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 '5 CENTS|Nov. 1882] 
 
 Six LECTURES 
 
 ON LIGHT 
 
 BY 
 
 JOHN TYNDALL 
 
 ILLUSTRATED 
 
 NEW YORK 
 
 THEHUnBOLDTPUBLI5HiN(iCOnPANY 
 
 NTBRKD AT THB NEW YORK POST OFFICE AS SECOND CLASS MATTER. 
 
tolwo 
 
 lOO.OOO SOLD. 
 
 Its History and Present Development. 
 By FREDRIK BJORNSTROM, M. D., 
 
 Head Physician of the Stockholm Hospital, Professor of Psychiatry, Late Royal Swed- 
 
 ish Medical Counselor. 
 Authorized Translation from the Second Swedish Edition. 
 
 BY BARON NILES POSEE, M. G., 
 
 Director of the Boston School of Gymnastics. 
 
 Paper Cover (No. 113 of The Humboldt Library), - 30 Cents 
 
 Cloth, Extra, " " " - 75 Cents 
 
 PRESS NOTICES. 
 
 The learned Swedish physician, Bjornstrom. Churchman. 
 
 It is a strange and mysterious subject, this hypnotism. The Sun. 
 
 Perhaps as concise as any work we have. S. California Practitioner. 
 
 We have found this book exceedingly interesting. California Homapath. 
 
 A concise, thorough, and scientific examination of a little-understood subject. Episco- 
 pal Recorder. 
 
 Few of the new books have more interest for scientist and layman alike. Sunday 
 Times (Boston). 
 
 The study of hypnotism is in fashion again. It is a fascinating and dangerous study. 
 Toledo Bee. 
 
 It is well .written, being concise, which is a difficult point to master in all translations. 
 Medical Bulletin (Philadelphia). 
 
 The subject will be fascinating to many, and it receives a cautious yet sympathetic 
 treatment in this book. Evangelist. 
 
 One of the most timely works of the hour. No physician who would keep up with the 
 times can afford to be without this work. Quarterly Journal of Inebriety. 
 
 Its aim has been to give all the information that may be said under the present state of 
 our knowledge. Every physician should read this volume. American Medical Journal 
 (St. Louis). 
 
 It is a contribution of decided value to a much-disussed and but little-analyzed subject 
 by an eminent Swedish alienist known to American students of European psychiatry. 
 Medical Standard (Chicago). 
 
 This is a highly interesting and instructive book. Hypnotism is on the onward march 
 to the front as a scientific subject for serious thought and investigation. The Medical Free 
 Press (Indianapolis). 
 
 Many of the mysteries of mesmerism, and all that class of manifestation, are here 
 treated at length, and explained as far as they can be with our present knowledge of psy- 
 chology. New York Journal of Commerce. 
 
 The marvels of hypnotic phenomena increase with investigation. Dr. Bjornstrom, in 
 this clear and well-written essay, has given about all that modern science has been able to 
 develop of these phenomena. Medical Visitor (Chicago). 
 
 It has become a matter of scientific research, and engages the attention of some of the 
 foremost men of the day, like Charcot, of Paris. It is interesting reading, outside of any 
 usefulness, and may take the place of a novel on the office table. Eclectic Medical Jour- 
 nal (Cincinnati). 
 
 This interesting book contains a scholarly account of the history, development, and 
 scientific aspect of hypnotism. As a whole, the book is of great interest and very instruc- 
 tive. It is worthy of careful perusal by all physicians, and contains nothing unfit to be read 
 by the laity. Medical and Surgical Reporter (Philadelphia). 
 
 To define the real nature of hypnotism is as difficult as to explain the philosophy of toxic 
 or therapeutic action of medicine more so, indeed. None the less, however, does it be- 
 hoove the practitioner to understand what it does, even if he cannot tell just what it is, or 
 how it operates. Dr. Bjornstrom's book aims to give a general review of the entire subject. 
 Medical Record. 
 
 THE HUHBOLDT PUBLISHING CO., 
 
 64 Fifth Avenue, New York. 
 
T 
 
 S\I LECTURES ON LIGHT. 
 
 BY Prof. JOHN TYNDALL, F.R.S. 
 
 PREFACE. 
 
 MY eminent friend, Prof. Joseph Henry, 
 of Washington, did me the honor of taking 
 these lectures under his personal direction, 
 and of arranging the times and places at 
 which they were to be delivered. 
 
 Deeming that my home-duties could not, 
 with propriety, 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, Prof. 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 l he time rendered unavoidable, the lec- 
 tures in New York were rendered coincident 
 with the period of the presidential election. 
 This was deemed unsatisfactory, and when 
 dhe fact was represented to me I -U once of- 
 fered to extend the time of my visit so as to 
 make the lectures in New York succeed 
 those in Washington. The proposition was 
 cordially accepted by my friends. 
 
 To me personally this modified arrange- 
 ment has proved in the highest degree satis- 
 factory. It gave me a much-needed holiday 
 at N iagara Falls ; it, moreover, rendered the 
 successive stages of my work a kind oigro?ut/i y 
 which reached its most impressive develop 
 in New York and Brooklyn. 
 
 In every city that I have visited, my recep- 
 tion has been that of a friend ; and, now 
 that my visit has become virtually a thing of 
 the past, I can look back upon it with unqual- 
 ified pleasure. It is a memory without a 
 stain an experience of deep and genuine 
 kindness on the part of the American people 
 never, on my part, to be forgotten. 
 
 This relates to what may be called thefos- 
 itive side of my visit to the circumstances 
 attending the work actually done. My only 
 drawback relates to work undone; for I carry 
 home with me the consciousness of having 
 been unable to respond to the invitations of 
 the great cities of the West ; thus, I fear, 
 causing, in many cases, disappointment. 
 Would that this could have been averted ! 
 But the character of the lectures, and the 
 weight of instrumental appliances which they 
 involved, entailed loss of time and heavy 
 labor. The need of rest alone would be a 
 sufficient admonition to me to pause here ; 
 but, besides this, each successive mail from 
 London brings me intelligence of work sus- 
 pended and duties postponed through my 
 absence. These are the considerations 
 which prevent me from responding, with a 
 warmth commensurate with their own, to 
 the wishes of my friends in the West. 
 
 On quitting England I had no intention 
 of publishing these lectures, and, except a 
 fragment or two, not a line of them was written 
 
 559 
 
SIX LECTURES ON LIGHT. 
 
 when I reached this city. They have been 
 begun, continued, and ended in New York, 
 and bear only too evident marks of the rapid- 
 ity of their production. I thought it, how- 
 ever, due, both to those who heard them with 
 such marked attention, and to those who wish- 
 ed to hear them, but were unable to do so, to 
 leave tnem behind me in an authentic form. 
 The execution of this work has cut me off 
 from many social pleasures ; it has also pre- 
 vented me from making myself acquainted with 
 institutions in the working of which I feel a 
 deep interest. But human power is finite, 
 and mine has been expended in the way which 
 1 deemed most agreeable, not to my more 
 intimate friends, 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 ex- 
 tended, for in every city I have visited willing 
 
 helpers were at hand. I must not, however, 
 omit the name of Mr. Rhecs, Professor 
 Henry's private secretary, who, not only in 
 Washington, but in Boston, gave me most 
 important assistance. To the trustees of the 
 Cooper Institute my acknowledgments are 
 due ; also to the directors of the Mercantile 
 Library at Brooklyn. I would add to these a 
 brief but grateful reference to my high-minded 
 friend and kinsman, General Hector Tyn- 
 dale, 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 pain occasioned by the death 
 of my junior assistant in Philadelphia. 
 
 Finally, I have to mention with warm com- 
 mendation the integrity, ability, and devo- 
 tion, with which, from first to last, I have 
 been aided by my principal assistant, Mr. 
 John Cottrell. 
 
 NEW YORK, February, 1873. 
 
 LECTURE I. 
 
 INTRODUCTORY : Uses of Experiment : Early Scien- 
 tific Notions : Sciences of Observation : Knowl- 
 edge of the Ancients Regarding Light: Nature 
 judged from Theory defective: Detects of the 
 Eye: Our Instruments: Rectilineal Propagation 
 of Light : Law of Incidence and Reflection : 
 Sterility of the Middle Ages: Retraction: Dis- 
 covery of Snell : Descartes and the Rainbow : 
 Newton's Experiments on the Composition of 
 Solar Light : His Mistake as regards Achroma- 
 tism : Synthesis of White Light : Yellow and 
 Blue Lights proved to produce White by their 
 Mixture : Colors of Natural Bodies : Absorption : 
 Mixture of Pigments contrasted with Mixture of 
 Lights. 
 
 SOME twelve years ago I published, in 
 England, a little book entitled the " Glaciers 
 of the Alps," and, a couple of years subse- 
 quently, a second volume, entitled " Heat as 
 a Mode of Motion." These volumes were 
 followed by others, written with equal plain- 
 ness, and with a similar aim, that aim being 
 to develop and deepen sympathy between 
 science and the world outside of science. I 
 agreed with thoughtful men* who deemed 
 it. good for neither world to be isolated from 
 the other, 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 pre- 
 viously been the pursuit and pleasure of my 
 life. 
 
 These books were, for the most part, re- 
 published by the Messrs. Appleton, under 
 
 the auspices of a man who is untiring in his 
 efforts to diffuse sound scientific knowledge 
 among the people of this country; who^e 
 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, of this city. Quite as rapidly as 
 in England, the aim of these works was un- 
 derstood 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 * to visit America, and last year 
 I was honored with a request so CQrdial, and 
 signed by five-and-twenty names so distin- 
 guished in science, in literature, and in ad- 
 ministrative position, that I at once resolved 
 to respond to it by braving, not only the dis- 
 quieting oscillations of the Atlantic, but the 
 far more disquieting ordeal of appearing in 
 person before the people of the United 
 States. 
 
 This request, conveyed to me by my ac- 
 complished friend, Professor Lesley, of Phil- 
 adelphia, and preceded by a letter of the 
 same purport from your scientific Nestor, 
 Professor Joseph Henry, of Washington, de- 
 sired 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 what 
 form such lectures ought to to take. In 
 
 * Among whom may be mentioned, specially, the * One of the earliest came from Mr. John Amory 
 late Sir Edmund Head, Bart. Lowell, of Boston. 
 
SIX LECTURES ON LIGHT 
 
 answer to nv inquiries, however, I was given 
 to understand (by Professor Youmans princi- 
 pally) that a course of experimental lectures 
 would materially promote scientific education 
 in this country, and I at once resolved to 
 meet this desire, as far as my time allowed. 
 
 Experiments have two uses a use in dis- 
 covery, and a use in tuition. They are the 
 investigator's language addressed to Nature, 
 to which she sends intelligible replies. These 
 replies, however, are, for the rrost part, at 
 first too feeble for the public ear ; for the in- 
 vestigator cares little for the loudness of Na- 
 ture's voice if he can only unravel its meaning. 
 But after the discoverer comes the teacher, 
 whose function it is eo to exalt and modify the 
 resu ts of the discoverer as to render them fit 
 for public presentation. This secondary 
 function I shall endeavor, in the present in- 
 stance, to fulfil. 
 
 I propose to ta<e a single department of 
 natural philosophy, and illustrate, by means 
 of it, the growth of scientific knowledge under 
 the guidance of experiment. I wish, in this 
 fi st lecture, to make yen acquainted with cer- 
 tain elementary phenomena ; then to point 
 out to you how those theoretic principles by 
 which phenomena are explained, take root, i 
 and flourish in the human mind, and after- ! 
 wards to apply these principles to the whole 
 b<-dy of knowledge covered by the lectures. 
 The science of optics lends itself to this mode 
 of reatment, and on it, therefore, I propose 
 to draw for the materials of the present course. 
 Jt 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 aLstruse discoveries of 
 modern times. 
 
 All our notions of Nature, however exalted 
 or however grotesque, have some foundation 
 in experience. The notion of personal voli- 
 tion in Nature had this basis. In the fury 
 and the serenity of natural phenomena the 
 t- iva^e saw the transcript of his own varying 
 moods, and he accordingly ascribed these 
 pnenoraena to beings of like passions with 
 hhnseit, but vastly transcending him in power. 
 
 Thus the notion of causality the assumption 
 that natural thi ngs did not com 2 of themselves, 
 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 
 fur the antecedents of phenomena all science 
 has sprung. 
 
 The development of man, indeed, is ulti- 
 mately due to his interaction with Nature. 
 Natural phenomena arrest his attention and 
 excite his questionings, the intellectual activity 
 tims provoked reacting on the intellect itself, 
 and adding to its strength. The quantity of 
 power added by any single effort of the in- 
 tellect may be indefinitely small ; but the in- 
 tegration of innumerable increments of this 
 kind has raised Intellectual power from its 
 rudiments to the magnitude it possesses to- 
 
 single increment is made good by the indefi- 
 nite number of such increments, summed up 
 in what may be regarded as practically infinite 
 time. 
 
 We will not now go back to man's first 
 intellectual gropings ; much less shall we 
 enter upon the thorny discussion as to how 
 the groping man arose. We will take him at 
 a certain stage of his development, when, by 
 evolution 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 of- 
 fered, and confining intellectual action to it. 
 The apparent motions of sun and stars first 
 drew towards them the questionings of the in- 
 tellect, and accordingly astronomy was the 
 first science developed. Slowly, and with diffi- 
 culty, the notion of natural fore-es took root 
 in the mind, the seedling of this notion being 
 the actual observation of electric and mag- 
 netic attractions. 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 principles to the 
 motions of the heavenly bod'es. We trace 
 the progress of astronomy through Hip- 
 parchus and Ptolemy; and, after a long halt, 
 through Copernicus, Galileo, Tycho Brahe, 
 and Kepler; w ile, 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 attracted the attention of the ancient 
 world. Light was a familiar phenomenon, 
 and from the earliest times we find men's 
 rr.inds 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, accordingly, 
 were far less successful in dealing with light 
 than in dealing with solar and stellar mo- 
 tions. Still, they did make some progress. 
 They satisfied themselves that light moved 
 in straight lines; they knew, also, that these 
 lines or rays of light were reflected from pol- 
 
 ished surfaces, and that the angle of inci- 
 dence was equal to the angle of reflection. 
 These two results of ancient scientific curios- 
 ity constitute the starting-point of our pres- 
 ent course of lectures. 
 
 But, in the first place, it may 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 blow burning of iron. It develops heat, 
 and, if the heat be preserved, a high temper- 
 ature may be thus attained. The destruc- 
 tion of the first Atlantic cable was probably 
 due to heat di.'V( luped in this way. Other 
 metals are stiil more combustible than iron. 
 You may light stiips of zinc in a candle- 
 flame, and cause them to burn almost like 
 
 day. In fact, the indefinite smallness of the strips of paper. But, besides combustion in 
 
SIX LECTURES ON LIGHT. 
 
 the air, we may also have combustion ii? a 
 liquid. 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 the present course. 
 
 Their generation merits a moment's atten- 
 tion. Before you is an instrument a small 
 voltaic battery in which zinc is immersed in 
 a. suitable liquid. Matters are so arranged 
 * hat an attraction is set up between the metal 
 and the oxygen, actual union, however, being 
 in the first instance avoided. Uniting the 
 two ends of the battery by a thick wire, the 
 attraction is satisfied, the oxygen unites with 
 the metal, the 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, I unite the 
 severed ends by a thin one. It glows with a 
 white heat. Whence comes that heat? The 
 question is well worthy of an answer. Sup- 
 pose in the first instance, when the thick wire 
 was employed, that we had permitted the ac- 
 tion to continue until 100 grains of zinc were 
 consumed, the amount of heat generated in 
 the battery would be capable of accurte nu- 
 merical expression. Let the action now con- 
 tinue, with this thin wire glowing, until 100 
 grains of zinc are consumed. Will the amount 
 (. f 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 va- 
 ries. By this arrangement, then, we are able 
 to burn our zinc at one place, and to exhibit 
 the heat and light of its combustion at a dis- 
 tant place. In New York, for example, we 
 have our grate and fuel ; but the heat and 
 'iijht of our fire may be made to appear at 
 Sun Francisco. 
 
 I now remove the thin wire and attach to 
 the severed ends of the thick one two thin 
 rods of coke. On bringing the rods together 
 we obtain a small star of light. Now, the 
 light to be employed in our lectures is a sim- 
 ple 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 light will give us all 
 the beams necessary for our experiments. 
 
 And here, in passing, let me refer to the 
 common delusion that the w6rks of Nature, j 
 the human eye included, are theoretically per- 
 fect. The degree of perfection of any organ 
 is determined by what it has to do. Looking 
 at the dazzling light from our large battery, 
 you see a globe of light, 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 sur. 
 rounded by a halo which nearly obliterates it. 
 This arises from an imperfection 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 labors 
 under a similar defect, and, when you looked 
 at the naked light from fifty cells, the blur of 
 light upon the retina was sufficient to destrov 
 the definition of the retinal image of the car- 
 bons. A long list of indictments might in- 
 deed be brought against the eye its opacity, 
 its want of symmetry, i~s lack of achroma- 
 tism, its absolute blindness, in pa:t. All 
 these taken together caused an eminent Ger- 
 man philosopher to say that, if any optician 
 sent him an instrument so full of defects, he 
 would send it back to him with the severesf 
 censure. But the eye is not to be judged 
 from the standpoint of theory. As a practi- 
 cal instrument, and taking the adjustments by 
 which its defects are neutralized into account, 
 it must ever remain a marvel to the reflecting 
 mind 
 
 The ancients, as I have said, were aware o/ 
 the rectilineal propagation of light. They 
 knew that an opaque body, placed between 
 the eye and a point of light, intecepted the 
 light of the point. Possibly the terms " ray " 
 and "beam" may have been suggested by 
 those straight spokes of light which, in cer- 
 tain states of the aimosphere, dart from the 
 sun at his rising and his setting. The recti- 
 lineal propagation of light may be illustrated 
 at home in th'S way: Make a small hole in a 
 closed window-shutter, before which stands a 
 house or a tree, and place within the dark- 
 ened room a white- screen at some distance 
 from the orifice. Every straight ray proceed- 
 ing from the house or tree stamps its color 
 upon the screen, and the sum of all the rays 
 for us an image ot the object. But, as tne 
 rays cross each other at the orifice, the image 
 is inverted. Here we may illustrate the sub- 
 ject thus: In front of our camera is a large 
 opening, closed at present by a sheet of tin- 
 foil. Pricking by means of a common sew- 
 ing-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 hun- 
 dred, a thousand a thousand. But, as the 
 apertures come closer to each other, that is to 
 say, as the tin-foil between the apertures van- 
 ishes, the images overlap more and more. 
 Removing the tin-foil altogether, the screen 
 becomes uniformly illuminated Hence the 
 light upon the screen may be regarded as the 
 overlapping of innumerable 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 super- 
 position of innumerable images of the sun. 
 
 The law that the angle of incidence is 
 equal to the angle of reflection is illustrated 
 in this simple way: A straight lath is placed 
 as an index perpendicular to a small looking- 
 
SIX LECTURES ON LIGHT. 
 
 glass capable of rotation. A beam of light 
 is received upon the glass and reflected back 
 upon the line of its incidence. Though the 
 incident and the reflected beams pass in 
 opposite directions, they do not jostle or dis- 
 place each other. The index being turned, 
 the mirror turns along with it, and at each 
 side of the index the incident and the 
 reflected beams are seen tracking themselves 
 tnrough the dust of the room. The mere 
 inspection of the two angles enclosed be- 
 twe.n the index and the two beams suffices 
 to show their equality. The same simple 
 apparatus enables us to illustrate a law of 
 great practical importance, name y, that, 
 when a mirror rotates, the angular velocity 
 of a beam reflected from it is tvvic: that of 
 the reflecting mirror. One experiment will 
 make this pla : n to you. The mirror is 
 now vertical, and both the incident and the 
 reflected beams are horizontal. Turning the 
 mirror through an angle of 45 the reflected 
 beam is vertical ; that is to say, it has moved 
 f)O r , or through twice the angle of the mirror. 
 
 One of the problems of science, on which 
 scientific progress mainly depends, is to help 
 trie senses of man by carrying them into re- 
 gions 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 miscroscope when we 
 want to explore motion and structure in their 
 infinitesimal dimensions. 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 
 wa ching the images of scales reflected from 
 the mirrors, the celebrated Gauss was able to 
 detect the slightest thrill or variation on the 
 part of the earth's magnetic force. The mi- 
 nute elongation of a bar of metal by the mere 
 warmth of the hand may be so magnified by 
 this method as to cause the index-beam to 
 move from the ceiling to the floor of this 
 room. The elongation of a bar of iron when 
 it is magnetized may be thus demonstrated. 
 By a similar arrangement the feeble attrac- 
 tions and repulsions of the diamagnetic force 
 have been made manifest; while in Sir William 
 Thompson's reflecting galvanometer the prin- 
 ciple receives one of its latest applications. 
 
 For more than 1,000 years no step was 
 taken in optics beyond this law of reflection. 
 The men of the Middle Ages, in fact, endeav- 
 ored on the o,e hand to develop the laws of 
 the universe 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 pertain- 
 ing to this one. Notwithstanding its demon- 
 strated failure during 1,500 years of trial, 
 there are still men among us who think the 
 riddle of the universe is to be solved by this 
 appeal to consciousness. And, like most 
 people who support a delusion, they maintain 
 
 theirs warmly, and show scant respect for 
 those who dissent from their views.* As re- 
 gards the refraction of light, the course of 
 real inquiry was resumed in 1100 by an Ara- 
 bian philosopher named Alhazen. Then it 
 was taken up in succession by Roger Bacon, 
 Vitellio, and Kepler. One of the most im- 
 portant occupations of science is the deter- 
 mination, by precise measurements, of the 
 quantitative relations of phenomena. The 
 value of such measurements depends 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 ob.-erv.i- 
 tions of his predecessors, look at them in ail 
 lights, and thus distill from them the princi- 
 ples whieh 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 the measurements of Vitellio. But 
 in the case of refraction he was not success- 
 ful. The principle, though a simple one, es- 
 caped him. It was firs discovered by Wille- 
 brod Snell, about the year 1621. 
 
 Less with the view of dwelling upon the 
 phenomenon itself than of introducing it to 
 you in a form which will render intelligible 
 the play of theoretic thought in Newton's 
 mind, I will show you the fact of refraction. 
 The dust of the air and the turbidity erf a liquid 
 may here be turned to account. A shallow 
 circula- vessel with a glass face, half filled 
 with water, rendered barely turbid by the 
 precipitation of a little mastic, is placed upon 
 its edge with its glass face vertical. Through 
 a slit in the hoop surrounding the vessel a 
 beam of light is admitted. It impinges upon 
 the water, enters it, and tracks itself through 
 the liquid in a sharp, bright band. 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 once reveals the track of the 
 incident-beam. If the incidence be vertical, 
 the beam is unrefracted. If oblique, its re- 
 fraction at the common surface of air and 
 water is rendered clearly visible. It is also 
 seen that reflection accompanies refraction, 
 the beam dividing itself at the point of inci- 
 dence into a refracted and a reflected portion. 
 
 The law by which Snell connected together 
 all the measurements executed up to his time, 
 is this : Let A B C D represent the outline 
 of our circular vessel (Fig. i), 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 
 
 * Schelling thus expresses his contempt for experi- 
 mental knowledge : " Newton's Optics is the greatest 
 illustration of a whole structure of fallacies, which in 
 all its parts is founded on observation and experi- 
 nent." There are some small imitators of 
 still in Germany. 
 
SIX LECTURES ON LIGHT. 
 
 along m' E, there is also refraction at E, the 
 beam striking the point n> '. From the ends 
 of the incident beams, let the perpendiculars 
 m o, m f o f be drawn upon B D, and from the 
 ends of the refracted beams let the perpen- 
 diculars p n,p f n f be also drawn. Measure 
 the lengths of o m and of p n and divide the 
 
 one by the other. You obtain a certain quo- 
 tient. In like manner divide m f o / by the 
 corresponding perpendicular p f n f \ vou ob- 
 tain in each case the same qiiotient. Snell, in 
 fact, found this quotient to be a constant 
 quantity for each particular substance, though 
 it varied in amount from substance to sub- 
 stance He called the quotient the index of 
 refraction. 
 
 This law is oue of the corner-stones of 
 optical science, and its applications to-day 
 <-<re million-fold. Immediately after its -dis- 
 covery, Descartes Applied it to the explana- 
 tion of the rainbow. The bow :s seen when 
 the back is turned to the sun. Draw a 
 straight line through the spectator's eye and 
 the sun, the bow is always seen at the same 
 angular distar.ee from this line. This was 
 the great difficulty. Why should the bow be 
 always and at all its parts, forty-one degrees 
 from this line? Taking a pen and calculat- 
 ing the track of every ray through a rain- 
 drop, Descartes found that, at one particular 
 angle, the rays emerged from the drop almost 
 parallel to each other; being t.;us enabled to 
 preserve their intensity through long atmos- 
 pheric distances; at all other angles the rays 
 quitted the drop divergent, and through this 
 divergence became so enfeebled as to be 
 practically .ost to the eye. The particular 
 angle here referred to was the foregoing 
 angle of forty-one degrees, which observa- 
 tion had proved to be invariably that of the 
 rainbow. 
 
 But in the rainbow a new phenomenon 
 was introduced the phenomenon of color. 
 And here we arrive at one of those points in 
 the history of science, when men's labors so 
 intermingle, that it is difficult to assign to 
 each worker his precise meed of honor. Des- 
 ca.tes was at the threshold of the discovery 
 of the composition of solar light. But he 
 failed to attain perfect clearness, and it is 
 
 certain that he did net enunciate the true 
 law. This was reserved for Newton, who 
 went to work in this way : Through the closed 
 window-shutter of a room he pierced an ori- 
 fice, and allowed a thin sunbeam to pass 
 through it. The beam stamped a round 
 image of the sun on the opposite white wall 
 of the room. In the path of this beam New- 
 ton placed a prism, expecting to seethe beam 
 refracted, but also expecting to see the image 
 of the sun, after refraction, round; to his 
 astonishment, it was drawn out to an image 
 whose length was five times its breadth; and 
 this image was divided into bands of differ- 
 ent colors. Newton saw immediately that 
 solar light was composite, not simple. His 
 image revealed to him the fact that some con- 
 stituents of the solar light were more deflect- 
 ed by the prism than others, and he conclud 
 ed, therefore, that white solar light was a 
 mixture of lights of different colors and of 
 different degrees of refrangibility. 
 
 Let us reproduce this celebrated experi- 
 ment. On the screen is now stamped a lu- 
 minous disk, which may stand for Newton's 
 image of the sun. Causing the beam which 
 produces the disk to pass through a prism, 
 we obtain Newton's elongated colored image, 
 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 
 prismatic colors. This drawing out of the 
 white light into its constituent colors is called 
 dispersion. 
 
 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. It is this stern conscientiousness in 
 testing its conclusions that gives adamantine 
 strength to science, and renders all assaults 
 on it unavailing. Newton completed his 
 proof by synthesis in this way : The spec- 
 trum 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 colors are refracted back and 
 eblended, the perfectly white image of the 
 slit is restored. Here, then, refraction and 
 dispersion are simultaneously abolished. Are 
 they always so ? Can we have the one with- 
 out the other ? It was Newton's conclusion 
 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. 
 Dolland subsequently proved that, by com- 
 bining two different kinds of glass, the colors 
 could be extinguished, still leaving a residue 
 of refraction, and he employed this residue 
 in the construction of achromatic lenses 
 lenses which yield no color which Newton 
 thought an impossibility. By setting a water- 
 prism water contained in a wedge-shaped 
 vessel with glass sides in opposition to a 
 prism of glass, this point can be illustrated 
 before you. We have first the position of the 
 unrefracted beam marked upon the screen ; 
 
SIX LECTURES ON LIGHT. 
 
 then we produce the water-spectrum ; finally, 
 by introducing' a flint glass prism, we refract 
 the beam back, until the color cisappears. 
 The image of the slit is now white ; but you 
 see that, though the dispersion is abolished, 
 the refraction is not. 
 
 This is the place to illustrate another point 
 bearing upon the instrumental means em- 
 ployed in these lectures. Note the position 
 ot the water-spectrum upon the screen. Alter- 
 ing, in no particular, the wedge-shaped ves- 
 sel, 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 color. 
 This will explain to you the use of this sub- 
 stance in our subsequent experiments. 
 
 The synthesis of white light may be 
 effected in three ways, which are now worthy 
 of special attention: Here, in the first in- 
 stance, we have a rich spectrum produced by 
 a prism of bisulphide of carbon. One face 
 of the prism is protected by a diaphragm 
 with a longitudinal siit, through which the 
 beam passes into the prism. It emerges de- 
 composed at the other side. I permit the 
 colors to pass through a cylindrical lens, 
 which so squeezes them together as to pro- 
 duce upon tiie screen a sharply-defined rect- 
 angular image of the longitudinal slit. In 
 that image the colors are re-blended, and you 
 see it perfectly white. Between the prism 
 and the cylindrical lens may be seen the 
 colors tracking- themselves through the dust 
 bf the room. Cutting off the more refrangi- 
 ble fringe by a card, the rectangle is seen 
 red ; cutting off the less refrangiole fringe, 
 the rectangle is seen blue. By means of a 
 thin glass prism, I deflect one portion of the 
 colors, and leave the residual portion. On 
 the screen are now two colored rectangles 
 produced in this way. These are comple- 
 mentary colors colors which, by their union, 
 produce white. Note that, by judicious 
 management, one of these colors is tendered 
 yellow, and the other bhie. I withdraw the 
 thin prism ; yellow falls upon blue, and we 
 have white as the result of their union. On 
 Dur way, we thus abolish the fallacy first 
 exposed by Hehnholtz, that the mixture ot 
 blue and yellow lights produces green. 
 
 Again, restoring the circular aperture, we 
 obtain once more a spectrum like that of 
 Newton. By means of a lens, we gather up 
 these colors, 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 colors of the 
 spectrum, we blend together the prismatic 
 colors in the eye itself, and thus produce the 
 impression of whiteness. 
 
 Having unravelled the interwoven con- 
 stituents of white light, we have next to 
 inquire, What part the constitution so 
 revealed enables this agent to play in Nature ? 
 
 To tt we owe all the phenomena of color ; 
 and yet not to it alone, for there must <be a 
 certain relationship between the ultimate par- 
 ticles of naturaJ bodies and light to enable 
 them to extract from it the luxuries of color. 
 But the function of natural bodies is here 
 selective, not creative. There is no color gen- 
 erated by any natural body whatever. Natural 
 bodies have showered upon them, in the 
 white light of the sun, the sum total of all 
 possible colors, and their action is limited to 
 the sifting of that total, the appropriating 
 from it of the colors 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 colors. 
 
 Let us begin our experimental inquiries here 
 by asking, What is the meaning of blackness ? 
 Pass a black ribbon in succession through the 
 colors of the spectrum ; it quenches all. 
 This is' the meaning of blackness it is the 
 result of the absorption of all 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 sent back to the eye. Place the same rib 
 bon in the green or blue of the spectrum ; ,t 
 is black as jet. It absorbs the green ar,d 
 blue light, and leaves the space on which they 
 fall a space of intense darkness. Place a 
 green ribbon in the green of the spectrum. 
 It shines vividly with its proper color ; transfer 
 it to the red, it is black as jet. Here it ab- 
 sorbs all the light that falls upon it, and offers 
 mere darkness to the eye. 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 color. Thus 
 the process through which natural bodies ac- 
 quire their colors is a negative one. The 
 colors are produced by subtraction, not by 
 addition. 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 trans- 
 mitted 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 li^ht. This other liquid 
 finally is purple because it destroys the gree.": 
 and the yellow, and allows the terminal colors 
 of the spectrum to pass unimpeded. From 
 the blending of the blue and the red this gor- 
 geous color is produced. 
 
 These experiments prepare us for the fur- 
 ther consideration of a point already adverted 
 to, and regarding which error has found cur- 
 rency for ages. You will find it stated in 
 books that blue and yellow lights mixed to- 
 gether produce green. But blue and yellow 
 have been ]ust proved to be complementary 
 colors, producing white by their mixture. 
 The mixture of blue and yellow pigments un- 
 
8 
 
 SIX LECTURES ON LIGHT. 
 
 doubtedly produces green, but the mixture of 
 pigments is totally different from the mixture 
 of lights. Helmholtz, who first proved yel- 
 low and blue to complementary colors, has 
 revealed the cause of the green in the case of 
 the pig-Rents. No natural color is pure. A 
 blue liquid or a blue powder permits not only 
 the blue to pass through it, but a portion of 
 the adjacent green. A yellow powder is 
 transparent not only to the yellow light, but 
 also in part transparent to the adjacent green. 
 Now, when blue and yellow are mixed to- 
 gether, 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 color to which both are 
 transparent, and the consequence is that, 
 when white light falls upon a mixture of yel- 
 low and blue powders, the green alone is sent 
 back to the eye. I have already shown you 
 that the fine blue ammonia-sulphate of copper 
 transmits a large portion of green, while cut- 
 ting off all the less refrangible light. A yel- 
 low solution of picric acid also allows the 
 green to pass, but quenches all the more re- 
 frangible light. What must occur when we 
 send a beam through both liquids? The 
 green band of the spectrum alone remains 
 upon the screen. 
 
 This question of absorption is one of the 
 most subtle and difficult in molecular physics. 
 \Ve 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, com- 
 posed of innumerable constituents, refrangi- 
 ble in different degrees. We find, secondly, 
 ihe atoms and molecules of bodies gifted 
 with the power of sifting solar light in the 
 most various ways, and producing by this 
 rifting the colors observed in nature and art. 
 To do this they must possess a molecular 
 structure commensurate in complexity with 
 that of light itself. Thirdly, we have :he 
 human eye and brain so organized as to be 
 able to take in and distinguish the multitude 
 of impressions thus generated. Thus, the 
 light at starting is complex; to sift and select 
 ii as they do natural bodies must be complex. 
 Finally, to take in the impressions thus gen- 
 crated, the human eye and brain must be j 
 highly complex. Whence this triple com 
 plexity ? If what are called material pur- 
 poses were the only end to be served, a much 
 simpler mechanism would be sufficient. But, 
 instead of simplicity instead of the princi- 
 ple of parsimony we have prodigality of re- 
 lation and adaptation, and this apparently 
 for the sole purpose of enabling us to see 
 things robed in the splendor of color. W'ould 
 it not seem that Nature harbored the inten- 
 tion of educating us for other enjoyments 
 than those derivable from meat and drink ? 
 At all events, whatever Nature meant and 
 it would be mere pres_mpnon to dogmatize 
 
 as to what she meant we find ourselves here 
 as the issue and upshot of her operations, en- 
 dowed with capacities to enjo\ not only the 
 materially useful, but endowed with others of 
 indefinite scope and application, which deal 
 alone with the beautiful and the true. 
 
 LECTURE II. 
 
 Origin of Physical Theories : Scope of the Imagina- 
 tion : Newton and the Emission Theory : Verifica- 
 tion of Physical Theories : The Luminiferous ; 
 Ether: Wave-Theory of Light: Thomas Young: 
 Fresnel and Arago : Conceptions of Wave-Motion: 
 Interference of Waves : Constitution of Sound- 
 Waves : Analogies of Sound and Light: Illustra- 
 tions of Wave-Motion : Interference of Sound- 
 Waves : Optical Illustrations: Pitch and Color: 
 Lengths of the Waves of Light and Rates of 
 Vibration of the Ether-Particles: Interference of 
 Light : Phenomena which first suggested the Un- 
 dulatory Theory: Hooke and the .Colors of Thin 
 Plates: The Soap-Bubble : Newton's Rings: 
 Theory of " Fits: " Its Explanation of the Kings : 
 Overthrow of the Theory: Colors of Mother-of- 
 Pearl. 
 
 WE might vary and extend our experi- 
 ments on light indefinitely, and they cer- 
 tainly would prove us to possess a wonderful 
 mastery over the phenomena. But the ves- 
 ture of the agent only would thus be re- 
 vealed, not the agent itself. The human 
 mind, however, is so constituted and so edu- 
 cated as regards natural things, that it can 
 never rest satisfied with this outward view of 
 them. Brightness and freshness take pos- 
 session of the mind when it is crossed by tne 
 light of principles, which show the facts of 
 Nature to be organically connected. 
 
 Let us, then, inquire what this thing is 
 that we have been generating, reflecting, re- 
 fracting, and analyzing. 
 
 In doing this, we shall learn that the life 
 of the experimental philosopher is twoiold. 
 He lives, in his vocation, a life of the senses, 
 using his hands, eyes, and ears in his experi- 
 ments, but such a question as that now before 
 us carries him beyond the margin of the 
 senses. He cannot consider, much less an- 
 swer, the question, "What is light?" with- 
 out transporting himself to a world which 
 undelies the sensible one, and out of which, 
 in accordance with rigid law, all optical phe- 
 nomena spring. To realize this subsensible 
 world, if I may use the term, the mh,d must 
 possess a certain pictorial power. It has to 
 visualize the invisible. It must be able to 
 form definite images of the things which that 
 subsensiDle 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 phenomena are accounted for ; a physical 
 theory has been enunciated which unites anu 
 explains them all. 
 
 This conception of physical theory implies, 
 as you perceive, the exercise of the imagina- 
 tion. Do not be afraid of this word, which 
 seems to render so many respectable people, 
 
SIX LECTURES ON LIGHT. 
 
 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 
 
 of elastic collision. The fact of optical re- 
 flection certainly occurred as if Jight consist- 
 ed of elastic particles, and this was Newton's 
 
 that they have suffered themselves to be mis- I sole justification for introducing them. 
 
 led by the popular definition of a great i But this is not all. In anotln r important 
 
 faculty instead of observing its operation in f particular, also. Newton's conceptions re- 
 
 garding the nature of light were influenced 
 by his previous knowledge. He had been 
 working at the phenomena of eravitation 
 and had made himself at home amid the oper- 
 ations 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, 
 as it may, Newton saw in 
 
 action of an attractive force exerted 
 the light-particles. He carried his 
 
 the 
 on 
 
 Be that 
 refraction 
 
 their own minds. Without imagination we 
 
 canr.ot take a step beyond the bourne of the 
 
 mere animal world, perhaps not even to the 
 
 edge of this. But, in speaking thus of 
 
 Imagination, I do not mean a riotous power 
 
 which deals capriciously with facts, but a 
 
 ivell-ordered and disciplined power, whose 
 
 sole function is to form conceptions which 
 
 the intellect imperatively demands. Imagina- 
 tion thus exercised never really severs itself 
 from the world of fact. This is the store- 
 house from which all its pictures are drawn ; 
 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 subsen- 
 uble world.* 
 
 1 will take, as an illustration of this sub- 
 ject, the case of Newton. Before he began 
 to deal with light, he was intimately ac- 
 quainted with the laws of elastic collision, 
 which all of you have seen more or less per- 
 fectly illustrated on a billiard-table. As re- 
 gards the collision of sens'ble masses, New- 
 ton knew the angle of incidence to be equal 
 
 to the angle of reflection, and he also knew j the refraction seen in our last lecture. Final- 
 that experiment, as shown in our last lecture, j ty, it was supposed that differences of color 
 had established the same law with regard to might be due to differences in the sizes of the 
 
 KU*. T T A i r _i * i : .:^i T*I_ _ _ ^.i i- ; i ^.i . . r 
 
 conception out with the most severe con- 
 sistency. Dropping vertically downwards 
 towards the earth's surface, the motion 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 come close to the surface, was, 
 according to Newton, also accelerated. Ap- 
 proaching such a surface obliquely, he sup- 
 posed'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, 
 
 light. He thus found in his previous knowl- 
 edge the material for theoretic images. He 
 had only to change the magnitude of concep- 
 tions already in his mind to arrive at the 
 Emission Theory of Light. He supposed 
 light to consist of elastic particles of incon- 
 ceivable minuteness shot out with inconceiv- 
 able rapidity by luminous bodies. Such par- 
 ticles impinging upon smooth surfaces were 
 reflected in accordance with the ordinary law 
 
 * The following charming extract, bearing upon 
 this point, was discovered and written out lor me by 
 my friend, Dr. Bence Jones, Hon. Secretary to the 
 Royal Institution 
 
 "In every kind of magnitude there is a degree or 
 sort to which our sense is proportioned, the percep- 
 tion and knowledge of which is of the greatest use to 
 mankind. The same is the groundwork of philoso- 
 phy : for, though all sorts and degrees are equally 
 the object of philosophical speculation, yet it is irom 
 those which are proportioned to sense that a philoso- 
 pher must set out in his inquiries, ascending or de- 
 
 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 veri- 
 fications of theory occur in that world. Lay- 
 ing the theoretic conception at the root of 
 matters, we determine by rigid deduction 
 what are the phenomena which must of neces- 
 sity grow out of this root. If the phenomena 
 thus deduced agree with those of the actual 
 world, it is a presumption in favor of the 
 theory. If as new classes of phenomena arise 
 they also are found to harmonize with theo 
 retic deduction, the presumption becomes still 
 stronger. If, finally, the theory confers pro- 
 phetic vision upon the investigator, enabling 
 him to predict the existence of phenomena 
 which have never yet been seen, and if those 
 
 scendmg afterwards, as his pursuits may r< quire. He V ai V. n V HV en ' an . a l " 
 
 does well indeed to take his views from many points j predictions be found on trial to be rigidly cor- 
 
 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 observation of sensible 
 things, he must begin with these, and must often re- 
 turn to them to examine his progress by them. 
 Here is his secure hold ; and as he sets out from 
 thenc, 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. Newton's Philosophical Dis- 
 ctverits. Written 1728 ; second editivn^ 1750 ; pp. 
 
 A 19 
 
 rect, the persuasion of the truth of the theory 
 becomes overpowering. Thus working back- 
 wards from a limited number of phenomena, 
 genius, by its own expansive force, reaches a 
 conception which covers all the phenomena. 
 There is no more wondertul performance of 
 the intellect than this. And we can render 
 no account of ii. Like the scriptural gift of 
 the Spirit, no man can tell whence it cometh. 
 The passage from fact to principle is some- 
 
10 
 
 SIX LECTURES ON LIGHT. 
 
 times 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 intoxication. To any one who 
 has experienced this pleasure, even in a mod- 
 erate 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 theory of 
 Newton, when the deductions from it were 
 brought face to face with natural phenomena ? 
 To the mind's eye, Newton's elastic particles 
 present themselves like particles of sensible 
 magnitude. The same reasoning applies to 
 both ; the same experimental checks exist for 
 both. Tested by experiment, then, Newton's 
 theory was found competent to explain many 
 facts, and with transcendent ingenuity its 
 author sought to make it account for all. He 
 so far succeeded, that men so celebrated as 
 Laplace and Malus, who lived till 1812, and 
 Biot and Brewster, who lived till our own 
 time, were found among his disciples. 
 
 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 lound is conveyed in waves or 
 pu'ses through the air ; and no sooner was this 
 truth well housed in the mind than it was trans- 
 formed into a theoretic conception. It 
 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 waves of sound we have the 
 air of our atmosphere ; but the stretch of im- 
 agination which filled all space with a luminif- 
 erous 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 other 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 indeed applied to various phenomena of 
 optics by the celebrated astronomer, Huy- 
 ghens. It was espoused and defended by the 
 celebrated mathematician, Euler. They were, 
 however, opposed by Newton, whose authority 
 Or shall I say 
 Not quite so. 
 
 Newton's preponderance was in some degree 
 due to the fact that, though Huyghe::s and 
 Euler were right in the main, tiiey did not 
 possess sufficient data to prove themselves 
 right. No human authority, however high, 
 
 at -he time bore them down. 
 it was authority merely ? 
 
 can maintain itself against the voice of Natuie 
 speaking through experiment. But the voice 
 of Nature may be an uncertain voice, through 
 the scantiness of data. This was the case at 
 the period now referred to, and at such a pe- 
 riod by the authority of Newton all antago- 
 nists were naturally overborne. 
 
 Still, this great Emission Theory, which 
 held its ground so long, resembled one of 
 those circles which, according to your coun- 
 tryman Emerson, the force of genius periodi- 
 cally draws round the operations of the in- 
 tellect, but which are eventually broken 
 through by pressure from behind. In the 
 year 1773 was born, at Milverton, in Somer- 
 setshire, one of the most remarkable men that 
 England ever produced. He was educated 
 for th profession of a phvsician, but was too 
 strong to be tied down to professional routine. 
 He devoted himself 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 fcil to the lot of this man to dis- 
 cover 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 pre 
 pared to detect any resemblance which might 
 reveal itself between the phenomena of light 
 and those of wave^motion. Such resem- 
 blances 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 nsme, perhaps, unfamiliar 
 to many of you. Permit me, by a kind of 
 geometrical construction which I once em- 
 ployed 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 line from Newton to Youns.,, which 
 shall form a tangent to the heads of both. 
 This line would slope downwards from New. 
 ton to Young, because Newton was certainly 
 the taller man of the two. But the slope 
 would not be steep, for the difference of stat- 
 ure was not excessive. The line would form 
 what engineers call a gentle gradient from 
 Newton to Young. Place underneath this 
 ine the biggest man born in the interval 
 Between both. He would not, in my opinion, 
 reach the line ; for if he did he would be 
 aller intellectually than Young, and there 
 \'as, I believe, none taller. But I do not 
 ant you to rest on English estimates of 
 young; the German, Helmholtz, a kindred 
 genius, thus speaks of him : " His was one 
 
SIX LECTURES ON LIGHT. 
 
 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 ex- 
 cited 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 
 ide s lay, therefore, buried and forgotten in 
 the folios of the Royal Society, until a new 
 'generation gradually and painfully made the 
 same discoveries, and proved the exactness 
 of his assertions and the truth of his demon- 
 strations." 
 
 It is quite true, as Helmholtz says, that 
 v oung was in advance of his age ; but some- 
 thing 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 countrymen deemed in fact a dreamer, 
 through the vigorous audacity of a writer 
 who had then possession of the public ear, 
 and who in the Edinburgh Review poured 
 ridicule upon Young and his speculations. 
 To the celebrated Frenchmen, Fresnel and 
 Arago, he was first indebted for the restitu- 
 tion of his rights, for they, especially Fresnel, 
 remade independently, as Helmholtz says, 
 and vastly extended his discoveries. To the 
 students of his works Young has long since 
 appeared in his true light, but these twenty 
 blank years pushed him from the public 
 mind, which became in turn filled with the 
 fame of Young's colleague at the Royal In 
 stitution, Davy, and afterwards with the 
 fame of Faraday. Carlyle refers to the re- 
 mark of Novalis, that a man's self-trust is 
 enormously increased the moment he finds 
 that others believe 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 investigator. 
 It remains to be stated thai his assailant was 
 Mr. Henry Brougham, afterwards Lord 
 Chancellor of England. 
 
 Our hardest work is now before us. And, 
 as I have often had occasion to notice that 
 capacity for hard work depends in a great 
 measure on the antecedent winding up of the 
 will and determination, I would call upon 
 you to gird up your loins for our coming 
 labors. If we succeed in climbing the hill 
 which faces us to-night, our future efforts 
 will be comparatively light. 
 
 In the earliest writings of the ancients we 
 find the notion that sound is conveyed by the 
 air. Aristotle gives expression to this no- 
 tion, and the great architect Vitruvius com- 
 pares 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 ci ntral 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 seiae 
 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 suc- 
 cession 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 after- 
 wards they become the front of the succeed- 
 ing 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 for- 
 ward, but simply rocked up and down. The 
 propagation of a wave is the propagation of 
 a form, and not the transference of the sub- 
 stance 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 oscillation. You 
 will notice that in this description the parti- 
 cles of water are made to vibrate across the 
 line of propagation.* 
 
 And now we have to take a step forward, 
 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 other. 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 ail the motions 
 imparted to it. If crest coincide with crest, 
 the wave is lifted to a double height; if fur- 
 row coincide with crest, the motions are in 
 opposition, and thei ; sum is zero. We have 
 then still water, which we shall learn pres- 
 ently corresponds to what we call darkness in 
 reference to our present subject. This action 
 of wave upon wave is technically called in- 
 terference, a term to be remembered. 
 
 Thomas Young's fundamental discovery in 
 optics was that the principle of Interference 
 applied to light. Long prior to his time, an 
 Italian philosopher, 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 
 
 * I do not wish to encumber the conception here 
 with the details of the motion, but I may draw atten- 
 tion to the beautiful model of Prsfessor Lyman, 
 
 water-waves. 
 
SIX LECTURES ON LIGHT. 
 
 quenched each other and darkened the spot. 
 This was a statement of fundamental signifi- 
 cance, but it required the discoveries and the 
 genius of Young to give it meaning. How 
 he did so, I will now try to make 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 ren- 
 dered more rare. Properly agitated, a tun- 
 ing-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 par- 
 celled out into spaces in which the air is con- 
 densed, 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 u series of such waves, and you 
 can imagine a second series sent through the 
 same air, and so related to the first that con- 
 densation coincides with condensation and 
 rarefaction with rarefaction. The conse- 
 quence of this coincidence would be a louder 
 sound than that produced by either system of 
 waves taken singly. But you can also ima- 
 gine a state of things where the condensa- 
 tions of the one system fall upon the rarefac- 
 tions 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, pro- 
 duce no sound. Thus, by adding sound to 
 sound we produce silence, as Grimaldi in his 
 experiment produced darkness by adding 
 light to light. 
 
 The analogy between sound and light here 
 at once flashes upon the mind. Young gen 
 eralized this observation. He discovered a 
 multitude of similar cases, and determined 
 their precise conditions. On the assumption 
 that light was wave-motion, all his experi- 
 ments on interference were explained ; on the 
 assumption that light was. flying particles, 
 nothing was explained. In the time of Huy- 
 ghens 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 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 differ- 
 ence between t.e waves of light and those of 
 sound. Could you see the air through which 
 sound-waves are passing, you would observe 
 every individual particle of air oscillating to 
 and fro in the direction of propagation. 
 Could you see the ether, you would also find 
 every individual particle making a small ex- 
 cursion to and fro, but here the motion, like 
 that assigned to the water-particles above re- 
 ferred to, would be across the line of propa- 
 
 gation. The vibrations of the air are longi- 
 tudinal, the vibrations of the ether are trans- 
 versal, 
 
 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. They are parts of a spiral, drawn upon 
 a circle of blackened glass, and, when the cir- 
 cle rotates, the spots move in successive pulses 
 over the screen. You have here clearly set 
 before you how the pulses travel incessantly 
 forward, while the particles that compose 
 them perform oscillations to and fro. This 
 is the picture of a sound-wave, in which the 
 vibrations are longitudinal. By another glass 
 wheel, we produce an image of a trans- 
 verse wave, and here we observe the waves 
 travelling in succession over the screen, while 
 each individual spot of light performs an ex- 
 cursion to and fro across the line of propaga- 
 tion. 
 
 Notice what follows when the glass wheel 
 is turned very quickly. Objectively consid- 
 ered, the transverse waves propagate them- 
 selves as before, but subjectively the effect is 
 totally changed. Because of the retention of 
 impressions upon the retina, the spots of light 
 simply describe a series of parallel luminous 
 lines upon the screen, the length of these 
 lines marking the amplitude of the vibration. 
 The impression of wave-motion has totally 
 disappeared. 
 
 The most familiar illustration of the inter- 
 ference of sound-waves is famished by the 
 beats produced by two musical sounds slightly 
 out of unison. These two tuning-forks are 
 now in perfect unison, and when they are 
 agitated together the two sounds flow without 
 roughness, as if they were but one. But, by 
 attaching to one of the forks a two-cent piece, 
 we cause it to vibrate a little more slowly 
 than its neighbor. Suppose that one of them 
 performs 101 vibrations in the time required 
 by the other to perform 100, and suppose 
 that at starting the condensations and rare- 
 factions of both forks coincide. At the loist 
 vibration of the quickest fork they will again 
 coincide, the quicker fork at this point hav- 
 ing gained one whole vibration, or one whole 
 wave upon the other. But a little reflection 
 will make it clear that, at the $oth vibration, 
 I the two forks are in opposition ; here the one 
 tends to produce a condensation where the 
 other tends to produce a rarefaction; 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. 
 I At the loist vibration we have again coinci 
 dence, and, therefore, augmented sound; at 
 | the isoth vibration we have again a quench- 
 j ing of the sound. Here the one fork is three 
 I half-waves in advance of the o: her In gen- 
 ! eral terms, the waves conspire when the one 
 series is an even number of half- wave lengths, 
 
SIX LECTURES ON LIGHT. 
 
 and they are destroyed when the one series is 
 an odd number of half-wave lengths in ad- 
 vance of the other. With two forks so cir- 
 cumstanced, we obtain those intermittent 
 shocks of sound separated by pauses of si- 
 lence, to which we give the name of beats. 
 
 I new wish to show you what may be 
 called the optical expression of those beats. 
 Attached to a large tuning-fork, F (Fig. 2), 
 is a small mirror, which shares the vibrations 
 of the fork, and on to the mirror is thrown a 
 thin beam of light, which shares the vibra- 
 tions of the mirror. The beam reflected 
 from the fork is received upon a piece of 
 looking-glass, and thrown back upon the 
 screen, where it stamps itself as a small lu- 
 minous disk. The agitation of the fork by a 
 violin-bow converts that disk into a band of 
 light, and if yo i simp.y move your heads to 
 and fro you cause the image of the band to 
 sweep over the retina, drawing it out to a sin- 
 uous line, thus proving the periodic character 
 of the motion which produces it. By a sweep 
 of the looking-glass, we can also cover the 
 screen from side to side by a luminous scroll, 
 in n, Fig 2, the depth of the sinuosities indi- 
 cating the amplitude of the vibration. 
 
 band of light gradually shortening as the vo 
 tiori subsides, until, when the motion ceases, 
 : we hare our luminous disk restored. Weight- 
 I ing one of the forks as we did before, with a 
 two-cent piece, sometimes the fork'] conspire, 
 and then you have the band of light drawn 
 out to its maximum length ; sometimes they 
 oppose each other, and then you have thi' 
 band of light diminished to a circle. Thus, 
 the beats which address the ear express them- 
 selves optically as the alternate elongation and 
 shortening of the band of light. If I move 
 the mirror of this second fork, you have a 
 sinuous line, as before ; but the sinuosities 
 are sometimes deep, and sometimes they al- 
 most disappear, as in Fig. 3, thus expressing 
 the alternate increase and diminution of the 
 sound, the intensity of which is expressed by 
 the depth of the sinuosities. To Lissajous w 
 owe this mode of illustration. 
 
 Instead of receiving the beam reflected from 
 the fork on a piece of looking-glass, we now 
 receive it upon a second mirror attached to a 
 second fork, and cast by it upon the screer. 
 Both forks now act in combination upon the 
 beam. The disk is drawn out, as befoie, the 
 
 The/VWfc of a sound is w 
 by the rapidity of the vibration, * tLe in fen- 
 si ty is by the amplitude. The rise of pitch 
 with the rapidity of the impulses may be illus- 
 trated by the syren, which consists of a per- 
 forated disk rotating over a cylinder into 
 which air is forced, and the end of which is 
 also perforated. When the perforations of 
 the disk coincide with those of the cylinder, a 
 puff escapes ; and, when the puffs succeed 
 each other with sufficient rapidity, the im- 
 pressions upon the auditory nerve link them, 
 selves together to a continuous musical note. 
 The more rapid the rotation of the disk the 
 quicker is the succession of the impulses, and 
 the higher the pitch of the note. Indeed, by 
 
14 
 
 SIX LECTURES ON LIGHT. 
 
 meins of the syren the number of vibrations 
 due to any musical no e, whether it be that of 
 an instrument, of the human voice, or of a 
 flying insect, may be accurately determined. 
 
 In front of our lamp now stands a very 
 homely instrument, S, Fig- 4, of this charac- 
 ter. The perforated disk is turned by a 
 wheel and band, and, when the two sets of 
 perforations coincide, a series of spots of 
 light, sharply defined by the lens L, ranged 
 on the circumference of a circle, is seen upon 
 the screen. On slowly turning the disk, a 
 flicker is produced by the alternate stoppage 
 and transmission of the light. At the same 
 time air is urged into the syren, and you hear 
 a fluttering sound corresponding to the flick- 
 ering light. But, by augmenting the rapid- 
 ity of rotation, the light, though intercepted 
 us before, appears perfectly steady, through 
 the persistence of impressions upon the 
 retina ; and, about the time when the optical 
 impression becomes continuous, the auditory 
 impression becomes equally so ; the puffs 
 from the syren linking themselves then to- 
 gether to a continuous musical note, which 
 rises in pitch with the rapidity of the rota- 
 tion. A movement of the head causes the 
 image of the spots to sweep over the retina, 
 producing beaded lines : the same effect is 
 produced upon our screen by the sweep of a 
 looking glass which has received the thin 
 beams from the syren. 
 
 In the undulatory theory, what pitch is to 
 the ear, color is to the eye. 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 accurately deduced. 
 This may, moreover, be done in many ways, 
 and., when the different determinations are 
 
 compared, the strictest harmony is found t 
 exist between them. The shortest waves of 
 the visible spectrum are those of the extreme 
 violet ; the longest, those of the extreme 
 red ; while the other colors are of intermedi- 
 ate 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 num- 
 bers, is 190,000 miles per second. Reducing 
 this to inches, and multiplying the number 
 .hus found by 36,918, we obtain the number 
 of waves of the extreme red in 190,000 miles. 
 All these waves enter the eye, and hit the 
 retina at the back of the eye in one second. 
 The number of shocks per second necessary 
 to the production of the impression of red is, 
 therefore, four hundred and fifty-one millions 
 of millions. In a similar manner, it may be 
 found that the number of shocks correspond- 
 ing to the impression of violet is seven hun- 
 dred and eighty-nine millions of millions. 
 All space is rilled with matter oscillating at 
 such rates. From every star waves of these 
 dimensions move with the velocity of light 
 like spherical shells outwards. And in the 
 ether, just as in the water, the motion of 
 every particle is the algebraic sum of all the 
 separate motions imparted to it. Still, one 
 motion does not blot the other out ; or, if 
 extinction occur at one point, it is atoned for 
 at some other point. Every star declares by 
 its light its undamaged individuality, as if it 
 alone had sent its thrills through space. 
 
 The principle of interference applies to the 
 waves of light as it does to the waves of 
 water and the waves of sound. And the 
 condLions 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, crest coincides with crest, 
 sinus with sinus, and the two systems blend 
 together to a single system of double ampli- 
 tude. If both series start at the same mo- 
 ment, one of them being, at starting, a whole 
 wave-length in advance of the other, they 
 also add themselves together, and we have 
 an augmented luminous effect. Just as in 
 the case of sound, 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, 
 or any odd number of half wave-lengths in 
 advance, then the crests of the one fail 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 their joint action the 
 ether remains perfectly still This stillness 
 of the ether is what we call darkness, which 
 corresponds, as already stated, with a dead 
 level in the case of water. 
 
 It was said in our first lecture, with refer- 
 ence to the colors produced by absorption, 
 
SIX LECTURES ON LIGHT. 
 
 15 
 
 tfcat the function of natural bodies is selec- 
 tive, not creative ; that they extinguish cer- 
 tain constituents of the white solar light, and 
 appear in the colors 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-extin- 
 quished, we may have in it the conditions 
 for the production of color. But 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. 
 
 The subject is most easily illustrated by the 
 class of phenomena which first suggested the 
 undulatory theory to the mind of Hooke. 
 These are the colors of thin films of all kinds, 
 which are known as the colors of thin plates. 
 In this relation no object in the world pos- 
 sesses a deeper scientific interest than a com- 
 mon soap-bubble. And here let me say 
 emerges one of the difficulties which the stu- 
 dent of pure science encounters in the pres- 
 ence of "practical" communities like those 
 of America and England ; it is not to be ex- 
 pected that such communities can entertain 
 any profound sympathy with labors which 
 seem so far removed from the domain of 
 practice as many of the labors of the man of 
 science are. Imagine Dr. Draper spending 
 his days in blowing soap-bubbles and in 
 studying their colors ! Would you show him 
 the necessary patience, or grant him the nec- 
 essary support ? And yet, be it remembered, 
 it was thus that Newton spent a large portion 
 of his time ; and that on such experiments 
 has been founded a theory, ths 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 labors you ought to abide. 
 
 Whence, then, are derived the colors of the 
 soap-bubble ? Imagine abeam of white light 
 impinging on the bubble. When it reaches 
 the first surface of the film, a known traction 
 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 re- 
 flected. 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 inter- 
 fere 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-ex- 
 tinction in the case of the longer waves, a 
 greater thickness of film is necessary than in 
 the case of the shorter ones. Different colors, 
 therefore, appear at different thicknesses of 
 the film. 
 
 Take with you a little bottle of spirit of 
 turpentine, and pour it into one of the ponds 
 in the Central Park. You will then see the 
 
 flashing of those colors over the surface of 
 the waier. 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 i uccession to the surface. 
 On reaching it, each drop spreads suddenly 
 out as a film, and glowing colors imm.ediately 
 flash forth upon the screen. The colors 
 change as the thickness of the film changes 
 by evaporation. They are also arranged in 
 zones in consequence of the gradual diminu- 
 tion of thickness from the centre outwards. 
 
 Any film whatever will produce these colors. 
 The film of air between two plates of window- 
 glass, squeezed together, exhibits rich fringes 
 of color. Nor is even air necessary ; the 
 mere rupture of optical continuity suffices. 
 Smite with an axe the black, transparent ice 
 black, because it is transparent 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 colors 
 of thin plates sometimes break like fire. The 
 colors are commonly seen in flawed crystals ; 
 they are also formed by the film of oxide 
 which collects upon molten lead. It is the 
 colors of thin plates that guide the tempering 
 of steel. But the origin of most historic in- 
 terest is, as already stated, the soap-bubble. 
 With one of those mixtures employed by the 
 eminent blind philosopher Plateau in his re- 
 searches on the cohesion figures of thin films, 
 we obtain in still air a bubble twelve or fifteen 
 inches in diameter. You may look at the 
 bubble itself, or you may look at its projec- 
 tion upon the screen, rich colors arranged in 
 zones are, in both cases, exhibited. Render- 
 ing the beam parallel, and permitting it to 
 impinge upon the sides, bottom, and top of 
 the bubble, gorgeous fans of color overspread 
 the screen, which rotate as the beam is carried 
 round the circumference of the bubble. By 
 this experiment the internal motions of the 
 film are also strikingly displayed. 
 
 Newton sought to measure the thickness of 
 he bubble corresponding to each of these 
 colors ; in fact, he sought to determine gen- 
 erally the relation of color to thickness. His 
 first care was to obtain a film of variable and 
 calculable depth. On a plano-convex glass 
 ens of very feeble curvature he laid a plate of 
 glass with a plane surface, thus obtaining a 
 ihn of air of gradually increasing depth from 
 the point of contact outwards. On looking at 
 he film in monochromatic light he saw sui- 
 -ounding the place of contact a series of bright 
 ings separated from each other by dark ones, 
 and becoming more closely packed together as 
 the distance from the point of contact aug- 
 mented. When he employed red light, his 
 ings had certain diameters ; when he em- 
 ployed blue light, the diameters were less. 
 Causing his glasses to pass through the spev 
 
SIX LECTURES ON LIGHT- 
 
 trurn from red to blue, the rings contracted ; 
 when the passage was from blue to red, the 
 rings expanded. When white light fell upon 
 the glasses, inasmuch as the colors were not 
 superposed, a series of iris-colored circles were 
 obtained. They became paler as the film be- 
 came thicker, until finally ihe colors became 
 so intimately reblended as to produce white 
 light. A magnified image of Newton's rings 
 is now before you, and, by employing in suc- 
 cession red, blue, and white lig"ht, we obtain 
 all the effects observed by Newton. 
 
 He compared the tints thus obtained v ith 
 the tints of the soao-bubble, and he calcu- 
 lated the corresponding thickness. How he 
 did this may be thus made plain to you : 
 Suppose the water of the ocean to be abso- 
 lutely smooth ; it would then accurately repre- 
 sent the earth's curved surface. Let a per- 
 fectly horizontal 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 lev- 
 elling operations, to allow for the curvature 
 of the earth. Newton's calculation was pre- 
 cisely similar. His plane glass was a tan- 
 gent 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 
 distances of his rings from the place of con- 
 tact, 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, couid have 
 worked with such marvellous exactitude. 
 
 To account for these rings was the great- 
 est difficulty that Newton ever encoun- 
 tered. 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 ques- 
 tion, Why, when a beam of light is incident 
 on a transparent body, are some of the light- 
 particles reflected and some transmitted ? 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 from one piece of 
 glass to fall upon another, it, as a general 
 rule, is also divided into a reflected and a 
 transmitted portion. Thus 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 ex 
 plain why it is that the self-same particle is 
 at one moment reflected and at the next mo- 
 ment transmitted. It could only be through 
 change in the condition of the particle 
 
 itself. The self-same particle, he affirmed, 
 was affected by " fits" of easy transmission 
 and reflection. 
 
 If you are willing to follow me while I un- 
 ravel this theory of fits, the most subtle, per- 
 haps, that ever entered the human mind, the 
 intellectual discipline will 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 cannot be a doubt 
 that his mind rested on a mechanical cause. 
 Nor can there be a doubt that, as in all 
 attempts at theorizing, he was compelled to 
 fall back upon experience for the materials of 
 his theory. His course of observation ana 
 of thought may have been this : From a 
 magnet he might obtain the notion of at- 
 tracted and repelled poles. What more 
 natural than that he should endow his light- 
 particles with such poles ? Turning their 
 attracted poles towards a transparent sub- 
 stance, the particles would be sucked in and 
 transmitted ; turning their repelled poles, 
 they would be driven away or reflected. 
 Thus, by the ascription of poles, the trans- 
 mission and reflection of the self-same parti- 
 cle at different times might be accounted 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, a? 
 I have said, measured the thickness corre- 
 sponding to every ring and -howed the differ- 
 ence of thickness between ring and ring. 
 Now, mark the result. For the sake of con- 
 venience, let us call the thickness of the film 
 of air corresponding to the first dark ring d, 
 then Newton found the distance correspond- 
 ing to the second dark ring 2 d; the thick 
 ness 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 meaning in this little distanced, 
 which turns up so constantly ? One can im- 
 agine the intense interest with which Newtoi* 
 pondered its meaning. Observe the probably 
 outcome of his thought. He had endowed 
 his light-particles with poles, but now he is 
 forced to introduce the notion of periodic re- 
 currence. How was this to be done ? JRy 
 supposing the light-particles animated, not 
 only with a motion of translation, but 
 also with a motion of rotation. Newton's 
 astronomical knowledge would render all such 
 conceptions familiar to him. The earth has 
 such a motion. In the time occupied in pass- 
 ing 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-particle must be supposed to 
 perform a complete rotation. True, the 
 light-particle is smaller than the planet and 
 the distance d. instead of being a n?i!!icm. snd 
 half of miles, is a little over the ninety-. 
 
SIX LECTURES ON LIGHT. 
 
 17 
 
 thousandth of an inch. But the two con- 
 ceptions are, in point of intellectual quality, 
 identical. 
 
 Imagine, then, a particle entering the film 
 of air where it possesses this precise thick- 
 ness. . 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 opposite side of the film and be lost to 
 the eye. All round the place of contact, 
 wherever the film possesses this precise thick- 
 ness, the light will equally disappear we 
 shall have a ring of darkness. 
 
 And now observe how well this conception 
 falls in with the law of proportionality dis- 
 covered by Newton. When the thickness of 
 the film is 2 d, the particle has time to per- 
 form 
 
 film ; when the thickness is 3 d y three com 
 plete somersaults ; when 10 d, ten complete 
 somersaults are performed. It is manifest 
 that in each of these cases, on arriving at the 
 
 sumed that the action which produces the al- 
 ternate bright and dark rings took place at a 
 single siirface ; that is, the second surface of 
 the film. The undulatory theory affirms 
 that the rings are caused by fie interference 
 of waves reflected from both surfaces. This 
 also has been demonstrated by experiment. 
 By proper devices we may abolish reflection 
 from one of the surfaces of the film, and 
 when this is done the rings vanish altogether. 
 
 Rings of feeble intensity are also formed by 
 transmitted light. These are referred by thft 
 undulatory theory to the interference of, 
 waves which have passed directly through the 
 film, with others w ; .ich have suffered two re- 
 /lections within the film. They are thus com- 
 pletely accounted for. 
 
 Newton, by the foregoing exceedingly 
 subtle assumption, vaulted over the difficulty 
 
 two complete somersaults within the I presented by the colors of thin plates. And, 
 
 as further difficulties in process of time thick- 
 ened round the theory, his disciples tried to 
 sustain it with an ingenuity worthy of their 
 master. The new difficulties were not an- 
 
 second surface of the film, the attracted pole 
 of the particle will be presented. It will, 
 therefore, 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 
 
 corresnond h<incr a1;r intprmprliarp h^tw^en I 
 
 ticipated by the theory, but were met by new 
 assumptions, until at length the Emission 
 Theory became what a distinguished writer 
 calls a " mob of hypotheses." In the pres- 
 ence of the phenomena of interference, the 
 theory finally broke down, while the whole of 
 these phenomena lie, as it were, latent in the 
 theory of undulation. Newton's 
 
 fits," for 
 
 correspond being also intermediate between j example, are immediately translatable into 
 those of the dark ones. Take the case of the the lengths of the ether-waves. We have 
 first bright ring. The thickness of the film 
 is y z d; in this interval the rotating particle 
 can perform only half a rotation. When, 
 therefore, it reaches the second surface of the 
 film, its repelled pole is presented ; it is, 
 therefore, driven back and reaches the eye. 
 At all distances round the centre correspond- 
 i-g to this thickness the same effect is pro- 
 
 duced, and the consequence is a ring of 
 brightness. The other bright rings are sim- 
 ilarly accounted for. At the second one, 
 where the thickness is l d, a rotation and 
 
 a half is performed ; at the third, two rota- 
 tions and a half ; and at each of these places 
 the particles present their repelled poles to 
 the lower surface of the film. They are there- 
 fore sent back to the eye, producing the im- 
 pression of brightness. Here, then, we hayj 
 unravelled the most subtle application that 
 Newton ever made of the Emission Theory. 
 
 It has been stated in the early part of this 
 lecture, that the Emission Theory assigned a 
 greater velocity to light in glass and water, 
 than in air or stellar space. Here it was at 
 direct issue with the theory of undulation, 
 which makes the velocity in air or stellar 
 space less than in glass or water. By an ex- 
 periment proposed by Arago, and executed 
 with comsummate skill by Foucault and 
 Fizeau, this question was b -ought to a crucial 
 test, and decided in favor of the theory of 
 undulation. In the present instance also the 
 two theories are at variance. Newton as- 
 
 the observed periodic recurrence as the thick- 
 ness varies so as to produce a retardation of 
 an odd or even number of semi-undulations.* 
 Numerous other colors are due to interfer- 
 ence. 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 same furrow, interfere 
 with and quench each other. But the ob- 
 liquity of reflection which extinguishes the 
 shorter waves does not extinguish the longer 
 ones, hence the phenomena of color. These 
 are called the colors of striated surfaces. 
 They are well illustrated by mother-of-pearl. 
 This shell is composed of exceedingly thin 
 layers, which, when cut across by the polish- 
 ing of the shell, expose their edges and fur- 
 nish the necessary small and regular grooves. 
 The most conclusive proof that the colors are 
 due to the mechanical state of tr.e surface is 
 to be found in the fact, established by Brew- 
 ster, that, by stamping the shell carefullv 
 
 * In the explanation of Newton's rings, something 
 besides thickness is to be taken into account. In the 
 case of the first surface of the film of air, the waves 
 pass from a denser to a rarer medium, while in the 
 case of the second surface, the waves pass from a 
 rarer to a denser medium. This difference at the 
 two reflecting surfaces can be proved to be equivalent 
 to the addition of half a -wave-length, to the thick- 
 ness of the film. To the absolute thickness, as de- 
 termined by Newton, half a wave-length is in each 
 case to be added. When this is done, the dark and 
 bright rings follow each other in exact accordance 
 with the law of interference already enunciated. 
 
SIX LECTURES ON LIGHT. 
 
 upon black sealing-wax, we transfer the 
 grooves, and produce upon the wax the colors 
 of -nother-of-pearl. 
 
 LECTURE III. 
 
 Relation 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 : Archi- 
 tecture of Crystals considered as an Introduction to 
 their Action upon Light: Notion of Atomic Po- 
 larity applied to Crystalline Structure : Experi- 
 mental Illustrations: Crystallization of Water: 
 Expansion by Heat and by Cold : Deportment of 
 VVater considered and explained: Molecular Ac- 
 tion illustrated by a Model: Force of Solidifica- 
 tion: Bearings of Crystallization on Optical Phe- 
 nomena : Refraction : Double Refraction : Po- 
 larization : Action of Tourmaline: Character of 
 the Beams emergent from Iceland Spar : Polariza- 
 tion by ordinary Refraction and Reflection : De- 
 polarization. 
 
 IN our last lecture we sought to familiarize 
 our minds with the characteristics of wave- 
 moiion. We drew a clear distinction between 
 the motio > of the wave itself and the motion 
 of its constituent particles. Passing through 
 water-waves and air-waves, we prepared our 
 mi ds for the conception of light-waves prop- 
 agated through the luminiferous ether. The 
 analogy of sound will fix the whole mechan- 
 ism in your minds. Here we have a vibrat- 
 vig body which originates the wave motion, 
 ,ve have, in the air, a vehicle which conveys 
 it, and we have the auditory nerve which re- 
 ceives the impressions of the sonorous waves. 
 In the case of light we have in the vibrating 
 atoms of the luminous body the originators of 
 the wave-motion, we have in the ether its 
 vehicle, while the optic nerve receives the im- 
 pression of the luminiferous waves. We 
 learned, also, that color is the analogue of 
 pitch, that the rapidity of atomic vibration 
 augmented, and the length of the ether-waves 
 decreased, in passing from the red to the blue 
 end of the spectrum. The fruitful principle 
 of interference we also found applicable to 
 the phenomena of light ; and we learned that, 
 in consequence of the different lengths of the 
 ether-waves, they were extinguished by dif- 
 ferent thicknesses of a transparent film, the 
 particular thickness which quenched one color 
 glowing, therefore, with the complementary 
 one. Thus the colors of thin plates were ac- 
 counted for. 
 
 But one of the objects of our last lecture, 
 and that not the least important, was to illus- 
 trate the manner in which scientific theories 
 are iormed. They, in the first place, take 
 their rise in the desire of the mind to pene- 
 trate tocthe sources of phenomena. This de- 
 sire has long been a part of human nature. 
 It prompted Caesar to say that he would ex- 
 change his victories for a Himpse of the 
 sources of the Nile ; it may be seen working 
 in Lucretius ; it impels Darwin to those dar- 
 fag speculations which of late years have so 
 ajrtnted the public mind. We have learned 
 tb.*i ' framing theories the imagination does 
 
 not create, but that it expands, diminishes, 
 moulds, and refines, as the case may be, 
 mater als 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 sub- 
 sensible medium, the ether, are presented to 
 the mind. But no theory escapes the condi- 
 tion. Newton took care not to encumber 
 gravitation with unnecessary physical concep- 
 tions ; but we have reason to 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 reve- 
 lation dissevered from the world of experi- 
 ence. 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 till further ; but here 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 u~ Vrht upon a table, and 
 bringing a magne needle near its bottom, 
 one end of the n*?'-"*: promptly retreats from 
 the magnet, v * the other as prom pi ly 
 approaches. leedle is held quivering 
 
 there by some ..x visible influence exerted 
 upon it. Raising the needle along the mag- 
 net, but still avoiding contact the rapidity 
 of its oscillations decreases, because the force 
 acting upon it becomes weaker. At the 
 centre the oscillations cease. Above the 
 centre, the end of the needle which had been 
 previously drawn towards the magnet re- 
 treats, and the opposite end approaches. As 
 we ascend higher, the oscillations become 
 mere violent, because the force becomes 
 stronger. At the upper end of the magnet, 
 
 It 
 
 FIG. 5. 
 
 as at the lower, the force reaches a maximum?. 
 \ UL all the lower half of the ma;.- net, froi.l 
 K to S (Fig. 5), attracts one end of the 
 
SIX LECTURES ON LIGHT. 
 
 19 
 
 needle, while all the upper half, from E to 
 N, attracts the opposite end. This double- 
 ness of the magnetic force is called polaritv, 
 and the points near the ends of the magnet in 
 which the forces seem concentrated re called 
 its pole s. 
 
 What, then, will occur if we break this 
 magnet in two at the centre E ! Will each 
 of the separate halves act as it did when it 
 formed part of the whole magnet ? No , 
 each half is in itself a perfect magnet, pos 
 sessing two poles. This may be proved by 
 breaking something of less value than the 
 magnet the steel of a lady's stays, for ex- 
 example, hardened and magnetized. It acts 
 like the magnet. When broken, each half 
 acts like the whole ; and when 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 
 limit ; you will be driven to prolong your 
 
 rection of the needle, and no other. A needle 
 of iron will answer as well as the magnetic 
 needle ; for the needle of iron is magnetized 
 by the magnet, and acts exactly like a needle 
 independently magnetized. 
 
 If we place two or more needles of iron 
 near the magnet, the action becomes more 
 complex, for the the iron 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, ar- 
 ranging themselves in definite forms, in obe- 
 dience to the magnetic action. 
 
 Placing a sheet of paper or glass over this 
 bar magnet and showering iron filings upon 
 the paper, I notice a tendency of the filings 
 io 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 th* 
 
 FIG. 6. 
 
 N is the nozzle of the lamp ; M a plane mirror, reflecting the beam upwards. At P, tle magnets and 
 iron tilings are placed ; L is a lens which forms an image of the magnets and filings ; and R is a total- 
 ly-reflecting prism which casts the image, G, upon the screen. 
 
 paper: each tap releases them for a moment! 
 and enables them to follow their bias. Bu 
 this is an experiment which can only be seen 
 by myself. To enable you to see it, I take a 
 pair of small magnets and by a simple optical 
 arrangement throw the images of the mag- 
 nets upon the screen. Scattering iron filings 
 over the glass plate to which the small magnets 
 are attached, and tapping the i^late, you see 
 the arrangement of the iron filings in those 
 magnetic curves which have been so long 
 familiar to scientific men.* 
 
 vision beyond that limit, and to contemplate 
 this thing that we call magnetic polarity as 
 resident in the ultimate particle of the mag- 
 net. Each atom is endowed with this polar 
 force. 
 
 Like all other forces, this force of magnet- 
 ism is amenable to mechanical laws ; and 
 knowing the direction 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 posi- 
 tion 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 di- 
 
 *Very beautiful specimens of these curves have 
 b^en recently obtained, and fixed, by Prof. May^r, 
 of Hoboken 
 
SIX LECTURES ON LIGHT. 
 
 The aspect of these curves so fascinated 
 Faraday that the greater portion of his intel- 
 lectual life was devoted to pondering ove 
 them. He invested the space through whic 
 they run with a kind of -materiality ; and th 
 probability is, that the progress of science b 
 connecting the phenomena of magnetisn 
 with the luminiferous ether, will prove thes 
 " lines of force," as Faraday loved to ca 
 the magnetic curves, to represent a conditio 
 of this mysterious substratum of all radian 
 action. 
 
 But it is not with the magnetic curves, a 
 such, that I now wish to occupy your atten 
 tion ; it is their relationship to theoretic con 
 ceptions that we have now to consider. B) 
 the action of the bar magnet urJon the needle 
 we obtain the notion of a polar forc^ ; by th 
 breaking of the strip of magnetized steel, w 
 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 aro attrac- 
 tive and two repulsive. The attractive poles 
 approach, the repulsive poles retreat ; the 
 consequence being a certain definite arrange- 
 ment of the particles with reference to each 
 other. 
 
 Now, this idea of structure, as produced 
 by polar force, opens a way for the intellect 
 into an entirely new region, and the reason you 
 are asked to accompany me into this region 
 is, that our next inquiry relates to the action 
 of crystals upon light. Before I speak of 
 this action, I wish you to realize 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 agglomera- 
 tion of pieces, which, on examination, pre- 
 sent 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, speci- 
 mens of beryl, topaz, emerald, tourmaline, 
 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 ob- 
 serve, are put together according 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 split 
 in certain directions before a knife-edge, ex- 
 p.sing smooth and shining surfaces, which 
 are called planes of cleavage ; and by follow- 
 ing these planes you sometimes reach an in 
 ternal 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 in- 
 evitable answer will be, that those crystals 
 are built by the play of polar forces with 
 which their ultimate molecules are endowed. 
 In virtue of these forces, 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. Dis- 
 solve nitre in water, and allow the water 
 slowly to evaporate; the nitre remains, and 
 the solution soon becomes so concentrated 
 that the liquid form can no longer be pre- 
 served. The nitre-molecules approach each 
 other, and come at length within the range 
 of their polar forces. They arrange them- 
 selves in obedience 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 
 hape. Alum crystallizes with the utmost 
 ease in this fashion. The resultant crystal 
 s, however, different in shape from that 01 
 nitre, because the poles of the molecules are 
 differently disposed; and, 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, 
 :hat thi crystallizing force shall act with de- 
 ibera f ion. There should be no hurry in its 
 Deration; but every molecule ought to be 
 )ermitted, without disturbance from its neigh- 
 >ors, to exercise its own molecular rights. If 
 he crystallization be too sudden, the regu 
 arity disappears. Water may be saturated 
 with sulphate of soda, dissolved when the 
 water is hot, and afterward permitted to cool. 
 When cold, the solution is supersaturated ; 
 hat is to say, more solid matter is contained 
 n it than corresponds to its temperature, 
 till the molecules show no signs of building 
 hemselves together. This is a very remaik- 
 .ble, though a very common fact. The 
 tiolecules in the centre of the liquid are so 
 lampered by the action of their neighbors 
 hat freedom to follow their own tendencies 
 s denied to them. Fix your mind's eye upon 
 
 molecule within the mass. It wishes to 
 nite with its neighbor to the right but it 
 rishes equally to unite with its neighbor to 
 he left ; the one tendency neutralizes the 
 ther, and it unites with neither. We have 
 ere, in fact, translated into molecular action 
 ic well-known suspension of animal volition 
 reduced by two equally inviting bundles of 
 ay. But, if a crystal of sulphate of soda be 
 ropped into the solution, the molecular in- 
 ecision ceases On the crystal the adjacent 
 lolecules will immediately precipitate them- 
 elves; on these again others will be precipi- 
 
SIX LF.CTURrs OX LIGHT. 
 
 tatod, and this not of precipitation will con- 
 tinue from the top of the flask to the bottom, 
 until the solution has, as far as possible, as- 
 sumed the solid form. The crystals here 
 formed are small, and confusedly arranged. 
 The process has been too hasty to a mit of 
 the pure and orderly action of the crystalliz- 
 ing- force. It typifies the state of a nation in 
 which natural and healthy change is resisted, 
 until society becomes, as it were, supersatu- 
 rated with the desire for change, the change 
 being effected through confusion and revolu- 
 tion, which a wise foresight might have 
 avoided. 
 
 Let me 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 bet- 
 ter form. The substance is common sal- 
 ammoniac, or chloride of ammonium, dis- 
 solved in water. Cleansing perfectly a glass 
 plate, the solution of the chloride is poured 
 over the glass, to which, when the plate is set 
 on edge, a thin film of the liquid adheres. 
 Warming the glass slightly, evaporation is 
 promoted ; 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 il- 
 luminating beam adds itself to that already 
 impa- ted to the glass plate, so that after a 
 moment or two the film can no longer exist in 
 the liquid condition. Molecule then closes 
 with molecule, and you have a most impres- 
 sive display of crystallizing energy overspread- 
 ing 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 througrh a 
 lens the subsequent recongelation of the film. 
 
 Here the crystallizing force is hampered by 
 the adhesion of the film to the glass ; never- 
 theless, the play of power is strikingly beau- 
 tiful. 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 crystals start from definite 
 nuclei in the centre of the film ; every small 
 crystalline particle which rests in the film fur- 
 nishes a starting-point. Throughout the pro- 
 cess 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 spiculae are unalterable. In 
 like manner you may find alum-crystals, 
 quartz-crystals, and all other crystals, dis- 
 torted in shape. They are thus far at the 
 mercy of the accidents of crystallization ; but 
 in one particular they assert their superiority 
 
 over all such accidents angular magnitude \ parallel to the planes of freezing, and send- 
 is always rigidly preserved. j ing a sunbeam through such a slab, it lique- 
 
 My second example of the action of crys- j fies internally at special points, round each 
 tallizing force is this: < y sending a voltaic ! point a six-petalled liquid flower of exquisite 
 current through a liquid, you know that we ( beauty being formed. Crowds of such flow- 
 decompose the liquid, and if it contains a ers are thus produced. 
 
 metal, we liberate this metal by the electro- 
 lysis. This small cell contains a solution of 
 acetate of lead, and this substance is chosen 
 because lead lends itself freely to this crys- 
 tallizing power. Into the cell dip two very 
 thin plati. um wires, and these are connected 
 by other wires with a small voltaic battery. 
 On sending the voltaic current through the 
 solution, the 1 ad will be si wly 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, appearing indeed like vegeta- 
 ble growths rendered so rapid as to be plain- 
 ly visible to the naked eye. On reversing the 
 current, these wonderful lead-fronds will dis- 
 solve, while from the other wire filaments of 
 lead dart through the liquid. In a moment 
 or two the growth of the lead-trees recom- 
 mences, 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 vegeta- 
 ble, and afterwards the animal ? Whatever 
 the answer to these questions may be, trust 
 me that the notions of the coming genera- 
 tions regarding this mysterious thing, which 
 some have called "brute matter," will be' 
 very different from those of the generations 
 past. 
 
 There is hardly a more beautiful and in- 
 structive example of this play of molecular 
 force tnan 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. 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 throughout. An inflexible power 
 binds spears and spiculse 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 con- 
 sisting of six rays, with this angle of 60 be- 
 tween every two of them. This structure 
 may be revealed in ordinary ice. In a sun- 
 beam, or, failing that, in our electric beam, 
 we have an instrument delicate enough to 
 unlock the frozen molecules without disturb- 
 ing the order of their architecture. Cutting 
 from clear, sound, regularly-frozen ice a slab 
 
SIX LECTURES ON LIGHT. 
 
 A moment's further devotion to the crys- 
 tallization of water will be well repaid ; for 
 the sum of qualities which renders ihis sub- 
 stance fitted to play its part in Nature may 
 well excite wonder and stimulate thought. 
 Like almost all other substances, water is ex- 
 panded by heat and contracted by cold. Let 
 this expansion and contraction be first illus- 
 trated : 
 
 A small fla'-k is filled with colored water, 
 and stopped with a cork. Through the cork 
 passes a glass tube water-tight, the liquid 
 standing at a certain height (/', Fig. 7) in the 
 tube. The flask and its tube resemble tho 
 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. 
 
 the definite temperature zi 39* Fahr, Crys, 
 
 tallization has virtually here commenced, the 
 
 molecules preparing themselves for the subse. 
 
 quent act of solidification which occurs at 
 
 32, and in which the expansion suddenly 
 
 culminates. In virtue of this expansion, 
 
 j ice, as you know, is lighter than water in the 
 
 | proportion of 8 to 9.* 
 
 It is my desire, in these lectures, to leaa 
 j you as closely as possible to tr.e limits 
 I hitherto attained by scientific thought, and, 
 | in pursuance rf this desire, I have now to 
 i invite your attention to a molecular problem 
 I of great interest, but of great complexity. I 
 j wish you to obtain sue i an insight of ^he 
 molecular world as shall give the intellect 
 satisfaction when reflecting on the deport- 
 ment of water before and during the act of 
 
 Projection of experiment : E is the nozzle of the lamp, 
 column. 
 
 Removing the lamp and piling a freezing 
 mixture in the vessel (B) round the flask, the 
 liquid column falls, thus showing the con- 
 traction 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 tie col- 
 umn remains stationary for son e seconds, 
 and afterwards begins to rise. The contrac- 
 tion 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 pro- 
 duced 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 
 
 L a converging lens, and z" z' the image of the liquid 
 
 crystallization. Consider, then, the ideal 
 case of a number of magnets deprived of 
 
 * In a little volume entitled ' l Forms of Water," 
 I have mentioned that cold iron floats upon molten 
 iron. lu company with my friend Sir William Arm- 
 strong, I had repeated opportunities of witnessing 
 this fact in his works at Elswick. in 1863. Faraday, 
 I remember, spoke to me subsequently of the com- 
 pleteness of iron castings as probably due to trie 
 swelling of the metal on solidification. Beyond 'his, 
 Ihavegiv.-n the subject no special attention, and I 
 know that many intelligent iron-foundt rs 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 re- 
 pulsion. Certain flies walk freely upon water in vir- 
 tue of an action of this kind. With bismuth, how- 
 ever, it is easy to burst iron bottles by the force of 
 solidification. 
 
SIX LECTURES ON LIGHT. 
 
 weight, but retaining their polar forces. If 
 we had a liquid of the specific gravity of 
 steel, we might, by making the magnets 
 float in it, realize this state of things, for in 
 s>uch a liquid the magnets would neither sink 
 nor swim. Now, the principle of gravitation 
 is that every particle of matter attracts every 
 other particle with a force varying as the in- 
 inverse square of the distance. In virtue of 
 the attraction of gravity, then, the magnets, 
 if perfectly free to move, would slowly ap- 
 proach each other. 
 
 But besides the unpolar force of gravity, 
 which belongs to matter in general, the mag- 
 nets are endowed with the polar force of 
 magnetism. For a time, however, the polar j 
 forces do not sensibly come into play. In 
 thi^ condition the magnets resemble our water 
 molecules at the temperature say of 50*. 
 
 with the force of contraction until the freezing 
 temperature is attained. Here the polar 
 forces suddenly and finally gain the victory. 
 The molecules close up and form solid crys- 
 tals, a considerable augmentation of volume 
 being the immediate consequence. 
 
 We can still further satisfy the intellect by 
 showing that these conceptions can be real- 
 ized by a model. The molecule of water is 
 composed of two atoms of hydrogen, united 
 to one of oxygen. We may assume the mole- 
 cide built up of these atoms to be pyramidal. 
 Suppose the triangles in Fig. 8 to be drawn 
 touching the sides of the molecule, and the 
 disposition of the polar forces to be that i-.di- 
 cated by the letters ; the points marked A 
 being attractive, and those marked R repel- 
 lent. In virtue of the general attraction of 
 the molecules, let them be d-rwn towards the 
 
 ,' 
 
 But the magnets come at length suffici"-,ily 
 near each other to enable their poles to nter- 
 act. From this point the action ceases* to be 
 a general attraction of the masses, .^n at- 
 traction of special points of the masses and a 
 repulsion of other points now come into play; 
 and it is easy to see that the rearrangement 
 of the magnets consequent upon the intro- 
 duction 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-mole- 
 cules. Like the magnets, they approach each 
 other as wholes, until the temperature ^9 is 
 reached. Previous to this temperature, 
 doubtless, the polar forces had begun to act, 
 and at this temperature their action exactly 
 balances the contraction due to cold. At 
 lower temperatures the polar forces predomi- 
 nate. But they carry on a gradual struggle 
 
 positions marked by the frill lines, and then 
 suppose the polar attractions and repulsions 
 to act. A will turn towards A', and R will 
 retreat from R x . The molecules will be caused 
 to rotate, their final positions being that shown 
 by the dotted lines. But the circle surround 
 ing the latter is larger than that surrounding 
 the full lines, which shows that the molecules 
 in their new positions n quire more room. In 
 this v. ay we obtain an image of the molecular 
 mechai ism active in the case of water. The 
 demand for more room is made with an energy 
 sufficient to overcome all ordinary resistances. 
 Your lead pipes yield readily to this power; but 
 iron does the same, and bomb-shells, as you 
 know, can be burst by the freezing of water. 
 Thick iron bottles filled with water and placed 
 in a freezing mixture are shivered into frag- 
 ments by the resistless vigor of molecular force. 
 
SIX LECTURES ON LIGHT. 
 
 We have now to exhibit the bearings of 
 crystallization upon optical phenomena. Ac- 
 cording to the undulatory theory, the velocity 
 of light in water and glass is less than in 
 air. Consider, then, a small portion of a 
 wave issuing from a point of light so distant 
 that the portion may be regarded as practi- 
 cally straight. Moving vertically downwards, 
 and impinging on an horizontal surface of 
 glass, the wave would go through the glass 
 without change of direction. But, as the 
 velocity in glass 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 glass will 
 be the first retarded, the other portions as 
 they enter the glass being retarded in succes- 
 sion. This retardation of the one end of the 
 wave causes 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 di- 
 rection. According to the undulatory theory, 
 light is thus refracted. 
 
 In water, for example, there is nothing in 
 the grouping of the molecules to interfere 
 with the perfect homogeneity of the ether ; 
 but, when water crystallizes 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 displayed in a particu- 
 larly impressive manner by Iceland spar, 
 which is crystallized carbonate of lime. The 
 difference of ethereal density in two direc- 
 tions in this crystal is very great, the separa- 
 
 I tion oi the beam into the two halves being, 
 
 therefore, particularly striking. 
 
 Before you is now projected an image of our 
 carbon -points. Introducing the spar, the beam 
 which builds the image is permitted to pass 
 
 j through it; instantly you have the single image 
 
 1 divided into two. Projecting an image of the 
 aperature through which the light issues from 
 the electric lamp, and introducing the spar, 
 two luminous disks, insteadof one, appear 
 immediately upon the screen. (See Fig. 9.,. 
 
 FIG. 9. 
 
 The two elements of rapidity of propaga- 
 tion, both of sound and light, in any sub- 
 stance whatever, are elasticity and density, 
 and the enormous velocity of light is attain- 
 able because the ether is at the same time of 
 infinitesimal density and of enormous elas- 
 ticity. It surrounds the atoms of all bodies, 
 but seems to be so acted upon by them that 
 its density is increased without a proportionate 
 increase of elasticity ; this would account for 
 the diminished velocity of light in refracting j 
 bodies. In virtue of the crystalline archi- ; 
 tecture that we have been considering, the j 
 ether in many crystals possesses different ! 
 densities in different directions ; and the con- 
 sequence is, that some of these media trans- 
 mit light with two different velocities. Now, 
 refraction depends wholly upon the change 
 of velocity on entering the refracting medium ; 
 and is greatest where the change of volicity 
 is greatest. Hence, as, in many crystals, we 
 have two different velocities, we have also 
 two different refractions, a beam of light being 
 divided by such crystals into two. This ef- 
 fect is called double refraction. 
 
 The two beams into which the spar divides 
 the single incident-beam do not behave alike. 
 One of them obeys the ordinary law of re- 
 fraction discovered by Snell, and this is 
 called the ordinary ray. The other does not 
 obey the ordinary law. Its index of refrac- 
 tion, for example, is not constant, nor do the 
 incident and refracted rays always lie in the 
 same plane. It is, therefore, called the ex- 
 traordinary ray. Pour water and bisulphide 
 of carbon into two cups of the same depth ; 
 looked at through the liquid, the cup that con- 
 tains the more strongly-refracting liquid will 
 appear shallower than the other. Place a 
 piece cf Iceland spar over a dot of ink ; two 
 dots are seen, but one appears nearer than the 
 other. The nearest dot belongs to the mosr 
 strongly-refracted ray, which in this case is 
 ths ordinary ray. Turn the spar round, and 
 the extraordinary image of the spot rotates 
 round the ordinary one. 
 
 The double refraction of Iceland spar was 
 first treated in a work published by Erasmus 
 Bartholitnus, in 1669. The celebrated Huy- 
 ghens sought to account for the phenomenon 
 
SIX LECTURES ON LIGHT. 
 
 on the principles of the wave theory, and he ! of the light being the consequence. The 
 
 succeeded in doing so. He made highly im- 
 portant observations on the distinctive charac- 
 ter of the two beams transmitted by the spar. 
 Newton, reflecting on the observations of 
 I luyghens, came to the conclusion that each 
 of the beams had two sides ; and from the 
 analogy of this two sidedness with the two 
 <->idedness of a magnet, wherein consists 'its 
 polarity, the two beams came subsequently to 
 be described as polarized. 
 
 We shall study this subject of the polariza- 
 tion of light with great ease and profit by 
 means of a crystal of tourmaline. But let us 
 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 prop- 
 agation. In the case of ordinary light we 
 are to figure the ether particles as vibrating 
 in all directions, or azimuths, as it is some- 
 times expressed, across this line. 
 
 Now, in a plate of tourmaline cut parallel 
 to the axis of the crystal, the beam of incident 
 light is divided into two, the one vibrating 
 parallel to the axis ot the crystal, the other at 
 right angles to the axis. 'The grouping of 
 *he molecules, and of the ether associated 
 wiih the molecules, reduces all the vibrations 
 incident upon the crystal to these two direc- 
 tions. One of these beams, namely that one 
 \vhose vibrations are perpendicular to the 
 c-ixis, is quenched with exceeding rapidity by 
 the tourma'ine, so '.hat, after having passed 
 thiough a very small thickness of the crystal, 
 the light emerges with all its vibrations re- 
 duced to a single plane. In this condition it 
 is what we call a beam of plane polarized 
 light. 
 
 A moment's reflection will show, if what 
 has been stated be correct, that, 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 
 
 image of a plate of tourmaline, / t (Fig. 10), 
 is now before you. I place parallel to it 
 anot'.er plate, t f t f : the green of the crystal 
 is a little deepened, nothing more. By means 
 of an endless screw, I now turn one of the 
 crystals gradually round ; as long as the two 
 plates are oblique to each other, a certain 
 portion of light gets through ; but, when they 
 are at right angles to each other, the space 
 common to both is a space of darkness, as 
 shown in Fig. n. 
 
 Let us return to a single plate ; and let me 
 say that it is on the green light transmitted 
 by the tourmaline that you are to fix your at- 
 tention. We have now to illustrate the two- 
 sidedness of that green light. The light sur- 
 rounding the green image being ordinary 
 light, is reflected by a plane glass mirror in 
 all directions ; the green light, on the con- 
 trary, is not so rcflecteJ. The image of the 
 tourmaline is row horizontal ; reflected up- 
 wards, it is still green ; reflected sideways, 
 the image is reduced to blackness, because of 
 the incompetency of the green 1 ght to be re- 
 flected in this direction. Making the plate 
 of tourmaline vertical and reflecting it as 
 before, in the upper image the light is 
 quenched ; in the side image you have now 
 the green. Picture the thing clearly. In 
 the one case the mirror receives the impact 
 ot the edges of the waves, ana the green light 
 is quenched. In the other case the sides of 
 the waves strike the mirror, and t e green 
 light is reflected. To render the extinction 
 complete, the light must be received upon 
 the mirror at a special angle. What this 
 an^le is we shall learn presently. 
 
 The quality of two-sidedness conferred 
 upon light by 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 the sun reflected 
 from the windows of the Luxembourg palace 
 in I'aris. I receive upon a plate of window- 
 glass the beam f i om our lamp ; a great por- 
 tion of the light reflected from the glass is 
 polarized ; the vibrations of this reflected 
 beam are executed, for the most part, paral- 
 lel to the surface of the glass, and, if the 
 glass be held so that the beam shall make an 
 angle of 58 with the perpendicular to the 
 glass, the whole ok. the reflected beam is polar- 
 ized. It was at this angle that the image 
 of the tourmaline was completely quenched 
 in our former experiments. It is called the 
 polarizing angle. 
 
 And now let us try to make substantially 
 the experiment of Malus. I receive the beam 
 from the lamp upon this plate of glass and 
 reflect it through the spar. Instead of two 
 images, you see but one. So that the light, 
 when polarized, as it now is, can only get 
 through the spar in one direction, and conse- 
 [uently produce <but one image. Why is 
 this? In the Iceland spar, as in the tour- 
 maline, all the vibrations of the ordinary light 
 
SIX LECTURES ON LIGHT. 
 
 are reduced to 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, emerg- 
 ent from the spar are polarized, their direc- 
 tions of vibration being at right angles to 
 each other. "When, therefore, the light was 
 polarized by reflection, the direction of vibra- 
 tion in the spar which corresponded to the 
 
 conclude? That the green light will be 
 transmitted along the latter, which is parallel 
 to the tourmaline, and not along the former, 
 which is perpendicular to it. Hence we n ay 
 infer that one image of the tourmaline will 
 show the ordinary green light of the crystal, 
 while the other image will be black. Let us 
 test our reasoning by experiment : it is veri- 
 fied to the letter. (Fig. 12.) 
 
 FIG. 
 
 direction of vibration of the polarized beam | 
 transmitted it, and that direction only. But j 
 one image, therefore, was possible under the 
 conditions. 
 
 And now you have it in your power to 
 check many of my statements, and you will 
 observe that ^uch logic as connect our experi- 
 ments is simply a transcript of the logic of 
 Nature. On the screen before you are the 
 
 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 brighter 
 and the bright one gradually darker; at an 
 angle of forty-five degrees both images are 
 equally bright (Fig. 13); while, when ninety 
 degrees have been obtained, the axis of the 
 crystal being then vertical, the bright and 
 
 two disks of light produced by the double re- 
 fraction of the spar. They are, as you 
 know, two images of the aperture through 
 which the light issues from the camera. 
 Placing the tourmaline in front of the aper- 
 ture, two images of the crystal will be ob- 
 tained ; but now let us reason out what is to 
 be expected from this experiment. The light 
 emergent from the tourmaline is polarized. 
 
 black images have changed places. ('Fig. 
 14.) 
 
 Given two beams transmitted through Ice- 
 land spar, it is perfectly manifest that we 
 have it in our power to determine instantly, 
 by means of a plate of tourmaline, the direc- 
 tions in which the ether-particles vibrate in 
 the two beams. I might place the double- 
 refracting spar in any position whatever. A 
 
 FIG. 14. 
 
 Placing the crystal with its axis horizontal, 
 the vibrations of the transmitted light will be 
 horizontal. Now the spar, as already stated, 
 has two perpendicular directions ot vibration, 
 one of which, at the present moment, is ver- 
 tical, the other horizontal. What are we to 
 
 minute's trial with the tourmaline would 
 enable you to determine the position which 
 yields a black and a bright image, and from 
 these you would at once infer the directions 
 of vih'ntion. 
 
 lurther, the two beams from the spar 
 
SIX LECTURES ON LIGHT. 
 
 27 
 
 being thus polarized, if they be suitably re- 
 ceived upon a plate of glass at the polarizing 
 angle, one of them will be reflected, the 
 other not. This is the conclusion of reason 
 from our previous knowledge; but you ob- 
 serve that reason is justified by experiment. 
 (Figs. 15 and 16.) 
 
 I have said that the whole of the beam re- 
 flected from glass at the polarizing angle is 
 polarized; a word must now be addfd regard- 
 ing the larger portion of the li^ht transmitted 
 by the glass. The transmitted beam contains 
 n ruantity of polarized light equal to that of 
 tl e reflected beam; but this quantity is only 
 a L action of the whole transmitted light. By 
 taking two plates of glass instead of one, we 
 
 (B is the birefracting spar, dividing the incident 
 liafnt into the two beams, o and e. G is the mirror). 
 T. i e beam is here reflected laterally. When the re- 
 nVction is upwards^ the other beam is reflected, as 
 shown in Fig. 16. 
 
 augment the quantity of the transmitted polar- 
 ized light; and, by taking a bundle of plates, we 
 
 so increase the quantity as to render the trans- 
 mitted beam, for all practical pu r poses, per- 
 fectly polarized. Indeed, bundles of glass 
 plates are often employed as a means of fur- 
 nishing polarized light. 
 
 One word more. When the tourmalines 
 are crossed, the space where they cross each 
 other is black. But we have seen that the 
 least obliquity on the part of the crystals per- 
 mits light to get through both. Now sup- 
 i ose, when the two plates are crossed, that 
 we interpose a third plate of tourmaline be- 
 tween them, wilh its axis obliqus to both. A 
 portion of the light transmitted by the first 
 plate will get through this intermediate one. 
 But, after it has got through, its plane of vi- 
 bration is changed: it is no longer perpendicu- 
 lar to the axis cf the crystal in front. Hence 
 it will get thiough that crystal. Thus, by 
 reasoning, we infer that the interposition of a 
 third plate of tourmaline 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 tne crossed tourmalines 
 I introduce a film of this crystal. You see 
 the edge of the film slowly descending, and 
 as it descends between the tourmalines, light 
 takes the place of darkness. The darkness, 
 in fact, se.med scraped away as if it were 
 something material. This effect has been 
 called and improperly called depolarization. 
 
 LECTURE IV. 
 
 Chromatic Phenomena produced by Crystals on Polar- 
 ized Light: The Nicoi Prism: Polarizer and Ana- 
 lyzer : Action of thick and thin Plates of Salenite : 
 Colors dependen. on Thickness: Resolution of Po- 
 larized Beam into two others by the Selenite : One 
 of them mure retarded than the other: Recom- 
 pounding of the two Systems of Waves by the Ana- 
 lyzer: Interference thus rendered possible : Conse- 
 quent Production of Colors: Action of Bodies 
 Mechanically strained or pressed : Action of Sono- 
 rous Vibrations : Action of Glass strained or pressed 
 by Heat: Circular Polarization: Chromatic Phe- 
 nomena produced by Quartz : The Magnetization 
 of Light : Rings surrounding the Axes of Crystals : 
 Biaxal and Uniaxal Crystals : Grasp of the Undu- 
 latory Theory. 
 
 We now stand upon the threshold of a new 
 and 'splendid optical domain. We have to 
 examine, this evening, the chromatic phe- 
 nomena produced by the action ot. crystals, 
 and double-refracting bodies gene: ally, upon 
 polarized light. For along 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 aston- 
 ishing. But these men had their hearts in 
 their work, and were on this account enabled 
 to extract great results from small instrumen- 
 tal appliances. But we have better apparatus 
 now. You have seen the two beams emer- 
 gent from Iceland spar, and have proved 
 them to be polarized. If we could abolish 
 
28 
 
 SIX LECTURES ON LIGHT. 
 
 cne of these beams, vre might employ the 
 other for experiments on polarized light. 
 
 These beams, as you know, are refracted 
 differently, and from this we are able to infer 
 that under some circumstances the one may 
 be totally reflected, and the other not An 
 optician, named Nicol, cut a crystal of Ice- 
 land spar in two in a certain direction. He 
 polished the severed urfaces, 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 per- 
 mitted to pass on. The invention of the 
 Nicol prism was a great step in practical op- 
 tics, and quite recently such prisms have 
 been constructed of a size which enables 
 audiences like the present to witness the 
 chromatic phenomena of polarized light to a 
 degree altogether unattainable 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, manu- 
 factured for me by Mr. Browning, who has 
 gained so high and well-merited a reputation 
 in the construction of spectroscopes. 
 
 These two Nicol prisms play the same 
 part as the crystals of tourmaline. Placed 
 with their directions of vibration parallel, 
 the light passes through both. When these 
 directions are crossed, the light is quenched. 
 Introducing a film of mica between the 
 prisms, the light is in pirt restored. But 
 notice, when the film of mica is thin, you 
 have sometimes not only light, but colored 
 light. Our work for some time to come will 
 be the examination of th^se colors. With 
 this view, I will take a representative crystal, 
 one easily dealt with; 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 produce^ no color. 
 With my penknife I take a thin splinter from transmitted vibration. 
 
 heart's-ease, the colors of which you might 
 safely defy the artist to reproduce. By turn- 
 ing the front Nicol ninety degrees round, we 
 pass through a colorless phase to a series of 
 colors complementary to the lorner ones. 
 Here, for example, is a rose tree with red 
 flowers and green leaves; turning the prism 
 ninety degrees round, we obtain a green 
 flower and red leaves. All these wonderful 
 chromatic effects have definite mechanical 
 causes in the motions of the ether. The 
 principle of interference, duly applied and 
 interpreted, explains them all. 
 
 By this time you have learned that the 
 word " light" may be used in two deferent 
 senses ; it may mean the impression made 
 upon consciousness, or it may mean the phys- 
 ical agent which makes the impression. It 
 is with the agent that we have to occupy our- 
 selves at present. That agent is the motion 
 of a substance which fills all space, and sur- 
 rounds the atoms and molecules of bodies. 
 To this interstellar and interatomic medium 
 definite mechanical properties are ascribed, 
 and we deal with it as a body possessed of 
 these p operties. In mechanics we have the . 
 composition and resolution of forces, and of 
 motions, extending to the composition and 
 resolution of -vibrations. We treat the lumi- 
 niferous ether on mechanical principles, and 
 from the composition, resolution, and inter- 
 ference of its vibrations, 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 famil- 
 iar. Let a vibration cross thiscrysta* oblique 
 to its axis ; we have seen by ex^r ^riuunc Mat 
 a portion of the light will pa-. '> through. 
 How much, we determine in this way : Draw 
 a straight line representing the intensity of 
 the vibration before it reaches the tourmaline, 
 and from the two ends of this line draw two 
 perpendiculars to the axis of the crystal ; the 
 distance between the feet of these two per- 
 pendiculars will represent the intensity of tne 
 
 this crystal and place it between the pri> 
 its image on the screen glows with thericuebi 
 colors. Turning the prism in front, these 
 colors gradually fade, disappear, but by con- 
 tinuing the rotation until the vibrating sec- 
 tions of the prisms are parallel, vivid colors 
 again appear, but these colors are comple- 
 mentary to the former ones. 
 
 Some patches of the splinter appear of one 
 color, some of another. These differences 
 ar,; due to the different thicknesses of the 
 film. If the thickness be uniform, the color 
 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 uni- 
 form color. It is easy, by shaping our films 
 so as to represent flowers or other objects, 
 
 Follow me now while I endeavor to make 
 
 clear to you what occurs when a 
 
 of 
 
 gypsum is placed between the Nicol prisms. 
 But, at the outset, let us establish still 
 further the analogy between the action of the 
 prisms and that of two plates of tourmaline, 
 The plates are now crossed, and you see that 
 by turning the film round, it may be placed 
 in a position where i*" has no power to Abolish 
 the darkness. Why is this? The answer is 
 that in the gypsum there are two directions, 
 at right angles to each other, which the wave* 
 of light aie constrained to follow, and that 
 now 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 aciion 
 upon the light. But now I turn the fil.n so 
 
 to exhibit such objects i.i colors unattainable as to render its direction of vibration oblique 
 by art. He i- e, for example, is a specimen of > to the axes ; then you see it has the power, 
 
SIX LECTURES ON LIGHT. 
 
 demonstrated in tne last lecture, of restoring 
 the light. 
 
 Let us now mount our Nicol prisms and 
 cross them as we crossed the tourmalines. 
 Introducing our film of gypsum between them, 
 you notice tnat in one particular position the 
 film has no power whatever 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. 
 
 Firstly, then, we have this first prism which 
 receives the light emergent from the electric 
 lamp, and which is called the polarizer. Then 
 we have the p ate of gypsum, placed at S 
 (Fig. 17), 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 an horizontal plane. 
 The two directions of vibration of '.he gypsum, 
 placed at S, are now oblique to the horizon. 
 Draw a rectangular cross upon paper to rep- 
 resent the two directions of vibration within 
 the gypsum. Draw an oblique line to repre- 
 sent the intensity of the vibration when it 
 reaches the gvpsum. Let fall from the two 
 ends of this line two perpendiculars on each 
 of the arms of the cross ; then the distances 
 between the feet of these perpendiculars rep- 
 resent the intensities of two rectangular vi- 
 brations which are the equivalents of the first 
 single vibration. Thus the polarized ray, 
 when it enters the gypsum, is resolved into 
 two others, vibrating at right angles to each 
 other. 
 
 Now, in one of those directions of vibration 
 the ether is more sluggish than in the other ; 
 and, as a consequence, the waves that follow 
 this direction are more retarded than the 
 others. The waves of both systems, in fact, 
 are shortened when they enter the gypsum, 
 but the one system is more shortened than the 
 
 other. You can readily imagine that in this 
 way the one system of waves may get half a 
 wave-length, or indeed any number of half 
 wave-lengths, in advance of the other. The 
 possibility of interference here flashes upon 
 the mind. A little consideration, however, 
 renders it evident that, as long as the vibra- 
 tions 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 can occur. But here, as 
 in the case of thin films, the different lengths 
 of the waves of light come into play. Red 
 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 and confer their colors on the 
 film of gypsum. Conversely, when the 
 shorter waves have been withdrawn, the 
 thickness is such that the longer waves re- 
 main. An elementary consideration suffices 
 to show that, when the directions of vibration 
 of prisms and gypsum enclose an angle of 
 forty-five degrees, the colors are at their 
 maximum brilliancy. When the film is 
 i urned from this direction, the colors gradu- 
 ally fade, until, at the point where the direc- 
 tions are parallel, they disappear altogether. 
 
 A knowledge of these phenomena 1'i best 
 obtained by means of a model of wood or 
 pasteboard representing the plate of gypsum, 
 ts planes of vibration, and also those of the 
 polarizer and analyzer. On these planes the 
 waves may be drawn, showing the resolution 
 of the first polarized ray into two others, and 
 then the reduction of tne two vibrations to a 
 common plane. Following out rigidly the 
 nte.-action of the two systems of waves, we 
 are taught by such a model that a'l the phe- 
 nomena of color, obtained when the planes of 
 vibration of the two Nicols are parallel, are 
 displaced by the complementary phenomena 
 when the Nicols are perpendicular to each 
 other. 
 
 In considering the next point, for the sake 
 of simplicity, we will operate with monochro- 
 matic light with red light, for example. 
 Supposing that a certain thickness of the gyp- 
 sum produces a retardation of half a wavo- 
 "ength, 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 are parallel, the retardatioi of 
 half a wave-length, or of any odd number of 
 half wave-lengths, produces extinction: at ail 
 thicknesses, on the other hand, which corre- 
 spond 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. S pposing, then, 
 
30 
 
 SIX LECTURES ON LIGHT. 
 
 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 light bands occur 
 between the dark ones. Experiment proves 
 the wedge-shaped crystal to show these bands; 
 but they are far better shown by this circular 
 film, which is so worked as to be thinnest at 
 the centre, gradually increasing in thickness 
 from the centre outwards. These splendid 
 rings of light and darkness are 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 employ- 
 ing white light may now be inferred : inas- 
 much as the red and the blue fall in different 
 places, we have iris-colored rings produced by 
 the white light. 
 
 Some of the cl^omatic effects of irregular 
 crystallization are beautiful in the extreme. 
 Could I introduce between our Nicols a pane 
 of glass covered by those frost-ferns which 
 the cold weather renders now so frequent, 
 rich colors would be the result. The beauti- 
 ful effects of irregular crystallization on glass 
 plates, now presented 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 in this way; 
 for their architecture implies an arrangement 
 of the ether which involves double refraction. 
 A film of horn, or the section of a shell, for 
 example, yields very beautiful colors in polar- 
 ized light. In a tree, the ether certainly pos- 
 sesses different degrees of elasticity along and 
 across the fibre; and, were wood transparent, 
 this peculiarity of molecular structure would 
 infallibly reveal itself by chromatic phe- 
 nomena like those that you have seen. But 
 not only do bodies built permanently by 
 Nature behave in this way, but it is possible, 
 as shown by Brewster, to confer, by strain or 
 by pressure, a temporary double-refracting 
 structure upon non-crystalline bodies, such as 
 common glass. 
 
 When I place this 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 longitudinally pressed. Both in the 
 strained portion and in the pressed portion 
 the ether is thrown into a condition which 
 would rend r the wood, were it transparent, 
 double refracting. Let us repeat the experi- 
 ment with a bar of glass. Between the 
 crossed Nicols I introduce such a bar. By 
 ihe dim residue of light lingering upon the 
 screen, you see the imjge of the glass, but it 
 has no effect 'ipon the light. I simply bend 
 
 the gla-s 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 ed^es most, 
 for here the strain and pressure are greatest. 
 In passing from strain to 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 exer- 
 cises no c ction upon the light. By employ- 
 ing 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 square between the prisms, 
 its neutrality is declared ; but it can hardly 
 be held sufficiently loosely 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. I now turn this 
 screw. Instantly the image of the square of 
 glass flashes out upon the screen. You see 
 luminous spaces separated from each other 
 by dark bands. Every pair of adjacent 
 luminous spaces is in opposite mechanical 
 conditions. On one side of the dark band 
 we have strain, on the other side pressure ; 
 while the dark band marks the neutral axis 
 between both. I now tighten the vise, and 
 you see color ; tighten still more, and tha 
 colors appear as rich as those presented by 
 crystals. Releasing the vise, the colors 
 suddenly vanish ; tightening suddenly, they 
 reappear. From the colors 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 
 colors here presented to you, the magnitude 
 of the pressure employed might be inferred. 
 Indeed, the late M. Werthtim, of Paris, in- 
 vented an instrument for the determination 
 of strains and pressures by the colors of 
 polarized light, which exceeded m accuracy 
 all other instruments of the kind. 
 
 You know that bodies are expanded by 
 heat and contracted 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 others 
 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 ihe 
 flame of a spirit-lamp, on reintroducing it 
 between the Nicols, light flashes out upon 
 the screen. Here, as in the case of mechan- 
 ical action, you have spaces of strain divided 
 by neutral axes from spaces of pressure. 
 
 Let us apply the heat more symmetrically. 
 This small square of glass is perforated at 
 
SIX LECTURES ON LIGHT. 
 
 the rentre, and into the orifice a bit of copper 
 wire is introduced. Placing the square be- 
 tween the prisms, and heating the copper, 
 the heat passes by conduction along the wire 
 to the glass, throug i which it spreads from 
 the centre outwards. You see a dim cross 
 bounding four luminous quadrants growing 
 up and becoming gradually black by compari- 
 son with the adjacent brightness. And as, in 
 the case of pressure, we produced colors, so 
 here als% by the proper application of heat, 
 gorgeous chromatic effects may be produced, 
 and they may be rendered permanent by first 
 heating the glass sufficiently, and then cool- 
 ing it, so that the chilled mass shall remain 
 in a state of strain and pressure. Two or 
 three examples will illustrate this point. The 
 colors, you observe, are quite as rich as those 
 obtained in the case of crystals. 
 
 And now we have to push these considera- 
 tions to a final illustration. Polarized light 
 may be turned to account in various ways as 
 an analyzer of molecular condition. A strip 
 of glass six feet long, two inches wide, and 
 a quarter of an inch thick, is held at the 
 centre between my finger and thumb. I 
 sweep over one of its halves a wet woolen 
 rag ; you hear an acute sound, due to the 
 vibrations of the glass. What is the condi- 
 tion of the glass while the sound is heard ? 
 This . its two halves lengthen and shorten in 
 quick succession. Its t*vo ends, therefore, 
 aie in a state of quick vibration ; but at the 
 centre the pulses from the two ends alter- 
 nately meet and retreat. Between their 
 opposing actions, the* glass at the centre is 
 kept motionless ; but, on the other hand, it 
 is alternately strained and compressed. The 
 state of the glass may be illustrated by a row 
 of spots of light, as the propagation of a 
 sonorous pulse was illustrated in a former 
 
 
 m 
 
 FIG. 1 8. 
 
 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 torn 
 asunder, the centre one not moving at all. 
 The condition of the sounding strip of glass 
 is here correct'y represented. In Fig. 18, A 
 B represents the glass rectangle with its 
 centre condensed ; while A' B' represents th 
 same rectangle with its centre rarefied. 
 
 If we introduce the glass s s' (Fig. 19) be- 
 tween the crossed Nicols, taking care to keep \ 
 the strip oblique to the direction 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 wiil 
 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 conclusion 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 fron of the 
 polarizer a plate of unannealed glass, you 
 have those beautiful colored 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 an experiment 
 which we owe to Biot. His apparatus, how- 
 ever, confined the observation of it to a single 
 person at a time. 
 
 But we have to follow the ether still 
 further. Suspended before you is a pendu- 
 lum, which, when drawn aside and then 
 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 pendulum oscil- 
 lates in a plane. But, if the rectangular 
 hock be imparted to the pendulum when it 
 is at the limit of its swing, then the com- 
 pounding of the two impulses causes the sus- 
 pended ball to describe not a straight line, 
 but an ellipse ; and, if the shock be compe- 
 tent of itself to produce a vibration of the 
 same amplitude as the first one, the ellipse 
 becomes a circle. But 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 
 "n my hand a plate of quartz cut from the 
 crystal perpendicular to its axis. This crys- 
 tal thus cut possesses the extraordinary power 
 of twisting the plane of vibration of a polar- 
 'zed rav to an extent dependent on the thick- 
 ness of the crystal. And the more refrangi- 
 ble the light the greater is the amount of 
 twisting, so that, when white light is em. 
 ployed, its constituent colors are thus drawn 
 
SIX LECTURES ON LIGHT. 
 
 asunder. Placing the quartz between the 
 polarizer and the analyzer, you see this 
 splendid color, and, turning the analyzer in 
 front, from right to left, the other colors 
 apoear in succession. Specimens of quartz 
 have been found which require the analyzer 
 to be turned from left to right, to obtain the 
 same succession of colors. Crystals of the 
 first class are therefore called right-handed, 
 and, of the second class, left handed crystals. 
 With profound sagacity, Fresnet, 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 circu- 
 larly polarized. Two such rays transmitted 
 along the axis of the crystal, and rotating in 
 
 although the mixture of blue and yellow pig 
 ments produces green, the mixture of blue 
 and ye.low lights produces white. By en- 
 larging our aperture, the two images pro- 
 duced by the spar are caused to approach 
 each other, and finally to overlap. The one 
 is now a vivid yellow, the other a vivid blue, 
 and you notice that where the colors arts 
 superposed we have a pure white. (See Fig. 
 20, where N is the nozzle of the lamp, Q the 
 quartz plate, L a lens, and B the birefracting 
 spar. The two images overlap at O, and 
 produce white by their mixture.) 
 
 This brings us to a point of our inquiries 
 which, though not capable of brilliant illus- 
 tration, is nevertheless so likely to affect pro- 
 foundly the future course of scientific thought 
 that I am unwilling to pass it over without 
 reference. I refer to the experiment which 
 Fa-'aday, its discoverer, called the magnetiza- 
 tion of light. The arrangement for thL 
 celebrated experiment is now before you. 
 
 FIG. 19. 
 
 opposite directions, when brought to inter- 
 ference by the analyzer, are demonstrably 
 competent to produce the observed phe- 
 nomena. 
 
 1 now abandon the analyzer, and put in its 
 place the piece of Iceland spar with which 
 we have already illustrated double refraction. 
 The two images of the carbon-points are 
 now before you. Introducing a plate of 
 quartz between the polarizer and the spar, 
 the two images glow with complementary 
 colors. Employing the image of an aperture 
 instead of that of the carbon-points, .we have 
 two complementary colored circles. As the 
 analyzer is caused to rotate, the colors pass 
 through various changes ; but they are al- 
 ways complementary to each other. If the 
 one be red, the other will be green ; if the 
 one be yellow, the other will be blue. Here 
 we have it in our power to demonstrate afresh 
 a statement made in a former lecture, that, i 
 
 We have first our electric lamp, then a Nice* 
 prism, to polarize the beam emergent from 
 the lamp ; then an electro-magnet, then a 
 second Nicol prism, 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 
 cylinder 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 intercepted 
 by the Nicol in front. I now excite the 
 magnet, and instantly light appears upon the 
 screen. On examination, we find that, by 
 the action of the magnet upon the ether con- 
 tained within the heavy glass, the plane ot 
 vibration is caused to rotate, thus enabling 
 the light to get through the analyzer. 
 
 The two classes into which quartz-crystals 
 are divided have been already mentioned. 
 In my hand I hold a compound piate, one- 
 
SIX LKC I UKi h 
 
 LIGHT. 
 
 *UtJ of it taken from a right-handed and the 
 other from a left-handed crystal. Placing 
 the plate in front of the polarizer, we turn 
 one of the Nicols until the two halves of the 
 plate show a common puce color. This 
 yields an exceedingly sensitive means of ren- 
 dering the action of a magr.et upon light 
 jiaible. By turning either the polarizer or 
 
 :he analyzer through the smallest angle, tne 
 uniformity of the col- r disappears, and the 
 two halves of the quartz show different colors. 
 The magnet also produces this effect. The 
 puce-colored circle is now befcre you on the 
 screen. (See Fig. 21 for the arrangement of 
 the experiment. N is he nozzle of the lamp, 
 H the first ?<ico), Q the biquartz plate, L a 
 lens, M the electro-magnet, and P the second 
 Xicol.) Exciting the magnet, one half of 
 the image becomes Suddenly red, the other 
 half green. Interrupting the current, the 
 two colors 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 tne current which 
 surrounds the magnet. Reversing the cur- 
 rent, the red and green reappear, but they 
 have changed places The red was for- 
 merly to the fight, and the green to the left ; 
 the green is now to the r ight, and the red to 
 the left. With the most exquisite ingenuity, 
 Faraday analyzed all those actions a. d stated 
 their laws. This experiment, however, long 
 remained rather as a scientific curiosity than as 
 a fruitful germ. That it would bear fruit of 
 the highest : mportance, Faraday felt pro- 
 foundly convinced, and recent jesearches are 
 on the way to verify his conviction: 
 
 A few words more are necessary to com- 
 plete our knowledge oi the wonderful inter- i 
 action between ponderable molecules and the ! 
 ether interfused among them. Symmetry of i 
 
 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 mol jles are ranged sym- 
 metrically around P ,ertain line, and not 
 around others. A' .g the former, therefore, 
 the ray is undiv .ed, while along ail the 
 others we have double refraction Ice is a 
 familiar examp'e ; it is built with perfect 
 symmetry around the perpendiculars to the 
 phnes of freezing, and a ray sent through 
 ice in this direction is not doubly refracted ; 
 whereas, in all o^her directions, it is. Ice- 
 land spar is another example of the same 
 kind : its molecules are built symmetrically 
 round the line uniting the two blunt angles 
 of the rhomb, in thf*> direction a ray suffers 
 no double refraction, in all others it does. 
 This direction of dou\ !c refraction is called 
 the optic axis oi the ci \Vi\l. 
 
 Hence, if a plate be oat from a crystal of 
 Iceland spar perpendicular to the axis, all 
 rays sent across this plate : n the dirction of 
 the axis will psoduce but cne image. But, 
 the moment we deviate from '.he parallelism 
 with the axis, double refraction sets in. If, 
 therefore, a beam that has b^cn rendered 
 conical by a converging lens be rent through 
 the spat so that the central ray c f the cone 
 passes along the axis, this ray or;\y will es- 
 cape double refraction. Each of the others 
 will be divided into an ordinary and extraor- 
 dinary ray, the one moving morf slowly 
 through the crystal than the other ; the one, 
 therefore, retarded with reference to the 
 other. Here, then, we have the condition/ 
 
34 
 
 SIX LECTURES ON LIGHT. 
 
 for interference, when the waves are reduced 
 by the analyzer to a common plane. A 
 highly beautiful and important source of 
 chromatic phenomena is thus revealed. 
 Placing the plate of spar between the crossed 
 prisms, we have upon the screen a beautiful 
 system of iris rings surrounding the end of 
 the optic axis, the circular bands of color 
 being intersected by a black cross. The 
 arms of th : s 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 cither 
 prism, cannot get through both. This com- 
 plete interception produces the arms of the 
 cross. With mono-chromatic light the rings 
 would be simply bright and black the 
 bright rings occurring at those thicknesses of 
 the spar which cause the rays to conspire ; 
 the black rings at those thicknesses which 
 cause them to quench each other. Here, 
 however, as elsewhere, the different lengths 
 of the light-waves give rise to iris-colors 
 when white light is employed. 
 
 Besides the regular crystals which produce 
 double refraction in no direction, and the 
 /</.*/ crystals which produce it in all direc- 
 tions 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. 
 \\henplatesot '"hese crystals, suitably cut, 
 arc plac. d bet wee the polarizer and analyzer, 
 ihe axes are sce-i si "ounded, not by circles, 
 but by curv s of anv '^er order and of a per- 
 fectly definite mathem. 'cal character. Each 
 band as proved experimentally by Herschel, 
 forms a lefnuiscata ; 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. 
 
 I have taken this somewhat wide range 
 over polarization itseli and over the phenom- 
 ena exhibited by crystals in polarized light, 
 in order to give you some notion of the firm- 
 ness and completeness of the theory which 
 grasps them all. Starting from the single 
 assumption of transverse unckilations, we 
 first of all determine the wave-lengths, and 
 find all the phenomena of color dependent 
 on this element. The wave-lengths may be 
 determined in many independent ways, and, 
 when the lengths so determined are compared 
 together, the strictest agreement is found to 
 exist between them. We follow the ether 
 into the most complicated cases of interac- 
 tion between it and ordinary matter, "the 
 theory is equal to them all. It makes not a 
 single new 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 , pre- 
 dicts and discloses unknown ones ; becomes ; 
 the guide of its former teacher Observation ; ! 
 
 and, enlightened by mechanical conceptions, 
 acquires an insight which pierces through 
 shape and color to force and cause. '* 
 
 But, while I have thus endeavored to illus- 
 trate before you the power of the undulatory 
 theory as a solver of all the difficulties of 
 optics, do I there! ore 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 is perfectly logical ; but let 
 us understand the precise value of the argu- 
 ment. 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, be- 
 lieved 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 just as logical as the first 
 argument. Wherein consists the strength of- 
 the theory of gravitation ? Solely in its com- 
 petence to account for all the phenomena of the 
 solar system. Wherein consists the strength 
 of the theory of undulation ? Solely in its 
 competence to disentangle and explain phe- 
 tnomena a hundred-fold more complex than 
 :hose of the solar system. Be as skeptical, 
 "f you like, regarding the undulatory theory ; 
 but if your skepticism be philosophical, it 
 will wrap the theory of gravitation in the 
 same or greater doubt. f 
 
 LECTURE V. 
 
 Range of vision incommensurate with Range of Radi- 
 ation ; The Ultraviolet 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 Rays here effective : Sir iVllliam Herschel's 
 Discovf ry of Dark Solar Rays: Invisible Rays the 
 Basis of tne 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 played in Nature 
 by Dark Rays: Identity of Light and Radiant 
 Heat: Invisible Images: Reflection, Refraction, 
 Plane Polarization, Depolarization, Circular Polar- 
 ization, Double Refraction, and Magnetization of 
 Radiant Heat. 
 
 THE first question that we have to con- 
 sider to-night is this , Is the eye, as an organ 
 of vision, commensurate with the whole 
 range of solar radiation is it capable of re- 
 ceiving visual impressions from all the rays 
 emitted by the sun ? The answer is nega- 
 tive. If we allowed ourselves to accept for a 
 
 * Whewell. 
 
 t 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 Report for 1862. 
 
SIX LECTURES ON LIGHT. 
 
 moment thav :-oaon o<" gradual growth, 
 artiehoi avion, arid asconaion, implied by the 
 ttrm e-cohitton, we might lairly conclude that 
 thre arcs'.ortj of visual i mpressions a waiting 
 ma i far greater than those ot which he is 
 now in possession. For example, he r e be- 
 yond the extreme violet of the spectrum ihere 
 is a vast efflux or rays which are totally use- 
 less as regards our present powers of \isicn. 
 But these ultra-violet waves, though incom- 
 petent to awaken the optic nerve, can so 
 shake the molecules of certain compound sub- 
 stances as to effect their decomposition. The 
 grandest example of the chemical action of 
 light, with which my friend Dr. Draper has 
 so indissolubly associated his name, is that of 
 the decomposition of carbonic acid in the 
 leaves of plants. All photography is founded 
 on such actions. There are substances on 
 which the ultra-violet waves exert a special 
 decomposing power ; and, by permitting the 
 invisible spectrum to fall upon surfaces pre- 
 pared with such substances, we reveal both 
 the existence and the extent of the ultra- 
 violet spectrum. 
 
 This mode of exhibiting the action of the 
 ultra-v.olet rays has be^n long known ; in- 
 deed, Thomas Young photographed the ultra- 
 violet rings of Newton. We have now to 
 demonstrate their presence in another way. 
 As a general rule, bodies transmit light or 
 absorb it, but there is a third case in which the 
 light falling upon the *x>dy is neither trans- 
 mitted nor absorbed, but converted into light of 
 another kind. Professor Stokes, the occupant 
 of the Chair of Newton in the University of 
 Cambridge, one of those original workers 
 who, though not widely known beyond 
 scientific circles, really constitute the core of 
 science has demonstrated this change of one 
 kind of light into another, and has pushed 
 his experiments so far as to render the iuvisi 
 'As rays visible. 
 
 A ion^ list of substances examined by 
 Stokes when excited by the invisible ultra- 
 violet waves, have been proved to emit light. 
 You know the rate of vibration corresponding 
 to the extreme violet of the spectrum ; you 
 are aware that, to produce the impression of 
 this color, the retina is struck 789 millions of 
 millions of times in a second. At this point, 
 the retina ceases to be useful as an orga i of 
 vision, for, though struck by waves of more 
 rapid recurr nee, they ?_r incompetent to 
 awaken the sensation of light. But, when 
 such non-visual waves are caused to impinge i 
 upon the molecules of certain substances I 
 en those of sulphate of quinine, for example 
 they compel those molecules, or their con- 
 stituent atoms, to vibrate ; and the peculiar- 
 ity is, that the vibrations thus set up are of 
 slower period than those of the exciting 
 waves. By this lowering of t c rate of vi- 
 oration through the intermediation of the sul- 
 phate of quinine, the invisible rays are ren- 
 dered visible. He-e we have our spectrum, 
 and beyond the violet I place this prepared 
 
 paper. The spectrum is immediately elonga- 
 ted by the generation of a new light beyond 
 the extreme violet. President Morton has 
 recently succeeded in discovering a substance 
 of great sensibility which he has named 
 Thallene, and he has been good enough ro 
 favor me with some paper saturated with a 
 solution of this substance. It causes a very 
 striking enlongation of the spectrum, the 
 ! new light generated being of peculiar bril- 
 liancy. To this change of the rays from a 
 higher to a lower refrangibility, Stokes has 
 given the name of Fluorescence. 
 
 By means of a deeply-colored violet glass, 
 we cut off almost the whole of the light of 
 our electric bean? ; 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 it I pour a solu- 
 tion of sulphate of quinine : opaque clouds, 
 to all appearance, instantly tumble down- 
 wards. But these are not clouds : there is 
 nothing precipitated here : the observed ac- 
 tion is a action of molecules, not of particles. 
 The medium before you is not a turbid me- 
 dium, for, when you look through it at a 
 luminious surface, it is perfectly clear. If we 
 paint upon a piece of paper a flower or a 
 bouquet with the sulphate of quinine, and ex- 
 pose it to the full beam, scarcely anything is 
 seen. But on interposing the violet glass, . 
 the design instantly flashes forth in strong 
 contrast with the deep surrounding violet. 
 Here is such a design prepared for me by 
 President Morton with his thallene : placed 
 in the violet light it exhioits a peculiarly 
 vivid and beautiful fluorescence. From the 
 experiments of Dr. Bence Jones, it would 
 seem that there is some .substance in the hu- 
 man body resembling the sulphate of quini.ie, 
 which causes ail 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 I plunge my ey* 
 into this violet beam, I am conscious of a 
 whitish-blue shimmer filling the space before 
 me. This is caused by fluorescent light gen- 
 erated in the eye itself ; looked at from with- 
 out, the crystalline lens at the same time 
 gleams vividly. 
 
 <* 
 
 But the waves from our incandescent car- 
 bon-points appeal to another sjnse than that 
 of vision. They not only produce light as a 
 sensation; they also produce heat. The mag- 
 nified image of the carbon points is now upon 
 the screen, and with a suitable instrument the 
 heating power of that instrument might be 
 demonstrated. Here, however, the heat is 
 spread over too large an area to be intense. 
 By pushing out the lens and causing a mova- 
 ble screen to approach our lamp, the im >ge 
 becomes smaller and smaller : the rays be- 
 come more concentrated, until finally they 
 are able to pierce black paper with a burning 
 ring. Rendering the beam parallel, and re- 
 ceiving it upon a concave mirror, the rays are 
 
SiX LECTURES ON LIGHT. 
 
 brought to a focus ; and paper placed at the through a glass cell containing water. The 
 focus is caused to smoke and burn. This beam is thus sifted of constituents, which, if 
 may be done by our common camera with its permitted lo fall upon the lens, would injure 
 lens, and by a concave mirror of very mod- its surface, and blur the focus. And this 
 erate po\v r. leads me to say an anticipate! y word regard- 
 
 \\ e v\i,l row adopt stronger measures with ing transparency. In our first lecture we 
 the radiatio. i from the electric la-rip In this entered fully into the production of colors 
 carm ra of blackened ti i is placed a lamp, in by absorption, and we spoke repeatedly 
 all particulars similar to those already em- of the quenching of the rays of light Did 
 ployed. But, instead of gathering up the this mean that the light was altogether 
 /ays from a carbon-point by a condensing annihilated? By 1,0 means. It was sim- 
 !ens placed in front of them, we gather them ply so lowered in relrangibility as to 
 up by a co :cave mirror, silvered in front, escape the vi-ual range. It was converted 
 and placed behind tLe carbons. By this into heat. Our red nbbon in the green of the 
 mirror \ve can cause the rays t > issue through spectrum quenched the green, but if suitably 
 the orifice in front, either parallel or conver- examined its tcm erature would have been 
 gent. They are r\ow prvallel, and therefore found laised. Oar. green ribbon in the red 
 to a certain extent diffu-ed. We place a of the spectrum quenched the red, but its 
 convex lens in the path of the beam ; the temperature at the same lime was augmented 
 light is converged to a focus, and at that to a degree exactly tquivalent to the light ex- 
 focus you s e that paper is not only pi.roed tinguished. Our black ribbon, when passed 
 and a burning ring foimed, but that it is in- through the spectrum, was found competent 
 stantly set ablaze. Many metals may be to quench ail its colors ; but at every stage of 
 burned up in the same way. In our first lee- its progress an amount ot heat was geneiated 
 ture thecombustibility of zinc was mentioned, in the ribbon exactly equivalent to the light 
 Macing a stiip <>( sheet zinc at this focus, it j lost. It is only ivhen absorption takes place 
 is instantly ignitid and burns with its charnc- | tkat heat is thus produced ; and heat is always 
 
 teristic purple flame. (In the annexed fi .u 
 m in' represents the concave mi-ror, L t 
 
 a result of ab orption. 
 
 Examine this water, then, in front of the 
 lamp, after the beam lias passed a little lime 
 through it : it is sensibly warm, and, if per- 
 mitted to remain there iong enough, it may 
 be made to boil. This is due to the absorp- 
 tion by ti.e water of a portion of the electric 
 beam. But a certain poition passes through 
 unabsorbed, and does not at all contribute to 
 the heating of the water Now, ice is also 
 transparent to the latter portion, and there- 
 fore is not melted by it ; hence, by employ- 
 ing this particular portion of the beam, we 
 are able 10 keep our lens intact, and to pro- 
 duce by means of it a shai ply-defined focus. 
 Placed at that focus, black paper instantly 
 burns, because the black paper absorbs the 
 light which had passed through the ice-lens 
 without absorption, la a subsequent lecture, 
 we shall endeavor to penetrate further into 
 the physical meaning ot these and other simi- 
 lar actions. I may add to these illustrations 
 of heating power, the ignition cf a diamond 
 in oxygen, by the concentrated beam of ihe 
 dectric lamp. The diamond, surrounded by 
 a hood of platinum to lessen the chilling due 
 to convection, is exposed at the focus. It is 
 
 lens, at the focus C of which combustion is 
 effected). Dr. Scoresby succeeded in ex 
 ploding gunpowder by the sun's r.ys con- 
 verged by large lenses of ice ; toe same effect rapidly raised to a white heat, and when re- 
 moved from ihe focus continues to glow like 
 a star. 
 
 this 1 eauiiful lens of! Placed in the path of the beam issuing from 
 the fucus of the lens I our lamp is a ceil with glass sides containing 
 
 may be produced with a small Kns, and wjgi 
 a terrestrial source of heat. In an iron mould 
 
 All the light of the beau; 
 
 we have fashion*, d 
 transparent ice. At 
 
 place a bit of black paper, with a little gun- a solution of alum. 
 
 cotton folded up within it. The paper ignites passes through this solution. The beam i^ 
 
 and the cotton explodes. Strange, is it not, received on a powerfully converging mirrot 
 
 that the beam should possess such heating silvered in front, and is brought to a focus by 
 
 power after having passed through so cold a the mirror. You can see the conical beam 
 substance? (of relUcied li^ht tracking itself through tae 
 
 In this experiment, you observe that, before dust of the room. I place at the focus a 
 
 the beam reaches the ice-lens, it has passed scrap of white paper ; it glows there with 
 
SIX LECTURES ON LIGHT. 
 
 dazzling Diigntness, but it is not even charred. 
 On removing the alum-cell, however, the 
 paper instantly inflames. There must, there- 
 fo-c, 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 
 
 These remarks apply, not only to the vis- 
 iblr emission examined by Dr. Draper, but 
 to the invisible emission which preceded 
 the appearance of any ligl t. In thj emis- 
 s on from the white-hot platinum wire now 
 before you the very waves exist with which 
 we st tried, only their intensity has been i i- 
 
 the 'ight which is absorbed and which pro- creasid a thousand-fold by the augmentation 
 vokcs combustion. What is this something? I of temperature necessary to the production 
 In the year 1800 Sir William Herschel j of this white light, i oth effects ar ; b mnd 
 
 passed a thermometer through the various 
 colors of the solar spectrum, an i marked the 
 rise of temperature corresponding to each 
 color. lie found the heating effect to aug- 
 ment from the violet to the red ; he di 1 not, 
 however, stop at the red. but pushed his 
 thermometer into the dark space beyond it 
 Here he found the temperature actually higher 
 than in any part of the visible spectrum. By 
 this important observation, he proved that 
 the sun emitted dark heat-rays which are en- 
 tirely unfit for the purposes c f vision. The 
 subject was subsequently taken up by See- 
 beck, Melloni, Miilkr, and others, and within 
 the 1 ist few years it h -s been found capable 
 of unexpected expansions and applications. 
 A muhod has been cbvised whereby the solar 
 or electric beam can be so filtered as to detach 
 from it and preserve intact this invisible 
 ultr -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 so. id bodies to incandes- 
 cence this non-visual emission is the neces- | act of chemical uni n is accompanied by an 
 sary basis of the visual. A platinum wire is I enormous diminution of its diathermancv, or 
 stretched in front of the table, and through \ perviousness to radiant heat. The researches 
 
 which established this result also proved the.. 
 
 elementary gases generally to be highly 
 
 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 only on 
 these conditions; hence Sir William Her- 
 schel's discovery of the invisible ultra-red 
 solar emission. 
 
 The invisible heat, emitted both by dark 
 brdiesand by luminous ones, flies through 
 space with the velocity of light, and is culled 
 radiant heat. Now, radiant heat may b; 
 made a subtle and powerful explorer of 
 molecular condition, and of late years it 
 has given a new significance to the art of 
 chemical combination. Take, for exam pi , 
 the air we breathe. It is a mixture of oxyg n 
 ai.d nitrogen; and with regard to radii. .t 
 heat it behaves like a vacuum, being i.icom 
 petent to absorb it in any sensible deg er. 
 But permit the same two gases to u .it; 
 chemic illy; without any augmentation of tn ; 
 quantity of matter, without altering the gase 
 ous condition, without interfering i i a.iy 
 way with the transparency of the g is, tlu 
 
 it an electric current flows. It is warmed by 
 the current, and may be felt to be warm by 
 the hand ; it also emits waves of heat, but no 
 1 ght. Augmenting the strength of the cur- 
 rent, the wire becomes hotter ; it finally 
 glows with a sober red light At this point 
 Dr. Draper many years ago began an inter- 
 esting investigation. He employed a vol- 
 taic current t:> heat his platinum, and he 
 studied by means of a prism the successive 
 introduction of the colors of the spectrum. 
 His first color, as here, was red ; then came 
 orange, then yellow, then green, and lastly 
 all the shades of blue. Thus as the tempera- 
 ture of the platinum was gradually aug- 
 mented, the at ms were caused to vibrate 
 mo: e rapidly, shorter waves wer3 thus pro- 
 duced, until finally he obtained the waves 
 
 corresponding to the entire spectrum, 
 each successive color was introduced, 
 
 As 
 the 
 
 colors preceding it became more vivid. Now, 
 the vividness, or intensity of light, like that 
 of sound, depends, not upon the length of the 
 wave, but on the amplitude of the vibration. 
 Hence, as the red grew more intense as the 
 more refrangible colors were introduced, we 
 ire 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. 
 
 transparent to radiant heat. This, again, 
 led to the proof of the diathermancy of ele- 
 mentary liquids, like bromine, an 1 of solu- 
 tions of the elements sulphur, phosphorus, 
 and iodine. A spectrum is now before you, 
 and you notice that this transparent bisul- 
 phide of carbon has no effect upon the colors. 
 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 i-> 
 perfectly transparent to the ultra- red emis- 
 sion with which we have now to deal. It, 
 therefore, is to be our ray-filter. 
 
 Placing the alum-cell again in front of the 
 electric lamp, we assure ourselves, as before, 
 of the utter inability of the concentrated 
 light to fire white paper. By introducing a 
 cell containing the solution of iodine, the 
 light is entirely cut off. On remov- 
 ing the alum-eel:, the paper at the dark focus 
 is in. tantly set en fire. Black paper is more 
 absorbent than, white for these ultra-red rays; 
 tuid the consequence is, that with it the sud- 
 denness and vigor of the combustion are aug- 
 mented Zinc is burnt up at the same place, 
 
SIX LECTURES ON LIGHT 
 
 while magnesium ribbon bursts into vivid 
 combustion 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 colors of the spectrum. 
 Before impinging 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 in part broken 
 up into shorter ones, being thus brought 
 within the, visual range. At the other end of 
 the spectrum, Stokes, by the interposition of 
 suitable substances, lowered the refrangibil- 
 ity 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 we give the name of Ca lores 
 fence. 
 
 At the perfectly invisible focus where these 
 effects are produced, the air may be as cold 
 as ice. Air, as already stated, does not ab- 
 sorb the radiant heat, and is therefore not 
 warmed by it. Place at the focus the most 
 sensitive air-thermometer : it is not affected 
 by th.e heat. 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, upon the other. I may add 
 iliat, with suitable precautions, the eye may 
 bj placed in a focus competent to heat plati- 
 nu into vivid redness, without experiencing 
 any damage, or the slightest sensation either 
 of li^ht or heat. 
 
 These ultra-red rays play a most important 
 part in Nature. I remove the iodine filter, 
 and concentrate the total beam. A test-tube 
 containing water is placed at the focus : it 
 immediately begins to sputter, and in a min- 
 ute 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 re- 
 commences. So that we here fix upon the 
 invisible ultra-red rays the heating of the wa- 
 ter. We are enabled now to understand the 
 momentous part played by these rays in Na- 
 ture. It is to them that we owe the warming 
 and the consequent evaporation of the tropi- 
 cal 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. Further, here is a large fla^k 
 containing a freezing mixture. T, u e aqueous 
 vapor of the air has been condensed and 
 frozen on the flask, which is now covered 
 with a white fur. Introducing-the alum-c II, ( 
 we place the coating of hoar-frost at the in- 
 tensely luminous focus ; not a spicul i r> * f V 
 
 , frost is melted. Introducing the iodine-cell, 
 and removing the alum, a broad space of the 
 frozen coating is instantly removed. Hence 
 we infer that the ice which feeds the Rhone, 
 the Rhine, and othei Divers which have 
 glaciers for t : ~.eir sources, is released from its 
 imprisonment upon the mountains by the 
 invisible ultra- red rays of the sun. 
 
 The growth of science is organic. The 
 end of to-day becomes to-morrow the means 
 to a remoter end. Every new discovery is 
 immediately made the basis of other^discov- 
 eries, or of new methods of investigation. 
 About fifty years ago, CErsted, of Copen- 
 hagen, discovered the deflection of a mag- 
 netic needle by an electric current ; and 
 Thomas Seebeck, of Berlin, discovered that 
 electric currents might be derived from heat. 
 Soon afterwards these discoveries were turned 
 to account by Nobilt and Melloni in the con- 
 struction of an apparatus which has vastly 
 augmented our knowledge of radiant heat. 
 The instrument is here. It is called a thermo- 
 electric pile; and it consists of thin bars of 
 bismuth and antimony soldered together in 
 pairs at their ends, but separated from each 
 other elswhere. From the ends of this ' 'pile" 
 wires pass to a coil of covered wire, within 
 and above which are suspended two magnetic 
 needles joined to a rigid system, and carefully 
 defended from currents of air. The heat, 
 then, acting on the pile, produces an electric 
 current ; t'he current, 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 seen. It 
 is now, however, strongly illuminated. Above 
 it is a lens which, if permitted, would form 
 an image of the needle and dial upon the 
 ceiling, where, however, it could not be con- 
 veniently seen. The beam is therefore re- 
 ceived upon a looking-glass, placed at the 
 proper angle, which throws the image upon 
 the screen. In this way the motions of this 
 small needle may be made visible to you all. 
 
 The delicacy of this instrument is such 
 that in a room like this it is exceedingly dif- 
 ficult to work with it. My assistant stands 
 several :eet off. I turn the pile towards him: 
 the heat from his face, even at this distance, 
 produces a deflection of 90. I turn the in- 
 strument towards a distant wall, which I 
 judge to be a little below the average temper- 
 ature 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, and which 
 long engaged the attention of some of our 
 foremost scientific workers, Forbes being the 
 first successful one the substantial identity 
 of light and radiant heat. 
 
 That they are identical in all respects can- 
 not of course be the case, for if they were 
 
SIX LECTURES ON LIGHT. 
 
 39 
 
 they would act in the same manner upon all 
 instruments, the eye included. The identity 
 meant is such as subsists between one color 
 and another, causing them to behave alike as 
 regards reflection, refraction, double refrac- 
 tion, and polarization. As regards reflection, 
 we may employ the looking glass used in our 
 first lecture. Marking any point in the track 
 of the reflected beam, and cutting off the 
 light by the iodine, on placing the pile at the 
 marked point, the needle immediately starts 
 aside. This is true for every position of the 
 mirror. So that both for light and heat the 
 same law of reflection holds good; for both 
 of them also the angular velocity of the re- 
 flected beam is twice that of the reflecting 
 mirror. Receiving the beam on a concave I 
 mirror, it is gathered up into a cone of' re- I 
 fleeted >ight; marking the apex of the cone, | 
 and cutting off the light, a moment's expo- 
 sure of the pile at the marked point produces 
 a violent deflection of the needle. (See Fig. 
 23, where m m is the mirror, P the pile, and 
 T the opaque solution.) 
 
 This beam of light now enters a right- 
 angled prism and is reflected at the hypothe- 
 nuse, in a direction perpendicular to its for- 
 mer one. The reflection here is total. Cut- 
 ;ing off the light, we prove the reflection of 
 the heat to be total also. The formation of 
 
 }J, <', Fig. 24, and placing oui pile D be- 
 hind the analyzer, neither heat nor light 
 reaches it; the needle remains undeflect- 
 ed. Introducing the iodine, the slight- 
 est turning of either prism causes the 
 heat to pass, and to announce it- 
 self by the deflection of the needle. Like 
 light, therefore, heat is polarized. Crossing 
 the Nicols again, the heat is intercepted and 
 
 invisible images by lenses and mirrors may 
 also be demonstrated. Concentrating the 
 beam, and cutting off the light, at the dark 
 focus the carbon-points burn their images 
 through a sheet of black paper. Placing a 
 sheet of platinized platinum at the focus, 
 when the concentration is strong an incan- 
 descent image of the points is immediately 
 stamped upon the platinum. 
 
 And now for polarization and its attendant 
 phenomena. Crossing our two Nicol prisms, 
 
 the needle returns to zero. Plunging into 
 the dark space between the prisms our plate 
 of mica, the needle instantly starts off, show- 
 ing that the mica acts upon the heat as it did 
 upon the light : we have in both cases the 
 same resolution and recompounding of vi- 
 brations. Removing the mica, the needle 
 falls to zero ; but, on introducing a plate of 
 quartz between the prisms, the consequent 
 deflection declares the circular polarization of 
 the heat. For double refraction it is neces- 
 sary that our images should not be too large 
 and diluted : here are the two disks pro- 
 duced by the splitting of the beam in Jce- 
 land spar. Marking the positions of the disks 
 and cutting off the light, the pile finds in its 
 places two heat-images. The needle now 
 stands near 90, and, on turning the spar, 
 the deflection remains constant. Transfer- 
 ring the pile to the other image, the deflec, 
 tion of 90 is maintained ; but on turning 
 the spar the needle now falls to zero. The 
 reason is manifest! Permitting the light to 
 pass, we find the luminous disk at some dis- 
 tance from the pile. We are dealing, in 
 fact, with the extraordinary beam which 
 rotates round the ordinary So that fcr heat 
 as well as for light we have double refraction, 
 and also an ordinary and extraordinary ray. 
 (In the adjacent figure, which shows the ex- 
 perimental arrangement, N is the nozzle of 
 
40 
 
 SIX LECTURES ON LIGHT. 
 
 the electric lamp. 1 a converging lens, B the 
 ^refracting spar, and F the thermo-electric 
 
 fi'i time permitted we might finish the series 
 <5r demonstrations by magnetizing a ray of 
 heat as we magnetized a ray of light. 
 
 We have finally to determine the position 
 and magnitude of the invisible adiatiou 
 which produces these results. For this pur- 
 pose we employ a particular form of the 
 thermo-electric pile. Its face is a rectangle, 
 which by movable side-pieces can be ren- 
 dered as narrow as desirable. Throwing a 
 concentrated spectrum upon a screen, 
 by means of an endless screw, we move this 
 rectangular pile through the entire spectrum. 
 Its surface is blackened so that it absorbs all 
 the light incident upon it, converting it into 
 
 a curve which exhibits the distribution of 
 heat in our spectrum. It is represented in 
 the adjacent figure. Beginning at the blue, 
 the curve rises, at first very gradually; then, 
 as it approaches the red more rapidly, the 
 line CD representing the strength of the ex- 
 treme 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 fading from the perception 
 of the pile. This figure is the result of 
 more than twelve careful series of measure- 
 ments, for each of which the curve was con- 
 structed. On superposing all these curves, 
 a 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- 
 
 FIG. 2$. 
 
 heat* and thus enabling it to declare its power 
 by the deflection of the magnetic needle. 
 
 When this instrument is brought to the 
 violet end of the spectrum, the heat is found 
 to be almost insensible. As the pile grad- 
 rally moves from the violet towards the red; 
 it encounters a gradually augmenting heat. 
 The red itself possesses the highest heating 
 power of all the colors 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, attains a max- 
 imum. From this point the heat falls some- 
 what more rapidly than it rose, and afterwards 
 gradually fades away. Drawing an hori- 
 zontal line to represent the length of the 
 spectrum, and' erecting along it, ft viYi-nu 
 points, perpendiculars proportional H length 
 to the heat existing at those points, we obtain 
 
 I live energies of the visible and invisible 
 I radiation The one is 7.7 times the other. 
 
 But in verification, as already stated, con- 
 sists 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 
 emission. In this way, it was found that the 
 energy of the invisible emission is eight times 
 that of the visible. No two methods could 
 be more opposed to each other, and hardly 
 j any two results could better harmonize. I 
 think, therefore, you may rely upon the ac- 
 curacy of the distribution of heat here as- 
 sig ied to the prismatic spectrum of the elec- 
 tric light. There is nothing vague in the 
 mode of" investigation, nor doubtful in its 
 conclusions. 
 
SIX LECTURES ON LIGHT. 
 
 4 
 
 LECTURE VI. 
 
 Or\&Mj^*c of Spectrum Analysis: Solar Chemistry: 
 Summary and Conclusions. 
 
 We have employed, as our source of light 
 in these lectures, the ends of two rods of coke 
 rendered incandescent by clectricitv. Coke 
 is particularly suitable for this purpose, be- 
 cause it can bear intense heat without fus on 
 or vaporization. It is also black, which 
 helps the light; for, other circumstances br- 
 ing equal, as shown experimentally by Bal- 
 
 four Stewart, the blacker the body the 
 brighter will be its light when incandescent. 
 Still, refractory as carbon is, if we closely ex- 
 amine our voltaic arc, or stream of light be- 
 tween the carbon-points, we should find ihere 
 incandescent carbon-vapor. We might also 
 detach the light of this vapor from the more 
 dazz'ing light of the solid points, and obtain 
 its ^ptctrum This would be not only less 
 brilliant, but of a totally different character 
 from the spectra that we have ; Iready seen* 
 Instead of being an unbroken succession of 
 colors from red to violet, the carbon -vapor 
 
' SlA LECTURES ON UC5IJT, 
 
 \vould yield a few bands of color with spaces 
 of darkness between them. 
 
 What is true of the carbon is true in a still 
 rr.o r e striking degree of the metals, the most 
 refractory of which can be fused, boiled, md 
 reduced to vapor by the electric current. 
 From the incandescent vapor the light, as a 
 general rule, flashes in groups of rays of 
 definite degrees of refrangibility, spaces ex- 
 isting between group and group, which are 
 unfilled by rays of any kind. But the con- 
 templation of the facts will render this sub- 
 ject rr.ore intelligible than words can make it. 
 Within the camera is now placed a cylinder 
 of carbon hollowed out at the trp to receive 
 a bit of metal; in the hollow is placed a frag- 
 ment of the mital thallium, and now you see 
 the arc of incandescent thallium-vapor upon 
 the screen. It is of a beautiful green color. 
 What is the meaning of that green? We 
 answer the question by subjecting the light 
 to prismatic analysis. Here you have its 
 sj cctrum, consisting of a single refracted 
 band. Light of one degree of refrangibility, 
 and that corresponding to green, is emitted 
 by the thallium-vapor. 
 
 We will now remove the thallium and put 
 a bit of silver in its place. The arc of silver 
 is not to be distinguished from that of thal- 
 lium; it is not only green, like the thallium- 
 vapor, but the same shade cf green. Arc 
 they, then, alike? Prismatic analysis en 
 ables us to answer the question. It is per- 
 fectly impossible to confound the spectrum 
 < f incandescent silver vapor with that of 
 thallium. Here are two green bands instead 
 of one. Adding to the silver in our camera 
 a 1 it of thallium, we obtain the light of both 
 metals, and you see that the green of the thal- 
 lium lies midway between the two greens of 
 the silver. Hence this similarity of color. 
 
 But you observe another interesting fact. 
 The thallium band is now far brighter than 
 the silver bands ; indeed, the latter have won- 
 dei fully degenerated since the bit of thallium 
 was put in. They are not at all so bright as 
 they were at first, 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 
 vapor facilitates the passage of the current to 
 such a degree as to render it almost incom- 
 petent to vaporize the silver. But the thal- 
 lium is gradually consumed ; its vapor di- 
 minishes, the resistance rises, until finally 
 you see the two silver bands as brilliant as 
 they were at first. The three bands of the 
 two metals are now of the same sensible 
 brightness. 
 
 We have in these bands a perfectly unal- 
 terable characteristic of these 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 oiher than the three green bands from 
 the mixture of both metals. Every known 
 metal has its bands, and in no known case 
 are the bands of two different metals alike. 
 Hence these spectra may be made a 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 
 us green bands, zinc gives us blue and red 
 bands ; brass, an alloy of copper and zinc, 
 gives us the bands of both metals, perfectly 
 unaltered in position or character. 
 
 But we are not confined to the metals ; the 
 salts of these metals yield the bands of the 
 metals. Chemical union is ruptured by a 
 sufficiently high heat, the vapor of the metal 
 is set free and yields its characteristic bands. 
 The chlorides of the metals are particularly 
 suitable for experiments of this character. 
 Common salt, for example, is a compound of 
 chlor ne and sodium ; in the electric lamp, it 
 yields the spectrum of the metal sodium 
 The chlorides of lithium and of strontium 
 yield in like manner the bands of these 
 metals. When, therefore, Bunsen and Kirch- 
 hoff, the celebrated founders of spectrum 
 analysis, after having established by an ex- 
 haustive examination the spectra of all known 
 substances, discovered a spectrum containing 
 bands different from any known bands, they 
 immediately inferred the existence of a new 
 metal. They were operating at the time 
 upon a residue obtained by evaporating one 
 of the mineral waters of Germany. In thaf 
 water they knew the new metal was con- 
 cealed, but vast quantities of it had to b& 
 evaporated before a residue could be obtained 
 sufficiently large to enable ordinary chemistry 
 to grapple with the metal. But they hunted 
 it down, and it now stands among chemical 
 substances as the metal Rubidium. They 
 subsequently discovered a second metal, 
 whic i they called Casium. Thus, having 
 first placed spectrum analysis on a safe foun- 
 dation, they demonstrated its capacity as an 
 agent of discovery. Soon af.erwards Mr. 
 Crookes, pursuing this same method, ob- 
 tained the salts of the thallium which yielded 
 that bright monoc romatic green band. The 
 metal itself was first isolated by a French 
 chemist. 
 
 All this relates to chemical discovery upon 
 earth, where the materials are in our own 
 hands. But Kirchhoff showed how spectrum 
 analysis might be applied to the investigation 
 of the sun and stars, and on bis way to this 
 result he solved a problem which had been 
 long an enigma to natural philosophers. A 
 spectrum is pure in which the colors do not 
 overlap each other. W T e purify the spectrum 
 by making our slit narrow and by augment- 
 ing 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 in- 
 
SIX LECTURES ON LIGHT. 
 
 dark lines. Four of them were 
 first seen by Dr. Wollaston, but they were 
 a r terwards multiplied anJ measured by Fraun- 
 |"-fer 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 philos- 
 ophers. (The principal lines 
 are lettered according to Fraun- 
 hofer in the annexed sketch of 
 the solar spectrum A, it may 
 be stated stands near the ex- 
 treme red, and J near the 
 extreme violet.) 
 
 Now, Kirchhoff had made 
 thoroughly clear to his mind 
 the princ pies which linked to- 
 gether the emission of light 
 and the absorption of light ; 
 he had proved their insepara- 
 bility for each particular kind 
 of light and heat. He had 
 proved, for every specific ray 
 of the spectrum, the doctrine 
 that the body emitting any ray 
 absorbed with special energy a 
 ray of the same refrangibility. 
 Consider, then, the effect of 
 knowledge, such as you now 
 possess, upon a mind prepared 
 like that of Kirchhoff. We 
 have seen the incandescent va- 
 pors of metals emitting defi- 
 nite groups of rays ; accord- 
 ing to Kirchhoff 's principle, 
 those vapors, if crossed by solar 
 light, ought to absorb !ays of 
 the same refrangibility as those 
 which they emit. He proved 
 this to be the case ; he was 
 able, by the interposition of a 
 vapor, to cut out of the solar 
 spectrum the band correspond- 
 ing in color to that vapor. 
 Now. the sun possesses a pho- 
 tosphere, or vaporous enve- 
 lope doubtless mixed with vi- 
 olentlv agitated clouds and 
 Kirchhoff saw that the power- 
 ful rays coming from the solid, or the 
 molten nucleus of the sun, must be inter- 
 cepted by this vapor. One dark band of 
 Fraunhof r, for example, occurs in the 
 yellow of the spectrum. Sodium vapor is 
 .demonstrably competent to produce that dark 
 band; hen- e Kirchhoff inferred the exist- 
 ence of sodium-vapor in the atmosphere of 
 the sun. In the cas,e of metals, which emit 
 a large number of bands, the absolute coi 
 c dence of every bright band of the nutal 
 with a dark Fraunhofer line, raises to tho 
 highest degree of certainty the inference that 
 the metal is present in the atmosphere of the 
 sun. In this way solar chemistry was found- 
 *d on spectrum analysis. 
 
 FIG. 27. 
 
 I But let me not skim so lightly over this 
 great subject. T have spoken of emis- 
 1 sion and absorption, and of the link that 
 I binds them. L<.t me endeavor to make plain 
 to you, through the analogy of sound, their 
 | physical meaning. I draw a fiddle-bow aciois 
 this tuning-fork, and it immediately fills the 
 room with a musical sound; this may be re- 
 garded as the radiation or emission of soun 1 
 from the fork. A few days ago, on sounui.ig 
 this fork, I noticed that, when its vibratio is 
 were quenched, the sound seemed to be con- 
 ! tinned, though more feebly. The sound ; p- 
 ! peared to come from under a distant tabl , 
 | where stood a number of tuning-forks of dif- 
 j ferent sizes and rates of vibration. One of 
 these, and one only, had been started by the 
 fork, and it was one whose rate of vibration 
 was the same as that of the fork which 
 started it. This is an instance of the ab- 
 sorption of sound of one fork by another. 
 j P.acing two forks near each other, sweeping 
 j the bow over one of them, and then quench- 
 | ing the agitateJ fork, the other c -ntinues to 
 j sound. Placing a cent-piece on each prong 
 of one of the forks, we destroy its perfect 
 synchronism \\ ith the oth, r, and then no co.n- 
 munication of sound from the one to the other 
 I is possible. 
 
 i will now do with //V^what has been here 
 done with sound. Placing a tin spoon con 
 taining sodium in a Bunsen's flame, we ol - 
 tain this intensely yellow light, which corr . 
 sponds in refrangibility with the yellow band 
 of the spectrum. Like our tuning-fork, it 
 emits waves at a special period. I will send 
 the white light from our lamp through that 
 flame, and prove before you that the yellow 
 flame intercepts the yellow of the spectrum 
 S S, Fig. 28; in other words, absorbs \v;ves 
 of the same period as its own, t'hus produc- 
 ing, to all intents and purposes, a dark 
 j Fraunhofer's band in the place of the yellow. 
 j (A Bunsen's flame contained within the chim- 
 ney Cis placed in front of the lamp L. Th i 
 tin spoon with its pellet of sodium is plunged 
 into the flame. Vivid combustion soon sets 
 in, and, whe n it does, the yellow of the spec- 
 trum, at D, is furrcwed by a dark ban '. 
 Withdrawing and introducing the sodium- 
 flame in rapid succession, the sudden disap 
 pearance a id reappearance of the strip of 
 dai kness aro observed). 
 
 Mentally, as well as physically, every agt 
 j of the world is the outgrowth and offspn g 
 of all preceding ages. Science proves itself 
 to be a genuine product of Nature by grow 
 ing according to this l?w. We have no so- 
 lution of continuity here. Every great dis- 
 covery has been duly prepared for in two 
 ways: first, by other discoveries which form 
 its prelude; and, secondly, through the 
 sharpening, by exercise, of the intellectual 
 instrument itself. Thus Ptolemy grew cue 
 of Hipparchus, Copernicus out of both, Kep- 
 ler out of all three, and Newton out of all 
 the four. Newton did not rise suddenly from 
 
SIX LECTURES ON LIGHT. 
 
 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-Ian-', as a 
 massive peak; still he is supported by it, and 
 a great part of his absolute height was the 
 height of humanity in his time. It is thus 
 
 'tn the dib-covx ~ies of Kirchhoff. Much had 
 beta previous!/ accomplished; this he mas- 
 tered, and the*: by the force of individual 
 genius went beyond it. He replaced uncer- 
 tainty by certaiuty, vagueness by definite- 
 ness, confusion Ly c r der; and I do not think 
 that Newton has a sn> er claim to the discov- 
 eries that have made h's name immortal than 
 Kirchhoff has to the credit of gathering up the 
 fragmentary knowledge of his time, of vastly 
 extending it, and of infusing into it the life 
 of great principles. Splendid results have 
 since been obtained with which the names of 
 Janssen, Huggins, Loci:.) ?r, Respighi, 
 Young, and others, are honorably associated, 
 out, splendid as they are, they are but the 
 sequel and application of the principles es- 
 tablished in his Heidelberg laboratory by the 
 celebrated German investigator. 
 
 SUMMARY AND CONCLUSION 
 
 My desire in these lectures has bf.n to 
 show you, with as little breach of continuity 
 as possible, the past growth and p/esent as 
 pect of a department of science, n which 
 have labored some of thj gre: test i itellccts 
 the world has ever seen. My f "iend Profes- 
 sor Henry, in introducing me atWaslvn^ton, 
 spoke of me as an apostle; but t.aeonl . apos- 
 tolate that I intended to fulfil was to place, 
 
 in plain words, my subject before you, and to 
 permit its own intrinsic attr ctions to act up- 
 on your minds. In the way of experiment, I 
 have tried to give you the best which, under 
 the circumstances, could be provided; but I 
 have sought to confer on each experiment a 
 distinct intellectual val.ie, for experiments 
 ought to be the representatives and exposi- 
 tors of thought a language addressed to the 
 eye as spoken words are to the ear. 16 as- 
 sociation with its context, nothing is more 
 impressive or instructive than a fit experi- 
 ment; 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 mas- 
 tery 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 necessarv muscular 
 exercise, so these early men speculated vague- 
 ly and confusedly regarding light, not having 
 as yet the discipline needed to give clearness 
 to their insight, and firmness to their grasp 
 of principles. They assured themselves of 
 th ; rectilineal propogation of light, and that 
 the angle of incidence was equal to the angle 
 of reflection. For more than a thousand 
 years I might ray, 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 fo'ce, which might have run 
 in the direction of science, was diverted in- 
 .o other directions. 
 
 The course of investigation as regards 
 j light was resumed in 1100 by an Arabian 
 ! philosopher named A.'hazan. Then it was 
 I taken up in succession by Roger Bacon, Vi- 
 I tellio, and Kepler. These men, though fail- 
 ing to detect the principle which ruied the 
 facts, kept the fire of investigation constantly 
 j burning. Then came the fundamental dis- 
 covery of Snell, that corner-stone of optics, 
 I as I have already called it, and immediately 
 afterward we have the application by Des 
 cartes of Snell's discovery to the explanation 
 of the rainbow. Then came Newton's 
 crowning experiments on the analysis and 
 I synthesis of white light, by vhich it was 
 ' proved to be compounded of various kinds 
 of light of different degrees of refrangibiiity. 
 In 1676 an impulse was given to optics by 
 astronomy. In that year Olaf Roemer, a 
 learned Dane, was engaged at the Observa- 
 tory of Paris in observing the eclipses of Ju- 
 piter's moons. He converted them into so 
 many signal-lamps, quenched when they 
 plunged into the shadow of the planet, and 
 relighted when they emerged from the shadow. 
 They enabled him to prove that light 
 requires time to pass through space, and to 
 j assign to it the astounding velocity of 190,- 
 I ooo miles a second. Then came the English 
 
SIX LECTURES OX LIGHT. 
 
 45 
 
 astronomer, Bradley, who noticed that the 
 fixed stars did not really appear to be fixed, 
 but dvscribe in the heavens evtry year a little 
 orbit resembling the earth's orbit. The re- 
 su ; t perplexe?! 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 ihe Thames in a boat, and 
 noticed that, as io:i r as his course remained 
 unchanged, the vane upo i his mast-head 
 showed the wind to be blowing constantly in 
 the same direction, but that the wind ap- 
 peared to vary with ev.-ry change in the di- 
 rection of his boat. " Here," as Whewell 
 s y , " was the image of his case The boat 
 was the earth, moving in its orbit, and the 
 vind was the light of a star." 
 
 We rnay ask in passi.ig, what, without the 
 .'acuity which formed the '"image," would 
 J5radley's wind and vane have been to him ? 
 A wind and vane, and nothing more. You 
 will immediately understand the meaning of 
 BraUey's discovery. Imagine yourself in a 
 noiionless railway-train with a shower of rain 
 descending vertically downward. The mo- 
 ment the train begins to move, the rain-drops 
 begin to s^ant, and the quicker the train the 
 greater is the obliquity. In a precisely sim- 
 ilar manner the rays from a star vertically 
 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 ner 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 "aberration of light," as his 
 discovery is called, enabled him to assign to 
 it a velocity almost identical with that de- 
 duced by Roe ner from a totally different 
 method of observation. Subsequently Fizeau, 
 employing not planetary or stellar distances, 
 but simply the breadth of the city of Paris, 
 determined the veloci y of light : while after 
 him Foucault a man of the rarest mechanical 
 genius solved the problem witn mt quitting 
 his private room. 
 
 Up to his demonstration of the composition 
 of white light, Newton had been everywhere 
 triumphant triumphant in the heavens, tri- 
 umphant on the earth, and his subsequent 
 experimental work is for the most part of 
 immortal value. But infallibility is not the 
 gift 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, .ana" that 
 you could not abolish the one without at the 
 same time abolishing the other. Here Dol- 
 land corrected him But Newtoa committed 
 a graver error than this. Science, as 1 
 sought to make clear to yo.i in our second 
 lecture, is on!y i i part a thing of the senses. 
 The roots of piienonuna are embedded in a 
 region beyond the reach of the sen es, and 
 less than the rout of the matter uii! never 
 
 satisfy the scientific mind. We find, accord- 
 ingly, in tiiis career of optics, the greater 
 minds constantly yearning to pass from the 
 phenomena to their causes to explore them 
 to their hidden roots. They thus entered 
 the region of theory, and here Newton, 
 though drawn from time to time towards the 
 truth, was drawn sii.l more strongly towards 
 the error, anJ made it his substantial choice. 
 His experiments are imperishable but his 
 theory has pa sed away. For a century it 
 stood like a dam across the course of discov- 
 ery ; but, like all barriers that rest upon 
 authority, and no. upon truth, the pressure 
 from behind increased, and eventually swept 
 the barrier away. This, as you know, was 
 done mainly through the labors 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 Pa!ace in Paris, discovered 
 the polarization of light by reflection. In 
 18 1 1 Arago discovered the splendid chro- 
 matic phenomena which we have had illus- 
 trated by plates of gvp-um i:i polarized lignt ; 
 he also discovered the rotation of the plane 
 of polarization by quartz-crystals. In 1813 
 Seebeck discoveied the polaiiz tion of light 
 by tourmaline. That same y^ar Brews. er 
 discovered those magnificent ban, s of co.or 
 that surround the axes of biaxal cry ta's. In 
 1814 Wollaston discovered the ri.i^s of Ice- 
 land spar. All these erkcU, \wiuh, without 
 a theoretic clue, would leave the human 
 mind in a hopeless jungle of phenonena 
 without 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 remarkable ver.fication fell to the 
 lot of the late Sir William Hamilton, of Dub- 
 lin, a profound mathematician, who. taking 
 up the theory where Fresnel had left it, ar- 
 rived at the conclusion that, at fo ir special 
 points at the surface of the ether-wave in 
 double-refracting crystals, lie ray was divided 
 not into two parts, but into an infinite num- 
 ber of parts ; forming at these points a con- 
 tinuous conical envelope instead of two 
 images. No human eye had ever seen this 
 envelope when Sir WillLm Hamilton inferred 
 its existence. Turning to nis fri.nJ Dr. 
 Lloyd, he asked him to test experimentally 
 the truth of his theoretic conclusion. Lloyd, 
 taking a crystal of arragonite, and following 
 with the most scrupulous exactness the indi- 
 cations of theory, cutting the crystal wheie 
 theory said it ought to be cut, observing it 
 .here theory said it ought to be observed, 
 found the luminous envelope which had pre 
 viousiy been a mere idea in the mind of the 
 malhem.ttici in. 
 
 .\e\ertneless this great theory cf undula- 
 tion, like many another truth, which in the 
 m has proved a blessing t> humanity. 
 
SIX LECTURES ON LIGHT. 
 
 had to establish, by hot conflict, its right to 
 exist* nee. Great names were arrayed against 
 it. It had been enunciated by Hooke, it had 
 been applied by Huyghens, it had been de- 
 fended by Euler. But they made no impres- 
 sion. And, indeed, the theory in their hands 
 was more an analogy than a demonstration. 
 It first took the form of n demonstrated verity 
 in the hand of Tho.nas Young. He broug it 
 the waves of light to bear upon each other, 
 causing them to support each other, and to 
 extinguish each ether at will. From their 
 mutual actions he determined their lengths, 
 and applied his determinations in all direc- 
 tions. He showed t at the standing difficulty 
 of polarization might be embraced by the 
 theory. After him came Fresnel, whose 
 transcendent mathematical abilities enat led 
 him to give the theory a generality unattained 
 by Young. He grasped the theory in its 
 entirety , followed the ether into its eddies 
 and estuaries in the hearts of crystals of the 
 mo>t complicated structure, and into bodies 
 subjected to strain and pressure. He showed 
 that the facts discovered by Malus, Arago, 
 Brewster, and Biot, were so many ganglia, 
 so to speak, of his theoretic organism, deriv 
 ing 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, leaving, however, behind him a 
 name immortal in the annals of science. 
 
 One word more I should like to say regard- 
 ing Fresnel. There are things, ladies and 
 gentlemen, better even than science. There 
 are matters of the character as well as matters 
 of thj intellect, and it is always a pleasure to 
 those who wish to think well 01 human na- 
 ture, when high intellect and upright charac- 
 ter are combined. They were, I believe, 
 combined in this young Frenchman. In 
 thos: hot conflicts of the undulatory theory, 
 he stood forth as a man of integrity, claiming 
 no more than his right, and ready to concede 
 their rights to others. He at once recognized 
 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. 1 should 
 like to read you a brief extract from a letter 
 written by Fresnel to Young in 1824, as it 
 ihrows a pleasant light upon the character of 
 the Frenc \ philosopher. '* For a long time," 
 says Fresnel, "that sensibility, or that 
 vanity, which people call love of glory, has 
 been much blunted in me. I labor much 
 less to catch the suffrages of the public than 
 to obtain that inward approv :i which has 
 always been the sweetest rewarxi of my efforts. 
 Without doubt, in moments of disgust and 
 discouragement, I have often needed the spur 
 ( f 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. theo- 
 
 retic truth, or the confirmation of a calcula- 
 tion by experiment." 
 
 This, ladies and gentlemen, is the core of 
 the whole matter as regard^ science. It 
 must be cultivated for its own sake, for the 
 pure love of truth, rather than for the ap- 
 plause or profit that it brings. And now my 
 occupation in America is wellnigh g~ne. 
 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? what 
 the prize of their high calling for whici they 
 struggled so assiduously? What urged them 
 to those battles and those victories over reti- 
 cent Nature which have become the heritage 
 of the human race ? It is never to be for- 
 gotten that not one of those great investiga- 
 tors, from Aristotle down to Stokes and 
 Xirchhoff, had any practical end in view, 
 according to the ordinary definition of the 
 word "practical." They di 1 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 may see to-day the issues of their 
 work in a thousand practical forms, and this 
 may be thought sufficient to justify it, if not 
 ennoble their efforts. But they did not work 
 for such issues > their reward wa ,of a totally 
 different kind. We love clothes, we love 
 luxuries, we love fine equipages, we lovt 
 money, and any man who can point to these 
 as the result of Us efforts in lite justifies 
 these efforts before all the world. In Ameri- 
 
 a 
 
 con- 
 
 fidently to this assembly whether such thing* 
 exhaust the demands of human nature? Tht 
 very presence here for six inclen ent nights 
 of this audience, embodying so much Oi the 
 mental force and refinement of this great 
 city, is an answer to my question I neetf 
 not tell such an assembly that there are joy; 
 of the intellect as well as joys of the body, 
 or that these pleasures of the spirit constu 
 tuted the reward of our great investigators. 
 Led on by the whisperings of natural truth, 
 through pain and self-denial, they often pur- 
 sued their work. With the ruling passion 
 strong in death, some of tnem, when no 
 longer able to hold a pen, dictated to their 
 friends the result of their labors, and, then 
 rested from them forever. 
 
 Could we have seen these men at work 
 without any knowledge of the consequences 
 of their work, what should we have thought 
 of then\? To many of their contemporaries 
 it would have appeared simply ridiculous to 
 see men, whose namei are now stars in the 
 firmanent of science, straining their atten- 
 tion to observe an effect of experiment al- 
 most too minute for detection, To the un- 
 
 . 
 
 ca and England more especially he is 
 " practical " man. But I would appeal co 
 
SIX LECTURES ON LIGHT. 
 
 initiated, they might well appear as big 
 children playing with not very amusing toys. 
 It is so to this hour. Could you watch the 
 true investigator your Henry or your Dra- 
 uer, for example in his laboratory, unless 
 animated by his spirit, you could hardly un- 
 derstand what keeps him there. Many of 
 the objects which rivet his attention might 
 jppear to you utterly tiivial ; and, if you were 
 cO step forward and 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 ot nations, the history of 
 science amply proves ; but the hope of its 
 doing so never was and never can be the 
 motive power of the investigator. 
 
 I know that 1 run some risk in speaking 
 thus before practical men. I knojv 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 
 t with avidity when it leads to useful appli- 
 cations." But what, I would ask, are the 
 hopes of useful applications which have 
 drawn you so many times to this place in 
 >pite 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 yot* to make a single 
 cent by science, am I among you to-night, 
 but because I tried to the best of my ability 
 iO present science to the world ss an intellec- 
 tual good. Surely no two terms were ever 
 so distorted and misapplied with refe.ence to 
 man in his higher relations as these terms 
 useful and practical. As if there were no 
 nakedness of the mind to be clothed as well 
 as nakedness of the body no hunger and 
 thirst of the intellect to s.itisfy. Let us ex- 
 pand the definitions of these terms until they 
 embrace all the needs of man, his highest in- 
 tellectual needs inclusive. It is specially on 
 this ground of its administering to the higher 
 needs of intellect, it is mainly because I be- 
 lieve it to be wholesome as a source of know- 
 
 tion. You are delighted, and with good 
 reason, with your electric telegrrphs, proud 
 of your steam-engines and your factories, 
 and charmed with the productions of pho- 
 tography. 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 forcts 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 
 peech on my part, I will answer this ques- 
 tion by an illustration. Stiip 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, I 
 fear, be very much in the condition of that 
 plastic dough. 
 
 At the present time there is a cry in Eng- 
 land for technical education, and it is the ex- 
 pression of a true national want ; but there 
 is no cry for original investigation. Still 
 without this, as surely as the stream dwindles 
 when the spring dries, so surely will "tech- 
 nical education" lose all foice of growth, all 
 power of reproduction. Our great investi- 
 gators 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 De Tocqutville. 
 who, having forgotten the scientific origin of 
 what they did, were at length compelled to 
 copy without variation the inventions of an 
 ancestry who, wiser than themselves, had 
 drawn their inspiration direct from Nature. 
 
 To keep society as regards science in 
 healthy play, three classes of workers are 
 necessary: Firstly, the investigator of natura/ 
 truth, whose vocation it is to pursue that 
 truth, and exteud the field of discovery for 
 the truth's own sake, and without reference 
 to practical ends. Secondly, the teacher of 
 natural truth, whose vocation it is to give 
 
 ledge, and as a means of discipline, that I I public diffusion to the knowledge already won 
 urge the claims of science this evening upon [ by the discoverer. Thirdly, the applier of 
 
 your attention. 
 
 But. with refen nee 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 elec- 
 tricity before Wheatslone and Morse ; where- 
 as, in point of fact, Watt and Wheatstone 
 and Morse, with all their practicality, were 
 the mere outcome of antecedent forces, which 
 acted without reference to practical ends. 
 This also, 1 think, me its u moment's atteu- 
 
 natural truth, whose vocation it is to make 
 scientific knowledge available for the needs, 
 comforts, and luxuries of life. These three 
 classes ought to coexist 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 .hj appli a 
 tions of science. Such applic uions. esj.i- 
 cially on this continent, are so as'ounding 
 they spread themselves so largely and um- 
 brageously before the public e-ye as to shut 
 
SIX LECTURES ON LIGHT. 
 
 out from view those workers who are engaged 
 in thj quieter ami profounder business of 
 original mvestiga'ion. 
 
 Take the electric telegraph as an example, 
 whi h 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 
 assuredly they have received it. But I should 
 be untrue to you and to myself if I failed lo 
 tell you that, however high in particular re- 
 spects their claims and qualities may be, prac- 
 tical men did not discover the electric tele- 
 gr j ph. Th-i discovery of the electric telegraph 
 impiie . the discovery of electricity itself, and 
 the development of its laws and phenomena. 
 Such discoveries are not made by practical 
 men, and they never will be m .do by them, 
 because their minds are bes<_'t with ideas 
 which, though of the highest value from one 
 point of view, are not those which slimulate 
 the original discoverer. 
 
 The ancients discovered the electricity of 
 arnber; and Gilbert, in the year 1600, ex- 
 tended the force to other bodies Then fol- 
 lowed other inquirers, your own Franklin 
 among the number. But this form of elec- 
 tricity, though tried, did not come into use 
 lor telegraphic purposes. Then appeared the 
 great Italian, Volto, who discovered the 
 source of electricity, which bears his name, 
 and applied the most profounu Lisight and 
 the most delicate experimental skill to its de- 
 velopment. Then arose the man who added 
 to the powers of his intellect all the graces of 
 the human heart, Michael Faraday, the dis- 
 coverer of the great domain of magneto- 
 electricity. CErsted discovered the deflection 
 ol the magnetic needle, and Arajpp and Stur- 
 eo i the magnetization of iron by the elec- 
 tric current. The voltaic circuit finally found 
 *ts theoretic Newton i i Ohm, while Henry, 
 of Princeton, who had the sagacity to recog- 
 
 our peril. For I say again, that, behind afi 
 your practical applications, 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 help- 
 less. In no case is the adage truer, "Other 
 men labored, but ye are entered, in o their 
 labors," than in the case, of,. the discoverer 
 and the applier of natural truth. But now a 
 word on the other side. While I say that 
 practical men are not the men to make the 
 necessary antecedent < iscoveries, the cases 
 are rare in which the discoverer knows how 
 lo turn his labors tc practical account. Dif- 
 ferent qualities of mind and different habits 
 of thought are needed in .he two cases; and. 
 while I wish to give emphatic utterance to 
 the claims of those whose claims, owing to 
 the simple fact of their intellectual elevation, 
 ar: often misunderstood, I am not here to 
 exalt the one class of workers at the expense 
 of the other. They are the necessary supple- 
 ments of each other; but remember that one 
 class is sure to be taken care of. Ail the ma- 
 terial rewards of society are already within 
 their reac.i; but it is at our peril th^twe neg- 
 lect to provide opportunity for those studies 
 and pursuits which have no such rewards, 
 and from wnich, therefore, the rising genius 
 of the country is incessantly tempted away. 
 
 Pasteur, one of the most eminent member? 
 of the Institute of Fiance, in accounting for 
 the disastrous overthrow of his country ana 
 the predominance of Germany in the late 
 war, expresses himself thus: " Few persons 
 comprehend the real origin of the marvels ei 
 industry and the wealth of nations. I need 
 no further proof of this than the employment, 
 more and more frequent in official language, 
 and in writing of all sorts, of the erroneous 
 expression applied science. The abandon- 
 ment of scientific c rcers by men capable of 
 pursuing them with distinction, was recent!*- 
 complaint d of. in the presence of a minister 
 
 uize the merits of Ohm while they were still I of the greatest talent. This statesman en- 
 
 decried in his own country, was at this time 
 ; .n the van of experimental inquiry. 
 
 In the works of these men you have all 
 rhe materials employed at this hour in all the 
 forms of the electric telegraph. Nay, more ; 
 Gauss, the celebrated astronomer, and Weber, 
 the celebrated natural philosopher, both pn> 
 fessors in the University of Gottingen, wish- 
 ing to establish a rapid mode of communication 
 between the observatory and the physical 
 cabinet of the university, did this by means 
 of an electric . telegraph. The force, in 
 shot t, 'had been discovered, it laws invest^ 
 gated and made sure, the most complete 
 mastery of i's phenomena had been attained, 
 nay, its applicability to teleg.aphic purposes 
 demonstrated, by men whose sole reward for 
 their labors was the noble joy of discovery, 
 and before your practical men appeared at 
 all upon the scene. 
 
 Are \v\; to ignore all this? We do so at 
 
 deavored 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 applied science. Nothing coaid be 
 more erroneous than this opinion, nothing, I 
 venture to say, more dangerous, even to 
 practical life, than the consequences which 
 might How from these words. They have 
 rested on my mind as a proof of the imperi- 
 ous necessity of reform in our superior edu 
 Cation There exists no category of the 
 sciences to which the name of applied science 
 could be given. ll r e have science ^ and the 
 applications of science which are united to- 
 gether as the tree and its (ruit." 
 
 And Cuvier, the great comparative, anato- 
 mist, 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 which 
 were pursued for their own sake, and solely 
 
LECTURES ON LIGHT. 
 
 through an ardor for knowledge. Those 
 who applied them could not have discoverec 
 them; those who discovered them had no in- 
 clination to pursue them to a practica. end. 
 Engaged in the high regions whither their 
 thoughts had carried them, they hardly per- 
 ceived these practical issues, though born of 
 their own deeds. These rising workshops, 
 these peopled colonies, those ships which fur- 
 row 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 concerns 
 them no more." 
 
 When the Pilgrim Fathers landed at Ply- 
 mouth Rock, and when Penn made his treaty 
 with the Indians, the new-comers had to 
 bui'.d their houses, to chasten the earth into 
 cultivation, and to take care of their souls. 
 In such a community, science, in its more 
 abstract forms, was not to be thought cf. 
 And, at the present hour, when your hardy 
 Western pioneers stand face to face with 
 stubborn Nature, piercing the mountains 
 and subduing the forest and the prairie, the 
 pursuit 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 con- 
 tinent is already raised far beyond this need. 
 The gentlemen of New York, Brooklyn, Bos- 
 ton, Philadelphia, Baltimore, and Washing- 
 ton, 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 
 
 words repeated, and, if necessary, laid to. 
 heart. In a work published in 1850, he says: 
 " It must be confessed that, among the civil- 
 ized peoples of our ^ge, there are few in 
 which the highest sciences have made so^ 
 little progress as in the United States. "* 
 He declares his conviction that, had you been 
 alone in the universe, you would speedily 
 have discovered that you cannot long make: 
 progress in practical science, without culti- 
 vating theoretic science at the same time. 
 But, according to De Tocqueville, you are- 
 not thus alone. He refuses to separate 
 America from its ancestral home ; and it is 
 here, he contends, that you collect the treas- 
 ures of the intellect, without taking the- 
 trouble to create them. 
 
 De Tocqueville evidently doubts the ca- 
 pacity of a democracy to foster genius as it 
 was fostered in the ancient aristocracies. 
 41 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 democractic societies as in aris- 
 tocracies. As for me," he continues, "I 
 can hardly believe it." He speaks of the un- 
 quiet feverishness of democratic commu- 
 nities, not in times of great excitement, for 
 such times may give an extraordinary impe- 
 tus to ideas, but in times of peace. There 
 s then, he says, "a small and uncomfortable- 
 agaitation, a sort of incessant attrition of 
 man against man, which troubles and dis- 
 tracts the mind without imparting to it either 
 animation or elevation." It rests with you 
 to prove whether these things are nece-sarily 
 so whether the highest scientific genius can- 
 not find in the midst of you a tranquil home. 
 
 as humanity has yet reached, may be I I should be loath to gainsay so keen an ob- 
 
 ' server and so profound a political writer, but, 
 
 justly demanded at their hands. They have 
 reached that maturity, as possessors of wealth 
 and leisure, when the investigator of natural 
 truth, for the truth's own sake, ought to find 
 among them promoters and protectors. 
 
 since my arrival in this country, I have been, 
 unable to see anything in the constitution of 
 society to prevent a student with the rcct ot 
 the matter in him from bestowing the most.' 
 
 Among the many grave problems before steadfast devotion on pure science. If great 
 them they have this to solve, whether a re- | scientific results are not achieved in Ameri- 
 public is able to foster the highest forms of ca, it is not to the small agitations of society 
 genius. You are familiar with the writings 
 
 that I should be disposed to ascribe the de- 
 
 of De Tocqueville, and must be aware of the ] feet, but to the fact that the men among you 
 intense sympathy which he felt for your insti- | who possess the endo vments necessary for 
 tutions ;. and this sympathy is all the more scientific inquiry are laden with duties of ad- 
 
 ministration or tuition so heavy as to be 
 utterly incompatible with the continuous and 
 
 valuable, from the philosophic candor 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, th_ 
 reverse of unfriendly venture to repeat be- 
 fore 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 thrre-and-tweniy years ago, and 
 perhaps would not write the same to-day ; 
 
 tranquil meditation which original investi- 
 gation demands. It may well be 
 
 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 honored 
 as he ought to be in this land ? I hardly 
 think they would. Still 1 do not think this 
 
 * II faut reconnaitre, que parmis les peuples civil- 
 ises de nos jours, il en esc pcu chez qui les hautea. 
 sciences aient fait moins de progres qu'aux Etats- 
 Unis, ou qui aient fourni moins de grands artistes, 
 
 jicriiaps wouiu 1101 wruciiic SKUIIC lu-uay , j de poetcs 7 Uustr e S , e t de celobrcs ecrivains. (De U- 
 but it will do nobody any harm to have his ' Democratic en Amenque, etc., tom-ii., p. 36.) 
 
SIX LECTURES ON LIGHT. 
 
 state of things likely to last. In America 
 there is a willingness on the part of individu- 
 als to devote their fortunes, in *,he matter of 
 education, to the service of the common- 
 wealth, which is without a parallel elsewhere; 
 and this willingness requires but wise di- 
 rection to enable you effectually to wipe away 
 the reproach of De Tocquaville. 
 
 Your most difficult proWem will be no f to 
 build institutions, but to make men ; not to 
 form the body, but to rind the spiritual em- 
 bers which shall kindle within that body 
 a living soul. You have scientific genius 
 among you ; not sown broadcast, believe 
 me, but still cattered here and there. Take 
 a!l unnecessary impediments out of its way. 
 Drawn by your kindness I have come here to 
 give 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 with- 
 out a stain. No lecturer was ever rewarded 
 
 as I have been. From 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. 
 It is not solely, or even chiefly, as lecturers, 
 but as investigators, that your men of genius 
 ought to be employed. Keep your sympa- 
 thetic eye upon the originator of knowledge. 
 | Give him the freedom necessary for his re- 
 j searches, not overloading him. either with the 
 I duties of tu tion or of administration, not de- 
 manding 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 that truth may* appear. 
 If you cast your bread thus upon the waters, 
 then be assured it will return to you, thougb 
 it may be after many days. 
 
 CONTENTS. 
 
 JL0CT. I. INTRODUCTORY- . . 2 
 
 II. Origin of Physical Theories ... 8 
 
 III. Relation of Theories to Experience . 18 
 IV. Chromai ; c Phenomena produced by 
 
 Crystals 27 
 
 LECT. V. Range of Vision and Range of Ra- 
 diation .......... 3^. 
 
 VI. Spectrum Analysis. ...... A, 
 
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 Wilson, Ph. D. 
 
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 Essays. By Richard A. Proctor. 
 
THE HUMBOLDT LIBRARY OF SCIENCE. 
 
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 NO. 46. 
 
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 Vignettes front Nature. By Grant 
 Allen. 
 
 The Philosophy of Style. By 
 
 Herbert Spencer. 
 
 Oriental Religions. By John Caird, 
 Pres. Univ. Glasgow, and Others. 
 
 Lectures on Evolution. (Illus- 
 trated}. By Prof. T. H. Huxley. 
 
 Six Lectures on Light. (Illus- 
 trated). By Prof. Tyndall. 
 
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 Archibald Geikie, F.R.S. 
 
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 The Evidence of Organic Evo- 
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 F.R.S. 
 
 Current Discussion in Science. 
 
 By W. M. Williams, F.C.S. 
 
 History of the Science of Poli- 
 tics. By Frederick Pollock. 
 
 Darwin and Hnmboldt. By Prof. 
 Huxley, Prof. Agassiz, and others. 
 
 The Dawn of Hitory. Parti. By 
 G. F. Keary, of the British Museum. 
 
 The Dawn of History. Part II. 
 
 The Diseases of Memory. 
 
 Th. Ribot. Translated from 
 French by J. Fitzgerald, M.A. 
 
 The Childhood of Religion. 
 
 Edward Clodd, F.R.A.S. 
 
 By 
 the 
 
 By 
 
 Life In Nature. (Illustrated). By 
 James Hinton. 
 
 The Sun : its Constitution, its Phe- 
 nomena, its Condition. By Judge 
 Nathan T. Carr. 
 
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 Exchange. By Prof. W. Stanley 
 Jevons, F.R.S. Parti. 
 
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 No. 52. 
 
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 No. 57- 
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 No. 60. 
 No. 61. 
 
 Money and the Mechanism of 
 Exchange. Part II. 
 
 Th Diseases of the Will. By 
 
 Th. Ribot. Translated from the 
 French by J. Fitzgerald, M.A. 
 
 Animal Automatism, and other 
 Essays. By Prof. T.H.Huxley, F.R.S. 
 
 The Birth and Growth of Myth. 
 
 By Edward Clodd, F.R.A.S. 
 
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 and other Essays. By William King- 
 don Clifford, F.R.S. 
 
 Illusions. 
 Illusions. 
 
 By James Sully. 
 Part II. 
 
 Part. I. 
 
 The Origin of Species. By Charles 
 Darwin. Part I. (Double number). 
 
 The Origin of Species. Part II. 
 (Double Number). 
 
 The Childhood of the World. 
 
 By Edward Clodd, F.R.A.S. 
 
 Miscellaneous Essays. By Richard 
 A. Proctor. 
 
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 World. By Prof. Geo. Rawlinson, 
 Univ. of Oxford. (Double number). 
 
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 Fowler, LL-D , President of Corpus 
 Christi Coll., Oxford. 
 
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 and Plants. By A. Russell Wal- 
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 on Archae- 
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THE HUMBOLDT LIBRARY OF SCIENCE. 
 
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 Richard A. Proctor. 
 
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 Would through the Action of 
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 M D. 
 
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 Th. Ribot Translated from the 
 French by J. Fitzgerald, M A. 
 
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 Thomas H. Huxley, and Grant Allen. 
 
 No. 97. 
 
 The Pleasures of Life. 
 
 John Lubbock. 
 
 By Sir 
 
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 Teachings of Science. By Wil- 
 liam Kingdon Clifford. 
 
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 Eaton Lowe ; Dr. Robert Brown, 
 F L S ; Geo. G. Chisholm, F R G S.; 
 and James Dallas, F L S. 
 
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 Sully and Geo. Croom Robertson. 
 
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 Theory of Co-operatloii. By 
 
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 of Legislators; The Great 
 Political superstition. By 
 
 Herbert Spencer. 
 
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 the University of Jena With a pref- 
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 By Richard Chenevix Trench. Parti. 
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 Part II. 
 
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 By Sir John Lubbock. 
 
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 Th. Ribot Translated from the 
 French by J Fitzgerald, M.A. 
 
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 M.D., Head Physician of the Stock- 
 holm Hospital, Professor of Psychia- 
 try. Late Royal Swedish Medical 
 Councillor. Authorized Translation 
 from the Second Swedish Edition by 
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 Gerald Molloy, D D., D.Sc. 
 
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 (Darwinism and Politics. By 
 
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 I Thomas Huxley, F.R.S. 
 
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 By Arnold Toynbee, Tutor of Baliol 
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THE HUMS O LOT LIBRARY OF SCIENCE. 
 
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