analyzer than that of 45 degrees, are 
 transmitted (52) and brought together under conditions 
 that may produce interferences (54). 
 
30 ON POLARIZED LIGHT. 
 
 K, the waves R and R e for red light of the ordinary and 
 extraordinary systems meeting in the same state of vibration, 
 occasioned by a difference of an even number of half un- 
 dulations, and thus forming a wave of doubled intensity for 
 red light (32). 
 
 L, M, the waves Y and Y e and B B e , for yellow and 
 blue of the ordinary and extraordinary systems respectively 
 meeting together, with a difference of an odd number of 
 half undulations, and thus neutralizing each other by in- 
 terferences. 
 
 N, red light, the result of the coincidence of the waves 
 for red light, and the neutralization by interferences of 
 those for yellow and blue respectively (32). 
 
 H, Fig. 16, dipolarized light, as H, Fig. 15. 
 
 I, the analyzer turned one quarter of a circle, its axis 
 being at right angles to that of I, Fig. 15. 
 
 K, the waves R R e for red light of the ordinary and the 
 extraordinary systems meeting together with a difference of 
 an odd number of half undulations, and thus neutralizing 
 each other by interference. 
 
 L, M, the waves Y Y e and B B e for yellow and blue of 
 the two systems severally meeting together in the same 
 state of vibration, occasioned by the difference of an even 
 number of half undulations, and forming by their coin- 
 cidences waves of doubled intensity for yellow and blue 
 light. 
 
 N, green light, the result of the coincidences of the waves 
 for yellow and blue light respectively, and the neutraliza- 
 tion by interference of those for red (32). 
 
 (570 By using a plate of selenite of uniform thickness, 
 the color will be uniform, whereas a plate of different thick- 
 nesses will produce different colors following the same 
 order as those of Newton's rings ; red being produced by 
 the thickest, violet by the thinnest, and intermediate colors 
 by intermediate thicknesses of the plates of selenite. 
 
 (58.) Hence, also, if we examine under the microscope 
 by polarized light any minute animal or vegetable tissues, 
 possessing by means of their unequal densities or elasticities 
 
ON POLARIZED LIGHT. 31 
 
 the properties of double refraction, there will be produced 
 colors varying according to the otherwise unappreciable 
 difference of density in the various parts of the tissues, 
 which may thus be distinguished and traced out in a much 
 more satisfactory manner than could be accomplished by 
 common light. Should, however, the doubly refracting 
 properties of the tissue be too feeble to produce a sufficient 
 difference of color, the effect may be considerably increased 
 by placing the object on a plate of selenite of uniform thick- 
 ness ; for which purpose a thickness of selenite producing 
 a bright purple or light blue color will be found to afford 
 the most agreeable contrast, and, as a single plate, to be 
 the most generally useful*. 
 
 (59.) If we use a doubly refracting prism as an analyzer, 
 and interpose a plate of selenite of uniform thickness, the 
 complementary colors will be seen at the same time, and 
 we are thus afforded a beautiful illustration of the decom- 
 position and recomposition of white light. For this pur- 
 pose a plate of selenite of that thickness which gives a 
 bright red (and its complementary color green) should be 
 selected, and laid on the stage of the microscope under a 
 thin brass plate, having three circular perforations of dif- 
 ferent diameters, the smallest being so adjusted, both with 
 respect to the power of the microscope and the separating 
 properties of the double refractor, as that the two circles 
 produced by double refraction shall just be distinctly sepa- 
 rated : the second and larger perforation is to be made at 
 such a distance from the first, and the third and largest at 
 such a distance from the second, as to be successively 
 brought into the field of view merely by the action of the 
 lever or rack- work. 
 
 (60.) If the selenite be viewed through the smallest per- 
 foration without the doubly refracting prism, a disc of 
 white light will be perceived. If the prism be now placed 
 
 * Mr. Darker has constructed a very convenient apparatus, consisting of 
 three plates of selenite of different thicknesses, made to revolve in a frame, so 
 as to produce, when required, different colors and their complementary tints. 
 
32 ON POLARIZED LIGHT. 
 
 over the eye-piece of the microscope, the white light will 
 be decomposed by interferences 5 a disc of red light will be 
 seen on one side, and a disc composed of yellow and blue, 
 forming by their combination green, or the complementary 
 color to red, will be observed on the other : and that these 
 colors are complementary, or that the one is what the other 
 wants to form white light, may be proved by bringing the 
 larger aperture into the field, when the diameters of the 
 discs of red and green light will be so increased as to over- 
 lap, and form by their union white light, while the parts 
 of each disc which do not combine will retain their re- 
 spective colors. If the prism be turned round on its axis, 
 the colors will change ; but white light will always be seen 
 where the complementary colors overlap and combine. 
 
 (61.) If, instead of a plate of selenite, we interpose be- 
 tween the polarizer and the analyzer one of the crystals of 
 calc. spar, quartz, topaz, nitre, arragonite, or the tourma- 
 line, provided the plates be cut in certain directions, colored 
 rings will be produced, which will change to their comple- 
 mentary colors at every quarter of a revolution of the ana- 
 lyzer. In some instances the rings will be intersected by 
 a cross, which will be black when the axis of the analyzer 
 is at right angles to that of the polarizer, and white when 
 in the same direction; for in one case the light will be 
 merely stopped, and in the other transmitted. 
 
 (62.) There is in every doubly refracting crystal at least 
 one direction in which no double refraction takes place, for 
 there must be one line where two opposing forces (34) meet, 
 and neutralize each other : this line is called by some the 
 optic axis, and by others, more intelligibly, the axis of [no] 
 double refraction. If, then, there be any part of a crystal 
 in which double refraction does not occur, the same effect 
 will be produced, when the light transmitted through such 
 part is analyzed, as though no doubly refracting crystal 
 had been interposed, and the light will be merely stopped 
 or transmitted by revolving the analyzer ; but as double 
 refraction takes place all around the axis, the systems of 
 
ON POLARIZED LIGHT. 33 
 
 waves thus produced will interfere when analyzed, and 
 occasion concentric colored rings. If a crystal possess 
 but one axis of [no] double refraction, one system of 
 colored rings, intersected by the effects of the light not 
 doubly refracted, and, consequently, merely stopped or 
 transmitted, will appear: if the crystal have two such 
 axes, two systems will be produced, though in some in- 
 stances they may be too widely separated to be observed 
 at the same time. 
 
 (63.) My friend, Dr. Pereira, has beautifully exemplified 
 this subject in his "LECTURES ON POLARIZED LIGHT;" 
 and has shewn that such is the mutual relation existing 
 between the forms and properties of crystals, that the form 
 at once indicates whether or not a crystal possesses the 
 property of double refraction, and, if a double refractor, 
 whether it has one or two axes of [no] double refraction. 
 
 To this work I refer those who, having stepped over 
 " the threshold/ 5 are inclined to study the higher branches 
 of this interesting subject. 
 
 (64.) In the diagram, Fig. 15, one tourmaline is sup- 
 posed to be used as a polarizer, and another as an analyzer; 
 but the most generally useful polarizer for the achromatic 
 microscope is a Nicols' prism ; a doubly refracting crystal 
 so prepared, that one of the images produced may be cut 
 off or thrown out of the field. If a polarizer of large dia- 
 meter should be required, a bundle consisting of from 16 
 to 18 plates of the thin glass used for covering microscopic 
 objects, adjusted at the polarizing angle, may be substi- 
 tuted for the Nicols' prism. The most convenient analyzer 
 is a thin plate of blue tourmaline ; a Nicols' prism may, 
 however, be used as an analyzer ; but if placed over the 
 eye-piece, the field of view is too much contracted ; and if 
 inserted in the body of the microscope, the prism can 
 neither be conveniently turned nor readily removed. 
 
 (65.) A bundle composed of eight or ten plates of thin 
 flat white window glass, the lowest plate being blackened 
 to absorb the transmitted light, is the most convenient 
 
 E 
 
34 ON POLARIZED LIGHT. 
 
 polarizer for the hydro-oxygen polariscope, because a field 
 of view to any extent may be thereby obtained. Perfect 
 polarization cannot, however, be produced by this means ; 
 for the light cannot fall upon and be reflected by all the 
 surfaces at precisely the same angle. A single plate of 
 blackened glass may be successfully substituted as a po- 
 larizer for the table polariscope ; but the advantage of the 
 complete polarization thus obtained is so much diminished 
 by the decreased intensity of the light reflected, that the 
 single plate can only be used with the hydro-oxygen polar- 
 iscope, for the purpose of proving that the light polarized 
 by the reflector is completely stopped by the analyzer at 
 the alternate quarters of its revolution. 
 
 (66.) When, also, it is required to exemplify the above 
 facts by the achromatic microscope, either a plate of tour- 
 maline must be used as a polarizer, or a plate of blackened 
 glass inclined at the polarizing angle* must be substituted 
 for both the Nicols' prism aud the ordinary reflector. 
 For if light reflected by a metallic surface be transmitted 
 through a Nicols' prism, it will be found that the emerging 
 ray is not entirely polarized in one plane ; and, therefore, 
 a plate of tourmaline or other analyzer will not entirely 
 cut off the light at every quarter of its revolution. 
 
 (67.) If animal structures or vegetable tissues are to be 
 examined by polarized light, the blue tourmaline is the best 
 analyzer ; and in order to admit more light, and interfere 
 less with the colors of the object, the plate may be ground 
 very thin without injuring its analyzing properties for such 
 
 * This is readily effected by inclining the body of the microscope itself at 
 the polarizing angle for glass, which is at once ascertained by holding at the 
 side of the microscope a card cut to the proper angle, and comparing the 
 angular position of each. If the blackened glass be then laid upon the ordi- 
 nary reflector, previously placed in a horizontal position, and the lamp moved 
 until the field be illuminated, the incident light must impinge upon the 
 glass at the proper angle, the angle of reflection being equal to the angle of 
 incidence. Messrs. Smith and Beck have recently adapted to the author's 
 microscope a very simple indicator, which correctly points out the polarizing 
 angle for glass, and supersedes the use of the card. 
 
ON POLARIZED LIGHT. 35 
 
 purpose ; but when the crystals of calc. spar, nitre, quartz, 
 &c. are to be examined, the analyzing plate must not be 
 ground so thin : for the more perfect the analyzer as well 
 as the polarizer, the more beautifully will 'the various 
 systems of colored rings be developed; and in this case also 
 the blue tourmaline is the best analyzer ; for although a 
 light orange-colored tourmaline appears to afford the best 
 definition, it interferes too much with the colors of the 
 rings*. The quality of a tourmaline does not, however, 
 depend merely on its color. Those are generally the best 
 which stop the most light when their axes are placed at 
 right angles, and yet admit the most when in the same 
 plane ; and certainly the less color, or the more it ap- 
 proaches to a neutral tint, the better. 
 
 (68.) Thus we have endeavoured to explain and illus- 
 
 * The effects of these crystals may be exhibited with the achromatic micro- 
 scope, by using the three-inch power racked down as near the Nicols' prism 
 as the works will permit ; and attaching the second eye-piece, taking off the 
 cap or stop, and adapting to the eye-lens a small stage for holding the crystals 
 and analyzer ; but the field of view thus obtained is too limited for observing 
 at the same time the widely separated systems of rings of arragonite. In 
 order to examine to the best advantage the different crystals, an adjustment 
 of the field of view is necessary. This is accomplished by constructing an 
 eye-piece with the field-lens attached to a separate tube sliding within the 
 one containing the eye-lens, and so arranged that the distance between the 
 lenses may be increased from 1| to 2 inches. Lenses of the same foci as 
 those used in the second eye-piece, but of somewhat larger diameters, and 
 fitted without a stop between them, answer the purpose very well. The field- 
 lens is to be drawn out when calc. spar is examined, and moved back, as near 
 to the eye-lens as the tubes will permit, when arragonite is viewed. An in- 
 termediate adjustment may be required for the inspection of other crystals, 
 and is readily obtained by this means. A superior illumination of the field 
 is effected by substituting for the three-inch power the lens usually employed 
 as the side illuminator, bringing it by the action of the universal joint 
 between the lower end of the microscope and the Nicols' prism. The illu- 
 mination can then also be adjusted, and the light condensed upon any parti- 
 cular part, or diffused more generally over the field, by moving the lens 
 nearer to, or farther from, the Nicols' prism. A plate of blackened glass 
 placed at the polarizing angle, as before described, may be very advantage- 
 ously substituted for the Nicols' prism, when a lamp is used for illuminating 
 the field. 
 
36 ON POLARIZED LIGHT. 
 
 trate, in a familiar manner, some of the phenomena con- 
 nected with the polarization of light, and have adopted 
 the language of the undulatory theory, because it will 
 enable the inquirer, at least, to comprehend what is meant 
 by the terms used; but, as we have before observed, whether 
 the theory be correct or not, the facts as illustrated by it 
 remain the same. Newton called certain spaces fits of 
 easy transmission and reflection ; these are now generally 
 supposed to indicate the length of half undulations. Future 
 philosophers may call them by some other name; but, 
 whatever theory is adopted, " these spaces/ 5 as it has been 
 observed by Sir John Herschell, " have a real existence, 
 being deduced by Newton from direct measurement, and 
 involve nothing hypothetical but the names given to them." 
 (69.) If, then, the undulatory theory can furnish the 
 student with such a material vehicle as will convey the 
 facts to his mind, and impress them on his memory, he 
 may safely receive it until some better be propounded, 
 when he can readily separate the facts from the terms, 
 and adopt a more suitable nomenclature. 
 
37 
 
 PART II. 
 
 DESCRIPTION OF, AND INSTRUCTIONS FOR USING, 
 
 THE TABLE AND HYDRO-OXYGEN 
 POLARISCOPE AND MICROSCOPE. 
 
 (70.) THIS instrument is so arranged that it may be 
 illuminated by a candle or argand lamp placed on the table, 
 as in the case of an ordinary microscope ; or, with the ad- 
 dition of suitable condensers, it may be attached to the 
 lantern of the hydro-oxygen apparatus, and used either as 
 a gas polariscope or microscope for illustrations in the 
 lecture room. 
 
 Fig. 18. Fig. 17. 
 
 Fig. 17. The polariscope. 
 
 Fig. 18. A cap with a ground glass shade. 
 
 On a scale of one quarter to an inch. 
 
 (71.) A B, Fig. 17, represents the body of the polar- 
 iscope formed of two tubes three inches diameter, each in- 
 clined at the polarizing angle for glass, viz. 56 45'. 
 
38 
 
 ON POLARIZED LIGHT. 
 
 C, the polarizing plates, consisting of eight pieces of 
 thin white window glass, the lowest being blackened to 
 absorb the transmitted light. These are attached to the 
 polariscope by screws, c, and removable at pleasure. 
 
 Fig. 18, a cap fitting on at A, formed by a ring enclosing 
 a piece of ground glass to disperse the light of the candle 
 or lamp used as an illuminator. 
 
 Fig. 19. 
 
 Fig. 21. 
 
 Fig. 19. The stage. Fig. 20. The power. Fig. 21. A higher power. 
 
 On a scale of one quarter to an inch. 
 
 Fig. 19, A, a tube fitting on at B, Fig. 17, and fixed by 
 a bayonet catch. B, the stage attached to and revolving 
 around the tube A, the objects being kept in position by 
 the springs c c. 
 
 Fig. 20, the lowest power to be screwed into the stage 
 B, Fig. 19. It is composed of two lenses, the first being 
 a piano convex lens of 2j inches diameter, and 3 inches 
 focus, and the second a lens slightly* crossed of 1 J inch 
 diameter, and 2 inches focus. 
 
 Fig. 21, a higher power, composed of two crossed lenses, 
 the first having a diameter of If inch, and focus of 2 
 inches; and the second a diameter of 1^ inch, and focus 
 of 2 inches. 
 
 (72.) The tourmaline, or other analyzing plate, turns 
 freely in a short tube, a #, projecting from the eye-lens of 
 the power, and the focus is adjusted by a rack and pinion. 
 
 (73.) A box 9 inches high and about 11 inches long, by 
 
 * By a crossed lens is understood a double convex lens, the two sides of 
 which are segments of circles of different diameters. 
 
ON POLARIZED LIGHT. 
 
 39 
 
 7 wide, will contain the whole, and may be conveniently 
 fitted as a stage to raise the polari scope to a convenient 
 height for illuminating and viewing objects on the table. 
 A sketch of the apparatus arranged for use has been taken 
 by my friend, Mr. Legg, by means of the camera lucida 
 attached to a microscope fitted with " Smith and Beckys 
 erecting piece/' and is represented in Fig. 22. 
 
 Fig. 22. 
 
 Fig. 22. The table polariscope arranged for use. 
 
 (74.) The instrument arranged with the lowest power 
 affords an extensive field of view, and is thus well adapted 
 for exhibiting the various effects produced by the different 
 forms and thicknesses of selenite and unannealed glass, and 
 for illustrating the interesting phenomena of polarized light 
 generally. It is also calculated for viewing such crystals 
 of salts as have been allowed spontaneously to crystallize 
 on glass; but the arrangement must be altered for ob- 
 serving the phenomena connected with the crystals of calc. 
 spar, nitre, quartz, &c. In the one case, the lens or power 
 is used to bring the image of the object to a focus ; in the 
 other, it is merely required to cause a great divergence of 
 the rays passing through the crystals; hence, the slide 
 containing them must either be placed immediately between 
 
40 ON POLARIZED LIGHT. 
 
 the eye-piece and analyzer by means of a small stage at- 
 tached to the tube a a of the power, or the stage and power 
 must be removed, and there must be inserted instead a 
 double convex lens of two inches focus, with a stage and 
 analyzer so arranged that the crystals may be placed just 
 within the focus of the lens, and immediately under the 
 
 Fig. 23. 
 
 F iff. 23. The condenser and stage for crystals. 
 On a scale of one quarter to an inch. 
 
 analyser, as Fig. 23. In either case, a much larger field 
 of view, and, consequently, a more beautiful display of 
 colored rings, will be developed than is obtained by the 
 achromatic microscope. A single plate of blackened glass 
 may be occasionally laid on the bundle of plates (with the 
 intervention of a piece of chamois leather, to prevent in- 
 jury), and substituted with advantage as a polarizer in 
 cases where perfect polarization is more important than 
 intensity of light (65). 
 
 (75.) By removing the analyzer and the polarizing plates, 
 and substituting for the former a cap or stop with an aper- 
 ture of ^ inch diameter, and for the latter a silvered re- 
 flector, or, in other words, a looking-glass, the polariscope 
 is immediately converted into a microscope of low power, 
 which will include within its field all the parts of a large 
 object, such as a butterfly with its wings extended, the 
 breathing apparatus of a dytiscus, a fern branch, dissected 
 leaf, &c. ; and thus exhibit at one view their relative pro- 
 portions and connections. 
 
 (76.) The polariscope may also be attached to the hydro- 
 oxygen lantern, and adapted for illustrations before a large 
 
ON POLARIZED LIGHT. 
 
 41 
 
 audience. An apparatus described below, fitted up for 
 this purpose at a comparatively small expense, has been 
 long used by the author, who considers it to possess great 
 power, and to be more convenient for private investigation 
 than any other, while it is equally well adapted for illus- 
 trations in the lecture-room. The arrangement of the 
 condensers and powers is the result of very many experi- 
 ments continued through several years, and, hence, a 
 particular description of them is given (Figs. 20, 21, 23, 
 25, 26, 27). 
 
 Fig. 
 
 Fig. 24. A section of the hydro-oxygen apparatus on a scale of one inch to a foot. 
 
 (770 A B, Fig. 24, represents a stand or frame upon 
 which are placed a Gurney's blow-pipe, and the lantern. 
 C, D, two of the upright pillars fixed into a stout stand, 
 
42 ON POLARIZED LIGHT. 
 
 and supporting a corresponding piece of wood, to which are 
 attached the tubes leading from the reservoir E to the 
 bladder F*, and from thence to the safety tubes, K. 
 
 G, a very light circular board strengthened by thin 
 pieces of wood placed vertically at right angles to each 
 other, and fastened with packthread to rings inserted 
 through the four wires connected with the pressure board H, 
 upon which are placed flat leaden weights. The pressure 
 is temporarily removed for the purpose of filling the bladder 
 F from the reservoir E, by raising the pressure board H, 
 and suspending it by two rings turned over corresponding 
 hooks in the upper part of the frame. 
 
 I, a stout brass rod passing through the middle of H, 
 to guide it. 
 
 K, Hemming' s safety tube screwed on the frame A B, 
 surmounted by a wire gauze safety cylinder and jet, and 
 connected by a tube of flexible metal with the bladder F. 
 
 L, a cylinder of soft lime or chalk, made to revolve con- 
 tinuously by clock-work, or occasionally by the hand. 
 
 M, a frame moving to and fro between headings on the 
 stand A B, so as to adjust the distance of the condensers 
 from the lime light. 
 
 N, the lantern attached to the frame M by a central screw 
 at the back, and furnished with two screws in front, admit- 
 ting a lateral and vertical adjustment. 
 
 O, a circular opening in a brass plate into which the 
 condensers are to be screwed. 
 
 Fig. 25. Tubes sliding the one within the other, con- 
 taining the condensers for the gas polariscope ; the first 
 being a crossed lens of 2| inches diameter, and 2 inches 
 focus ; and the second a slightly crossed lens of the same 
 diameter, and 9 inches focus. 
 
 Fig. 26. Tubes containing the condensers for the gas 
 microscope of the same diameters as those of Fig. 25, the 
 
 Light indian rubber bags of suitable forms may be substituted for the 
 bladders E and F. 
 

 Fig. 25. 
 
 OX POLARIZED LIGHT. 
 
 Fig. 26. 
 
 43 
 
 Fiy. 25. Condensers for the gas polariscope. 
 Fig. 20. Condensers for the gas microscope. 
 
 Fig. 27. The lowest power adapted for the gas polariscope or microscope. 
 On a scale of one quarter to an inch. 
 
 first being a plano-convex lens of 3 inches focus, and the 
 second a crossed lens of 4 inches focus. 
 
 Fig. 27. The lowest power, Fig. 20, adapted for the gas 
 apparatus by substituting an eye lens (a) of 1^ inch dia- 
 meter and 2^ inches focus, and placing it nearer the field- 
 lens. 
 
 (78.) The condensers, Fig. 25, are to be screwed into the 
 lantern, and the polariscope, Fig. 1 7 3 which is made to slide 
 over the condensers, must be turned sideways, and fastened 
 in a horizontal position by a bayonet catch: the stage, 
 Fig. 19, and power. Fig. 21, are then attached, and the 
 whole apparatus arranged at such an angle to the screen 
 as to counteract the angular position of the polariscope. 
 
 (79.) The most useful power for the gas polariscope or 
 microscope is that of Fig. 21, though the power, fig. 27, 
 maybe employed for very large objects; but with reference 
 to the crystals of calc. spar, nitre, &c. one of the modes 
 described in sec. 74 must be adopted. 
 
 (80.) When the instrument is to be used as a gas micro- 
 scope, the condensers and polariscope must be removed, 
 and the condensers, Fig. 26, screwed into the lantern; the 
 stage and power are then attached and fastened with the 
 bayonet catch as before described. 
 
 (81.) If the power, Fig. 2/, be substituted for that of 
 Fig. 21, and the microscope be placed at a distance of six 
 
44 ON POLARIZED LIGHT. 
 
 or seven feet from the screen, the image of a butterfly with 
 its wings extended may be so magnified as to occupy a 
 space of fifteen feet in diameter, and yet be clearly defined. 
 The chromatrope slides are also well exhibited with this 
 power. 
 
 (82.) Very small objects may, nevertheless, be beauti- 
 fully displayed by changing the powers and condensers. 
 The crossed lens of Fig. 25, and the eye-lens of the power, 
 Fig. 20, which is never used as such with the gas apparatus, 
 appear to form the best condensers for the high powers ; 
 these need not, however, be separately fitted up, as there 
 is no difficulty in changing the relative situations of any of 
 the lenses used as condensers ; but in all cases they must 
 be especially arranged with their most convex surfaces 
 towards each other, and these may be immediately distin- 
 guished by observing the difference between the images of 
 the flame of a lamp, or other object, as reflected by the 
 respective surfaces, for the more convex the lens the smaller 
 will be the reflected image. The bars of a window frame 
 will be reflected in parallel lines by the plain side of a lens, 
 but will appear to converge in proportion to the convexity 
 of the reflecting surface, and thus a slight difference of 
 convexity may be readily detected. It is also important 
 that the condensers should be allowed a sufficient space 
 for expansion in their cells ; if they be burnished in, or 
 screwed up too tightly, they will most probably be cracked 
 by the heat of the flame and incandescent lime. 
 
 (83.) Condensers of greater diameters than three inches 
 may be used ; but numerous experiments have tended to 
 prove that no adequate advantage is thereby obtained, 
 unless the apparatus be supplied with the separate gases 
 under very great pressure ; and, except under the same 
 circumstances, cylinders of soft lime or chalk are preferable 
 to those of hard lime, as the latter require to be submitted 
 to the action of the gases under much greater pressure 
 than the former, in order to produce an equally intense 
 degree of light. 
 
ON POLARIZED LIGHT. 45 
 
 The advantages of the above arrangement are 
 
 (84.) That the apparatus occupies a small space, is rea- 
 dily prepared for use, and can be managed without the 
 fatigue and inconvenience of filling large bags with gas, 
 and of procuring and lifting heavy weights. 
 
 (85.) That one quart of oxygen* and two of hydrogen 
 will be amply sufficient to enable the inquirer at any time 
 to submit his suggestions to the test of experiment. 
 
 (86.) That the proper relative proportions of the gases 
 are definitely fixed in the first instance. 
 
 (87.) That a waste of gas is prevented by the facility 
 with which it can be turned off and re-lighted. 
 
 (88.) That an increased degree of light for producing 
 any particular effect may be immediately obtained by the 
 action of the hand on the pressure board. 
 
 (89.) That Mr. Gurney's apparatus may be detached, 
 and, if furnished with his water safety chamber and jet, 
 may be used for illustrations in the lecture-room, as the 
 safest and most powerful hydro-oxygen blowpipe known. 
 
 (90.) Lastly, that the whole may be conveniently re- 
 moved, as required for the purposes of the lecturer. 
 
 The supposed disadvantages are 
 
 * In consequence of the greatly reduced price of chlorate of potass, oxygen 
 gas is now procured with as much facility as hydrogen. The best mode of 
 obtaining it is to mix one portion of the black oxide of manganese with 
 twice its weight of chlorate of potass, both previously reduced to a fine 
 powder, which may be kept in a bottle for use as required. If a four-ounce 
 glass retort be about half filled with this mixture, and the heat of a spirit 
 lamp be applied, a large quantity of pure oxygen will be liberated in. a few 
 minutes. The operation does not succeed so well with a larger retort; but if 
 four of these small retorts be used successively, as rruch oxygen will be 
 obtained in half an hour as will displace six or seven pails of water. The 
 residuum forms into a solid mass, but, being readily soluble in water, it may 
 then be emptied into a vessel, and allowed to settle, when the water may be 
 poured off, and the manganese, which only acts mechanically, may be dried, 
 and again repeatedly used. 
 
 The most convenient method of receiving the gases, is to fix, within two 
 inches of the bottom of a small trough or tub, a zinc receiver, furnished with 
 two stop cocks at the top, to which bladders may be attached, and alternately 
 changed, as one or the other becomes filled with gas. 
 
46 ON POLARIZED LIGHT. 
 
 (91.) First, that it is dangerous to use the gases previ- 
 ously mixed. 
 
 (92.) A series of experiments continued during many 
 years has, however, proved that, while the bladder contain- 
 ing the mixed gases is under pressure, the flame cannot be 
 made to pass the safety chambers, and, consequently, an 
 explosion cannot take place ; and even if through extreme 
 carelessness or design, as by the removal of pressure, or the 
 contact of a spark with the bladder, it should occur, it can 
 produce no other than the momentary effect of the alarm 
 occasioned by the report ; whereas, when the gases are 
 used in separate bags under a pressure of two or three half 
 hundred weights, if the pressure on one of the bags be 
 accidentally removed or suspended, the gas from the other 
 will be forced into it, and, if not discovered in time, will 
 occasion an explosion of a very dangerous character; or 
 if, through carelessness, one of the partially emptied bags 
 should be filled up with the wrong gas, effects of an equally 
 perilous nature would ensue. 
 
 (93.) And, secondly, that as the bladder F must be fre- 
 quently re-filled, the light cannot be continuous. 
 
 (94.) This is true, and forms some objection to the plan, 
 where an apparatus is to be permanently fixed for public 
 exhibition, though it might be obviated by adapting a 
 second bladder to the safety tubes ; but the objection does 
 not apply in the case of the lecturer, whose assistant will 
 have many opportunities of keeping up the supply of gas, 
 while the lecturer himself explains his subject, or changes 
 either the object or the power. 
 
 COMl'TON AND IUTCHIL, I'KlNTtiKS, MIDDLE STREET, CLOTH FAIR. 
 
PRICES OF SMITH & BECK'S 
 ACHROMATIC MICROSCOPES, 
 
 EXCLUSIVE OF OBJECT GLASSES AND APPARATUS. 
 
 *. d. 
 
 No. 1. Improved large Microscope, with limb continued under the 
 stage, and cylinder with rackwork, for applying all illu- 
 minating apparatus with greater ease and accuracy, 
 mounted on two pillars and cones, with revolving base 
 .to tripod ; body with quick and slow motions, graduated 
 sliding tube, and two corrected Huyghenian eye-pieces ; 
 stage half-inch thick, with vertical and horizontal actions 
 (given by rackwork and screw, or lever), sliding and re- 
 volving planes and spring clamping piece ; diaphragm 
 with revolving and removable fittings ; large plane and 
 concave mirrors, with lengthening arm ; pliers, joint 
 and forceps, and two glass plates . . . 21 
 
 Upright Honduras Mahogany Case for ditto, with one box 
 
 for apparatus . . . . . 2 10 
 
 Ditto, with two boxes . . . . .330 
 
 Solid Spanish ditto, with one box . . 3 10 
 
 Ditto, with two boxes . . . . 4 10 
 
 Solid Spanish Mahogany Case for ditto, with box for ap- 
 paratus, 58 drawers for 1248 objects, with numbered 
 knobs and porcelains, one sliding and two folding doors, 
 with glass panels . . . . . 15 
 
 No. 2. Improved smaller Microscope, on the same principle as 
 No. 1, but with single pillar, on tripod or firm folding 
 stand . . . . . . 15 15 
 
 Solid flat Mahogany Case . . . 1 10 
 
 Ditto ditto with drawer . . 1 15 
 
 No. 3. Large Microscope, similar to No. 1, but without limb 
 
 and cylinder under stage . . . .16160 
 
 Cases the same as for No. 1. 
 No. 4. Smaller Microscope, similar to No. 2, but without limb 
 
 and cylinder under stage . . . . 12 12 
 
 Cases the same as for No. 2. 
 No. 5. Best Student's Microscope, with uprights and joints, 
 
 complete actions, one eye-piece, and case . .990 
 
 No. 6. The above, with plain stage, consisting of sliding piece 
 and clamping spring, one eye-piece, other actions as 
 above, and case . . . . .660 
 
 No. 7. Smaller Student's Microscope, with actions similar to No. 6, 
 
 and case . . . . 5 10 
 
 No. 8. Small Microscope for Travelling, with all the actions to 
 stage and body, two eye-pieces, double mirror, and 
 packed in a case 10 inches by 4^ inches, and 3^ inches 
 
 deep 10 10 
 
 No. 9. The above, with plain stage . . . .770 
 
s. d. 
 
 No. 10. Plain Microscope for Hospital use, with quick and slow 
 motions to body, one eye-piece, and packed in mahogany 
 case . . . 3 10 
 
 No. 11. Dissecting Achromatic Microscope, double rackwork and 
 limb, f inch object glass, two eye -pieces, erecting glasses 
 and case, power from 5 to 100 linear 
 
 No. 12. Darwin's Improved Single Microscope, with one doublet, 
 three single lenses, and three Coddington lenses, Dis- 
 secting apparatus, complete, and packed in a case 
 
 12 12 
 
 10 
 
 ACHROMATIC OBJECT GLASSES FOR THE MICROSCOPE. 
 
 Focal 
 length. 
 
 Linear Magnifying 
 With Eye-piece 
 
 Power, nearly* 
 N.l N.2 N.3 
 
 Angle of 
 aperture 
 about 
 
 Price. 
 
 Lieber- 
 knhn 
 addi- 
 tional. 
 
 1^ inch 
 inch 
 
 T 4 y inch 
 Ditto 
 3 inch 
 
 3- inch 
 Ditto 
 J inch 
 Ditto 
 T V inch 
 
 Draw-tube closed 
 Add for each inch 
 of tube drawn out 
 
 Tube closed. . . 
 
 20 
 4 
 GO 
 7 
 120 
 12 
 do. 
 205 
 25 
 240 
 30 
 do. 
 450 
 40 
 do. 
 500 
 50 
 
 45 
 6 
 105 
 12 
 210 
 20 
 do. 
 360 
 35 
 430 
 45 
 do. 
 760 
 60 
 do. 
 920 
 70 
 
 80 
 8 
 180 
 20 
 350 
 35 
 do. 
 620 
 60 
 720 
 80 
 do. 
 1300 
 115 
 do. 
 1500 
 130 
 
 13 degrees 
 27 degrees 
 
 55 degrees 
 65 degrees 
 70 degrees 
 
 85 degrees 
 100 degrees 
 90 degrees 
 110 degrees 
 120 degrees 
 
 s. d. 
 300 
 
 330 
 
 550 
 660 
 550 
 
 660 
 770 
 770 
 880 
 10 10 
 
 15s. 
 11s. 
 
 10s. 
 10s. 
 6s. 
 
 Add for each inch 
 of tube 
 
 
 Add for each inch 
 of tube 
 
 ditto 
 
 Add for each inch 
 of tube 
 
 Tube closed. . 
 
 Add for each inch 
 of tube 
 
 ditto 
 Tube closed . 
 
 Add for each inch 
 of tube 
 
 ditto 
 
 Tube closed 
 Add for each inch 
 of tube 
 
 
 With the inch object glass and the erecting glasses, employing eye-pieces No. 1 and 2, 
 the magnifying power will range from 5 to 1 50. 
 
APPARATUS, 
 
 FOR THE MICROSCOPES No. 1 and 2 
 
 (Of lower price, on account of their more simple adaptation to those 
 instruments). 
 
 s. d. 
 
 Brasswork of Achromatic Condenser, with adjustments . .100 
 
 Combination of lenses for ditto, with 60 of aperture . . 1100 
 
 Ditto ditto ditto 75 ,, . 2 10 
 
 Ditto ditto ditto ditto with revolving 
 
 diaphragm for regulating the aperture . .400 
 
 Wenham's Parabolic Reflector, with sliding stop, and fittings . 1 15 
 Glass Parabolic Reflector, with fittings . . . 1 12 6 
 
 Amici's Prism for oblique light . . . . 1 10 
 
 Polarizing Apparatus, with selenite of one tint, and with revolving 
 
 fitting . . . . . . 2 10 
 
 Darker's Series of Selenites, extra to ditto . . .150 
 
 Bundle of Thin Glass for polarizing by reflection or transmission 110 
 
 ADDITIONAL APPARATUS FOR THE MICROSCOPES 
 IN GENERAL. 
 
 s. d. 
 
 Extra Eye-piece for large instruments . . . 17 6 
 
 Ditto for smaller ditto . . . .0150 
 
 Ditto for Hospital ditto . . . 13 
 
 Indicator to Eye-piece . . . . . .050 
 
 Erecting Glasses, for varying power, and for dissection . .100 
 
 Brasswork for Achromatic Condenser . . . 1 10 
 
 Combination of Lenses for ditto . . . from 110 
 
 Right-angled Prism . . . . . 1 10 
 
 Nachet's Prism . . . . . . 18 
 
 Parabolic Reflector, with sliding stop . . . 1 12 6 
 
 Rackwork Fitting for ditto . . . . .220 
 
 Sliding ditto ditto . . . . .0150 
 
 Bull's-eye Lens, on stand . . . . .110- 
 
 Side Silver Reflector . . . . . .110 
 
 Side Condenser, with ball and socket fitting . . .0180 
 
 Ditto on stand . . . . .0120 
 
 Polarizing Apparatus, with selenite . . . .2100 
 
 Darker's Selenite Stage . . . . .220 
 
 Double Image Prism, with fitting to eye-piece . . .0150 
 Two ditto, ditto, with selenite, and brass plate with holes . 220 
 Crystals to shew rings round the optic axis, and fitted to eye- 
 piece ...... each 10 6 
 
 Tourmalines ...... from 10 
 
 Wollaston's Camera Lucida, and fittings . . .100 
 
 Steel Disc . . ditto . . . .0150 
 
 Micrometer for Stage, mounted in brass . . .0100 
 
 Ditto ditto, unmounted . . . .050 
 
 Ditto for Eye-piece, with fittings and adjusting screw . .100 
 
 Three Dark Wells, and holder . . . . 12 6 
 
 Black and White Discs, fitted to a semicircle, with lengthening 
 
 arm and sliding socket . . . . .1150 
 
 Compressorium . . . . . .100 
 
 Wenham's ditto for Parabolic Reflector . . . 7 ft 
 
 Screw Live Box . . . . . 14 6 
 
 Large Live Box . . . . . . .086 
 
 Small ditto .... 6 6 
 
 Large Glass Trough, with wedge and spring complete . 8 6 
 
 Small ditto . . . . .050 
 
s. d. 
 
 Set of three Glass Tubes . . . .026 
 
 Glass Plates, with hollows and ledges . . . from 016 
 
 Three- pronged Forceps and Joint . . . 14 
 
 Usual Forceps, with joint . . . . .086 
 
 Large Brass Pliers, and smaller ditto . .050 and 026 
 
 Best Argand Lamp, with blue chimney . . .150 
 
 Small Camphine ditto, on improved principle . . .220 
 
 Iron Table, with revolving top for the Microscope . .550 
 
 MICROSCOPIC OBJECTS. 
 
 Vegetable Preparations : 
 
 Recent: Cells, Cuticles, Ducts, Fibre, Membrane, Spores, 
 Sporules, Tissues, Spiral and other Vessels, Hairs, Leaves, 
 Petals, Fungi, Sections of Woods, &c. . each specimen 016 
 Fossil: Sections of various exogenous and endogenous 
 
 Woods ..... each section 1 6 
 
 Slides, with two and three sections . . 030 and 046 
 
 Desmidieae and Algae .... each slide 016 
 
 Diatomaceae : Recent : Several hundred varieties, including species 
 of Campilodiscus, Cocconema, Epithemia, Navicula, Suri- 
 rella, Synedra, &c. Fossil : Specimens from various loca- 
 lities in the British Islands, Germany, Italy, North America, 
 the East and West Indies, New Zealand, &c. .016 
 
 Spicules and Gemmules of Sponges and Gorgonias . .016 
 
 Zoophites, many species . . . . .020 
 
 Shells, sections of various species . . . .016 
 
 Echinus Spines, Sections in great variety . . .016 
 
 Entomological Preparations : 
 
 Antennae, Eyes, Feet, Hairs, Scales, Skins, Spiracles, Stings, 
 Stomachs, Tongues, Tracheae, Wings, &c. Specimens of 
 numerous Acari and Parasites . . . .016 
 
 Hairs, Whiskers of various Animals mounted whole or in section, 
 
 Quills, Feathers of Birds, &c. . . . .016 
 
 Objects from human and other Bodies . . . .016 
 
 Anatomical Preparations : 
 
 Blood Discs, Pigment Cells, Skin, Muscular Fibre, Tissues, 
 
 &c. ...... from 1 6 
 
 Bones: 
 
 Transverse and Vertical Sections of 60 or 70 Recent and 
 
 Fossil Mammals, Birds, Reptiles, and Fishes each slide 016 
 Teeth : 
 
 Transverse and Vertical Sections of about 30 varieties, Recent 
 
 and Fossil ..... each 020 
 
 Injected Preparations ..... from 026 
 
 Polariscope Objects, about 100, selected from Animal, Vegetable, 
 
 and Mineral Substances .... each 016 
 
 Mineralogical : 
 
 Sections of Limestones, Oolites, Flints, Agates, Sic. each from 016 
 
 CABINETS FOR OBJECTS, 
 
 In which the Specimens lie flat, and with porcelain Labels to the Drawers. 
 
 Best Spanish Mahogany Cabinet, with glass panel, and to hold 
 
 1000 objects . . . . . .700 
 
 Honduras ditto, without glass panel . . . .660 
 
 Best Spanish Mahogany Cabinet, with glass panel, and to hold 
 
 750 objects . . . .5150 
 
 Honduras ditto, without glass panel . . . .500 
 
5 
 
 t. d. 
 Best Spanish Mahogany Cabinet, with glass panel, and to hold 
 
 500 objects . . . . . .440 
 
 Honduras ditto, without glass panel . . . 3 10 
 
 Cabinets made to any size, and of every description. 
 
 Cardboard Boxes, to hold two dozen objects . . each 010 
 
 Ditto ditto one dozen ditto . . ,,009 
 
 Ditto ditto half dozen ditto . . ,,006 
 
 Single Mahogany Racks, for objects . . per foot 006 
 
 Double ditto ditto 1 
 
 INSTRUMENTS USED IN PREPARING OBJECTS. 
 
 Wood-cutting Machine, with knife . . . .110 
 
 Instrument for Cutting Circles of thin Glass . . .150 
 
 Diamond for Cutting thin Glass . . . .0150 
 
 Ditto for plate and window Glass . . . .0120 
 
 Ditto for Writing . . . . 5s. 6d. & 7 6 
 
 Instrument for Making Cells of gold size and other fluids . 076 
 
 Page's Wooden Forceps, for holding the glass slides when warmed 5 <> 
 Quekett's Forceps for deep jars . . . .076 
 
 Iron work of Dredge, for deep water fishing . . .150 
 
 Small collecting Bottles . . per doz. Is. 6d. to 3 
 
 Small Dissecting Knives .... each 030 
 
 Valentine's Knife, for making sections of soft substances . 0150 
 
 Spring Scissors, 7s. 6d. ; Curved ditto, 5s. ; Straight ditto, 3s. 
 Needle Holders, 5s. ; Hooks, 2s. 6d. ; Points, Is. 6d. 
 Cutting Forceps, 5s. 6d. ; Spring ditto, 3s. 
 Combination of Lenses, mounted in tortoiseshell, for pocket hand 
 
 magnifiers . . . . . 10 6 
 
 Ditto, with small brass stand . . . . .0180 
 
 Coddington Lenses, in various mountings . . from 066 
 
 MATERIALS USED IN MOUNTING OBJECTS. 
 
 Canada Balsam, Asphalt, Gold Size, Glycerine, &c. Is. and 2s. bottles. * 
 Thin Glass, in circles, 6s. per oz. ; in squares, 4s. per oz. ; ditto, mixed, 5s. 
 Plate-glass Slips, 3 inches by 1 inch, with ground edges, Is. per dozen. 
 Glass Cells, square, round, oblong, oval, and with solid bottoms, 2s. 6d. and 
 
 3s. per dozen. 
 Labels for covering objects, 3s. per hundred. 
 
 WOODWARD'S TABLE AND HYDRO-OXYGEN 
 
 POLABISCOPE AND MICROSCOPE. 
 
 s. d. 
 
 Woodward's Table Polariscope and Microscope, with polarizing 
 bundle, black glass, silvered reflector, and ground glass 
 shade ; large stage, with complete fittings, and two powers 
 with rackwork motions, smaller stage for crystals ; and 
 box as stand, with complete fittings for apparatus . 10 
 
 The above fitted as a Hydro- Oxygen Apparatus, with lantern, 
 safety jet, bladders, and pressure boards ; the Polariscope 
 and Microscope Condensers, and an eye-lens to adapt 
 to the lowest power .... extra 15 
 Clock for the Movement of Lime Cylinder . . .440 
 
 Lime Cylinders ..... per dozen 036 
 
 Tourmalines for the above instrument . . . from 100 
 
SELENITE OBJECTS. 
 
 Gothic Window, in imitation of stained glass 
 
 Parrot, Pheasant, or Peacock 
 
 Bird of Paradise .... 
 
 Jem Crow, Changeable Lady, Casper, Harlequin, 
 or Robin Hood 
 
 Miller turned Sweep, or Bunch of Grapes . 
 
 Chameleon ..... 
 
 Butterfly or Heartsease . 
 
 Shamrock, motto "Erin-go-bragh;" or Thistle, motto 
 "Dinna forget" 
 
 The Flower Forget-me-not, motto " Forget-me-not" 
 
 Flag (Iris) or Rose 
 
 Tulip, or Rosette 
 
 Uneven tints of Selenite, Is. ; Even ditto, 5s. 
 
 Single wedges, 5s. ; double ditto, 7s. 6d. 
 
 Rings (Newton's) . . . from 
 
 Crosses and .Discs of various colors for superposing 
 
 Crystals cut to show rings round the optic axis, in 
 
 sets of six .... from 
 
 Glass, Quartz, and other Prisms ; together with all 
 matters relating to Polarization. 
 
 Butterflies, Insects, and parts of ditto, Ferns, 
 Mosses, Sections of Woods, &c. for the Mi- 
 croscope ..... from 
 
 
 in 1 
 
 
 2 
 
 I 
 
 
 
 
 lotto 
 
 ot" . 
 
 s . 
 
 
 15 
 
 10 
 
 
 15 
 
 7 
 
 d. 
 
 
 
 
 
 
 6 
 
 to 
 to 
 to 
 
 to 
 to 
 to 
 
 j 
 10 
 6 
 4 
 
 2 
 1 
 
 
 
 
 
 
 
 
 
 3- 
 
 
 
 
 
 
 
 15 
 10 
 
 7 
 10 
 7 
 5 
 
 <t. 
 
 
 
 
 
 
 
 
 
 6 
 
 
 6 
 
 
 
 5 to 7 6* 
 10 
 
 18 Oto 3 
 
 1 6 to 5 
 
 ASTRONOMICAL TELESCOPES & PORTABLE ACHROMATIC DITTO, 
 
 DAY, NIGHT, AND NAVAL. 
 
 SPECTACLES, EYE GLASSES, READING GLASSES, 
 OPERAS, &c. 
 
 Magic Lanterns and Philosophical Apparatus. 
 
 CAMERA LUC IDAS AND DRAWING INSTRUMENTS. 
 
 Barometers, Thermometers, Hygrometers, &c. 
 

UtNtMAL LIBHAHY - U.C. BE 
 
 THE UNIVERSITY OF CALIFORNIA LIBRARY