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LIGHT 
 
LONDON ; PRINTED BY 
 
 SPOTTI3WOOD3 AND CO., NEW-STREET SQUARE 
 AND PARLIAMENT STREET 
 
LIBRA U L 
 
 UNIVERSITY OF 
 
 CALIFORNIA. 
 
SIX LECTURES ON LIGHT 
 
 DELIVERED IX AMERICA IN 1872-1873 
 
 BY 
 
 JOHN TYNDALL, D.C.L. LL.T). F.RS. 
 
 PROFESSOR OP NATURAL PHILOSOPHY IX THE ROYAL INSTITUTION 
 
 SECOND EDITION 
 
 /LI 15 II. A U 1 
 
 UN1VKKS.TY OF, 
 
 iiife^^ 
 
 LONDON 
 LONGMANS, GEEEN, AND CO. 
 
 1875 
 

CAT. 
 
 PBEFACE 
 
 TO THE 
 
 SECOND ENGLISH EDITION. 
 
 THE REASONS for giving these Lectures are briefly set 
 forth in the Introduction to the first of them ; while 
 the grounds of their publication are stated in the 
 6 Preface to the American Edition.' 
 
 During their delivery, the experimental facts were 
 before the audience, forming visible links in the 
 logic of each discourse. Here, by the use of plain 
 language, I have endeavoured to reproduce distinct 
 images of these facts, and to show them in their proper 
 relations. 
 
 With a view to this end I have sought to raise the 
 Wave-theory of Light to adequate clearness in the 
 reader's mind, and to show its power as an organizer of 
 optical phenomena. 
 
 From what has been recently written on such ques- 
 tions, it is to be inferred that the origin, scope, and 
 
VI PEEFACE TO THE 
 
 warrant, of physical theories generally, constitute a 
 theme of considerable interest to thoughtful minds. 
 On these points I have ventured, particularly in the 
 second and third Lectures, to state the views which my 
 own reflections have suggested to me. 
 
 To produce a systematic treatise on Light was, of 
 course, quite wide of my aim. My desire rather was to 
 throw into a small compass, an exposition for which I 
 should have been grateful at a certain period of my 
 own studies. I wished in the first place, as the prime 
 condition of all satisfactory progress, to clear the reader's 
 mind of all indistinctness regarding elementary facts 
 and conceptions ; and to whet incidentally the desire 
 for further knowledge. 
 
 I wished, moreover, for the sake of that numerous 
 portion of the community who are interested in the 
 material results of science, to trace effects to their causes, 
 by showing how such results receive their primary 
 vitalization from the thoughts of men with no material 
 end in view. The ' Summary and Conclusion,' which 
 might be read as an introduction, is for the most part 
 devoted to this object. I have added in an Appendix 
 three brief Addresses by distinguished Americans, which 
 possess more than a passing interest. 
 
 To the first English edition of these Lectures, Dr. 
 Young's ' Reply to the Edinburgh Eeviewers ' was ap- 
 pended. Numbers of scientific men were, to my 
 knowledge, but imperfectly acquainted with this great 
 
SECOND ENGLISH EDITION. vii 
 
 discussion ; while the general public knew nothing 
 whatever about it. The end contemplated having been 
 gained, the ' Reply ' is here omitted ; and in lieu of 
 it a portrait of Dr. Young, engraved with great success 
 by Mr. Adlard, forms the frontispiece of the volume. 
 
 JOHN TYNDALL. 
 
 ROYAL INSTITUTION: May 1875. 
 
PBEFACE 
 
 TO 
 
 THE AMERICAN EDITION. 
 
 MY eminent friend Professor JOSEPH HENRY, of the 
 Smithsonian Institution, Washington, did me the 
 honour of taking these Lectures under his personal 
 direction, and of arranging the times and places at 
 which they were to be delivered. 
 
 Believing that my home duties could hardly be 
 suspended for a longer period, I did not, at the outset, 
 expect to be able to prolong my visit to the United 
 States beyond the end of 1872. 
 
 Thus limited as to time, Professor HENRY began 
 in the North, and, proceeding southwards, arranged 
 for the successive delivery of the lectures in Boston, 
 New York, Philadelphia, Baltimore, and Washington. 
 
 By this arrangement, which circumstances at the 
 time rendered unavoidable, the lectures in New York 
 would have been rendered coincident with the period of 
 the presidential election. This was deemed unsatisfac- 
 
X PREFACE TO 
 
 tory ; and the fact being represented to me, I at once 
 offered to extend the time of my visit so as to make the 
 lectures in New York succeed those in Washington. 
 This proposition was cordially accepted by my friends. 
 
 To me personally this modified arrangement has 
 proved both pleasant and beneficial. It gave me a 
 much-needed and delightful holiday at Niagara Falls ; 
 it, moreover, rendered the successive stages of my 
 work a kind of growth^ which reached its most im- 
 pressive development in New York and Brooklyn. 
 
 My reception throughout has been that of a friend 
 by friends ; and now that my visit has become virtually 
 a thing of the past, I look back upon it as a memory 
 without a single stain of unpleasantness. Excepting 
 one inexorable event, nothing has occurred that I could 
 wish not to have occurred ; while from beginning to 
 end I have been met by expressions of good-will on the 
 part of the American people never, on my part, to be 
 forgotten. Indeed, ' good-will ' is not the word to ex- 
 press the kindness manifested towards me in the United 
 States. 
 
 Would that it had been in my power to meet the wishes 
 of my friends more completely, by responding to the invi- 
 tations sent to me from the great cities of the Interior and 
 the West, and from Canada. But the character of the 
 lectures, and their weight of instrumental appliances, 
 involved such heavy labour that the need of rest alone 
 would be a sufficient reason for my pausing here. 
 
THE AMEKICAN EDITION. xi 
 
 Besides this, each successive mail from London brings 
 me intelligence of work suspended and duties post- 
 poned through my absence. 
 
 The Royal Institution possesses an honorary secretary 
 who has devoted the best years of an active professional 
 life and the best energies of a strong mind to its 
 interests. And if anything of the kind should ever be 
 founded here, the heartiest wish that I could offer for 
 its success would be, that it may be served with the 
 singleness of purpose, and self-sacrificing love, bestowed 
 by its managers and its members on the Royal Institu- 
 tion ; and by none more unceasingly than by Dr. BENCE 
 JONES. But he, on whom I might rely, is now struck 
 down by a distressing illness ; 1 and, though others are 
 willing to aid me in all possible ways, there can be no 
 doubt as to my line of duty. I ought to be at home. 
 I ask my friends in the Interior and the West, and in 
 Canada, to take these things into consideration ; and to 
 think of me not as one insensible to their kindness, 
 but as one who, with a warmth commensurate with their 
 own, would comply with all their wishes if he could. 
 
 One other related point deserves mention. On quit- 
 ting England I had no intention of publishing these 
 Lectures, and, except a fragment or two, they were wholly 
 unwritten when I arrived in this city. Since that time, 
 besides lecturing in New York, Brooklyn, and New 
 
 1 He died, working for the Institution to the last, on Sunday morning, 
 April 20, 1873. 
 
xii PKEFACE TO 
 
 Haven, the Lectures have been written out and carried 
 through the Press. Many evidences of the rapidity, of 
 their production will appear ; but I thought it due to 
 those who listened to them with such unwavering atten- 
 tion, as also to those who wished to hear them, but were 
 unable to do so, to leave them behind me in an approxi- 
 mately authentic form. 
 
 The constant application which this work rendered 
 necessary has cut me off from many social pleasures ; it 
 has prevented me from making myself acquainted with 
 the working of institutions in which I feel a deep 
 interest, and from availing myself of the generous 
 hospitality offered to me by the clubs of New York. 
 In short, it has made me an unsociable man. But, find- 
 ing social pleasure and hard work incompatible, I took 
 the line of devoting such energy as I could command, 
 not to the society of my intimate friends alone, but to 
 the people of the United States. 
 
 In the opening lecture are mentioned the names of 
 gentlemen to whom I am under lasting obligations for 
 their friendly, and often laborious aid. The list might 
 readily be extended, for in every city visited willing 
 helpers were at hand. I must not, however, omit the 
 name of Mr. RHEES, Professor HENRY'S private secretary, 
 who not only in Washington, but in Boston, gave rne 
 most important assistance. To the Trustees of the 
 Cooper Institute my acknowledgments are due ; and to 
 the Directors of the Mercantile Library at Brooklyn. 
 
THE AMEEICAN EDITION. xiii 
 
 I would add to these a brief but grateful reference to 
 my high-minded friend and kinsman General HECTOR 
 TYNDALE, for his long-continued care of me, and for the 
 thoughtful tenderness by which he and his family 
 softened, both to me and to the parents of the youth, 
 the grief occasioned by the death rff my junior assistant 
 in Philadelphia. 
 
 Finally, I have to mention with warm commendation 
 the integrity, ability, and devotion with which, from 
 first to last, I have been aided by my principal assistant, 
 Mr. JOHN COTTRELL. 
 
 JOHN TYNDALL. 
 
 NEW YORK: February 1873 
 
CONTENTS. 
 
 LECTUKE I. 
 
 TAGE 
 
 Introductory Uses of Experiment Early Scientific Notions 
 Sciences of Observation Knowledge of the Ancients regarding 
 Light Defect of the Eye Our Instruments Eectilineal Pro- 
 pagation of Light Law of Incidence and Reflection Sterility 
 of the Middle Ages Refraction Discovery of Snell Partial and 
 Total Reflection Velocity of Light Rcemer, Bradley, Foucault, 
 and Fizeau Principle of Least Action Descartes and the Rain- 
 bow Newton's Experiments on the Composition of Splar Light 
 His Mistake regarding Achromatism Synthesis of "White Light 
 Yellow and Blue Lights produce White by their Mixture 
 Colours of Natural Bodies Absorption Mixture of Pigments 
 contrasted with Mixture of Lights 1 
 
 LECTURE II. 
 
 Origin of Physical Theories Scope of the Imagination Newton* 
 and the Emission Theory Verification of Physical Theories 
 The Luminiferqus,Eth^rj- Wave-theory of Light Thomas 
 Young Fresnel and Arago Conception of Wave-motion 
 Interference of Waves Constitution of Sound-waves Analogies 
 of Sound and Light Illustrations of Wave-motion Interference 
 of Sound Waves Optical Illustrations Pitch and Colour 
 Lengths of the Waves of Light and Rates of Vibration of the 
 Ether-particles Interference of Light Phenomena which first 
 suggested the Undulatory Theory Boyle and Hooke The 
 Colours of thin Plates The Soap-bubble Newton's Rings 
 Theory of ' Fits ' Its Explanation of the Rings Overthrow of 
 the Theory Diffraction of Light Colours produced by Diffrac- 
 tion Colours of Mother-of-pearl 42 
 
xvi CONTENTS. 
 
 LECTUKE III. 
 
 PAGE 
 
 Eelation of Theories to Experience Origin of the Notion of the 
 Attraction of Gravitation Notion of Polarity, how generated 
 Atomic Polarity Structural Arrangements due to Polarity 
 Architecture of Crystals considered as an Introduction to their 
 Action upon Light Notion of Atomic Polarity applied to Crys- 
 talline Structure Experimental Illustrations Crystallization 
 of Water Expansion by Heat and by Cold Deportment of 
 "Water considered and explained Bearings of Crystallization on 
 Optical Phenomena Refraction Double Eefraction Polariza- 
 tion Action of Tourmaline Character of the Beams emergent 
 from Iceland Spar Polarization by ordinary Refraction and 
 Reflection Depolarization 95 
 
 LECTURE IV. 
 
 Chromatic Phenomena produced by Crystals in Polarized Light 
 The Nicol Prism Polarizer and Analyzer Action of Thick and 
 Thin Plates of Selenite Colours dependent on Thickness Reso- 
 lution of Polarized Beam into two others by the Selenite One 
 of them more retarded than the other Recompounding of the 
 two Systems of Waves by the Analyzer Interference thus 
 rendered possible Consequent Production of Colours Action 
 of Bodies mechanically strained or pressed Action of Sonorous 
 Vibrations Action of Glass strained or pressed by Heat Cir- 
 cular Polarization Chromatic Phenomena produced by Quartz 
 The Magnetization of Light Rings surrounding the Axes of 
 Crystals-r-Biaxal and Uniaxal Crystals Grasp of the Undu- 
 latory Theory The Colour and Polarization of Sky-light 
 Generation of Artificial Skies 124 
 
 LECTURE V. 
 
 Range of Vision not commensurate with Range of Radiation The 
 Ultra-violet Rays Fluorescence Rendering invisible Rays 
 visible Vision not the only Sense appealed to by the Solar and 
 Electric Beam Heat of Beam Combustion by Total Beam at 
 the Foci of Mirrors and Lenses Combustion through Ice-lens 
 Ignition of Diamond Search for the effective Rays Sir 
 William Herschel's Discovery of dark Solar Rays Invisible Rays 
 the Basis of the Visible Detachment by a Ray-filter of the 
 Invisible Rays from the Visible Combustion at Dark Foci 
 Conversion of Heat-rays into Light-rays Calorescence Part 
 
CONTENTS. xvii 
 
 played in Nature by Dark Kays Identity of Light and Radiant 
 Heat Invisible Images Reflection, Refraction, Plane Polariza- 
 tion, Depolarization, Circular Polarization, Double Refraction, 
 and Magnetization *of Radiant Heat 162 
 
 LECTURE VI. 
 
 Principles of Spectrum Analysis Prismatic Analysis of the Light 
 of Incandescent Vapours Discontinuous Spectra Spectrum 
 Bands proved by Bunsen and Kirchhoff to be characteristic of the 
 Vapour Discovery of Rubidium, Caesium, and Thallium 
 Relation of Emission to Absorption The Lines of Fraunhofer 
 . Their Explanation by Kirchhoff Solar Chemistry involved in 
 this Explanation Foucault's Experiment Principles of Ab- 
 sorption Analogy of Sound and Light Experimental Demon- 
 stration of this Analogy Recent Applications of the Spectro- 
 scope Summary and Conclusion . . . . . .192 
 
 APPENDIX. 
 
 President Barnard's Address . . . . * . . . 229 
 
 Professor Draper's Address 235 
 
 President White's Remarks 238 
 
 Professor Tyndall's Remarks 242 
 
 Measurement of the Waves of Light 247 
 
 Water Crystallization . 249 
 
 On the Spectra of Polarized Light 250 
 
 INDEX . 265 
 
ILLUSTRATIONS. 
 
 THOMAS YOUNG ........ Frontispiece 
 
 PLUMES PRODUCED BY THE CRYSTALLIZATION OF 
 
 WATER . To face p. 249 
 
1, 1 
 
 ON LIGHT. 
 
 LECTURE I. 
 
 INTRODUCTORY USES OF EXPERIMENT EARLY SCIENTIFIC NOTIONS 
 
 SCIENCES OF OBSERVATION KNOWLEDGE OF THE ANCIENTS REGARD- 
 ING LIGHT DEFECTS OF THE EYE OUR INSTRUMENTS RECTILINEAL 
 PROPAGATION OF LIGHT LAW OF INCIDENCE AND REFLECTION STERIL- 
 ITY OF THE MIDDLE AGES REFRACTION DISCOVERY OF SNELL 
 
 PARTIAL AND TOTAL REFLECTION VELOCITY OF LIGHT RCEMER, 
 
 BRADLEY. FOUCAULT, AND F1ZEAU PRINCIPLE OT LEAST ACTION 
 DESCARTES AND THE RAINBOW NEWTON'S EXPERIMENTS ON THE 
 COMPOSITION OF SOLAR LIGHT HIS MISTAKE AS REGARDS ACHROMAT- 
 ISM SYNTHESIS OF WHITE LIGHT YELLOW AND BLUE LIGHTS PRODUCE 
 
 WHITE BY THEIR MIXTURE COLOURS OF NATURAL BODIES AB3ORP* 
 
 TION MIXTURE OF PIGMENTS CONTRASTED WITH MIXTURE OF 
 LIGHTS. 
 
 1. Introduction. 
 
 SOME twelve years ago I published, in England, a 
 little book entitled the ' Glaciers of the Alps,' and, 
 a couple of years subsequently, a second book, en- 
 titled ' Heat as a Mode of Motion.' These volumes were 
 followed by others, written with equal plainness, and with 
 a similar aim, that aim being to develope and deepen 
 sympathy between science and the world outside of 
 science. I agreed with thoughtful men l who deemed 
 it good for neither world to be isolated from the other, 
 
 1 Among whom may be mentioned, especially, the late Sir Edmund 
 Head, Bart., with whom I had many conversations on this subject. 
 
 B 
 
2 ON LIGHT. I.ECT. 
 
 or unsympathetic towards the other, and, to lessen 
 this isolation, at least in one department of science, I 
 swerved aside from those original researches which had 
 previously been the pursuit and pleasure of my life. 
 
 The works here referred to were, for the most part, 
 republished by the Messrs. Appleton of New York, 1 
 under the auspices of a man who is untiring in his 
 efforts to diffuse sound scientific knowledge amorg the 
 people of the United States : whose energy, ability, 
 and single-mindedness, in the prosecution of an arduous 
 task, have won for him the sympathy and support 
 of many of us in ' the old country.' I allude to 
 Professor Youmans. Quite as rapidly as in England, 
 the aim of these works was understood and appreciated 
 in the United States, and they brought me from this 
 side of the Atlantic innumerable evidences of good- 
 will. Year after year invitations reached me 2 to 
 visit America, and last year I was honoured with a 
 request so cordial, signed by five-and-twenty names so 
 distinguished in science, in literature, and in adminis- 
 trative position, that I at once resolved to respond to 
 it by braving not only the disquieting oscillations 
 of the Atlantic, but the far more disquieting ordeal 
 of appearing in person before the people of the 
 United States. 
 
 This invitation, conveyed to me by my accom- 
 plished friend Professor Lesley, of Philadelphia, and 
 preceded by a letter of the same purport from your 
 scientific Nestor, the celebrated Joseph Henry, of 
 
 1 At whose hands it gives me pleasure to state I have always ex- 
 perienced honourable and liberal treatment. 
 
 One of the earliest of these came from Mr. John Amory Lowell of 
 Boston. 
 
i. INTRODUCTORY, USES OF EXPERIMENT. 3 
 
 Washington, desired that I would lecture in some of 
 the principal cities of the Union. This I agreed to 
 do, though much in the dark as to a suitable subject. 
 In answer to my inquiries, however, I was given to 
 understand that a course of lectures showing the uses of 
 experiment in the cultivation of Natural Knowledge 
 would materially promote scientific education in this 
 country. And though such lectures involved the selec- 
 tion of weighty and delicate instruments, and their 
 transfer from place to place, I at once resolved to meet 
 the wishes of my friends as far as the time and means 
 at my disposal would allow. 
 
 2. Subject of the Course. Source of Light 
 employed. 
 
 Experiments have two great uses a use in dis- 
 covery and verification, and a use in tuition. They 
 were long ago defined as the investigator's language 
 addressed to Nature, to which she sends intelligible 
 replies. These replies, however, usually reach the ques- 
 tioner in whispers too feeble for the public ear. But 
 after the discoverer comes the teacher, whose function 
 it is so to exalt and modify the experiments of his pre- 
 decessor as to render them fit for public presentation. 
 This secondary function I shall endeavour, in the present 
 instance, to fulfil. 
 
 I propose to take a single department of natural 
 philosophy, and illustrate, by means of it, the growth 
 of scientific knowledge under the guidance of experi- 
 ment. I wish, in this first lecture, to make you 
 acquainted with certain elementary phenomena ; then 
 to point out to you how those theoretic principles by 
 which phenomena are explained, take root, and flourish 
 
 B 2 
 
4 ON LIGHT. LECT. 
 
 in the human mind, and afterwards to apply these prin- 
 ciples to the whole body of knowledge covered by the 
 lectures. The science of optics lends itself to this 
 mode of treatment, and on it, therefore, I propose to 
 draw for the materials of the present course. It will 
 be best to begin with the few simple facts regarding 
 light which were known to the ancients, and to pass 
 from them in historic gradation to the more abstruse 
 discoveries of modern times. 
 
 All our notions of Nature, however exalted or how- 
 ever grotesque, have some foundation in experience. 
 The notion of personal volition in Nature had this basis. 
 In the fury and the serenity of natural phenomena the 
 savage saw the transcript of his own varying moods, 
 and he accordingly ascribed these phenomena to beings 
 of like passions with himself, but vastly transcending 
 him in power. Thus the notion of causality the as- 
 sumption that natural things did not come of them- 
 selves, but had unseen antecedents lay at the root of 
 even the savage's interpretation of Nature. Out of 
 this bias of the human mind to seek for the ante- 
 cedents of phenomena all science has sprung. 
 
 We will not now go back to man's first intellectual 
 gropings ; much less shall we enter upon the thorny dis- 
 cussion as to how the groping man arose. We will take 
 him at a certain st age of his development, when, by evo- 
 lution or sudden endowment, he became possessed of the 
 apparatus of thought and the power of using it. For 
 a time and that historically a long one he was limited 
 to mere observation, accepting what Nature offered, 
 and confining intellectual action to it alone. The ap- 
 parent motions of sun and stars first drew towards them 
 the questionings of the intellect, and accordingly astro- 
 
i. PE.OGRESS OF THE ANCIENTS. 5 
 
 nomy was the first science developed. Slowly, and with 
 difficulty, the notion of natural forces took root in the 
 human mind. Slowly, and with difficulty, the science 
 of mechanics had to grow out of this notion ; and slowly 
 at last came the full application of mechanical princi- 
 ples to the motions of the heavenly bodies. We trace 
 the progress of astronomy through Hipparchus and 
 Ptolemy ; and, after a long halt, through Copernicus, 
 Galileo, Tycho Brahe, and Kepler ; while from the high 
 table-land of thought raised by these men Newton shoots 
 upward like a peak, overlooking all others from his 
 dominant elevation. 
 
 But other objects than the motions of the stars at- 
 tracted the attention of the ancient world. Light was 
 a familiar phenomenon, and from the earliest times we 
 find men's minds busy with the attempt to render some 
 account of it. But without experiment, which belongs 
 to a later stage of scientific 'development, little progress 
 could be made in this subject. The ancients, accord- 
 ingly, were far less successful in dealing with light 
 than in dealing with_sojar and stellar motions. Still 
 they did make some progress. They satisfied them- 
 selves that light moved in -straight lines ; they knew 
 also- that light was reflected from polished surfaces, and " 
 that the angle of incidence of the rays of light was 
 equal to the angle of reflection. These two results of 
 ancient scientific curiosity constitute the starting-point 
 of our present course of lectures. 
 
 But in the first place it will be useful to say a few 
 words regarding the source of light to be employed in 
 our experiments. The rusting of iron is, to all intents 
 and purposes, the slow burning of iron. It developes 
 
6 ON LIGHT. LECT. 
 
 heat, and, if the heat be preserved, a high temperature 
 may be thus attained. The destruction of the first 
 Atlantic cable was probably due to heat developed in 
 this way. Other metals are still more combustible 
 than iron. You may light strips of zinc in a candle 
 flame, and cause them to burn almost like strips of 
 paper. But we must now expand our definition of 
 combustion, including under this term not only com- 
 bustion in air, but also combustion in liquids. Water, 
 for example, contains a store of oxygen, which may 
 unite with and consume a metal immersed in it ; it is 
 from this kind of combustion that we are to derive the 
 heat and light employed in our present course. 
 
 The generation of this light and of this heat merits 
 a moment's attention. Before you is an instrument 
 a small voltaic battery in which zinc is immersed in 
 a suitable liquid. An attractive force is at this 
 moment exerted between the metal and the oxygen 
 of the liquid ; actual union, however, being in the 
 first instance avoided. IJniting the two ends of the 
 battery by a thick wire, the attraction is satisfied, 
 the oxygen unites with the rnetal, zinc is consumed, 
 and heat, as usual, is the result of the combustion. A 
 power which, for want of a better name, we call an 
 electric current, passes at the same time through the 
 wire. 
 
 Cutting the thick wire in two, let the severed ends 
 be united by a thin one. It glows with a white heat. 
 Whence comes that heat ? The question is well worthy 
 of an answer. Suppose in the first instance, when 
 the thick wire is employed, that we permit the action 
 to continue until 100 grains of zinc are consumed, the 
 amount of heat generated in the battery would be 
 
i. SOURCE OF LIGHT. 7 
 
 capable of accurate numerical expression. Let the 
 action then continue, with the thin wire glowing, until 
 100 grains of zinc are consumed. Will the amount of 
 heat generated in the battery be the same as before ? 
 No, it will be less by the precise amount generated in 
 the thin wire outside the battery. In fact, by adding 
 the internal heat to the external, we obtain for the 
 combustion of 100 grains of zinc a total which never 
 varies. We have here a beautiful example of that law of 
 constancy as regards natural energies, the establishment 
 FIG. 1. 
 
 of which is the greatest achievement of modern scientific 
 philosophy. By this arrangement, then, we are able to 
 burn our zinc at one place, and to exhibit the effects 
 of its combustion at a distance. In New York, for 
 example, we may have our grate and fuel; but the heat 
 and light of our fire may be made to appear at San 
 Francisco. 
 
 Removing the thin wire and attaching to the severed 
 ends of the thick one two rods of coke, we obtain, on 
 bringing the rods together (as in fig. 1 ), a small star of 
 
3 ON LIGHT. LECT. 
 
 light. Now, the light to be employed in our lectures 
 is a simple exaggeration of this star. Instead of being 
 produced by ten cells, it is produced by fifty. Placed 
 in a suitable camera, provided with a suitable lens, 
 this powerful source will give us all the light necessary 
 for our experiments. 
 
 And here, in passing, I am reminded of the common 
 delusion that the works of Nature, the human eye in- 
 cluded, are theoretically perfect. The eye has grown 
 for ages toivards perfection ; but ages of perfect- 
 ing may be still before it. Looking at the dazzling 
 light from our large battery, I see a luminous globe, 
 but entirely fail to see the shape of the coke-points 
 whence the light issues. The cause may be thus made 
 clear : On the screen before you is projected an image 
 of the carbon points, the whole of the lens in front 
 of the camera being employed to form the image. It 
 is not sharp, but surrounded by a halo which nearly 
 obliterates the carbons. This arises frarn an imperfec- 
 tion of the lens, called its spherical aberration, due to 
 the fact that the circumferential and central rays have 
 not the same focus. The human eye labours under a 
 similar defect, and from this and other causes it arises 
 that when the naked light from fifty cells is looked at, 
 the blur of light upon the retina is sufficient to destroy 
 the definition of the retinal image of the carbons. A 
 long list of indictments might indeed be brought against 
 the eye its opacity, its want of symmetry, its lack of 
 achromatism, its absolute blindness, in part. All these 
 taken together caused Helmholtz to say that, if any 
 optician sent him an instrument so full of defects, he 
 would be justified in sending it back with the severest 
 censure. But the eye is not to be judged from the 
 
i. RECTILINEAL PROPAGATION. 
 
 stand-point of theory. It is not perfect, as I have said, 
 but on its way to perfection. As a practical instrument, 
 and taking the adjustments by which its defects are 
 neutralized into account, it must ever remain a marvel 
 to the reflecting mind. 
 
 3. Rectilineal Propagation of Light. Elementary 
 Experiments. Law of Reflection. 
 
 The ancients were aware of the rectilineal pro- 
 pagation of light. They knew that an opaque body, 
 placed between the eye and a point of light, intercepted 
 the light of the point. Possibly the terms ' ray ' and 
 ' beam ' may have been suggested by those straight 
 spokes of light which, in certain states of the atmo- 
 sphere, dart from the sun at his rising and his setting. 
 The rectilineal propagation of light may be illustrated 
 by permitting the solar light to enter by a small 
 aperture in a window-shutter a dark room in which / 
 a little smoke has been diffused. In pure air you can- / 
 not see the beam, but in smoke you can, because 
 the light, which passes unseen through the air, is scat- 
 tered and revealed by the smoke particles, among which 
 the beam pursues a straight course. 
 
 The following instructive experiment depends on the 
 rectilineaj. propagation of light. Make a small hole in . 
 a closed windoAV-shutter, before which stands a house or 
 a tree, and place within the darkened room a white 
 screen at some distance from the orifice. Every straight 
 ray proceeding from the house or tree stamps its colour 
 upon the screen, and the sum of all the rays will, there- 
 fore, be an image of the object. But, as the rays cross 
 each other at the orih'ce, the image is inverted. At 
 present we may illustrate and expand the subject thus : 
 
10 
 
 ON LIGHT. 
 
 LECT. 
 
 In front of our camera is a large opening (L, fig. 2), 
 from which the lens has been removed, and which is 
 closed at present by a sheet of tin-foil. Pricking by 
 means of a common sewing-needle a small aperture in 
 the tin-foil, an inverted image of the carbon-points 
 starts forth upon the screen. A dozen apertures will 
 give a dozen images, a hundred a hundred, a thousand 
 a thousand. But, as the apertures come closer to each 
 other, that is to say, as the tin-foil between the aper- 
 tures vanishes, the images overlap more and more. 
 
 FIG. 2. 
 
 Removing the tin-foil altogether, the screen becomes 
 uniformly illuminated. Hence the light upon the. 
 screen may be regarded as the overlapping of innumer- 
 able images of the carbon-points. In like manner the 
 light upon every white wall on a cloudless day may be 
 regarded as produced by the superposition of innumer- 
 able images of the sun. 
 
 The law that the angle of incidence is equal to the 
 angle of reflection has a bearing upon a theory, to be 
 subsequently mentioned, which renders its simple illus- 
 tration here desirable. A straight lath (pointing to 
 
KEFLECTION OF LIGHT. 
 
 11 
 
 the figure 5 in fig. 3) is fixed as an index peipendicular 
 to a small looking-glass (M) capable of rotation. 
 A beam of light is first received upon the glass and re- 
 flected back along the line of its incidence. The index 
 being turned, the mirror turns along with it, and at 
 each side of the index the incident and the reflected 
 beams (L o, o E) track themselves through the dust of 
 the room. The mere inspection of the two angles 
 
 FIG. 3. 
 
 enclosed between the index and the two beams suffices 
 to show their equality, while if the graduated quadrant 
 be consulted, the arc from 5 to m is found accurately 
 equal to the arc from 5 to n. A card placed edgeways 
 upon a table without inclination to the right or to the 
 left is said to be perpendicular to the plane of the table. 
 The complete expression of the law of reflection is that 
 the angles of incidence and reflection are equal ; and 
 that the incident and reflected rays always lie in a 
 plane perpendicular to the reflecting surface. 
 
 This simple apparatus enables us to illustrate another 
 
 il- 
 
 
12 ON LIGHT. LKCT. 
 
 law of great practical importance, namely, that, when a 
 mirror rotates, the angular velocity of a beam reflected 
 from it is twice that of the reflecting mirror. A simple 
 experiment will make this plain. The arc (m n, fig. 3 ) 
 before you is divided into ten equal parts, and when 
 the incident beam and the index cross the zero of the 
 graduation, both the incident and reflected beams are 
 horizontal. , Moving the index of the mirror to 1, 
 the reflected beam cuts the arc at 2 ; moving the index 
 to 2, the arc is cut at 4 ; moving the index to 3, the arc 
 is cut at 6 ; moving the index to 4, the arc is cut at 8 ; 
 finally, moving the index to 5, the arc is cut at 10 (as 
 in the figure). In every case the reflected beam 
 moves through twice the angle passed over by the 
 mirror. 
 
 One of the problems of science, on which scientific 
 progress mainly depends, is to help the senses of man, 
 by carrying them into regions which could never be 
 attained without such help. Thus we arm the eye with 
 the telescope when we want to sound the depths of 
 space, and with the microscope when we want to ex- 
 plore motion and structure in their infinitesimal dimen- 
 sions. Now, this law of angular reflection, coupled 
 with the fact that a beam of light possesses no weight, 
 gives us the means of magnifying small motions to an 
 extraordinary degree. Thus, by attaching mirrors to 
 his suspended magnets, and by watching the images of 
 divided scales reflected from the mirrors, the celebrated 
 Grauss was able to detect the slightest thrill of variation 
 on the part of the earth's magnetic fore. By a similar 
 arrangement the feeble attractions and repulsions of 
 the diamagnetic force have been made manifest. The 
 minute elongation of a bar of metal by the mere warmth 
 
i. StIRIT OF THE MIDDLE AGES. 13 
 
 of the hand may be so magnified by this method as to 
 cause the index-beam to move through 20 or 30 feet. 
 The lengthening of a bar of iron when it is magnetized 
 may be also thus demonstrated. Helmholtz long ago 
 employed this method to render evident to his students 
 the classical experiments of Du Bois Eaymond on animal 
 electricity ; while in Sir William Thomson's reflecting 
 galvanometer the principle receives one of its latest, and 
 most important applications. 
 
 *> The Refraction of Light. Total Reflection. 
 
 For more than a thousand years no step was taken 
 in optics beyond this law of reflection. The men of 
 the Middle Ages, in fact, endeavoured on the one hand 
 to develope the laws of the universe a priori out of 
 their own consciousness, while many of them were so 
 occupied with the concerns of a future world that they 
 looked with a lofty scorn on all things pertainiag to 
 this one. Speaking of the natural philosophers of his 
 time, Eusebius says, ' It is not through ignorance of the 
 things admired by them, but through contempt of their 
 useless labour, that we think little of these matters, 
 turning our souls to the exercise of better things.' So 
 also Lactantius ' To search for the causes of things ; 
 to inquire whether the sun be as large as he seems ; 
 whether the moon is convex or concave ; whether the 
 stars are fixed in the sky, or float freely in the air ; 
 of what size and of what material are the heavens ; 
 whether they be at rest or in motion ; what is the mag- 
 nitude of the earth ; on what foundations is it suspended 
 or balanced ; to dispute and conjecture upon such 
 matters is just as if we chose to discuss what we think 
 
14 ON LIGHT. LECT. 
 
 of a city in a remote country, of which we never heard 
 but the name.' ' 
 
 As regards the refraction of light, the course of 
 real inquiry was resumed in 1100 by an Arabian 
 philosopher named Alhazen. Then it was taken up in 
 succession by Roger Bacon, Vitellio, and Kepler. One 
 of the most important occupations of science is the 
 determination, by precise measurements, of the quan- 
 titative relations of phenomena ; the value of such 
 measurements depending greatly upon the skill and 
 conscientiousness of the man who makes them. Vitellio 
 appears to have been both skilful and conscientious, 
 while Kepler's habit was to rummage through the 
 observations of his predecessors, to look at them in all 
 lights, and thus distil from them the principles which 
 united them. He had done this with the astronomical 
 measurements of Tycho Brahe, and had extracted from 
 them the celebrated ' laws of Kepler.' He did it also 
 with Vitellio's measurements of refraction. But in 
 this case he was not successful. The principle, though 
 
 1 The spirit of thos^ ancient heroes of the faiHi is still to be found 
 in unexpected places. In the April number of the Contemporary Review, 
 after describing how modern science ca,me to be what it is, my friend 
 Dr. Acland puts the following language into the mouth of Bishop 
 Wilson : ' What is surprising to me is the labour that you have taken 
 to attain so very little. You deserve for this the utmost credit a reason- 
 able being can desire; for you, being so accurate and so painstaking, 
 senm well aware of the uncertainty of some of your data, and of the pos- 
 sible futility, therefore, of some of your conclusions. For I am told that, 
 with all your pains, your sciences contain within them so many examples 
 of proved errors, that, being candid men, you must often feel the material 
 ground under your feet to be very slippery.' Rebelling thus expresses 
 his contempt for experimental knowledge: 'Newton's Optics is the 
 greatest illustration of a whole structure of fallacies, which in all its 
 parts is founded on observation and experiment.' There are some small 
 imitators of Schelling still in Germany. 
 
i. REFRACTION OF LIGHT, SNELL. 15 
 
 a simple one, escaped him, and it was first discovered 
 by Willebrord Snell, about the year 1621. 
 
 Less with the view of dwelling upon the phenome- 
 non itself than of introducing it in a form which will 
 render intelligible to you, subsequently, the play of 
 theoretic thought in Newton's mind, the fact of refrac- 
 tion may be here demonstrated. I will not do this by 
 drawing the course of the beam with chalk on a black 
 
 FIG. 4. 
 
 board, but by causing it to mark its own white track 
 before you. A shallow circular vessel (R I Or, fig. 4), 
 with a glass face, half filled with water rendered barely 
 turbid by the admixture of a little milk or the precipi- 
 tation of a little mastic, is placed with its glass face 
 vertical. By means of a small plane reflector (M), and 
 through a slit (I) in the hoop surrounding the vessel, a 
 beam of light is admitted in any required direction. 
 It impinges upon the water (at 0), enters it, and tracks 
 itself through the liquid in a sharp, bright band (0 G). 
 Meanwhile the beam passes unseen through the air 
 above the water, for the air is not competent to scatter 
 the light. A puff of tobacco smoke into this space at 
 
16 
 
 ON LIGHT. 
 
 once reveals the track of the incident-beam. If the 
 incidence be vertical, the beam is unrefracted. If 
 oblique, its refraction at the common surface of air and 
 water (at 0) is rendered clearly visible. It is also 
 seen that reflection (along K) accompanies refraction, 
 the beam dividing itself at the point of incidence into 
 a refracted and a reflected portion. 1 
 
 The law by whiclTSnell connected together all the 
 
 measurements executed up to his time, is this : Let 
 A B C D (Fig. 5) represent the outline of our circular 
 vessel, A C being the water-line. When the beam is 
 incident along B E, which is perpendicular to A C, 
 there is no refraction. When it is incident along m E, 
 there is refraction : it is bent at E and strikes the 
 circle at n. When it is incident along m' E, there is 
 also refraction at E, the beam striking the point n f . 
 From the ends of the incident beams, let the perpen- 
 diculars m o, m' o' be drawn upon B D, and from the 
 
 1 It will be subsequently shown how this simple apparatus may be 
 employed to determine the ' polarising angle ' of a liquid. 
 
i. PARTIAL REFLECTION. 17 
 
 ends of the refracted beams let the perpendiculars 
 p n, p' n' be also drawn. Measure the lengths of o m 
 and of p n, and divide the one by the other. You 
 obtain a certain quotient. In like manner divide m' o' 
 by the corresponding perpendicular p' n' ; you obtain 
 in each case the same quotient. Snell, in fact, found 
 this quotient to be a constant quantity for each par- 
 ticular substance, though it varied in amount from 
 substance to substance. He called the quotient the 
 index of refraction* 
 
 In all cases where the light is incident from air 
 upon the surface of a solid or a liquid, or, more gene- 
 rally still, when the incidence is from a less highly 
 refracting to a more highly refracting medium, the 
 reflection is partial. In this case the most powerfully 
 reflecting substances either transmit or absorb a portion 
 of the incident light. At a perpendicular incidence 
 water reflects only 18 rays out of every 1,000; glass 
 reflects only 25 rays, while mercury reflects 666. 
 When the rays strike the surface obliquely the reflec- 
 tion is augmented. At an incidence of 40, for ex- 
 ample, water reflects 22 rays, at 60 it reflects 65 rays, 
 at 80 333 rays ; while at an incidence of 89^, where 
 the light almost grazes the surface, it reflects 721 rays 
 out of every 1,000. Thus, as the obliquity increases, 
 the reflection from water approaches, and finally quite 
 overtakes, the reflection from mercury ; but at no inci- 
 dence, however great, when the incidence is from air, 
 is the reflection from water, mercury, or any other sub- 
 stance, total. 
 
 Still, total reflection may occur, and with a view to 
 understanding its subsequent application in the 1ST! col's 
 prism, it is necessary to state when it occurs. This 
 
 c 
 
18 
 
 ON LIGHT. 
 
 leads me to the enunciation of a principle which under- 
 lies all optical phenomena the principle of reversi- 
 bility. 1 In the case of refraction, for instance, when 
 the ray passes obliquely from air into water, it is bent 
 towards the perpendicular ; when it passes from water 
 to air, it is bent from the perpendicular, and accurately 
 reverses its course. Thus in fig. 5, if m E n be the track 
 taken by a ray in passing from air into water, n E m 
 will be its track in passing from water into air. Let 
 us push this principle to its consequences. Supposing 
 the light, instead of being incident along m E or m' E, 
 were incident as close as possible along c E (fig. 6) ; 
 suppose, in other words, that it just grazes the surface 
 
 V 
 
 before entering the water. After refraction it will pur- 
 sue the course E n". Conversely, if the light start from 
 n", and be incident at E, it will, on escaping into the 
 air, just graze the surface of the water. The question 
 now arises, what will occur supposing the ray from the 
 water follows the course n'" E, which lies beyond n" E ? 
 The answer is, it will not quit the water at all, but will 
 be totally reflected (along E x). At the under surface 
 
 1 From this principle Sir John Herschel deduces in a simple and 
 elegant manner the fundamental law of reflection. See FamiHar 
 Lectures, p. 236. 
 
i. TOTAL REFLECTION. 19 
 
 of the water, moreover, the law is just the same as at 
 its upper surface, the angle of incidence (DE-TI'") being 
 here also equal to the angle of reflection (D E x). 
 
 Total reflection may be thus simply illustrated : 
 Place a shilling in a drink ing-glass, and tilt the glass 
 so that the light from the shilling shall fall with the 
 necessary obliquity upon the water surface above it. 
 Look upwards towards that surface, and you see the 
 image of the shilling shining there as brightly as the 
 shilling itself. Thrust the closed end of a glass test- 
 ^tube into water, and incline the tube. When the in- 
 clination is sufficient, horizontal light falling upon the 
 tube cannot enter the air within it, but is totally re- 
 flected upward 5 when looked down upon, such a tube 
 looks quite as bright as burnished silver. Pour a little 
 water into the tube ; as the liquid rises, total reflection 
 is abolished, and with it the lustre, leaving a gradually 
 diminishing shining zone, which disappears wholly 
 when the level of the water within the tube reaches 
 that without it. Any glass tube, with its end stopped 
 water-tight, will produce this effect, which is both 
 beautiful and instructive. 
 
 Total reflection never occurs except in the attempted 
 passage of a ray from a more refracting to a less re- 
 fracting medium ; but in this case, when the obliquity 
 is sufficient, it always occurs. The mirage of the desert, 
 and other phantasmal appearances in the atmosphere, 
 are in part due to it. When, for example, the sun 
 heats an expanse of sand, the layer of air in contact 
 with the sand becomes lighter and less refracting than 
 the air above it : consequently, the rays from a distant 
 object, striking very obliquely on the surface of the 
 heated stratum, are sometimes totally reflected upwards, 
 
 c 2 
 
20 ON LIGHT. LECT. 
 
 thus producing images similar to those produced by 
 water. I have seen the image of a rock called Mont 
 Tombeline distinctly reflected from the heated air of the 
 strand' of Normandy near Avranches ; and by such de- 
 lusive appearances the thirsty soldiers of the French 
 army in Egypt were greatly tantalized. 
 
 The angle which marks the limit beyond which total 
 reflection takes place is called the limiting angle (it is 
 marked in fig. 6 by the strong line E T&"). It must evi- 
 dently diminish as the refractive index increases. For 
 water it is 48-^, for flint glass 38 41/ , and for diamond 
 23 42'. Thus all the light incident from two complete 
 quadrants, or 180, in the case of diamond, is con- 
 densed into an angular space of 47 22' (twice 23 42') 
 by refraction. Coupled with its great refraction, are 
 the great dispersive and great reflective powers of dia- 
 mond ; hence the extraordinary radiance of the gem, 
 both as regards white light and prismatic light. 
 
 5. Velocity of Light. Aberration. Principle 
 of least Action. 
 
 In 1676 an impulse was given to optics by astronomy. 
 In that year Olav Eoemer, a learned Dane, was engaged 
 at the Observatory of Paris in observing the eclipses of 
 Jupiter's moons. The planet, whose distance from the 
 sun is 475,693,000 miles, has four satellites. We are 
 now only concerned with the one nearest to the planet. . 
 Rcemer watched this moon, saw it move round in front 
 of the planet, pass to the other side of it, and then 
 plunge into Jupiter's shadow, behaving like a lamp sud- 
 denly extinguished : at the second edge of the shadow 
 he saw it reappear, like a lamp suddenly lighted. The 
 moon thus acted the part of a signal light to the 
 
i. VELOCITY OF LIGHT, ECEMER. 21 
 
 astronomer, and enabled him to tell exactly its time of 
 revolution. The period between two successive light- 
 ings up of the lunar lamp he found to be 42 hours, 28 
 minutes, and 35 seconds. 
 
 This measurement of time was so accurate, that 
 having determined the moment when the moon emerged 
 from the shadow, the moment of its hundredth appear- 
 ance could also be determined. In fact, it would be 
 100 times 42 hours, 28 minutes, 35 seconds, after the 
 first observation. 
 
 Eoemer's first observation was made when the earth 
 was in the part of its orbit nearest Jupiter. About 
 six months afterwards, the earth being then at the 
 opposite side of its orbit, when the little moon ought 
 to have made its hundredth appearance, it was found 
 unpunctual, being fully 15 minutes behind its calcu- 
 lated time. Its appearance, moreover, had been grow- 
 ing gradually later, as the earth retreated towards the 
 part of its orbit most distant from Jupiter. Roemer 
 reasoned thus : ' Had I been able to remain at the 
 other side of the earth's orbit, the moon might have 
 appeared always at the proper instant ; an observer 
 placed there would probably have seen the moon 15 
 minutes ago, the retardation in my case being due to 
 the fact that the light requires 15 minutes to travel 
 from the place where my first observation was made to 
 my present position.' 
 
 This flash of genius was immediately succeeded by 
 another. ' If this surmise be correct,' Roemer reasoned, 
 4 then as I approach Jupiter along the other side of the 
 earth's orbit, the retardation ought to become gradu- 
 ally less, and when I reach the place of my first obser- 
 vation, there ought to be no retardation at all.' He 
 
22 ON LIGHT.^ 
 
 found this to be the case, and thus not only proved that 
 light required time to pass through space, but also 
 determined its rate of propagation. 
 
 The velocity of light, as determined by Roemer, is 
 192,500 miles in a second. 
 
 For a time, however, the observations and reasonings 
 of Rcerner failed to produce conviction. They were 
 doubted by Cassini, Fontenelle, and Hooke. Subse- 
 quently came the unexpected corroboration of Roemer by 
 the English astronomer, Bradley, who noticed that the 
 fixed stars did not really appear to be fixed, but that they 
 describe little orbits in the heavens every year. The 
 result perplexed him, but Bradley had a mind open to 
 suggestion, and capable of seeing, in the smallest fact, 
 a picture of the largest. He was one day upon the 
 Thames in a boat, and noticed that as long as his 
 course remained unchanged, the vane upon his mast- 
 Lead showed the wind to be blowing constantly in the 
 same direction, but that the wind appeared to vary with 
 every change in the direction of his boat. ' Here,' as 
 Whewell says, ' was the image of his case. The boat 
 was the earth, moving in its orbit, and the wind was 
 the light of a star,' 
 
 We may ask in parsing, what without the faculty 
 which formed the ' image,' would Bradley's wind and 
 vane have been to him ? A wind and vane, and nothing 
 more. You will immediately understand the meaning 
 of Bradley's discovery. Imagineyourself in a motion- 
 less railway-train, with a shower of rain descending 
 vertically downwards. The moment the train begins 
 to move the rain-drops begin to slant, and the quicker 
 the motion of the train the greater is the obliquity. In 
 a precisely similar manner the rays from a star verti- 
 
i. ABERRATION OF LIGHT, BRADLEY. 23 
 
 cally overhead are caused to slant by the motion of the 
 earth through space. Knowing the speed of the train, 
 and the obliquity of the falling rain, the velocity of 
 the .drops may be calculated ; and knowing the speed 
 of the earth in her orbit, and the obliquity of the rays 
 due to this cause, we can calculate just as easily the 
 velocity of light. Bradley did this, and the ' aberra- 
 tion of light,' as his discovery is called, enabled him 
 to assign to it a velocity almost identical with that 
 deduced by Roemer fro"rn a totally different method 
 of observation. Subsequently Fi^eau, and quite recently 
 Cornu, employing not planetary or stellar distances, but 
 simply the breadth of 'the city of Paris, determined the 
 velocity of light : while Foucault a man of the rarest 
 mechanical genius solved the problem without quitting 
 his private rooter- Owing to an error in the determi- 
 nation of the earth's distance from the sun, the velocity 
 assigned to light by both Kcemer and Bradley is too 
 great. With a close approximation to accuracy it may 
 be regarded as 186,000 miles a second. 
 
 By Robmer's discovery, the notion entertained by 
 Descartes, and espoused by Hooke, that light is pro- 
 pagated instantly through space, was overth rown. But 
 the establishment of its motion through stellar space 
 led to speculations regarding its velocity in transparent 
 terrestrial substances. The index of refraction of a ray 
 passing from air in to -water is -J. Newton assumed these 
 numbers to mean that the velocity of light in water 
 being 4, its veloci^rn- - air is 3 ; and he deduced the 
 phenomena of refraction from this assumption. The 
 reverse has since been proved to be the case that is to 
 say, the velocity of light in water being 3, its velocity 
 in air is 4 ; but both in Newton's time and ours the 
 
24 ON LIGHT. LECT. 
 
 same great principle determined, and determines, the 
 course of light in all cases. In passing from point to 
 point, whatever be the media in its path, or however it 
 may be reflected, light takes the course which occupies 
 least time. Thus in fig. 4, taking its velocity in air 
 and in water into account, the light reaches Or from I 
 more rapidly by travelling first to 0, and there changing 
 its course, than if it proceeded straight from T to Gr. 
 This is readily comprehended, because in the latter case 
 it would pursue a greater distance through the water, 
 which is the more retarding medium. 
 
 6. Descartes' Explanation of the Rainbow. 
 Snell's law of refraction is one of the corner-stones of 
 optical science, and its applications to-day are million- 
 fold. Immediately after its discovery Descartes applied it 
 to the explanation of the rainbow. A beam of solar light 
 falling obliquely upon a rain-drop is refracted on enter- 
 ing the drop. It is in part reflected at the back of the 
 drop, and on emerging it is again refracted. By these 
 two refractions, at its entrance and at its emergence, the 
 beam of light is decomposed, quitting the drop resolved 
 into coloured constituents. The light thus reaches 
 the eye of an observer facing the drop, and with his 
 back to the sun. 
 
 Conceive a line drawn from the sun to the observer's 
 eye, and prolonged beyond the -observer. Conceive 
 another line drawn through the eye, enclosing an angle 
 of 42| with the line drawn from the sun, and prolonged 
 to the falling shower. Along this second line a rain- 
 drop, at its remote end, when struck by a sunbeam, will 
 send a ray of red light. Every other drop similarly 
 situated, that is, every drop at an angular distance of 
 
i. THE RAINBOW, DESCARTES. 25 
 
 42| from the line aforesaid, will do the same. A cir- 
 cular band of red light is thus formed, which may be re- 
 garded as the boundary of the base of a cone, having 
 the rays which form the band for its surface, and its 
 apex at the observer's eye. Because of the magnitude 
 of the sun, the angular width of this red band will be 
 half a degree. 
 
 From the eye of the observer conceive another line, 
 to be drawn, enclosing an angle, not of 42, but of 
 40^, with the prolongation of the line drawn to the 
 sun. Along this line a solar beam striking a rain-drop 
 will send violet light to the eye. All drops at the same 
 angular distance will do the same, and we shall there- 
 fore obtain a band of violet light of the same width as 
 the red band. These, two bands constitute the limiting 
 colours of the rainbow, and between them the bands 
 corresponding to the other colours lie. 
 
 Thus the line drawn from the eye to the middle 
 of the bow, and the line drawn through the eye to the 
 sun, always enclose an angle of about 41 ; to account 
 for this was the great difficulty, which remained un- 
 solved up to the time of Descartes. 
 
 Taking a pen in hand, and calculating by means 
 of Snell's law the track of every ray through a rain- 
 drop, Descartes found that, at one particular angle, 
 the rays, reflected at its back, emerged from the drop 
 almost parallel to each other. They were thus enabled 
 to preserve their intensity through long atmospheric 
 distances. At all other angles the rays quitted the drop 
 divergent, and through this divergence became so 
 enfeebled as to be practically lost to the eye. The 
 angle of parallelism here referred to was that of forty- 
 one degrees, which observation had proved to be in- 
 variably associated with the rainbow. 
 
26 OX LIGHT. LECT. 
 
 From what has been said, it is clear that two ob- 
 servers standing* beside each other, or one above the 
 other, nay, that even the two eyes of the same observer, 
 do not see exactly the same bow. The position of the 
 base of the cone changes with that of its apex. And 
 here we have no difficulty in answering a question often 
 asked namely, whether a rainbow is ever seen reflected 
 in water. Seeing two bows, the one in the heavens, 
 the other in the water, you might be disposed to infer 
 that the one bears the same relation to the other that a 
 tree upon the water's edge bears to its reflected image. 
 The rays, however, which reach an observer's eye after 
 reflection, and which form a bow, would, were their 
 course uninterrupted, converge to a point vertically 
 under the observer, and as far below the level of the 
 water as his eye is above it. But under no cir- 
 cumstances could an eye above the water-level, and one 
 below it, see the same bow in other words, the self- 
 same drops of rain cannot form the reflected bow 
 and the bow seen directly in the heavens. The re- 
 flected bow, therefore, is not, in the usual optical sense 
 of the term, the image of the bow seen in the sky. 
 
 7. Analysis and Synthesis of Light. Doctrine 
 of Colours. 
 
 In the rainbow a new phenomenon was introduced 
 the phenomenon of colour. And here we arrive 
 at one of those points in the history of science, when 
 great men's labours so intermingle that it is difficult 
 to assign to each worker his precise meed of honour. 
 Descartes was at the threshold of the discovery of the 
 composition of solar light ; but for Newton was 
 reserved the enunciation of the true law. He went 
 
ANALYSIS OF LIGHT, NEWTON. 
 
 27 
 
 to work in this way : Through the closed window- 
 shutter of a room he pierced an orifice, and allowed 
 a thin sunbeam to pass through it. The beam stamped 
 a round white image of the sun on the opposite wall 
 of the room. In the path of this beam Newton placed 
 a prism, expecting to see the beam refracted, but also 
 expecting to see the image of the sun, after refraction, 
 still round. To his astonishment, it was drawn out to 
 an image with a length five times its breadth. It was, 
 
 FIG. 7. 
 
 moreover, no longer white, but divided into bands of 
 different colours. Newton saw immediately that solar 
 light was composite, not simple. His elongated image 
 revealed to him the fact that some constituents of the 
 light were more deflected by the prism than others, 
 and he concluded, therefore, that white solar light was 
 a mixture of lights of different colour?, of different 
 degrees of refrangibility. 
 
 Let us reproduce this celebrated experiment. On 
 the screen is now stamped a luminous disk, which may 
 
28 ON LIGHT. LECT. 
 
 stand for Newton's image of the sun. Causing the beam 
 (from L, fig. 7) which produces the disk to pass through 
 a lens (E) which forms an image of the aperture, and 
 then through a prism (P), we obtain Newton's coloured 
 image, with its red and violet ends, which he called a 
 spectrum. Newton divided the spectrum into seven 
 parts red, orange, yellow, green, blue, indigo, violet ; 
 which are commonly called the seven primary or pris- 
 matic colours. The drawing out of the white light 
 into its constituent colours is called cfytp&rtfLon. 
 
 This was the first analysis of solar light by Newton ; 
 but the scientific mind is fond of verification, and never 
 neglects it where it is possible. Newton completed his 
 proof by synthesis in this way : The spectrum now 
 before you is produced by a glass prism. Causing the 
 decomposed beam to pass through a second similar 
 prism, but so placed that the colours are refracted back 
 and reblended, the perfectly white luminous disk is 
 restored. 
 
 In this case, refraction and dispersion are simulta- 
 neously abolished. Are they always s ? Can we have 
 the one without the other ? It was Newton's conclu- 
 sion that we could not. Here he erred, and his error, 
 which he maintained to the end of his life, retarded 
 the progress of optical discovery. Dollond subse- 
 quently proved that, by combining two different kinds 
 of glass, the colours can be extinguished, still leaving 
 a residue of refraction, and he employed this residue 
 in the construction of achromatic lenses lenses 
 yielding no colour which Newton thought an impossi- 
 bility. By setting a water-prism water contained in 
 a wedge-shaped vessel with glass sides (B,*fig. 8) in 
 
i. ACHROMATISM, DOLLOND. 29 
 
 opposition to a wedge of glass (to the right of B), this 
 point can be illustrated before you. We have first 
 of all the position (dotted) of the unrefracted beam 
 marked upon the screen ; then we produce Ihe narrow 
 water-spectrum (W); finally, by introducing a flint- 
 glass prism, we refract the beam back, until the colour 
 disappears (at A). The image of the slit is now ivhite ; 
 but though the dispersion is abolished, there remains 
 a very sensible amount of refraction. 
 
 FIG. 8. 
 
 This is the place to illustrate another point bearing 
 upon the instrumental means employed in these lec- 
 tures. Bodies differ widely from each other as to their 
 powers of refraction and dispersion. Note the position 
 of the water-spectrum upon the screen. Altering in 
 no particular the wedge-shaped vessel, but simply 
 substituting for the water the transparent bisulphide 
 of carbon, you notice how much higher the beam is 
 thrown, and how much richer is the display of colour. 
 
30 OX LIGHT. 
 
 LECT, 
 
 To augment the size of our spectrum we here employ 
 (at L) a slit, instead of a circular aperture. 1 
 
 The synthesis of white light may be effected in 
 three ways, all of which are worthy of attention : 
 Here, in the first instance, we have a rich spectrum 
 produced by the decomposition of the beam (from L, 
 fig. 9). One face of the prism (P) is protected by a 
 
 FIG. 9. 
 
 diaphragm (not shown in the figure), with a longitu- 
 dinal slit, through which the beam passes into the prism. 
 
 1 The low dispersive power of water masks, as Helmholtz has re- 
 marked, the imperfect achromatism of the eye. "With the naked eye I 
 can see a distant Line disk sharply defined, but not a red one. I can also 
 see the lines which mark the upper and lower boundaries of a horizon- 
 tally refracted spectrum sharp at the blue end, but ill-defined at the red 
 end. Projecting a luminous disk upon a screen, and covering one semi- 
 circle of the aperture with a red and the other with a blue or green glass, 
 the difference between the apparent sizes of the two semicircles is in 
 my case, and in numerous other cases, extraordinary. Many per- 
 sons, however, see the apparent sizes of the two semicircles reversed. 
 If with a spectacle glass I correct the dispersion of the red light over 
 the retina, then the blue ceases to give a sharply-defined image. Thus 
 examined the departure of the eye from achromatism appears very gross 
 indeed. 
 
i. COMPLEMENTAEY COLOURS, HELMHOLTZ. 31 
 
 It emerges decomposed at the other side. I permit the 
 colours to pass through a cylindrical lens (C), which 
 so squeezes them together as to produce upon the 
 screen a sharply-defined rectangular image of the 
 longitudinal slit. In that image the colours are re- 
 blended, and it is perfectly white. Between- the prism 
 and the cylindrical lens may be seen the colours, 
 tracking themselves through the dust of the room. 
 Cutting off the more refrangible fringe by a card, the 
 rectangle is seen red ; cutting off the less refrangible 
 fringe, the rectangle is seen blue. By means of a thin 
 glass prism (W), I deflect one portion of the colours, and 
 leave the residual portion. On the screen are now two 
 coloured rectangles produced in this way. These are 
 corhplementary colours colours which, by.their union, 
 produce white. Note that, by judicious management, 
 one of these colours is rendered yellow, and the other 
 blue. I withdraw the thin prism ; yellow and blue 
 immediately commingle, and we have white as the result 
 of their union* On our way, then, we remove the 
 fallacy, first exposed by Helmholtz, that the mixture 
 of blue and yellow lights produces green. 
 
 Restoring the circular aperture, we obtain once 
 more a spectrum like that of Newton. By means 
 of a lens, we gather up these colours, and build them 
 together, not to an image of the aperture, but to an 
 image of the carbon-points themselves. 
 
 Finally, in virtue of the persistence of impressions 
 upon the retina, by means of a rotating disk, on which 
 are spread in sectors the colours of the spectrum, we 
 blend together the prismatic colours in the eye itself, 
 and thus produce the impression of whiteness. 
 
 Having unravelled the interwoven constituents of 
 
32 ON LIGHT. LECT. 
 
 white light, we have next to inquire, What part the 
 constitution so revealed enables this agent to play in 
 Nature ? To it we owe all the phenomena of colour, 
 and yet not to it alone ; for there must be a certain rela- 
 tionship between the ultimate particles of natural bodies 
 and white light, to enable them to extract from it the 
 luxury of colour. But the function of natural bodies is 
 here selective, not creative. There is no colour generated 
 by any natural body whatever. Natural bodies have 
 showered upon them, in the white light of the sun, 
 the sum total of all possible colours, and their action 
 is limited to the sifting of that total, the appropri- 
 ating from it of the colours which really belong to them, 
 and the rejecting of those which do not. It will fix 
 this subject in your minds if I say, that it is the portion 
 ' of light which they reject, and not that which belongs 
 to them, that gives bodies their colours. 
 
 Let us begin our experimental inquiries here by 
 asking, What is the meaning of blackness ? Pass a 
 black ribbon through the colours of the spectrum ; it 
 quenches all of them. The meaning of blackness is 
 , thus revealed it is the result of the absorption of all 
 j the constituents of solar light. Pass a red ribbon 
 through the spectrum. In the red light the ribbon is a 
 vivid red. Why ? Because the light that enters the 
 ribbon is not quenched or absorbed, but in great part 
 sent back to the eye. Place the same ribbon in the green 
 of the spectrum ; it is black as jet. It absorbs the green 
 light, and leaves the space on which it falls a space of 
 intense darkness. Place a green ribbon in the green 
 of the spectrum. It shines vividly with its proper 
 colour ; transfer it to the red, it is black as jet. Here 
 
i. COLOURS PRODUCED BY ABSORPTION. 83 
 
 it absorbs all the light that falls upon it, and offers 
 mere darkness to the eye. 
 
 Thus, when white light is employed, the red sifts 
 it by quenching the green, and the green sifts it 
 by quenching the red, both exhibiting the residual 
 colour. The process through which natural bodies 
 acquire their colours is therefore a negative one. > 
 The colours are produced by subtraction, not by addi^/ 
 tion. This red glass is red because it destroys all the' 
 more refrangible rays of the spectrum. This blue 
 liquid is blue because it destroys all the less refrangible 
 rays. Both together are opaque because the light 
 transmitted by the one is quenched by the other. In 
 this way, by the union of two transparent substances 
 we obtain a combination as dark as pitch to solar light. 
 This other liquid, finally, is purple because it destroys 
 the green and the yellow, and allows the terminal 
 colours of the spectrum to pass unimpeded. From the 
 blending of the blue and the red this gorgeous purple 
 is produced. 
 
 One step further for the sake of exactness. The light 
 which falls upon a body is divided into two portions, 
 one of which is reflected from the surface of the 
 body ; and this is of the same colour as the incident 
 light. If the incident light be white the superficially 
 reflected light will also be white. Solar light, for 
 example, reflected from the surface of even a black body, 
 is white. The blackest camphine smoke in a dark 
 room through which a sunbeam passes from an aperture 
 in the window-shutter, renders the track of the beam 
 white, by the light scattered from the surfaces of the 
 soot particles. The moon appears to us as if 
 
 ' Clothed in white samite, mystic, wonderful ; ' 
 D 
 
34 ON LIGHT. LECT. 
 
 but were she covered with the blackest velvet she would 
 still hang in the heavens as a white orb, shining upon 
 our world substantially as she does now. 
 
 8. Colours of Pigments as distinguished from 
 Colours of Light. 
 
 The second portion of the light enters the body, 
 and upon its treatment there the colour of the body 
 depends. And here a moment may properly be given 
 to the analysis of the action of pigments upon light. 
 They are composed of particles mixed with a vehicle ; 
 but how intimately soever the particles may be blended, 
 they still remain particles, separated it may be by 
 exceedingly minute distances, but still separated. To 
 use the scientific phrase, they are not optically continu- 
 ous. Now, wherever optical continuity is ruptured we 
 have reflection of the incident light. It is the multi- 
 tude of reflections at the limiting surfaces of the 
 particles that prevents light from passing through glass, 
 or rock-salt, when these transparent substances are 
 pounded into powder. The light here is exhausted in 
 a waste of echoes, not extinguished by true absorption. 
 It is the same kind of reflection that renders the 
 thunder-cloud so impervious to light. Such a cloud is 
 composed of particles of water mixed with particles of 
 air, both separately transparent, but practically opaque 
 when thus mixed together. 
 
 In the case of pigments, then, the light is reflected 
 at the limiting surfaces of the particles, but it is in 
 p^rt absorbed within the particles. The reflection is 
 necessary to send the light back to the eye ; the absorp- 
 tion is necessary to give the body its colour. The same 
 
i. COLOUKS OF PIGMENTS. 35 
 
 remarks apply to flowers. The rose is red in virtue, 
 not of the light reflected from its surface, but of light 
 which has entered its substance, which has- been re- 
 flected from surfaces within, and which in returning 
 through the substance has had its green extinguished. 
 A similar process in the case of hard green leaves ex- 
 tinguishes the red, and sends green light from the body 
 of the leaves to the eye. 
 
 All bodies, even the most transparent, are more 
 or less absorbent of light. Take the case of water : 
 in small quantities it does not sensibly affect light. 
 A glass cell of clear water interposed in the track of 
 our beam does not perceptibly change any one of the 
 colours of the spectrum derived from the beam. Still 
 absorption, though insensible, has here occurred, and 
 to render it sensible we have only to increase the depth 
 of the water through which the light passes. Instead of 
 a cell an inch thick, let us take a layer, ten or fifteen 
 feet thick : the colour of the water is then very evident. 
 By augmenting the thickness we absorb more of the 
 light, and by making the thickness very great we absorb 
 the light altogether. Lampblack or pitch can do 
 no more, and the only difference between them and 
 water is that a very small depth in their case suffices 
 to extinguish all the light, j The difference between 
 the highest known transparency, and the highest known 
 opacity, is one of degree merely. I 
 
 If, then, we render water sufficiently deep to quench 
 all the light ; and if from the interior of the water no 
 light reaches the eye, we have the condition necessary 
 to produce blackness. Looked properly down upon 
 there are portions of the Atlantic Ocean to which one 
 would hardly ascribe a trace of colour : at the most a 
 
 D 2 
 
36 ON LIGHT. LECT. 
 
 tint of dark indigo reaches the eye. The water, in 
 fact, is practically black, and this is an indication both 
 of its depth and purity. But the case is entirely 
 changed when the ocean contains solid particles in a 
 state of mechanical suspension, capable of sending 
 light back to the eye. 
 
 Throw, for example, a white pebble into the blackest 
 Atlantic water ; as it sinks it becomes greener and 
 greener, and, before it disappears, it reaches a vivid 
 blue green. Break such a pebble into fragments, these 
 will behave like the unbroken mass : grind the pebble 
 to powder, every particle will yield its modicum of 
 green ; and if the particles be so fine as to remain 
 suspended in the water, the scattered light will be a 
 uniform green. Hence the greenness of shoal water. 
 You go to bed with the black water of the Atlantic 
 around you. You rise in the morning, find it a vivid 
 green, and correctly infer that you are crossing the 
 bank of Newfoundland. Such water is found charged 
 with fine matter in a state of mechanical suspension. 
 The light from the bottom may sometimes come into 
 play, but it is not necessary. The subaqueous foam 
 generated by the screw or paddle-wheels of a steamer 
 also sends forth a vivid green. The foam here fur- 
 nishes a reflecting surface, the water between the eye 
 and it F the absorbing medium. 
 
 Nothing can be more superb than the green of the 
 Atlantic waves when the circumstances are favourable 
 to the exhibition of the colour. As long as a wave 
 remains unbroken no colour appears, but when the foam 
 just doubles over the crest like an Alpine snow-cornice, 
 under the cornice we often see a display of the most 
 exquisite green. It is metallic in its brilliancy. But 
 
i. COLOUR OF WATER. 37 
 
 foam is necessary to its production. The foam is first 
 illuminated, and it scatters the light in all direc- 
 tions ; the light which passes through the higher 
 portion of the wave alone reaches the eye, and gives 
 to that portion its matchless colour. The folding of the 
 wave, producing, as it does, a series of longitudinal 
 protuberances and furrows which act like cylindrical 
 lenses, introduces variations in the intensity of the 
 light, and materially enhances its beauty. 
 
 We are now prepared for the further consideration 
 of a point already adverted to, and regarding which 
 error long found currency. You will find it stated 
 in many books that blue and yellow lights mixed 
 together produce green. But blue and yellow have 
 been just proved to be complementary colours, pro- 
 ducing white by their mixture. The mixture of blue 
 and yellow pigments undoubtedly produces green,, but \ 
 the mixture of pigments is totally different from the~~f 
 mixture of lights. 
 
 Helmholtz has revealed the cause of the green in 
 the case of a mixture of blue and yellow pigments. No 
 natural colour is pure. A blue liquid or a blue powder 
 permits not only the blue to pass through it, but a por- 
 tion of the adjacent green. A yellow powder is trans- 
 parent not only to the yellow light, but also in part to 
 the adjacent green. Now, when blue and yellow are 
 mixed together, the blue cuts off the yellow, the orange, 
 and the red ; the yellow, on the other hand, cuts off the 
 violet, the indigo, and the blue. Green is the only 
 colour to which both are transparent, and the conse- 
 quence is that, when white light falls upon a mixture 
 of yellow and blue powders, the green alone is sent 
 back to the eye. You have already seen that the fine 
 
38 ON LIGHT. LECT. 
 
 blue ammonia-sulphate of copper transmits a large 
 portion of green, while cutting off all the less re- 
 frangible light. A yellow solution of picric acid also 
 allows the green to pass, but quenches all the more 
 refrangible light. What must occur when we send a 
 beam through both liquids ? The experimental answer 
 to this question is now before you : the green band of 
 the spectrum alone remains upon the screen. 
 
 The impurity of natural colours is strikingly illus- 
 trated by an observation recently communicated to 
 me by Mr. Woodbury. On looking through a blue 
 glass at green leaves in sunshine, he saw the super- 
 ficially reflected light blue. The light, on the con- 
 trary, which came from the body of the leaves was 
 crimson. On examination, I found that the glass em- 
 ployed in this observation transmitted both ends of the 
 spectrum, the red as well as the blue, and that it 
 quenched the middle. This furnished an easy explana- 
 tion of the effect. In the delicate spring foliage the blue 
 of the solar light is for the most part absorbed, and a 
 light, mainly yellowish green, but containing a con- 
 siderable quantity of red, escapes from the leaf to the 
 eye. On looking at such foliage through the violet 
 glass, the green and the yellow are stopped, and the red 
 alone reaches the eye. Thus regarded, therefore, the 
 leaves appear like faintly-blushing roses, and present a 
 very beautiful appearance. With the blue ammonia- 
 sulphate of copper, which transmits no red, this effect 
 is not obtained. 
 
 As the year advances the crimson gradually hardens 
 to a coppery red ; and in the dark green leaves of old 
 ivy it is almost absent. Permitting a concentrated 
 beam of white light to fall upon fresh leaves in a dark 
 
T. COLOURS OF FOLIAGE. 39 
 
 room, the sudden change from green to red, and from 
 red back to green, when the violet glass is alternately 
 introduced across the beam and withdrawn, is very 
 surprising. Looked at through the same glass, the 
 meadows in May appear of a warm purple. With a 
 solution of permanganate of potash, which, while it 
 quenches the centre of the spectrum, permits its ends 
 to pass more freely than the violet glass, striking effects 
 are also obtained. 1 
 
 This question of absorption, considered with refer- 
 ence to its molecular mechanism, is one of the most 
 subtle and difficult in physics. We are not yet in a 
 condition to grapple with it, but we shall be by-and- 
 by. Meanwhile we may profitably glance back on 
 the web of relations which these experiments reveal 
 to us. We have in the first place in solar light an agent 
 of exceeding complexity, composed of innumerable 
 constituents, refrangible in different degrees. We find, 
 secondly, the atoms and molecules of bodies gifted 
 with the power of sifting solar light in the most vari- 
 ous ways, and producing by this sifting the colours 
 observed in nature and art. To do this they must pos- 
 sess a molecular structure commensurate in complexity 
 with that of light itself. Thirdly, we have the human 
 eye and brain, so organized as to be able to take in and 
 
 1 Both in foliage and in flowers we have striking differences of ab- 
 sorption. The copper Leech and the green beech, for example, take in 
 different rays. But the very growth of the tree is due to some of the 
 rays thus taken in. Are the chemical rays, then, the same in the 
 copper and the green beech ? In two such flowers as the primrose and 
 the violet, where the absorptions, to judge by the colours, are almost com- 
 plementary, are the chemically active rays the same ? The general re- 
 lation of colour to chemical action is worthy of the application of the 
 method by which Dr. Draper proved so conclusively the chemical potency 
 of the yellow rays. 
 
40 ON LIGHT. LECT. 
 
 distinguish the multitude of impressions thus generated. 
 The light, therefore, at starting is complex ; to sift and 
 select it as they do, natural bodies must be complex; while 
 to take in the impressions thus generated, the human 
 eye and brain, however we may simplify our conceptions 
 of their action, 1 must be highly complex. Whence this 
 triple complexity ? If what are called material pur- 
 
 1 Young, Helmholtz, and Maxwell reduce all differences of hue to 
 combinations in different proportions of three primary colours. It is 
 demonstrable by experiment that from the red, green, and violet all the 
 other colours of the spectrum may be obtained. 
 
 Sir Charles Wheatstone has recently drawn my attention to a work 
 by Christian Ernst Wunsch, Leipzig, 1792, in which the author an- 
 nounces the proposition that there are neither five nor seven, but only 
 three simple colours in white light. Wunsch produces five spectra, 
 with five prisms and five small apertures, and he mixes the colours first 
 in pairs, and afterwards in other ways and proportions. His result is that 
 'red is a simple colour incapable of being decomposed; that orange is 
 compounded of intense red and weak green ; that yellow is a mixture of 
 intense red and intense green ; that green is a simple colour ; that blue 
 is compounded of saturated green and saturated violet ; that indigo is a 
 mixture of saturated violet and weak green ; while violet is a pure simple 
 colour. He also finds that yellow and indigo blue produce white by 
 their mixture. Yellow with bright blue (hochblau) also produces white, 
 which seems, however, to have a tinge of green, while the pigments of 
 these two colours when mixed always give a more or less beautiful 
 green. Wunsch very emphatically distinguishes 'the mixture of pigments 
 from that of lights. Speaking of the generation of yellow, he says, 
 ' I say expressly red and green light, because I am speaking about light- 
 colours (Lichtfarben), and not abotit pigments.' However faulty his 
 theories may be, Wiinsch's experiments appear in the main to be 
 precise and conclusive. Nearly ten years subsequently Young adopted 
 red, green, and violet as the three primary colours, each of them 
 capable of producing three sensations, one of which, however, pre- 
 dominates over the two others. Helmholtz adopts, elucidates, and 
 enriches this notion. (Popular Lectures, p. 249. The beautiful paper 
 of Helmholtz on the mixture of colours, translated by myself, is pub- 
 lished in the 'Philosophical Magazine 'for 1852. Maxwell's excellent 
 memoir on the Theory of Compound Colours is published in the ' Philo- 
 sophical Transactions,' vol. 150, p. 57.) 
 
PRIMARY COLOURS. 
 
 41 
 
 poses were the only end to be served, a much simpler 
 mechanism would be sufficient. But, instead of sim- 
 plicity, we have prodigality of relation and adaptation 
 and this apparently for the sole purpose of enabling us 
 to see things robed in the splendours of colour. Would 
 it not seem that Nature harboured the intention of edu- 
 cating us for other enjoyments than those derivable 
 from meat and drink ? At all events, whatever Nature 
 meant and it would be mere presumption to dogmatize 
 as to what she meant we find ourselves here, as 
 the upshot of her operations, endowed with capacities 
 to enjoy not only the materially useful, but endowed 
 with others of indefinite scope and application, which 
 deal alone with the beautiful and the true. 
 
 LI 15 li A ,v . 
 
 IS 1 V K I!S IT Y <>K| 
 
 CALIK" ' SI A. J 
 
 ^v_ .-_ ;___- 
 
42 ON- LIGHT. LECT. 
 
 LECTUEE II. 
 
 ORIGIN OF PHYSICAL THEORIES SCOPE OF THE IMAGINATION NEWTON 
 AND THE EMISSION THEORY VERIFICATION OF PHYSICAL THEORIES 
 
 THE LUMINIFEROUS ETHER WAVE-THEORY OF LIGHT THOMAS YOUNG 
 
 FRESNEL AND ARAGO CONCEPTION OF WAVE-MOTION INTERFERENCE 
 OF WAVES CONSTITUTION OF SOUND-WAVES ANALOGIES OF SOUND 
 
 AND LIGHT ILLUSTRATIONS OF WAVE-MOTION INTERFERENCE OF 
 
 SOUND WAVES OPTICAL ILLUSTRATIONS PITCH AND COLOUR LENGTHS 
 OF THE WAVES OF LIGHT AND RATES OF VIBRATION OF THE ETHER- 
 PARTICLES INTERFERENCE OF LIGHT PHENOMENA WHICH FIRST 
 
 SUGGESTED THE UNDULATORY THEORY BOYLE AND HOOKB THE 
 COLOURS OF THIN PLATES THE SOAP-BUBBLE NEWTON's RINGS 
 
 THEORY OF ' FITS ' ITS EXPLANATION OF THE RINGS OVERTHROW 
 
 OF THE THEORY DIFFRACTION OF LIGHT COLOURS PRODUCED BY 
 DIFFRACTION COLOURS OF MOTHER-OF-PEARL. 
 
 1. Origin and Scope of Physical Theories. 
 
 WE might vary and extend our experiments on 
 Light indefinitely, and they certainly would prove 
 us to possess a wonderful mastery over the phenomena. 
 But the vesture of the agent only would thus be re- 
 vealed, not the agent itself. The human mind, how- 
 ever, is so constituted and so educated, as regards 
 natural things, that it can never rest satisfied with this 
 outward view of them. Brightness and freshness take 
 possession of the mind when it is crossed by the light 
 of principles, shewing the facts of Nature to be organ- 
 ically connected. 
 
 Let us, then, inquire what this thing is that we 
 
ii. CONCEPTION OF PHYSICAL THEORY. 43 
 
 have been generating, reflecting, refracting and analyz- 
 ing. 
 
 In doing this, we shall learn that the life of the 
 experimental philosopher is twofold. He lives, in his 
 vocation, a life of the senses, using his hands, eyes, and 
 ears in his experiments : but such a question as that 
 now before us carries him beyond the margin of the 
 senses. He cannot consider, much less answer, the 
 question, ' What is light ? ' without transporting him- 
 self to a world which underlies the sensible one, and 
 out of which spring all optical phenomena. To realize 
 this subsensible world, if I may use the term, the mind 
 must possess a certain pictorial power. It must be 
 able to form definite images of the things which that 
 world contains ; and to say that, if such or such a state 
 of things exist in that world, then the phenomena 
 which appear in ours must, of necessity, grow out of 
 this state of things. If the picture be correct, the phe- 
 nomena are accounted for ; a physical theory has been 
 enunciated which unites and explains them all. 
 
 This conception of physical theory implies, as you 
 perceive, the exercise of the imagination. Do not be 
 afraid of this word, which seems to render so many 
 respectable people, both in the ranks of science and 
 out of them, uncomfortable. That men in the ranks of 
 science should feel thus is, I think, a proof that they 
 have suffered themselves to be misled by the popular 
 definition of a great faculty instead of: observing its 
 operation in their own minds. Without imagination 
 we cannot take a step beyond the bourne of the mere 
 animal world, perhaps not even to the edge of this one. 
 But, in speaking thus of imagination, I do not mean a 
 riotous power which deals capriciously with facts, but 
 
44 ON LIGHT. 
 
 LECT. 
 
 a well-ordered and disciplined power, whose sole func- 
 tion is to form conceptions which the intellect im- 
 peratively demands. Imagination, thus exercised, never 
 really severs itself from the world of fact. This is the 
 storehouse from which the materials for all its pictures 
 are derived ; and the magic of its art consists, not in 
 creating things anew, but in so changing the magnitude, 
 position, and other relations of sensible things, as to 
 render them fit for the requirements of the intellect in 
 the subsensible world. 1 
 
 Descartes imagined space to be filled with some- 
 thing that transmitted \ighiinstantaneously. Firstly, 
 because, in his experience, no measurable interval was 
 known to exist between the appearance of a flash of 
 light, however distant, and its effect upon consciousness ; 
 and secondly, because, as far as his experience went, no 
 physical power is conveyed from place to place without 
 
 1 The following charming extract, bearing upon this point, was dis- 
 covered and written out for me by my deeply lamented friend Dr. 
 Bence Jones, late Hon. Secretary to the Royal Institution : 
 
 ' In every kind of magnitude there is a degree or sort to which our 
 sense is proportioned, the perception and knowledge of which is of the 
 greatest use to mankind. The same is the groundwork of philosophy; 
 for, though all sorts and degrees are equally the object of philosophical 
 speculation, yet it is from those which are proportioned to sense that a 
 philosopher must set out in his inquiries, ascending or descending after- 
 wards as his pursuits may require. He does well indeed to take his 
 views from many points of sight, and supply the defects of sense by a 
 well-regulated imagination ; nor is he to be confined by any limit in 
 space or time ; but, as his knowledge of Nature is founded on the ob- 
 servation of sensible things, he must begin with these, and must often 
 return to them to examine his progress by them. Here is his secure 
 hold ; and as he sets out from thence, so if he likewise trace not often 
 his steps backwards with caution, he will be in hazard of losing his way 
 in the labyrinths of Nature.' (Maclaurin : An Account of Sir I. New- 
 ton's Philosophical Discoveries. Written 1728; second edition, 1750; 
 pp. 18, 19.) 
 
ii. BASIS OF THE EMISSION THEORY. 45 
 
 a vehicle. But his imagination helped itself farther 
 by illustrations drawn from the world of fact. ' When,' 
 he says, ' one walks in darkness with staff in hand, the 
 moment the distant end of the staff strikes an obstacle 
 the hand feels it. This explains what might otherwise 
 be thought strange, that the light readies us instan- 
 taneously from the sun. I wish thee to believe that 
 light in the bodies that we call luminous is nothing 
 more than a very brisk and violent motion, which, by 
 means of the air and other transparent media, is con- 
 veyed to the eye exactly as the shock through the 
 walking-stick reaches the hand of a blind man. This 
 is instantaneous, and would be so even if the intervening- 
 distance were greater than that between earth and 
 heaven. It is therefore no more necessary that any- 
 thing material should reach the eye from the luminous 
 object, than that something should be sent from the 
 ground to the hand of the blind man when he is con- 
 scious of the shock of his staff.' The celebrated Robert 
 Hooke first threw doubt upon this notion of Descartes, 
 but afterwards substantially espoused it. The belief in 
 instantaneous transmission was destroyed by the dis- 
 covery of Kcemer referred to in our last lecture. 
 
 2. The Emission Theory of Light. 
 
 The case of Newton still more forcibly illustrates 
 the position, that in forming physical theories we draw 
 for our materials upon the world of fact. Before he 
 began to deal with light, he was intimately acquainted 
 with the laws of elastic collision, which all of you have 
 seen more or less perfectly illustrated on a billiard- table. 
 As regards the collision of sensible masses, Newton knew 
 
46 ON LIGHT. I.ECT. 
 
 the angle of incidence to be equal to the angle of re- 
 flection, and he also knew that experiment, as shewn in 
 our last lecture (fig. 3), had established the same law 
 with regard to light. He thus found in his previous 
 knowledge the material for theoretic images. He had 
 only to change the magnitude of conceptions already 
 in his mind to arrive at the Emission Theory of Light. 
 He supposed light to consist of elastic particles of in- \ 
 conceivable minuteness shot out with inconceivable 
 rapidity by luminous bodies, and that such particles 
 impinging upon smooth surfaces were reflected in ac- 
 cordance with the ordinary law of elastic collision. 
 The fact of optical reflection certainly occurred as if 
 light consisted of such particles, and this was Newton's 
 sole justification for introducing them. 
 
 But this is not all. In another important particu- 
 lar, also, Newton's conceptions regarding the nature of 
 light were influenced by his previous knowledge. He 
 had been pondering over the phenomena of gravitation, 
 and had made himself at home amid the operations of 
 this universal power. ' Perhaps his mind at this time was 
 too freshly and too deeply imbued with these notions 
 to permit of his forming an unfettered judgment re- 
 garding the nature of light. Be that as it may, Newton 
 saw in Eefraction the action of an attractive force ex- 
 erted on the light-particles. He carried his conception 
 out with the most severe consistency. Dropping ver- 
 tically downwards towards the earth's surface, the mo- 
 tion of a body is accelerated as it approaches the earth. 
 Dropping in the same manner downwards on a horizontal 
 surface, say through air on glass or water, the velocity 
 of the light-particles, when they came close to the sur- 
 face, was, according to Newton, also accelerated. Ap- 
 
ir. TEST OF THEOEY. 47 
 
 preaching such a surface obliquely, he supposed the 
 particles, when close to it, to be drawn down upon it, 
 as a projectile is drawn by gravity to the surface of the 
 earth. This deflection was, according to Newton, the 
 refraction seen in our last lecture (fig. 4). Finally, it 
 was supposed that differences of colour might be due to 
 differences in the size of the particles. This was the 
 physical theory of light enunciated and defended by 
 Newton ; and you will observe that it simply consists 
 in the transference of conceptions born in the world of 
 the senses to a subsensible world. 
 
 But, though the region of physical theory lies thus 
 behind the world of senses, the verifications of theory 
 occur in that world. Laying the theoretic conception 
 at the root of matters, we determine by deduction what 
 are the phenomena which must of necessity grow out of 
 this root. If the phenomena thus deduced agree with 
 those of the actual world, it is a presumption in favour 
 of the theory. If, as new k classes of phenomena arise, 
 they also are found to harmonize with theoretic de- 
 duction, the presumption becomes still stronger. If, 
 finally, the theory confers prophetic vision upon the 
 investigator, enabling him to predict the occurrence of 
 phenomena which have never yet been seen, and if those 
 predictions be found on trial to be rigidly correct, the 
 persuasion of the truth of the theory becomes over- 
 powering. 
 
 Thus working backwards from a limited number of 
 phenomena, genius, by its own expansive force, reaches 
 a conception which covers them all. There is no 
 more wonderful performance of the intellect than 
 this ; but we can render no account of it. Like the 
 scriptural gift of the Spirit, no man can tell whence 
 
48 ON LIGHT. 
 
 LECT. 
 
 it cometh. The passage from fact to principle is 
 sometimes slow, sometimes rapid, and at all times a 
 source of intellectual joy. When rapid, the pleasure 
 is concentrated and becomes a kind of ecstasy or in- 
 toxication. To any one who has experienced this 
 pleasure, even in a moderate degree, the action of 
 Archimedes when he quitted the bath, and ran naked, 
 crying * Eureka ! ' through the streets of Syracuse, 
 becomes intelligible. 
 
 How, then, did it fare with the Emission Theory when 
 the deductions from it were brought face to face with 
 natural phenomena? Tested by experiment, it was 
 found competent to explain many facts, and with tran- 
 scendent ingenuity its author sought to make it account 
 for them all. He so far succeeded, that men so cele- 
 brated -as Laplace and Mains, who lived till 1812, and 
 Biot and Brewster, who lived till our own time, were 
 found among his disciples. 
 
 3. The Undulatory Theowj of Light. 
 
 Still, even at an early period of the existence of the 
 Emission Theory, one or two great names were found 
 recording a protest against it ; and they furnish another 
 illustration of the law that, in forming theories, the 
 scientific imagination must draw its materials from the 
 world of fact and experience. It was known long ago 
 that sound is conveyed in waves or pulses through the 
 air ; and no sooner was this truth well housed in the 
 mind than it was transformed into a theoretic concep- 
 tion. It was supposed that light, like sound, might 
 also be the product of wave-motion. But what, in this 
 case, could be the material forming the waves ? For the 
 
ii. BASIS OF THE UNDULATORY THEORY. 49 
 
 waves of sound we have the air of our atmosphere ; but 
 the stretch of imagination which filled all space with a 
 luminiferous ether trembling with the waves of light 
 was so bold as to shock cautious minds. In one of my 
 latest conversations with Sir David Brewster, he said to 
 me that his chief objection to the undulatory theory 
 of light was that he could not think the Creator guilty 
 of so clumsy a contrivance as the filling of space with 
 ether in order to produce light. This, I may say, is 
 very dangerous ground, and the quarrel of science with 
 Sir David, on this point, as with many estimable persons 
 on other points, is, that they profess to know too much 
 about the mind of the Creator. 
 
 This conception of an ether was advocated, and in- 
 deed applied to various phenomena of Coptics, by the 
 celebrated astronomer, Huyghens. It was espoused 
 and defended by the celebrated mathematician, Euler 
 They were, however, opposed by Newton, whose au- 
 thority at the time bore them down. Or shall we say it 
 was authority merely ? Not quite so. Newton's pre- 
 ponderance was in some degree due to the fact that, 
 though Huyghens and Euler were right in the main, 
 they did not possess sufficient data to prove themselves 
 right. No human authority, however high, can main- 
 tain itself against the voice of Nature speaking through 
 experiment. But the voice of Nature may be an un- 
 certain voice, through the scantiness of data. This was 
 the case at the period now referred to, and at such a 
 period by the authority of Newton all antagonists were 
 naturally overborne. 
 
 Still, this great Emission Theory, which held its 
 ground so long, resembled one of those circles which, 
 according to your countryman Emerson, the force of 
 
50 ON LIGHT. LECT. 
 
 genius periodically draws round the operations of the 
 intellect, but which are eventually broken through by 
 pressure from behind. In the year 1 773 was born, at 
 Milverton, in Somersetshire, one of the most remarkable 
 men that England ever produced. He was educated for 
 the profession of a physician, but was too strong to be 
 tied down to professional routine. He devoted him- ' 
 self to the study of natural philosophy, and became in 
 all its departments a master. He was also a master of 
 letters. Languages, ancient and modern, were housed 
 within his brain, and, to use the words of his epitaph, 
 ' he first penetrated the obscurity which had veiled for 
 ages the hieroglyphics of Egypt.' * It fell to the lot of 
 this man to discover facts in optics which Newton's 
 theory was incompetent to explain, and his mind 
 roamed in search of a sufficient theory. He had made 
 himself acquainted with all the phenomena of wave- 
 motion ; with all the phenomena of sound ; working 
 successfully in this domain as an original discoverer. 
 Thus informed and disciplined, he was prepared to 
 detect any resemblance which might reveal itself be- 
 tween the phenomena of light and those of wave-motion. 
 Such resemblances he did detect ; and, spurred on by 
 the discovery, he pursued his speculations and his 
 experiments, until he finally succeeded in placing on an 
 immovable basis the Undulatory Theory of Light. 
 
 The founder of this great theory was Thomas Young, 
 a name, perhaps, unfamiliar to many of you, but which 
 ought to be familiar to you all. Permit me, there- 
 fore, by a kind of geometrical construction which I once 
 ventured to employ in London, to give you a notion 
 of the magnitude of this man. Let Newton stand 
 erect in his age, and Young in his. Draw a straight 
 
n. 'YOUNG, FRESNEL, ARAGO, BROUGHAM. 51 
 
 line from Newton to Young, tangent to the heads of 
 both. This line would slope downwards from Newton to 
 Young, because Newton was certainly the taller man 
 of the two. But the slope would not be steep, for the 
 difference of stature was not excessive. The line 
 would form what engineers call a gentle gradient from 
 Newton to Young. Place underneath this line the 
 biggest man born in the interval between both. It may 
 be doubted whether he would reach the line ; for if he did 
 he would be taller intellectually than Young, and there 
 was probably none taller. But I do not want you to 
 rest on English estimates of Young; the German, 
 Helmholtz, a kindred genius, thus speaks of him : ' His 
 was one of the most profound minds that the world 
 has ever seen; but he had the misfortune to be too 
 much in advance of his age. He excited the wonder 
 of his contemporaries, who, however, were unable to 
 follow him to the heights at which his daring intellect 
 was accustomed to soar. His most important ideas 
 lay, therefore, buried and forgotten in the folios of the 
 Eoyal Society, until a new generation gradually and 
 painfully made the same discoveries, and proved the 
 exactness of his assertions and the truth of his de- 
 monstrations.' 
 
 It is quite true, as Helmholtz says, that Young was 
 in advance of his age ; but something is to be added 
 which illustrates the responsibility of our public writers. 
 For twenty years this man of genius was quenched 
 hidden from the appreciative intellect of his country- 
 men deemed in fact a dreamer, through the vigorous 
 sarcasm of a writer who had then possession of the 
 public ear, and who in the Edinburgh Revieiu poured 
 ridicule upon Young and his speculations. To the cele- 
 
 E 2 
 
52 ON LIGHT. LECT. 
 
 brated Frenchmen Fresnel and Arago he was first 
 indebted for the restitution of his rights ; for they, 
 especially Fresnel, remade independently, as Helm- 
 holtz says, and vastly extended his discoveries. To 
 the 'students of his works Young has long since ap- 
 peared in his true light, but these twenty blank years 
 pushed him from the public mind, which became in turn 
 tilled with the fame of Young's colleague at the Koyal 
 Institution, Davy, and afterwards . with the fame of 
 Faraday. Carlyle refers to a remark of Novalis, that 
 a man's self-trust is enormously increased the moment 
 he finds that others believe in him. If the opposite 
 remark be true if it be a fact that public disbelief 
 weakens a man's force there is no calculating the 
 amount of damage these twenty years of neglect may 
 have done to Young's productiveness as an investi- 
 gator. It remains to be stated that his assailant was 
 Mr. Henry Brougham, afterwards Lord Chancellor of 
 England. 
 
 4. Wave-motion, Interference of Waves, ' Whirlpool 
 Rapids ' of Niagara. 
 
 Our hardest work is now before us. But the 
 capacity for hard work depends in a great measure 
 on the antecedent winding up of the will ; I would 
 call upon you, therefore, to gird up your loins for our 
 coming labours. If we succeed in climbing the hill 
 which faces us to-night, our future difficulties will not 
 be insurmountable. 
 
 In the earliest writings of the ancients we find the 
 notion that sound is conveyed by the air. Aristotle 
 gives expression to this notion, and the great architect 
 
n. WAVE-MOTION. 53 
 
 Vitruvius compares the waves of sound to waves of 
 water. But the real mechanism of wave-motion was 
 hidden from the ancients, and indeed was not made 
 clear until the time of Newton. The central difficulty 
 of the subject was, to distinguish between the motion 
 of the wave itself, and the motion of the particles which 
 at any moment constitute the wave. 
 
 Stand upon the sea-shore and observe the advancing 
 rollers before they are distorted by the friction *of the 
 bottom. Every wave has a back and a front, and, if 
 you clearly seize the image of the moving wave, you 
 will see that every particle of water along the front of 
 the wave is in the act of rising, while every particle 
 along its back is in the act of sinking. The particles 
 in front reach in succession the crest of the wave, and 
 as soon as the crest is passed they begin to fall. They 
 then reach the furrow or sinus of the wave, and can 
 sink no farther. Immediately afterwards they become 
 the front of the succeeding wave, rise again until they 
 reach the crest, and then sink as before. Thus, while 
 the waves pass onward horizontally, the individual 
 particles are simply lifted up and down vertically. 
 Observe a sea-fowl, or, if you are a swimmer, abandon 
 yourself to the action of the waves ; you are not carried 
 forward, but simply rocked up and down. The propa- 
 gation of a wave is the propagation of a form, and not 
 the transference of the substance which constitutes the 
 wave. 
 
 The length of the wave is the distance from crest to 
 crest, while the distance through which the individual 
 particles oscillate is called the amplitude of the oscil- 
 lation. You will notice that in this description the 
 
54 ON LIGHT. LECT. 
 
 particles of water are made to vibrate across the line of 
 propagation. 1 
 
 And now we have to take a step forwards, and it is 
 the most important step of all. You can picture two 
 series of waves proceeding from different origins 
 through the same water. When, for example, you 
 throw two stones into still water, the ring- waves pro- 
 ceeding from the two centres of disturbance intersect 
 each o4her. Now, no matter how numerous these waves 
 may be, the law holds good that the motion of every 
 particle of the water is the algebraic sum of all the 
 motions imparted to it. If crest coincide with crest 
 and furrow with furrow, the wave is lifted to a double 
 height above its sinus ; if furrow coincide with crest, 
 the motions are in opposition, and their sum is zero. 
 We have then still water. This action of wave upon 
 wave is technically called interference, a term to be 
 remembered. 
 
 To the eye of a person conversant with these princi- 
 ples, nothing can be more interesting than the crossing 
 of water ripples. Through their interference the water- 
 surface is sometimes shivered into the most beautiful 
 mosaic, trembling rhythmically as if with a kind of 
 visible music. When waves are skilfully generated in 
 a dish of mercury, a strong light thrown upon the 
 shining surface, and reflected on to a screen, reveals 
 the motions of the liquid metal. The shape of the 
 vessel determines the forms of the figures produced. In 
 
 1 I do not wish to encumber the conception here with the details of 
 the motion, but I may draw attention to the beautiful model of Prof. 
 Lyman, wherein waves are shown to be produced by the circular motion 
 of the particles. This, as proved by the brothers Weber, is the real 
 motion in the case of water-waves. 
 
ii. INTERFERENCE OF WATER-WAVES. 55 
 
 a circular dish, for example, a disturbance at the centre 
 propagates itself as a series of circular waves, which, 
 after reflection, again meet at the centre. If the point 
 of disturbance be a little way removed from the centre, 
 the interference of the direct and reflected waves 
 produces the magnificent chasing shown in the annexed 
 
 FIG. 10. 
 
 figure. 1 The light reflected from such a surface yields 
 a pattern of extraordinary beauty. When the mercury 
 is slightly struck by a needle-point in a direction 
 concentric with the surface .of the vessel, the lines of 
 light run round in mazy coils, interlacing and unravel- 
 ling themselves in a wonderful manner. When the vessel 
 is square, a splendid chequer-work is produced by the 
 
 1 Copied from Weber's Wellenlehre. 
 
56 ON LIGHT. LECT. 
 
 crossing of the direct and reflected waves. Thus, in 
 the case of wave-motion, the most ordinary causes give 
 rise to most exquisite effects. The words of your coun- 
 tryman, Emerson, are perfectly applicable here: 
 
 ' Thou can'st not wave thy staff in the air, 
 
 Or dip thy paddle in the lake, 
 But it carves the brow of beauty there, 
 
 And the ripples in rhymes the oars forsake.' 
 
 The most impressive illustration of the action of 
 waves on waves that I have ever seen occurs near 
 Niagara. For a distance of two miles, or thereabouts, 
 below the Falls, the river Niagara flows unruffled 
 through its excavated gorge. The bed subsequently 
 narrows, and the water quickens its motion. At the 
 place called the ' Whirlpool Rapids,' I estimated the 
 width of the river at 300 feet, an estimate confirmed 
 by the dwellers on the spot. When it is remembered 
 that the drainage of nearly half a continent is com- 
 pressed into this space, the impetuosity of the river's 
 escape through this gorge may be imagined. 
 
 Two kinds of motion are here obviously active, a 
 motion of translation and a motion of undulation the 
 race of the river through its gorge, and the great waves 
 generated by its collision with the obstacles in its way* 
 In the middle of the stream, the rush and tossing are 
 most violent ; at all events, the impetuous force of the 
 individual waves is here most strikingly displayed. 
 Vast pyramidal heaps leap incessantly from the river, 
 some of them with such energy as to jerk their summits 
 into the air, where they hang suspended as bundles of 
 liquid pearls, which, when shone upon by the sun, are of 
 indescribable beauty. 
 
IT. 'WHIRLPOOL RAPIDS' OF NIAGARA. 57 
 
 The first impression, and, indeed, the current ex- 
 planation of these Rapids is, that the central bed of the 
 river is cumbered with large boulders, and that the 
 jostling, tossing, and wild leaping of the water there 
 are due to its impact against these obstacles. A very 
 different explanation occurred to me upon the spot. 
 Boulders derived from the adjacent cliffs visibly cumber 
 the sides of the river. Against these the water rises 
 and sinks rhythmically but violently, large waves being 
 thus produced. On the generation of each wave there 
 is an immediate compounding of the wave-motion with 
 the river -motion. The ridges, which in still water 
 would proceed in circular curves round the centre o 
 disturbance, cross the river obliquely, and the result is, 
 that at the centre waves commingle which have really 
 been generated at the sides. This crossing of waves 
 may be seen on a small scale in any gutter after rain ; 
 it may also be seen on simply pouring water from a 
 wide-lipped jug. Where crest and furrow cross each 
 other, the wave is annulled ; where furrow and 
 furrow cross, the river is ploughed to a greater depth ; 
 and where crest and crest aid each other, we have that 
 astonishing leap of the water which breaks the co- 
 hesion of the crests, and tosses them shattered into 
 the air. The phenomena observed at the Whirlpool 
 Rapids constitute, in fact, one of the grandest illustra- 
 tions of the principle of interference, 
 
 5. Analogies of Sound and Light. 
 
 Thomas Young's fundamental discovery in optics 
 was that the principle of Interference was applicable to 
 light. Long prior to his time an Italian philosopher, 
 
58 ON LIGHT. LECT. 
 
 Grimaldi, had stated that under certain circumstances 
 two thin beams of light, each of which, acting singly, 
 produced a luminous spot upon a white wall, when caused 
 to act together, partially quenched each other and 
 darkened the spot. This was a statement of fundamental 
 significance, but it required the discoveries and the 
 genius of Young to give it meaning. How he did so will 
 gradually become clear to you. You know that air is 
 compressible ; that by pressure it can be rendered more 
 dense, and that by dilatation it can be rendered more 
 rare. Properly agitated, a tuning-fork now sounds in 
 a manner audible to you all, and most of you know that 
 *the air through which the sound is passing is parcelled 
 out into spaces in which the air is condensed, followed 
 by other spaces in which the air is rarefied. These 
 condensations and rarefactions constitute what we call 
 waves of sound. You can imagine the air of a room 
 traversed by a series of such waves, and you can imagine 
 a second series sent through the same air, and so related 
 to the first that condensation coincides with condensa- 
 tion and rarefaction with rarefaction. The consequence 
 of this coincidence would be a louder sound than that 
 produced by either system of waves taken singly. But 
 you can also imagine & state of things where the con- 
 densations of the one system fall upon the rarefactions 
 of the other system. In this case the two systems 
 would completely neutralize each other. Each of them 
 taken singly produces sound ; both of them taken 
 together produce no sound. Thus, > by adding sound 
 to sound we produce silence, as Grrimaldi in his experi- 
 ment produced darkness by adding light to light. 
 
 The analogy between sound and light here flashes 
 upon the mind. Young generalized this observation. He 
 
ii. LONGITUDINAL WAVES. 59 
 
 discovered a multitude of similar cases, and determined 
 their precise conditions. On the assumption that 
 light was wave-motion, all his experiments on inter- 
 ference were explained ; on the assumption that light 
 was flying particles, nothing was^explained. In the 
 time of Huyghens and Euler a medium had been 
 assumed for the transmission of the waves of light ; 
 but Newton raised the objection that, if light consisted 
 of the waves of such a medium, shadows could not 
 
 FIG. 11. 
 
 exist. The waves, he contended, would bend round 
 opaque bodies and produce the motion of light behind 
 them, as sound turns a corner, or as waves of water 
 wash round a rock. It was proved that the bending 
 round referred to by Newton actually occurs, but that 
 the inflected waves abolish each other by their mutual 
 interference. Young also discerned a fundamental 
 difference between the waves of light and those of 
 sound. Could you see the air through which sound- 
 
60 ON LIGHT. 
 
 LECT, 
 
 waves are passing, you would observe every individual 
 particle of air oscillating to and fro in the direction of 
 propagation. Could you see the luminiferous ether, 
 you would also find every individual particle making a 
 small excursion to and fro ; but here the motion, like 
 that assigned to the water-particles above referred to, 
 would be across the line of propagation. The vibra- 
 tions of the air are longitudinal, those of the ether 
 transversal. 
 
 It is my desire that you should realize with clearness 
 the character of wave-motion, both in ether and in air. 
 And, with this view, I bring before you an experiment 
 wherein the air-particles are represented by small spots 
 of light (R 0, fig. 11). They are derived from a clean 
 spiral, drawn upon a circle of blackened glass (D), so 
 that when the circle rotates, the spots move in successive 
 pulses over the screen. 1 In this experiment you have 
 clearly set before you how the pulses travel incessantly 
 forward, while their component particles perform oscilla- 
 tions to and fro. This is the picture of a sound-wave, in 
 which the vibrations are longitudinal. By another glass 
 wheel (I), fig. 12) we produce an image of a transverse 
 wave (0 R), and here we observe the waves travelling in 
 succession over the screen, while each individual spot of 
 light performs an excursion to and fro across the line of 
 propagation. 
 
 Notice what follows when the glass wheel is turned 
 very quickly. Objectively considered, the transverse 
 waves propagate themselves as before, but subjectively 
 the effect is totally changed. Because of the reten- 
 tion of impressions upon the retina, the spots of light 
 
 1 The apparatus is constructed by that excellent acoustic mechanician, 
 M. Rudolf Konig, of Paris. 
 
n. TRANSVERSE WAVES. 61 
 
 simply describe a series of parallel luminous lines upon 
 the screen, the length of these lines marking the ampli- 
 tude of the vibration. Here the impression of wave- 
 motion has totally disappeared. 
 
 The most familiar illustration of the interference of 
 sound-waves is furnished by the beats produced by 
 two musical sounds slightly out of unison. When two 
 tuning-forks in perfect unison are agitated together 
 
 FIG. 12. 
 
 I 
 
 the two sounds flow without roughness, as if they 
 were but one. But, by attaching with wax to one 
 of the forks a little weight, we cause it to vibrate 
 more slowly than its neighbour. Suppose that one 
 of them performs 101 vibrations in the time re- 
 quired by the other to perform 100, and suppose that 
 at starting the condensations and rarefactions of both 
 forks coincide. . At the 101st vibration of the quickest 
 fork they will again coincide, that fork at this point 
 having, gained one whole vibration, or one whole wave- 
 
62 ON LIGHT. -L-ECT. 
 
 length upon the other. But a little reflection will 
 make it clear that, at the 50th vibration, the two forks 
 are in opposition ; here the one tends to produce a 
 condensation where the other tends to produce a rare- 
 faction ; by the united action of the two forks, therefore, 
 the sound is quenched, and we have a pause of silence. 
 This occurs where one fork has gained half a wave- 
 length upon the other. At the 101st vibration, as 
 already stated, we have coincidence, and, therefore, 
 augmented sound ; at the 1 50th vibration we have 
 again a quenching of the sound. Here the one fork is 
 three half-waves in advance of the other. In general 
 terms, the waves conspire when the one series is an 
 even number of half-wave lengths, and they destroy each 
 ojjber when the one series is an odd number of half- wave 
 lengths in advance of the other. With two forks so cir- 
 cumstanced, we obtain those intermittent shocks of 
 sound separated by pauses of silence, to which \ve give 
 the name of beats. By a suitable arrangement, more- 
 over, it is possible to make one sound wholly extinguish 
 another. Along four distinct lines, for example, the 
 vibrations of the two prongs of a tuning-fork completely 
 blot each other out. 1 
 
 The pitch of sound is wholly determined by the 
 rapidity of the vibration, as the intensity is by the am- 
 plitude. What pitch is to the ear in acoustics, colour 
 is to the eye in the undulatory theory of light. Though 
 never seen, the lengths of the waves of light have been 
 determined. Their existence is proved by their effects, 
 and from their effects also their lengths may be accu- 
 rately deduced. This may, moreover, be done in many 
 
 1 Sound, 1st and 2nd ed., Lecture VII.; and 3rd ed., Chap. VIII. 
 Longmans. 
 
. INTERFERENCE OF SOUND. 63 
 
 ways, and, when the different determinations are com- 
 pared, the strictest harmony is found to exist between 
 them. This consensus of evidence is one of the strong- 
 est points of the undulatory theory. The shortest 
 waves of the visible spectrum are those of the extreme 
 violet ; the longest, those of the extreme red ; while the 
 other colours are of intermediate pitch or wave-length. 
 The length of a wave of the extreme red is such that it 
 would require 36,918 of them, placed end to end, to 
 cover one inch, while 64,631 of the extreme violet 
 waves would be required to span the same distance. 
 
 Now, the velocity of light, in round numbers, is 
 190,000 miles per second. Reducing this to inches, 
 and multiplying the number thus found by 36,918, we 
 find the number of waves of the extreme red, in 
 190,000 miles, to be four hundred and fifty-one millions 
 of millions. All these waves enter the eye, and strike 
 the retina at the back of the eye in one second. In a 
 similar manner, it may be found that the number of 
 shocks corresponding to the impression of violet is seven 
 hundred and eighty-nine millions of millions. 
 
 All space is filled with matter oscillating at such 
 rates. From every star waves of these dimensions 
 move, with the velocity of light, like spherical shells 
 in all directions. And in ether, just as in water, the 
 motion of every particle is the algebraic sum of ail the 
 separate motions imparted to it. One motion does not 
 blot out the other ; or, if extinction occur at one point, 
 it is strictly atoned for, by augmented motion, at some 
 other point. Every star declares by its light its un- 
 damaged individuality, as if it alone had sent its thrills 
 through space. 
 
64 ON LIGHT. 
 
 LECT. 
 
 6. Interference of Light. 
 
 The principle of interference, as proved by Young, 
 applies to the waves of light as it does to the waves 
 of water and the waves of sound. And the conditions 
 of interference are the same in all three. If two 
 series of light-waves of the same length start at the 
 same moment from a common origin (say A, tig. 13), 
 crest coincides with crest, sinus with sinus, and the two 
 
 7\ /\ /\ 
 
 \J \7 
 
 systems blend together to a single system (A m n) of 
 double amplitude. If both series start at the same 
 moment, one of them being, at starting, a whole wave- 
 length in advance of the other, they also add them- 
 selves together, and we have an augmented luminous 
 effect. The same occurs when the one system of waves 
 is any even number of semi-undulations in advance of 
 the other. But if the one system be half a wave-length 
 (as at A' a', fig. 14), or any odd number of half wave- 
 lengths in advance, then the crests of the one fall upon 
 the sinuses of the other ; the one system, in fact, tends 
 to lift the particles of ether at the precise places where 
 the other tends to depress them ; hence, through the 
 joint action of these opposing forces (indicated by the 
 arrows) the light-ether remains perfectly still. This 
 
n. INTERFERENCE OF LIGHT. 65 
 
 stillness of the ether is what we call darkness, which 
 corresponds with a dead level in the case of water. 
 
 FIG. 14. 
 
 It was said in our first lecture, with reference to 
 the colours produced by absorption, that the function 
 of natural bodies is selective, not creative ; that they ex- 
 tinguish certain constituents of the white solar light, 
 and appear in the colours of the unextinguished light. 
 It must at once flash upon your minds that, inasmuch as 
 we have in interference an agency by which light may 
 be self-extinguished, we may have in it the conditions 
 for the production of colour. Bub this would imply that 
 certain constituents are quenched by interference, while 
 others are permitted to remain. This is the fact ; and 
 it is entirely due to the difference in the lengths of the 
 waves of light. 
 
 7. Colours of thin Films. Observations of Boyle 
 , and Hooke. 
 
 This subject may be illustrated by the class of 
 phenomena which first suggested the undulatory theory 
 to the mind of Hooke. These are the colours of thin 
 transparent films of all kinds, known as the colours 
 of thin plates. In this relation no object in the world 
 possesses a deeper scientific interest than a common 
 soap-bubble. And here let me say emerges one of the 
 difficulties which the student of pure science encounters 
 in the presence of ' practical ' communities like those of 
 
 F 
 
66 ON LIGHT. LECT. 
 
 America and England ; it is not to be expected that 
 such communities can entertain any profound sympathy 
 with labours which seem so far removed from the domain 
 of practice as many of the labours of the man of science 
 are. Imagine Dr. Draper spending his days in blowing 
 soap-bubbles and in studying their colours ! Would 
 you show him the necessary patience, or grant him the 
 necessary support ? And yet be it remembered it was 
 thus that minds like those of Boyle, Newton and Hooke 
 were occupied ; and that on such experiments has been 
 founded a theory, the issues of which are incalculable. 
 I see no other way for you, laymen, than to trust the 
 scientific man with the choice of his inquiries ; he stands 
 before the tribunal of his peers, and by their verdict on 
 his labours you ought to abide. 
 
 Whence, then, are derived the colours of the soap- 
 bubble ? Imagine a beam of white light impinging 
 on the bubble. When it readies the first surface of the 
 film, a known fraction of the light is reflected back. 
 But a large portion of the beam enters the film, reaches 
 its second surface, and is again in part reflected. The 
 waves from the second surface thus turn back and hotly 
 pursue the waves from the first surface. And, if the 
 thickness of the film be such as to cause the necessary 
 retardation, the two systems of waves interfere with each 
 other, producing augmented or diminished light, as the 
 case may be. 
 
 But, inasmuch as the waves of light are of different 
 lengths, it is plain that, to produce self- extinction in 
 the case of the longer waves, a greater thickness of 
 film is necessary than in the case of the shorter ones. 
 Different colours, therefore, must appear at different 
 thicknesses of the film. 
 
ii. COLOUES OF THIN PLATES. 67 
 
 Take with you a little bottle of spirit of turpentine, 
 and pour it into one of your country ponds. You 
 will then see the flashing of those colours over the 
 surface of the water. On a small scale we produce them 
 thus : A common tea-tray is filled with water, beneath 
 the surface of which dips the end of a pipette. A beam 
 of light falls upon the water, and is reflected by it to 
 the screen. Spirit of turpentine is poured into the 
 pipette; it descends, issues from the end in minute 
 drops, which rise in succession to the surface. On 
 reaching it, each drop spreads suddenly out as a film, 
 and glowing colours immediately flash forth upon the 
 screen. The colours change as the thickness of the 
 film changes by evaporation. They are also arranged 
 in zones, in consequence of the gradual diminution of 
 thickness from the centre outwards. 
 
 Any film whatever will produce these colours. The 
 film of air between two plates of glass squeezed together, 
 exhibits, as shown by Hooke, rich fringes of colour. A 
 particularly fine example of these fringes is now before 
 you. Nor is even air necessary ; the rupture of optical 
 continuity suffices. Smite with an axe the black, trans- 
 parent ice black, because it is pure and of great depth 
 under the moraine of a glacier ; you readily produce in 
 the interior flaws which no air can reach, and from these 
 flaws the colours of thin plates sometimes break like fire. 
 But the source of most historic interest is, as already 
 stated, the soap-bubble. With one of these mixtures 
 employed by the eminent blind philosopher Plateau in 
 his researches on the cohesion figures of thin films, we 
 obtain in still air a bubble ten or twelve inches in 
 diameter. You may look at the bubble itself, or you 
 may look at its projection upon the screen; rich colours 
 
 F 2 
 
68 ON LIGHT. T.ECT. 
 
 arranged in zones are, in both cases, exhibited. Ren- 
 dering the beam parallel, and permitting it to impinge 
 upon the sides, bottom, and top of the bubble, gorgeous 
 fans of colour overspread the screen, rotating as the 
 beam is carried round the circumference of the bubble. 
 By this experiment the internal motions of the film are 
 also strikingly displayed. 
 
 Not in a moment are great theories elaborated : the 
 facts which demand them are first called into pro- 
 minence by observant minds ; then, to the period of 
 observation, succeeds a period of pondering and of 
 tentative explanation. By such efforts the human 
 mind is gradually prepared for the final theoretic 
 illumination. The colours of thin plates, for ex- 
 ample, occupied the attention of the celebrated Robert 
 Boyle. In his ' Experimental History of Colours ' he 
 contends against the schools which affirmed that colour 
 was ' a penetrative quality that reaches to the inner- 
 most parts of the object,' adducing opposing facts. 
 4 To give you a first instance,' he says, ' I shall need 
 but to remind you of what I told you a little after 
 the beginning of this essay, touching the blue and red 
 and yellow that may be produced upon a piece of 
 tempered steel ; for these colours, though they be very 
 vivid, yet if you break the steel they adorn they will 
 appear to be but superficial.' He then describes, in 
 phraseology which shows the delight he took in his 
 work, tne following beautiful experiment : 
 
 c We took a quantity of clean lead, and melted it 
 with a strong fire, and then immediately pouring it out 
 into a clean vessel of convenient shape and matter 
 ( we used one of iron, that the great and sudden heat 
 might not injure it), and then carefully and nimbly 
 
n. -BOYLE'S OBSERVATIONS. 69 
 
 taking off the scum that floated on the top, we per- 
 ceived, as we expected, the smooth and glossy surface 
 of the melted matter to he adorned with a very glorious 
 colour, which being as transitory as delightful, did 
 almost immediately give place to. another vivid colour, 
 and that was as quickly succeeded by a third, and this, 
 as it were, chased away by a fourth ; and so these wonder- 
 fully vivid colours successively appeared and vanished 
 till the metal ceasing to be hot enough to hold any 
 longer this pleasing spectacle, the colours that chanced 
 to adorn the surface when the lead thus began to cool 
 remained upon it, but were so superficial that how 
 little soever we scraped off the surface of the lead, we 
 did, in such places, scrape off all the colour.' ' These 
 things,' he adds, ' suggested to me some thoughts or 
 ravings which I have not now time to acquaint you, 
 with.' l 
 
 He extends his observations to chemical essential 
 oils and spirit of wine, ' which being shaken till they 
 have good store of bubbles, those bubbles will (if atten- 
 tively considered) appear adorned with various and 
 lovely colours, which all immediately vanish upon the 
 retrogressing of the liquid which affords these bubbles 
 their skins into the rest of the oil.' He also refers to 
 the colours of glass films. ' I have seen one that was 
 skilled in fashioning glasses by the help of a lamp blow- 
 ing some of them so strongly as to burst them : where- 
 upon it was found that the tenacity of the metal was 
 such that before it broke it suffered itself to be reduced 
 into films so extremely thin that they constantly showed 
 upon their surfaces the varying colours of the rainbow.' 2 
 
 1 Boyle's Works, Birch's edition, p. 675. * Page 743. 
 
70 ON LIGHT. LECT. 
 
 Subsequent to Boyle the colours of thin plates 
 occupied the attention of the celebrated Robert Hooke, 
 in whose writings we find a dawning of the undulatory 
 theory. He describes with great distinctness the colours 
 obtained with thin flakes of ' Muscovy glass' (talc), also 
 those surrounding flaws in crystals where optical con- 
 tinuity is destroyed. He shows very clearly the de- 
 pendence of the colour upon the thickness of the film, 
 and proves by microscopic observation that plates of a 
 uniform thickness yield uniform colours. ' If,' he says, 
 ' you take any small piece of the Muscovy glass, and 
 with a needle, or some other convenient instrument, 
 cleave it oftentimes into thinner and thinner laminye, 
 you shall find that until you come to a determinate 
 thinness of them they shall appear transparent and 
 colourless ; but if you continue to split and divide them 
 further, you shall find at last that each plate shall 
 appear most lovely tinged or imbued with a determinate 
 colour. If, further, by any means you so flaw a pretty 
 thick piece that one part begins to cleave a little from 
 the other, and between these two there be gotten some 
 pellucid medium, those laminated or pellucid bodies 
 that fill that space shall exhibit several rainbows or 
 coloured lines, the colours of which will be disposed 
 and ranged according to the various thicknesses of the 
 several parts of the plate.' He then describes fully 
 and clearly the experiment with pressed glasses already 
 referred to : 
 
 ' Take two small pieces of ground and polished look- 
 ing-glass plate, each about the bigness of a shilling : 
 take these two dry, and with your forefingers and 
 thumbs press them very hard and close together, and 
 ymi shall find that when they approach each other 
 
ir. HOOKE'S OBSERVATIONS. 71 
 
 very near there will appear several irises or coloured 
 lines, in the same manner almost as in the Muscovy 
 glass ; and you may very easily change any of the 
 colours of any part of the interposed hody by pressing 
 the plates closer and harder together, or leaving them 
 more lax that is, a part which appeared coloured with 
 a red may be presently tinged with a yellow, blue, 
 green, purple, or the like. Any substance,' he says, 
 4 provided it be thin and transparent, will show these 
 colours.' Like Boyle, he obtained them with glass 
 films ; he also ' produced them with bubbles of pitch, 
 rosin, colophony, turpentine, solutions of several gums, 
 as gum arabic in water, any glutinous liquor, as wort, 
 wine, spirit of wine, oyl of turpentine, glare of snails, 
 &c.' 
 
 Hooke's writings show that even in his day the idea 
 that both light and heat are modes of motion had taken 
 possession of many minds. ' First,' he says, ' that all kind 
 of fiery burning bodies have their parts in motion I think 
 will be very easily granted me. That the spark struck 
 from a flint and steel is in rapid agitation I have else- 
 where made probable ; . , . . that heat argues a motion 
 of the internal parts is (as I said before) generally 
 
 granted ; and that in all extremely hot shining 
 
 bodies there is a very quick motion that causes light, 
 as well as a more robust that causes heat, may be 
 argued from the celerity wherewith the bodies are dis- 
 solved. Next, it must be a vibrative motion. 1 His 
 reference to the quick motion of light and the more 
 robust motion of heat is a remarkable stroke of sagacity ; 
 but Hooke's direct insight is better than his reasoning ; 
 for the proofs he adduces that light is 'a vibrating 
 motion' have no particular bearing upon the question. 
 
79, ON LIGHT. LECT. 
 
 Still the Undulatoiy Theory was undoubtedly dawn- 
 ing upon the mind of this remarkable man . In endea- 
 vouring to account for the colours of thin plates, he again 
 refers to the relation of colour to thickness : he dwells 
 upon the fact that the film which shows these colours 
 must be transparent, proving this by showing that 
 however thin an opaque body was rendered no colours 
 were produced. ' This,' he says, ' I have often tried by 
 pressing a small globule of mercury between two smooth 
 plates of glass, whereby I have reduced that body to a 
 much greater thinness than was requisite to exhibit 
 the colours with a transparent body.' Then follows 
 the sagacious remark that to produce the colours 
 'there must be a considerable reflecting body adjacent 
 to the tinder or further side of the lamina or plate : 
 for this I always foutid, that the greater that reflection 
 was, the more vivid were the appearing colours. From 
 which observations,' he continues, ' it is most evident, 
 that the reflection from the under or further side of 
 the body is the principal cause of the production of 
 these colours.' 
 
 He draws a diagram, correctly representing the 
 reflection at the two surfaces of the film ; but here 
 his clearness ends. He ascribes the colours to a 
 coalescence or confusion of the two reflected pulses ; 
 the principle of interference being unknown to him, he 
 could not go further in the way of explanation. 
 
 8. Newton's Rings. Relation of Colour to 
 Thickness of Film. 
 
 In this way, then, by the active operation of different 
 minds, facts are observed, examined, and the precise 
 
ii. NEWTON'S OBSERVATIONS. 73 
 
 conditions of their appearance determined. All such 
 work in science is the prelude to other work ; and the 
 efforts of Boyle and Hooke cleared the way for the 
 optical career of Newton. He conquered the difficulty 
 which Hooke had found insuperable, and determined by 
 accurate measurements the relation of the thickness of 
 the film to the colour of displays. In doing this his 
 first care was to obtain a film of variable and calculable 
 depth. On a plano-convex glass lens (D B E, fig. 15) 
 
 FIG. 15. 
 
 of very feeble curvature he laid a plate of glass (AC) 
 with a plane surface, thus obtaining a film of air of 
 gradually increasing depth from the point of contact (B) 
 outwards. On looking at the film in monochromatic 
 light he saw, with the delight attendant on fulfilled 
 prevision, surrounding the place of contact a series 
 of bright rings separated from each other by dark 
 ones, and becoming more closely packed together as 
 the distance from the point of contact augmented 
 (as in fig. 16). When he employed red light, his rings 
 had certain diameters ; when he employed blue light, 
 the diameters were less. In general terms, the more 
 refrangible the light the smaller were the rings. rB 1 
 Causing his glasses to pass through the spectrum 
 from red to blue, the rings gradually contracted ; 
 when the passage was from blue to red, the 
 rings expanded. This is a beautiful experiment, and 
 appears to have given Newton the most lively satis- 
 faction. When white light fell upon the glasses. 
 
74 OX LIGHT. LECT. 
 
 inasmuch as the colours were not superposed, a series 
 of iris-coloured circles was obtained. A magnified 
 image of Newton's rings is now before you, and, by 
 employing in succession red, blue, and white light, we 
 obtain all the effects observed by Newton. You notice 
 that in monochromatic light the rings run closer and 
 
 FIG. 16. 
 
 closer together as they recede from the centre. This is 
 due to the fact that at a distance the film of air thickens 
 more rapidly than near the centre. When white light 
 is employed, this closing up of the rings causes the 
 various colours to be superposed, so that after a certain 
 thickness they are blended together to white light, the 
 rings then ceasing altogether. It needs but a moment's 
 reflection to understand that the colours of thin plates 
 are never unmixed or monochromatic. 
 
 Newton compared the tints obtained in this way 
 with the tints of his soap-bubble, and he calculated the 
 corresponding thickness. Itow he did this may be thus 
 made plain to you : Suppose the water of the ocean to 
 
IT. NEWTON'S RINGS. 75 
 
 be absolutely smooth ; it would then accurately repre- 
 sent the earth's curved surface. Let a perfectly hori- 
 zontal plane touch the surface at any point. Knowing 
 the earth's diameter, any engineer or mathematician 
 in this room could tell you how far the sea's surface 
 will lie below this plane, at the distance of a yard, ten 
 yards, a hundred yards, or a thousand yards from the 
 point of contact of the plane and the sea. It is common, 
 indeed, in levelling operations, to allow for the curva- 
 ture of the earth. Newton's calculation was precisely 
 similar. His plane glass was a tangent to his curved 
 one. From its refractive index and focal distance he 
 determined the diameter of the sphere of which his 
 curved glass formed a segment, he measured the dis- 
 tances of his rings from the place of contact, and he 
 calculated the depth between the tangent plane and 
 the curved - surface, exactly as the engineer would 
 calculate the distance between his tangent plane and 
 the surface of the sea. The wonder is, that, where 
 such infinitesimal distances are involved, Newton, with 
 the means at his disposal, could have worked with such 
 marvellous exactitude. 
 
 To account for these rings was the greatest difficulty 
 that Newton ever encountered. He quite appreciated 
 the difficulty. Over his eagle-eye there was no film no 
 vagueness in his conceptions. At the very outset his 
 theory was confronted by the question, Why, when a 
 beam of light is incident on a transparent body, are 
 some of the light-particles reflected and some trans- 
 mitted ? Is it that there are two kinds of particles, 
 the one specially fitted for transmission and the other 
 for reflection ? This cannot be the reason ; for, if 
 we allow a beam of light which has been reflected 
 
76 ON LIGHT. LECT. 
 
 from one piece of glass to fall upon another, it, as a 
 general rule, is also divided into a reflected and a trans- 
 mitted portion. The particles once reflected are not 
 always reflected, nor are the particles once transmitted 
 always transmitted. Newton saw all this ; he knew he 
 had to explain why it is that the self-same particle is at 
 one moment reflected and at the next moment trans- 
 mitted. It could only be through some change in the 
 condition of the particle itself. The self-same par- 
 ticle, he affirmed, was affected by ' fits' of easy trans- 
 mission and reflection. 
 
 9. Thzory of ' Fits ' applied to Newtorfs Rings. 
 
 If you are willing to follow me in an attempt to 
 reveal .the speculative groundwork of this theory of 
 fits, the intellectual discipline will, I think, repay you 
 for the necessary effort of attention. Newton was chary 
 of stating what he considered to be the cause of the 
 fits, but there can hardly be a doubt that his mind 
 rested on a physical cause. Nor can there be a doubt 
 that here, as in all attempts at theorising, he was 
 compelled to fall back upon experience for the material* 
 of his theory. Let us attempt to restore his course of 
 thought and observation. A magnet would furnish 
 him with the notion of attracted and repelled poles ;- 
 and he who habitually saw in the visible an image of 
 the invisible would naturally endow his light-particles 
 with such poles. Turning their attracted poles towards 
 a transparent substance, the particles would be sucked 
 in and transmitted ; turning their repelled poles, they 
 would be driven away or reflected. Thus, by tj 
 ascription of poles, the transmission and reflection 
 
ii. THEORY OF 'FITS.' 77 
 
 the self-same particle at different times might be ac- 
 counted for. 
 
 Regard these rings of Newton as seen in pure red 
 light: they are alternately bright and dark. The film 
 of air corresponding to the outermost of them is not 
 thicker than an ordinary soap-bubble, and it becomes 
 thinner on approaching the centre ; still Newton, as I 
 have said, measured the thickness corresponding to 
 every ring, and showed the difference of thickness be- 
 tween ring and ring. Now, mark the result. For the 
 sake of convenience, let us call the thickness of the film 
 of air corresponding to the first dark ring d ; then 
 Newton found the distance corresponding to the second 
 dark ring 2 d ; the thickness corresponding to the third 
 dark ring 3 d ; the thickness corresponding to the tenth 
 dark ring 10 d, and so on. Surely there must be some 
 hidden meaLing in this little distance d, which turns 
 up so constantly ? One can imagine the intense interest 
 with which Newton pondered its meaning. Observe 
 the probable outcome of his thought. He had endowed 
 his light- particles with poles, but now he is forced to 
 introduce the notion of periodic recurrence. Here his 
 power of transfer from the sensible to the subsensible 
 would render it easy for him to suppose the light-par- 
 ticles animated, not only with a motion of translation, 
 but also with a motion of rotation. Newton's astrono- 
 mical knowledge rendered all such conceptions familiar 
 to him. The earth has such a double motion. In the 
 time occupied in passing over a million and a half of 
 miles of its orbit that is, in twenty-four hours our 
 planet performs a complete rotation, and, in the time 
 required to pass over the distance d, Newton's light- 
 tide must be supposed to perform a complete rota- 
 
 < *^H 
 
 W 
 
78 ON LIGHT. LKCT. 
 
 tion. True, the light-particle is smaller than the planet, 
 and the distance d, instead of being a million and a 
 half of miles, is a little over the ninety thousandth of 
 an inch. But the two conceptions are, in point of in- 
 tellectual quality, identical. 
 
 Imagine, then, a particle entering the film of air 
 where it possesses this precise thickness. To enter the 
 film, its attracted end must be presented. Within the 
 film it is able to turn once completely round ; at the 
 other side of the film its attracted pole will be again 
 presented ; it will, therefore, enter the glass at the op- 
 posite side of the film and be lost to the eye. All round 
 the place of contact, wherever the film possesses this 
 precise thickness, the light will equally disappear we 
 shall therefore have a ring of darkness. 
 
 And now observe how well this conception falls i 
 with the law of proportionality discovered by Newton. 
 When the thickness of the film is 2 d, the particle has 
 time to perform two complete rotations within the 
 film ; when the thickness is 3 d, three complete rota- 
 tions ; when 10 d, ten complete rotations are per 
 formed. It is manifest that in each of these cases, on 
 arriving at the second surface of the film, the attracted 
 pole of the particle will be presented. It will, there- 
 fore, be transmitted ; and, because no light is sent to 
 the eye, we shall have a ring of darkness at each of 
 these places. 
 
 The bright rings follow immediately from the same 
 conception. They occur between the dark rings, the 
 thicknesses to which they correspond being also inter- 
 mediate between those of the dark ones. Take the case 
 of the first bright ring. The thickness of the film is 
 ^ d ; in this interval the rotating particle can perform 
 
ii. APPLICATION OF THEORY. 79 
 
 only half a rotation. When, therefore, it reaches the 
 second surface of the film, its repelled pole is pre- 
 sented ; it is, therefore, driven back and reaches the 
 eye. At all distances round the centre correspond- 
 ing to this thickness the same effect is produced, and 
 the consequence is a ring of brightness. The other 
 bright rings are similarly accounted for. At the second 
 one, where the thickness is 1^ d, a rotation and a half 
 is performed; at the third, two rotations and a half; 
 and at each of these places the particles present their 
 repelled poles to the lower surface of the film. They 
 are therefore sent back to the eye, and produce there 
 the impression of brightness. This analysis, though 
 involving difficulties when closely scrutinised, enables 
 us to see how the theory of fits may have grown into 
 consistency in the mind of Newton. 
 
 It has been already stated that the Emission Theory 
 assigned a greater velocity to light in glass and water 
 than in air or stellar space ; and that on this point 
 it was at direct issue with the theory of undulation, 
 which makes the velocity in air or stellar space greater 
 than in glass or water. By an experiment proposed by 
 Arago, and executed with consummate skill by Fou- 
 cault and Fizeau, this question was brought to a crucial 
 test, and decided in favour of the theory of undula- 
 tion. 
 
 In the present instance also the two theories are at 
 variance. Newton assumed that the action which pro- 
 duces the alternate bright and dark rings took place at a 
 single surface ; that is, the second surface of the film. 
 The undulatory theory affirms that the rings are caused 
 by the interference of waves reflected from both sur- 
 faces. This also has been demonstrated by experiment. 
 
80 ON LIGHT. 
 
 IJBCT. 
 
 By a proper arrangement, as we shall afterwards learn, 
 we may abolish reflection from one of the surfaces of 
 the film, and when this is done the rings vanish alto- 
 gether. 
 
 Rings of feeble intensity are also formed by trans- 
 mitted light. These are referred by the undulatory 
 theory to the interference of waves which have passed 
 directly through the film, with others which have suf- 
 fered two reflections within the film. They are thus 
 completely accounted for. 
 
 10. The Diffraction of Light. 
 
 Newton's espousal of the emission theory is said to 
 have retarded scientific discovery. It might, however, 
 be questioned . whether, in the long run, the errors 
 of great men have not really their effect in ren- 
 dering intellectual progress rhythmical, instead of 
 permitting it to remain uniform, the ' retardation ' in 
 each case being the prelude to a more impetuous 
 advance. It is confusion and stagnation, rather than 
 error, that we ought to avoid. Thus, though the undu- 
 latory theory was held back for a time, it gathered 
 strength in the interval, and its development within 
 the last half century has been so rapid and trium- 
 phant as to leave no rival in the field. We have now 
 to turn to the investigation of new classes of pheno- 
 mena, of which it alone can render a satisfactory 
 account. 
 
 Newton, who was familiar with the idea of an ether, 
 and who introduced it in some of his speculations, 
 objected, as already stated, that if light consisted of 
 waves shadows could not exist ; for that the waves 
 would bend round the edges of opaque bodies and 
 
ii. DIFFKACTION. 81 
 
 agitate the ether behind them. He was right in 
 affirming that this bending ought to occur, but wrong 
 in supposing that it does not occur. The bending is 
 real, though in all ordinary cases it is masked by the 
 action of interference. This inflection of the light 
 receives the name of Diffraction. 
 
 To study the phenomena of diffraction it is necessary 
 that our source of light should be a physical point, 
 or a fine line ; for when luminous surfaces are employed, 
 the waves issuing from different points of the surface 
 obscure and neutralize each other. A point of light of 
 high intensity is obtained by admitting the parallel rays 
 of the sun through an aperture in a window-shutter, and 
 concentrating the beam by a lens of short focus. The 
 small solar image at the focus constitutes a suitable 
 point of light. The image of the sun formed on the 
 convex surface of a glass bead, or of a wateh-glass 
 blackened within, though less intense, will also answer. 
 An intense line of light is obtained by. admitting the 
 sunlight through a slit, and sending it through a 
 strong cylindrical lens. The slicjs of lightis contracted to 
 a physical line at the focus of the lens. A glass tube 
 blackened within and placed in the light, reflects from 
 its surface a luminous line which, though less intense, 
 also answers the purpose. 
 
 In the experiment now to be described a vertical 
 slit of variable width is placed in front of the electric 
 lamp, and this slit is looked at from a distance through 
 another vertical slit, also of variable aperture, and held 
 in the hand. 
 
 The light of the lamp being, in the first place, 
 rendered monochromatic by placing a pure red glass in 
 front of the slit, when the eye is placed in the straight 
 
 G 
 
82 ON LIGHT. LECT, 
 
 line drawn through both slits an extraordinary appear- 
 ance (shown in fig. 17) is observed. Firstly, the slit 
 in front of the lamp is seen as a vivid rectangle of light, 
 but right and left of it is a long series of rectangles, 
 decreasing in vividness, and separated from each other 
 by intervals of absolute darkness. 
 
 The breadth of these bands is seen to vary with the 
 width of the slit held before the eye. When the slit 
 is widened the bands become narrower, and they crowd 
 more closely together ; when the slit is narrowed, the 
 
 FIG. 17. 
 
 individual bands widen and also retreat from each other, 
 leaving between them wider spaces of darkness than 
 before. 
 
 Leaving everything else unchanged, let a blue glass 
 or a solution of ammonia-sulphate of copper, which 
 gives a very pure blue, be placed in the path of the 
 light. A series of blue bands is thus obtained, exactly 
 like the former in all respects save one ; the blue 
 rectangles are narrower, and they are closer together 
 than the red ones. 
 
 If we employ colours of intermediate refrangibilities, 
 which we may do by causing the different colours of a 
 spectrum to shine through the slit, we obtain bands of 
 colour intermediate in width and occupying interme- 
 
IT. 
 
 DIFFRACTION BANDS. 
 
 83 
 
 diate positions between those of the red and blue. The 
 aspect of the bands in red, green, and violet light is 
 represented in fig. 18. When white light, therefore, 
 passes through the slit the various colours are not 
 superposed, and instead of a series of monochromatic 
 bands, separated from each other by intervals of dark- 
 ness, we have a series of coloured spectra placed side- 
 by side. When the distant slit is illuminated by a 
 candle flame, instead of the more intense electric light, 
 or when a distant platinum wire raised to a white heat 
 
 FIG. 18. 
 
 Ill III 1 
 
 by an electric current is employed, substantially the 
 same effects are observed. 
 
 11. Application of the Wave-theory to the Phe- 
 nomena of Diffraction. 
 
 Of these and of a multitude of similar effects the 
 Emission Theory is incompetent to offer any satisfactory 
 explanation. Let us see how they are accounted for by 
 the Theory of Undulation. 
 
 And here, with the view of reaching absolute clear- 
 ness, I must make an appeal to that faculty the 
 importance of which I have dwelt upon so earnestly 
 
 G 2 
 
84 ON LIGHT JJSCT. 
 
 here and elsewhere the faculty of imagination. Figure 
 yourself upon the sea-shore, with a well-formed wave 
 advancing. Take a line of particles along the front of 
 the wave, all at the same distance below the crest ; they 
 are all rising in the same manner and at the same rate. 
 Take a similar line of particles on the back of the wave, 
 they are all falling in the same manner and at the 
 same rate. Take a line of particles along the crest, 
 they are all in the same condition as regards the motion 
 of the wave. The same is true for a line of particles 
 along the furrow of the wave. 
 
 The particles referred to in each of these cases re- 
 spectively being in the same condition as regards the 
 motion of the wave, are said to be in the same phase 
 of vibration. Bat if you compare a particle on the 
 front of the wave with one at the back ; or more 
 generally, if you compare together any two particles 
 not occupying the same position in the wave, their 
 conditions of motion not being the same, they are said 
 to be in different phases of vibration . If one of the 
 .particles lie upon the crest, and the other on the furrow 
 of the wave, then, as one is about to rise and the other 
 about to fall, they are said to be in opposite phases of 
 vibration. 
 
 There is still another point and it is one of the 
 utmost importance as regards our present subject to 
 be cleared up. Let O (fig. 19) be a point in still 
 water which, when disturbed, produces a series of 
 circular waves : the disturbance necessary to produce 
 these waves is simply an oscillation up and down of the 
 point 0. Let m n be the position of the ridge of one 
 of the waves at any moment, and m f n' its position a 
 second or two afterwards. Now every particle of water, 
 
ii. PRINCIPLE OF HUYGHENS. 85 
 
 as the wave passes it, oscillates, as we have learned, up 
 and down. If, then, this oscillation be a sufficient 
 origin of wave-motion, then each distinct particle of 
 
 the wave m n ought to give birth to a series of circular 
 waves. This is the important point up to which I 
 wished to lead you. Every particle of the wave m n does 
 act in this way. Taking each particle as a centre, and 
 surrounding it by a circular wave with a radius equal 
 to the distance between m n and m f n' , the coalescence 
 of all these little waves would build up the larger 
 ridge m' n f exactly as we find it built up in nature. 
 Here, in fact, we resolve the wave-motion into its 
 elements, and having succeeded in doing this we shall 
 have no great difficulty in applying our knowledge to 
 optical phenomena. 
 
 Now let us return to our slit, and, for the sake of 
 simplicity, we will first consider the case of monochro- 
 matic light. Conceive a series of waves of ether 
 advancing from the first slit towards the second, and 
 finally filling the second slit. When each wave passes 
 through the latter it not only pursues its direct course 
 
ON LIGHT. 
 
 LECt. 
 
 to the retina, but diverges right and left, tending to 
 throw into motion the entire mass of the ether 
 behind the slit. In fact, as already explained, every 
 point of the ivave which fills the slit is itself a centre 
 of a new wave-system, which is transmitted in all 
 directions through the ether behind the slit. This is 
 the celebrated principle of Huyghens : we have now 
 to examine how these secondary waves act upon each 
 other. 
 
 Let us first regard the central band of the series. "Let 
 A P (fig. 20) be the width of the aperture held before the 
 
 eye, grossly exaggerated of course, and let the dots across 
 the aperture represent ether particles, all in the same 
 phase of vibration. Let E T represent a portion of the 
 retina; From O, in the centre of the slit, let a per- 
 pendicular R be imagined drawn upon the retina. The 
 motion communicated to the point R will then be the 
 sum of all the motions emanating in this direction 
 from the ether particles in the slit. Considering the 
 extreme narrowness of the aperture, we may, without 
 sensible error, regard all points of the wave A P as 
 equally distant from R. No one of the partial 
 
ii. EXPLANATION OF BANDS. 87 
 
 waves lags sensibly behind the others : hence, at R, and 
 in its immediate neighbourhood, we have no sensible 
 reduction of the light by interference. This undi- 
 minished light produces the brilliant central band of 
 the series. 
 
 Let us now consider those waves which diverge 
 laterally behind the slit. In this case, the waves from 
 the two sides of the slit have, in order to converge 
 upon the retina, to pass over unequal distances. Let 
 A P (fig. 21) represent, as before, the width of the 
 
 second slit. We have now to consider the action of 
 the various parts of the wave A P upon a point R' of 
 the retina, not situated in the line joining thie slits. 
 Let us take the particular case in which the difference 
 in path from the two marginal points A, P, to the retina 
 is a whole wave-length of the red light ; how must this 
 difference affect the final illumination of the retina ? 
 
 Let us fix our attention upon the particular oblique 
 line that passes through the centre of the slit to the 
 retina at R'. The difference of path between the waves 
 which pass along this line and those from the two 
 
88 ON LIGHT. LBCT. 
 
 margins is, in the case here supposed, half a wave- 
 length. Make e R' equal to P R', join P and e, and 
 draw d parallel to P e. A e is then the length of a 
 wave of light, while A d is half a wave-length. Now 
 the least reflection will make it clear that not only 
 is there discordance between the central and marginal 
 waves, but that every line of waves such as x R', on 
 the one side of OR 7 , finds a line x' R' upon the other 
 side of R, from which its path differs by half an 
 undulation, with which, therefore, it is in complete 
 discordance. The consequence is that the light on the 
 one side of the central line will completely abolish the 
 light on the other side of that line, absolute darkness 
 being the result of their coalescence. The first dark 
 interval of our series of bands is thus accounted for. 
 It is produced by an obliquity of direction which causes 
 the paths of the marginal waves to be a whole wave- 
 length different from each other. 
 
 When the difference between the paths of the mar- 
 ginal waves is half a wave-length, a partial destruction 
 of the light is effected. The luminous intensity corre- 
 sponding to this obliquity is a little less than one-half 
 accurately 0'4 that of the undiffracted light, 
 
 If the paths of the marginal waves be three semi- 
 undulations different from each other, and if the whole 
 beam be divided into three equal parts, two of these 
 parts will, for the reasons just given, completely neu- 
 tralize each other, the third only being effective. 
 Corresponding, therefore, to an obliquity which pro- 
 duces a difference of three semi-undulations in the 
 marginal waves, we have a luminous band, but one of 
 considerably less intensity than the undiffracted cen- 
 tral band. 
 
ii. DIFFRACTION THROUGH SEVERAL APERTURES. 89 
 
 With a marginal difference of path of four semi- 
 undulations we have a second extinction of the entire 
 beam, because here the beam can be divided into four 
 equal parts, every two of which quench each other. 
 A second space of absolute darkness will therefore 
 correspond to the obliquity producing this difference. 
 In this way we might proceed further, the general 
 result being that, whenever the direction of wave- 
 motion is such as to produce a marginal difference of 
 path of an even number of semi-undulations, we have 
 complete extinction ; while, when the marginal dif- 
 ference is an odd number of semi-undulations, we have 
 only partial extinction, a portion of the beam remaining 
 as a luminous band. 
 
 A moment's reflection will make it plain that the 
 wider the slit the less will be the obliquity of direction 
 needed to produce the necessary difference of path. 'With 
 a wide slit, therefore, the bands, as observed, will be closer 
 together than with a narrow one. It is also plain that 
 the shorter the wave, the less will be the obliquity re- 
 quired to produce the necessary retardation. The maxima 
 and minima of violet light must therefore fall nearer to 
 the centre than the maxima and minima of red light. 
 The maxima and minima of the other colours fall 
 between these extremes. In this simple way the 
 undulatory theory completely accounts for the extra- 
 ordinary appearance above referred to. 
 
 When a slit and telescope are used, instead of the 
 slit and naked eye, the effects are magnified and ren- 
 dered more brilliant. Looking, moreover, through a 
 properly adjusted telescope with a small circular aper- 
 ture in front of it, at a distant point of light, the point 
 is seen encircled by a series of coloured bands. If 
 
90 
 
 ON LIGHT. 
 
 LECT. 
 
 monochromatic light be used, these bands are simply 
 bright and dark, but with white light the circles display 
 iris-colours. If a slit be shortened so as to form a 
 square aperture, we have two series of spectra at right 
 angles to each other. The effects, indeed, are capable 
 of endless variation by varying the size, shape, and 
 number of the apertures through which the point of 
 
 FIG. 22. 
 
 light is observed. Through two square apertures, with 
 their corners touching each other as at A, Schv erd 
 observed the appearance shown in fig. 22. Adding wo 
 others to them, as at B, he observed the appearai ce 
 represented in fig. 23. The position of every ba id 
 of light and shade in such figures has been calculated 
 from theory by Fresnel, Fraunhofer, Herschel, Schwerd, 
 and others, and completely verified by experiment. 
 
SCHWERD'S OBSEKVATIONS. 
 
 91 
 
 Vour eyes could not tell you with greater certainty of 
 the existence of these bands than the theoretic calcu- 
 lation. 
 
 The street-lamps at night, looked at through the 
 meshes of a handkerchief, show diffraction phenomena. 
 The diffraction effects obtained in looking through a 
 bird's feathers are, as shown by Schwerd, very brilliant. 
 
 FH> 23. 
 
 The ' idescence of certain Alpine clouds is also an effect 
 of c T ffraction which may be imitated by the spores 
 of ^ycopodium. When shaken over a glass plate 
 thesp spores cause a point of light, looked at through 
 the dusted plate, to be surrounded by coloured circles, 
 which rise to actual splendour when the light becomes 
 intense. Shaken in the air the spores produce the same 
 
92 ON LIGHT. LECT. 
 
 effect. The diffraction phenomena obtained during 
 the artificial precipitation of clouds from the vapours 
 of various liquids in an intensely illuminated tube are 
 exceedingly fine. 
 
 . One of the most interesting cases of diffraction by 
 small particles that ever came before me was that of 
 an artist whose vision was disturbed by vividly-coloured 
 circles. He was in great dread of losing his sight ; 
 assigning as a cause of his increased fear that the 
 circles were becoming larger and the colours more 
 vivid. I ascribed the colours to minute particles in 
 the humours of the eye, and ventured to encourage 
 him by the assurance that the increase of size and 
 vividness on the part of the circles indicated that the 
 diffracting particles were becoming smaller, and that 
 they might finally be altogether absorbed. The predic- 
 tion was verified. It is needless to say one word on the 
 necessity of optical knowledge in. the case of the prac- 
 tical oculist. . , . . . 
 
 Without oreakirig ground on the chromatic pheno- 
 mena presented by crystals, two other sources of colour 
 may be mentioned here. By interference in the earth's 
 atmosphere -the light of, a star, as shown by Arago, is 
 self-extinguished, the twinkling of the star and the 
 changes of colour which it undergoes being due to this 
 cause. Looking at such a star through an opera- 
 glass, and shaking the glass so as to cause the 
 image of the star to pass rapidly over the retina, 
 you produce a row of coloured beads, the spaces 
 between which correspond to the periods of extinction. 
 Fine scratches drawn upon glass or polished metal 
 reflect the waves of light from their sides ; and 
 some, being reflected from opposite sides of the 
 
ii. COLOURS OF STRIATED SURFACES. 93 
 
 same scratch, interfere with and quench each other. 
 But the obliquity of reflection which extinguishes 
 the shorter waves does not extinguish the longer 
 ones, hence the phenomena of colour. These are 
 called the colours of striated surfaces. They are 
 beautifully illustrated by mother-of-pearl. This shell 
 is composed of exceedingly thin layers, which, when cut 
 across by the polishing of the shell, expose their edges 
 and furnish the necessary small and regular grooves. 
 The most conclusive proof that the colours are due to 
 the mechanical state of the surface is to be found in 
 the fact, established by Brewster, that by stamping the 
 shell carefully upon black sealing-wax, we transfer the 
 grooves, and produce upon the wax "the colours of 
 mother-of-pearl. 
 
 LI i; ii V>v 
 
 v^ i A- i. i?< I T Y O F 
 
94 
 
 ON LIGHT. 
 
in. 
 
 LECTUEE III. 
 
 RELATION Of THEORIES TO EXPERIENCE ORIGIN OF TUB NOTION OF THE 
 ATTRACTION OF GRAVITATION NOTION OF POLARITY, HOW GENERATED 
 ATOMIC POLARITY STRUCTURAL ARRANGEMENTS DUE TO POLARITY 
 ARCHITECTURE OF CRYSTALS CONSIDERED AS AN INTRODUCTION TO 
 
 THEIR ACTION UPON LIGHT NOTION OF ATOMIC POLARITY APPLIED TO 
 
 CRYSTALLINE STRUCTURE EXPERIMENTAL ILLUSTRATIONS CRYSTAL- 
 LIZATION OF WATER EXPANSION BY HEAT AND BY COLD DEPORTMENT 
 
 OF WATER CONSIDERED AND EXPLAINED BEARINGS OF CRYSTALLI/A- 
 
 TION ON OPTICAL PHENOMENA REFRACTION DOUBLE REFRACTION- 
 POLARIZATION ACTION OF TOURMALINE CHARACTER OF THE BEAMS 
 
 EMERGENT FROM ICELAND SPAK POLARIZATION BY ORDINARY RE- 
 FRACTION AND REFLECTION DEPOLARIZATION. 
 
 1. Derivation of Theoretic Conceptions from 
 Experience. 
 
 ONE of the objeets of our last lecture, and that not the 
 least important, was to illustrate the manner in which 
 scientific theories are formed. They, in the first place, 
 take their rise in the desire of the mind to penetrate 
 to the sources of phenomena. From its infinitesi- 
 mal beginnings, in ages long past, this desire has 
 grown and strengthened into an imperious demand of 
 man's intellectual nature. It long ago prompted 
 Caesar to say that he would exchange his victories for 
 a glimpse of the sources of the Nile ; it wrought itself 
 into the atomic theories of Lucretius ; it impels Darwin 
 to those daring speculations which of late years have 
 so agitated the public mind. But in no case in framing 
 
96 ON LIGHT. 
 
 LKCT. 
 
 theories does the imagination create its materials. It 
 expands, diminishes, moulds and refines, as the case 
 may be, materials derived from the world of fact and 
 observation. 
 
 This is more evidently the case in a theory like that 
 of light, where the motions of a subsensible medium, 
 the ether, are presented to the mind. But no theory 
 escapes the condition. Newton took care not to en- 
 cumber the idea of gravitation with unnecessary physi- 
 cal conceptions; but we know that he indulged in 
 them, though he did not connect them with his 
 theory. But even the theory as it stands did not 
 enter the mind as a revelation dissevered from the 
 world of experience. The germ of the conception 
 that the sun and planets are held together by a force 
 of attraction is to be found in the fact that a 
 magnet had been previously seen to attract iron. The 
 notion of matter attracting matter came thus from 
 without, not from within. In our present lecture the 
 magnetic force must serve us as the portal into a new 
 subsensible domain; but in the first place we must 
 master its elementary phenomena. 
 
 The general facts of magnetism are most simply 
 illustrated by a magnetized bar of steel, commonly 
 called a bar magnet. Placing such a magnet upright 
 upon a table, and bringing a magnetic needle near its 
 bottom, one end of the needle is observed to retreat 
 from the magnet, while the other as promptly ap- 
 proaches. The needle is held quivering there by some 
 invisible influence exerted upon it. Eaising the needle 
 along the magnet, but still avoiding contact, the ra- 
 pidity of its oscillations decreases, because the force 
 acting upon it becomes weaker. At the centre the oscil- 
 
in. EXTENSION OF MAGNETISM TO MOLECULES. 97 
 
 lations cease. Above the centre, the end of the needle 
 which had been previously drawn towards the magnet 
 retreats, and the opposite end approaches. As we as- 
 cend higher, the oscillations become more violent, 
 because the force becomes stronger. At the upper end 
 of the magnet, as at the lower, the force reaches a 
 maximum ; but all the lower half of the magnet, from 
 E to S (^fig. 25), attracts one end of the needle, while 
 all the upper half, from E to N, attracts the opposite 
 end. This doubleness of the magnetic force is called 
 
 FIG. 25. 
 
 polarity, and the points near the ends of the magnet in 
 which the forces seem concentrated are called its poles. 
 What, then, will occur if we break this magnet in 
 two at the centre E ? Shall we obtain two magnets, 
 each with a single pole ? No ; each half is in itself a 
 perfect magnet, possessing two poles. This may be 
 proved by breaking something of less value than the 
 magnet the steel of a lady's stays, for example, 
 hardened and magnetized. It acts like the magnet. 
 When broken, each half acts like the whole ; and when 
 
 H 
 
<J8 ON LIGHT. LECT. 
 
 these parts are again broken, we have still the perfect 
 magnet, possessing, as in the first instance, two poles. 
 Push your breaking to its utmost sensible limit, you 
 cannot stop there. The bias derived from observa- 
 tion will infallibly carry you beyond the bourne of 
 the senses, and compel you to regard this thing 
 that we call magnetic polarity as resident in the 
 ultimate particles of the steel. You come to the 
 conclusion that each atom of the magnet is endowed 
 with this polar force. 
 
 Like all other forces, this force of magnetism is 
 amenable to mechanical laws ; and, knowing the direc- 
 tion and magnitude of the force, we can predict its 
 action. Placing a small magnetic needle near a bar 
 magnet, it takes up a determinate position. That 
 position might be deduced theoretically from the 
 mutual action of the poles. Moving the needle round 
 the magnet, for each point of the surrounding space 
 there is a definite direction of the needle, and no 
 other. A needle of iron will answr as well as the 
 magnetic needle ; for the needle of iron is magnetized 
 by the magnet, and acts exactly like a steel needle 
 independently magnetized. 
 
 If we place two or more needles of iron near the mag- 
 net, the action becomes more complex, for then the 
 needles are not only acted on by the magnet, but 
 they act upon each other. And if we pass to smaller 
 masses of iron to iron filings, for example we find 
 that they act substantially as the needles, arranging 
 themselves in definite forms, in obedience to the mag- 
 netic action. 
 
 Placing a sheet of paper or glass over a bar 
 magnet and showering iron filings upon the paper, I 
 
POLARITY AND STRUCTURE. 
 
 99 
 
 notice a tendency of the filings to arrange themselves 
 in determinate lines. They cannot freely follow this 
 tendency, for they are hampered by the friction against 
 the paper. They are helped by tapping the paper ; 
 
 FIG. 26. 
 
 N is the nozzle of Hie lamp ; M a plane mirror, reflecting the beam upwards. At 
 P the magnets and iron filings are placed ; L is a lens which forms an image of 
 the magnets and filings ; and B, is a totally-reflecting prism, which casts the image 
 G upon the screen. 
 
 each tap releasing them for a moment, and enabling 
 them to follow their tendencies. But this is an experi- 
 ment which can only be seen by myself. To enable you 
 all to see it, I take a pair of small magnets and by a 
 simple optical arrangement throw the magnified images 
 of the magnets upon the screen. Scattering iron filings 
 over the glass plate to which the small magnets are 
 attached, and tapping the plate, you see the arrange- 
 
 H 2 
 
100 ON LIGHT. LBCT. 
 
 merit of the iron filings in those magnetic curves which 
 have been so long familiar to scientific men. 1 
 
 The aspect of these curves so fascinated Faraday 
 that the greater portion of his intellectual life was de- 
 voted to pondering over them. He invested the space 
 through which they run with a kind of materiality ; 
 and the probability is that the progress of science, by 
 connecting the phenomena of magnetism with the lumi- 
 niferous ether, will prove these ' lines of force,' as 
 Faraday loved to call them, to represent a condition of 
 this mysterious substratum of all radiant action. 
 
 But it is not the magnetic curves, as such, but 
 their relationship to theoretic conceptions that we have 
 now to consider. By the action of the bar magnet upon 
 the needle we obtain a notion of a polar force ; by the 
 breaking of the strip of magnetized steel, we attain the 
 notion that polarity can attach itself to the ultimate 
 particles of matter. The experiment with the iron 
 filings introduces a new idea into the mind ; the idea, 
 namely, of structural arrangement. Every pair of 
 filings possesses four poles, two of which are attractive 
 and two repulsive. The attractive poles approach, the 
 repulsive poles retreat ; the consequence being a certain 
 definite arrangement of the particles with reference to 
 each other. 
 
 2. Theory of Crystallization. 
 
 Now, this idea of structure, as produced by polar 
 force, opens a way for the intellect into an entirely new 
 
 1 Very beautiful specimens of these curves hare been recently 
 obtained and /.red by my distinguished friend, Prof. Mayer, of Hoboken, 
 to whom I am indebted for the original of the woodcut placed in front 
 of this Lecture. 
 
in. CRYSTALS BUILT BY POLAR FORCE. 101 
 
 region, and the reason you are asked to accompany me 
 into this region is, that our next enquiry relates to the 
 action of crystals upon light. Prior to speaking of this 
 action, I wish you to realise intellectually the process 
 of crystalline architecture. Look then into a granite 
 quarry, and spend a few minutes in examining the 
 rock. It is not of perfectly uniform texture. It is 
 rather an agglomeration of pieces, which, on examina- 
 tion, present curiously-defined forms. You have there 
 what mineralogists call quartz, you have felspar, you 
 have mica. In a mineralogical Cabinet, where these 
 substances are preserved separately, you will obtain 
 some notion of their forms. You will see there, also, 
 specimens of beryl, topaz, emerald, tomrmaline, heavy 
 spar, fluor-spar, Iceland spar possibly 'a 'full-formed 
 diamond, as it quitted the hand of Nature, not yet 
 having got into the hands of the lapidary. 
 
 These crystals, you will observe, are put together ac- 
 cording to law ; they are not chance productions ; and, if 
 you care to examine them more minutely, you will find 
 their architecture capable of being to some extent 
 revealed. They often split in certain directions before 
 a knife-edge, exposing smooth and shining surfaces, 
 which are called planes of cleavage ; and by following 
 these planes you sometimes reach an internal form, 
 disguised beneath the external form of the crystal. 
 Ponder these beautiful edifices of a hidden builder. 
 You cannot help asking yourself how they were built ; 
 and familiar as you now are with the notion of a polar 
 force, and the ability of that force to produce structural 
 arrangement, your inevitable answer will be, that those 
 crystals are built by the play of polar forces with which 
 their molecules are endowed. In virtue of these forces, 
 
.102 ON LIGHT. LECT. 
 
 atom lays itself to atom in a perfectly definite way, 
 the final visible form of the crystal depending upon 
 this play of its molecules. 
 
 Everywhere in Nature we observe this tendency to 
 run into definite forms, and nothing is easier than to 
 give scope to this tendency by artificial arrangements. 
 Dissolve nitre in water, and allow the water slowly to 
 evaporate ; the nitre remains, and the solution soon 
 becomes so concentrated that the liquid condition can 
 no longer be preserved. The nitre-molecules approach 
 each other, and come at length within the range of 
 their polar forces. They arrange themselves in obedi- 
 ence to these forces, a minute crystal of nitre being at 
 first produced. On this crystal the molecules continue 
 to deposit themselves from the surrounding liquid. 
 The crystal grows, and finally we have large prisms of 
 nitre, each of a perfectly definite shape. Alum crys- 
 tallizes with the utmost ease in this fashion. The 
 resultant crystal is, however, different in shape from 
 that of nitre, because the poles of the molecules are 
 differently disposed. If they be only nursed with 
 proper care, crystals "of these substances may be caused 
 to grow to a great size. 
 
 The condition of perfect crystallization is, that the 
 crystallizing force shall act with deliberation. There 
 should be no hurry in its operations ; but every mole- 
 cule ought to be permitted, without disturbance from 
 its neighbours, to exercise its own rights. If the crys- 
 tallization be toq sudden, the regularity disappears. 
 Water may be saturated with sulphate of soda, dissolved 
 when the water is hot, and afterwards permitted to cool. 
 When cold the solution is supersaturated ; that is to say, 
 more solid matter is contained in it than corresponds to 
 
in. ILLUSTRATIONS OF CRYSTALLIZATION. 103 
 
 its temperature. Still the molecules show no sign of 
 building themselves together. 
 
 This is a very remarkable, though a very common 
 fact. The molecules in the centre of the liquid are so 
 hampered by the action of their neighbours that freedom 
 to follow their own tendencies is denied to them. Fix your 
 mind's eye upon a molecule within the mass. It wishes 
 to unite with its neighbour to the right, but it wishes 
 equally to unite with its neighbour to the left ; the 
 one tendency neutralizes the other, and it unites with 
 neither. But, if a crystal of sulphate of soda be dropped 
 into the solution, the molecular indecision ceases. On 
 the crystal the adjacent molecules will immediately 
 precipitate themselves ; on these again others will be 
 precipitated, and this act of precipitation will continue 
 from the top of the flask to the bottom, until the 
 solution has, as far as possible, assumed the solid form. 
 The crystals here produced are small, and confusedly 
 arranged. The process has been too hasty to admit of 
 the pure and orderly action of the crystallizing force. 
 It typifies the state of a nation in which natural and 
 healthy change is resisted, until society becomes, as it 
 were, supersaturated with the desire for change, the 
 change being then effected through confusion and revo- 
 lution. 
 
 Let ine illustrate the action of crystallizing force by 
 two examples of it : Nitre might be employed, but 
 another well-known substance enables me to make the 
 experiment in a better form. The substance is com- 
 mon sal-ammoniac, or chloride of ammonium, dissolved 
 in water. Cleansing perfectly a glass plate, the solu- 
 tion of the chloride is poured over the glass, to which, 
 when the plate is set on edge, a thin film of the liquid 
 
104 ON LIGHT. LECT. 
 
 adheres. Warming the glass slightly, evaporation is 
 promoted, but by evaporation the water only is removed. 
 The plate is then placed in a solar microscope, and an 
 image of the film is thrown upon a white screen. The 
 warmth of the illuminating beam adds itself to that 
 already imparted to the glass plate, so that after a 
 moment or two the dissolved salt can no longer exist in 
 the liquid condition. Molecule then closes with mole- 
 cule, and you have a most impressive display of crystal- 
 lizing energy overspreading the whole screen. You 
 may produce something similar if you breathe upon the 
 frost-ferns which overspread your window-panes in 
 winter, and then observe through a pocket lens the sub- 
 sequent recongelation of the film. 
 
 In this case the crystallizing force is hampered by 
 the adhesion of the film to the glass ; nevertheless, the 
 play of power is strikingly beautiful. Sometimes the 
 crystals start from the edge of the film and run through 
 it from that edge, for, the crystallization being once 
 started, the molecules throw themselves by preference 
 on the crystals already formed. Sometimes the crys- 
 tals start from definite nuclei in the centre of the film ; 
 every small crystalline particle which rests in the film 
 furnishing a starting-point. Throughout the process 
 you notice one feature which is perfectly unalterable, 
 and that is, angular magnitude. The spiculse branch 
 from the trunk, and from these branches others shoot ; 
 but the angles enclosed by the spiculse are unalterable. 
 In like manner you may find alum-crystals, quartz- 
 crystals, and all other crystals, distorted in shape. They 
 are thus far at the mercy of the accidents of crystalliza- 
 tion ; but in one particular they assert their superiority 
 
HI. CRYSTALLIZATION OF WATER. 105 
 
 over all such accidents angular magnitude is always 
 rigidly preserved. 
 
 My second example of the action of crystallizing force 
 is this : By sending a voltaic current through a liquid, 
 you know that we decompose the liquid, and if it con- 
 tains a metal, we liberate this metal by the electrolysis. 
 This small cell contains a solution of acetate of lead, 
 which is chosen for our present purpose, because lead lends 
 itself freely to this crystallizing power. Into the cell 
 are dipped two very thin platinum wires, and these are 
 connected by other wires with a small voltaic battery. 
 On sending the voltaic current through the solution, 
 the lead will be slowly severed from the atoms with 
 which it is now combined ; it will be liberated upon 
 one of the wires, and at the moment of its liberation it 
 will obey the polar forces of its atoms, and produce 
 crystalline forms of exquisite beauty. They are now 
 before you, sprouting like ferns from the wire, appear- 
 ing indeed like vegetable growths rendered so rapid as 
 to be plainly visible to the naked eye. On reversing 
 the current, these wonderful lead-fronds will dissolve, 
 while from the other wire filaments of lead dart through 
 the liquid. In a moment or two the growth of the lead- 
 trees recommences, but they now cover the other wire. 
 
 In the process of crystallization, Nature first reveals 
 herself as a builder. Where do her operations stop ? 
 Does she continue by the play of the same forces to 
 form the vegetable, and afterwards the animal ! What- 
 ever the answer to these questions may be, trust me that 
 the notions of the coming generations regarding this mys- 
 terious thing, which some have called 'brute matter,' 
 will be very different from those of the generations past. 
 
106 ON LIGHT. LECT. 
 
 There is hardly a more beautiful and instructive 
 example of this play of molecular force than that 
 furnished by the case of water. You have seen the 
 exquisite fern-like forms produced by the crystallization 
 of a film of water on a cold window-pane. 1 You have 
 also probably noticed the beautiful rosettes tied together 
 by the crystallizing force during the descent of a snow- 
 shower on a very calm day. The slopes and summits 
 of the Alps are loaded in winter with these blossoms 
 of the frost. They vary infinitely in detail of beauty, 
 but the same angular magnitude is preserved through- 
 out : an inflexible power binding spears and spiculae to 
 the angle of 60 degrees. 
 
 The common ice of our lakes is also ruled in its 
 deposition by the same angle. You may sometimes 
 see in freezing water small crystals of stellar shapes, 
 each star consisting of six rays, with this angle of 
 60 between every two of them. This structure may 
 be revealed in ordinary ice. In a sunbeam, or, 
 failing that, in our electric beam, we have an in- 
 strument delicate enough to unlock the frozen mole- 
 cules without disturbing the order of their architecture. 
 Cutting from clear, sound, regularly-frozen ice a slab 
 parallel to the planes of freezing, and sending a sun- 
 beam through such a slab, it liquefies internally at 
 special points, round each point a six-petalled liquid 
 flower of exquisite beauty being form eel. Crowds of 
 such flowers are thus produced. From an ice-house we 
 sometimes take blocks of ice presenting misty spaces in 
 the otherwise continuous mass; and when we enquire 
 
 1 A specimen of the plumes produced by water crystallization is 
 figured, and an account of it given, .in the Appendix. 
 
in. EXPANSION BY COLD ; PKOPOSED EXPLANATION. 107 
 
 into the cause of this mistiness, we find it to be due to 
 myriads of small six-petalled flowers, into which the ice 
 has been resolved by the mere heat of conduction. 
 
 A moment's further devotion to the crystallization 
 of water will be well repaid ; for the sum of qualities 
 which renders this substance fitted to play its part in 
 Nature may well excite wonder and stimulate thought. 
 Like almost all other substances, water is expanded by 
 heat and contracted by cold. Let this expansion and 
 contraction be first illustrated : 
 
 A small flask is filled with coloured water, and 
 stopped with a cork. Through the cork passes a glass 
 tube water-tight, the liquid standing at a certain 
 height in the tube. The flask and its tube resemble 
 the bulb and stem of a thermometer. Applying the 
 heat of a spirit-lamp, the water rises in the tube, and 
 finally trickles over the top. Expansion by heat is thus 
 illustrated. 
 
 Removing die lamp and piling a freezing mixture 
 round the flask, the liquid column falls, thus showing 
 the contraction of the water by the cold. But let 
 the freezing mixture continue to act : the falling of 
 the column continues to a certain point; it then 
 ceases. The top of the column remains stationary for 
 some seconds, and afterwards begins to rise. The con- 
 traction has ceased, and expansion by cold sets in. Let 
 the expansion continue till the liquid trickles a second 
 time over the top of the tube. The freezing mixture has 
 here produced to all appearance the same effect as the 
 flame. In the case of water, contraction by cold ceases, 
 and expansion by cold sets in at the definite tempera- 
 ture of 3 Fahr. Crystallization has virtually here 
 commenced, the molecules preparing themselves for the 
 
108 ON LIGHT. LECT. 
 
 subsequent act of solidification which occurs at 32, and 
 in which the expansion suddenly culminates. In virtue 
 of this expansion, ice, as you know, is lighter than 
 water in the proportion of 8 to 9. 1 
 
 A molecular problem of great interest is here in- 
 volved, and I wish now to place before you, for the 
 satisfaction of your minds, a possible solution of the 
 problem : 
 
 Consider, then, the ideal case of a number of magnets 
 deprived of weight, but retaining their polar forces. If 
 we had a mobile liquid of the specific gravity of steel, 
 we might, by making the magnets float in it, realize this 
 state of things, for in such a liquid the magnets would 
 neither sink nor swim. Now, the principle of gravi- 
 tation enunciated by Newton is that every particle of 
 matter, of every kind, attracts every other particle with 
 a force varying as the inverse square of the distance. In 
 virtue of the attraction of gravity, then, the magnets, if 
 perfectly free to move, would slowly approach each other. 
 
 But besides the unpolar force of gravity, which be- 
 longs to matter in general, the magnets are endowed 
 with the polar force of magnetism. For a time, however, 
 the polar forces do not come sensibly into play. In this 
 
 1 In a little volume entitled ' Forms of Water,' I have mentioned 
 that cold iron floats upon molten iron. In company with my friend Sir 
 William Armstrong, I had repeated opportunities of witnessing this 
 fact in his works at Elswick, 1863. Faraday, I remember, spoke to me 
 subsequently of the completeness of iron castings as probably due to the 
 swelling of the metal on solidification. Beyond this, I have given the 
 subject no special attention ; and I know that many intelligent iron- 
 founders doubt the fact of expansion. It is quite possible that the 
 solid floats because it is not wetted by the molten iron, its volume being 
 virtually augmented by capillary repulsion. Certain flies walk freely 
 upon water in virtue of an action of this kind. With bismuth, however, 
 it is easy to burst iron bottles by the force of soli din' cation. 
 
in. UNDULATORY THEORY OF REFRACTION. 109 
 
 condition the magnets resemble our water-molecules 
 at the temperature say of 50. But the magnets come 
 at length sufficiently near each other to enable their 
 poles to interact. From this point the action ceases 
 to be solely a general attraction of the masses. An 
 attraction of special points of the masses and a repul- 
 sion of other points now come, into play ; and it is 
 easy to see that the rearrangement of the magnets con- 
 sequent upon the introduction of these new forces may 
 be such as to require a greater amount of room. This, I 
 take it, is the case with our water-molecules. Like the 
 magnets, they approach each other for a time as ivholes. 
 Previous to reaching the temperature 39 Fahr., the 
 polar forces had doubtless begun to act, but it is at 
 this temperature that their action exactly balances the 
 contraction due to cold. At lower temperatures, as 
 regards change of volume, the polar forces predominate. 
 But they carry on a struggle with the force of contrac- 
 tion until the freezing temperature is attained. The 
 molecules then close up to form solid crystals, a con- 
 siderable augmentation of volume being the immediate 
 consequence. 
 
 3. Ordinary Refraction of Light explained by 
 the Undulatory Theory. 
 
 We have now to exhibit the bearings of this act of 
 crystallization upon optical phenomena. According to 
 the undulatory theory, the velocity of light in water and 
 glass is less than in air. Consider, then, a small por- 
 tion of a wave issuing from a point of light so distant 
 that the portion may be regarded as practically plane. 
 Moving vertically downwards, and impinging on an 
 horizontal surface of glass or water, the wave would go 
 
110 
 
 ON LIGHT. 
 
 LKCT. 
 
 through the medium without change of direction. But, 
 as the velocity in glass and water is less than the 
 velocity in air, the wave would be retarded on passing 
 into the denser medium. 
 
 But suppose the wave, before reaching the glass, to 
 be oblique to the surface ; that end of the wave which 
 first reaches the medium will be the first retarded by 
 it, the other portions as they enter the glass being re- 
 tarded in succession. It is easy to see that this 
 retardation of the one end of the wave must cause it 
 to swing round and change its front, so that when the 
 wave has fully entered the glass its course is oblique to 
 
 its original direction. According to the undulatory 
 theory, light is thus refracted. 
 
 With these considerations to guide us, let us follow 
 the course of a beam of monochromatic light through 
 our glass prism. The velocity in air is to its velocity in 
 glass as 3 : 2. Let ABC (fig. 27) be the section of our 
 prism, and a b the section of a plane wave approach- 
 ing it in the direction of the arrow. When it reaches 
 c d, one end of the wave is on the point of entering the 
 
in. APPLICATION OF THEORY TO CRYSTALS. Ill 
 
 glass, and while the portion of the wave still in the air 
 passes over the distance c e, the wave in the glass will 
 have passed over only ^wo-thirds of this distance, or 
 d f. The line ef now marks the front of the wave. 
 Immersed wholly in the glass it pursues its way to g h, 
 where the end g of the wave is on the point of escaping 
 into the air. During the time required by the end h 
 of the wave to pass over the distance h k to the surface 
 of the prism, the other end (/, moving more rapidly, 
 will have reached the point i. The wave, therefore, 
 has again changed its front, so that after its emergence 
 from the prism it will pass on to I m, and subsequently 
 in the direction of the arrow. The refraction of the 
 beam is thus completely accounted for ; and it is, more- 
 over, based upon actual experiment, which proves that 
 the ratio of the velocity of light in glass to its velocity 
 in air is that here mentioned. It is plain that if the 
 change of velocity on entering the glass was greater, 
 the refraction also would be greater. 
 
 4. Double Refraction of Light explained by the 
 Undulatory Theory. 
 
 The two elements of rapidity of propagation, both 
 of sound and light, in any substance whatever, are 
 elasticity and density, the speed increasing with the 
 former and diminishing with the latter. The enormous 
 velocity of light in stellar space is attainable because 
 the ether is at the same time of infinitesimal density 
 and of enormous elasticity. Now the ether surrounds 
 the atoms of all bodies, but it is not independent of them. 
 In ponderable matter it acts as if its density were in- 
 creased without a proportionate increase of elasticity ; 
 
112 ON LIGHT. 
 
 I.ECT. 
 
 and this accounts for the diminished velocity of light in 
 refracting bodies. We here reach a point of cardinal im- 
 portance. In virtue of the crystalline architecture that 
 we have been considering, the ether in many crystals 
 possesses different densities, and hence different elastici- 
 ties, in two different directions ; and the consequence is, 
 that some of these media transmit light with two diffe- 
 rent velocities. But as refraction depends wholly upon 
 the change of velocity on enteringthe refracting medium, 
 and is greatest where the change of velocity is greatest, 
 we have in many crystals two different refractions. 
 By such crystals a beam of light is divided into two. 
 This effect is called double refraction. 
 
 In ordinary water, for example, there is nothing 
 in the grouping of the molecules to interfere with the 
 perfect homogeneity of the ether ; but, when water crys- 
 tallizes to ice, the case is different. In a plate of ice 
 the elasticity of the ether in a direction perpendicular to 
 the surface of freezing is different from what it is 
 parallel to the surface of freezing ; ice is, therefore, a 
 double refracting substance. Double refraction is dis- 
 played in a particularly impressive manner by Iceland 
 spar, which is crystallized carbonate of lime. The 
 difference of ethereal density in two directions in this 
 crystal is very great, the separation of the beam into 
 the two halves being, therefore, particularly striking. 
 
 I am unwilling to quit this subject before raising' it 
 to unmistakable clearness in your minds. The vibra- 
 tions of light being transversal, the elasticity concerned 
 in the propagation of any ray is the elasticity at right 
 angles to the direction of propagation. In Iceland 
 spar there is one direction round which the crystalline 
 molecules are symmetrically built. This direction is 
 
III. 
 
 DOUBLE REFRACTION. 
 
 113 
 
 called the axis of the crystal. In consequence of this 
 symmetry the elasticity is the same in all directions 
 perpendicular to the axis, and hence a ray transmitted 
 along the axis suffers no double refraction. But the 
 elasticity along the axis is greater than the elasticity 
 at right angles to it. Consider, then, a system of 
 waves crossing the crystal in a direction perpendicular 
 to the axis. Two directions of vibration are open to 
 such waves : the ether particles can vibrate parallel 
 to the axis or perpendicular to it. They do both, and 
 hence immediately divide themselves into two systems 
 propagated with different velocities. Double refraction 
 is the necessary consequence. 
 
 By means of Iceland spar cut in the prop;; Direction, 
 
 FIG. 28. 
 
 double refraction is capable of easy illustration. Causing 
 the beam which builds the image of our carbon-points 
 to pass through the spar, the single image is instantly 
 divided into two. Projecting (by the lens E, fig. 28) 
 an image of the aperture (L) through which the light 
 issues from the electric lamp, and introducing the spar 
 
 I 
 
114 ON LIGHT. LECT. 
 
 (P), two luminous disks (E 0) appear immediately 
 upon the screen instead of one. 
 
 The two beams into which the spar divides the 
 single incident-beam have been subjected to the closest 
 examination. They do not behave alike. One of them 
 obeys the ordinary law of refraction discovered by Snell, 
 . and is, therefore, called the ordinary ray : its index of 
 refraction is 1*654. The other does not obey this law. 
 Its index of refraction, for example, is not constant, 
 but varies from a maximum of 1'654 to a minimum 
 of 1*483 ; nor in this case do the incident and refracted 
 rays always lie in the same plane. It is, therefore, 
 called the extraordinary ray. In calc-spar, as just 
 stated, the "ordinary ray is the most refracted. One 
 consequence of this merits a passing notice. Pour 
 water and bisulphide of carbon into two cups of the 
 same depth ; the cup that contains the more strongly- 
 refracting liquid will appear shallower than the other. 
 Place a piece of Iceland spar over a dot of ink ; two 
 dots are seen, the one appearing nearer than the other 
 to the eye. The nearest dot belongs to the most 
 strongly-refracted ray, exactly as the nearest cup- 
 bottom belongs to the most highly refracting liquid. 
 When you turn the spar round, the extraordinary image ^ 
 of the dot rotates round the ordinary one, which 
 remains fixed. This is also the deportment of ouy two 
 disks upon the screen. 
 
 5. Polarization of Light explained by the 
 Undulatory Theory. 
 
 The double refraction of Iceland spar was first 
 treated in a work published by Erasmus Bartholinus, in 
 1669. The celebrated Huyghens sought to account for 
 
in. POLAKIZATION OF LIGHT. 115 
 
 this phenomenon on the principles of the wave theory, 
 and he succeeded in doing so. He, moreover, made 
 highly important observations on the distinctive cha- 
 racter of the two beams transmitted by the spar, 
 admitting, with resigned candour, that he had not 
 solved them, and leaving that solution to future times. 
 Newton, reflecting on the observations of Huyghens, 
 came to the conclusion that each of the beams trans- 
 mitted by Iceland spar had two sides ; and from the 
 analogy of this two-sidedness with the two-endedness 
 of a magnet, wherein consists its polarity, the two 
 beams came subsequently to be described as polarized. 
 
 We may begin the study of the polarization of 
 light, with ease and profit, by means of a crystal of 
 tourmaline. But we must start with a clear conception 
 of an ordinary beam of light. It has been already 
 explained that the vibrations of the individual ether- 
 particles are executed across the line of propagation. 
 "In the case of ordinary light we are to figure the ether- 
 particles as vibrating in all directions, or azimuths, as 
 it is sometimes expressed, across this line. 
 
 Now, in the case of a plate of tourmaline cut 
 parallel to the axis of the crystal, a beam of light 
 incident upon the plate is divided into ,two, the one 
 vibrating parallel to the axis of the crystal, the other 
 at right angles to the axis. The grouping of the 
 molecules, and of the ether associated with the mole- 
 cules, reduces all the vibrations incident upon the 
 crystal to these two directions. One of these beams, 
 namely, that whose vibrations are perpendicular to 
 the axis, is quenched with exceeding rapidity by the 
 tourmaline. To such vibrations many specimens of 
 the crystal are highly opaque ; so that, after having 
 
116 ON LIGHT. LKCT. 
 
 passed through a very small thickness of the tourmaline, 
 the light emerges with all its vibrations reduced to a 
 single plane. In this condition it is what we call 
 plane polarized light. 
 
 A moment's reflection will show that, if what is 
 here stated be correct, on placing a second plate of 
 tourmaline with its axis parallel to the first, the light 
 will pass through both ; but that, if the axes be crossed, 
 the light that passes through the one plate will be 
 quenched by the other, a total interception of the light 
 being the consequence. Let us test this conclusion by 
 experiment. The image of a plate of tourmaline (t t, 
 fig. 29) is now before you. I place parallel to it another 
 
 FIG. 29. 
 
 t 
 
 plate (if t f ) : the green of the crystal is a little 
 deepened, nothing more ; this agrees with our conclu- 
 sion. By means of an endless screw, I now turn one of 
 the crystals gradually round, and you observe that as 
 long as the two plates are oblique to each other, a 
 certain portion of light gets through ; but that when 
 they are at right angles to each other, the space 
 
^/ 
 
 in. DISCOVEEY OF MALUS. 117 
 
 common to both is a space of darkness (fig. 30). Our con- 
 clusion, arrived at prior to experiment, is thus verified. 
 
 Let us now return to a single plate ; and here let 
 me say that it is on the green light transmitted by the 
 tourmaline that you are to fix your attention. We have^X 
 to illustrate the two-sidedness of that green light, in 
 contrast to the all-sidedness of ordinary light. The 
 light surrounding the green image, being ordinary light, 
 is reflected by a plane glass mirror in all directions ; 
 the green light, on the contrary, is not so reflected. 
 The image of the tourmaline is now horizontal ; re- 
 flected upwards, it is still green; reflected sideways, 
 the image is reduced to blackness, because of the in- 
 competency of the green light to be reflected in this 
 direction. Making the plate of tourmaline vertical, 
 and reflecting it as before, it is in the upper image that 
 the light is quenched ; in the side image you have now 
 the green. This is a result of the greatest significance. 
 If the vibrations of light were longitudinal, like those 
 of sound, you could have no action of this kind ; and 
 this very action compels us to assume that the vibra- 
 tions are transversal. Picture the thing clearly. In the 
 one case the mirror receives, as it were, the impact of the 
 edges of the waves, the green light being then quenched. 
 In the other case the sides of the waves strike the mir- 
 ror, and the green light is reflected. To render the 
 extinction complete, the light must be received upon 
 the mirror at a special angle. What this angle is we 
 shall learn presently. 
 
 The quality of two-sidedness conferred upon light 
 by bi-refracting crystals may also be conferred upon it 
 by ordinary reflection. Malus made this discovery in 
 1808, while looking through Iceland spar at the light of 
 
118 ON LIGHT. 
 
 LECT. 
 
 the sun reflected from the windows of the Luxembourg 
 palace in Paris. I receive upon a plate of window-glass 
 the beam from our lamp ; a great portion of the light 
 reflected from the glass is polarized. The vibrations of 
 this reflected beam are executed, for the most part, 
 parallel to the surface of the glass, and when the glass 
 is held so that the beam shall make an angle of 58 
 with the perpendicular to the glass, the ivhole of the 
 reflected beam is polarized. It was at this angle that 
 the image of the tourmaline was completely quenched in 
 our former experiment. It is called the polarizing angle. 
 
 Sir David Brewster proved the angle of polarization 
 of a medium to be that particular angle at which the 
 refracted and reflected rays inclose a right angle. 1 The 
 polarizing angle augments with the index of refraction. 
 For water it is 52J ; for glass, as already stated, 58 ; 
 while for diamond it is 68. 
 
 And now let us try to make substantially the 
 experiment of Malus. The beam from the lamp is 
 received at the proper angle upon a plate of glass 
 and reflected through the spar. Instead of two images, 
 you see but one. So that the light, when polarized, as 
 it now is by reflection, can only get through the spar in 
 one direction, and consequently produce but one image. 
 Why is this ? In the Iceland spar, as in the tourmaline, 
 all the vibrations of the ordinary light are reduced to 
 
 1 This beautiful law is usually thus expressed : The index of refrac- 
 tion of any substance is the tangent of its polarizing angle. With the 
 aid of this law and an apparatus similar to that figured at page 15, we 
 can readily determine the index of refracting any liquid. The refracted 
 and reflected beams being visible, they can readily be caused to enclose 
 a right angle. The polarizing angle of the liquid may be thus found 
 with the sharpest precision. It is then only necessary to seek out its 
 natural tangent to obtain the index cf refraction. 
 
in. BEAMS FROM SPAR TESTED B}T TOURMALINE. 119 
 
 two planes at right angles to each other ; but, unlike 
 the tourmaline, both beams are transmitted with equal 
 facility by the spar. The two beams, in short, emergent 
 from the spar, are polarized, their directions of vibration 
 being at right angles to each other. It is important to 
 remember this. When, therefore, the light was polar- 
 ized by reflection, the direction of vibration in the spar 
 which coincided with the direction of vibration of the 
 polarized beam transmitted it, and that direction only. 
 Only one image, therefore, was possible under the con- 
 ditions. 
 
 You will now observe that such logic as connects 
 our experiments is simply a transcript of the logic of 
 Nature. On the screen before you are two disks of 
 light produced by the double refraction of Iceland spar. 
 They are, as you know, two images of the aperture 
 through which the light issues from the camera. Plac- 
 ing the tourmaline in front of the aperture, two images 
 of the crystal will also be obtained ; but now let us 
 reason out beforehand what is to be expected from this 
 experiment. The light emergent from the tourmaline 
 is polarized. Placing the crystal with its axis hori- 
 zontal, the vibrations of its transmitted light will be 
 horizontal. Now the spar, as already stated, has two 
 directions of vibration, one of which at the present 
 moment is vertical, tfce other horizontal. What are 
 we to conclude ? That the green light will be trans- 
 mitted along the latter, which is parallel to the axis of 
 the tourmaline, and not along the former, which is 
 perpendicular to that axis. Hence we may infer that 
 one image of the tourmaline will show the ordinary 
 green light of the crystal, while the other image will 
 
120 
 
 ON LIGHT. 
 
 XECT. 
 
 be black. Tested by experiment, our reasoning is veri- 
 fied to the letter (fig. 31). 
 
 FIG. 31. 
 
 Let us push our test still further. By means of an 
 endless screw, the crystal can be turned ninety degrees 
 round. The black image, as I turn, becomes gradually 
 
 FIG. 32. 
 
 brighter, and the bright one gradually darker ; at an 
 angle of forty-five degrees both images are equally 
 bright (fig. 32); while, when ninety degrees have been 
 
 FIG. 
 
 obtained, the axis of the crystal being then vertical, 
 the bright and black images have changed places, 
 
m. TESTED BY REFLECTION AND REFRACTION. 121 
 
 exactly as reasoning would have led us to suppose 
 (fig. 33). 
 
 Given the two beams transmitted through Iceland 
 spar, it is perfectly manifest that we have it in our 
 power to determine instantly, by means of a plate of 
 tourmaline, the directions in which the ether-particles 
 vibrate in the two beams. The double refracting spar 
 might be placed in any position whatever. A minute's 
 trial with the tourmaline would enable you to deter- 
 mine the position which yields a black and a bright 
 image, and from this you would at once infer the direc- 
 tions of vibration. 
 
 Let us reason still further together. The two 
 beams from the spar being thus polarized, it is plain 
 
 FIG. 34. 
 
 (B 'is the hi-refracting spar, dividing the incident light into the two beams o and e, 
 G is the mirror.) The beam is here reflected laterally. When the reflection is up- 
 wards, the other beam is reflected as shown in fig. 35. 
 
 that if they be suitably received upon a plate of glass 
 at the polarizing angle, one of them will be reflected, 
 the other not. This is a simple inference from our pre- 
 vious knowledge ; but you observe that the inference is 
 justified by experiment. (Figs. 34 and 35.) 
 
122 
 
 ON LIGHT. 
 
 I have said that the whole of the beam reflected 
 from glass at the polarizing angle is polarized ; a word 
 must now be added regarding the far larger portion of 
 the light which is transmitted by the glass. The 
 
 PIG. 35. 
 
 transmitted beam contains a quantity of polarized 
 light equal to the reflected beam : but this is only 
 a fraction of the whole transmitted light. By tak- 
 ing two plates of glass instead of one, we augment 
 the quantity of the transmitted polarized light ; and by 
 taking a bundle of plates, we so increase the quantity 
 as to render the transmitted beam, for all practical pur- 
 poses, perfectly polarized. Indeed, bundles of glass 
 plates are often employed as a means of furnishing 
 polarized light. Interposing -such a bundle at the 
 proper angle into the paths of the two beams emergent 
 from Iceland spar, that which, in the last experiment, 
 failed to be reflected, is here transmitted. The plane 
 of vibration of this transmitted light is at right angles 
 to that of the reflected light. 
 
 One word more. When the tourmalines are crossed, 
 the space where they cross each other is black. But 
 
HI. DEPOLARIZATION. 123 
 
 we have seen that the least obliquity on the part of 
 the crystals permits light to get through both. Now 
 suppose, when the two plates are crossed, that we in- 
 terpose a third plate of tourmaline between them, with 
 its axis oblique to both. A portion of the light trans- 
 mitted by the first plate will get through this inter- 
 mediate one. But, after it has got through, its plane 
 of vibration is changed : it is no longer perpendicular 
 to the axis of the crystal in front. Hence it will get 
 through that crystal. Thus, by pure reasoning, we 
 infer that the interposition of a third plate of tourma- 
 line will in part abolish the darkness produced by the 
 perpendicular crossing of the other two plates. I have 
 not a third plate of tourmaline ; but the talc or mica 
 which you employ in your stoves is a more convenient 
 substance, which acts in the same way. Between the 
 crossed tourmalines, I introduce a film of this crystal 
 with its axis oblique to theirs. You see the edge of the 
 film slowly descending, and as it descends, light takes 
 the place of darkness. The darkness, in fact, seems 
 scraped away, as if it were something material. This 
 effect has been called, naturally but improperly, 
 depolarization. Its proper meaning will be disclosed 
 in our next lecture. 
 
 These experiments and reasonings, if only thoroughly 
 studied and understood, will form a solid groundwork 
 for the analysis of the splendid optical phenomena next 
 to be considered. 
 
124 ON LIGHT. 
 
 LECT. 
 
 LECTURE IV. 
 
 CHROMATIC PHENOMENA PRODUCED BY CRYSTALS IN POLARIZED LIGHT 
 
 THE NICOL PRISM POLARIZER AND ANALYZER ACTION OF THICK 
 
 AND THIN PLATES OF SELENITE COLOURS DEPENDENT ON THICK- 
 NESSRESOLUTION OF POLARIZED BEAM INTO TWO OTHERS BY THE 
 SELENITE ONE OF THEM MORE RETARDED THAN THE OTHER RE- 
 COMPOUNDING OF THE TWO SYSTEMS OF WAVES BY THE ANALYZER 
 - INTERFERENCE THUS RENDERED POSSIBLE CONSEQUENT PRODUC- 
 TION OF COLOURS ACTION OF BODIES MECHANICALLY STRAINED OR 
 
 PRESSED ACTION OF SONOROUS VIBRATIONS ACTION OF GLASS 
 
 STRAINED OR PRESSED BY HEAT CIRCULAR POLARIZATION CHROMA- 
 TIC PHENOMENA PRODUCED BY QUARTZ THR MAGNETIZATION OF 
 
 LIGHT RINGS SURROUNDING THE AXES OF CRYSTALS BIAXAL AND 
 UNIAXAL CRYSTALS GRASP OF THE UNDULATORY THEORY THE COLOUR 
 AND POLARIZATION OF SKY-LIGHT GENERATION OF ARTIFICIAL SKIES. 
 
 1. Action of Crystals on Polarized Light: the 
 Nicol Prism. 
 
 WE have this evening to examine and illustrate the 
 chromatic phenomena produced by the action of crystals, 
 and double-refracting bodies generally, upon polarized 
 light, and to apply the Undulatory Theory to their eluci- 
 dation. For a long time investigators were compelled 
 to employ plates of tourmaline for this purpose, and 
 the progress they made with so defective a means of 
 inquiry is astonishing. But these men had their hearts 
 in their work, and were on this account enabled to 
 extract great results from small instrumental appliances. 
 But for our present purpose we need far larger appa- 
 ratus ; and, happily, in these later times this need has 
 
iv. THE NICOL'S PEISM. 125 
 
 been to a great extent satisfied. We have seen and 
 examined the two beams emergent from Iceland spar, 
 and have proved them to be polarized. If, at the 
 sacrifice of half the light, we could abolish one of these, 
 the other would place at our disposal a beam of polarized 
 light, incomparably stronger than any attainable from 
 tourmaline. 
 
 The beams, as you know, are refracted differently, and 
 from this, as made plain in 4. Lecture I., we are able 
 to infer that the one may be totally reflected, when 
 the other is not. An able optician, named Nicol, cut a 
 crystal of Iceland spar in tfSL halves in a certain direc- 
 tion. He polished the severed surfaces, and reunited 
 them by Canada balsam, the surface of union being 
 so inclined to the beam traversing the spar that the 
 ordinary ray, which is the most highly refracted, was 
 totally reflected by the balsam, while the extraordinary 
 ray was permitted to pass on. 
 
 Let bx,cy (fig. 36) represent the section of an elon- 
 gated rhomb of Iceland spar cloven from the crystal. Let 
 this rhomb be cut along the plane b c ; and the two 
 severed surfaces, after having been polished, reunited 
 by Canada balsam. We learned, in our first lecture, 
 that total reflection only takes place when a ray seeks 
 to escape from a more refracting to a less refracting 
 medium, and that it always, under these circumstances, 
 takes place when the obliquity is sufficient. Now the 
 refractive index of Iceland spar is, for the extraordinary 
 ray less, and for the ordinary greater, than for Canada 
 balsam. Hence, in passing from the spar to the balsam, 
 the extraordinary ray passes from a less refracting to 
 a more refracting medium, where total reflection cannot 
 occur'; while the ordinary ray passes from a more 
 
126 
 
 ON LIGHT. 
 
 LECT. 
 
 refracting to a less refracting medium, where total 
 reflection can occur. The requisite obliquity is secured 
 by making the rhomb of such a length that the plane 
 
 FIG. 36. 
 
 ^ 
 
 of which b c is the section shall be perpendicular, or 
 nearly so, to the two end surfaces of the rhomb 
 b x, c y. 
 
 The invention of the Nicol prism was a great step in 
 practical optics, and quite recently such prisms have 
 been constructed of a size and purity which enable 
 audiences like the present to witness the chromatic phe- 
 nomena of polarized light to a degree altogether unat- 
 tainable a short time ago. The two prisms here before 
 you belong to my excellent friend Mr. William Spottis- 
 woode, and they were manufactured by Mr. Ladd. I 
 have with me another pair of very noble prisms, still 
 larger than these, manufactured for me by Mr. Browning, 
 
iv. THICK AND THIN PLATES 01' SELENITE. 127 
 
 who has gained so high and well-merited a reputation 
 in the construction of spectroscopes. 1 
 
 2. Colours of Films of Selenite in Polarized Light. 
 
 These two Nicol prisms play the same part as the 
 two plates of tourmaline. Placed with their directions 
 of vibration parallel, the light passes through both ; 
 while when these directions are crossed the light is 
 quenched. Introducing a film of mica between the 
 prisms, the light, as in the case of the tourmaline, is 
 restored. But notice, when the film of mica is thin 
 you have sometimes not only light, but coloured light. 
 Our work for some time to come will consist of the ex- 
 amination of such colours. With this view, I will take 
 a representative crystal, one easily dealt with, because 
 it cleaves with great facility the crystal gypsum, or 
 selenite, which is crystallized sulphate of lime. Between 
 the crossed Nicols I place a thick plate of this crystal ; 
 like the mica, it restores the light, but it produces no 
 colour. With my penknife I take a thin splinter from 
 the crystal and place it between the prisms ; the image 
 of the splinter glows with the richest colours. Turning 
 the prism in front, these colours gradually fade and 
 disappear, but, by continuing the rotation until the 
 vibrating sections of the prisms are parallel to each 
 other, vivid colours again arise, but these colours are 
 complementary to the former ones. 
 
 Some patches of the splinter appear of one colour, 
 some of another. These differences are due to the 
 different thicknesses of the film. As in the case of 
 
 1 The largest and purest prism hitherto made has been recently con- 
 structed for Mr. Spottiswoode by Messrs. Tisley & Spiller. 
 
128 ON LIGHT. LECT. 
 
 Hooke's thin plates, if the thickness be uniform, the 
 colour is uniform. Here, for instance, is a stellar shape, 
 every lozenge of the star being a film of gypsum of 
 uniform thickness : each lozenge, you observe, shows a 
 brilliant and uniform colour. It is easy, by shaping 
 our films so as to represent flowers or other objects, to 
 exhibit such objects in hues unattainable by art. Here, 
 for example, is a specimen of heart's-ease, the colours of 
 which you might safely defy the artist to reproduce. 
 By turning the front Nicol 90 degrees round, we pass 
 through a colourless phase to a series of colours com- 
 plementary to the former ones. This change is still 
 more strikingly represented by a rose-tree, which is 
 now presented in its natural hues a red flower and 
 green leaves ; turning the prism 90 degrees round, we 
 obtain a green flower and red leaves. All these wonder- 
 ful chromatic effects have definite mechanical causes in 
 the motions of the ether. The principle of interference 
 duly applied and interpreted explains them all. 
 
 3. Colours of Crystals in Polarized Light explained 
 by the Undulatory Theory. 
 
 By this time you have learned that the word ' light ' 
 may be used in two different senses; it may mean 
 the impression made upon consciousness, or it may 
 mean the physical agent which makes the impression. 
 It is with the agent that we have to occupy ourselves 
 at present. That agent is a substance which fills all 
 space, and surrounds the atoms and molecules of bodies. 
 To this interstellar and interatomic medium definite 
 mechanical properties are ascribed, and we deal with it 
 in our reasonings and calculations as a body possessed of 
 
iv. THEORETIC ANALYSIS OF VIBRATIONS. 129 
 
 these properties. In mechanics we have the composition 
 and resolution of forces and of motions, extending to the 
 composition and resolution of vibrations. We treat the 
 luminiferous ether on mechanical principles, and, from 
 the composition, resolution, and interference of its vi- 
 brations we deduce all the phenomena displayed by 
 crystals in polarized light. 
 
 Let us take, as an example, the crystal of tourmaline, 
 with which we are now so familiar. Let a vibration 
 cross this crystal oblique to its axis. Experiment has 
 assured us that a portion of the light will pass through. 
 The quantity which passes we determine in this way. Let 
 A B (fig 37) be the axis of the tourmaline, and let a b 
 
 FIG. 37. 
 
 ,1 
 
 V 
 
 represent the amplitude of the ethereal vibration before 
 it reaches A B. From a and b let the two perpendicu- 
 lars a c and b d be drawn upon the axis : then c d will 
 be the amplitude of the transmitted vibration. 
 
 I shall immediately ask you to follow me while 
 I endeavour to explain the effects observed when 
 a film of gypsum is placed between the two Nicol's 
 prisms. But, prior to this, it will be desirable to esta- 
 blish still further the analogy between the action of the 
 prisms and that of the two plates of tourmaline. The 
 magnified images of these plates, with their axes at right- 
 angles to each other, are now before you. Introducing 
 between them a film of selenite, you observe that by 
 turning the film round it may be placed in a position 
 
 K 
 
130 
 
 ON LIGHT. 
 
 LECT. 
 
 where it has no power to abolish the darkness of the 
 superposed portions of the tourmalines. Why is this ? 
 The answer is, that in the gypsum there are two direc- 
 tions, at right angles to each other, in which alone vibra- 
 tions can take place, and that in our present experiment 
 one of these directions is parallel to one of the axes 
 of the tourmaline, and the other parallel to the other 
 axis. When this is the case, the film exercises no 
 sensible action upon the light. But now I turn the 
 film so as to render its directions of vibration oblique 
 to the two tourmaline axes ; then, you. see it exercises 
 the power, demonstrated in the last lecture, of restoring 
 the light. 
 
 Let us now mount our Nicol's prisms, and cross 
 them as we crossed the tourmalines. Introducing 
 
 FIG. 38. 
 
 our film of gypsum between them, you notice that in 
 one particular position the film has no power what- 
 ever over the field of view. But, when the film is 
 turned a little way round, the light passes. We have 
 now to understand the mechanism by which this is 
 effected. 
 
jv. APPLICATION OF THEOEY TO SELENITE. 131 
 
 Firstly then, we have a prism which receives 
 the light from the electric lamp, and which is called 
 the polarizer. Then we have the plate of gypsum 
 (supposed to be placed at S, fig. 38), and then the 
 prism in front, which is called the analyzer. On its 
 emergence from the first prism, the light is polarized ; 
 and, in the particular case now before us, its vibrations 
 are executed in a horizontal plane. We have to ex- 
 amine what occurs when the two directions of vibration 
 in the gypsum are oblique to the horizon. Draw a 
 rectangular cross (A B, C D, fig. 39) to represent these 
 
 FIG. 39. 
 
 / 
 
 I A 1 J > \i A 1 \ 
 
 two directions. Draw a KB 3 (a b) to represent the 
 amplitude of the vibration on the emergence of the 
 light from the first Nieol. Let fall from the two ends 
 of this line two perpendiculars on each of the arms of 
 the cross ; then the distances (c d, e f) between the 
 feet of these perpendiculars represent the amplitudes of 
 two rectangular vibrations, which are the components 
 of the first single vibration. Thus the polarized ray, 
 when it enters the gypsum, is resolved into its two 
 equivalents, which vibrate at right angles to each 
 other. 
 
 In one of these two rectangular directions the ether 
 
 x 2 
 
132 ON LIGHT. LKCT. 
 
 within the gypsum is more sluggish than in the 
 other ; and, as a consequence, the waves that follow 
 this direction are more retarded than the others. In 
 fact, in both cases the undulations are shortened when 
 they enter the gypsum, but in the one case they 
 are more shortened than in the other. You can 
 readily imagine that in this way the one system of 
 waves may get half a wave-length, or indeed any num- 
 ber of half-wave lengths, in advance of the other. The 
 possibility of interference here at once flashes upon the 
 mind. A little consideration, however, will render it 
 evident that, as long as the vibrations are executed at 
 right angles to each other, they cannot quench each 
 other, no matter what the retardation may be. This 
 brings us at once to the part played by the analyzer. 
 Its sole function is to recompound the two vibrations 
 emergent from the gypsum. It reduces them to a 
 single plane, where, if one of them be retarded by 
 the proper amount, extinction will occur. 
 
 But here, as in the case of thin films, the different 
 lengths of the waves of light come into play. Eed will 
 require a greater thickness to produce the retardation 
 necessary for extinction than blue ; consequently, when 
 the longer waves have been withdrawn by interference, 
 the shorter ones remain, the film of gypsum shining 
 with the colours which they confer. Conversely, when 
 the shorter waves have been withdrawn, the thickness 
 is such that the longer waves remain. An elementary 
 consideration suffices to show that, when the directions 
 of vibration of the prisms and the gypsum enclose an 
 angle of forty-five degrees, the colours are at their maxi- 
 mum brilliancy. When the film is turned from this 
 direction, the colours gradually fade, until, at the point 
 
iv. RELATION OF THICKNESS TO COLOUR. 133 
 
 where the directions of vibration are parallel, they dis- 
 appear altogether. 
 
 The best way of obtaining a knowledge of these phe- 
 nomena is to construct a model of thin wood or paste- 
 board, representing the .plate of gypsum, its planes of 
 vibration, and also those of the polarizer and analyzer. 
 Two parallel pieces of the board are to be separated by 
 an interval which shall represent the thickness of the 
 film of gypsum. Between them, two other pieces, 
 intersecting each other at a right angle, are to repre- 
 sent the planes of vibration within the film ; while at- 
 tached to the two parallel surfaces outside are two other 
 pieces of board to represent the planes of vibration of 
 the polarizer and analyzer. On the two intersecting 
 planes the waves are to be drawn, showing the resolu- 
 tion of the first polarized beam into two others, and 
 then the subsequent reduction of the two systems of vi- 
 brations to a common plane by the analyzer. Follow- 
 ing out rigidly the interaction of the two systems of 
 waves, we are taught by such a model that all the phe- 
 nomena of colour obtained by the combination of the 
 waves when the planes of vibration of the two Nicols 
 are parallel are displaced by the cmnplementury phe- 
 nomena when the planes of vibration are perpendicular 
 to each other. 
 
 In considering the next point, we will operate, for 
 the sake of simplicity, with monochromatic light with 
 red light, for example, which is easily obtained pure by 
 red glass. Supposing a certain thickness of the gypsum 
 produces a retardation of half a wave-length, twice this 
 thickness will produce a retardation of two half wave- 
 lengths, three times this thickness a retardation of three 
 half-wave lengths, and so on. Now, when the Nicols 
 
134 ON LIGHT. LECT. 
 
 are parallel, the retardation of half a wave-length, or 
 of any odd number of half wave-lengths, produces ex- 
 tinction; at all thicknesses, on the other hand, which 
 correspond to a retardation of an even number of half 
 wave-lengths, the two beams support each other, when 
 they are brought to a common plane by the analyzer. 
 Supposing, then, that we take a plate of a wedge-form, 
 which grows gradually thicker from edge to back, 
 we ought to expect in red light a series of recurrent 
 bands of light and darkness ; the dark bands occurring 
 at thicknesses which produce retardations of one, three, 
 five, etc., half wave-lengths, while the bright bands 
 occur between the dark ones. Experiment proves the 
 wedge-shaped film to show these bands. They are also 
 beautifully shown by a circular film, so worked as to 
 be thinnest at the centre, and gradually increasing in 
 thickness from the centre outwards. A splendid series 
 of rings of light and darkness is thus produced. 
 
 When, instead of employing red light, we employ 
 blue, the rings are also seen : but, as they occur at 
 thinner portions of the film, they are smaller than the 
 rings obtained with the red light. The consequence 
 of employing white light may be now inferred ; inas- 
 much as the red and the blue fall in different places, 
 we have iris-coloured rings produced by the white 
 light. 
 
 Some of the chromatic effects of irregular crystal- 
 lization are beautiful in the extreme. Could I intro- 
 duce between our Nicols a pane of glass covered by 
 those frost-ferns which the cold weather renders now so 
 frequent, rich colours would be the result. The beau- 
 tiful effects of the irregular crystallization of tartaric 
 acid and other substances on glass plates, now presented 
 
iv. COMPLEMENTARY PHENOMENA. 135 
 
 to you, illustrate what you might expect from the 
 frosted window-pane. And not only do crystalline 
 bodies act thus upon light, but almost all bodies that 
 possess a definite structure do the same. As a general 
 rule, organic bodies act thus upon light ; for their 
 architecture implies an arrangement of the molecules, 
 and of the ether, which involves double refraction. A 
 film of horn, or the section of a shell, for example, 
 yields very beautiful colours in polarized light. In a 
 tree, the ether certainly possesses different degrees of 
 elasticity along and across the fibre ; and, were wood 
 transparent, this peculiarity of molecular structure 
 would infallibly reveal itself by chromatic phenomena 
 like those that you have seen. 
 
 4. Colours produced by Strain and Pressure. 
 
 But not only do natural bodies behave in this way, 
 but it is possible, as shown by Brewster, to confer, by 
 artificial strain or pressure, a temporary double-refract- 
 ing structure upon non crystalline bodies, such as 
 common glass. This is a point worthy of illustration. 
 When I place a bar of wood across my knee and seek to 
 break it, what is the mechanical condition of the bar ? 
 It bends, and its convex surface is strained longitudi- 
 nally ; its concave surface, that next my knee, is longitu- 
 dinally pressed. Both in the strained portion and in the 
 pressed portion the ether is thrown into a condition 
 which would render the wood, were it transparent, double- 
 refracting. For, in cases like the present, the drawing 
 of the molecules asunder longitudinally is always ac- 
 companied by their approach to each other laterally ; 
 while the longitudinal squeezing is accompanied by 
 
136 ON LIGHT. LECT. 
 
 lateral retreat. Each half of the bar exhibits this anti- 
 thesis, and is therefore double-refracting. 
 
 Let us now repeat this experiment with a bar of 
 glass. Between the crossed Nicols I introduce such a 
 bar. By the dim residue of light lingering upon the 
 screen, you see the image of the glass, but it has no 
 effect upon the light. I simply bend the glass bar 
 with my finger and thumb, keeping its length oblique 
 to the directions of vibration in the Nicols. Instantly 
 light flashes out upon the screen. The two sides of 
 the bar are illuminated, the edges most, for here the 
 strain and pressure are greatest. In passing from 
 longitudinal strain to longitudinal pressure, we cross a 
 portion of the glass where neither is exerted. This is 
 the so-called neutral axis of the bar of glass, and along 
 it you see a dark band, indicating that the glass along 
 this axis exercises no action upon the light. By em- 
 ploying the force of a press, instead of the force of my 
 finger and thumb, the brilliancy of the light is greatly 
 augmented. 
 
 Again, I have here a square of glass which can be 
 inserted into a press of another kind. Introducing 
 the uncompressed square between the prisms, its neu- 
 trality is declared ; but it can hardly be held suffi- 
 ciently loosely in the press to prevent its action from 
 manifesting itself. Already, though the pressure is 
 infinitesimal, you see spots of light at the points where 
 the press is in contact with the glass. On turning a 
 screw the image of the square of glass flashes out upon 
 the screen. Luminous spaces are seen separated from 
 each other by dark bands. 
 
 Every two adjacent luminous spaces are in oppo- 
 site mechanical conditions. On one side of the dark 
 
iv. EFFECT OF MECHANICAL STRAINS AND PRESSURES. 137 
 
 band we have strain, on the other side pressure ; 
 while the dark band marks the neutral axis between 
 both. I now tighten the vice, and you see colour ; 
 tighten still more, and the colours appear as rich as 
 those presented by crystals. Eeleasing the vice, the 
 colours suddenly vanish ; tightening suddenly, they 
 reappear. From the colours of a soap-bubble Newton 
 was able to infer the thickness of the bubble, thus 
 uniting by the bond of thought apparently incongruous 
 things. From the' colours here presented to you, the 
 magnitude of the pressure employed might be in- 
 ferred. Indeed, the late M. Wertheim, of Paris, 
 invented an instrument for the determination of strains 
 and pressures, by the colours of polarized light, which 
 exceeded in accuracy all previous instruments of the 
 kind. 
 
 And now we have to push these considerations to a 
 final illustration. Polarized light may be turned to 
 account in various ways as an analyzer of molecular 
 condition. It may, for instance, be applied to reveal 
 the condition of a solid body when it becomes sonorous. 
 A strip of glass six feet long, two inches wide, and a 
 quarter of an inch thick, is held at the centre between 
 the finger and thumb. On sweeping a wet woollen rag 
 over one of its halves, you hear an acute sound due to the 
 vibrations of the glass. What is the condition of the 
 glass while the sound is heard ? This : its two halves 
 lengthen and shorten in quick succession. Its two ends, 
 therefore, are in a state of quick vibration ; but at the 
 centre the pulses from the two ends alternately meet 
 and retreat from each other. Between their opposing 
 actions, the glass at the centre is kept motionless ; 
 but, on the other hand, it is alternately strained afid 
 
LECT. 
 
 138 ON LIGHT, 
 
 compressed. The state of the glass may be illustrated by 
 FIG. 40. 
 
 a row of spots of light, as the propagation of a sonorous 
 FIG. 41. 
 
IT. ACTION OF SONOROUS VIBRATIONS. 139 
 
 pulse was illustrated in our second lecture. By a simple 
 mechanical contrivance the spots are made to vibrate to 
 and fro : the terminal dots have the largest amplitude 
 of vibration, while those at the centre are alternately 
 crowded together and drawn asunder, the centre one 
 not moving at all. (In fig. 40, A B may be taken to 
 represent the glass rectangle with its centre condensed ; 
 while A' B' represents the same rectangle with its 
 centre rarefied. The ends of the strip suffer neither 
 condensation nor rarefaction.) 
 
 If we introduce the strip of glass (s s', fig. 41) between 
 the crossed Nicols, taking care to keep it oblique to the 
 directions of vibration of the Nicols, and sweep our wet 
 rubber over the glass, this may be expected to occur : 
 At every moment of compression the light will flash 
 through ; at every moment of strain the light will also 
 flash through ; and these states of strain and pressure 
 will follow each other so rapidly that we may expect a 
 permanent luminous impression to be made upon the 
 eye. By pure reasoning, therefore, we reach the con- 
 clusion that the light will be revived whenever the glass 
 is sounded. That it is so, experiment testifies : at every 
 sweep of the rubber, a fine luminous disk (o) flashes 
 out upon the screen. The experiment may be varied 
 in this way : Placing in front of the polarizer a plate of 
 unannealed glass, you have a series of beautifully 
 coloured rings, intersected by a black cross. Every 
 sweep of the rubber not only abolishes the rings, but 
 introduces complementary ones, the black cross being, 
 for the moment, supplanted by a white one. This is a 
 modification of a beautiful experiment which we owe 
 to Biot. His apparatus, however, confined the obser- 
 vation of it to a single person at a time. 
 
140 ON LIGHT. LECT. 
 
 5. Gjlours of Unannealel Glass. 
 
 Bodies are usually expanded by heat and con- 
 tracted by cold. If the heat be applied with perfect 
 uniformity, no local strains or pressures come into play ; 
 but, if one portion of a solid be heated and other 
 portions not, the expansion of the heated portion intro- 
 duces strains and pressures which reveal themselves 
 under the scrutiny of polarized light. When a square 
 of common window-glass is placed between the 
 Nicols, you see its dim outline, but it exerts no 
 action on the polarized light. Held for a moment 
 over the flame of a spirit-lamp, on reintroducing 
 it between the Nicols, light flashes out upon the 
 screen. Here, as in the case of mechanical action, 
 you have luminous spaces of strain divided by dark 
 neutral axes from spaces of pressure. 
 
 Let us apply the heat more symmetrically. A 
 small square of glass is perforated at the centre, and 
 into the orifice a bit of copper wire is introduced. 
 Placing the square between the prisms, and heating 
 the wire, the heat passes by conduction to the 
 glass, through which it spreads from the centre out- 
 wards. You immediately see, bounding four lumi- 
 nous quadrants, a dim cross, which becomes gradually 
 blacker by comparison with the adjacent brightness. 
 And as, in the case of pressure, we produced colours, so 
 here also, by the proper application of heat, gorgeous 
 chromatic effects may be produced. The condition 
 necessary to the production of these colours may be 
 rendered permanent by first heating the glass suffi- 
 ciently, and then cooling it, so that the chilled mass 
 
iv. ACTION OF UNANNEALED GLASS. 141 
 
 shall remain in a state of permanent strain and pressure. 
 Two or three examples will illustrate this point. Figs. 
 
 FIG. 42. FIG. 43. 
 
 42 and 43 represent the figures obtained with two 
 pieces of glass thus prepared. Two rectangular pieces 
 
 FIG. 44. 
 
142 ON LIGHT. LECT. 
 
 of unannealed glass, crossed and placed between the 
 polarizer and analyzer, exhibit the beautiful iris fringes 
 represented in fig. 44. 
 
 6. Circular Polarization. 
 
 But we have to follow the ether still farther into its 
 hiding-places. Suspended before you is a pendulum, 
 which, when drawn aside and liberated, oscillates to and 
 fro. If, when the pendulum is passing the middle point 
 of its excursion, I impart a shock to it tending to drive 
 it at right angles to its present course, what occurs ? 
 The two impulses compound themselves to a vibration 
 oblique in direction to the former one, but the pen- 
 dulum still oscillates in a plane. But, if the rect- 
 angular shock be imparted to the pendulum when it is 
 at the limit of its swing, then the compounding of the 
 two impulses causes the suspended ball to describe not 
 a straight line, but an ellipse ; and, if the shock be 
 competent of itself to produce a vibration of the same 
 amplitude as the first one, the ellipse becomes a circle. 
 
 Why do I dwell upon these things ? Simply to make 
 known to you the resemblance of these gross mechanical 
 vibrations to the vibrations of light. I hold in my hand 
 a plate of quartz cut from the crystal perpendicular to its 
 axis. The crystal thus cut possesses the extraordinary 
 power of twisting the plane of vibration of a polarized 
 ray to an extent dependent on the thickness of the 
 crystal. And the more refrangible the light the greater 
 is the amount of twisting ; so that, when white light 
 is employed, its constituent colours are thus drawn 
 asunder. Placing the quartz between the polarizer 
 and analyzer, you see this vivid red, and, turning the 
 analyzer in front, from right to left, the other colours 
 
iv. ACTION OF QUARTZ CRYSTALS. 143 
 
 of the spectrum, appear in succession. Specimens of 
 quartz have been found which require the analyzer to 
 be turned from left to right to obtain the same succes- 
 sion of colours. Crystals of the first class are therefore 
 called right-handed, and of the second class, left-handed 
 crystals. 
 
 With profound sagacity, Fresnel, to whose genius 
 we mainly owe the expansion and final triumph of the 
 undulatory theory of light, reproduced mentally the 
 mechanism of these crystals, and showed their action to 
 be due to the circumstance that, in them, the waves of 
 , ether so act upon each other as to produce the condition 
 represented by our rotating pendulum. Instead of 
 being plane polarized, the light in rock crystal is cir- 
 cularly polarized. Two such rays, transmitted along 
 the axis of the crystal, and rotating in opposite direc- 
 tions, when brought to interference by the analyzer, 
 are demonstrably competent to produce all the observed 
 phenomena. 
 
 7. Complementary Colours of Si-refracting Spar in 
 Circularly Polarized Light. Proof that Yellow and 
 Blue are Complementary. 
 
 I now remove the analyzer, and put in its place the 
 piece of Iceland spar with which we have already illus- 
 trated double refraction. The two images of the car- 
 bon-points are now before you, produced, as you know, 
 by two beams vibrating at right angles to each other. 
 Introducing a plate of quartz between the polarizer 
 and the spar, the two images glow with complementary 
 colours. Employing the image of an aperture instead 
 of that of the carbon-points, we have two coloured 
 
144 
 
 ON LIGHT. 
 
 LECT. 
 
 circles. As the analyzer is caused to rotate, the colours 
 pass through various changes ; but they are always 
 complementary. When the one is red, the other is 
 green ; when the one is yellow, the other is blue. 
 Here we have it in our power to demonstrate afresh a 
 statement made in our first lecture, that, although the 
 mixture of blue and yellow pigments produces green, 
 the mixture of blue and yellow lights produces white. 
 By enlarging our aperture, the two images produced 
 by the spar are caused to approach each other, and 
 finally to overlap. The one is now a vivid yellow, 
 
 FIG. 45. 
 
 the other a vivid blue, and you notice that where the 
 colours are superposed we have a pure white. (See fig. 
 45, where N is the end of the polarizer. Q the quartz 
 plate, L a lens, and B the bi-refracting spar. The two 
 images overlap at 0, and produce white by their mix- 
 ture.) 
 
 8. The Magnetization of Light. 
 
 This brings us to a point of our inquiries which, 
 though rarely illustrated in lectures, is nevertheless 
 so likely to affect profoundly the future course of 
 
iv. MAGNETIZATION OF LIGHT. 145 
 
 scientific thought that I am unwilling to pass it over 
 without reference. I refer to the experiment which 
 Faraday, its discoverer, called the ' magnetization of 
 light.' The arrangement for this celebrated experiment 
 is now before you. We have first our electric lamp, 
 then a Nicol prism, to polarize the beam emergent 
 from the lamp ; then an electro-magnet, then a second 
 Nicol, and finally our screen. At the present moment 
 the prisms are crossed, and the screen is dark. I 
 place from pole to pole of the electro-magnet a cylin- 
 der of a peculiar kind of glass, first made by Faraday, 
 and called Faraday's heavy glass. Through this glass 
 the beam from the polarizer now passes, being inter- 
 cepted by the Nicol in front. On exciting the magnet 
 light instantly appears upon the screen. By the action 
 of the magnet upon the ether contained within the 
 heavy glass, the plane of vibration is caused to rotate, 
 the light being thus enabled to get through the 
 analyzer. 
 
 The two classes into which quartz-crystals are di- 
 vided have been already mentioned. In my hand I hold 
 a compound plate, one half of it taken from a right- 
 handed, and the other from a left-handed crystal. 
 Placing the plate in front of the polarizer, I turn one 
 of the Nicols until the two halves of the plate show a 
 common puce colour. This yields an exceedingly sensi-' 
 tive means of rendering visible the action of a magnet 
 upon light. By turning either the polarizer or the 
 analyzer through the smallest angle, the uniformity of 
 the colour disappears, and the two halves of the quartz 
 show different colours. The magnet produces an effect 
 equivalent to this rotation. The puce-coloured circle 
 is now before you on the screen. (See fig. 46, where 
 
 L 
 
146 
 
 ON LIGHT. 
 
 N is the nozzle of the lamp, H the first Nicol, Q the 
 biquartz plate, L a lens, M the electro-magnet, with 
 the heavy glass across its poles, and P the second Nicol.) 
 Exciting the magnet, one half of the image becomes 
 suddenly red, the other half green. Interrupting the 
 current, the two colours fade away, and the primitive 
 puce is restored. 
 
 The action, moreover, depends upon the polarity 
 of the magnet, or, in other words, on the direction of 
 the current which surrounds the magnet. Eeversing 
 the current, the red and green reappear, but they 
 have changed places. The red was formerly to the 
 right, and the green to the left ; the green is now to 
 'the right, and the red to the left. With the most ex- 
 quisite ingenuity, Faraday analyzed all those actions 
 and stated their laws. This experiment, however, long 
 remained rather a scientific curiosity than a fruitful 
 germ. That it would bear fruit of the highest impor- 
 tance, Faraday felt profoundly convinced, and recent re- 
 searches are on the way to verify his conviction. 
 
RINGS ROUND AXES OF CRYSTALS. 147 
 
 9. Iris-rings surrounding the Axes of Crystals. 
 
 A few words more are necessary to complete our 
 knowledge of the wonderful interaction between pon- 
 derable molecules and the ether interfused among them. 
 Symmetry of molecular arrangement implies symmetry 
 on the part of the ether ; atomic dissymmetry, on the 
 other hand, involves the dissymmetry of the ether, and, 
 as a consequence, double refraction. In a certain class 
 of crystals the structure is homogeneous, and such 
 crystals produce no double refraction. In certain other 
 crystals the molecules are ranged symmetrically round 
 a certain line, and not around others. Along the 
 former, therefore, the ray is undivided, while along all 
 the others we have double refraction. Ice is a familiar 
 example : its molecules are built with perfect symmetry 
 around the perpendiculars to the planes of freezing, 
 and a ray sent through ice in this direction is not 
 doubly refracted; whereas, in all other directions, it is. 
 Iceland spar is another example of the same kind : its 
 molecules are built symmetrically round the line unit- 
 ing the two blunt angles of the rhomb. In this direc- 
 tion a ray suffers no double refraction, in all others it 
 does. This direction of no double refraction is called 
 the optic axis of the crystal. 
 
 Hence, if a plate be cut from a crystal of Iceland spar 
 perpendicular to the axis, all rays sent across this plate 
 in the direction of the axis will produce but one image. 
 But, the moment we deviate from the parallelism with 
 the axis, double refraction sets in. If, therefore, a 
 beam that has been rendered conical by a converging 
 lens be sent through the spar so that the central ray of 
 
 L 2 
 
148 ON LIGHT. 
 
 LECT; 
 
 the cone passes along the axis, this ray only will escape 
 double refraction. Each of the others will he divided 
 into an ordinary and an extraordinary ray, the one 
 moving more slowly through the crystal than the 
 other; the one, therefore, retarded with reference to 
 the other. Here, then, we have the conditions for 
 interference, when the waves are reduced by the ana- 
 lyzer to a common plane. 
 
 Placing the plate of Iceland spar between the crossed 
 Nicol's prisms, and employing the conical beam, we have 
 FIG. 47. 
 
 upon the screen a beautiful system of iris-rings sur- 
 rounding the end of the optic axis, the circular bands 
 of colour being intersected by a black cross (fig. 47). 
 /The arms of this cross are parallel to the two directions 
 of vibration in the polarizer and analyzer. It is 
 easy to see that those rays whose planes of vibration 
 within the spar coincide with the plane of vibration 
 of either prism, cannot get through both. This com- 
 plete interception produces the arms of the cross. 
 With monochromatic light the rings would be simply 
 bright and black the bright rings occurring at those 
 thicknesses of the spar which cause the rays to con- 
 
IV. 
 
 UNIAXAL AND BIAXAL CEYSTALS. 
 
 149 
 
 spire ; the black rings at those thicknesses which cause 
 them to quench each other. Turning the analyzer 90 
 round, we obtain the complementary phenomena. The 
 black cross gives place to a bright one, and every dark 
 ring is supplanted also by a bright one (fig. 48). Here, 
 
 FIG. 48. 
 
 as elsewhere, the different lengths of the light-waves 
 give rise to iris-colours when white light is employed. 
 Besides the regular crystals which produce double 
 
 FIG. 49. 
 
 refraction in no direction, and the uniaxal crystals 
 which produce it in all directions but one, Brewster 
 discovered that in a large class of crystals there are 
 two directions in which double refraction does not take 
 place. These are called biaxal crystals. When plates 
 of these crystals, suitably cut, are placed between the 
 
150 ON LIGHT. LBCT. 
 
 polarizer and analyzer, the axes (A A', fig 49) are seen 
 surrounded, not by circles, but by curves of another order 
 and of a perfectly definite mathematical character. Each 
 band, as proved experimentally by Herschel, forms a 
 lemniscata ; but the experimental proof was here, as 
 in numberless other cases, preceded by the deduction 
 which showed that, according to the undulatory theory, 
 the bands must possess this special character. 
 
 10. Power of the Undulatory Theory. 
 
 I have taken this somewhat wide range over polar- 
 ization itself, and over the phenomena exhibited by 
 crystals in polarized light, in order to give you some 
 notion of the firmness and completeness of the theory 
 which grasps them all. Starting from the single 
 assumption of transverse undulations, we first of all 
 determine the wave-lengths, and find all the pheno- 
 mena of colour dependent on this element. The wave- 
 lengths may be determined in many independent ways. 
 Newton virtually determined them when he measured 
 the periods of his Fits : the length of a fit, in fact, is 
 that of a quarter of an undulation. The wave-lengths 
 may be determined by diffraction at the edges of a slit 
 (as in the Appendix to these Lectures) ; they may be 
 deduced from the interference fringes produced by 
 reflection ; from the fringes produced by refraction ; also 
 by lines drawn with a diamond upon glass at measured 
 distances asunder. And when the lengths determined 
 by these independent methods are compared together, 
 the strictest agreement is found to exist between them. 
 
 With the wave-lengths at our disposal, we fallow the 
 ether into the most complicated cases of interaction 
 
iv. GRASP OF THE UNDULATORY THEORY. 151 
 
 between it and ordinary matter, ' the theory is equal 
 to them all. It makes not a single new physical 
 hypothesis ; but out of its original stock of principles 
 it educes the counterparts of all that observation shows. 
 It accounts for, explains, simplifies the most entangled 
 cases ; corrects known laws and facts ; predicts and dis- 
 closes unknown ones ; becomes the guide of its former 
 teacher Observation ; and, enlightened by mechanical 
 conceptions, acquires an insight which pierces through 
 shape and colour to force and cause.' l 
 
 But, while I have thus endeavoured to illustrate be- 
 fore you the power of the undulatory theory as a solver 
 of all the difficulties of optics, do I therefore wish you 
 to close your eyes to any evidence that may arise 
 against it? By no means. You may urge, and justly 
 urge, that a hundred years ago another theory was held 
 by the most eminent men, and that, as the theory then 
 held had to yield, the undulatory theory may have to 
 yield also. This seems reasonable ; but let us under- 
 stand the precise value of the argument. In similar 
 language a person in the time of Newton, or even in 
 our time, might reason thus: Hipparchus and Ptolemy, 
 and numbers of great men after them, believed that 
 the earth was the centre of the solar system. But this 
 deep-set theoretic notion had to give way, and the 
 theory of gravitation may, in its turn, have to give 
 way also. This is j ust as reasonable as the first argu- 
 ment. Wherein consists the strength of the theory of 
 gravitation ? Solely in its competence to account for 
 all the phenomena of the solar system. Wherein con- 
 sists the strength of the theory of undulation ? Solely 
 in its competence to disentangle and explain phenomena 
 
 Whewell. 
 
152 ON LIGHT. LE:?T. 
 
 a hundred-fold more complex than those of the solar 
 system. Accept if you will the scepticism of Mr. 
 Mill 1 regarding the undulatory theory ; but if your 
 scepticism be philosophical, it will wrap the theory of 
 gravitation in the same or greater doubt. 2 
 
 11. The Blue of the Sky. 
 
 I am unwilling to quit these chromatic phenomena 
 without referring to a source of colour which has often 
 come before me of late in the blue of your skies at 
 noon, and the deep crimson of your horizon after the 
 set of sun. I will here summarise and extend what I have 
 already said upon this subject in another place. Proofs 
 of the most cogent description could be adduced 
 to show that the blue light of the firmament is 
 reflected light. That light comes to us across the direc- 
 tion of the solar rays, and even against the direction of 
 the solar rays ; and this lateral and opposing rush of 
 wave-motion can only be due to the rebound of the 
 waves from the air itself, or from something suspended 
 in the air. The solar light, moreover, is not reflected by 
 the sky in the proportions which produce white. The 
 sky is blue, which indicates an excess of the smaller waves. 
 The blueness of the air has been given as a reason for 
 the blueness of the sky ; but then the question arises. 
 How, if the air be blue, can the light of sunrise and sun- 
 set, which travels through vast distances of air, be yellow, 
 orange, or even red ? The passage of the white solar 
 
 1 Removed from us since these words were written. 
 
 2 The only essay known to me on the Undulatory Theory, from the 
 pen of an American writer, is an excellent one by President Barnard, 
 published in the Smithsonian Eeport for 1862. 
 
iv. SCATTERING BY SMALL PARTICLES. 153 
 
 light through a blue medium could by no possibility 
 redden the light ; the hypothesis of a blue air is there- 
 fore untenable. In fact the agent, whatever it be, 
 which sends us the light of the sky, exercises in so 
 doing a dichroitic action. The light reflected is blue, 
 the light transmitted is orange or red. A marked dis- 
 tinction is thus exhibited between reflection from the 
 sky and that from an ordinary cloud, which exercises 
 no such dichroitic action. 
 
 The cloud, in fact, takes no note of size on the part 
 of the waves of ether, but reflects them all alike. Now 
 the cause of this may be that the cloud particles are so 
 large in comparison with the size of the waves of ether 
 as to scatter them all indifferently. A broad cliff re- 
 flects an Atlantic roller as easily as a ripple produced 
 by a sea-bird's wing ; and in the presence of large re- 
 flecting surfaces, the existing differences of magnitude 
 among the waves of ether may also disappear. But 
 supposing the reflecting particles, instead of being very 
 large, to be very small, in comparison with the size of 
 the waves. Then, instead of the whole wave being 
 fronted and in great part thrown back, a small portion 
 only is shivered off by the obstacle. Suppose, then, 
 such minute foreign particles to be diffused in our at- 
 mosphere. Waves of all sizes impinge upon them, 
 and at every collision a portion of the impinging wave 
 is struck off. All the waves of the spectrum, from the 
 extreme red to the extreme violet, are thus acted upon ; 
 but in what proportions will they be scattered ? Large- 
 ness is a thing of relation ; and the smaller the wave, 
 the greater is the relative size of any particle on which 
 the wave impinges, and the greater also the relative re- 
 flection. 
 
154 ON LIGHT. LBCT. 
 
 A small pebble placed in the way of the ring-ripples 
 produced by heavy rain-drops on a tranquil pond will 
 throw back a large fraction of each ripple incident upon 
 it, while the fractional part of a larger wave thrown 
 back by the same pebble might be infinitesimal. Now 
 to preserve the solar light white, its constituent pro- 
 portions must not be altered ; but in the scattering of 
 the light by these very small particles we see that the 
 proportions are altered. The smaller waves are in 
 excess, and, as a consequence, in the scattered light 
 blue will be the predominant colour. The other 
 colours of the spectrum must, to some extent, be asso- 
 ciated with the blue : they are not absent, but deficient. 
 We ought, in fact, to have them all, but in diminishing 
 proportions, from the violet to the red. 
 
 We have thus reasoned our way to the conclusion, 
 that were particles, small in comparison to the size of 
 the ether waves, sown in our atmosphere, the light scat- 
 tered by those particles would be exactly such as we 
 observe in our azure skies. And, indeed, when this 
 light is analysed, all the colours of the spectrum 
 are found in the proportions indicated by our con- 
 clusion. 
 
 By its successive collisions with the particles the 
 white light is more and more robbed of its shorter 
 waves ; it therefore loses more and more of its due 
 proportion of blue. The result may be anticipated. 
 The transmitted light, where short distances are in- 
 volved, will appear yellowish. But as the sun sinks 
 towards the horizon the atmospheric distance increases, 
 and consequently the number of the scattering particles. 
 They weaken in succession the violet, the indigo, the 
 blue, and even disturb the proportions of green. The 
 
iv. ARTIFICIAL SKY. 155 
 
 transmitted light under such circumstances must pass 
 from yellow through orange to red. This also is 
 exactly what we find in nature. Thus, while the re- 
 flected light gives us, at noon, the deep azure of the 
 Alpine skies, the transmitted light gives us, at sunset, 
 the warm crimson of the Alpine snows. 
 
 But can small particles be really proved to act in the 
 manner indicated ? No doubt of it. Each one of you 
 can submit the question to an experimental test. 
 Water will not dissolve resin, but spirit will ; and when 
 spirit which holds resin in solution is dropped into 
 water, the resin immediately separates in solid particles, 
 which render the water milky. The coarseness of this 
 precipitate depends on the quantity of the dissolved 
 resin. Professor Briicke has given us the proportions 
 which produce particles particularly suited to our pre- 
 sent purpose. One gramme of clean mastic is dissolved 
 in eighty-seven grammes of absolute alcohol, and the 
 transparent solution is allowed to drop into a beaker 
 containing clear water briskly stirred. An exceedingly 
 fine precipitate is thus formed, which declares its 
 presence by its action upon light. Placing a dark sur- 
 face behind the beaker, and permitting the light to fall 
 into it from the top or front, the medium is seen to be 
 of a very fair sky-blue. A trace of soap in water gives 
 a tint of blue. London milk makes an approximation 
 to the same colour through the operation of the same 
 cause : and Helmholtz has irreverently disclosed the 
 fact that a blue eye is simply a turbid medium. 
 
 12. Artificial Sky. 
 
 But we have it in our power to imitate far more 
 closely the natural conditions of this problem. We can 
 
156 ON LIGHT. LECT. 
 
 generate in air artificial skies, and prove their perfect 
 identity with the natural one, as regards the exhibition 
 of a number of wholly unexpected phenomena. It has 
 been recently shown in a great number of instances that 
 waves of ether issuing from a strong source, such as the 
 sun or the electric light, are competent to shake asun- 
 der the atoms of gaseous molecules. The apparatus 
 used to illustrate this consists of a glass tube about a 
 yard in length, and from 2^ to 3 inches internal diame- 
 ter. The gas or vapour to be examined is introduced 
 into this tube, and upon it the condensed beam of the 
 electric lamp is permitted to act. The vapour is so 
 chosen that one, at least, of itsproductsof decomposition, 
 as soon as it is formed, shall be precipitated to a kind 
 of cloud. By graduating the quantity of the vapour, this 
 precipitation may be rendered of any degree of fineness, 
 forming particles distinguishable by the naked eye, or 
 particles which are probably far beyond the reach of 
 our highest microscopic powers. 1 have no reason to 
 doubt that particles may be thus obtained whose 
 diameters constitute but a very small fraction of the 
 length of a wave of violet light. 
 
 Now, in all such cases when suitable vapours are 
 employed in a sufficiently attenuated state, no matter 
 what the vapour may be, the visible action commences 
 with the formation of a blue cloud. Let me guard my- 
 self at the outset against all misconception as to the use 
 of this term. The blue cloud here referred to is totally 
 invisible in ordinary daylight. To be seen, it requires 
 to be surrounded by darkness, it only being illuminated 
 by a powerful beam of light. This cloud differs in 
 many important particulars from the finest ordinary 
 clouds, and might justly have assigned to it an inter- 
 
tv. POLARIZATION OF SKY-LIGHT. 157 
 
 mediate position between these clouds and true cloud- 
 less vapour. 
 
 It is possible to make the particles of this actinic 
 cloud grow from an infinitesimal and altogether ultra- 
 microscopic size to particles of sensible magnitude ; and 
 by means of these, in a certain stage of their growth, 
 we produce a blue which rivals, if it does not transcend, 
 that of the deepest and purest Italian sky. Introduc- 
 ing into our tube a quantity of mixed air and nitrite 
 of butyl vapour sufficient to depress the mercurial 
 column of an air-pump one-twentieth of an inch, 
 adding a quantity of air and hydrochloric acid sufficient 
 to depress the mercury half an inch further, and send- 
 ing through this compound and highly attenuated atmo- 
 sphere, the beam of the electric light ; gradually within 
 the tube arises a splendid azure, which strengthens for 
 a time, reaches a maximum of depth and purity, and 
 then, as the particles grow larger, passes into whitish 
 blue. This experiment is representative, and it illus- 
 trates a general principle. Various other colourless 
 substances of the most diverse properties, optical and 
 chemical, might be employed for this experiment. The 
 incipient cloud, in every case, would exhibit this superb 
 blue ; thus proving to demonstration that particles of 
 infinitesimal size, without any colour of their own, and 
 irrespective of those optical properties exhibited by the 
 substance in a massive state, are competent to produce 
 the blue colour of the sky. 
 
 1 3. Polarization of Sky-light. 
 
 But there is another subject connected with our 
 firmament, of a more subtle and recondite character 
 
158 ON LIGHT. LECT. 
 
 than even its colour. I mean that ' mysterious and 
 beautiful phenomenon,' the polarization of the light 
 of the sky. Looking at various points of the blue 
 firmament through a NicoFs prism, and turning the 
 prism round its axis, we soon notice variations of 
 brightness. In certain positions of the prism, and 
 from certain points of the firmament, the light appears 
 to be wholly transmitted, while it is only necessary to 
 turn the prism round its axis through an angle of 
 ninety degrees to materially diminish the intensity of 
 the light. Experiments of this kind prove that the 
 blue light sent to us by the firmament is polarized, 
 and on close scrutiny it is also found that the direction 
 of most perfect polarization is perpendicular to the 
 solar rays. Were the heavenly azure like the ordinary 
 light of the sun, the turning of the prism would have 
 no effect upon it ; it would be transmitted equally 
 during the entire rotation of the prism. The light of 
 the sky is in great part quenched, because it is in great 
 part polarized. 
 
 The same phenomenon is exhibited in perfection by 
 our actinic clouds, the only condition necessary to its- 
 production being the smallness of the particles. In all 
 cases, and with all substances, the cloud formed at the 
 commencement, when the precipitated particles are 
 sufficiently fine, is blue. In all cases, moreover, this 
 fine blue cloud polarizes perfectly the beam which 
 illuminates it, the direction of polarization enclosing 
 an angle of 90 with the axis of the illuminating 
 beam. 
 
 It is exceedingly interesting to observe both the 
 growth and the decay of this polarization. For ten or 
 fifteen minutes after its first appearance the light from 
 
iv. POLARIZATION BY ARTIFICIAL SKY. 159 
 
 a vividly illuminated incipient cloud, looked at hori- 
 zontally, is absolutely quenched by a Nicol's prism 
 with its longer diagonal vertical. But as the sky-blue 
 is gradually rendered impure by the introduction of 
 particles of too large a size, in other words, as real 
 clouds begin to be formed, the polarization begins to 
 deteriorate, a portion of the light passing through the 
 prism in all its positions, as it does in the case of sky- 
 light. It is worthy of note that for some time after 
 the cessation of perfect polarization the residual light 
 which passes, when the Nicol is in its position of 
 minimum transmission, is of a gorgeous blue, the 
 whiter light of the cloud being extinguished. When 
 the cloud texture has become sufficiently coarse to ap- 
 proximate to that of ordinary clouds, the rotation of 
 the Nicol ceases to have any sensible effect on the 
 quantity of the light discharged at right angles to the 
 beam. 
 
 The perfection of the polarization in a direction 
 perpendicular to the illuminating beam was also illus- 
 trated by the following experiment executed with many 
 vapours. A Nicol's prism large enough to embrace 
 the entire beam of the electric lamp was placed 
 between the lamp and the experimental tube. Send- 
 ing the beam polarized by the Nicol through the 
 tube, I placed myself in front of it, the eyes being on a 
 level with its axis, my assistant occupying a similar 
 position behind the tube. The short diagonal of the 
 large Nicol was in the first instance vertical, the plane 
 of vibration of the emergent beam being therefore also 
 vertical. As the light continued to act, a superb blue 
 cloud visible to both my assistant and myself was slowly 
 formed. But this cloud, so deep and rich when looked at 
 
160 ON LIGHT. LECT. 
 
 from the positions mentioned, utterly disappeared when 
 looked at vertically downwards, or vertically upwards. 
 Keflection from the cloud was not possible in these 
 directions. When the large Nicol was slowly turned 
 round its axis, the eye of the observer being on the 
 level of the beam, and the line of vision perpendicular 
 to it, entire extinction of the light emitted hori- 
 zontally occurred when the longer diagonal of the 
 large Nicol was vertical. But a vivid blue cloud 
 was seen when looked at downwards or upwards. This 
 truly fine experiment, which I should certainly have 
 made without suggestion, was, as a matter of fact, first 
 definitely suggested by a remark addressed to me in 
 a letter by Professor Stokes. 
 
 All the phenomena of colour and of polarization 
 observable in the case of skylight are manifested by 
 those actinic clouds ; and they exhibit additional phe- 
 nomena which it would be neither convenient to 
 pursue, nor perhaps possible to detect, in the actual 
 firmament. They enable us, for example, to follow 
 the polarization from its first appearance on the barely 
 visible blue to its final extinction in the coarser cloud. 
 These changes, as far as it is now necessary to refer to 
 them, may be thus summed up : 
 
 1. The actinic cloud, as long as it continues blue, 
 discharges polarized light in all directions, but the 
 direction of maximum polarization, like that of sky- 
 light, is at right angles to the direction of the illumin- 
 ating beam. 
 
 , 2. As long as the cloud remains distinctly blue the 
 light discharged from it at right angles to the illumi- 
 nating beam is perfectly polarized. It may be utterly 
 quenched by a Nicol's prism, the cloud from which it 
 
iv. SUMMARY OF PHENOMENA. 161 
 
 issues being caused to disappear. Any deviation from 
 the perpendicular enables a portion of the light to get 
 through the prism. 
 
 3. The direction of vibration of the polarized light 
 is at right angles to the illuminating beam. Hence a 
 plate of tourmaline, with its axis parallel to the beam, 
 stops the light, and with the axis perpendicular to the 
 beam transmits the light. 
 
 4. A plate of selenite placed between the Nicol and 
 the actinic cloud shows the colours of polarized light ; 
 in fact, the cloud itself plays the part of a polarizing 
 Nicol. 
 
 5. The particles of the blue cloud are immeasurably 
 small, but they increase gradually in size, and at a certain 
 period of their growth cease to discharge perfectly 
 polarized light. For some time afterwards the light 
 that reaches the eye through the Nicol is of a mag- 
 nificent blue, far exceeding in depth and purity that 
 of the purest sky ; thus the waves that first feel the 
 influence of size, at both limits of the polarization, 
 are the shortest waves of the spectrum. These are 
 the first to accept polarization, and they are the first 
 to escape from it. 
 
 L I B Li A K V 
 
 UN J VKUSITY 
 
 CALIFORNIA, 
 
162 ON LIGHT. LECT. 
 
 LECTUKE V. 
 
 KANGE OF VISION NOT COMMENSURATE WITH RANGE OF RADIATION THE 
 ULTRA-VIOLET RAYS FLUORESCENCE RENDERING INVISIBLE RAYS 
 VISIBLE VISION NOT THE ONLY SENSE APPEALED TO BY THE SOLAR 
 
 AND ELECTRIC BEAM HEAT OF BEAM CONBUSTION BY TOTAL BEAM 
 
 AT THE FOCI OF MIRRORS AND LENSES COMBUSTION THROUGH ICE- 
 LENS IGNITION OF DIAMOND SEARCH FOR THE RAYS HERE EFFEC- 
 TIVE SIR WILLIAM HERSCHEL'S DISCOVERY OF DARK SOLAR RAYS 
 
 INVISIBLE RAYS THE BASIS OF THE VISIBLE DETACHMENT BY A RAY- 
 FILTER OF THE INVISIBLE RAYS FROM THE VISIBLE COMBUSTION AT 
 DARK FOCI CONVERSION OF HEAT-RAYS INTO LIGHT-RAYS CALOR- 
 ESCENCE PART PLAYED IN NATURE BY DARK RAYS IDENTITY OF LIGHT 
 AND RADIANT HEAT INVISIBLE IMAGES REFLECTION, REFRACTION, 
 PLANE POLARIZATION, DEPOLARIZATION, CIRCULAR POLARIZATION, 
 DOUBLE REFRACTION, AND MAGNETIZATION OF RADIANT HEAT. 
 
 1. Range of Vision and of Radiation. 
 
 THE first question that we have to consider to-night 
 is this : Is the eye, as an organ of vision, commensurate 
 with the whole range of solar radiation is it capable 
 of receiving visual impressions from all the rays emitted 
 by the sun ? The answer is negative. If we allowed 
 ourselves to accept for a moment that notion of gradual 
 growth, amelioration, and ascension, implied by the 
 tj3rm evolution, we miglit fairly conclude that there 
 are stores of visual impressions awaiting man, far 
 greater than those now in his possession. Eitter dis- 
 covered in 1801 that beyond the extreme violet of the 
 spectrum there is a vast efflux of rays which are totally 
 
v. ULTRA-VIOLET KAYS. 163 
 
 useless as regards our present powers of vision. These 
 ultra-violet waves, however, though incompetent to 
 awaken the optic nerve, can shake asunder the mole- 
 cules of certain compound substances on which they 
 impinge, thus producing chemical decomposition. 
 
 But though the blue, violet, and ultra-violet rays 
 can act thus upon certain substances, the fact is hardly 
 sufficient to entitle them to the name of ' chemical rays,' 
 usually applied to distinguish them from the other 
 constituents of the spectrum. As regards their action 
 upon the salts of silver, and many other substances, 
 they may perhaps merit this title* ; but in the case .of 
 the grandest example of the chemical action of light 
 the decomposition of carbonic acid in the leaves of 
 plants, with which my eminent friend Dr. Draper has 
 so indissolubly associated his name the yellow rays are 
 found to be most active. 
 
 There are substances, however, on which the violet 
 and ultra-violet waves exert a special decomposing 
 power ; and, by permitting the invisible spectrum to 
 fall upon surfaces prepared with suc&i substances, we 
 reveal both the existence and the" extent of the ultra- 
 violet spectrum. ^- 
 
 2. Ultra-violet Rays : -FluoPescence* 
 
 The method of exhibiting the action of the ultra- 
 violet rays by their chemicftl^action has been long 
 known ; indeed, Thomas Young photographs d the ultra- 
 violet rings of Newton. We have now to demonstrate 
 their presence in another way. As a general rule, 
 bodies either transmit light or absorb it ; butffl&here is a 
 third case in which the light falling upon tge body "is 
 
 M 2 
 
164 ON LIGHT. LECT. 
 
 neither transmitted nor absorbed, but converted into 
 light of another kind. Professor Stokes, the occupant 
 of the chair of Newton in the University of Cambridge, 
 has demonstrated this change of one kind of light into 
 another, and has pushed his experiments so far as to 
 render the invisible rays visible. 
 
 A large number of substances examined by Stokes, 
 when excited by the invisible ultra-violet waves, have 
 been proved to emit light. You know the rate of vibra- 
 tion corresponding to the extreme violet of the spectrum ; 
 you are aware that to produce the impression of this 
 colour, the retina is struck 789 millions of millions of 
 times in a second. At this point, the retina ceases to 
 be useful as an organ of vision, for though struck by 
 waves of more rapid recurrence, they are incompetent 
 to awaken the sensation of light. But when such non- 
 visual waves are caused to impinge upon the molecules 
 of certain substances on those of sulphate of quinine, 
 for example they compel those molecules, or their 
 constituent atoms, to vibrate ; and the peculiarity is, 
 that the vibrations thus set up are of slower period 
 than those of the exciting waves. By this lowering of 
 the rate of vibration through the intermediation of the 
 sulphate of quinine, the invisible rays are brought 
 within the range of vision. We shall subsequently 
 have abundant opportunity for learning that trans- 
 parency to the visible b} 7 no means involves transparency 
 to the invisible rays. Our bisulphide of carbon, for 
 example, which, employed in prisms, is so eminently 
 suitable for experiments on the visual rays, is by no 
 means so suitable for these ultra-violet rays. Flint 
 glass is better, and rock crystal is better than flint 
 
v. FLUOKESCENCE. 165 
 
 glass. A glass prism, however, will suit our present 
 purpose. 
 
 Casting by means of such a prism a spectrum, not 
 upon the white surface of our screen, but upon a sheet 
 of paper which has been wetted with a saturated 
 solution of the sulphate of quinine, and afterwards dried, 
 an obvious extension of the spectrum is revealed. We 
 have, in the first instance, a portion of the violet 
 rendered whiter and more brilliant; but, besides this, 
 we have the gleaming of the colour where, in the case 
 of unprepared paper, nothing is seen. Other substances 
 produce a similar effect. A substance, for example, re- 
 cently discovered by President Morton, and named by 
 him Thctllene, produces a very striking elongation of 
 the spectrum, the new light generated being of peculiar 
 brilliancy. 
 
 Fluor spar and some other .substances, when raised to 
 a temperature still under redness, emit light. During the 
 ages which have elapsed since their formation, this capa- 
 city of shaking the ether into visual tremors appears to 
 have been enjoyed by these substances. Light has been 
 potential within them all this time.; and, as well ex- 
 plained by Draper, the heat, though not itself of visual 
 intensity, can unlock the molecules so as to enable them 
 to exert their long-latent power of vibration. This de- 
 portment of fluor spar determined Stokes in his choice 
 of a name for his great discovery : he called this ren- 
 dering visible of the ultra-violet rays Fluorescence. 
 
 By means of a deeply-coloured violet glass, we cut 
 off almost the whole of the light of our electric beam ; 
 but this glass is peculiarly transparent to the violet and 
 ultra-violet rays. The violet beam now crosses a large 
 jar filled with water, into which I pour a solution of 
 
166 ON LIGHT. LECT. 
 
 sulphate of quinine. Clouds, to all appearance opaque, 
 instantly tumble downwards. Fragments of horse- 
 chestnut bark thrown upon the water also send down 
 beautiful cloud-like striae. But these are not clouds : 
 there is nothing precipitated here : the observed action 
 is an action of molecules, not of particles. The me- 
 dium before you is not a turbid medium, for when you 
 look through it at a luminous surface it is perfectly 
 clear. 
 
 If we paint upon a piece of paper a flower or a 
 bouquet with the sulphate of quinine, and expose it to 
 the full beam, scarcely anything is seen. But on inter- 
 posing the violet glass, the design instantly flashes forth 
 in strong contrast with the deep surrounding violet. 
 A most beautiful example of such a design has been pre- 
 pared for me with his thallene by President Morton : 
 placed in the violet light it exhibits a peculiarly 
 brilliant fluorescence. From the experiments of Dr. 
 Bence Jones, it would seem that there is some sub- 
 stance in the human body resembling the sulphate of 
 quinine, which causes all the tissues of the body to be 
 more or less fluorescent. The crystalline lens of the 
 eye exhibits the effect in a very striking manner. 
 When, for example, I plunge my eye into this violet 
 beam, I am conscious of a whitish-blue shimmer filling 
 the space before me. This is caused by fluorescent 
 light generated in the eye itself. Looked at from with- 
 out, the crystalline lens at the same time is seen 
 to gleam vividly. 
 
 Long before its physical origin was understood this 
 fluorescent light attracted attention. Boyle, as Sir 
 Charles Wheatstone has been good enough to point 
 out to me, describes it with great fullness and exact- 
 
v, BCTXLE AND GOETHE. 167 
 
 ness. ' We have sometimes,' he says, c found in the 
 shops of our druggists a certain wood which is there 
 called Lignum Nephriticum, because the inhabitants 
 of the country where it grows are wont to use the 
 infusion of it, made in fair water, against the stone in 
 the kidneys. This wood may afford us an experiment 
 which, besides the singularity of it, may give no small 
 assistance to an attentive considerer towards the detec- 
 tion of the nature of colours. Take Lignum Nephri- 
 ticum, and with a knife cut it into thin slices : put 
 about a handful of these slices into two or three or 
 four pounds of the purest spring water. Decant this 
 impregnated water into a glass phial ; and if you hold 
 it directly between the light and your eye, you shall 
 see it wholly tinted with an almost golden colour. 
 But if you hold this phial from the light, so that your 
 eye be placed betwixt the window and the phial, the 
 liquid will appear of a deep and lovely ceruleous 
 colour.' 
 
 ' These,' he continues, ' and other phenomena which 
 I have observed in this delightful experiment, divers 
 of my friends have looked upon, not without some 
 wonder ; and I remember an excellent oculist, finding 
 by accident in a friend's chamber a phial full of 
 this liquor, which I had given that friend, and having 
 never heard anything of the experiment, nor having 
 anybody near him who could tell him what this strange 
 liquor might be, was a great while apprehensive, as he 
 presently afterwards told me, that some strange new 
 distemper was invading his eyes. And I confess that 
 the unusualness of the phenomenon made me very 
 solicitous to find out the cause of this experiment ; and 
 though I am far from pretending to have found it, yet 
 
168 ON LIGHT. LECT. 
 
 my enquiries have, I suppose, enabled me to give such 
 hints as may lead your greater sagacity, to the discovery 
 of the cause of this wonder.' l 
 
 Groethe in his ' Farbenlehre ' thus describes the 
 fluorescence of horse-chestnut bark : ' Let a strip of 
 fresh horse-chestnut bark be taken and clipped into a 
 glass of water ; the most perfect sky-blue will be imme- 
 diately produced.' 2 Sir John Herschel first noticed 
 and described the fluorescence of the sulphate of quin- 
 ine, and showed that the light proceeded from a thin 
 stratum of the solution adjacent to the surface where 
 the light enters it. He showed, moreover, that the 
 incident beam, although not sensibly weakened in lumi- 
 nous power, lost, in its transmission through the solution 
 of sulphate of quinine, the power of producing the 
 blue fluorescent light. Sir David Brewster also 
 worked at the subject ; but to Professor Stokes we are 
 indebted not only for its expansion, but for its full 
 and final explanation. 
 
 3. The Heat of the Electric Beam. Ignition through 
 a Lens of Ice. Possible Cometary Temperature. 
 
 But the waves from our incandescent carbon-points 
 appeal to another sense than that of vision. They not 
 only produce light, but heat, as a sensation. The 
 magnified image of the carbon-points is now upon the 
 screen; and with a suitable instrument the heating 
 power of the rays which form that image might be 
 readily demonstrated. In this case, however, the heat 
 is spread over too large an area to be very intense. 
 
 1 Boyle's Works, Birch's edition, vol. i. pp. 729 and 730. 
 2 Werke, b. xxix. p. 24. 
 
v, HEAT OF ELECTRIC BEAM. 169 
 
 Pushing out the lens, and causing a movable screen to 
 approach our lamp, the image is seen to become 
 smaller and smaller ; the rays at the same time be- 
 coming more and more concentrated, until finally 
 they are able to pierce black paper with a burning ring. 
 Pushing back the lens so as to render the rays parallel 
 and receiving them upon a concave mirror, they are 
 brought to a focus ; paper placed at that focus is caused 
 to smoke and burn. Heat of this intensity may be 
 obtained with our ordinary camera and lens, and a 
 concave mirror of very moderate power. 
 
 We will now adopt stronger measures with the 
 radiation. In this larger camera of blackened tin is 
 
 FIG. 50. 
 
 placed a lamp, in all particulars similar to those already 
 employed. But instead of gathering up the rays from 
 the carbon-points by a condensing lens, we gather them 
 up by a concave mirror (m w', fig 50), silvered in front 
 and placed behind the carbons (P), By this mirror we 
 can cause the rays to issue through the orifice in front 
 of the camera, either parallel or convergent. They 
 are now parallel, and therefore, to a certain extent, 
 
170 ON LIGHT. LECT. 
 
 diffused. We place a convex lens (L) in the path of 
 the beam ; the light is converged to a focus (C), and at 
 that focus paper is not only pierced and a burning ring 
 formed, but it is instantly set ablaze. 
 
 Many metals may be burned up in the same way. 
 In our first lecture the combustibility of zinc was men- 
 tioned. Placing a strip of sheet-zinc at this focus, it 
 is instantly ignited, burning with its characteristic 
 purple flame. And now I will substitute for our glass 
 lens (L) one of a more novel character. In a smooth 
 iron mould a lens of pellucid ice has been formed. 
 Placing it in the position occupied a moment ago by 
 the glass lens, I can see the beam brought to a sharp 
 focus. At the focus I place a bit of black paper, with 
 a little gun-cotton folded up within it. The paper 
 immediately ignites and the cotton explodes. Strange, 
 is it not, that the beam should possess such heating 
 power after having passed through so cold a substance ? 
 In his arctic expeditions Dr. Scoresby succeeded in 
 exploding gunpowder by the sun's rays converged by 
 large lenses of ice ; here we have succeeded in pro- 
 ducing the effect with a small lens, and with a terres- 
 trial source of heat. 
 
 In this experiment, you observe that, before the beam 
 reaches the ice-lens, it has passed through a glass cell 
 containing water. The beam is thus sifted of con- 
 stituents, which, if permitted to fall upon the lens, 
 would injure its surface, and blur the focus. And this 
 leads me to say an anticipatory word regarding trans- 
 parency. In our first lecture we entered fully into the 
 production of colours by absorption, and we spoke re- 
 peatedly of the quenching of the rays of light. Did 
 this mean that the light was altogether annihilated ? 
 
v. HEBSCHBL'S DISCOVERY. 171 
 
 By no means. It was simply so. lowered in refr Eligi- 
 bility as to escape the visual range. It was converted 
 into heat. Our red ribbon in the green of the spectrum 
 quenched the green, but if suitably examined its tem- 
 perature would have been found raised. Our green 
 ribbon in the red of the spectrum quenched the red, 
 but its temperature at the same time was augmented 
 to a degree exactly equivalent to the light extinguished. 
 Our black ribbon, when passed through the spectrum, 
 was found competent to quench all its colours ; but at 
 every stage of its progress an amount of heat was 
 generated in the ribbon exactly equivalent to the light 
 lost. It is only when absorption takes place that 
 heat is thus produced : and heat is always a result of 
 absorption. 
 
 Examine the water, then, in front of the lamp after 
 the beam has passed through it : it is sensibly warm, 
 and, if permitted to remain there long enough, it might 
 be made to boil. This is due to the absorption, by the 
 water, of a certain portion of the electric beam. But a 
 portion passes through unabsorbed, and does not at all 
 contribute to the heating of the water. Now, ice is 
 also in great part transparent to these latter rays, and 
 therefore is but little melted by them. Hence, by 
 employing this particular portion of the beam, we 
 are able to keep our lens intact, and to produce by 
 means of it a sharply-defined focus. Placed at that 
 focus, white paper is not ignited, because it fails to 
 absorb the rays emergent from the ice-lens. At the 
 same place, however, black paper instantly burns, be- 
 cause it absorbs the transmitted light. 
 
 And here it may be useful to refer to an estimate by 
 Newton, based upon doubtful data, but repeated by 
 
172 ON LIGHT. LECT. 
 
 various astronomers of eminence since his time. The 
 comet of 1680, when nearest to the sun, was only a sixth 
 of the sun's diameter from his surface. Newton esti- 
 mated its temperature, in this position, to be more than 
 two thousand times that of molten iron. Now it is clear 
 from the foregoing experiments that the temperature of 
 the comet could not be inferred from its nearness to the 
 sun. If its power of absorption were sufficiently low, 
 the comet might carry into the sun's neighbourhood the 
 temperature of stellar space. 
 
 4. Combustion of Diamond by Radiant Heat. 
 
 Faraday thus describes the burning of a diamond in 
 oxygen by the concentrated rays of the sun. It was 
 effected at Florence, in presence of Sir Humphry Davy, 
 on Tuesday the 27th of March, 1814: 'To-day we made 
 the grand experiment of burning the diamond, and 
 certainly the phenomena presented were extremely 
 beautiful and interesting. A glass globe containing 
 about 22 cubical inches was exhausted of air, and filled 
 with pure oxygen. The diamond was supported in the 
 centre of this globe. The Duke's burning-glass was the 
 instrument used to apply heat to the diamond. It con- 
 sists of two double convex lenses, distant from each 
 other about 3J feet; the large lens is about 14 or 15 
 inches in diameter, the smaller one about 3 inches in 
 diameter. By means of the second lens the focus is 
 very much reduced, and the heat, when the sun shines 
 brightly, rendered very intense. The diamond was 
 placed in the focus and anxiously watched. On a 
 sudden Sir H. Davy observed the diamond to burn 
 
v. ULTRA-RED RAYS. 173 
 
 visibly, and when removed from the focus it was found 
 to be in a state of active and rapid combustion.' 
 
 The combustion of the diamond had never been 
 effected by radiant heat from a terrestrial source. I 
 tried to accomplish this before crossing the Atlantic, 
 and succeeded in doing so. The small diamond now in 
 my hand is held by a loop of platinum wire. To pro- 
 tect it as far as possible from air currents, and also to 
 concentrate the heat upon it, it is surrounded by a hood 
 of sheet platinum. Bringing ajar of oxygen underneath, 
 I cause the focus of the electric beam to fall upon the 
 diamond. A small fraction of the time expended in 
 the experiment described by Faraday, suffices to raise 
 the diamond to a brilliant red. Plunging it then into 
 the oxygen, it glows like a little white star, and it would 
 continue to burn and glow until wholly consumed. 
 The focus can also be made to fall upon the diamond 
 in oxygen, as in the Florentine experiment : the result 
 is the same. It is simply to secure more complete 
 mastery over the position of the focus, so as to cause it 
 to fall accurately upon the diamond, that the mode of 
 experiment here described was resorted to. 
 
 5. Ultra-red Rays : Calorescence. 
 
 In the path of the beam issuing from our lamp I 
 now place a cell with glass sides containing a solution of 
 alum. All the light of the beam passes through this 
 solution. This light is received on a powerfully con- 
 verging mirror silvered in front, and brought to a focus 
 by the mirror. You can see the conical beam of re- 
 flected light tracking itself through the dust of the 
 room. A scrap of white paper placed at the focus 
 
174 ON LIGHT. LECT. 
 
 shines there with dazzling brightness, but it is not even 
 charred. On removing the alum cell, however, the 
 paper instantly inflames. There must, therefore, be 
 something in this beam besides its light. The light is 
 not absorbed by the white paper, and therefore does 
 not burn the paper ; but there is something over and 
 above the light which is absorbed, and which provokes 
 combustion. What is this something ? 
 
 In the year 1800 Sir William Herschel passed a 
 thermometer through the various colours of the solar 
 spectrum, and marked the rise of temperature corre- 
 sponding to each colour. He found the heating effect 
 to augment from the violet to the red ; he did not, how- 
 ever, stop at the red, but pushed his thermometer into 
 the dark space beyond it. Here he found the tempera- 
 ture actually higher than in any part of the visible 
 spectrum. By this important observation, he proved 
 that the sun emitted heat-rays which are entirely unfit 
 for the purposes of vision. The subject was subse- 
 quently taken up by Seebeck, Melloni, Miiller, and 
 others, and within the last few years it has been found 
 capable of unexpected expansions and applications. I 
 have devised a method whereby the solar or electric 
 beam can be so filtered as to detach from it, and pre- 
 serve intact, this invisible ultra-red emission, while the 
 visible and ultra-violet emissions are wholly intercepted. 
 We are thus enabled to operate at will upon the purely 
 ultra-red waves. 
 
 In the heating of solid bodies to incandescence this 
 non-visual emission is the necessary basis of the visual. 
 A platinum wire is stretched in front of the table, and 
 through it an electric current flows. It is warmed by 
 the current, and may be felt to be warm by the hand. 
 
v. HEAT FILTERED FROM LIGHT. 175 
 
 It emits waves of heat, but no light. Augmenting the 
 strength of the current, the wire becomes hotter; it 
 finally glows with a sober red light. At this point Dr. 
 Draper many years ago began an interesting investiga- 
 tion. He employed a voltaic current to heat his 
 platinum, and he studied, by means of a prism, the suc- 
 cessive introduction of the colours of the spectrum. 
 His first colour, as here, was red ; then came orange, 
 then yellow, then gre,en, and lastly all the shades of 
 blue. Thus as the temperature of the platinum was 
 gradually augmented, the atoms were caused to vibrate 
 more rapidly ; shorter waves were thus introduced, until 
 finally waves were obtained corresponding to the entire 
 spectrum. As each successive colour was introduced, 
 the colours preceding it became more vivid. Now the 
 vividness or intensity of light, like that of sound, de- 
 pends not upon the length of the wave, but on the am- 
 plitude of the vibration. Hence, as the less refrangible 
 colours grew more intense as the more refrangible ones 
 were introduced, we are forced to conclude that side by 
 side with the introduction of the shorter waves we had 
 an augmentation of the amplitude of the longer ones. 
 
 These remarks apply not only to the visible emission 
 examined by Dr. Draper, but to the invisible emission 
 which precedes the appearance of any light. In the 
 emission from the white-hot platinum wire now before 
 you the very waves exist with which we started, only 
 their intensity has been increased a thousand-fold by 
 the augmentation of temperature necessary to the pro- 
 duction of this white light. Both effects are bound 
 together : in an incandescent solid, or in a molten 
 solid, you cannot have the shorter waves without this 
 intensification of the longer ones. A sun is possible 
 
176 ON LIGHT. LECT. 
 
 only on these conditions ; hence Sir William Herschel's 
 discovery of the invisible ultra-red solar emission. 
 
 The invisible heat, emitted both by dark bodies and 
 by luminous ones, flies through space with the velocity 
 of light, and is called radiant heat. Now, radiant heat 
 may be made a subtle and powerful explorer of mole- 
 cular condition, and, of late years, it has given a new 
 significance to the act of chemical combination. Take, 
 for example, the air we breathe. It is a mixture of 
 oxygen and nitrogen ; and it behaves towards radiant 
 heat like a vacuum, being incompetent to absorb it in 
 any sensible degree. But permit the same two gases 
 to unite chemically ; then, without any augmentation 
 of the quantity of matter, without altering the gaseous 
 condition, without interfering in any way with the 
 transparency of the gas, the act of chemical union is 
 accompanied by an enormous diminution of its diather- 
 mancy, or perviousness to radiant heat. 
 
 The researches which established this result also 
 proved the elementary gases, generally, to be highly 
 transparent to radiant heat. This, again, led to the 
 proof of the diathermancy of elementary liquids, like 
 bromine, and of solutions of the solid elements sulphur, 
 phosphorus, and iodine. A spectrum is now before 
 you, and you notice that the transparent bisulphide of 
 carbon has no effect upon the colours. Dropping into 
 the liquid a few flakes of iodine, you see the middle of 
 the spectrum cut away. By augmenting the quantity of 
 iodine, we invade the entire spectrum, and finally cut 
 it off altogether. Now, the iodine, which proves itself 
 thus hostile to the light, is perfectly transparent to the 
 ultra-red emission with which we have now to deal. 
 It, therefore, is to be our ray-filter. 
 
v. CALORESCENCE. 177 
 
 Placing the alum-cell again in front of the electric 
 lamp, we assure ourselves, as before, of the utter in- 
 ability of the concentrated light to fire white paper. 
 Introducing a cell containing the solution of iodine, the 
 light is entirely cut off; and then, on removing the 
 alum-cell, the white paper at the dark focus is instantly 
 set on fire. Black paper is more absorbent than white 
 for these rays ; and the consequence is, that with it the 
 suddenness and vigour of the combustion are augmented. 
 Zinc is burnt up at the same place, magnesium bursts 
 into vivid combustion, while a sheet of platinized 
 platinum placed at the focus is heated to whiteness. 
 
 Looked at through a prism, the white-hot platinum 
 yields all the colours of the spectrum. Before im- 
 pinging upon the platinum, the waves were of too slow 
 recurrence to awaken vision ; by the atoms of the 
 platinum, these long and sluggish waves are broken up 
 into shorter ones, being thus brought within the visual 
 range. At the other end of the spectrum, by the 
 interposition of suitable substances, Professor Stokes 
 lowered the refrangibility, so as to render the non- 
 visual rays visual, and to this change he gave the name 
 of Fluorescence. Here, by the intervention of the 
 platinum, the refrangibility is raised, so as to render 
 the non-visual visual, and to this change I have given 
 the name of Calorescence. 
 
 At the perfectly invisible focus where these effects 
 are produced, the air may be as cold as ice. Air, as 
 already stated, does not absorb the radiant heat, and is 
 therefore not warmed by it. Nothing could more 
 forcibly illustrate the isolation, if I may use the term, 
 of the luminiferous ether from the air. The wave- 
 motion of the one is heaped up, without sensible effect 
 
 N 
 
178 ON LIGHT. LECT. 
 
 upon the other. I may add that, with suitable pre- 
 cautions, the eye may be placed in a focus competent 
 to heat platinum to vivid redness, without experiencing 
 any damage, or the slightest sensation either of light 
 or heat. 
 
 The important part played by these ultra-red rays in 
 Nature may be thus illustrated : I remove the iodine 
 filter, and concentrate the total beam upon a test-tube 
 containing water. It immediately begins to sputter, 
 and in a minute or two it boils. What boils it ? 
 Placing the alum solution in front of the lamp, the 
 boiling instantly ceases. Now, the alum is pervious to 
 all the luminous rays ; hence it cannot be these rays 
 that caused the boiling. I now introduce the iodine, 
 and remove the alum ; vigorous ebullition immediately 
 recommences at the invisible focus. So that we here 
 fix upon the invisible ultra-red rays the heating of the 
 water. 
 
 We are thus enabled to understand the momentous 
 part played by these rays in Nature. It is to them 
 that we owe the warming and the consequent evapora- 
 tion of the tropical ocean ; it is to them, therefore, that 
 we owe our rains and snows. They are absorbed close 
 to the surface of the ocean, and warm the superficial 
 water, while the luminous rays plunge to great depths 
 without producing any sensible effect. But we can 
 proceed further than this. Here is a large flask con- 
 taining a freezing mixture, which has so chilled the 
 flask, that the aqueous vapour of the air has been con- 
 densed and frozen upon it to a white fur. Introducing 
 the alum-cell, and placing the coating of hoar-frost at 
 the intensely luminous focus, not a spicula of the daz- 
 zling frost is melted. Introducing the iodine-cell, and 
 
v. THE THERMO-PILE. 179 
 
 removing the alum, a broad space of the frozen coating 
 is instantly melted away. Hence we infer that the snow 
 and ice, which feed the Rhone, the Rhine, and other 
 rivers with glaciers for their sources, are released from 
 their imprisonment upon the mountains by the in- 
 visible ultra-red rays of the sun. 
 
 6. Identity of Light and Radiant Heat. Reflection 
 from Plane and Curved Surfaces. Total Reflec- 
 tion of Heat. 
 
 The growth of science is organic. That which to- 
 day is an end becomes to-morrow a means to a remoter 
 end. Every new discovery in science is immediately 
 made the basis of other discoveries, or of new methods 
 of investigation. Thus about fifty years ago, CErsted, 
 of Copenhagen, discovered the deflection of a magnetic 
 needle by an electric current ; and about the same 
 time Thomas Seebeck, of Berlin, discovered thermo- 
 electricity. These great discoveries were soon after- 
 wards turned to account, by Nobili and Melloni, in the 
 construction of an instrument which has vastly aug- 
 mented our knowledge of radiant heat. This instru- 
 ment, which is called a thermo-electric pile, con- 
 sists of thin bars of bismuth and antimony, soldered 
 alternately together at their ends, but separated from 
 each other elsewhere. From the ends of this ' thermo- 
 pile ' wires pass to a galvanometer, which consists of a 
 coil of covered wire, within and above which are sus- 
 pended two magnetic needles, joined to a rigid system, 
 and carefully defended from currents of air. 
 
 The action of the arrangement is this : the heat, falling 
 on the pile, produces an electric current ; the current, 
 
 N 2 
 
180 ON LIGHT. LECT. 
 
 passing through the coil, deflects the needles, and the 
 magnitude of the deflection may be made a measure of 
 the heat. The upper needle moves over a graduated dial 
 far too small to be directly seen. It is now, however, 
 strongly illuminated ; and above it is a lens which, if 
 permitted, would form an image of the needle and dial 
 upon the ceiling. There, however, it could not be con- 
 veniently viewed. The beam is therefore received upon 
 a looking-glass, placed at the proper angle, which throws 
 the image upon a screen. In this way the motions of 
 this small needle may be made visible to you all. 
 
 The delicacy of this apparatus is such that in a room 
 filled, as this room now is, with an audience physically 
 warm, it is exceedingly difficult to work with it. My 
 assistant stands several feet off. I turn the pile towards 
 him : the heat from his face, even at this distance, pro- 
 duces a deflection of 90. I turn the instrument 
 towards a distant wall, judged to be a little below the 
 average tempeiature of the room. The needle descends 
 and passes to the other side of zero, declaring by this 
 negative deflection that the pile feels the chill of the 
 wall. Possessed of this instrument, of our ray-filter, 
 and of our large Nicol prisms, we are in a condition to 
 investigate a subject of great philosophical interest ; 
 one which long engaged the attention of some of our 
 foremost scientific workers the substantial identity 
 of light and radiant heat. 
 
 That they are identical in all respects cannot of 
 course be the case, for if they were they would act 
 in the same manner upon all instruments, the eye 
 included. The identity meant is such as subsists 
 between one colour and another, causing them to 
 behave alike as regards reflection, refraction, double 
 
v. IDENTITY OF LIGHT AND HEAT. 181 
 
 refraction, and polarization. Let us here run rapidly 
 over the resemblances of light and heat. As regards 
 reflection from plane surfaces, we may employ a looking- 
 glass to reflect the light. Marking any point in the 
 track of the reflected beam, cutting off the light by 
 the dissolved iodine, and placing the pile at the marked 
 point, the needle immediately starts aside, showing that 
 the heat is reflected in the same direction as the light. 
 This is true for every position of the mirror. Resuming, 
 for example, the experiments made with the apparatus 
 
 employed in our first lecture (fig. 3, p. 11); moving 
 the index* attached to the mirror along the divisions 
 of our graduated arc (M 0), and determining by the 
 pile the positions of the invisible reflected beam, we 
 prove that the angular velocity of the heat-beam, like 
 that of the light-beam, is twice that of the mirror. 
 
 As regards reflection from curved surfaces, the 
 identity also holds good. Receiving the beam from 
 our electric lamp on a concave mirror (m m, fig. 51), 
 it is gathered up into a cone of reflected light ; mark- 
 
182 
 
 ON LIGHT. 
 
 LECT. 
 
 ing the apex of the cone by a pointer, and cutting off 
 the light by the iodine solution (T),a mom3nt's exposure 
 of the pile (P) at the marked point produces a violent 
 deflection of the needle. 
 
 The common and total reflection of a beam of radiant 
 heat may be simultaneously demonstrated. From the 
 nozzle of the lamp (L, fig. 52) a beam impinges upon 
 a plane mirror (M N), is reflected upwards, and enters 
 a right-angled prism, of which a b c is the section. 
 
 FIG. 52. 
 
 It meets the hypothennse at an obliquity greater 
 than the limiting angle, 1 and is therefore totally re- 
 flected. Quenching the light by the ray-filter at F, 
 and placing the pile at P, the totally-reflected heat- 
 beam is immediately felt by the pile, and declared by 
 the galvanometric deflection. 
 
 1 Defined in Leetvu*e I. 
 
v. INVISIBLE IMAGES, POLARIZATION OF HEAT. 183 
 
 7. Invisible Images formed by Radiant Heat. 
 
 Perhaps no experiment more conclusively proves the 
 substantial identity of light and radiant heat, than the 
 formation of invisible heat-images. Employing the 
 mirror already used to raise the beam to its highest 
 state of concentration, we obtain, as is well known, an 
 inverted image of the carbon points, formed by the 
 light rays at the focus. Cutting off the light by the 
 
 FIG. 53. 
 
 
 ray-filter, and placing at the focus a thin sheet of 
 platinized platinum, the invisible rays declare their 
 presence and distribution by stamping upon the plati- 
 num a white-hot image of the carbons, (See fig. 53.) 
 
 8. Polarization of Heat. 
 
 Whether radiant heat be capable of polarization or 
 not was for a long time a subject of discussion. Berard 
 had announced affirmative results, but Powell and 
 Lloyd failed to verify them. The doubts thus thrown 
 upon the question were removed by the experiments 
 
184 
 
 ON LIGHT. 
 
 of Forbes, who first established the polarization and 
 ' depolarization ' of heat. The subject was subsequently 
 followed up by Melloni, an investigator of consummate 
 ability, who sagaciously turned to account his own dis- 
 covery, that the obscure rays of luminous sources are 
 in part transmitted by black glass. Intercepting by a 
 plate of this glass the light from an oil flame, and 
 operating upon the transmitted invisible heat, he 
 obtained effects of polarization, far exceeding in mag- 
 nitude those which could be obtained with non -lumi- 
 nous sources. At present the possession of our more 
 perfect ray-filter, and more powerful source of heat, 
 
 FIG. 54. 
 
 enables us to pursue this identity question to its utmost 
 practical limits. 
 
 Mounting our two Nicols (B and C, fig. 54) in front 
 of the electric lamp, with their principal sections 
 crossed, no light reaches the screen. Placing our 
 thermo-electric pile (D) behind the prisms, with its 
 face turned towards the source, no deflection of the 
 galvanometer is observed. Interposing between the 
 
v. DOUBLE REFRACTION OF HEAT. 185 
 
 lamp (A) and the first prism (B) our ray-filter, the 
 light previously transmitted through the first Nicol 
 is quenched ; and now the slightest turning of either 
 Nicol opens a way for the transmission of the heat, 
 a very small rotation sufficing to send the needle up 
 to 90. When the Nicol is turned back to its first 
 position, the needle again sinks to zero, thus demon- 
 strating in the plainest manner the polarization of 
 the heat. 
 
 When the Nicols are crossed and the field is dark, 
 you have seen, in the case of light, the effect of introduc- 
 ing a plate of mica between the polarizer and analyzer. 
 In two positions the mica exerts no sensible influence ; 
 in all others it does. A precisely analogous deportment 
 is observed as regards radiant heat. Introducing our 
 ray-filter, the thermo-pile, playing the part of an eye as 
 regards the invisible radiation, receives no heat when 
 lS the eye receives no light ; but when the mica is so 
 turned as to make its planes of vibration oblique to 
 those of the polarizer and analyzer, the heat immedi- 
 ately passes through. So strong does the action be- 
 come, that the momentary plunging of the film of mica 
 into the dark space between the Nicols suffices to send 
 the needle up to 90. This is the effect to which the 
 term ' depolarization ' has been applied ; the experi- 
 ment really proving that with light and heat we have 
 the same resolution by the plate of mica, and recom- 
 pounding by the analyzer, of the ethereal vibrations. 
 
 Removing the mica and restoring the needle once 
 more to 0, I introduce between the Nicols a plate of 
 quartz cut perpendicular to the axis; the immediate 
 deflection of the needle declares the transmission of the 
 heat, and when the transmitted beam is properly 
 
186 
 
 ON LIGHT. 
 
 LECT. 
 
 examined, it is found to be circularly polarized, exactly 
 as a beam of light is polarized under the same con- 
 ditions. 
 
 9. Double Refraction of Heat. 
 
 I will now abandon the Nicols, and send through 
 the piece of Iceland spar (B, fig. 55), already employed 
 to illustrate the double refraction of light, our sifted 
 
 <* 
 
 FIG. 55. 
 
 
 beam of invisible heat. To determine the positions of 
 the two images, let us first operate upon the total 
 beam. Marking the places of the light-images, we 
 introduce between N and L our ray-filter (not in the 
 figure) and quench the light. Causing the pile to 
 approach one of the marked points, the needle remains 
 unmoved until the point has been attained ; here the 
 pile at once detects the heat. Pushing the pile across 
 
v. MAGNETIZATION OF HEAT. 187 
 
 the interval separating the two marks, the needle first 
 falls to 0, and then rises again to 90 in the second 
 position. This proves the double refraction of the 
 heat. 
 
 I now turn the Iceland spar : the needle remains 
 fixed : there is no alteration of the deflection. Pass- 
 ing the pile rapidly across to the other mark, the 
 deflection is maintained. Once more I turn the spar, 
 but now the needle falls to 0, rising, however, again 
 to 90 after a rotation of 360. We know that in the 
 case of light the extraordinary beam rotates round the 
 ordinary one ; and we have here been operating on the 
 extraordinary heat-beam, which, as regards double re- 
 fraction, behaves exactly like a beam of light. 
 
 
 I 10. Magnetization of Heat. 
 
 To render our series of comparisons complete, we must 
 demonstrate the magnetization of heat. But here a 
 slight modification of our arrangement will be necessary. 
 In repeating Faraday's experiment on the magnetiza- 
 tion of light, we had, in the first instance, our Nicols 
 crossed and the field rendered dark, a flash of light ap- 
 pearing upon the screen when the magnet was excited. 
 Now the quantity of light transmitted in this case is 
 really very small, its effect being rendered striking 
 through contrast with the preceding darkness. When 
 we so place the Nicols that their principal sections en- 
 close an angle of 45, the excitement of the magnet 
 causes a far greater positive augmentation of the light, 
 though the augmentation is not so well seen through 
 lack of contrast, because here, at starting, the field is 
 illuminated. 
 
188 ON LIGHT. LECT. 
 
 In trying to magnetize our beam of heat, we will 
 adopt this arrangement. Here, however, at the outset, 
 a considerable amount of heat falls upon one face of the 
 pile. This it is necessary to neutralize, by permitting 
 rays from another source to fall upon the opposite face 
 of the pile. The needle is thus brought to zero. Cut- 
 ting off the light by our ray-filter, and exciting the mag- 
 net, the needle is instantly deflected, proving that the 
 magnet has opened a door for the heat, exactly as in 
 Faraday's experiment it opened a door for the light. 
 Thus, in every case brought under our notice, the sub- 
 stantial identity of light and radiant heat has been 
 demonstrated. 
 
 By the refined experiments of Knoblauch, who worked 
 long and successfully at this question, the double refrac- 
 tion of heat, by Iceland spar, was first demonstrated ; 
 but though he employed the luminous heat of the sun, 
 the observed deflections were exceedingly small. So, 
 likewise, those eminent investigators De la Povostaye 
 and Desains succeeded in magnetizing a beam of heat ; 
 but though, in their case also, the luminous solar heat 
 was employed, the deflection obtained did not amount 
 to more than two or three degrees. With obscure 
 radiant heat the effect, prior to these experiments, had 
 not been obtained ; but, with the arrangement here de- 
 scribed, we obtain deflections from purely invisible heat, 
 equal to 150 of the lower degrees of the galvanometer. 
 
 11. Distribution of Heat in the Electric Spectrum. 
 
 We have finally to determine the position and mag- 
 nitude of the invisible radiation which produces these 
 results. For this purpose we employ a particular form 
 
v. DISTRIBUTION OF HEAT IN SPECTRUM. 189 
 
 of the thermo-pile. Its face is a rectangle, which by 
 movable side-pieces can be rendered as narrow as de- 
 sirable. Throwing a small and concentrated spectrum 
 upon a screen, by means of an endless screw we move 
 the rectangular pile through the entire spectrum, and 
 determine in succession the thermal power of all its 
 colours. 
 
 When this instrument is brought to the violet end 
 of the spectrum, the heat is found to be almost insen- 
 sible. As the pile gradually moves from the violet 
 towards the red, it encounters a gradually augmenting 
 heat. The red itself possesses the highest heating 
 power of all the colours of the spectrum. Pushing the 
 pile into the dark space beyond the red, the heat rises 
 suddenly in intensity, and at some distance beyond 
 the red it attains a maximum. From this point the 
 heat falls somewhat more rapidly than it rose, and after- 
 wards gradually fades away. 
 
 Drawing a horizontal line to represent the length of 
 the spectrum, and erecting along it, at various points, 
 perpendiculars proportional in length to the heat exist- 
 ing at those points, we obtain a curve which exhibits 
 the distribution of heat in our spectrum. It is repre- 
 sented in the adjacent figure. Beginning at the blue, 
 the curve rises, at first very gradually ; towards the red 
 it rises more rapidly, the line C D (fig 56, next page) 
 representing the strength of the extreme red radiation. 
 Beyond the red it shoots upwards in a steep and massive 
 peak to B ; whence it falls, rapidly for a time, and after- 
 wards gradually fades from the perception of the pile. 
 This figure is the result of more than twelve careful 
 series of measurements, for each of which the curve 
 was constructed. On superposing all these curves, a 
 
190 
 
 ON LIGHT. 
 
Y. VERIFICATION OF RESULT. 191 
 
 satisfactory- agreement was found to exist between them. 
 So that it may safely be concluded that the areas of the 
 dark and white spaces, respectively, represent the rela- 
 tive energies of the visible and invisible radiation. 
 The one is 7*7 times the other. 
 
 But in verification, as already stated, consists the 
 strength of science. Determining in the first place the 
 total emission from the electric lamp ; then by means 
 of the iodine filter determining the ultra-red emission ; 
 the difference between both gives the luminous emis- 
 sion. In this way, it is found that the energy of 
 the invisible emission is eight times that of the visible. 
 No two methods could be more opposed to eacli other, 
 and hardly any two results could better harmonize. I 
 think, therefore, you may rely upon the accuracy of the 
 distribution of heat here assigned to the prismatic 
 spectrum of the electric light. There is nothing vague 
 in the mode of investigation, or doubtful in its con- 
 clusions. 
 
 ,; 
 
 
 
 
192 ON LIGHT. 
 
 LECT. 
 
 LECTUEE VI. 
 
 PRINCIPLES OF SPECTRUM ANALYSIS PRISMATIC ANALYSIS OF THE LIGHT 
 
 OF INCANDESCENT VAPOURS DISCONTINUOUS SPECTRA SPECTRUM 
 
 BANDS PROVED BY BUNSEN AND KIRCHHOFF TO BE CHARACTERISTIC OF 
 
 THE VAPOUR DISCOVERY OF RUBIDIUM, CESIUM, AND THALLIUM 
 
 RELATION OF EMISSION TO ABSORPTION THE LINES OF FRAUNHOFER 
 
 THEIR EXPLANATION BY KIRCHHOFF SOLAR CHEMISTRY INVOLVED 
 
 IN THIS EXPLANATION FOUCAULT's EXPERIMENT PRINCIPLES OF 
 ABSORPTION ANALOGY OF SOUND AND LIGHT EXPERIMENTAL DE- 
 MONSTRATION OF THIS ANALOGY RECENT APPLICATIONS OF THE 
 SPECTROSCOPE SUMMARY AND CONCLUSION. 
 
 WE have employed as our source of light in these 
 lectures the ends of two rods of coke, rendered incan- 
 descent by electricity. Coke is particularly suitable for 
 this purpose, because it can bear intense heat without 
 fusion or vaporization. It is also black, which helps the 
 light ; for, other circumstances being equal, as shown 
 experimentally by Professor Balfour Stewart, the blacker 
 the body the brighter will be its light when incandes- 
 cent. Still, refractory as carbon is, if we closely ex- 
 amined our voltaic arc, or stream of light between the 
 carbon-points, we should find there incandescent carbon- 
 vapour. And if we could detach the light of this vapour . 
 from the more dazzling light of the solid points, we 
 should find its spectrum not only less brilliant, but of a 
 totally different character from the spectra that we have 
 already seen. Instead of being an unbroken succession 
 of colours from .red to violet, the carbon- vapour would 
 
vi. SPECTKA OF INCANDESCENT VAPOURS. 193 
 
 yield a few bands of colour with spaces of darkness be- 
 tween them. 
 
 What is true of the carbon is true in a still more 
 striking degree of the metals, the most refractory 
 of which can be fused, boiled, and reduced to vapour by 
 the electric current. From the incandescent vapour the 
 light, as a general rule, flashes in groups of rays of 
 definite degrees of refrangibility, spaces existing be- 
 tween group and group, which are unfilled by rays 
 of any kind. But the contemplation of the facts will 
 render this subject more intelligible than words cau 
 make it. Within the camera is now placed a cylinder 
 of carbon hollowed out at the top to receive a bit of 
 metal ; in the hollow is placed a fragment of the metal 
 thallium. Down upon this we bring the upper carbon 
 point, and then separate the one from the other. A 
 stream of incandescent thallium vapour passes between 
 them, the magnified image of which is now seen upon 
 the screen. It is of a beautiful green colour. What is 
 the meaning of that green ? We answer the question 
 by subjecting the light to prismatic analysis. Sent 
 through the prism, its spectrum is seen to consist of a 
 single refracted band. Light of one degree of refrangi- 
 bility, and that corresponding to this particular green, is 
 emitted by the thallium vapour. 
 
 We will now remove the thallium and put a bit of 
 silver in its place. The arc of silver is not to be dis- 
 tinguished from that of thallium ; it is not only green, 
 but the same shade of green. Are they then alike ? 
 Prismatic analysis enables us to answer the question. 
 However impossible it is to distinguish the one colour 
 from the other, it is equally impossible to confound the 
 spectrum, of incandescent silver vapour with that of 
 
 o 
 
194 ON LIGHT. LECT. 
 
 thallium. In the case of silver, we have two green bands 
 instead of one. 
 
 If we add to the silver in our camera a bit of thal- 
 lium, we shall obtain the light of both metals. After 
 waiting a little, we see that the green of the thallium lies 
 midway between the two greens of the silver. Hence 
 this similarity of colour. 
 
 But why have we to wait a little before we see 
 this effect ? The thallium band at first almost masks 
 the silver bands by its superior brightness. Indeed, 
 the silver bands have wonderfully degenerated since 
 the bit of thallium was put in, and for a reason worth 
 knowing. It is the resistance offered to the passage 
 of the electric current from carbon to carbon, that calls 
 forth the power of the current to produce heat. If the 
 resistance were materially lessened, the heat would be 
 materially lessened ; and if all resistance were abolished, 
 there would be no heat at all. Now, thallium is a much 
 more fusible and vaporizable metal than silver ; and its 
 vapour facilitates the passage of the current, to such a 
 degree, as to render it almost incompetent to vaporize the 
 more refractory silver. But the thallium is gradually con- 
 sumed ; its vapour diminishes, the resistance rises, until 
 finally you see the two silver bands as brilliant as they 
 were at first. 1 
 
 We have in these bands a perfectly unalterable 
 characteristic of the two metals. You never get other 
 bands than these two green ones from the silver, never 
 other than the 'single green band from the thallium, 
 never other than the three green bands from the 
 mixture of both metals. Every known metal has its 
 
 1 This circumstance ought not to be lost sight of in the examination 
 of compound spectra. Other similar instances might be cited. 
 
vi. SPECTRUM ANALYSIS. 195 
 
 own particular bands, and in no known case are the bands 
 of two different metals alike in refrangibility. It follows, 
 therefore, that these spectra may be made a sure test 
 for the presence or absence of any particular metal. 
 If we pass from the metals to their alloys, we find no 
 confusion. Copper gives green bands ; zinc gives blue 
 and red bands ; brass, an alloy of copper and zinc, gives 
 the bands of both metals, perfectly unaltered in position 
 or character. 
 
 But we are not confined to the metals themselves ; 
 the salts of these metals yield the bands of the metals. 
 Chemical union is ruptured by a sufficiently high heat ; 
 the vapour of the metal is set free, and it yields its cha- 
 racteristic bands. The chlorides of the metals are parti- 
 cularly suitable for experiments of this character. Com- 
 mon salt, for example, is a compound of chlorine and 
 sodium ; in the electric lamp it yields the spectrum of 
 the metal sodium. The chlorides of copper, lithium, 
 and strontium yield, in like manner, the bands of these 
 metals. 
 
 When, therefore, Bunsen and Kirchhoff, the cele- 
 brated founders of spectrum analysis, Sifter having estab- 
 lished by an exhaustive examination the spectra of all 
 known substances, discovered a spectrum containing bands 
 different from any known bands, they immediately in- 
 ferred the existence of a new metal. They were operat- 
 ing at the time upon a residue, obtained by evaporating 
 one of the mineral waters of Germany. In that water they 
 knew the unknown metal was concealed, but vast quanti- 
 ties of it had to be evaporated before a residue could be 
 obtained, sufficiently large to enable ordinary chemistry to 
 grapple with the metal. They, however, hunted it down, 
 and it now stands among chemical substances as the metal 
 
 o 2 
 
196 ON LIGHT. LECT. 
 
 Rubidium. They subsequently discovered a second 
 metal, which they called Ccesium. Thus, having first 
 placed spectrum analysis on a sure foundation, they de- 
 monstrated its capacity as an agent of discovery. Soon 
 afterwards Mr. Crookes, pursuing the same method, 
 discovered the bright green band of thallium, and ob- 
 tained the salts of the metal which yielded it. The 
 metal itself was first isolated in ingots by M. Lamy ? a 
 French chemist. 
 
 All this relates to chemical discovery upon earth, 
 where the materials are in our own hands. But it was 
 soon shown how spectrum analysis might be applied 
 to the investigation of the sun and stars ; and this 
 result was reached through the solution of a problem 
 which had been long an enigma to natural philosophers. 
 The scope and conquest of this problem we must now 
 endeavour to comprehend. A spectrum is pure in which 
 the colours do not overlap each other. We purify the 
 spectrum by making our slit narrow, and by augmenting 
 the number of our prisms. When a pure spectrum of 
 the sun has been obtained in this way, it is found to be 
 furrowed by innumerable dark lines. Four of them were 
 first seen by Dr. Wollaston, but they were afterwards 
 multiplied and measured by Fraunhofer with such 
 masterly skill, that they are now universally known as 
 Fraunhofer's lines. To give an explanation of these 
 lines was, as I have said, a problem which long chal- 
 lenged the attention of philosophers, and to KirchhofT, 
 Professor of Physics in the University of Heidelberg, 
 belongs the honour of having first conquered this 
 problem. 
 
 (The positions of the principal lines, lettered accord- 
 ing to Fraunhofer, are shown in the annexed sketch 
 
FRAUNHOFER'S LINES. 
 
 197 
 
 (fig. 57) of the solar spectrum. A is supposed to stand 
 near the extreme red, and J near the extreme violet.) 
 
 The brief memoir of two pages, in which this im- 
 mortal discovery is recorded, was communicated to 
 the Berlin Academy on October 27, 1859. FIG. 57. 
 Fraunhofer had remarked in the spectrum -> 
 of a candle flame two bright lines, which 
 coincide accurately, as to position, with the 
 double dark line D of the solar spectrum. 
 These bright lines are produced with par- x 
 ticular intensity by the yellow flame derived 
 from a mixture of salt and alcohol. They 
 are in fact the lines of sodium vapour. 
 KirchhofT produced a spectrum by permit- 
 ting the sunlight to enter his telescope by a 
 slit and prism, and in front of the slit he 
 placed the yellow sodium flame. As long 
 as the spectrum remained feeble, there always 
 appeared two bright lines, derived from the 
 flame, in the place of the two dark lines D 
 of the spectrum. In this case, such absorp- t 
 tion as the flame exerted upon the sunlight 
 was more than atoned for by the radiation 
 from the flame. When, however, the solar 
 spectrum was rendered sufficiently intense, 
 the bright bands vanished, and the two dark 
 Fraunhofer lines appeared with much greater c 
 sharpness and distinctness than when the 
 flame was not employed. 
 
 This result, be it noted, was not due to o 
 any real quenching of the bright lines of the D 
 flame, but to the augmentation of the in- 
 tensity of the adjacent spectrum. The experi- 
 
 I 
 
198 ON LIGHT. user. 
 
 ment proved to demonstration, that when the white light 
 sent through the flame was sufficiently intense, the 
 quantity which the flame absorbed was far in excess of 
 that which it radiated. 
 
 Here then, is a result of the utmost significance. 
 Kirchhoff immediately inferred from it that the salt 
 flame, which could intensify so remarkably the dark 
 lines of Fraunhofer, ought also to be able to produce 
 them. The spectrum of the Drummond light is known 
 to exhibit the two bright lines of sodium, which, however, 
 gradually disappear as the modicum of sodium, contained 
 as an impurity in the incandescent lime, is exhausted. 
 Kirchhoff formed a spectrum of the lime-light, and after 
 the two bright lines had vanished, he placed his salt 
 flame in front of the slit. The two dark lines D imme- 
 diately started forth. Thus, in the continuous spectrum 
 of the lime-light, he evoked, artificially, the lines D of 
 Fraunhofer. 
 
 Kirchhoff knew that this was an action not peculiar 
 to the sodium flame, and he immediately extended his 
 generalization to all coloured flames which yield sharply 
 defined bright bands in their spectra. White light, with 
 all its constituents complete, sent through such flames, 
 would, he inferred, have those precise constituents 
 absorbed, whose refrangibilities are the same as those 
 of the bright bands ; so that after passing through such 
 flames, the white light, if sufficiently intense, would 
 have its spectrum furrowed by bands of darkness. On 
 the occasion here referred to, Kirchhoff also succeeded 
 in reversing a bright band of lithium. 
 
 The long-standing difficulty of Fraunhofer's lines fell 
 to pieces in the presence of facts and reflections like 
 these, which also carried with them an immeasurable 
 
TI. SOLAR CHEMISTRY. 199 
 
 extension of the chemist's power. Kirchhoff saw that 
 from the agreement of the lines in the spectra of terres- 
 trial substances with Fraunhofer's lines, the presence 
 of these substances in the sun and fixed stars might 
 be immediately inferred. Thus, the dark lines D in 
 the solar spectrum proved the existence of sodium 
 in the solar atmosphere ; while the bright lines dis- 
 covered by Brewster in a nitre flame, which had 
 been proved to coincide exactly with certain dark lines 
 between A and B in the solar spectrum, proved the 
 existence of potassium in the sun. 
 
 All subsequent research verified the accuracy of these 
 first daring conclusions. In his second paper, commu- 
 nicated to the Berlin Academy before the close of 1859, 
 KirchhofF proved the existence of iron in the sun. 
 The bright lines of the spectrum of iron vapour are 
 exceedingly numerous, and 65 of them were subse- 
 quently proved by KirchhofF to be absolutely identical 
 
 o 
 
 in position with 65 dark Fraunhofer's lines. Angstrom 
 and Thalen pushed the coincidences to 450 for iron, 
 while, according to the same excellent investigators, the 
 following numbers express the coincidences, in the case 
 of the respective metals to which they are attached : 
 
 . 33 
 . 19 
 
 4 
 
 2 
 2 
 
 . 7 
 
 The probability is overwhelming that all these sub- 
 stances exist in the atmosphere of the sun. 
 
 KirchhofF's discovery profoundly modified the con- 
 ceptions previously entertained regarding the consti- 
 tution of the sun, leading him to views of that con- 
 
 Calcium . 
 
 75 
 
 Nickel . 
 
 Barium 
 
 11 
 
 Cobalt 
 
 Magnesium 
 Manganese 
 Titanium . 
 
 4 
 57 
 118 
 
 Hydrogen 
 Aluminium 
 Zinc 
 
 Chromium 
 
 18 
 
 Copper 
 
200 ON LIGHT. 
 
 LECT. 
 
 stitution which, though they may be modified in detail, 
 will, I believe, remain substantially valid to the end of 
 time. The sun consists of a nucleus which is surrounded 
 by a flaming atmosphere of lower temperature. That 
 nucleus may, in part, be clouds, mixed with, or under- 
 lying true vapour. The light of the nucleus would 
 give us a continuous spectrum, like that of the Drum- 
 mond light ; but having to pass through the photo- 
 sphere, as Kirchhoff's beam passed through the sodium 
 flame, those rays of the nucleus which the photosphere 
 can emit are absorbed, and shaded, lines, corresponding 
 to the rays absorbed, occur in the spectrum. Abolish 
 the solar nucleus, and we should have a spectrum 
 showing a bright line in the place of every dark line of 
 Fraunhofer, just as, in the case of KirchhorTs second 
 experiment, we should have the bright sodium lines of 
 the flame if the lime light were withdrawn. These 
 lines of Fraunhofer are therefore not absolutely dark, 
 but dark by an amount corresponding to the difference 
 between the light intercepted and the light emitted by 
 the photosphere. " 
 
 Almost every great scientific discovery is approached 
 contemporaneously by many minds, the fact that one 
 mind usually confers upon it the distinctness of demon- 
 stration being an illustration, not of genius isolated, but 
 of genius in advance. Thus Foucault, in 1849, came 
 to the verge of Kirchhoffs discovery. By converging 
 an image of the sun upon a voltaic arc, and thus ob- 
 taining the spectra of both sun and arc superposed, 
 he found that the two bright lines which, owing to the 
 presence of a little sodium in the carbons, or in the 
 air, are seen in the spectrum of the arc, coincide with 
 the dark lines D of the solar spectrum. The lines D he 
 
vi. PHYSICAL CAUSE OF ABSORPTION. 201 
 
 found to be considerably strengthened by the passage 
 of the solar light through the voltaic arc. 
 
 Instead of the image of the sun, Foucault then pro- 
 jected upon the arc the image of one of the solid in- 
 candescent carbon points, which of itself would give a 
 continuous spectrum ; and he found that the lines 
 D were thus generated in that spectrum. Foucault's 
 conclusion from this admirable experiment was ' that 
 the arc is a medium which emits the rays D on its 
 own account, and at the same time absorbs them 
 when they come from another quarter.' Here he 
 stopped. He did not extend his observations beyond 
 the voltaic arc ; he did not offer any explanation of the 
 lines of Fraunhofer ; he did not arrive at any conception 
 of solar chemistry, or of the constitution of the sun. 
 His beautiful experiment remained a germ without 
 fruit, until the discernment, ten years subsequently, of 
 the whole class of phenomena to which it belongs, 
 enabled Kirchhoff to solve these great problems. 
 
 Soon after the publication of KirchhofFs discovery, 
 Professor Stokes, who, ten years prior to the discovery, 
 had nearly anticipated it, borrowed an illustration from 
 sound, to show the reciprocity of radiation and absorp 
 tion. A stretched string responds to aerial vibra- 
 tions which synchronize with its own. A great number 
 of such strings stretched in space would roughly repre- 
 sent a medium ; and if the note common to them all 
 were sounded at a distance they would absorb the vibra- 
 tions corresponding to that note. That is to say, they 
 would absorb the vibrations which they can emit. 
 
 When a violin-bow is drawn across this tuning-fork, 
 the room is immediately filled with a musical sound, 
 which may be regarded as the radiation or emission 
 
202 ON LIGHT. LECT. 
 
 of sound from the fork. A few days ago, on sound- 
 ing this fork, I noticed that when its vibrations were 
 quenched, the sound seemed to be continued, though 
 more feebly. It appeared, moreover, to come from 
 under a distant table, where stood a number of tuning- 
 forks of different sizes and rates of vibration. One of 
 these, and one only, had been started by the sounding 
 fork, and it was the one whose rate of vibration was the 
 same as that of the fork which started it. This is an 
 instance of the absorption of the sound of one fork by 
 another. Placing two unisonant forks near each other, 
 sweeping the bow over one of them, and then quench- 
 ing the agitated fork, the other continues to sound; 
 this other can re-excite the former, and several transfers 
 of sound between the : two forks can be thus effected. 
 Placing a, cent-piece on each prong of one of the forks, 
 we destroy* its perfect synchronism with the other, and 
 no such communication of sound from the one to the 
 other is then possible. 
 
 I have now to bring before you, on a suitable scale, 
 the demonstration that we can do with light what has 
 been here done with sound. For several days in 1861 
 I endeavoured to accomplish this, with only partial 
 success. In iron dishes a mixture of dilute alcohol 
 and salt was placed, and warmed so as to promote 
 vaporization. The vapour was ignited, and through 
 the yellow flame thus produced the beam from the elec- 
 tric lamp was sent ; but a faint darkening only of the 
 yellow band of a projected spectrum could be obtained. 
 A trough was then made which, when fed with the salt 
 and alcohol, yielded a flame ten feet thick ; but the re- 
 sult of sending the light through this depth of flame 
 was still unsatisfactory. Eemembering that the direct 
 
vi. EXPERIMENTAL ILLUSTRATION. 203 
 
 combustion of sodium in a Bunsen's flame produces a 
 yellow far more intense than that of the salt flame, and 
 inferring that the intensity of the colour indicated the 
 copiousness of the incandescent vapour, I sent through 
 the flame from metallic sodium the beam of the 
 electric lamp. The success was complete; and this 
 experiment I wish now to repeat in your presence! 1 
 
 Firstly then you notice, when a fragment of sodium 
 is placed in a tin spoon and introduced into a Bun- 
 sen's flame, an intensely yellow light is produced. It 
 corresponds in refrangibility with the yellow band of 
 the spectrum. Like our tuning-fork, it emits waves 
 of a special period. When the white light from the 
 electric lamp is sent through that flame, you will have 
 ocular proof that the yellow flame intercepts the yellow 
 of the spectrum ; in other words, that it absorbs waves 
 of the same period as its own, thus producing, tof all in- 
 tents and purposes, a dark Fraunhofer's band in the place 
 of the yellow. 
 
 In front of the slit (at L, fig. 58) through which the 
 beam issues is placed a Bunsen's burner (6) protected by 
 a chimney (C). This beam, after passing through a lens, 
 traverses the prism (P) (in the real experiment there 
 was a pair of them), is there decomposed, and forms a 
 vivid continuous spectrum (S S) upon the screen. In- 
 troducing a tin spoon with its pellet of sodium into the 
 Bunsen's flame, the pellet first fuses, colours the flame 
 intensely yellow, and at length bursts into violent 
 combustion. At the same moment the spectrum is 
 
 1 The dark band produced when the sodium is placed within the 
 lamp was observed on the same occasion. Then was also observed for 
 the first time the magnificent blue band of lithium which the Bunsen's 
 flame fails to bring out. 
 
204 ON LIGHT. LECT. 
 
 furrowed by an intensely dark band (D), two incbes wide 
 and two feet long. Introducing and withdrawing tbe 
 sodium flame in rapid succession, the sudden appearance 
 and disappearance of the band of darkness is shown in 
 a most striking manner. In contrast with the adjacent 
 brightness this band appears absolutely black, so vigor- 
 ous is the absorption. The blackness, however, is but 
 relative, for upon the dark space falls a portion of the 
 light of the sodium flame. 
 
 FIG. 58. 
 
 I have already referred to the experiment of Fou- 
 cault ; but other workers also had been engaged on the 
 borders of this subject before it was taken up by Bunsen 
 and Kirchhoff. With a few modifications here intro- 
 duced, I have already used the following language re- 
 garding the precursors of the discovery of spectrum 
 analysis, and solar chemistry : ' Mr. Talbot had ob- 
 served the bright lines in the spectra of coloured flames, 
 and both he an4 Sir John Herschel pointed out the pos- 
 sibility of making prismatic analysis a chemical test of 
 exceeding delicacy, though not of entire certainty. 
 More than a quarter of a century ago Dr. Miller gave 
 
TI. KIKCHHOFF AND HIS PKECUESOKS. 205 
 
 drawings and descriptions of the spectra of various 
 coloured flames. Wheatstone, with his accustomed 
 acuteness, analyzed the light of the electric spark, and 
 proved that the metals between which the spark passed 
 determined the bright bands in its spectrum. In an 
 investigation described by Kirchhoff as ' classical,' Swan 
 had shown that ^^ of a grain of sodium in a Bunsen's 
 flame could be detected by its spectrum. He also 
 proved the constancy of the bright lines in the spectra 
 of hydro-carbon flames. Masson published a prize essay 
 on the bands of the induction spark ; while Van der 
 Willigen, and more recently Pliicker, have also given 
 us beautiful drawings of spectra obtained from the 
 same source. 
 
 ' But none of these distinguished men betrayed the 
 least knowledge of the connection between the bright 
 bands of the metals, and the dark lines of the solar 
 spectrum; nor could spectrum analysis be said to be 
 placed upon anything like a safe foundation prior to 
 the researches of Bunsen and Kirchhoff. The man 
 who, in a published paper, came nearest to the philo- 
 
 o 
 
 sophy of the subject was Angstrom. In that paper 
 translated by myself, and published in the "Philoso- 
 phical Magazine" for 1855, he indicates that the rays 
 which a body absorbs are precisely those which it can 
 emit, when rendered luminous. In another place, he 
 speaks of one of his spectra giving the general im- 
 pression of the reversal of the solar spectrum. But his 
 memoir, philosophical as it is, is distinctly marked by 
 the uncertainty of his time. Foucault, Thomson, and 
 Balfour Stewart have all been near the discovery, while, 
 as already stated, it was almost hit by the acute but 
 unpublished conjecture of Stokes.' 
 
206 ON LIGHT. 
 
 LECT. 
 
 Mentally, as well as physically, every year of the 
 world's age is the outgrowth and off spring of all preceding 
 years. Science proves itself to be a genuine product 
 of Nature by growing according to this law. We have 
 no solution of continuity here. All great discoveries 
 are duly prepared for in two ways : first, by other dis- 
 coveries which form their prelude ; and, secondly, by the 
 sharpening of the enquiring intellect. Thus Ptolemy 
 grew out of Hipparchus, Copernicus out of both, Kepler 
 out of all three, and' Newton out of all the four. New- 
 ton did not rise suddenly from the sea-level of the 
 intellect to his amazing elevation. At the time that 
 he appeared, the table-land of knowledge was already 
 high. He juts, it is true, above the table-land, as a 
 massive peak ; still he is supported by it, and a great 
 part of his absolute height is the height of humanity 
 in his time. It is thus with the discoveries of Kirch- 
 hoff. Much had been previously accomplished; this 
 he mastered, and then by the force of individual genius 
 went beyond it. He replaced uncertainty by certainty, 
 vagueness by definiteness, confusion by order; and I 
 do not think that Newton has a surer claim to the 
 discoveries that have made his name immortal, than 
 Kirchhoff has to the credit of gathering up the frag- 
 mentary knowledge of his time, of vastly extending it, 
 and of infusing into it the life of great principles. 
 
 With one additional point we will wind up our illus- 
 trations of the principles of solar chemistry. Owing 
 to the scattering of light by matter floating mechani- 
 cally in the earth's atmosphere, the sun is seen not 
 sharply defined, but surrounded by a luminous glare. 
 Now, a loud noise will drown a whisper, an intense 
 light will quench a feeble one, and so this circumsolar 
 
vi. ROSE-COLOURED SOLAR PROMINENCES. 207 
 
 glare prevents us from seeing many striking appearances 
 round the border of the sun. The glare is abolished in 
 total eclipses, when the moon comes between the earth 
 and the sun, and there are then seen a series of rose- 
 coloured protuberances, stretching sometimes tens of 
 thousands of miles beyond the dark edge of the moon. 
 They are described by Vassenius in the ' Philosophical 
 Transactions' for 1733; and were probably observed 
 even earlier than this. In 1842 they attracted 
 great attention, and were then compared to Alpine 
 snow-peaks reddened by the evening sun. That 
 these prominences are flaming gas, and principally 
 hydrogen gas, was first proved by M. Janssen during 
 an eclipse observed in India, on the 18th of August, 
 1868. 
 
 But the prominences may be rendered visible in full 
 sunshine ; and for a reason easily understood. You 
 have seen in these lectures a single prism employed 
 to produce a spectrum, and you have seen a pair of 
 prisms employed. In the latter case, the dispersed 
 white light, being diffused over about twice the area, 
 had all its colours proportionately diluted. You have 
 also seen one prism and a pair of prisms employed to 
 produce the bands of incandescent vapours; but here 
 the light of each band, being absolutely monochro- 
 matic, was incapable of further dispersion by the second 
 prism, and could not therefore be weakened by such 
 dispersion. 
 
 Apply these considerations to the circumsolar region. 
 The glare of white light round the sun can be dispersed 
 and weakened to any extent, by augmenting the number 
 of prisms ; while a monochromatic light, mixed with 
 this glare, and masked by it, would retain its intensity 
 
208 ON LIGHT. LECT. vi. 
 
 unenfeebled by dispersion. Upon this consideration 
 has been founded a method of observation, applied in- 
 dependently by M. Janssen in India and by Mr. Lockyer 
 in England, by which the monochromatic bands of the 
 prominences are caused to obtain the mastery, and to 
 appear in broad daylight. By searching carefully and 
 skilfully round the sun's rim, Mr. Lockyer has proved 
 these prominences to be mere local juttings from a 
 fiery envelope which entirely clasps the sun, and which 
 he has called the Chromosphere. 
 
 It would lead us far beyond the object of these lec- 
 tures to dwell upon the numerous interesting and impor- 
 tant results obtained by Secchi, Eespighi, Young, and 
 other distinguished men who have worked at the 
 chemistry of the sun and its appendages. Nor can 
 I do more at present than make a passing reference 
 to the excellent labours of Dr. Huggins in connexion 
 with the fixed stars, nebulae, and comets. They, more 
 than any others, illustrate the literal truth of the 
 statement, that the establishment of spectrum analysis, 
 and the explanation of Fraunhofer's lines, carried with 
 them an immeasurable extension of the chemist's 
 range. But my object here is to make principles plain, 
 rather than to follow out the details of their illustration. 
 This latter would be a task requiring only time for its 
 execution, but requiring more of it than I have now at 
 my command. 
 
209 
 
 SUMMARY AND CONCLUSION. 
 
 MY desire in these lectures has been to show you, with as 
 little breach of continuity as possible, something of the 
 past growth and present aspect of a department of science, 
 in which have laboured some of the greatest intellects 
 the world has ever seen. My friend Professor Henry, 
 in introducing me at Washington, spoke of me as an 
 apostle ; but the only apostolate that I intended to 
 fulfil was to place, in plain words, my subject before 
 you, and to permit its own intrinsic attractions to act 
 upon your minds. I have sought to confer upon 
 each experiment a distinct intellectual value, for 
 experiments ought to be the representatives and 
 expositors of thought a language addressed to the 
 eye as spoken words are to the ear. In association 
 with its context, nothing is more impressive or instruc- 
 tive than a fit experiment ; but, apart from its context, 
 it rather suits the conjuror's purpose of surprise, than 
 that purpose of education which ought to be the ruling 
 motive of the scientific man. 
 
 And now a brief summary of our work will not be 
 out of place. Our present mastery over the laws and 
 phenomena of light has its origin in the desire of man 
 to know. We have seen the ancients busy with this 
 problem, but, like a child who uses his arms aimlessly, 
 for want of the necessary muscular exercise, so these 
 early men speculated vaguely and confusedly regarding 
 
210 ON LIGHT. 
 
 natural phenomena, not having had the discipline needed 
 to give clearness to their insight, and firmness to their 
 grasp of principles. They assured themselves of the rec- 
 tilineal propagation of light, and that the angle of inci- 
 dence was equal to the angle of reflection. For more 
 than a thousand years I might say, indeed, for more 
 than fifteen hundred years subsequently the scientific 
 intellect appears as if smitten with paralysis, the fact 
 being that, during this time, the mental force, which 
 might have run in the direction of science, was diverted 
 into other directions. 
 
 The course of investigation, as regards light, was 
 resumed in 1100 by an Arabian philosopher named 
 Alhazan. Then it was taken up in succession by Roger 
 Bacon, Vitellio, and Kepler. These men, though fail- 
 ing to detect the principle which ruled the facts, kept 
 the fire of investigation constantly burning. Then 
 came the fundamental discovery of Snell, that corner- 
 stone of optics, as I have already called it, and imme- 
 diately afterwards we have the application by Descartes 
 of Snell's discovery to the explanation of the rainbow. 
 Following this we have the overthrow, by Raemer, of 
 the notion of Descartes, that light was transmitted 
 instantaneously through space. Then came Newton's 
 crowning experiments on the analysis and synthesis of 
 white light, by which it was proved to be compounded 
 of various kinds of light of different degrees of re- 
 frangibility. 
 
 Up to his demonstration of the composition of white 
 light, Newton had been everywhere triumphant tri- 
 umphant in the heavens, triumphant on the earth, and 
 his subsequent experimental work is, for the most 
 part, of immortal value. But infallibility is not the 
 
CONCLUSION. 211 
 
 property of man, and, soon after his discovery of the 
 nature of white light, Newton proved himself human. 
 He supposed that refraction and dispersion went hand in 
 hand, and that you could not abolish the one without 
 at the same time abolishing the other. Here Dollond 
 corrected him. 
 
 But Newton committed a graver error than this. 
 Science, as I sought to make clear to you in our 
 second lecture, is only in part a thing of the senses. 
 The roots of phenomena are embedded in a region be- 
 yond the reach of the senses, and less than the root 
 of the matter will never satisfy the scientific mind. 
 We find, accordingly, in this career of optics the great- 
 est minds constantly yearning to break the bounds of 
 the senses, and to trace phenomena to their subsensible 
 foundations. Thus impelled, they entered the region of 
 theory, and here Newton, though drawn from time to 
 time towards the truth, was drawn still more strongly 
 towards the error, and made it his substantial choice. 
 His experiments are imperishable, but his theory has 
 passed away. For a century it stood like a dam across 
 the course of discovery ; but, like all barriers that rest 
 upon authority, and not upon truth, the pressure from 
 behind increased, and eventually swept the barrier away. 
 This, as you know, was done mainly through the labours 
 of Thomas Young, and his illustrious French fellow- 
 worker Fresnel. 
 
 In 1808 Malus, looking through Iceland spar at 
 the sun reflected from the window of the Luxembourg 
 Palace in Paris, discovered the polarization of light 
 by reflection. In 1811 Arago discovered the splendid 
 chromatic phenomena which we have had illustrated 
 by the deportment of plates of gypsum in polarized 
 
 p 2 
 
2} 2 ON LIGHT. 
 
 light ; he also discovered the rotation of the plane of 
 polarization by quartz-crystals. In 1813 Seebeck dis- 
 covered the polarization of light by tourmaline. That 
 same year Brewster discovered those magnificent bands 
 of colour that surround the axes of biaxal crystals. 
 In 1814 Wollaston discovered the rings of Iceland spar. 
 All these effects, which, without a theoretic clue, would 
 leave the human mind in a jungle of phenomena with- 
 out harmony or relation, were organically connected by 
 the theory of undulation. 
 
 The theory was applied and verified in all direc- 
 tions, Airy being especially conspicuous for the severity 
 and conclusiveness of his proofs. The most remark- 
 able verification fell to the lot of the late Sir William 
 Hamilton, of Dublin, who, taking up the theory where 
 Fresnel had left it, arrived at the conclusion that at 
 four special points of the ' wave-surface ' in double- 
 refracting crystals, the ray was divided, not into two 
 parts, but into an infinite number of parts ; forming at 
 these points a continuous conical envelope instead of 
 two images. No human eye had ever seen this 
 envelope when Sir William Hamilton inferred its ex- 
 istence. He asked Dr. Lloyd to test experimentally 
 the truth of his theoretic conclusion. Lloyd, taking 
 a crystal of arragonite, and following with the most 
 scrupulous exactness the indications of theory, cutting 
 the crystal where theory said it ought to be cut, observ- 
 ing it where theory said it ought to be observed, dis- 
 covered the luminous envelope which had previously 
 been a mere idea in the mind of the mathematician. 
 
 Nevertheless this great theory of undulation, like 
 many another truth, which in the long run has proved 
 a blessing to humanity, had to establish, by hot con- 
 
CONCLUSION. 213 
 
 flict, its right to existence. Great names were arrayed 
 against it. It had been enunciated by Hooke, it had 
 been applied by Huyghens, it had been defended by 
 Euler. But they made no impression. And, indeed, 
 the theory in their hands was more an analogy than a 
 demonstration. It first took the form of a demon- 
 strated verity in the hands of Thomas Young. He 
 brought the waves of light to bear upon each other, 
 causing them to support each other, and to extinguish 
 each other at will. From their mutual actions he de- 
 termined their lengths, and applied his knowledge in 
 all directions. He finally showed that the difficulty of 
 polarization yielded to the grasp of theory. 
 
 After him came Fresnel, whose transcendent mathe- 
 matical abilities enabled him to give the theory a 
 generality unattained by Young. He seized it in 
 its entirety ; followed the ether into the hearts of 
 crystals of the most complicated structure, and into 
 bodies subjected to strain and pressure. He showed 
 that the facts discovered by Mains, Arago, Brewster, 
 and Biot were so many ganglia, so to speak, of his 
 theoretic organism, deriving from it sustenance and 
 explanation. With a mind too strong for the body 
 with which it was associated, that body became a wreck 
 long before it had become old, and Fresnel died, leav- 
 ing, however, behind him a name immortal in the 
 annals of science. 
 
 One word more I should like to say regarding 
 Fresnel. There are things better even than science. 
 Character is higher than Intellect, but it is especially 
 pleasant to those who wish to think well of human 
 nature when high intellect and upright character are 
 found combined. They were, I believe, combined in 
 
214 ON LIGHT. 
 
 this young Frenchman. In those hot conflicts of the 
 undulatory theory, he stood forth as a man of integ- 
 rity, claiming* no more than his right, and ready to 
 concede their rights to others. He at once recog- 
 nized and acknowledged the merits of Thomas Young. 
 Indeed, it was he, and his fellow-countryman Arago, 
 who first startled England into the consciousness of the 
 injustice done to Young in the Edinburgh Review. 
 
 I should like to read to you a brief extract from a 
 letter written by Fresnel to Young in 1824, as it 
 throws a pleasant light upon the character of the 
 French philosopher. 6 For a long time,' says Fresnel, 
 ' that sensibility, or that vanity, which people call love 
 of glory has been much blunted in me. I labour 
 much less to catch the suffrages of the public, than to 
 obtain that inward approval which has always been the 
 sweetest reward of my efforts. Without doubt, in 
 moments of disgust and discouragement, I have often 
 needed the spur of vanity to excite me .to pursue my 
 researches. But all the compliments I have received 
 from Arago, De la Place, and Biot never gave me so 
 much pleasure as the discovery of a theoretic truth, or 
 the confirmation of a calculation by experiment.' 
 
 This, then, is the core of the whole matter as regards 
 science. It must be cultivated for its own sake, for the 
 pure love of truth, rather than for the applause or profit 
 that it brings. And now my occupation in America is 
 well-nigh gone. Still I will bespeak your tolerance for 
 a few concluding remarks, in reference to the men who 
 have bequeathed to us the vast body of knowledge of 
 which I have sought to give you some faint idea in these 
 lectures. What was the motive that spurred them on ? 
 
CONCLUSION. 215 
 
 What urged them to those battles and those victories 
 over reticent Nature, which have become the heritage 
 of the human race ? It is never to be forgotten that 
 not one of those great investigators, from Aristotle down 
 to Stokes and Kirch hofF, had any practical end in view, 
 according to the ordinary definition of the word ' prac- 
 tical.' They did not propose to themselves money as 
 an end, and knowledge as a means of obtaining it. For 
 the most part, they nobly reversed this process, made 
 knowledge their end, and such money as they possessed 
 the means of obtaining it. 
 
 We see to-day the issues of their work in a thousand 
 practical forms, and this may be thought sufficient 
 to justify, if not ennoble their efforts. But they did 
 not work for such issues ; their reward was of a totally 
 different kind. In what way different ? We love 
 clothes, we love luxuries, we love fine equipages, we love 
 money, and any man who can point to these as the re- 
 sult of his efforts in life, justifies these results before all 
 the world. In America and England, more especially, he 
 is a "practical' man. But I would appeal confidently to 
 this assembly whether such things exhaust the demands 
 of human nature ? The very presence here for six 
 inclement nights of this great audience, embodying so 
 much of the mental force and refinement of this vast 
 city, 1 is an answer to my question. I need not tell such 
 an assembly that there are joys of the intellect as well as 
 joys of the body, or that these pleasures of the spirit 
 constituted the reward of our great investigators. Led 
 on by the whisperings of natural truth, through pain 
 
 1 New York: for more than a decade no such weather had been ex- 
 perienced. The snow was so deep that the ordinary means of locomo- 
 tion were for a time suspended. 
 
216 ON LIGHT. 
 
 and self-denial, they often pursued their work. With 
 the ruling passion strong in death, some of them, when 
 no longer able to hold a pen, dictated to their friends 
 the results of their labours, and then rested from them 
 for ever. 
 
 Could we have seen these men at work, without any 
 knowledge of the consequences of their work, what 
 should we have thought of them ? To the uninitiated, 
 in their day, they might often appear as big children 
 playing with soap-bubbles and other trifles. It is 
 so to this hour. Could you watch the true investi- 
 gator your Henry or your Draper, for example in 
 his laboratory, unless animated by his spirit, you could 
 hardly understand what keeps him there. Many of 
 the objects which rivet his attention might appear to 
 you utterly trivial; and, if you were to ask him 
 what is the use of his work, the chances are that 
 you would confound him. He might not be able 
 to express the use of it in intelligible terms. He 
 might not be able to assure you that it will put a 
 dollar into the pocket of any human being, living or to 
 come. That scientific discovery may put not only 
 dollars into the pockets of individuals, but millions into 
 the exchequers of nations, the history of science amply 
 proves ; but the hope of its doing so never was, and 
 it never can be, the motive power of the investigator. 
 
 I know that some risk is run in speaking thus before 
 practical men. I know what De Tocqueville says of 
 you. ' The man of the North,' he says, ' has not only 
 experience, but knowledge. He, however, does not 
 care for science as a pleasure, and only embraces it 
 with avidity when it leads to useful applications.' But 
 what, I would ask, are the hopes of useful applications 
 
CONCLUSION. 217 
 
 which have caused you so many times to fill this place, 
 in spite of snow-drifts and biting cold ? What, I may 
 ask, is the origin of that kindness which drew me from 
 my work in London to address you here, and which, if 
 I permitted it, would send me home a millionaire ? Not 
 because I had taught you to make a single cent by 
 science am I here to-night, but because I tried to the 
 best of my ability to present science to the world as an 
 intellectual good. Surely no two terms were ever so 
 distorted and misapplied with reference to man, in his 
 higher relations, as these terms useful and practical. 
 Let us expand our definitions until they embrace all 
 the needs of man, his highest intellectual needs inclu- 
 sive. It is specially on this ground of its adminis- 
 tering to the higher needs of the intellect ; it is mainly 
 because I believe it to be wholesome, not only as a 
 source of knowledge but as a means of discipline, that 
 I urge the claims of science upon your attention. 
 
 But with reference to material needs and joys, surely 
 pure science has also a word to say. People sometimes 
 speak as if steam had not been studied before James 
 Watt, or electricity before Wheatstone and Morse ; 
 whereas, in point of fact, Watt and Wheatstone and 
 Morse, with all their practicality, were the mere out- 
 come of antecedent forces, rvvhich acted without refer- 
 ence to practical ends. This also, I think, merits a 
 moment's attention. You are delighted, and with good 
 reason, with your electric telegraphs, proud of your 
 steam-engines and your factories,^ and charmed with 
 the productions of photography. You see daily, with 
 just elation, the creation of new forms of industry 
 new powers of adding to the wealth and comfort of 
 society. Industrial England is heaving with forces 
 
218 ON LIGHT. 
 
 tending to this end ; and the pulse of industry beats 
 still stronger in the United States. And yet, when 
 analyzed, what are industrial America and industrial 
 England ? 
 
 If you can tolerate freedom of speech on my part, 
 I will answer this question by an illustration. Strip 
 a strong arm, and regard the knotted muscles when the 
 hand is clenched and the arm bent. Is this exhibition 
 of energy the work of the muscle alone ? By no means. 
 The muscle is the channel of an influence, without which 
 it would be as powerless as a lump of plastic dough. 
 It is the delicate unseen nerve that unlocks the power 
 of the muscle. And without those filaments of genius, 
 which have been shot like nerves through the body of 
 society by the original discoverer, industrial America, 
 and industrial England, would be very much in the con- 
 dition of that plastic dough. 
 
 At the present time there is a cry in England for 
 technical education, and it is a cry in which the most 
 commonplace intellect can join, its necessity is so 
 obvious. But there is no cry for original investi- 
 gation. Still without this, as surely as the stream 
 dwindles when the spring dies, so surely will ' technical 
 education' lose all force of growth, all power of repro- 
 duction. Our great investigators have given us 
 sufficient work for a time ; but if their spirit die out, 
 we shall find ourselves eventually in the condition of 
 those Chinese mentioned by Be Tocqueville, who, 
 having forgotten the scientific origin of what they 
 did, were at length compelled to copy without varia- 
 tion the inventions of an ancestry wiser than them- 
 selves, who had drawn their inspiration direct from 
 Nature. 
 
CONCLUSION. 219 
 
 Both England and America have reason to bear those 
 things in mind, for the largeness and nearness of ma- 
 terial results are only too likely to cause both countries 
 to forget the small spiritual beginnings of such results, 
 in the mind of the scientific discoverer. You mul- 
 tiply, but he creates. And if you starve him, or "other- 
 wise kill him nay, if you fail to secure for him free 
 scope and encouragement you not only lose the mo- 
 tive power of intellectual progress, but infallibly sever 
 yourselves from the springs of industrial life. 
 
 What has been said of technical operations holds 
 equally good for education, for here also the original 
 investigator constitutes the fountain-head of know- 
 ledge. It belongs to the teacher to give this knowledge 
 the requisite form ; an honourable and often a difficult 
 task. But it is a task which receives its final sanctifica- 
 tion, when the teacher himself honestly tries to add a 
 rill to the great stream of scientific discovery. Indeed, 
 it may be doubted whether the real life of science can 
 be fully felt and communicated by the man who has not 
 himself been taught by direct communion with Nature. 
 We may, it is true, have good and instructive lectures 
 from men of ability, the whole of whose knowledge is 
 second-hand, just as we may have good and instructive 
 sermons from intellectually able and unregenerate men. 
 But for that power of science, which corresponds to 
 what the Puritan fathers would call experimental re- 
 ligion in the heart, you must ascend to the original 
 investigator. 
 
 To keep society as regards science in healthy play, 
 three classes of workers are necessary : Firstly, the in- 
 vestigator of natural truth, whose vocation it is to pur- 
 sue that truth, and extend the field of discovery for the 
 
220 ON LIGHT. 
 
 truth's own sake, and without reference to practical 
 ends. Secondly, the teacher of natural truth, whose 
 vocation it is to give public diffusion to the knowledge 
 already won by the discoverer. Thirdly, the applier of 
 natural truth, whose vocation it is to make scientific 
 knowledge available for the needs, comforts, and luxu- 
 ries of civilized life. These three classes ought to co- 
 exist and interact. Now, the popular notion of science, 
 both in this country and in England, often relates not 
 to science strictly so called, but to the applications of 
 science. Such applications, especially on this continent, 
 are so astounding they spread themselves so largely 
 and umbrageously before the public eye that they often 
 shut out from view those workers who are engaged in 
 the quieter and profounder business of original investi- 
 gation. 
 
 Take the electric telegraph as an example, which 
 has been repeatedly forced upon my attention of late. 
 I am not here to attenuate in the slightest degree the 
 services of those who, in England and America, have 
 given the telegraph a form so wonderfully fitted for 
 public use. They earned a great reward, and they 
 have received it. But I should be untrue to you and 
 to myself if I failed to tell you that, however high in 
 particular respects their claims and qualities may be, 
 your practical men did not discover the electric tele- 
 graph. The discovery of the electric telegraph implies 
 the discovery of electricity itself, and the develop- 
 ment of its laws and phenomena. Such discoveries 
 are not made by practical men, and they never will 
 be made by them, because their minds are beset by 
 ideas which, though of the highest value from one 
 
CONCLUSION. 221 
 
 point of view, are not those which stimulate the original 
 discoverer. 
 
 The ancients discovered the electricity of amber ; and 
 Gilbert, in the year 1600, extended the discovery to 
 other bodies. Then followed Boyle, Von Guericke, 
 Gray, Canton, Du Fay, Kleist, Cunaeus, and your 
 own Franklin. But their form of electricity, though 
 tried, did not come into use for telegraphic pur- 
 poses. Then appeared the great Italian Volta, who 
 discovered the source of electricity which bears his 
 name, and applied the most profound insight, and 
 the most delicate experimental skill, to its develop- 
 ment. Then arose the man who added to the powers 
 of his intellect all the graces of the human heart, 
 Michael Faraday, the discoverer of the great domain of 
 magneto-electricity. (Ersted discovered the deflection 
 of the magnetic needle, and Arago and Sturgeon the 
 magnetization of iron by the electric current. The 
 voltaic circuit finally found its theoretic Newton in 
 Ohm; while Henry, of Princeton, who had the sagacity 
 to recognise the merits of Ohm while they were still 
 decried in his own country, was at this time in the van 
 of experimental inquiry. 
 
 In the works of these men you have all the materials 
 employed at this hour, in all the forms of the electric 
 telegraph. Nay, more ; Gauss, the celebrated astrono- 
 mer, and Weber, the celebrated natural philosopher, 
 both professors in the University of Gottingen, wishing 
 to establish a rapid mode of communication between 
 the observatory and the physical cabinet of the uni- 
 versity, did this by means of an electric telegraph. 
 Thus, before your practical men appeared upon the 
 scene, the force had been discovered, its laws investi- 
 
222 ON LIGHT. 
 
 gated and made sure, the most complete mastery of its 
 phenomena had been attained nay, its applicability to 
 telegraphic purposes demonstrated by men whose sole 
 reward for their labours was the noble excitement of 
 research, and the joy attendant on the discovery of 
 natural truth. 
 
 Are we to ignore all this ? We do so at our peril. 
 For I say again 'that, behind all our practical appli- 
 cations, there is a region of intellectual action to which 
 practical men have rarely contributed, but from which 
 they draw all their supplies. Cut them off from this 
 region, and they become eventually helpless. In no case 
 is the adage truer, ' Other men laboured, but ye are 
 entered into their labours,' than in the case of the dis- 
 coverer and applier of natural truth. But now a word 
 on the other side. While practical men are not the men 
 to make the necessary antecedent discoveries, the cases 
 are rare,. though, in our day, not absent, in which the dis- 
 coverer knows how to turn his labours to practical ac- 
 count. Different qualities of mind and habits of thought 
 are usually needed in the two cases ; and while I wish 
 to give emphatic utterance to the claims of those whose 
 position, owing to the simple fact of their intellectual 
 elevation, is often misunderstood, I am not here to 
 exalt the one class of workers at the expense of the 
 other. They are the necessary complements of each 
 other. But remember that one class is sure to be taken 
 care of. All the material rewards of society are already 
 within their reach, while that same society habitually 
 ascribes to them intellectual achievements which were 
 never theirs. This cannot but act to the detriment of 
 those studies out of which, not only our knowledge 
 of nature, but our present industrial arts themselves 
 
CONCLUSION. 223 
 
 have sprung, and from which the rising genius of the 
 country is incessantly tempted away. 
 
 Pasteur, one of the most eminent members of the 
 Institute of France, in accounting for the disastrous 
 overthrow of his country and the predominance of 
 Grermany in the late war, expresses himself thus : ' Few 
 persons comprehend the real origin of the marvels of 
 industry and the wealth of nations. I need no further 
 proof of this than the employment more and more fre- 
 quent in official language, and in writing of all sorts, 
 of the erroneous expression applied science. The 
 abandonment of scientific careers by men capable of 
 pursuing them with distinction, was recently deplored 
 in the presence of a minister of the greatest talent. 
 The statesman endeavoured to show that we ought not 
 to be surprised at this result, because in our day the 
 reign of theoretic science yielded place to that of ap- 
 plied science. Nothing could be more erroneous than 
 this opinion, nothing, I venture to say, more dangerous, 
 even to practical life, than the consequences which might 
 flow from these words. They have rested in my mind 
 as a proof of the imperious necessity of reform in our 
 superior education. There exists no category of the 
 sciences, to which the name of applied science could 
 be rightly given. We have science, and the appli- 
 cations of science, which are united together as the 
 tree and its fruit.' 
 
 And Cuvier, the great comparative anatomist, writes 
 thus upon the same theme : ' These grand practical 
 innovations are the mere applications of truths of a 
 higher order, not sought with a practical intent, but 
 pursued for their own sake, and solely through an 
 ardour for knowledge. Those who applied them could 
 
-224 ON LIGHT. 
 
 not have discovered them ; those who discovered them 
 had no inclination to pursue them to a practical end. 
 Engaged in the high regions whither their thoughts 
 had carried them, they hardly perceived these practical 
 issues, though born of their own deeds. These rising 
 workshops, these peopled colonies, those ships which 
 furrow the seas this abundance, this luxury, this 
 tumult all this comes from discoverers in science, and 
 it all remains strange to them. At the point where 
 science merges into practice they abandon it ; it con- 
 cerns them no more.' 
 
 When the Pilgrim Fathers landed at Plymouth 
 Kock, and when Penn made his treaty with the Indians, 
 the new-comers had to build their houses, to chasten 
 the earth into cultivation, and to take care of their 
 souls. In such a community science, in its more ab- 
 stract forms, was not to be thought of. And at the 
 present hour, when your hardy Western pioneers stand 
 face to face with stubborn Nature, piercing the moun- 
 tains and subduing the forest and the prairie, the pur- 
 suit of science, for its own sake, is not to be expected. 
 The first need of man is food and shelter ; but a vast 
 portion of this continent is already raised far beyond 
 this need. The gentlemen of New York, Brooklyn, 
 Boston, Philadelphia, Baltimore, and Washington, have 
 already built their houses, and very beautiful they are : 
 they have also secured their dinners, to the excellence 
 of which I can also bear testimony. They have, in 
 fact, reached that precise condition of well-being and 
 independence when a culture, as high as humanity has 
 yet reached, may be justly demanded at their hands. 
 They have reached that maturity, as possessors of 
 wealth and leisure, when the investigator of natural 
 
CONCLUSION. 225 
 
 truth, for the truth's own sake, ought to find among 
 them promoters and protectors. 
 
 Among the many problems before them they have 
 this to solve, whether a republic is able to foster the 
 highest forms of genius. You are familiar with the 
 writings of De Tocqueville, and must be aware of 
 the intense sympathy which he felt for your institu- 
 tions ; and this sympathy is all the more valuable from 
 the philosophic candour with which he points out not only 
 your merits, but your defects and dangers. Now if I 
 come here to speak of science in America in a critical 
 and captious spirit, an invisible radiation from my 
 words and manner will enable you to find me out, and 
 will guide your treatment of me to-night. But if I in 
 no unfriendly spirit in a spirit, indeed, the reverse of 
 unfriendly venture to repeat before you what this 
 great historian and analyst of democratic institutions 
 said of America, I am persuaded that you will hear me 
 out. He wrote some three and twenty years ago, and, 
 perhaps, would not write the same to-day ; but it will 
 do nobody any harm to have his words repeated, and, if 
 necessaiy, laid to heart. 
 
 In a work published in 1850, De Tocqueville says: 'It 
 must be confessed that, among the civilized peoples of our 
 age, there are few in which the highest sciences have made 
 so little progress as in the United States.' l He declares 
 his conviction that, had you been alone in the universe, 
 you would soon have discovered that you carmot long 
 make progress in practical science, without cultivating 
 
 1 ' II faut reeounaltre que parmi les peuples civilises de nos jours il 
 en est peu chez qui les hautes sciences aient fait moins de progres qu'aux 
 Etats-Unis, ou qui aient fourni moins de grands artistes, de poetes illus- 
 tres et de celelres ecrirains.' (' De la Democratic en Amerique,' ttc., 
 tome ii. p. 36.) 
 
 Q 
 
226 ON LIGHT. 
 
 theoretic science at the same time. But, according to 
 De Tocqueville, you are not thus alone. He refuses to 
 separate America from its ancestraj. home ; and it is 
 there, he contends, that you collect the treasures of the 
 intellect, without taking the trouble to create them. 
 
 De Tocqueville evidently doubts the capacity of a 
 democracy to foster genius as it was fostered in the 
 ancient aristocracies. 'The future,' he says, 'will 
 prove whether the passion for profound knowledge, so 
 rare and so fruitful, can be born and developed so 
 readily in democratic societies as in aristocracies. As 
 for me,' he continues, G I can hardly believe it.' He 
 speaks of the unquiet feverishness of democratic com- 
 munities, not in times of great excitement, for such 
 times may give an extraordinary impetus to ideas, 
 but in times of peace. There is then, he says, ' a small 
 and uncomfortable agitation, a sort of incessant attri- 
 tion of man against man, which troubles and distracts 
 the mind without imparting to it either loftiness or 
 animation.' It rests with you to prove whether these 
 things are necessarily so whether scientific genius 
 cannot find, in the midst of you, a tranquil home. 
 
 I should be loth to gainsay so keen an observer and 
 so profound a political writer, but, since my arrival in 
 this country, I have been unable to see anything in 
 the constitution of society, to prevent a student, with 
 the root of the matter in him, from bestowing the most 
 steadfast devotion on pure science. If great scientific 
 results are not achieved in America, it is not to the small 
 agitations of society that I should be disposed to ascribe 
 the defect, but to the fact that the men among you who 
 possess the endowments necessary for profound scientific 
 inquiry, are laden with duties of administration, or 
 
CONCLUSION. . 227 
 
 tuition, so heavy as to be utterly incompatible with the 
 continuous and tranquil meditation which original inves- 
 tigation demands. It -may well be asked whether Henry 
 would have been transformed into an administrator, or 
 whether Draper would have forsaken science to write 
 history, if the original investigator had been honoured 
 as he ought to be in this land. I hardly think they would. 
 Still I do not imagine this state of things likely to last. 
 In America there is a willingness on the part of indi- 
 viduals to devote their fortunes, in the matter of educa- 
 tion, to the service of th^ commonwealth, which is pro- 
 bably without a parallel elsewhere : and this willingness 
 requires but wise direction to enable you effectually to 
 wipe away the reproach of De Tocqueville. 
 
 Your most difficult problem will be not to build 
 institutions, but to discover men. You may erect 
 laboratories and endow them ; you may furnish them 
 with all the appliances needed for enquiry ; in so do- 
 ing you are but creating opportunity for the exercise 
 of powers which come from sources entirely beyond your 
 reach. You cannot create genius by bidding for it. In 
 biblical language, it is the gift of Grod ; and the most 
 you could do, were your wealth, and your willingness to 
 apply it, a million-fold what they are, would be to 
 make sure that this glorious plant shall have the free- 
 dom, light, and warmth necessary for its development. 
 We see from time to time a noble tree dragged down 
 by parasitic runners. These the gardener can remove, 
 though the vital force of the tree itself may lie beyond 
 him : and so, in many a case, you men of wealth can 
 liberate genius from the hampering toils which the 
 struggle for existence often casts around it. 
 
 Drawn by your kindness, I have come here to give 
 Q 2 
 
228 ON LIGHT. 
 
 these lectures, and now that my visit to America has 
 become almost a thing of the past, I look back upon 
 it as a memory without a single stain. No lecturer was 
 ever rewarded as I have been. P'rom this vantage- 
 ground, however, let me remind you that the work of 
 the lecturer is not the highest work ; that, in science, 
 the lecturer is usually the distributor of intellectual 
 wealth amassed by better men. And though lecturing 
 and teaching, in moderation, will in general promote 
 their moral health, it is not solely, or even chiefly, as 
 lecturers, but as investigators, that your highest men 
 ought to be employed. You have scientific genius 
 amongst you not sown broadcast, believe me, it is sown 
 thus nowhere but still scattered here and there. Take 
 all unnecessary impediments out of its way. Keep your 
 sympathetic eye upon the originator of knowledge. 
 Give him the freedom necessary for his researches, not 
 overloading him, either with the duties of tuition or 
 of administration, not demanding from him so-called 
 practical results .-- above all things, avoiding that 
 question which ignorance so often addresses to genius, 
 ' What is the use of your work ? ' Let him make 
 truth his object, however unpractical for the time 
 being it may appear. If you cast your bread thus 
 upon the waters, then be assured it will return to 3 7 ou, 
 though it may be after many days. 
 
APPENDIX. 
 
 MY work in the United States was wound up by a social 
 meeting in New York, under the presidency of the Hon. W. 
 M. Evarts, a name as familiar to English as to American ears. 
 Of the able addresses delivered on that occasion, I here present 
 three, 1 which have a special bearing upon scientific and edu- 
 cational questions. The first by Dr. Barnard, the learned 
 President of Columbia College, New York ; the second by Dr. 
 Draper, so well and favourably known in England, both as a 
 historian and man of science; and the third by Dr. White, 
 President of Cornell University. To these I have ventured to 
 add a few remarks of my own, made upon the same occasion, 
 
 PRESIDENT BARNARD'S ADDRESS. 
 
 I am expected to deal, this evening, with a theme which, 
 under the actual circumstances, it is somewhat difficult to 
 handle. The degree to which our systems of education tend 
 to foster or discourage original investigation. into the truths of 
 Nature is a topic which might better befit an assembly more 
 gravely disposed than the present. Dulce est desipere in loc > 
 it is pleasant to put on the cap and bells when circumstances 
 favour, says Horace, and he says quite truly ; but he does not 
 say, difficile est sapere inter pocula it is hard to imitate the 
 solemnity of Minerva's bird when champagne is on the board, 
 
 as I think he ought to have said, and as he would, perhaps, 
 
 
 
 1 With certain personal references omitted. 
 
230 APPENDIX. 
 
 have said if prosody had allowed, and which would have been 
 equally true. I shall not aim at such an imitation. I do not 
 mean to be didactic if I can help it. If I am so, I trust you 
 will be indulgent. 
 
 I say, then, that our long- established and time-honoured 
 system of liberal education and when I speak of the system 
 I mean the who?e system, embracing not only the colleges, 
 but the tributary schools of lower grade as well^does not 
 tend to form original investigators of Nature's truths ; and 
 the reason that it does not is, that it inverts the natural 
 order of proceeding in the business of mental culture, and 
 fails to stimulate in season the powers of observation. And 
 when I say this, I must not be charged with treason to my 
 craft at least not wich treason spoken for the first time here, 
 for I have uttered the same sentiment more than once before 
 in the solemn assemblies of the craft itself. 
 
 I suppose, Mr. President, that at a very early period of 
 your life you may have devoted, like so many other juvenile 
 citizens, a portion of your otherwise unemployed time to 
 experiments in horticulture. In planting leguminous seeds 
 you could not have failed to observe that the young plants 
 come up with their cotyledons on their heads. If. in ponder- 
 ing this phenomenon, you arrived at the same conclusion that 
 I did, you must have believed that Nature had made a 
 mistake, and so have pulled up your plants and replanted 
 them upside-down. Men and women are but children of a 
 larger growth. They see the tender intellect shooting up in 
 like manner, with the perceptive faculties all alive at top ; 
 and they, too, seem to think that Nature has made a mistake, 
 and so they treat the mind as the child treats his bean-plant, 
 and turn it upside-down to make it grow better. They bury 
 the promising young buds deep in a musty mould formed of 
 the decay of centuries, under the dehision that out of such 
 debris they may gather some wholesome nourishment ; when 
 we know all that they want is the light and warmth of the 
 sun to stimulate them and the free air of heaven in which 
 
APPENDIX. 231 
 
 to unfold themselves. What heartless cruelty pursues the 
 little child-martyr every day and all the day long, at home or 
 at school alike ; in this place bidden to mind his book and not 
 to look out of the window in that, told to hold his tongue 
 and to remember that children must not ask questions ! A 
 lash from a whalebone switch upon the tender little fingers 
 too eagerly outstretched could not sting more keenly, or be 
 felt with a sharper sense of wrong, than such a rebuke coming 
 across the no less eagerly extended tentacles of the dawning 
 and inquiring intellect. 
 
 Now, a system of education founded on a principle like 
 this is not going to fit men to engage successfully in that haz- 
 ardous game of life, in which, in Prof. Huxley's beautiful 
 simile, we are all of us represented as playing with an unseen 
 antagonist, who enforces against us relentlessly every minutest 
 rule of the game, whether known to us or not. Still less can 
 it fit them to bring to light new rules of this difficult game, 
 never yet detected by any human intelligence. Yet it is pre- 
 cisely of this kind of men that the world has present need. 
 For grand as are the triumphs of scientific investigation 
 already achieved, it is impossible to doubt that there are still 
 grander yet behind to reward the zealous labourers of the time 
 to come. I know that it sometimes seems to us otherwise. I 
 know that the very grandeur of the achievements of the 
 past makes us sometimes doubtful of the future ; for it is 
 generally true that the portals of Nature's secret chambers, 
 yet unexplored, are only dimly discernible before they are 
 unlocked. 
 
 I remember a time it is now long gone by when this 
 sceptical feeling as to the possibilities of large scientific 
 progress in the time to come was extremely prevalent, so 
 prevalent that a learned professor of a neighbouring college 
 thought it worth his while to combat, in an energetic public 
 address, the discouraging notion that Nature has no longer 
 any important secrets to yield. Subsequently history has mag- 
 nificently corroborated his argument. For that was a time 
 
232 APPENDIX. 
 
 when, as yet, no Faradav had drawn a living spark from the 
 lifeless magnet ; no Daniel, or Grove, or Bunsen, had given 
 us an enduring source of electro-dynamic power; no Ohm 
 had taught us how to measure such a power when obtained ; 
 no Bessell had detected the parallaxes of the fixed stars ; no 
 Adams or Leverrier had thrown his grapple into space, and 
 felt the influence of an unseen planet trembling, to use the 
 beautiful language of Herschel, along the delicate line of his 
 analysis ; no Draper, or Daguerre, or Talbot, had revealed 
 the wonders of actinism ; no Mayer or Joule had laid a sure 
 foundation for the grand doctrine of the conservation of 
 force ; no Carpenter had unravelled the intricacies of nervous 
 physiology, or analyzed the relations of mind and brain ; no 
 Agassiz had ridden down the Alps on the backs of the gla- 
 ciers and proved their steady flow ; no Darwin had lifted the 
 veil from the mysteries of organic development ; no Schiapa- 
 relli or Newton had put the harness of universal gravitation 
 upon the wayward movements of the shooting-stars; no 
 Mallet had presented an intelligible theory of volcanic flames 
 and of the earth's convulsive tremors ; no Kirchhoff had fur- 
 nished a key to the intimate constitution of celestial bodies 
 or a gauge of stellar drift ; no Huggins, or Secchi, or Young, 
 had applied the key thus presented to enter the secret cham- 
 bers of the sun, the comets, the fixed stars, and the nebulas; 
 no Stokes had made the darkness visible which lies beyond 
 the violet. In short, that period of presumed scientific omni- 
 science seems now, as we look back to it, but the faint dawn- 
 ing of a day of glorious discovery, which we dare not, even 
 yet, pronounce to be approaching its meridian. 
 
 How much of all this has been due to our systems of edu- 
 cation ? Among the great promoters of scientific progress 
 before or since, how large is the number who may, in strict 
 propriety, be said to have educated themselves ? Take, for 
 illustration, such familiar names as those of William Herschel, 
 and Franklin, and Eumford, and Rittenhouse, and Davy, and 
 Faraday, and Henry. Is it not evident that Nature herself, 
 
APPENDIX. 233 
 
 to those who will follow her teachings, is a better guide to 
 the study of her own phenomena than all the training of our 
 schools? And is not this because Nature invariably begins 
 with the training of the observing faculties ? Is it not be- 
 cause the ample page which she spreads out before the 
 learner is written all over, not with words, but with substan- 
 tial realities ? Is it not because her lessons reach beyond the 
 simple understanding and impress the immediate intuition ? 
 That what she furnishes is something better than barren in- 
 formation passively received ; it is positive knowledge actively 
 gathered ? 
 
 If, then, in the future we would fit man properly to cul- 
 tivate Nature, and not leave scientific research, as. to a great 
 extent, we have done hei'etofore, to the hazard of chance, we 
 must cultivate her own processes. Our earliest teachings 
 must be things, and not words. The objects first presented 
 to the tender mind must be such as address the, senses, and 
 such as it can grasp. Store it first abundantly with the ma- 
 terial of thought, ami the process of thinking will be spon- 
 taneous and easy. 
 
 This is not to depreciate the value of other subjects or of 
 other modes of culture. It is only to refer them to their 
 proper place. Grammar, philology, logic, human history, 
 belles-lettres, philosophy all these things will be seized with 
 avidity and pursued with pleasure by a mind judiciously pre- 
 pared to receive them. On this point we shall do well to 
 learn, and I believe we are beginning to learn something, 
 from contemporary peoples upon the Continent of Europe. 
 Object- teaching is beginning to be introduced, if only spar- 
 ingly, into our primary schools. It should be so introduced 
 universally. And in all our schools, but especially in those 
 in which the foundation is laid of what is called a liberal 
 education, the knowledge of visible things should be made to 
 precede the study of the artificial structure of language, and 
 the intricacies of grammatical rules and forms. 
 
 The knowledge of visible things I repeat these words 
 
234 APPENDIX. 
 
 that I may emphasize them, and, when I repeat them, ob- 
 serve that I mean knowledge of visible things, and not infor- 
 mation about them knowledge acquired by the learner's 
 own conscious efforts, not crammed into his mind in set 
 forms of words out of books. Our methods of education 
 manifest a strong tendency in these modern times to degener- 
 ate in such a sort of cramming. Forty years ago, the printed 
 .helps to learning now supplied to the young men of our col- 
 leges in so lavish profusion were almost unknown ; and teach- 
 ers lent about as little aid, at least during the earlier years, as 
 books. What the student learned then he learned for him- 
 self by positive hard labour. Now we have made the task so 
 easy, we have built so many royal roads to learning in all its 
 departments, that it may be well doubted if the young men 
 of our day, with all their helps, acquire as much as those of 
 that earlier period acquired without them. 
 
 The moral of this experience is, that mental culture is not 
 secured by pouring information into passive recipients ; it 
 comes from stimulating the mind to gather knowledge for 
 itself. When our systems of education shall have been re- 
 modelled from top to bottom, with due attention to this 
 principle, then, if we have minds among us which are capable 
 of pursuing Nature into her yet uncaptured strongholds, we 
 .shall find them out and set them at their work. Then 
 neither 'mute, inglorious Miltons' on the one hand, nor on 
 the other silent, unsuspected Keplers, nor Newtons ' guilt- 
 less ' of universal gravitation, shall live unconscious of their 
 powers, or die and make no sign. Then the progress of 
 science will no longer be dependent, as in the past it has been 
 to so great a degree, on the chance struggles of genius rebel- 
 ling against circumstances, such as have given us a Herschel, 
 a Franklin, a Hugh Miller, or a Henry ; nor will the world 
 be any more astonished to see the most brilliant of the 
 triumphs of the intellect achieved by men who have cloven 
 their own way to the forefront, in defiance of all its educa- 
 tional traditions. 
 
APPENDIX. 235 
 
 PROFESSOR DRAPER'S ADDRESS. 
 
 When I was in London a year or two ago I passed some 
 pleasant hours with my friend Prof. Tyndall. Among these, 
 I think that, perhaps, the most pleasant were those of one 
 afternoon that we spent together in the laboratory of the 
 Royal Institution, where Davy discovered potassium and 
 sodium, and decomposed the. earths; where Young first 
 announced the grand and fertile principle of interference, and 
 placed on firm foundations the wave-theory of light ; where 
 Faraday made his great discoveries in electricity and magne- 
 tism. On that occasion Dr. Tyndall was showing me the 
 action of ether-waves of short period upon gaseous matter, 
 clouds formed by actinic decomposition. I saw the superb 
 sky-blue light and verified its polarized condition. It was 
 like the light of heaven. 
 
 Well, as I laid down the Nicol prism we had been using, 
 1 could not help thinking that there was an unseen Presence 
 in the place : a genius loci that inspired men to make such 
 discoveries. Who was it that brought that genius there ? 
 
 At the time of the American Revolution there resided in 
 the town of Rumford, N. H., one Benjamin Thompson, who 
 occupied himself in teaching a school. He embraced, as we 
 Americans would say, the wrong side of the question on that 
 occasion he sided with the king's Government. He went 
 to England, became a man of mark, and was knighted. Then 
 he went on the Continent, again distinguished himself by his 
 scientific attainments, again was titled, and this time, in 
 memory of his American home, was called Count Rumford. 
 
 On his return to London, Count Rumford founded the 
 Royel Institution, and thus to a native American the world 
 owes that establishment which has been glorified by Davy, 
 and Young, and Faraday. Had it not been for Rumford, 
 Davy might have spent his life in filling gas-bags for Dr. 
 Beddoes's patients, and Faraday might have been a book- 
 binder. 
 
236 APPENDIX. 
 
 But if Benjamin Thompson, an American, founded the 
 Royal Institution, James Smithson, an Englishman, shortly 
 afterwards, founded that noble Institution in Washington 
 which bears his name, and which, under the enlightened care 
 of Prof. Henry, has so greatly ministered to the advancement 
 and diffusion of science. You, sir, have called on me to 
 respond to your toast, 'English and American Science,' Mid 
 I think these facts show you how closely they have been 
 associated. 
 
 Now Prof. Tyndall is on the point of leaving us. When 
 he gets back to A.lbemarle Street, he will remember Broad- 
 way. I am sure that you will all join me in wishing him a 
 pleasant voyage over the Atlantic. But I wish him some- 
 thing better than that, I will add a safe return to America. 
 There is a great deal for him to do here yet. He may tell his 
 friends that he has been to America, but he must not tell 
 them that he has seen the Americans. We who are living 
 on the Atlantic verge of the continent are only modified 
 Europeans very slightly modified, indeed. One must go 
 beyond the Alleghanies yes, and over to the Pacific coast, 
 before he can say he has seen what the American really is. 
 I suppose that Dr. Tyndall has finished his glacier expe- 
 ditions to Switzerland. Is there nothing here that can 
 tempt him ? He and other members of the Alpine Club need 
 not go about the streets of London weeping like so many 
 broken-hearted Alexanders that there are no more worlds 
 to conquer. Let them take a look at the Rocky Mountains 
 and tell us what they think of them. Dr. Tyndall is a lover 
 of Nature. Well ! we can show him all kinds of scenery, 
 from where the half-frozen Mackenzie is lazily flowing through 
 a waste of snows on its way toward the Arctic Ocean, to where 
 oranges are growing on the Gulf. Or, if he is tired of inani- 
 mate Nature, and is in the mood of Dr. Johnson you know 
 the story. Boswell said to Johnson one day : ' See ! What a 
 beautiful afternoon ; let us take a walk in the green fields.' 
 * No, I won't,' replied the grim and gruff lexicographer. 
 
APPENDIX. 237 
 
 . c I've seen green fields ; one green field is like another green 
 field. They are all alike. No, sir 1 I'll walk down Cheap- 
 side. I like to look at men ; ' if Dr. Tyndall is in that mood, 
 can we not satisfy his curiosity ? Another friend of mine, 
 Mr. Froude, has set us all talking about Ireland. We can 
 show Dr. Tyndall how we take the Irish immigrant, in his 
 corduroy knee-breeches, his smashed-down hat, and his shil- 
 lalah in his fist, and in a generation or so turn him into an 
 ornament of professional life, make him a successful merchant, 
 or familiarize him with all the amenities of elegant society. 
 If that's not enough, we will show him how we take the 
 German, and, wonderful to be said, make him half forget 
 his fatherland and half his mother-tongue, and become an 
 English-speaking American citizen. If that's not enough, 
 we will show him how we have purged the African, the 
 woolly- headed black man, of the paganism of his forefathers, 
 and are now trying our hand at Darwinizing him into a re- 
 spectable voter. If that's not enough, we will show him 
 how, in the trans-Mississippi plains, we are improving the 
 red Indian alas, I fear my friend will say, improving him 
 off the face of the earth ! If that's not enough, we will show 
 him \vhere we have got tens of thousands of Chinese, with 
 picks and shovels, digging Pacific railways. We are mixing 
 European and Asiatic, red Indians and black Africans to- 
 gether, and I suppose certain English naturalists will tell us 
 that the up?hot of the thing will be a survival of the fittest. 
 In San Francisco we can show Dr. Tyndall the church, the 
 chapel, the joss-house, all in a row ; and perhaps, considering 
 his forlorn, celibate condition, he may be conscience-stricken 
 when we display before his astonished eyes the much-mar- 
 ried men of Mormondom. 
 
 Nowhere in the world are to be found more imposing 
 political problems than those to be settled here ; nowhere a, 
 greater need of scientific knowledge. I am not speaking of 
 ourselves alone, but also of oiir Canadian friends on the other 
 side of the St. Lawrence. We must join together in generous 
 
238 APPENDIX. 
 
 emulation of the best that is done in Europe. In her Ma- 
 jesty's representative, Lord Duiferin, they will find an eager 
 appreciation of all that they may do. Together we must try 
 to refute what De Tocqueville has said about us, that com- 
 munities such as ours can never have a love of pure science. 
 But, whatever may be the glory of our future intellectual 
 life, let us both never forget what we owe to England. Hers 
 is the language that we speak, hers are all our ideas of liberty 
 and law. To her literature as to a fountain of light we re- 
 pair. The torch of science that is shining here was kindled 
 at her midnight lamp. 
 
 PRESIDENT WHITE'S REMARKS. 
 
 There is a legend well known to most of us and which 
 has an advantage over most legends, in that it is substantially 
 true that a very distinguished man of science in this country 
 was once approached by an eminent practical man, and urged 
 to turn his great powers in scientific investigation and ex- 
 position to effect in making a fortune. 
 
 And to the great surprise of that man of business, the man 
 of science responded, ' but, my dear sir, / have no time to 
 waste in making money' 
 
 Of all the recent great results of science, I think, sir, that 
 those words have struck deepest and sped farthest in the 
 average carnal mind on our side the A^antic. 
 
 1 No time to waste in making money ! ' I have stood, sir, 
 in the presence of a very eminent man of affairs one whose 
 word is a power in the great marts of the world, and watched 
 him as he heard for the first time this astonishing dictum. 
 He stood silent apparently in awe. The words seemed to 
 reverberate among the convolutions of his brain, and to be 
 re-echoed far away, back, from depth to depth among the 
 deepest recesses of his consciousness ' No time to waste in 
 making money ! ' 
 
 The toast, sir, to which you ask me to speak is, ' The 
 Relation of Science to Political Progress.' 
 
APPENDIX. 239 
 
 Now, sir, I maintain that the true spirit of scientific re- 
 search, embracing as it does zeal in search for truth, devotion 
 to duty which such a search imposes, faith in good as the 
 normal and necessary result of such a search that such a 
 spirit is, at this moment, one of the most needed elements in 
 the political progress of our country. 
 
 I might go on to show how usefully certain scientific 
 methods might be brought to bear on the formation of politi- 
 cal judgments, and in determining courses of political action. 
 I might show how even a very moderate application of scien- 
 tific principles would save us from what is constantly going 
 on in municipal, State, or national legislation the basing of 
 important statutes, to-day, on the supposition that two and 
 two make four, and to-morrow on the theory that two and 
 two make forty ; but the hour is late, and I spare you ; I 
 will confine myself simply to the value, in our political pro- 
 gress, of the spirit and example of some of the scientific 
 workers of our day and generation. 
 
 What is the example which reveals that spirit ? It is an 
 example of zeal zeal in search for the truth, sought for truth's 
 sake and not for the sake of material advantage ; it is an 
 example of thoroughness of the truth sought in its whole- 
 ness, not in dilutions or adaptations, or suppressions, supposed 
 to be healthy for this man's mind, or that man's soul ; it is 
 an example of bravery the fearlessness that leads a truth- 
 seeker to brave all outcry and menace ; it is an example of 
 devotion to duty ; without which, for a steady force, as Prof. 
 Tyndall just now observed, no worthy scientific work can be 
 accomplished ; and, finally, an example of faith of a high 
 and holy faith that the results of earnest truth-seeking cannot 
 be other than good faith that truth and goodness are insep- 
 arable faith that there is a Power in the universe which 
 forbids any honest truth-seeking to lead to lasting evil. A 
 faith, this is, which has had its ' noble army of martyrs r 
 from long before Roger Bacon down to this present mar- 
 tyrs not less real than that devoted saint, from whom, as I 
 
240 APPENDIX. 
 
 understand, our guest takes his name, who perished in the 
 flames as a martyr to religious duty. 
 
 What I maintain, then, is, that this zeal for truth as truth, 
 this faith in the good as for ever allied to the true, this de- 
 votion to duty, as the result of such faith and zeal, consti- 
 tute probably the most needed element at this moment in the 
 political regeneration of this country, and that, therefore, the 
 example of our little army of true devotees of science has an 
 .exceeding preciousness. 
 
 Said a justly distinguished sen'ator to me yesterday, in 
 Washington : ' The true American idea of education is, to 
 give all children a good and even start ; then to hold up the 
 prizes of life before them ; then to say to them : " Go in and 
 win ; let the smartest have the prizes." ' 
 
 Who of the common herd shall dispute the conclusions 
 of a senator beneath the great cast-iron dome at Washing- 
 ton ? But here, in this presence, I may venture to say that 
 such a theory of education is one of the main causes of our 
 greatest national danger and disgrace. No theory can be 
 more false, or, in the long run, more fatal. Look at it for 
 a moment : 
 
 We are greatly stirred, at times, as this fraud or that 
 scoundrel is dragged to light, and there rise cries and moans 
 over the corruption of the times; but, my friends, these 
 frauds and these scoundrels are not the * corruption of the 
 times.' They are the mere pustules which the body politic 
 throws to the surface. Thank God, that there is vitality 
 enough left to throw them to the surface ! The disease is 
 below all this ; infinitely more wide-spread. 
 
 What is that disease ? I believe that it is, first of all, 
 indifference indifference to truth as truth ; next, scepticism, 
 by which I do not mean inability to believe this or that 
 dogma, but the scepticism which refuses to believe that there 
 is any power in the universe strong enough, large enough, good 
 enough, to make the thorough search for truth safe in every 
 line of investigation ; next, infidelity, by which I do not mean 
 
APPENDIX. 241 
 
 want of fidelity to this or that dominant creed, but want of 
 fidelity to that which underlies all creeds, the idea that the 
 true and the good are one ; and, finally, materialism, by which 
 I do not mean this or that scientific theory of the universe, 
 but that devotion to the mere husks and rinds of good, that 
 struggle for place and pelf, that faith in mere material com- 
 fort and wealth which eats out of human hearts all patriotism, 
 and which is the very opposite of the spirit that gives energy 
 to scientific achievement. 
 
 The education which our senatorial friend approved 
 leads naturally to just this array of curses. 
 
 On the other hand, I believe that the little army of scien- 
 tific men furnish a very precious germ from which better 
 ideas may spring. 
 
 And we should strengthen them. We have already mul- 
 titudes of foundations and appliances for the dilution of 
 truth -Tor the stunting of truth for the promotion of half- 
 truths for the development o this or that side of truth, 
 
 We have no end of intellectual hot-house arrangements 
 for the cultivation of the plausible rather than the true ; and 
 therefore it is that we ought to attach vast value to the men 
 who with calmness and determination seek THE TRUTH, in 
 its wholeness, on whatever line of investigation, not dilut- 
 ing it or masking it. 
 
 Their zeal, their devotion, their faith, furnish one of those 
 very protests which are most needed against that low tone 
 of political ideas which in its lower strata is political cor- 
 ruption. Their life gives that very example of a high spirit, 
 aim, and work, which the time so greatly needs. 
 
 The reverence for scientific achievement, the revelation 
 of the high honours which are in store for those who seek for 
 truth in science the inevitable comparison between a life 
 devoted to that great pure search, on the one hand, and a life 
 devoted to place-hunting or pelf-grasping on the other all 
 these shall come to the minds of thoughtful men in lonely 
 
 K 
 
242 APPENDIX. 
 
 garrets of our cities, in remote cabins on our prairies, and 
 thereby shall come strength and hope for higher endeavour. 
 
 And, Mr. Chairman, as this influence for good spreads 
 and strengthens, I have faith that gratitude will bring in 
 results for political good of yet another kind. 
 
 Many predecessors of our friend have, as literary men, 
 strengthened the ties that bind together the old land and 
 the new ; and I trust that love, admiration, and gratitude, 
 between men of science on both sides the Atlantic, may add 
 new cords and give strength to old cords which unite the 
 hearts of the two great English-speaking nations. 
 
 PROFESSOR TYNDALL'S REMARKS. 
 
 There is one point in your speech, Mr. President, which 
 requires simple honesty and little wit on my part to respond to. 
 That point is symbolized by those united flags of America and 
 England which I now see before me. You spoke of the 
 sympathy existing between the intellect of England an dt v at 
 of the United States, and of the smallness of our differences 
 compared with the area of our coincidences. Coming from you, 
 sir, these words had a peculiar weight and worth to me. 
 I am persuaded that they are not the words of mere con- 
 ventional compliment, but that they embody your con- 
 victions. And I am equally persuaded that they are the 
 expression of a fact which will become more and more prom- 
 inent as time rolls on, and as international knowledge is 
 increased. 
 
 During my four months' residence in the United States I 
 have not heard a single whisper hostile to England ; and this 
 accounts for a certain change of feeling on my part, accompanied 
 by a corresponding change o expression in my lectures. At a 
 time when the political relations of America and England 
 were critical in the extreme, I received from the United States 
 letters expressing the emphatic opinion that if men of science, 
 
APPENDIX. 243 
 
 on both sides of the Atlantic, could be persuaded to inter- 
 change friendly visits, it would, as far as the United States 
 were concerned, do more than diplomacy to soften the asperities 
 arising out of political differences. I said something bearing 
 upon this point in Boston ; but < f late nothing. And this, 
 because I saw that any reference to it would have been out of 
 place ; resembling, as Mr. Emerson would say, the sound of a 
 scythe in December, when there is nothing to mo\v. We are 
 not angelson either side of the Atlantic, nor am I aware that 
 we desire to be angels ; but as men I believe there exists 
 between us a strength of brotherhood competent to weld to- 
 gether the two kindred nations almost as closely as the various 
 parts of your own vast community are welded to each other. 
 
 And now let us turn for a moment to science. The 
 interest shown in the lectures to which you have so kindly 
 referred is not, in my opinion, the creation of the hour. 
 Every such display of public sympathy must have its prelude, 
 during which men's minds are prepared, a desire for knowledge 
 created, an intelligent curiosity aroused. Then in the nick of 
 time comes a person, who, though but an accident, touches a 
 spring which permits tendency to flow into fact, and public 
 feeling to pass from the potential to the actual. In no other 
 way can I account for my four months' experience in the 
 United States. The soil had been prepared, and the good seed 
 sown long before I came among you. And it is on the belief 
 that the subject has a root deeper than the curiosity of the 
 hour, tbat I found my hopes of its not passing rapidly from 
 the public mind. 
 
 It would be a great thing for this land of incalculable 
 destinies to supplement its achievements in . the industrial 
 arts by those higher investigations from which our mastery 
 over Nature and over industrial art itself has been derived, and 
 which, when applied in a true catholic spirit to man himself, 
 will assuredly render him permanently healthier and nobler 
 than he now is. To no other country is the cultivation of 
 science, in its highest forms, of more importance than to yours. 
 
 B 2 
 
244 APPENDIX. 
 
 In no other country would it exert a more benign and ele- 
 vating influence. What, then, is to be done toward so desir- 
 able a consummation ? Here, I think, you must take counsel 
 of your leading scientific men. As regards physical science, 
 I think, they are likely to assure you that it is not the 
 statical element of buildings that you require, so much as 
 the dynamical element of brains. Making use as far as 
 possible of existing institutions,* let chairs be founded, suffi- 
 ciently, but not luxuriously endowed, which shall have original 
 research for their main object and ambition. With such 
 vital centres among you, all your establishments of education 
 would feel their influence ; without such centres, even your 
 primary instruction will never flourish as it ought. I would 
 by no means sever tuition from investigation ; but, as in the 
 institution to which I have the honour to belong, the one 
 ought, in the cases now in view, to be made subservient to 
 the other. The Royal Institution gives lectures indeed, it 
 lives in part by lectures, though mainly by the contributions 
 of its members, and the bequests of its friends. But the chief 
 feature of its existence a feature never lost sight of by its 
 wise and honourable Board of Managers is that it should 
 be a school of research and discovery. Though a by-law gives 
 them the power to interfere, for the twenty years during which I 
 have been there, no manager or member of the Institution has 
 ever interfered wit'i my researches: It is this wise freedom, 
 accompanied by a never-failing sympathy, extended to the 
 great men who preceded me, that has given to the Royal Insti- 
 tution its imperishable renown. 
 
 As to the source of the funds necessary for founding 
 such chairs as those referred to, it is not for me to offer an 
 opinion. Without raising the disputed question of State aid, 
 it is possible in this country to do a great deal without it. 
 The willingness of American citizens to throw their fortunes 
 into the cause of public education is, as I have already stated, 
 without a parallel in my experience. Hitherto their efforts 
 
APPENDIX. 245 
 
 have been directed to the practical side of science ; and this is 
 why I sought in my lectures to show the dependence of practice 
 upon principles. On the ground, then, of mere practical, 
 material utility, pure science ought to be cultivated. But 
 assuredly among your men of wealth there are those willing 
 to listen to an appeal on higher grounds. Into this plea I 
 would pour all my strength. Not as a servant of Mammon do 
 I ask you to take science to your hearts, but as the strengthener 
 and enlightener of the mind of man. 
 
 Might I now address a word or two to those who in the 
 ardour of youth feel themselves drawn towards science as a 
 vocation. They must, I think, be prepared to suffer a little at 
 times for the sake of scientific righteousness, not refusing, 
 should occasion demand it, to live low and lie hard to achieve 
 the object of their lives. I do not here urge upon my younger 
 friends any thing that I should have been unwilling~to do 
 myself when young. A simple statement of my student-life 
 on the Continent would prove this to demonstration. And it 
 is with the view of giving others the chance that I then 
 enjoyed, among my noble and disinterested German teachers, 
 that I propose, after deducting, with strict accuracy, the sums 
 which have been actually expended on my lectures, to devote 
 every cent of the money which you have so generously poured 
 in upon me, to the education of young American philosophers 
 in Germany. I ought not, for their sake, to omit one additional 
 motive which upheld me during my student life a sense 
 of duty. Every young man of high aims must, I think, 
 have a spice of this principle within him. There are sure 
 to be hours in his life when his outlook will be dark, his 
 work difficult, and his intellectual future uncertain. Over 
 such periods, when the stimulus of success is absent, he 
 must be carried by his sense of duty. It may not be so 
 quick an incentive as glory, but it is a nobler one, and gives 
 a tone to character which the hope of glory cannot impart. 
 That unflinching devotion to work, without which no real 
 eminence in science is now attainable, implies the writing at 
 
246 APPENDIX. 
 
 certain times of the stern resolve upon the student's character : 
 4 1 work, not because I love to work, but because I ought to 
 work.' In science, however, love and duty are sure to be re- 
 conciled in the end. 
 
 And now, gentlemen, all is nearly over, and in a day or 
 two I quit these shores. I read a day or two ago an article 
 in the Galaxy, in which the writer, who had been in Eng- 
 land, and who had had what you call i a good time ' in 
 England, spoke nevertheless of the deep pleasure of reaching 
 his own home. The words struck a sympathetic chord with- 
 in me. And it is a curious psychical fact, that this home- 
 yearning, in my case, is not only unopposed, but is actually 
 aided by the feeling that since I came to this country Amer- 
 ica has been a home to me. It is not a case of two opposing 
 attractions, but a case in which, one of the attractions being 
 satisfied, I am left not only free to be acted on, but more 
 ready to be acted on by the other. Were there any linger- 
 ing doubt, as to my visit, at the bottom of my mind; did I 
 feel that I had blundered and w r ith the best and purest in- 
 tentions I might, through an error of judgment, have blun- 
 dered so as to cause you discontent, I should now be wish- 
 ing to abolish the doubt or to repair the blunder. This 
 would be so much withdrawn from the pleasurable thought 
 of home. But there is no drawback of this kind ; and, 
 therefore, as I have said, the fulness of my content here, en- 
 hances the prospective pleasure of meeting my older friends. 
 By some means or other the people of this country have be- 
 gotten and fostered a strange confidence in me towards them. 
 I feel as if I, a simple scientific student, who never taught 
 the world to be a cent richer, who merely sought to present 
 science to the world as an intellectual good, am leaving, not a 
 group of friends merely, not merely a friendly city, but a 
 iriendly continent behind me. The very disappointment of the 
 West I take as a measure of the West's goodwill. Tested 
 and true friends are aAvaiting me at the other side, and, think- 
 ing of them and you, the pure cold intellect is for the 
 
APPENDIX. 247 
 
 moment deposed, and the * human heart ' is master of the 
 situation. But lest it, in the waywardness of strong emotion, 
 should utter any thing which the re-enthroned intellect of 
 to-morrow might condemn, I will pause here hoping, not 
 for the entire consummation, for that would be a hope too 
 daring, but hoping, as the generations pass, that the attachment 
 which binds me to America, on the one side, and ' the Old 
 Country.' on the other, may be more and more approached and 
 realized by the nations themselves. 
 
 MEASUREMENT OF THE WAVES OF LIGHT. 
 
 THE diffraction fringes described in Lecture II., instead of 
 being formed on the retina, may be formed on a screen, or 
 upon ground glass, when they can be looked at through a 
 magnifying lens from behind, or they can be observed in the 
 air when the ground glass is removed. Instead of permitting 
 them to form on the. retina, we will suppose them formed on 
 a screen. This places us in a condition to understand, even 
 without trigonometry, the solution of the important problem 
 of measuring the length of a wave of light. 
 
 We will suppose the screen so distant that the rays falling 
 upon it from the two margins of the slit are sensibly parallel. 
 We have learned in Lecture II. that the first of the dark 
 bands corresponds to a difference of marginal path of one un- 
 dulation ; the second dark- band to a difference of path of two 
 undulations ; the third dark band to a difference of three 
 undulations, and so on. Now the angular distance of the 
 bands from the centre is capable of exact measurement ; this 
 distance depending, as already stated, on the width of the slit. 
 With a slit 1'35 l millimeters wide, Schwerd found the angular 
 distance of the first dark band from the centre of the field to 
 
 1 The millimeter is about th of an inch. 
 
248 APPENDIX. 
 
 be V 38" ; the angular distances of the second, third, fourth 
 dark band being twice, three times, four times this quantity. 
 Let A B, fig. 59, be the plate in which the slit is cut, and 
 C D the grossly exaggerated width of the slit, with the beam 
 of red light proceeding from it at the obliquity corresponding 
 to the first dark band. Let fall a perpendicular from one 
 
 FIG. 59. 
 
 A CD B 
 
 edge, D, of the slit on the marginal ray of the other edge at 
 d. The distance; G d, between the foot of this perpendicular 
 and the other edge is the length of a wave of the light. The 
 angle C D d, moreover, being equal to R C B/, is, in the case 
 now under consideration, V 38". From the centre D, with 
 the width D C as radius, describe a semicircle ; its radius 
 D C being 1*35 millimeters, the length of this semicircle is found 
 by an easy calculation to be 4*248 millimeters. The length 
 C d is so small that it sensibly coincides with the arc of the 
 circle. Hence the length of the semicircle is to 'the length 
 C d of the wave as 180 to 1' 38", or, reducing all to seconds, 
 as 648, 000" to 98". Thus, we have the proportion 
 
 648, 000 : 98 : : 4-248 to the wave-length C d. 
 
 Making the calculation, we find the wave-length for this 
 particular kind of light to be 0'000643 of a millimeter, or 
 0-000026 of an inch. 
 
J 'J/U> , ,. . 
 
 p 'v*,< siv ,: ' 
 
PLUMES PRODUCED BY" THE CRYSTALLIZATION OF WATER. 
 Ph.otograpi.ed by Professor Lockett. 
 
APPENDIX. 249 
 
 WA TER CR YSTA LLIZA TION. 
 
 THE following letter from my excellent friend Professor 
 Joseph Henry refers to a surprising case of crystallization 
 here figured, and for which I am indebted J ) the kindness of 
 Professor Loekett ; 
 
 1 Smithsonian Institution, "Washington : 
 
 'March 24, 1873. 
 
 ' MY DEAR PROFESSOR TYNDALL, Accompanying this I 
 send you a photograph, at the request of Professor S. H. 
 Loekett, of the Louisiana S "^ University, of which the 
 following is bis explanation : 
 
 ' " In my drawing; room I kept a wash-basin in which to 
 rinse out the colour from my water-colour brushes. This 
 colour gradually formed a uniform sediment of an indefinite 
 tint over the bottom of the basin. On the night of the 26th 
 of December last, which was an unusually cold one for this 
 climate, the water in the basin froze. On the melting of the 
 ice the next day, the beautiful figure you see on the pho- 
 tographs was left in the sediment. I carefully poured the 
 water from the basin, let the sediment dry. and thus perfectly- 
 preserved the figure. It has been accurately photographed 
 by an artist in this city. The negative is preserved, and if 
 you would like to have any more copies they can readily be 
 obtained. 
 
 ' " We are not much accustomed in this warm country of 
 ours to the beautiful ' forms of water,' and this has struck me 
 as a little remarkable, and worthy of being kept." 
 
 ' The fact that the results have been produced by coloured 
 sediment indicates a method of exhibiting the effects of crys- 
 tallization in an interesting manner. 
 
 ' JOSEPH HENRY, 
 ' Secretary, Smithsonian Institution.' 
 
250 APPENDIX. 
 
 ON THE SPECTRA OF POLARIZED LIGHT. 
 
 Mr. William Spottiswoode has recently introduced 
 to the members of the Royal Institution, in a very 
 striking form, a series of experiments on the spectra of 
 polarized light. With his large Nicol's prisms he first 
 repeated and explained the experiments of Foucault 
 and Fizeau, and subsequently enriched the subject by 
 very beautiful additions of his own. I here append a 
 portion of the abstract of his discourse : 
 
 ' It is well known that if a plate of selenite sufficiently thin 
 be placed between two such Nicol's prisms, or, more technically 
 speaking, between a polarizer and analyzer, colour will be 
 produced. And the question proposed is, What is the nature 
 of that colour? is it simply a pure colour of the spectrum, or 
 is it a compound, and if so, what are its component parts ? 
 The answer 'given by the wave theory is in brief this : In its 
 passage through the selenite plate the rays have been so 
 separated in the direction of their vibrations and in the 
 velocity of their transmission, that, when re-compounded by 
 means of the analyzer, they have in some instances neutralized 
 one another. If this be the case, the fact ought to be visible 
 when the beam emerging from the analyzer is dispersed by 
 the prism ; ibr then we have the rays of all the different 
 colours ranged side by side, and if any be wanting, their 
 absence will be shown by the appearance of a dark band in 
 their place in the spectrum. But not only so ; the spectrum 
 ought also to give an account of the other phenomena ex- 
 hibited by trie selenite whea the analyzer is turned round, 
 viz. that when the angle of turning amounts to 45, all trace 
 of colour disappears ; and also that when the angle amounts 
 to 90, colour reappears, not, however, the original colour, but 
 one complementary to it. 
 
APPENDIX. 251 
 
 ' You see in the spectrum of the reddish light produced by 
 the selenite a broad but dark band in the blue; when tl.e 
 analyzer is turned round the band becomes less and less dark , 
 until when the angle of turning amounts to 45 it has entirely 
 disappeared. At this stage each part of the spectrum has its 
 own proportional intensity, and the whole produces the 
 colourless irnap;e seen without the spectroscope. Lastly, as 
 the turning of the analyzer is continued, a dark band appears 
 in the red, the part of the spectrum complementary to that 
 occupied by the first band ; and the darkness is most complete 
 when the turning amounts to 90. Thus we have from the 
 spectroscope a complete account of what has taken place to 
 produce the original colour and its changes. 
 
 ' It is further well known that the colour produced by a 
 selenite, or other crystal plate, is dependent upon the thickness 
 of the plate. And, in fact, if a series of plates be taken, 
 giving different colours, t beir spectra are found to show bands 
 arranged in different positions. The thinner plates show bands 
 in the parts of the spectrum nearest to the violet, where the 
 waves are shorter, and consequently give rise to redder 
 colours ; while the thicker show bands nearer to the red, 
 where the waves are longer and consequently supply bluer 
 tints. 
 
 ' When the thickness of th,e plate is continually increased, 
 so that the colour produced has gone through the complete 
 cycle of the spectrum,, a further increase of thickness causes a 
 reproduction of the colours in. the same order ; but it will be 
 noticed that at each recurrence of the cycle the tints become 
 paler, until when a number of cycles have been performed, 
 and the thickness of the plate is considerable, all trace % of 
 colour is lost. Let us now take a series of plates, the first two, 
 of which, as you see, give colours ; with the others which are 
 successively of greater thickness the tints are so feeble that 
 they can scarcely be distinguished. The spectrum of the first 
 shows a single band ; that of the second, two; showing that 
 the second series of tints is not identical with the first, but 
 
252 APPENDIX. 
 
 that it is produced by the extinction of two colours from the 
 components of white light. The spectra of the others show- 
 series of bands more and more numerous in proportion to the 
 thickness of the plate, an array which may be increased inde- 
 finitely. The total light, then, of which the spectrum is 
 deprived by the thicker plates is taken from a greater number 
 of its parts ; or, in other words, the light which still remains 
 is distributed more and more evenly over the spectrum ; and 
 in the same proportion the sum total of it approaches more 
 and more nearly to white light. 
 
 ' These experiments were made more than thirty years ago 
 by the French philosophers^ MM. Foucault and Fizeau. 
 
 ' If instead of selenite, Iceland spar, or other ordinary 
 crystals, we use plates of quartz cut perpendicularly to the 
 axis, and turn the analyzer round as before, the light, instead 
 of exhibiting only one colour and its complementary with an 
 intermediate stage in which colour is absent, changes con- 
 tinuously in tint ; and the order of the colour depends partly 
 upon the direction in which the analyzer is turned, and partly 
 upon the character of the crystal,, i.e. whether it is right- 
 handed or left-handed. If we examine the spectrum in this 
 case we find that the dark band never disappears, but marches 
 from one end of the spectrum to another, or vice versa, pre- 
 cisely in such a direction as to give rise to the tints seen by 
 direct projection. 
 
 ' The kind of polarization effected by the quartz plates is 
 called circular, while that effected by the other class of crystals 
 is called plane, on account of the form of the vibrations exe- 
 cuted by the molecules of aether ; and this leads us to ex- 
 amine a little more closely the nature of the polarization of 
 different parts of these spectra of polarized light. 
 
 i Now, two things are clear : first, that if the light be plane- 
 polarized, that is, if all the vibrations throughout the entire 
 ray are rectilinear and in one plane, they must in all their bear- 
 ings have reference to a particular direction in space, so that 
 they will be differently affected by different positions of the 
 
APPENDIX. 253 
 
 analyzer. Secondly, that if the vibrations be circular, they 
 will be affected in precisely the same way (whatever that may 
 be) in all positions of the analyzer. This statement merely 
 recapitulates a fundamental point in polarization. In fact, 
 plane-polarized light is alternately transmitted and extinguished 
 by the analyzer as it is turned through 90 ; while circularly- 
 polarized light [if we could get a single ray] remains to all ap- 
 pearance unchanged. And if we examine carefully the spectrum 
 of light which has passed through a selenite,or other ordinary 
 crystal, we shall find that, commencing with two consecutive 
 bands in position, the parts occupied by the bands and those 
 midAvay between them are plane polarized, for they become 
 alternately dark and bright ; while the intermediate parts, i.e. 
 the parts at one-fourth of the distance from one band to the 
 next, remain permanently bright. These are, in fact, circu- 
 larly polarized. But it would be incorrect to conclude from 
 this experiment alone that such is really the case, because the 
 same appearance would be seen if those parts were unpolarized, 
 i.e. in the condition of ordinary lights. And on such a sup- 
 position we should conclude with equal justice that the parts 
 on either side of the parts last mentioned (e.g. the parts sepa- 
 rated by eighth parts of the interval between two bands) were 
 partially polarized. But there is an instrument of very simple 
 construction, called a " quarter-undulation plate," a plate 
 usually of mica, whose thickness is an odd multiple of a 
 quarter of a wave length, which enables us to discriminate 
 between light unpolarized and circularly polarized. The exact 
 mechanical effect produced upon the ray could hardly be ex- 
 plained in detail within our present limits of time ; but suffice 
 it for the present to say that when placed in a proper position, 
 the plate transforms plane into circular and circular into plane 
 polarization. That being so, the parts which were originally 
 banded ought to remain bright, and those which originally 
 remained bright ought to become banded during the rotation 
 of the analyzer. The general effect to the eye will conse- 
 
254 APPENDIX. 
 
 quently be a general shifting of the bands through one-fourth 
 of the space which separates each pair. 
 
 1 Circular polarization, like circular motion generally, may 
 of course be of two kinds, which differ only in the direction 
 of the motion. And, in fact, to convert the circular polariza- 
 tion produced by this plate from one of these kinds to the ether 
 (say from right-handed to left-handed, or vice versa), we 
 have only to turn the plate round through 90. Conversely 
 right-handed circular polarization will be changed by the 
 plate into plane polarization in one direction, while left- 
 handed will be changed into plane at right angles to the first. 
 Hence, if the plate be turned round through 90 }ve shall see 
 that the bands are shifted in a direction opposite to that in 
 which they were moved at first. In this therefore we have 
 evidence not only that the polarization immediately on either 
 side of a band is circular; but also that that immediately on 
 the one side is right-handed, while that immediately on the 
 other is left-handed. [ ! ] 
 
 ' If time permitted, I might enter still further into datail, 
 and show that the polarization between the p]ane and the 
 circular is elliptical, and even the positions of the longer and 
 shorter axes and the direction of motion in each case. But 
 sufficient has, perhaps, been said for our present purpose. 
 
 ' Before proceeding to the more varied forms of spectral 
 bands, which I hope presently to bring under your notice, I 
 should like to ask your attention for a few minutes to the 
 peculiar phenomena exhibited w r hen two plates of selenite 
 giving complementary colours are used. The appearance of 
 the spectrum varies with the relative position of the plates. 
 If they are similarly placed that is, as if they were one plate 
 of crystal they will behave as a single plate, whose thickness 
 is the sum of the thicknesses of each, and will produce double 
 
 [ l At these points the two rectangular vibrations into which the 
 original polarized ray is resolved by the plates of gypsum act upon each 
 other like the two rectangular impulses .imparted to our pendulum in 
 Lecture IV., one being given when the pendulum* is at the limit of its 
 swing. Vibration is thus converted into rotation.] 
 
APPENDIX. 255 
 
 the number of bands which one alone would give ; and when 
 the analyzer is turned, the bands will disappear and re-appear 
 in their complementary positions, as usual in the case of plane- 
 polarization. If one of them be turned round through 45, a 
 single band will be seen at a particular position in the spec- 
 trum. This breaks into two, which recede from one another 
 towards the red and violet ends respectively, or advance 
 towards one another according to the direction in which the 
 analyzer is turned. If the plate be turned through 45 in 
 the opposite direction, the effects will be reversed. The 
 darkness of the bands is, however, not equally complete 
 during their whole passage. Lastly, if one of the plates be 
 turned through 90, no bands will be seen, and the spectrum 
 will be alternately bright and dark, as if no plates were used, 
 except only that the polarization is itself turned through 90. 
 
 'If a wedge-shaped crystal be used, the bands, instead of 
 being straight, will cross the spectrum diagonally, the direc- 
 tion of the diagonal (dexter or sinister) being determined by 
 the position of the thicker end of the wedge. If two similar 
 wedges be used with their thickest ends together, they will 
 act as a wedge whose angle and whose thickness is double of 
 the first. If they be placed in the reverse position they will act 
 as a flat plate, and the bands will again cross the spectrum in 
 straight lines at right angles to its length. 
 
 ' If a concave plate be used the bands will dispose them- 
 selves in a fanlike arrangement, their divergence depending 
 upon the distance of the slit from the centre of concavity. 
 
 ' If two quartz wedges, one of which has the optic axis 
 parallel to the edge of the refractory angle, and the other 
 perpendicular to it, but in one of the planes containing the 
 angle (Babinet's Compensator), the appearances of the bands 
 are very various. 
 
 ' The diagonal bands, beside sometimes doubling themselves 
 as with ordinary wedges, sometimes combine so as to form 
 longitudinal (instead of transverse) bands ; and sometimes 
 cross one another so as to form a diaper ra'tern with bright 
 
256 APPENDIX. 
 
 compartments in a dark framework, and vice versa, according 
 to the position of the plates. 
 
 1 The effects of different dispositions of the interposed crys- 
 tals might be varied indefinitely ; but enough has perhaps 
 been said to show the delicacy of the method of spectrum 
 analysis as applied to the examination of polarized light.' 
 
 The singular and beautiful effect obtained with a 
 circular plate of selenite, thin at the centre, and 
 gradually thickening towards the circumference, is 
 easily connected with a similar effect obtained with 
 Newton's rings. Let a thin slice of light fall upon the 
 glasses which show the rings, so as to cover a narrow 
 central vertical zone passing through them all. The 
 image of this zone upon the screen is crossed by por- 
 tions of the iris rings. Subjecting the reflected beam 
 to prismatic analysis, the resultant spectrum may be 
 regarded as an indefinite number of images of the zone 
 placed side by side. In the image before dispersion 
 we have iris-rings, the extinction of the light being 
 nowhere complete ; but when the different colours are 
 separated by dispersion, each colour is crossed trans- 
 versely by its own system of dark interference bands, 
 which become gradually closer with the increasing 
 refrangibility of the light. The complete spectrum, 
 therefore, appears furrowed by a system of continuous 
 dark bands, crossing the colours transversely, and ap- 
 proaching each other as they pass from red to blue. 
 
 In the case of the plate of selenite, a slit is placed 
 in front of the polarizer, and the film of selenite is 
 held close to the slit, so that the light passes through 
 the central zone of the film. As in the case of New- 
 ton's rings, the image of the zone is crossed by iris- 
 
APPENDIX. 257 
 
 coloured bands ; but when subjected to prismatic dis- 
 persion, the light of the zone yields a spectrum furrowed 
 by bands of complete darkness exactly as in the case of 
 Newton's rings, and for a similar reason. This is the 
 beautiful effect described by Mr. Spottiswoode as the 
 fanlike arrangement of the bands the fan opening 
 out at the red end of the spectrum. 
 
 PEEFACE DU TEADUCTEUE FEANgAIS. 
 
 LA LUMIERE de M. Tyndall ne ressemble a aucun des Traites 
 qu'il m'a ete donne d'etudier et d'analyser, en si grand nombre, 
 dans mon ' Repertoire d'Optique moderne ' et plus tard. 
 
 Ce n'est pas un Traite elementaire, car il comprend les 
 phenomenes les plus delicats, ceux nieme que nos programmes 
 du baccalaureat ecartent de 1'enseignement classique ; et ce 
 n'est pas non plus un Traite d'Optique superieure, puisqu'il 
 est la redaction de Lecons faites devant un auditoire d'hommes 
 et de femmes du monde. 
 
 Ce n'est pas un Traite d'Optique physique, car il donne la 
 raison mecanique de chaque phenomene, et ce n'est pas un 
 Traite d'Optique mathematique, car le calcul n'y joue aucun 
 role. 
 
 Ce n'est pas un Traite d'Optique experimentale, car le 
 raisonnement domine tout, car 1'analyse et la synthese y 
 prennent une part considerable; et ce n'est pas un Traite 
 d'Optique rationnelle, car il se resume tout entier dans la 
 production des phenomenes sur la plus large echelle qu'on 
 puisse imaginer, en les rendant visibles a un immense audi- 
 toire. 
 
 Qu'est-il done et comment le caracteriserons-nous ? En 
 disant qu'il est tout a la, fois, dans son unite merveilleuse, 
 elementaire et superieur, physique et mathematique, experi- 
 mental ei rationnel ; qu'il constitue un mode d'enseignement 
 
 s 
 
258 APPENDIX. 
 
 sui generis, analytique et synthetique a la fois, vraiment 
 nouveau et admirable en soi. 
 
 Singuliere dans le fond, la Lumiere est non moins siagu- 
 liere dans la forme. Au premier aspect, et parce qu'elle est 
 en reaiite la redaction de Leons irnprovisees, cette forme 
 semble laisser beaucoup a desirer : on la dirait imparfaite ; 
 rnais, a mesure qu'on se 1'assimile la Lumiere etonne et ravit 
 par sa verite, sa precision, sa nettete, et 1'on est force de 
 reconnaitre qu'elle est en son genre un modele incomparable 
 de perfection. 
 
 Ce qui la specific et lui donne une superiorite incontes- 
 table, c'est son mode d'exposition, d'une transparence en 
 quelque sorte infinie. Elle donne la vision intuitive des 
 faits et plus encore de la raison des faits ou du rnecanisme 
 des phenomenes. Je croyais savoir 1'Optique, que j'ai etudiee 
 et redigee sous toutes ses formes depuis trente ans ; maisje 
 suis force de reconnaitre que je la sais incomparablement 
 mieux depuis que j'ai traduit la Lumiere. Les petites 
 fornrales de M. Tyndall penetrent beaucoup mieux au cosur 
 des phenomenes que les plus savantes equations des Traites 
 d'Optique mathematique. 
 
 Comme chefs-d'oeuvre du genre je citerai la troisieme 
 Le9on, sur la polarite et la polarisation rectiligne, et la qua- 
 trieme, sur la polarisation chromatique et les interferences. 
 Jamais enseignement n'avait fait sur mon esprit un effet aussi 
 extraordinaire. Non-seulement je voyais les phenomenes, mais 
 je m'identifiais avec eux ; ils n'etaient plus que des modifi- 
 cations de mon intelligence, ma propre pensee. La sensation 
 que j'eprouvais alors etait vraiment delicieuse. La vue 
 intuitive des phenomenes, de leurs causes, de leur mecanisme 
 est en effet le triomphe supreme de I'enseignement. Et voila 
 surtout a quel point de vue je recommande la Lumiere de 
 M. Tyndall comme le seul livre qui, a ma connaissance, initie 
 pleinement aux mysteres de 1'Optique. Je voudrais le voir 
 dans les mains de tous les amis de la Science. Elle n'est pas 
 aussi profonde, aussi savante, aussi encyclopedique que la 
 Chaleur ; mais elle va bien plus directement au but et Fatteint 
 
APPENDIX. 259 
 
 plus completement. J'ai fait d'ailleurs tout ce que j'ai pu 
 pour que, sans avoir une couleur trop anglaise, ma traduction 
 rendit parfaitement le texte original. 
 
 Me sera-t-il permis de faire remarquer que, dans son 
 expose de 1'analyse spectrale, M. Tyndall fait la part vraiment 
 trop belle a M. Kirchhoff? S'il est vrai que le celebre phy- 
 sicien de Heidelberg a fait, le premier, la ehimie so.laire, il 
 n'est pas vrai au meme degre qu'il ait fait le premier, par 
 1'analyse spectrale, la ehimie des substances terrestres. M. 
 Tyndall parle bien de quelques precurseurs de M. Kirchhoff, 
 mais il oublie les principaux, M. Pliicker par exernple, qui 
 apprit a MM. Kirchhoff et Bunsen a analyser par le prisme la 
 lumiere des flammes qu'ils s'efforeaient peniblement de resoudre 
 dans ses principes constituants a 1'aide de verres colores ou 
 absorbants. Le Memoire de M. Kirchhoff a ete lu a 1' Academic 
 de Berlin, le 27 octobre 1859 ; et neuf ans auparavant, comme 
 sir William Thomson, le collegue et glorieux emule de M. 
 Tyndall, a bien voulu le rappeler dans son discours inaugural 
 de 1' Association Britannique (Edimbourg, septembre 1871), 
 je disais (' Repertoire d'Optique moderne,' publie en Janvier 
 1860, t. III. p. 243) : ' M. Foucault a publie une Note curieuse 
 sur les spectres produits par les corps qui brulent entre les 
 deux pointes de charbon fixees aux poles d'une forte pile. 
 Nous avons repete ces belles experiences avec M. Soleil ; nous 
 pouvons meme dire que nous leur avons donne un plus grand 
 eclat en projetant ces spectres sur un ecran, sans leur rien 
 faire perdre de leur splendeur. Dans la combustion de 
 1'argent et du cuivre, il y a une raie verte qui surpasse en 
 intensite les rayons les plus illumines du spectre solaire. Le 
 spectre du cuivre etait iacilement reconnu par ses raies vertes, 
 celui du zinc par ses merveilleuses raies violettes. Dans la 
 flamme du laiton, compose de cuivre et de zinc, on admirait 
 les raies vertes du cuivre et les raies violettes du zinc ; le 
 maillechort a presente des apparences beaucoup plus splendides : 
 I'o3il ne se lassait pas de contempler toutes les raies lurnineuses 
 des metaux qui entrent dans la composition de cet alliage 
 multiple. AVEC UN PEU D'EXPERIENCE ON ARRIVE A FMRE, 
 
 s 2 
 
260 APPENDIX. 
 
 PAR L'OBSERVATION DES RAIES, L' ANALYSE SIKON QUANTITATIVE, 
 DU MOINS QUALITATIVE des combinaisons les plus complexes de 
 metaux tres-dissemblables.' 
 
 Dans mon cornpte rendu d'une matinee scientifique donnee 
 par M. Soleil et moi, dans les salons de M. Smile de Girardin, 
 en 1849, j'avais decrit ces memes experiences et affirme plus 
 nettement que les spectres de radiation ou d'absorption de tons 
 les corps de la nature avaient leurs raies sombresou brillantes, 
 caracteristiques et specifiques de leur nature intime. Ma con- 
 viction a cet egard etait si profonde, que je priai mon illustre 
 maitre A. Cauchy de formuler la theorie generale de ces raies. 
 II le fit dans une Note tres-courte, mais tres-explicite, pub- 
 liee dans les ' Comptes rendus des seances de 1' Academic des 
 Sciences' et inseree en 1847 dans le second volume de mon 
 * Repertoire d'Optique,' p. 238 et 239. Qu'il me soit permis de 
 reproduire ici les quelques lignes qui .la terminaient et qui 
 n'ont pas ete assez remarquees : ' Observons encore que 1'etat 
 initial d'un system (, de molecules ou plutot d'une portion de ce 
 systeme etant arbitraire, le systeme d'ondes planes qui repre- 
 sente cet etat initial, et qui s'en deduit par une formule con- 
 nue, pent varier a 1'infini comme cet etat lui-meme. II en 
 resulte que, parmi les ondes planes correspondant aux di- 
 verses longueurs d'ondulations, les unes doivent etre tres- 
 sensibles, tandis que les autres peuverit 1'etre beaucoup moins 
 et disparaitre presque entierement. On ne devra done pas 
 etre surpris de voir, dans la theorie de la lumiere, les rayons 
 doues de refrangibilites diverses, lorsqu'on les dispersera par 
 le moyen du prisme, offrir des intensites variables non- 
 seulement avec les longueurs d'ondulations correspondantes, 
 mais encore avec la nature des corps dont ils emanent ou 
 qu'ils traversent, et 1'on devrait s'etonner, au contraire, qu'il 
 en fut autrement. Ainsi doivent etre evidemment expliquees 
 les raies brillantes et obscures decouvertes dans le spectre 
 solaire et dans ceux que fournissent les autres corps lumineux. 
 Les raies du spectre ne doivent pas nous surprendre : leur 
 absence seule, dans le plus grand nombre des cas, serait 
 inexplicable.' Voila ce que nous ecrivions en 1847. La 
 
APPENDIX. 261 
 
 cliimie spectral e etait done a cette epoque une verite essen- 
 tielle et incontestable: M. Kirchhoff n'avait done pas a la 
 creer. Cette affirmation ne ressortira pas moins d'une autre 
 citation que je tiens a faire ici : M. Adolphe Erman, dans un 
 Memoire adresse a I'Acadernie des Sciences de Paris, en 
 octobre 1844, sur les raies d'absorption du chlore, de 1'iode et 
 du brorne, disait en termes formels: 'II est etonnant que la 
 theorie des phenomenes d'absorption (les phenomenes solaires 
 expliques par KirchhofF sont des phenomenes d'absorption) ait 
 encore fait si peu de progres, puisque la marche des recherches 
 qui doivent y conduire etait nettement indiquee d'avance. II 
 me sernble, en effet, que ces recherches doivent se borner: 
 1 a decomposer, a Faide du prisme, la lumiere sur laquelle 
 1'absorption a agi; 2 a caracteriser les rayons qui ont ete 
 eteints par le seul moyen que 1'Optique nous fournit pour cet 
 effet, je veux dire par la mesure des longueurs d'ondes ; et 3 
 a voir si les longueurs d'ondes des rayons observes sont liees 
 par quelques lois qui expliquent leur disparition.' C'est la. 
 evidemment toute 1'analyse spectrale, plus complete qu'on ne 
 1'a faite jusqu'ici. En dehors de ces vues theoriques, il y 
 avait a decouvrir le fait qu'une flamme arrete les ondes qu'elle 
 peut elle-meme engendrer, et la gloire de cette decouverte est 
 essentiellement francaise : elle appartient a M. Leon Foucault ; 
 M. Kirchhoff n'a iait que la fbrmuler rnieux et la generaliser. 1 
 
 II est dans la Lumiere quelques passages hasardes que je 
 ne puis laisser passer sans explications, precisement parce que, 
 en les traduisant, j'en assume a un certain degre la responsa- 
 bilite. 
 
 Page 9, ligne 11, M. Tyndall se fait 1'echo d'une boutade 
 humonstique d'une de ses pi us grand es ad mi ratio us, M. Helm- 
 holtz: 'On pourrait en realite dresser centre I'o2il une longue 
 liste d'accusations : son opacite, son defaut de symetrie, son 
 rnanque d'achromatisme, sa ceciteabsolue ou partielle. Toutes 
 ces raisons prises ensemble amenerent M. Helmholtz a dire, 
 
 1 What M. Foucault and M. Kirchhoff respectively accomplished is 
 stated in this volume I have awarded to M. Kirchhoff nothing that 
 can be justly denied to him. J.T. 
 
262 APPENDIX. 
 
 que, si tin opticien lui livrait un instrument si plein de defauts 
 il se croirait autorise a le renvoyer avec les reproches les plus 
 severes.' Sous cette forme, 1'appreciation de Foeil est vraiment 
 impardonnable. L'oeil n'est pas essentiellement ou absolument 
 achromatique : cela est vrai, cela meme est necessairement 
 vrai, puisque aucune oeuvre finie ne pent etre infiniment 
 parfaite et que la perfection absolue est le propre de 1'etre 
 infini. Mais, par cela meme qu'aucun homme n'a conscience 
 de ce defaut d'achromatisme, qu'il faut, pour le mettre en 
 evidence, des experiences tres-delicatesfaites avec de puissants 
 instruments ; qu'il ne modifie en rien pratiquement les couleurs 
 des objets, 1'oeil est exactement ce qu'il doit etre. On ne 
 pourrait probablement faire disparaitre ces imperfections 
 inseparables de tout etre cree et fini sans en faire naitre 
 d'autres beaucoup plus grandes. M. Helmholtz attribue le 
 defaut d'achromatisme de Fceil au fait que la den site des 
 milieux de 1'ceil ne surpasse guere la densite de 1'eau. Or 
 connait-il assez la constitution de 1'oeil et les innombrables 
 conditions qu'il doit remplir pour affirmer qu'une densite plus 
 grande de ses milieux n'aurait pas des inconvenients tres- 
 graves, n'amenerait pas des epanchements ou des infiltrations ? 
 Je remercie M. Tyndall d'avoir oppose a ce jugement incon- 
 sidere cette conclusion tres-sage (page 9) : ' Comme instrument 
 pratique et en faisant entrer en ligne de compte les accommo- 
 dements par lesquels ses defauts sont neutralises, 1'oeil n'en 
 reste pas moins une merveille pour tout esprit capable de re- 
 flexion? 
 
 Page 132, M. Tyndall semble regarder comme fondee et 
 insoluble Fobjection faite a 1'Eglise catholique, de prouver sa 
 divinite ou son infaillibilite par 1'authenticite de 1'Ecriture 
 sainte, et d'affirmer 1'authenticite et 1'inspiration divine des 
 saintes Ventures par 1'autorite de 1'Eglise infaillible, ce qui 
 constituerait un cercle vicieux. On a prouve, dep'uis bien 
 longfemps, que ce cercle vicieux n'existe pas pour nous 
 catholiques. En effet nous avons, independamment de toute 
 citation des Ecritures, par la tradition et la succession du 
 ministere apostolique, la certitude de 1'apostolicite de 1'Eglise 
 
APPENDIX. 263 
 
 roinaine, et aussi du fait que les apotres ont donne aux 
 eglises qu'ila ont fondees tels ou tels livres, et non d'autres, 
 comme Ecriture sainte et parole de Dieu ; en un mot nous 
 prouvons, par la tradition non interrompue, 1'authenticite et, 
 par consequent, 1'inspiration des saintes Ecritures, puis, par 
 1'inspiration -divine de I'Ecriture, I'infaillibilite de I'flglise. 
 Ce n'est pas la un cercle vicieux, c'est, an contraire, a 1'egard 
 des protestants, qui admettent la divinite de I'Ecriture sainte, 
 a priori, au point de recuser toute autre preuve, mi argument 
 personnel. Ce sont les protestants qui tombent, eux, dans un 
 cercle vicieux, en prouvant la divinite de 1'^criture par une 
 pretendue 2 jersua ^ on inlerieure du Saint-Esprit, et se pre- 
 tendent assures de cette assistance par le temoignage des 
 Ecritures qui la leur promettent. Si M. Tyndall s'etait 
 interdit cette comparaison, je me serais de mon cote interdit 
 cette observation que je regrette d'etre force de. consigner ici. 
 Non erat hie locus! 
 
 Pages 112 et 113, M. Tyndall se hasarde a dire: * C'est 
 par cet acte de cristallisation que la Nature se revele d'abord 
 a nous comme architecte. Ou s'arreteront ses operations? 
 continuera-t-elle, par le jeu des mernes forces, a former des 
 vegetaux et des animaux ? Quelle que puisse etre la reponse 
 a ces questions, croyez-moi, les notions des generations a 
 venir, sur cette chose mysterieuse que quelques-uns appellent 
 la matiere brute, seront tres-differentes de celles des gene- 
 rations passees.' C'est le germe de cette confession etrange 
 qui lui est echappee dans son discours d' inauguration de 
 Belfast : ' Quand je prolonge ma vision en arriere, a travers 
 les limites de 1'evidence experimentale, je discerne en cette 
 matiere, que, dans notre ignorance et sans le respect dft a son 
 createur, nous avons jusqu'ici couverte d'opprobres, la promesse 
 et la puissance d'engendrer toutes les formes et toutes les qualites 
 de la vie." 1 Nous avons explique cet ecart d'un esprit tres- 
 eleve, par cette remarque bien simple, qu'apres avoir perdu 
 la notion du Dieu createur, et fait la matiere eternelle, il 
 devait fatalement lui attribuer les proprietes et les facultes 
 divines ; et que, sans s'en douter, il rendait hommage au Dieu 
 
264 APPENDIX. 
 
 des Chretiens, devenu pour lui le Dieu inconnu, Ignoto 
 Deo. 
 
 Dans 1'Appendice sur les rapports de la cristallisation avec 
 la vie, M. Tyndall s'abandonne de nouveau a ses conceptions 
 materialistes, qui n'aboutissent, helas ! qu'a faire mieux com- 
 prendre que la vie est restee, pour cet esprit eminent, un 
 mystere impenetrable. II se complait a la representer comme 
 le resultat des attractions et des repulsions polaires, dont il 
 dote les atomes et les molecules des corps a la ia9on des 
 aimants. Mais, meme dans la vie de son arbre, il y a autre 
 chose que des directions moleculaires, effets de forces plutot 
 statiques que dynamiques: il y a mouvement et transport, 
 qui supposent une force vive. J'aurais voulu qu'il eut place 
 le phenomena de la vie a cote des phenomenes de la sensation 
 et de la pensee, dont il dit : * Le cerveau de 1'homme lui- 
 meme est assurement un assemblage de molecules arrangees 
 suivant des lois physiques ; mais, si vous me demandez de 
 deduire de cette assemblage le plus petit des phenomenes de 
 la sensation ou de la pensee, je me prosterne dans la poussiere, 
 et je reconnais 1'impuissance humaine. Cette fois la specu- 
 lation etendrait ses ailes bien au dela de la region ou il n'est 
 plus de milieu capable de soutenir ?on vol.' 
 
 Sur le terrain purement scientifique, nul plus que moi 
 n'admire en M. Tyndall le penseur original et profbnd, le 
 maitre eminemment habile ; mais, avec son esprit si eclaire 
 et si eleve, ii comprendra qu'en le combattant sur le terrain 
 de la religion et de la metaphysique, j'use d'un droit legitime, 
 je rernplis un devoir sacre. 
 
 F. MOIGNO. 
 
INDEX. 
 
 ABS 
 
 I BSORPTION, principles of, 202 
 * Addresses at Social Meeting in 
 
 New York, 229 
 Air} 7 , Sir George, severity and con- 
 
 clusiveness of his proofs, 212 
 Alhazen,his inquiry respecting light, 
 
 14,210 
 Analyzer, polarizer and, 131, recom- 
 
 pounding of the two systems of 
 
 waves by the analyzer, 133 
 Angstrom, his paper on spectrum 
 
 analysis, 205 
 
 Arago, Fran9ois, and Dr. Young, 52 
 his discoveries respecting light, 
 
 211, 212 
 Atomic polarity, 98-101 
 
 BACON, Eoger, his inquiry re- 
 specting light, 14, 210 
 
 Barnard, President, his address at 
 Social Meeting in New York, 229 
 
 Bartholinus, Erasmus, on Iceland 
 spar, 114 
 
 Berard on polarization of heat, 
 183 
 
 Blackness, meaning of, 32 
 
 Boyle, Robert, his observations on 
 colours, 68, 69 
 
 his remarks on fluorescence, 
 166-168 
 
 Bradley, James, discovers the aber- 
 ration of light, 22, 23 
 
 CKY 
 
 Brewster, Sir David, his chief objec- 
 tion to the undulatory theory of 
 light, 49 
 
 his discovery in biaxal crystals, 
 212 
 
 Brougham, Mr. (afterwards Lord), 
 ridicules Dr. T. Young's specula- 
 tions, 51, 52 
 
 Browning, Mr., his prisms, 126 
 
 P/ESIUM, discovery of, 196 
 ^ Calorescence, 177 
 Clouds, actinic, 155-157 
 
 polarization of, 158 
 Colours of thin plates, 67 
 
 Boyle's observations on, 68, 69 
 
 Hooke on the colours of thin 
 plates, 70 
 
 of striated surfaces, 92, 93 
 Comet of 1680, Newton's estimate 
 
 of the temperature of, 171 note 
 Crookes, Mr., his discovery of thal- 
 lium, 196 
 
 Crystals, action of, upon light, 
 100 
 
 built by polar force, 101 
 
 illustrations of crystallization, 102 
 
 architecture of, considered as an 
 introduction to their action upon 
 light, 101 
 
 bearings of crystallization upon 
 optical phenomena, 109 
 
266 
 
 INDEX. 
 
 CRY 
 
 Crystals, rings surrounding the axes 
 of, uniaxal and biaxal, 149 
 
 Cuvier on ardour for knowledge, 
 223 
 
 DE TOCQUEVILLE, writings of, 
 218, 225, 226 
 
 Descartes, his explanation of the 
 rainbow, 24, 25 
 
 his ideas respecting the transmis- 
 sion of light, 44 
 
 his notion of light, 210 
 Diamond, ignition of a, in oxygen, 
 
 172 
 
 Diathermancy, 176 
 Diffraction of light, phenomena of, 81 
 
 bands, 81, 82 
 
 explanation of, 83 
 
 colours produced by, 92 
 Dollond, his experiments on achro- 
 
 matism, 28 
 
 Draper, Dr., his investigation on 
 heat, 175; his address at Social 
 Meeting in New York, 235 
 
 Drummond light, spectrum, of, 198 
 
 EARTH, daily orbit of, 77 
 Electric beam, heat of the, 
 
 169 
 
 Electricity, discoveries in, 220, 221 
 Emission theory of light, bases of 
 
 the, 46 
 Newton espouses the theory, and 
 
 the results of this espousal, 
 
 79 
 Ether, Huyghens and Euler advo- 
 
 cate and defend the conception of 
 
 an, 49, 59 
 
 objected to by Newton, 59 
 Euler espouses and defends the 
 
 conception of an ether, 49, 59 
 
 .. GRA 
 
 Euse'bius on the natural philosophers 
 
 of his time, 13 
 Expansion by cold, 107 
 Experiment, uses of, 3 
 Eye, the, its imperfections, grown for 
 
 ages towards perfection, 8 
 imperfect achromatism of the, 
 
 30 note 
 
 FARADAY, Michael, his dis- 
 covery of magneto-electricity, 
 221 
 ' Fits,' theory of, 76 
 
 its explanation of Newton's rings, 
 77 
 
 overthrow of the theory, 80 
 Fizeau determines the velocity of 
 
 Tight, 23 
 
 Fluorescence, Stokes's discovery of, 
 164 
 
 the name, 176 
 
 Forbes, Professor, polarizes and 
 
 depolarizes heat, 184 
 Foucault. determines the velocity of 
 
 light, 23 
 
 his experiments on absorption, 
 200, 201, 204 
 
 Fraunhofer, his theoretical calcula- 
 tions respecting diffraction, 91 
 
 his lines, 196 
 
 their explanation by Kirch- 
 
 hoff, 197 
 Fresnel, and Dr. Young, 52 
 
 his theoretical calculations re- 
 specting diffraction, 90 
 
 his mathematical abilities and 
 immortal name, 213 
 
 OETHE, on fluorescence, 168 
 J Gravitation, origin of the no- 
 tion of the attraction of, 96 
 
INDEX. 
 
 267 
 
 GRA 
 Gravitation, strength of the theory 
 
 of, 150 
 GrimalcU, his discovery -with respect 
 
 to light, 58 
 Young's generalizations of, 58 . 
 
 HAMILTON, Sir William, of 
 Dublin, his discovery of coni- 
 cal refraction, 212 
 Heat, generation of, 6 
 
 Dr. Draper's investigation re- 
 specting, 175 
 
 Helmholtz, his estimate of the genius 
 of Young, 5 1 
 
 on the imperfect achromatism of 
 the eye, 30 note, 31 
 
 reveals the cause of green in the 
 case of pigments, 37 
 
 Henry, Professor Joseph, his invita- 
 tion, 2 
 
 Herschel, Sir John, his theoretical 
 calculations respecting diffraction, 
 90 
 
 first notices and describes the 
 fluorescence of sulphate of quinine, 
 168 
 
 his experiments on spectra, 
 204 
 
 Herschel, Sir William, his experi- 
 ments on the heat of the vari- 
 ous colours of the solar spectrum, 
 174 
 
 Hooke, Eobert, on the colours of 
 thin plates, 70 
 
 his remarks on the idea that 
 light and heat are modes of 
 motion, 71 
 
 Horse-chestnut bark, fluorescence 
 
 of, 168 
 
 Huggins Dr., his labours, 208 
 Huyghens advocates the conception 
 
 of ether, 49, 59 
 
 'LAC 
 
 Huyghens, his celebrated principle, 
 86 
 
 on the double refraction of Ice- 
 land spar, 114 
 
 TCELAND spar, 112 
 -*- double refraction caused by, 
 113 
 
 this double refraction first 
 treated by Erasmus Eartholinus, 
 114 
 
 character of the beams emergent 
 from, 117 
 
 tested by tourmaline, 119 
 
 Knoblauch's demonstration of 
 the double refraction of, 188 
 
 Ice-lens, combustion through, 170 
 Imagination, scope of the, 43 
 
 note by Maclaurin on this point, 
 44 note 
 
 TANSSEN, M., on the rose-col cured 
 V solar prominences, 207 
 Jupiter, Rcemer's observations of 
 
 the moons of, 20 
 Jupiter's distance from the sun, 20 
 
 EEPLER, his investigations on 
 the refraction of light, 14, 210 
 Kirchhoff, Professor, his explanation 
 
 of Fraunhofer's lines, 197 
 his precursors, 204 
 his claims, 206 
 
 Knoblauch, bis demonstration of 
 the double refraction of heat of 
 Iceland spar, 188 
 
 T ACTANTIUS, on the natural 
 J-J philosophers of his time, 13 
 
268 
 
 INDEX. 
 
 LAM 
 
 Lamy, M., isolates thallium in 
 
 ingots, 196 
 
 Lesley, Professor, his invitation, 2 
 Light familiar to the ancients, 5 
 
 generation of, 6, 7 
 
 spherical aberration of, 8 
 
 the rectilineal propagation of, 
 and mode of producing it, 9 ' 
 
 illustration showing that the 
 angle of incidence is equal to the 
 angle of reflection, 10, 11 
 
 sterility of the Middle Ages, 13 
 
 history of refraction, 14 
 
 demonstration of the fact of 
 refraction, 15 
 
 partial and total reflection of, 
 17-20 
 
 velocity of, 21 
 
 Bradley's discovery of the aber- 
 ration of light, 22, 23 
 
 principle of least action, 23 
 
 Descartes and the rainbow, 24 
 
 Newton's analysis of, 27 
 
 synthesis of white light, 30 
 complementary colours, 31 
 
 yellow and blue lights produce 
 white by their mixture, 31 
 
 what is the meaning of black- 
 ness? 32 
 
 analysis of the action of pigments 
 upon, 34 
 
 absorption, 35 
 
 mixture of pigments contrasted 
 with mixture of lights, 37 
 
 Wiinsch on three simple colours 
 in white light, 41 
 
 Newton arrives at the emission 
 theory, 46 
 
 Young's discovery of the undula- 
 tory theory, 50 
 
 illustrations of wave-motion, 52, 
 59 
 
 interference of sound-waves, 61 
 
 L1G 
 
 Light, velocity of, 63 
 
 principle of interference of waves 
 of, 64 
 
 phenomena which first suggested 
 the undulatory theory, 65-72 
 
 soap-bxibbles and their colours, 
 65-73 
 
 Newton's rings, 73-79 
 
 his espousal of the emission 
 theory, and the results of this 
 espousal, 79, 80 
 
 transmitted light, 80 
 
 diffraction, 81, 92 
 
 origin of the notion of the attrac- 
 tion of gravitation, 96 
 
 polarity, how generated, 97 
 
 action of crystals upon, 101 
 
 refraction of, 110 
 
 elasticity and density, 111 
 
 double refraction, 112 
 
 chromatic phenomena produced 
 by crystals in polarised, 124 
 
 the Nicol prism, 125 
 
 mechanism of, 129 
 
 vibrations. 129 
 
 I composition and resolution of 
 vibrations, 131 
 
 polarizer and analyzer, 131 
 
 recompounding the two systems 
 of waves by the analyzer, 133 
 
 interference thus rendered possi- 
 ble, 134 
 
 chromatic phenomena produced 
 by quartz, 142 
 
 magnetization of, 145 
 
 | rings surrounding the axis of 
 crystals, 146 . 
 
 colour and polarization of sky, 
 150-60 
 
 range of vision incommensurate 
 with range of radiation, 162 
 
 effect of thallene on the spec- 
 trum, 165 
 
INDEX. 
 
 269 
 
 LIG 
 
 Light, fluorescence, 165 
 
 transparency. 171 
 
 the ultra-red rays, 174 
 
 part played by Nature by these 
 rays, 178 
 
 conversion of heat-rays into light- 
 rays, 179 
 
 identity of radiant heat and, 180 
 
 polarization of heat, 183 
 
 principles of spectrum analysis, 
 192 
 
 spectra of incandescent vapours, 
 193 
 
 Fraunhofer'slines,andKirchhoff's 
 explanation of them, 197 
 
 solar chemistry, 198-200 
 
 demonstration of analogy be- 
 tween sound and, 201, 202 
 
 Kirchhoff and his precursors, 204 
 
 rose-coloured solar prominences, 
 207 
 
 results obtained by various 
 workers, 208 
 
 summary and conclusion, 209 
 
 measurement of the waves of, 
 247 
 
 polarised, the spectra of, 250 
 Lignum Nephriticum, fluorescence 
 
 of, 167 
 Lloyd, Dr., on polarization of heat, 
 
 183, 212 
 Lockyer, Mr., on the rose-coloured 
 
 solar prominences, 208 
 Lycopodium.diffraction effects caused 
 
 by the spores of, 91 
 
 MAGNETIZATION of light, 145 
 Malus, his discovery respect- 
 ing reflected light through Iceland 
 spar, 117 
 
 discovers the polarization of light 
 by reflection, 211 
 
 NIC 
 Masson, his essay on the bands of 
 
 the induction spark, 205 
 Melloni, on the polarization of heat, 
 
 184 
 Metals, combustion of, 5, 6 
 
 spectrum analysis of, 193 
 
 spectrum bands proved by Bun- 
 sen and Kirchhoff to be charac- 
 teristic of the vapour of, 195 
 
 Mill, John Stuart, his scepticism 
 regarding the undulatory theory, 
 152 
 
 Miller, Dr., his drawings and de- 
 scriptions of the spectra of various 
 coloured flames, 204 
 
 Morton, Professor, his discovery of 
 thallene, 165 
 
 Mother-of-pearl, colours of, 93 
 
 "YTATUKE, a savage's interpreta- 
 
 -^ tion of, 4 
 
 New York, addresses at Social 
 
 Meeting in, 229 
 Newton, Sir Isaac, his experiments 
 
 on the composition of solar light, 
 
 28 
 
 his spectrum, 28 
 
 dispersion, 28 
 
 arrives at the emission theory of 
 light, 46 
 
 his objection to the conception of 
 an ether espoused and defended 
 by Huyghens and Euler, 59 
 
 his optical career, 73 
 
 his rings, 73-79 
 
 espouses the emission theory, 79 
 - - effects of this espousal, 80 
 
 his rings explained by the theory 
 of ' fits,' 76 
 
 his idea of gravitation, 96 
 
 hi s. errors, 211 
 Nicol prism, the, 125 
 
270 
 
 INDEX. 
 
 OCE 
 
 OCEAN, colour of the, 35 
 (Ersted, discovers the deflec- 
 tion of a magnetic needle by an 
 electric current, 178 
 Optics, science of, 3 
 
 PASTEUR referred to, 223 
 
 f- Physical theories, origin of, 
 
 42-45 
 Pigments, analysis of the action of, 
 
 upon light. 34 
 
 mixture of, contrasted with mix- 
 ture of lights, 37 
 
 Helmholtz reveals the cause of the 
 green in the case of mixed blue 
 and yellow pigments, 37 
 
 impurity of natural colours, 38 
 Pitch of sound, 61 
 
 Pliicker, his drawings of spectra, 
 
 205 
 Polariscope, stained glass in the, 
 
 134, 135 
 
 unannealed glass in the, 139 
 Polarity, notion of, how generated, 
 
 97 
 
 atomic, 98 
 
 structural arrangements due to, 
 100 
 
 polarization of light, 115 
 
 tested by tourmaline, 119 
 
 and by reflection and refraction, 
 121 
 
 depolarization, 123 
 Polarization of light, 115 
 
 circular, 143 
 
 sky-light, 152, 158, 160 
 
 of artificial sky, 159 
 
 of radiant heat, 183 
 Polarizer and analyzer, 131 
 Poles of a magnet, 97 
 
 Powell, Professor, on polarization 
 
 of heat, 181 
 Prism, the Nicol, 125 
 
 SEE 
 
 QUARTZ, chromatic phenomena 
 produced by, 1 42 
 
 T)ADIANTheat, 176 
 -*-' diathermancy, or pervious- 
 ness to radiant heat, 176 
 
 conversion of heat-rays into 
 light-rays, 177 
 
 formation of invisible heat- 
 images, 183 
 
 polarization of, 183 
 
 double refraction, 185 
 
 magnetization of, 187 
 Rainbow, Descartes' explanation of 
 
 the, 24, 25 
 
 Refraction, demonstration of, 15 
 Rpfraction of light, 110 
 
 double, 112 ' 
 
 Reflection, partial and total, 17-20 
 Respighi, results obtained by, 208 
 Ritter, his discovery of the ultra- 
 violet rays of the sun, 162 
 Roemer, Olav, his observations of 
 Jupiter's moons, 20 
 
 his determination of the velocity 
 of light, 21 
 
 Rubidium, discovery of, 195 
 Rusting of iron, what it is, 5 
 
 QCHWERD, his observations re- 
 ^ specting diffraction, 91 
 Science, growth of, 179, 206 
 Schelling, his contempt for experi- 
 mental knowledge, 14 note 
 Scoresby, Dr., succeeds in exploding 
 gunpowder by the sun's rays con- 
 veyed by large lenses of ice, 1 70 
 Secchi, results obtained by, 208 
 Seebeck, Thomas, discovers thermo- 
 electricity, 179 
 
 discovers the polarization of 
 light by tourmaline, 212 
 
INDEX. 
 
 271 
 
 SEL 
 Selenite, experiments with thick 
 
 and thin plates of, 127 
 Silver spectrum, analysis of, 193, 194 
 Sky-light, colour and polarization of, 
 
 152. 157 
 
 generation of artificial skies, 155 
 Snell, Willebrord, his discovery, 15 
 
 his law, 16, 24 
 Soap-bubbles and their colours, 65, 
 
 66 
 
 Sound, early notions of the ancients 
 respecting, 52 
 
 interference of waves of, 61 
 
 pitch of, 61 
 
 analogies of light and, 63 
 
 demonstration of analogy be- 
 tween, and light, 201, 202 
 
 Sonorous vibrations, action of, 137 
 Spectrum analysis, principles of, 192 
 Spectra of incandescent vapours, 193 
 
 discontinuous, 194, 195 
 
 of polarized light, 250 
 Spectrum bands proved by Bunsen 
 
 and Kirchhoff to be characteristic 
 of the vapour, 195 
 
 its capacity as an agent of dis- 
 covery, 196 
 
 analysis of the sun and stars, 196 
 Spottiswoode, Mr. William, 126 
 Stewart, Professor Balfpur, 205 
 Stokes, Professor, results of his ex- 
 amination of substances excited 
 by the ultra-violet waves, 164 
 
 his discovery of fluorescence, 165 
 
 on fluorescence, 168 
 
 nearly antici pates KirchhofFs dis- 
 covery, 201, 205 
 
 Striated surfaces, colovrs of, 93 
 Sulphate of quinine first noticed and 
 
 described by Sir John Herschel, 168 
 Sun, chemistry of the, 199 
 Sun, rose-coloured solar prominences, 
 
 207 
 
 UND 
 
 TALBOT, Mr., his experiments, 
 204 
 Tartaricacid,irregular crystallization 
 
 of, and its effects, 134 
 Thallene, its effect on the spectrum, 
 
 165 
 
 Thallium, spectrum analysis of, 193, 
 194 
 
 discovery of, 196 
 
 isolated in ingots by M. Lamy, 
 196 
 
 Theory, relation of, to experience, 
 
 95 
 
 Thermo-electric pile, 179 
 Thermo-electricity, discovery of, 179 
 Tombeline, Mont, inverted image of, 
 
 20 
 Tourmaline, polarization of light by 
 
 means of, 115 
 
 Transmitted light, reason for, 80 
 Transparency, remarks on, 170 
 Tyndall, Professor, his remarks at 
 
 Social Meeting in New York, 242 
 
 TTLTR A- VIOLET sun-rays, dis- 
 U covered by Hitter, 162 
 
 effects of, 163 
 Ultra-red rays of the solar spectrum, 
 
 174 
 
 part played by the, 176 
 
 Undulatory theory of light, bases of 
 
 the, 48 
 Sir David Brewster's chief 
 
 objection to the, 49 
 
 Young's foundation of the, 50 
 
 phenomena which first sug- 
 gested tho, 65, 72 
 Undulatory theory of light, Mr. 
 
 Mill's scepticism regarding the, 
 
 152 
 a demonstrated verity in 
 
 the hands of Young, 213 
 
272 
 
 INDEX. 
 
 YAS 
 
 TTASSENIUS describes the rose- 
 ' coloured solar prominences in 
 1733, 207 
 
 Vitellio, his skill and conscientious- 
 ness, 14 
 
 his investigations respect inglight, 
 210 
 
 Voltaic battery, use of, and its 
 production of heat, 6, 7 
 
 TTTATER, crystallization of, 104, 
 W 249 
 
 deportment of, considered and 
 
 explained, 108, 109 
 Waves of water, 52 
 
 length of a wave, 53 
 
 interference of waves, 54-56 
 Wertheim, M., his instrument for 
 
 the determination of strains and 
 pressures by the colours of polar- 
 ized light, 137 
 Wheatstone, Sir Charles. 166 
 
 his analysis of the light of the 
 electric spark, 205 
 
 Whirlpool Rapids, illustration of 
 the principle of the interference of 
 waves at the, 57 
 
 YOU 
 
 White, President, his remarks at 
 Social Meeting in New York, 238 
 
 Willigen, Van der, his drawings of 
 spectra, 205 
 
 Wollaston, Dr., 196 
 
 discovers the rings of Iceland 
 spar, 212 
 
 Woodbury, Mr., on the impurity of 
 
 natural colours, 38 
 Wiinsch, Christian Ernst, on the 
 
 three simple colours in white 
 
 lights, 41 
 
 his experiments, 41 
 
 YOUNG, Dr. Thomas, his dis- 
 covery of Egyptian hierogly- 
 phics, 50 ; and the undulatory 
 theory of light, 50 
 Young, Dr. Thomas, Helmholtz's 
 estimate of him, 51 
 
 ridiculed by Brougham in the 
 'Edinburgh Review,' 51 
 
 generalizes Grimaldi's observa- 
 tion on light, 58 
 
 photographs the ultra-violet rings 
 of Newton, 163 
 
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