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LIGHT, VISIBLE AND INVISIBLE 
 
MACMILLAN AND CO., Limited 
 
 LONDON • BOMBAY • CALCUTTA • MADRAS 
 
 MELBOURNE 
 
 THE MACMILLAN COMPANY 
 
 NEW YORK • BOSTON • CHICAGO 
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 THE MACMILLAN CO. OF CANADA, Ltd. 
 
 TORONTO 
 
 
LIGHT 
 
 VISIBLE AND INVISIBLE 
 
 A SERIES OF LECTURES 
 
 DELIVERED AT THE ROYAL INSTITUTION OF 
 
 GREAT BRITAIN, AT CHRISTMAS, 1896 
 
 WITH ADDITIONAL LECTURES 
 
 BY 
 
 SILVANUS P: THOMPSON, ^N 
 
 D.Sc, F.R.S., M.R.I. 
 
 PRINCIPAL OF, AND PROFESSOR OF PHYSICS IN, THE CITY AND GUILDS 
 TECHNICAL COLLEGE, FINSBURY, LONDON 
 
 SECOND EDITION, ENLARGED 
 
 *«W COLLEGE UBHahY 
 I CJMrrNUT HHJL, MASS. 
 
 MACMILLAN AND CO., LIMITED 
 
 ST. MARTIN'S STREET, LONDON 
 
 1928 
 
 ttOSTON COLL!»E tlBRAt^" 
 CHESTNUT HILL, MASS. 
 
COPYRIGHT 
 
 First Edition i8g7 
 Second Edition, enlarged^ 1910, 1912, 1919, 1921, 1928 
 
 255899 
 
 PRINTED IN GREAT BRITAIN 
 EY R. & R. CLARK, LIMITED, EDINBURGH 
 
INTRODUCTION 
 
 Two things are expected of a lecturer who undertakes 
 a course of Christmas lectures at the Royal Institution. 
 In the first place his discourses must be illustrated to 
 the utmost extent by experiments. In the second, 
 however simple the language, in which scientific facts 
 and principles are described, every discourse must 
 sound at least some note of modernity, must reflect 
 some wave of recent progress in science. 
 
 So in undertaking a course of lectures in Optics in 
 the year 1896 the lecturer ventured to proceed on 
 certain lines which may, perhaps, seem strange to the 
 sedate student whose knowledge of optics has been 
 acquired on the narrower basis of the orthodox text- 
 book. The ideas developed in the first lecture arose 
 from the conviction that the time-honoured method of 
 teaching geometrical optics — a method in which the 
 wave-nature of light is steadily ignored — is funda- 
 
VI LIGHT 
 
 mentally wrong. For the sake of students and teachers 
 of optics he has added to Lecture I. an Appendix, in 
 which the newer ideas are further developed. Other 
 Appendices have been added to the later Lectures, 
 with the aim of filling up some of the gaps left in the 
 subjects as treated in the lecture theatre. 
 
 Now that the electromagnetic nature of all light- 
 waves has been fully demonstrated, no apology is needed 
 for bringing into the fifth Lecture a few of the experi- 
 mental points upon which that demonstration rests. 
 That these fundamental points can be given without 
 any great complication of either thought or language 
 is in itself the strongest argument for making that 
 demonstration an essential feature at an early stage in 
 the teaching of the science. 
 
 Many of the ideas which must be grasped, for 
 example that of the polarisation of light, are popularly 
 supposed to be extremely difficult; whereas the difficulty 
 lies not in the ideas themselves so much as in the 
 language in which they are generally set forth. In an 
 experience lasting over a good many years, the author 
 has found that the main points in the phenomena of 
 polarisation are quite easily grasped by persons of 
 ordinary intelligence — even by children — provided they 
 are presented in a modern way devoid of pedantic 
 
INTRODUCTION vil 
 
 terms, and illustrated by appropriate models. A 
 similar remark would equally apply to other parts of 
 optics, such as interference and diffraction, which are 
 barely alluded to in the present lectures. Many 
 branches are necessarily omitted altogether from so 
 brief a course : amongst them the entire subject of 
 spectrum analysis, the construction and theory of optical 
 instruments, and the greater part of the subject of 
 colour vision. No attempt was made to include these 
 topics, and no apology is needed for their omission. 
 Whatever value these discourses may possess must 
 depend upon the things they include, not upon those 
 which they do not. 
 
 At the request of the Publishers the author has added 
 a lecture on Radium which he has several times delivered 
 in different places in 1903- 1904, together with the lecture 
 On the Manufacture of Light which was given to a 
 popular audience at the Meeting of the British Associa- 
 tion in the city of York in 1906. 
 
 London, May 19 10. 
 
CONTENTS 
 
 LECTURE I 
 
 LIGHT AND SHADOWS 
 
 How light-waves travel — Experiments with the ripple-tank — How 
 shadows are cast — How to make light -waves converge and 
 diverge — Measurement of brightness of lights — Reflexion of 
 light by mirrors — Formation of images — Regular and irregular 
 reflexion — Diffuse reflexion by paper and rough surfaces — 
 Multiple images — Refraction of light — Lenses — The eye as an 
 optical instrument — Some curious optical experiments — Inver- 
 sion of Images — The magic mirror of Japan — English magic 
 mirrors ......... Page I 
 
 Appendix — The general method of geometrical optics . . 55 
 
 LECTURE II 
 
 THE VISIBLE SPECTRUM AND THE EYE 
 
 Colour and wave-length — Rainbow tints — The spectrum of visible 
 colours — Spectrum made by prism — Spectrum made by grat- 
 ing—Composition of white light — Experiments on mixing 
 
X LIGHT 
 
 colours — Analysis of colours — Blue and yellow mixed make 
 white, not green — Complementary tints — Contrast tints pro- 
 duced by fatigue of eye — Other effects of persistence of vision 
 — Zoetrope — Animatograph o . . . . Page 71 
 
 Appendix — Anomalous refraction and dispersion . . . 100 
 
 LECTURE III 
 
 POLARISATION OF LIGHT 
 
 Meaning of polarisation — How to polarise waves of light — Illus- 
 trative models — Polarisers made of glass, of calc-spar, and of 
 slices of tourmaline — How any polariser will cut off polarised 
 light — Properties of crystals — Use of polarised light to detect 
 false gems — Rubies, sapphires, and amethysts — Polarisation by 
 double-refraction — Curious coloured effects, in polarised light, 
 produced by colourless slices of thin crystals when placed 
 between polariser and analyser — Further study of comple- 
 mentary and supplementary tints — Exhibition of slides by 
 polarised light — Effects produced on glass by compression, and 
 by heating ...... o . . 105 
 
 Appendix — The elastic-solid theory of light .... 156 
 
 LECTURE IV 
 
 THE INVISIBLE SPECTRUM (ULTRA-VIOLET PART) 
 
 The spectrum stretches invisibly in both directions beyond the 
 visible part — Below the red end are the invisil)le longer waves 
 that will warm bodies instead of illuminating them — These are 
 called the calorific or infra-red waves. Beyond the violet end 
 of the visible spectrum are the invisible shorter waves that 
 
/ 
 CONTENTS xi 
 
 produce chemical efifects — These are called actinic or ultra- 
 violet waves — How to sift out the invisible ultra-violet light from 
 the visible light— How to make the invisible ultra-violet light 
 visible — Use of fluorescent screens — Reflexion, refraction, and 
 polarisation of the invisible ultra-violet light — Luminescence : 
 the temporary kind called Fluorescence, and the persistent 
 kind called Phosphorescence — How to make "luminous 
 paint " — Experiments with phosphorescent bodies — Other pro- 
 perties of invisible ultra-violet light — Its power to diselectrify 
 electrified bodies — Photographic action of visible and of in- 
 visible light — The photography of colours — Lippmann's dis> 
 covery of true colour-photography — The reproduction of the 
 colours of natural objects by trichroic photography — Ives's 
 photochromoscope . ...... Page i6o 
 
 Appendix — Table of wave-lengths and frequencies , . 190 
 
 LECTURE V 
 
 THE INVISIBLE SPECTRUM (iNFRA-RED PART) 
 
 How to sift out the invisible mfra-red light from the visible light — 
 Experiments on the absorption and transmission of invisible 
 infra-red light — It is cut off by transparent glass, but trans- 
 mitted by opaque ebonite — Use of radiometer — Use of thermo- 
 pile and bolometer — "Heat-indicating" paint — Experiments 
 on the reflexion, refraction, and polarisation of invisible infra- 
 red light — Discovery by Hertz of propagation of electric waves 
 — Plertzian waves are really gigantic waves of invisible light 
 — Experiments on the properties of Hertzian waves ; their 
 reflexion, refraction, and polarisation — Inference that all light- 
 waves, visible and invisible, are really electric waves of different 
 sizes . . . . . . . . . . 192 
 
 Appendix — The electromagnetic theory of light . , . 230 
 
xii LIGHT 
 
 LECTURE VI 
 
 RONTGEN LIGHT 
 
 Rontgen's Discovery — Production of light in vacuum tubes by 
 electric discharges — Exhaustion of air from a tube — Geissler- 
 tube phenomena — The mercurial pump — Crookes's-tube pheno- 
 mena — Properties of Kathode light — Crookes's shadows — De- 
 flection of Kathode light by a magnet — Luminescent and 
 mechanical effects — Lenard's researches on Kathode rays in air 
 — Rontgen's researches — The discovery of X-rays by the lumin- 
 escent effect — Shadows on the luminescent screen— Transpar- 
 ency of aluminium — Opacity of heavy metals — Transparency of 
 flesh and leather — Opacity of bones^ Absence of reflexion, re- 
 fraction, and polarisation — Diselectrifying effects of Rontgen 
 rays — Improvements in PvOntgen tubes — Speculations on the 
 nature of Rontgen light — Seeing the invisible . . Page 238 
 
 Appendix — Other kinds of invisible Hght .... 277 
 
 LECTURE VII 
 
 RADIUM AND ITS RAYS 
 
 Emission by certain substances of radiations that will penetrate 
 opaque screens — Properties of uranium salts — The Becquerel 
 rays — Radio-activity— Examination by electroscope — Researches 
 of the Curies — Madame Curie discovers /^olofimm and radium 
 in pitchblende — Experiments with radium — Separation by 
 magnetic field of the three kinds of rays emitted by radium — 
 Strutt's radium clock — Crookes's spinthariscope — Researches 
 of P. Curie on heat emitted by radium, and of Rutherford on 
 disintegration of radium atom . , . . . 2S1 
 
CONTENTS xiii 
 
 LECTURE VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 Primitive sources of light — Invention of gas-lighting — Invention of 
 electric -lighting — Cause of incandescence — Incandescence by 
 electricity — Luminescence — Luminous efficiency— Photometry 
 — The Photometer — Inequality of distribution of light from lamps 
 — Inequality of composition of lights — The teaching of the 
 spectrum — Spectra of incandescent solids and vapours — Sensi- 
 tiveness of the eye to radiations of particular wave-lengths — 
 Absorption and emission — Measurement of emission — Bad 
 economy of ordinary sources of light — Light of the fire-fly — 
 Temperature, and quality of radiation — Emissivity of the rare 
 earths — High-pressure incandescent gas-lighting — Efficiency of 
 glow-lamps — New kinds of glow-lamps — New kinds of arc- 
 lamps — Electric vapour-lamps — Cost of manufacture of light — 
 Cheapest form of light — Future progress — Sunlight after all 
 
 Page 302 
 
 INDEX 371 
 
LECTURE I 
 
 LIGHTS AND SHADOWS 
 
 How light-waves travel — Experiments with the ripple-tank — How 
 shadows are cast — How to make light -waves converge and 
 diverge — Measurement of brightness of lights — Reflexion of 
 light by mirrors — Formation of images — Regular and irregular 
 reflexion — Diff'use reflexion by paper and rough surfaces — 
 Multiple images — Refraction of light— Lenses — The eye as an 
 optical instrument — Some curious optical experiments — Inver- 
 sion of images — The magic mirror of Japan — English magic 
 mirrors. 
 
 Light, as is known both from astronomical observations 
 and from experiments made with optical apparatus, travels 
 at a speed far exceeding that of the swiftest motion of any 
 material thing. Try to think of the swiftest thing on the 
 face of the earth. An express train at full speed, per- 
 haps, occurs to you. How far will it go while you 
 count up to ten ? Counting distinctly I take just over 
 5J seconds. In that time an express train would have 
 travelled 500 feet ! Yet a rifle-bullet would have gone 
 farther. There is something that goes quicker than any 
 actual moving thing. A sound travels faster. In the 
 same time a sound would travel a mile. Do you say 
 that a sound is only a movement in the air, a mere aerial 
 
 B 
 
2 LIGHT LECT. 
 
 wave ? That is quite true. Sound consists ' of waves, 
 or rather of successions of waves in the air. None of 
 you who may have listened to the delightful lectures of 
 Professor M'Kendrick in this theatre last Christmas will 
 have forgotten that ; or how he used the phonograph to 
 record the actual mechanical movements impressed by 
 those air-waves as they beat against the sensitive surface 
 of the tympanum. 
 
 But this Christmas we have to deal with waves of a 
 different kind — waves of light instead of waves of sound 
 — and though we are still dealing with waves, yet they 
 are waves of quite a different sort, as we shall see. 
 
 In the first place, they travel very much faster than 
 waves of sound in the air. During that 5|- seconds, 
 while an express train could go 500 feet, or while a 
 sound would travel a mile, light would travel a million 
 miles ! A million miles ! How shall I get you to think 
 of that distance ? An express train going 60 miles an 
 hour would take 16,666^ hours, which is the same thing 
 as 694 days 10 hours 40 minutes. Suppose you were 
 now — 29th December 1896, 3 o'clock — to jump into 
 an express train, and that it went on and on, not only 
 all day and all night, but all through next year, day after 
 day, and all through the year after next, month after 
 month, until November, and that it did not stop till 
 24th November 1898 at 20 minutes before 2 o'clock 
 in the morning; by that time — nearly two years — you 
 would have travelled just a million miles. But the 
 space that an express train takes a year and eleven 
 months to travel, light travels in 5 J seconds — just while 
 you count ten ! 
 
1 LIGHTS AND SHADOWS 3 
 
 And not only are the waves of light different from 
 those of sound in their speed — they are different in size. 
 As compared with sound-waves they are very minute 
 ripples. The invisible waves of sound are of various 
 sizes, their lengths differing with the pitch of the sound. 
 The middle c of the pianoforte has a wave-length of 
 about 4 feet 3 inches, while the shrill notes that you can 
 sing may be only a few inches long. A shrill whistle 
 makes invisible ripples about half an inch long in the air. 
 But the waves of light are far smaller. The very largest 
 waves of all amongst the different kinds of visible light 
 — the red waves — are so small that you could pack 
 39,000 of them side by side in the breadth of one inch ! 
 And the waves of other colours are all smaller. How 
 am I to make you grasp the smallness of these wavelets ? 
 What is the shortest thing you can think of? The thick- 
 ness of a pin ? Well, if a pin is only a hundredth 
 part of an inch thick it is still 390 times as broad as a 
 ripple of red light. The thickness of a human hair? 
 Well, if a hair is only a thousandth part of an inch thick 
 it is still 39 times as big as the size of a wave of red 
 light. 
 
 Now, from the facts that waves of light travel so fast, 
 and are so very minute, there follow some very important 
 consequences. One consequence is that the to-and-fro 
 motions of these little ripples are so excessively rapid — 
 millions of millions of times in a second — that there is 
 no possible way of measuring their frequency : we can 
 only calculate it. Another consequence is that it is 
 very difficult to demonstrate that they really are waves. 
 While a third consequence of their being so small is 
 
4 LIGHT LECT. 
 
 that, unlike big waves, they don't spread much round 
 the edges of obstacles. 
 
 You have doubtless all often watched the waves on 
 the sea, and the ripples on a pond, and know how when 
 the waves or the ripples in their travelling strike against 
 an obstacle, such as a rock or a post, they are parted by 
 it, pass by it, and run round to meet behind it. But 
 when waves of light meet an obstacle of any ordinary 
 size they don't run round and meet on the other side of 
 it — on the contrary, the obstacle casts a shadow behind 
 it. If the waves of light crept round into the space 
 behind the obstacle, that space would not be a dark 
 shadow. 
 
 Well, but that is a question after all of the relative 
 
 sizes of the obstacle and of the waves. Sea waves may 
 
 meet behind a rock or a post, because the rock or the 
 
 post may not be much larger than the wave-length.^ 
 
 But if you think of a big stone breakwater — much bigger 
 
 in its length than the wave-length of the waves, — you 
 
 know that there may be quite still water behind it ; in 
 
 that sense it casts a shadow. So again with sound-waves; 
 
 ordinary objects are not infinitely bigger than the size of 
 
 ordinary sound-waves. The consequence is that the 
 
 sound-waves in passing them will spread into the space 
 
 behind the obstacle. Sounds don't usually cast sharp 
 
 acoustic shadows. If a band of musicians is playing in 
 
 front of a house, you don't find, if you go round to the 
 
 ^ Note that the scientific term " wave-length " means the length 
 from the crest of one wave to the crest of the next. This, on the 
 sea, may be 50 feet or more. In the case of ripples on a pond, it 
 may be but an inch or two. Many people would call it the breadth 
 of the waves rather than the length. 
 
I LIGHTS AND SHADOWS 5 
 
 back of the house, that all sound is cut off. The sounds 
 spread round into the space behind. But if you notice 
 carefully you will observe that while the house does not 
 cut off the big waves of the drum or the trombone, it 
 does perceptibly cut off the smaller waves of the flute or 
 the piccolo. And Lord Rayleigh has often shown in 
 this theatre how the still smaller sound-waves of ex- 
 cessively shrill whistles spread still less into the space 
 behind obstacles. You get sharp shadows when the waves 
 are very small compared with the size of the obstacle. 
 
 Perhaps you will then tell me that if this argument 
 is correct, you ought not, even with light-waves, to get 
 sharp shadows if you use as obstacles very narrow 
 obstacles, such as needles or hairs. Well, though per- 
 haps you never heard it, that is exactly what is found to 
 be the case. The shadow of a needle or a hair, when 
 light from a single point or a single narrow slit is allowed 
 to fall upon it, is found not to be a hard black shadow. 
 On the contrary, the edges of the shadow are found to 
 be curiously fringed, and there is light right in the very 
 middle of the shadow caused by the waves passing by 
 it, spreading into the space behind and meeting there. 
 
 However, all this is introductory to the subject of 
 shadows in general. If we don't take special precau- 
 tions, or use very minute objects to cast shadows, we 
 shall not observe any of these curious effects. The 
 ordinary shadows cast by a bright light proceeding 
 from any luminous point are sharp-edged ; in fact, the 
 waves, in ordinary cases, act as though they did not 
 spread into the shadows, but travelled simply in straight 
 lines. 
 
6 LIGHT LECT. 
 
 Let me try to illustrate the general principle of the 
 
 <<Jl.JUL. 
 
 />■/ 
 
 Fig, I. 
 
 . 1 
 
 travelling of ripples by use of a shallow tank of water, 
 
 ^ Ripple-tanks for illustrating the propagation of waves have long 
 been known. Small tanks were used at various times by Professor 
 Tyndall. See also Professor Poynting, F. R.S., in Nature^ 29th 
 May 1884, p. 1 19. 
 
I LIGHTS AND SHADOWS 7 
 
 on the surface of which I can produce ripples at will. 
 An electric lamp placed underneath it throws up shadows 
 of the ripples upon a slanting translucent screen, and 
 you can see, for yourselves how the ripples spread from 
 the centre of disturbance in concentric circles, each circle 
 enlarging, and the ripples following one after another at 
 regular distances apart. That distance is what we call 
 the "wave-length." 
 
 If I use the tip of my finger to produce a disturbance, 
 the ripples travel outward in all directions at an equal 
 speed. Each wave-front is therefore a circle. If, however, 
 I use to produce the disturbance a straight wooden ruler, 
 it will set up straight wavelets that follow one another in 
 parallel ranks. These we may describe as plane waves, 
 as distinguished from curved .ones. Notice how they 
 march forward, each keeping its distance from that in 
 front of it. 
 
 Now, if you have ever watched with care the ripples 
 on a pond, you will know that though the ripples march 
 forward, the water of which these ripples are composed 
 does not — it merely rises up and down as each ripple 
 comes by. The proof is simple. Throw in a bit of 
 cork as a float. If the water were to flow along, it would 
 take the cork with it. But no ; see how the cork rides 
 the waves. It is the motion only that travels forward 
 across the surface — the water simply swings to -and - 
 fro, or rather up and down, in its place. Now that 
 this has once been brought to your attention, you 
 will be able to distinguish between the two kinds of 
 movement — the apparent motion of the waves as they 
 travel along the surface, and the actual motion of the 
 
8 
 
 LIGHT 
 
 LECT. 
 
 particles in the waves, which is ahvays of an oscillatory 
 kind. 
 
 Here is a model of a wave-motion that will make the 
 difference still clearer. At the top a row of little white 
 
 Fig. 2. 
 
 balls (Fig. 2) is arranged upon stems to which, in 
 regular order one after the other, is given an oscillatory 
 motion up and down. Not one of these white particles 
 travels along, l^^ach simply oscillates in its own place. 
 
I LIGHTS AND SHADOWS 9 
 
 Yet the effect is that of a travelling wave, or rather set 
 of waves. The direction in which the wave travels is 
 transverse to the displacements of the particles. The 
 length from crest to crest of the waves is about 4 inches. 
 Their velocity of travelling depends, of course, on the 
 speed with which I turn the handle of the apparatus. 
 The amplitude of the displacement of each of the balls 
 is not more than one inch up or down from the centre 
 line. 
 
 Perhaps now you will be able to think of the little 
 wavelets of light, marching in ranks so close that there 
 are 40,000 or 50,000 of them to the inch, and having a 
 velocity of propagation of 185,000 miles a second. 
 
 Now let me state to you two important principles of 
 wave-motion — all-important in the right understanding 
 of the behaviour of waves of light. 
 
 (i) The first is that waves always^ march at right 
 angles to their own front. This is how a rank of 
 soldiers march — straight forward in a direction square 
 to the line into which they have dressed. It was so 
 with the water-ripples that you have already seen. 
 
 (2) The second principle is that every point of any 
 wave-front may be regarded as a new source or centre 
 from which waves will start forward in circles. Look 
 at the sketch (Fig. 3). From P as a centre ripples 
 are travelling outward in circles, for there has been a 
 disturbance at P. Now if there is placed in the way of 
 
 ^ Always, that is to say, in free media, in gases, liquids, and 
 non-crystalline solids. In crystals, where the structure is such that 
 the elasticity differs in different directions, it is possible to have 
 waves niEtrching obliquely to their own front. 
 
lO 
 
 LIGHT 
 
 LECT. 
 
 these ripples a screen, S, or obstacle, with a hole in it, 
 all the wave-fronts that come that way will be stopped 
 or reflected back, except that bit of each wave-front 
 that comes to the gap in the screen. That particular 
 bit will go on into the space beyond, but will spread at 
 equal speed in all directions, giving rise to a new but 
 fainter set of ripples which will be again of circular form, 
 
 \ \ 
 
 \ \ 
 
 • •»» — >■ ' ^^ — > I 
 
 I >»^ — 5^ - »> > , ^ — > I 
 
 Fig. 3. 
 
 having their centre however not at P but at the gap in 
 the screen. This too I can readily illustrate to you in 
 my ripple-tank. 
 
 The first of these two principles is really a conse- 
 quence of the second, and of anpther principle (that 
 of " interference ") which concerns the overlapping of 
 waves. Of these we may now avail ourselves to find 
 how waves will march if we know at any moment the 
 
LIGHTS AND SHADOWS 
 
 II 
 
 Fm. 4. 
 
 shape of the wave -front. Suppose (Fig. 4) we knew 
 that at a certain moment the wave-front of a set of 
 ripples had got as far 
 as the curved hne FF, 
 and that we wanted to 
 know where it would 
 be an instant later. If 
 we know how fast the 
 wave travels we can 
 think of the time taken 
 to travel some short 
 space such as half an 
 inch. Take then a pair 
 of compasses and open 
 them out to half an 
 inch. Then put the 
 point of the compasses at some part — say a — of the curve 
 FF, and strike out the piece of circle as shown at a'. 
 That is where the disturbance would spread to in that 
 short interval of time if the bit of wave-front at a had 
 alone been allowed to spread forward. But the bit at b 
 is also spreading, so we must strike another arc, using b 
 as centre, and another at ^, and another at </, and so on, 
 using the same radius for all of them. And now we see 
 that if all these bits, instead of acting each separately, 
 are acting at the same time, the wavelets from each will 
 overlap and give us one large enveloping curve at GG ; 
 the effect being the same as though the wave-front FF 
 had itself marched forward to GG. Those parts of the 
 wavelets that tend to spread cross-ways in the over- 
 lapping balance one another ; for instance, part of the 
 
12 LIGHT LECT. 
 
 wavelet from a tends to cross downwards in front of r, 
 while a part of the wavelet from e tends to cross upwards 
 to an equal amount. These sideway effects cancel one 
 another, with the result that the effect is the same on 
 the whole as though the bit of wave at c had simply 
 marched straight forward to c. 
 
 Perhaps you will say that if this is true then when 
 light-waves meet an obstacle some light ought to spread 
 round into the shadow at the edges. And so it does as 
 has already been said. But, owing to the exceeding 
 smallness of the light-waves compared with the dimen- 
 sions of ordinary objects, the spreading is so slight as to 
 be unnoticed. In fact, except when we are dealing 
 with the shadows of very thin objects, like hairs and 
 pins, or with mere edges, the light behaves as though it 
 simply travelled in straight lines.^ 
 
 Our next business is to show how ripples can be 
 made to diverge and converge. If we take a point as 
 our source of the ripples, then they will of themselves 
 spread or diverge from that point in all directions in 
 circles, each portion of each wave-front having a bulg- 
 ing form. If we take as the source a flat surface, so as 
 to get plane waves, they march forward as plane waves 
 
 ^ This is all that is meant by the old statement that light travels 
 in "rays." There really are no rays. The harder one tries to 
 isolate a "ray" by itself, by letting light go first through a narrow 
 slit or pinhole, and then passing it through a second slit or pinhole, 
 the more do we find it impossible ; for then we notice the tend- 
 encies to spread more than ever. If the word "ray" is to be 
 retained at all in the science of optics, it must be understood to 
 mean nothing more than the geometrical line along which a piece 
 of the wave-front marches. 
 
LIGHTS AND SHADOWS 
 
 13 
 
 Fig. 5. 
 
 without either diverging or converging. If, however, we 
 can in any way so manage our experiments as to get 
 ripples with a hollow front 
 instead of a bulging front, 
 then the succeeding ripples 
 will converge as they march. 
 This is shown in Fig. 5. 
 Suppose FF is a hollow wave- 
 front marching forward to- 
 ward the right. Think of the 
 bit of wave-front at a. After 
 a short interval of time it 
 would spread (were it alone) 
 to^^'. Similarly^ would spread 
 to b\ and so on, so that when 
 all these separate wavelets overlap, the effect is the same 
 as though there the wave-front FF had marched to GG, 
 closing in as it marches. After the lapse of another 
 equally short interval it will have closed in to HH. It is 
 clear that, on the principle that waves alw^ays march at 
 right angles to their own front, they tend all to march 
 inwards and meet at a new centre somewhere at Q. 
 Suppose you ranged a row of soldiers in a curve like 
 FF, and told each soldier to march straight forward be- 
 tween his comrades. If each soldier were to march at 
 right angles to the curved line, they would all be march- 
 ing toward a common centre, and would close in against 
 one another ! 
 
 Now it is obviously easy to make waves of light 
 diverge — they do so of themselves if the source of light 
 be a point. We shall see later how to make them con- 
 
14 LIGHT LECT. 
 
 verge ; but, meantime, we will use what we know about 
 divergence to help us to measure the relative brightness 
 of two lights. 
 
 Here is a little electric glow-lamp. The shopman 
 who sold it to me says that when it is supplied with 
 electric current at the proper pressure,^ it will give as 
 much light as sixteen candles. I switch on the current 
 and it shines. I light a standard candle,^ so that you 
 can compare the brightness for yourselves. Do you 
 think that the glow-lamp is really sixteen times as bright 
 as the candle? Your eye is really a very unreliable 
 judge ^ of the relative brightness. We must, therefore, 
 find some way of balancing the brighter and the less 
 bright lights against one another. The instrument for 
 doing this is called a photometer. 
 
 ^ Electric pressure, or "voltage," is measured in terms of the 
 unit of electric pressure called the "volt." The usual electric pres- 
 sure of the conductors which branch from the supply-mains into a 
 house is lOO volts. 
 
 ^ The standard candle prescribed by the regulations of the 
 Board of Trade as the legal standard of light in Great Britain is a 
 sperm candle burning 120 grains of spermaceti per hour. 
 
 ^ This unreliability of the eye to form a numerically correct 
 judgment is partly dependent on the physiological fact that the sen- 
 sation is never numerically proportional to the stimulus. Though 
 the stimulus be 16 times as great, the sensation perceived by the 
 brain is not 16 times as great. The rule (Fechner's law) is that 
 the sensation is proportional to the natural logarithm of the 
 stimulus. The natural logarithm of 16 is 277 ; that is to say, the 
 light that is 16 times as bright as I ,candle only produces a sensa- 
 tion 277 times as great. A single light of 100 candle brilliancy 
 only produces a sensation 4*6 times as great as that of I candle. 
 Besides this the iris diaphragm of the eye automatically reduces the 
 size of the pupil when a brighter light shines into the eye, making 
 the eye less sensitive. 
 
LIGHTS AND SHADOWS 
 
 15 
 
 But before we can understand the photometer we 
 must first think about the degree of illumination which 
 a light produces when it falls upon a white surface. I 
 take here a piece of white cardboard one inch square. 
 If I hold it close to my candle it catches a great 
 deal of the light, and is brightly illuminated. If I 
 hold it farther away it is less brightly illuminated. 
 We can, therefore, alter the illumination of the sur- 
 face by altering the distance. But we cannot use this 
 principle for calculations about brightness until we 
 know the rule that connects the distance with the 
 degree of illumination ; and that rule depends upon the 
 way in which light spreads when it starts from a point. 
 
 Fig. 6. 
 
 Suppose we think of the whole quantity of light that is 
 spreading all round from a point. Of all that amount 
 of light what fraction will be caught by this square inch 
 of cardboard when I hold it a foot away? Not very 
 much. But now think of that same amount of light as 
 as it goes on spreading. Fig. 6 shows you that by the 
 time that the light has travelled out from the centre to 
 double the distance it will have spread (according to the 
 law of rectilinear propagation discussed above) so that 
 the diverging beam is now twice as broad each way. 
 It will now cover a cardboard square that is 2 inches 
 each way, or that has 4 square inches of surface. So if 
 the same amount of light that formerly fell on i square 
 inch ^s now spread over 4 square inches of surface, it 
 
i6 LIGHT LECT. 
 
 follows that each of those 4 square inches is only 
 illuminated one quarter as brightly as before. If you 
 had a bit of butter to spread upon a piece of bread — 
 and then you were told that you must spread the same 
 piece of butter over a piece of bread of four times the 
 surface, you know that the layer of butter would be 
 only the quarter as thick ! And so again, if I let the 
 light spread still farther, by the time it has gone three 
 times as far it will have spread over nine times the 
 surface, and the degree of illumination on any one square 
 inch at that treble distance will be only one-ninth part 
 as great as at first. This is the so-called law of "inverse 
 squares," and is simply the geometrical consequence^ 
 of the circumstance that the light is spreading from a 
 point. Now we are ready to deal with the balancing of 
 two lights. By letting two lights shine on a piece of card- 
 board, or rather on two neighbouring pieces, and then 
 altering the distance of one of the lights until both 
 pieces of card are equally illuminated, we can get a 
 balance of effects, and then calculate from the squares 
 of the distances how bright the lights were. The eye, 
 which is a very bad judge of relative unequal bright- 
 nesses is really a very fair judge (and by practice can be 
 trained to be a very accurate judge) of the equality of 
 illumination of two neighbouring patches. But we must 
 make our arrangements so that only one light shines 
 
 ^ The fact that a candle flame is not a mere point introduces a 
 measurable error in photometry. It cannot be too clearly under- 
 stood that the law of inverse - squares is never applicable strictly 
 except to effects spreading from points. This criticism applies also 
 to the use or misuse of the law of inverse-squares in magnetism and 
 electricity. 
 
LIGHTS AND SHADOWS 
 
 17 
 
 upon each patch. One simple way of doing this is to 
 let each light cast a 
 shadow of a stick on 
 a white surface, so that 
 each light shines into 
 the shadow cast by 
 the other. If you alter 
 the distances till the 
 shadows are equally 
 dark, then you know 
 that the illumination 
 of each is equal. But 
 a better way is to 
 arrange matters that 
 the two illuminated 
 patches are actually 
 superposed. Here is 
 a very simple and 
 effective way of doing 
 it. Two pieces of 
 white cardboard, A 
 and B (Fig. 7), form- 
 ing a V-shape, are set 
 upon a stand, between 
 the two lights that are 
 to be compared. One 
 light shines upon the 
 surface of A, and the 
 other upon the surface 
 of B. Through A are 
 cut a number of slots or holes, so that the illuminated 
 
 G 
 
i8 LIGHT LECT. 
 
 surface of B is seen through the slots in A. If the 
 illumination of A is duller than that of B the slots will 
 seem dark between the brighter bars of the front card ; 
 but if the illumination of A is brighter than that of B 
 then the slots will seem bright between dull bars.^ By 
 moving one of the lights nearer or farther away, the 
 respective illuminations can be altered until balance is 
 obtained ; and then the relative values are calculated 
 from the squares of the distances. With this photometer 
 let us now test our electric lamp to see if it is really 
 worth sixteen candles. I put it on the photometer 
 bench and move it backward and forward till the lights 
 balance. You see it balances when rather less than 
 four times as far away as the standard candle. It is, 
 therefore, of not quite sixteen candle-power. 
 
 Another very simple and accurate photometer is 
 made by taking two small slabs of paraffin wax (such as 
 candles are made of) and putting them back to back 
 
 ^ This form of photometer is a modification by Mr. A. P. 
 Trotter, M.A. , of Cape Town, of the reHef photometer invented in 
 1883 by the author and Mr. C. C. Starling. To prevent error 
 arising from internal reflexion the back of the card A should be 
 blackened. By setting the support at a fixed distance from the 
 standard light on the left side, and altering, as needed to obtain 
 balance, the distance of the light of which the brightness is to be 
 measured, it is possible to make the instrument direct-reading ; the 
 scale to the right of the support being graduated so as to read not 
 the actual distances but their squares. For instance, if the distance 
 of the middle slot from the standard light be I metre, then on the 
 other side the graduation must read I at I metre ; 4 at 2 metres ; 
 9 at 3 metres, and so forth. Accuracy of reading is promoted by 
 the circumstance that when balance has been found for the middle 
 slot of A the slots to the left of the middle will look darker, and 
 those to the right brighter than the central one. 
 
LIGHTS AND SHADOWS 
 
 19 
 
 with a sheet of tin-foil or black paper between them. 
 They are then placed (as in Fig. 8) on the graduated 
 bench between the lights whose 
 brightness- is to be compared to- 
 gether, and set in such a way that 
 one light shines on one paraffin 
 slab, and the other light on the 
 other slab, as in Fig. 9. If the 
 illuminations on the two sides 
 balance the edges of the slabs will seem equally bright. 
 But if the illumination on one face is stronger than 
 that on the other then that paraffin slab which is 
 more highly illuminated will seem brighter at its edge 
 than the other.^ This is because of the translucent or 
 
 Fig. 9. 
 
 ^ This paraffin slab photometer is the invention of Dr. Joly, 
 F.R.S., of Dublin. It is an exceedingly satisfactory instrument. 
 
 ^Iitll^?-J. > »>_._ ^1 
 
 7K 
 
 / w \, 
 
 _^^ Light No. 2 
 
 Fig. 
 
 Either of these two forms of instrument here described is preferable 
 to the old-fashioned "grease-spot" photometer of Bunsen. But 
 both are surpassed in accuracy by the precision -photometer of 
 
20 LIGHT LECT. 
 
 semi-opaque property of paraffin wax, which results in 
 a diffusion of the light laterally. With this photometer 
 it is very easy to balance the brightness of two lights, 
 even if their tint be not quite identical. In Germany, 
 they employ as standard, instead of a sperm candle, the 
 little Hefner lamp filled with a chemical liquid known 
 as amyl-acetate. But it has — as you see — the serious 
 disadvantage of giving out a light which is unfortunately 
 of a redder tint than most of our other lights. To be 
 quite suitable, the lamp that we choose as a standard of 
 light ought to be not only one that will give out a fixed 
 quantity of light, but one that is irreproachable in the 
 quality of its whiteness : it should be a standard of 
 white light. Perhaps now that acetylene gas is so 
 easily made it may serve as a standard, for as yet 
 none of the proposed electric standards seem quite 
 satisfactory. 
 
 Let us pass on to the operation of reflecting light by 
 means of mirrocs. A piece of polished metal such as 
 
 Brodhun and Lummer, which can, however, only be described here 
 very briefly. It gives determinations that can be reHed on to within 
 one-half of one per cent. The two lights to be compared are caused 
 to shine on the two opposite faces of a small opaque white screen, W 
 (Fig. lo). The eye views these two sides, as reflected in two small 
 mirrors, M^^ and M^, by means of a special prism-combination, con- 
 sisting, as shown, of two right-angled prisms of glass, A and B, 
 which are cemented together with balsam over only a small part of 
 their hypotenuse surfaces; the light from M^ can pass direct through 
 this central portion to the eye, but the uncemented portions of the 
 hypotenuse surface of B act by total internal reflexion and bring the 
 light from Mo to the eye. The eye, therefore, virtually sees a patch 
 of one surface of W surrounded by a patch of the other surface of 
 W, and hence can judge very accurately as to whether they are 
 equally illuminated or not. 
 
LIGHTS AND SHADOWS 
 
 21 
 
 silver, or a silvered glass, will reflect the waves of light, 
 and so, though in an inferior degree, will any other 
 material if only its surface be sufficiently smooth. By 
 sufficiently smooth I mean that the ridges or scratches 
 or roughnesses of its surface are decidedly smaller than 
 the wave-length of the light. If the scratches or ridges 
 on a surface are in width less than a quarter of the wave- 
 length (in the case of light, therefore, less than about 
 2o"Fooo inch) they do not cause any breaking up of 
 
 the waves ; and such surfaces are, for optical purposes, 
 quite "smooth." Indeed that is the usual way of 
 polishing things. You scratch them all over with some 
 sort of very fine powder that makes scratches finer than 
 of an inch. 
 
 20^000 
 
 Now the rebound of waves when they beat against a 
 polished surface, whether that surface be a flat one or 
 a curved one, can be studied by applying the same 
 principles of wave-motion that we have already learned. 
 In Fig. II we have light starting from a point at P and 
 
22 LIGHT LECT. 
 
 spreading. If a smooth obstacle, SS, is placed in the 
 path of these waves they will meet it, but some parts of 
 the wave-front will meet it before other parts. Think 
 of the bit of the wave-front that meets the mirror at a. 
 If it had not been stopped, it would after a brief 
 moment of time have got as far as a . But having 
 bounded back from the surface it will set up a wavelet 
 that will spread backwards at the same rate. Therefore, 
 draw with your compasses the wavelet d\ using as radius 
 the length a a. The next bit of the wave -front b 
 reaches the surface of the mirror a little later. The 
 length from thence to b' is therefore a little shorter than 
 a a. So take that shorter length as radius and strike 
 out the wavelet b" . Completing the set of wavelets in 
 the same way we get the final curve of the reflected 
 wave, which you see will now march backwards as 
 though it had come from some point Q on the other 
 side of the mirror. In fact, if the mirror is a flat one, 
 Q will be exactly as far behind the surface as P is in 
 front of it. We call the point Q tlie "image" of the 
 point P. This reflexion of ripples as though they had 
 come from a point behind the mirror I can show you 
 by aid of my ripple-tank. I put in a flat strip of lead to 
 serve as a reflector — see how the waves as they come up 
 to it march off with their curvature reversed, as though 
 they had started from some point behind the reflecting 
 surface. 
 
 Again I can show you the same thing with a candle 
 and a looking-glass. You know that we can test the 
 direction in which light is coming by looking at the 
 direction in which a shadow is cast by it. If I set up 
 
1 LIGHTS AND SHADOWS 23 
 
 (Fig. 12) this little dagger on a whitened board I can 
 see which way its shadow falls. If now I place a candle 
 beside it on the board at P it casts a shadow of the 
 dagger on the side away from P. Next, set up a piece 
 of silvered mirror glass a little farther along the board. 
 We have now two shadows. One is the direct shadow 
 which was previously cast ; the other is the shadow cast 
 by the waves that have been reflected in the mirror, and 
 
 Fig. 
 
 you see by the direction in which this second shadow 
 falls that it falls just as if the light had come from a 
 second candle placed at Q, just as far behind the mirror 
 as P is in front. Let us put an actual second candle at 
 Q, and then take away the mirror, and you see the 
 second shadow in the same place and of the same shape 
 as before. So we have proved by direct experiment that 
 our reasoning about the waves was correct. Indeed, 
 
24 LIGHT LECT. 
 
 you have only to look into a flat mirror, and examine 
 the images of things in it, to satisfy yourselves about 
 the rule. The images of objects are always exactly 
 opposite the objects, and are each as far behind the 
 mirror as the object is in front. Probably you have 
 all heard of the savage prince captured by sailors, 
 who, when he was taken on board ship and shown a 
 mirror hanging on a wall, wanted to run round to 
 see the other savage prince whom he saw on the other 
 side ! 
 
 If instead of using flat mirrors we use curved ones, 
 w^e find different rules to be observed. That is because 
 the curved surfaces print new curvatures on the wave- 
 fronts, causing them to alter their lines of march. There 
 are, as you know, tw^o sorts of curvatures. The surface 
 may bulge out — ^in which case we call it convex ; or it 
 may be hollowed — in which case we call it a concave 
 surface. 
 
 In my ripple tank I now place a curved piece of 
 metal with its bulging side toward the place where I 
 make the ripples. Suppose now I send a lot of plane 
 ripples to beat against this surface ; the part of the 
 wave-front that strikes first against the bulging curve is 
 the earliest to be reflected back. The other parts strike 
 the surface later, and when reflected back have fallen 
 behind ; so that the ripples come back curved — the 
 curved mirror has, in fact, imprinted upon the ripples 
 a curvature twice as great as its own curvature. This 
 can be seen from Fig. 13, where we consider the straight 
 ripples marching to meet the bulging reflector. The 
 middle point M of the bulging surface meets the advan- 
 
LIGHTS AND SHADOWS 
 
 25 
 
 cing wave first and turns that bit back. If there had 
 been no obstacle the wave would, after a short interval 
 of time, have got as far as A. But where will it actually 
 go to ? The bit that strikes M will go back as far as 
 B ; the bit marked a will go on a little, and then be 
 
 reflected back. Take 
 
 C 
 your compasses again 
 
 and measure the dis- 
 tance it still has to go 
 to a ^ and then turn- 
 ing the compasses 
 strike out the arc a. 
 Do the same for the 
 bits marked b and c, 
 and you will find the 
 overlapping wavelets 
 
 Fig. 13. 
 
 to give you the new outline of the reflected wave, 
 which marches backwards as though it had started from 
 the point marked F. This point F is half-way between 
 M and the centre of curvature of the surface. The 
 centre is marked C in the drawing. 
 
 So, again, if I use as reflector a hollow or concave- 
 curved surface, it will imprint upon the waves a concave 
 form, the imprinted curvature being twice as great as 
 the curvature of the reflecting surface. But now we 
 come upon a new effect. See in my ripple-tank how, 
 when the straight ripples beat against the concave 
 surface, so that the middle part of the wave-front is the 
 last to rebound, all the other parts have already re- 
 bounded and are marching back, the returning ripples 
 being curved inwards. In fact, you see that, being 
 
26 
 
 LIGHT 
 
 LECT. 
 
 themselves now curved ripples with hollow wave-fronts, 
 they converge inwards upon one another, and march back 
 toward the point F. A bit of the wave -front at P 
 marches straight until it strikes the mirror at R. Then 
 instead of going on to Q it is reflected inward and 
 travels to F, toward which point other parts of the wave 
 also travel. Here then we have found a real focus or 
 meeting point of the waves ; not, as in the preceding 
 cases, a virtual focus from which the waves seemed to 
 
 Fig. 14. 
 
 come. We have then learned that, for ripples at least, 
 a concave mirror may produce a real convergence to a 
 point. 
 
 Let us at once show that the same thing can be done 
 with light-waves by using a concave silvered mirror. 
 
 From my optical lantern, with its internal electric 
 lamp, my assistant causes a broad beam of light to 
 stream forth. The air is dusty, and each little particle 
 of dust catches a portion of the beam, and helps you 
 to see which way it is marching. Li this beam I hold a 
 
eoSTON COLlf«E » >BKAK\ 
 CHESTNUT HJLL. MASS. 
 
 LIGHTS AND SHADOWS 
 
 27 
 
 concave silvered mirror. At once you see how by print- 
 ing a curvature upon the waves it forces the beam to 
 converge (Fig, 15) upon a point here in mid-air. That 
 point is the focus. You will further notice that by 
 turning the mirror about I can shift the position of the 
 
 Fig. 15. 
 
 focus, and concentrate the waves in different places at 
 will. 
 
 If I replace the concave mirror by a convex one, I 
 shall cause a divergence of the waves. No longer is 
 there any real focus, but the waves now march away 
 as if they had come from a virtual focus behind the 
 mirror at F (Fig. 16), precisely as we saw for the ripples 
 in the ripple-tank. 
 
 We have now got as far as the making of real images 
 
< r* 
 
 28 
 
 LIGHT 
 
 LECT. 
 
 by so changing the shapes of the wave-fronts and their 
 consequent Unes of march as to cause them to converge to 
 focal points. Let us try a few more experiments on the 
 formation of images. Removing from the optical lantern 
 all its lenses, let us simply leave inside it the electric 
 lamp. You know that in this lamp there are two pencils 
 of carbon, the tips of which do not quite touch, and 
 
 Fig. 16. 
 
 which are made white-hot by the flow of the electric 
 current between them. I cover up the opening in front 
 of the lantern by a piece of tin-foil, and in this I now 
 stab a small round hole with a pointed stiletto. At 
 once you see thrown on the screen an image (Fig. 17) 
 of the two white-hot tips of the carbon pencils. The 
 positive carbon has a flat end, the negative tip is pointed. 
 That image is inverted as a matter of fact, and its forma- 
 tion on the screen is a mere consequence of the rectilinear 
 
JHUdTOi^ COLLEGE LIBJialtV 
 . CMMTOUT HaL, MAm 
 
 I LIGHTS AND SHADOWS 29 
 
 propagation of the light. If I stab another hole we shall 
 have another image. This time I have pierced a square 
 hole, but the second image is just the same as the first, 
 and does not depend on the shape of the hole. I pierce 
 again a three-cornered hole — still another image. If I 
 pierce a whole lot of holes I get just as many images, 
 and they are arranged in a sort of pattern, which exactly 
 corresponds to the pattern of holes I have pierced in 
 the tin-foil. 
 
 Now if I wanted to produce one single bright image 
 instead of a lot of little images scattered about, I must in 
 
 Fig. 
 
 17- 
 
 some way manage so to turn these various beams that 
 they shall all converge upon the same region of the screen. 
 In other words, the formation of bright images can be 
 effected by using some appliance which will imprint a 
 convergence upon the waves. You know that a concave 
 mirror will do this. . Very well, let me use a concave 
 mirror. See how, when we choose one of the proper 
 curvature to converge the light upon the screen, it blends 
 all the images together, and gives us one bright image. 
 We may remove our tin-foil cap altogether, so as to work 
 
3© LIGHT LECT. 
 
 with the whole beam, and we get a still more brilliant 
 image of the carbon points. 
 
 Substituting for the arc-lamp a group of little coloured 
 electric glow-lamps, I cause their beams to be reflected 
 out into the room by my concave mirror, and here, by 
 trying with a hand -screen of thin translucent paper, 
 you see how I can find the real image of the group of 
 lamps. This image is inverted ; and being in this case 
 formed at a distance from the mirror greater than that 
 of the object, it is magnified. If the object is removed 
 to a greater distance the image comes still nearer in ; 
 and is then of diminished size, though still inverted. 
 
 So far we have been dealing with the regular reflexion 
 that takes place at properly polished surfaces. But if 
 the surfaces are not properly polished — that is, if their 
 ridges or scratches or roughnesses are not sensibly smaller 
 than the size of waves, then, though they may still 
 reflect, the reflexion is irregular. White paper reflects 
 in this diffuse way. You do not get any definite images, 
 because the slight roughnesses of the texture break 
 up the wave-fronts and scatter them in all directions. 
 That is why a white sheet of paper looks white from 
 whichever aspect you regard it. If the substance is one 
 which, like silk, has a definite fibre or grain that reflects 
 a little better in one direction than in another, then the 
 quantity of light reflected will depend partly upon the 
 direction in which the grain catches the light, and partly 
 upon the angle at which the light is inclined to the 
 surface. This is easily demonstrated by examining the 
 appearance of a piece of metal electrotyped in exact 
 facsimile of a piece of silk fabric. Here is such a 
 
I LIGHTS AND SHADOWS 31 
 
 piece. It was deposited ^ in a gutta-percha mould cast 
 upon a piece of figured silk brocade ; it reproduces the 
 exact shimmer of silk, because it reproduces the grain 
 of the silk in its operation of partial reflexion. If silk 
 is woven with warp of one colour and weft of another, 
 the different colours are better reflected at certain angles 
 — hence the effect produced by "shot" silk. 
 
 To illustrate the property of diffuse reflexion let me 
 show you two simple experiments. Here is a piece of 
 mirror. Upon it I paint with Chinese white the word 
 LIGHT. The letters look white on a dark background. 
 But if I use it to reflect upon the wall a patch of light 
 from the electric lamp the letters come out black. The 
 light that fell on those parts was scattered in all direc- 
 tions — so those parts looked white to you, but they 
 have diffused the waves instead of directing them 
 straight to the wall as the other smooth parts of the 
 surface do. 
 
 Let me prove to you how much light is really reflected 
 from a piece of paper. I have merely to shine my 
 lamp upon this piece of white paper, and hold it near 
 the cheek of this white marble bust for you to see for 
 yourselves what an amount of light it actually reflects 
 upon the object. Exchanging the white paper for a 
 
 ^ Made at the Technical College, Finsbury, by Mr. E. Rousseau, 
 instructor in electro-deposition. His process of casting, in a molten 
 compound of gutta-percha, the matrices, which are afterwards metal- 
 lised to receive the deposit in the electrotype bath, is distinctly superior 
 to the commercial process of taking moulds in a hydraulic press. 
 On one occasion he took for me a mould of a Rowland's diffraction 
 grating, having 14,400 parallel lines to the inch. Like the original 
 it showed most gorgeous diffraction colours. 
 
32 
 
 LIGHT 
 
 LECT. 
 
 piece of red paper, — that is to say of paper that reflects 
 red waves better than waves of any other colour, — and 
 you see how the red light is thrown back upon the 
 bust, and brings an artificial blush to its cheek. 
 
 If light is reflected from one mirror to another one 
 standing at an angle with the first, two or more images 
 
 Fig. i8. 
 
 may be formed, according to the position of the mirrors. 
 Here (Fig. i8) are two flat mirrors hinged together 
 like the leaves of a book. If I open them out to an 
 angle equal to one-third of a circle — namely, 120° — and 
 then place a candle between them, each mirror will make 
 an image, so that, when you peep in between the mirrors, 
 there will seem to be three candles. If I fold the mirrors 
 a little nearer, so that they enclose a quadrant of a circle 
 
I LIGHTS AND SHADOWS 33 
 
 — or are at right angles — then there will seem to be four 
 candles, one real one and three images. If I shut the 
 angle up to 72° — or one-fifth of a circle — then there will 
 seem to be five candles. Or to 60° — one-sixth of a circle 
 — then there appear six candles. This is the principle 
 of the toy called the Kaleidoscope^ with which some most 
 beautiful and curious combinations of patterns and 
 colours can be obtained by the multiplication of images. 
 Even with two such mirrors as these some quaint effects 
 are possible. When nearly shut up, a single light 
 between them seems to be drawn out into a whole 
 ring of images. Open them out to 72° or to a right 
 angle, and try the effect of putting your two. hands sud- 
 denly between the mirrors. Ten hands or eight hands 
 (according to the angle chosen) simultaneously appear 
 as if by magic. Place between the mirrors a wedge of 
 Christmas cake, and shut up the mirrors till they touch 
 the sides of the wedge, — you will see a whole cake 
 appear. 
 
 It is now time to pass on to another set . of optical 
 effects which depend upon the rate at which the waves 
 travel. I have told you how fast they travel in the air — 
 186,400 miles a second, or (if you will calculate it out 
 by a reduction sum) one foot in about the thousand- 
 millionth part of one second. Well, but light does not 
 go quite so fast through water as through air — only 
 about three-fourths as fast ; that is, it goes in water only 
 at the rate of about 138,000 miles a second, or only 
 about nine inches in the thousand-millionth part of a 
 second. And in common glass it goes still slower. On 
 the average — for glasses differ in their com.position, and 
 
34 
 
 LIGHT 
 
 LECT. 
 
 therefore in the retardation they produce on light-waves 
 — Hght only goes about two-thirds as fast as in air. That 
 is, while light would travel one foot through air, it would 
 only travel about eight inches through glass. 
 
 Now as a consequence of this difference in speed 
 it follows quite simply that if the waves strike obliquely 
 against the surface of water or of glass that part of the 
 wave-front that enters first into the denser medium 
 goes more slowly, and the other part which is going on 
 for a little longer time though air gains on the part that 
 entered first, and so the direction of the wave-front is 
 changed, and the line of march is also changed. Let 
 us study it a little more precisely. If waves of light 
 I proceeding from a point 
 
 ^% P strike against the top 
 
 ; \\ surface of a block of 
 
 glass, as in Fig. 19, how 
 will the retardation that 
 they experience on enter- 
 ing affect their march ? 
 Suppose that at a certain 
 moment a ripple has got 
 as far as FF'. If it had 
 been going on through 
 air it would, after a very 
 short interval of time, have got as far as GG'. But 
 it has struck against the glass, and the part that goes 
 in first instead of going as far as G' will only get 
 two-thirds as far. So once more take your compasses, 
 and strike off a set of arcs for the various wavelets, 
 in each case taking as the arc two -thirds of the dis- 
 
I LIGHTS AND SHADOWS 35 
 
 tance that the Ught would have had to go if after 
 passing the surface it could have gone on to GG'. The 
 overlapping wavelets build up the new wave-front HG', 
 which you notice is a flatter curve, and has its centre 
 somewhere farther back at Q. In fact, the effect of the 
 glass in retarding the wave is to flatten its curvature and 
 alter its march, so that in going on through the glass it 
 will progress as though it had come not from P, but 
 from Q, a point i|- times as far away. Consider the 
 bit of wave-front that has been marching down the line 
 PG'. When it enters the ' glass its line of march is 
 changed — instead of going on along G'A it goes more 
 steeply down G'B, as though it had come from Q. This 
 abrupt change of direction along a broken path, caused 
 by the entrance into a denser ^ medium, is known by the 
 term refraction. Glass refracts more than water does ; 
 heavy crystal glass (containing lead) refracts more than 
 the light sorts of glass used for window-panes and bottles; 
 while many other substances have a still higher refrac- 
 tivity. 
 
 Now, we can use this property of the refracting sub- 
 stances to produce convergence and divergence of light- 
 waves, because, as you see, when we want to imprint a 
 curvature on the wave-fronts, we can easily do this by 
 using the retardation of water or of glass. Suppose we 
 wanted to alter a plane-wave so as to make it converge 
 to a focus, what we have got to do is to retard the 
 middle part of the wave-front a little, so that the other 
 
 ^ "Denser," in its optical sense, means the same thing as more 
 retarding. Compare with what is said on p. 62 in the Appendix to 
 this Lecture. 
 
36 
 
 LIGHT 
 
 LECT. 
 
 parts shall gain on it. It will then be concave in shape, 
 and therefore will march to a focus. What sort of a 
 piece of glass will do this ? A mere window-pane wil^ 
 not. A thick slab will not, seeing it is equally thick all 
 over. Clearly it must be a piece of glass that is thicker 
 at one part than another. Well, suppose w^e take a 
 piece of glass that is thicker in the middle than at the 
 edges, what will it do ? Suppose that, as in Fig. 20, 
 we have some plane-waves coming along, and that we 
 put in their path a piece of glass that is flat on one face 
 
 and bulging on the other face. Think of the time when 
 a wave-front has arrived at GG. A moment later where 
 will it be ? The middle part that strikes at M will be going 
 through glass to B. This distance MB we know will be 
 only two-thirds as great as the distance to which it would 
 go in air. Had it gone on in air it would have gone as 
 far as A, the length MA being drawn ij times as great 
 as MB. The edge parts of the wave-front go almost 
 wholly through air, and will gain on the middle part. 
 So the new wave-front, instead of being flat through 
 HAH, will be curved concavely in the shape HBH ; 
 
1 LIGHTS AND SHADOWS 37 
 
 and as a result the wave will march on converging to 
 meet at F in a real focus.^ It would be the same if the 
 piece of glass were turrj^d round the other way, with its 
 bulging face toward the light ; it would still imprint a 
 concavity on the advancing wave and make it converge 
 to a focus. This is exactly how a burning-glass acts. 
 
 With my ripple -tank I am able to imitate these 
 effects, but not very accurately, because the only way I 
 have of slowing the ripples is to make the water shal- 
 lower where retardation is to be produced. This I do 
 by inserting a piece of plate glass cut to the proper shape. 
 Where the ripples pass over the edge of the submerged 
 piece of glass they travel more slowly. Where they meet* 
 the edge obliquely the direction of their march is changed 
 — they are refracted. Where they pass over a lens- 
 shaped piece they are converged toward a focus. 
 
 It is, however, more convincing to show these things 
 with light-waves themselves. Let me first show you 
 refraction upon the optical circle by the aid (Fig. 21) of 
 a special apparatus ^ for directing the beam toward the 
 centre at any desired angle. Placing a large optical 
 circle with its face toward you and its back to the lantern, 
 I can throw the light obliquely upon the top surface of 
 
 ^ From Fig. 20 it is easy to see that the curvature of the im- 
 pressed HAH is just half (if MB = § MA) of the curvature of the 
 glass surface. Hence it follows that the focal length of the plano- 
 convex lens (if of glass having a refractivity of I J) is equal to twice 
 the radius of curvature of the lens-surface. In the case of double- 
 convex lenses, each face imprints a curvature upon the wave as it 
 passes through. See Appendix to Lecture I. p. 65. 
 
 ^ This apparatus, which can be fitted to any ordinary lantern, 
 consists of three mirrors at 45° carried upon an arm affixed to a 
 
38 
 
 LIGHT 
 
 LECT. 
 
 a piece of glass, the under surface of which has been 
 
 ground to a semi-cylinder 
 (jj^ig. 22). The refracted 
 beam emerges at a differ- 
 ent angle, its line of march 
 having been made more 
 steeply oblique by the 
 retardation of the glass. 
 If you measure the angles 
 not in degrees but by the 
 straight distances across 
 the circle, you will find 
 that, for the kind of glass 
 
 I am using, the proportion between the length CD (the 
 
 sleeve that fits the condenser-tube, as in Fig. 21. The beam after 
 three reflexions comes radially back across the axis of the con- 
 
 FlG. 22. 
 
 densers ; and by turning the arm around in the condenser-tube can 
 be used at any angle. 
 
LIGHTS AND SHADOWS 
 
 39 
 
 sine of refraction) and the length AB (the sine of inci- 
 dence) is ahvays just the proportion of 2 to 3, whatever 
 the obliquity of the incident beam. When the incident 
 beam falls at grazing incidence most of it is reflected 
 and never enters the glass, and the part that does enter 
 is refracted down at an angle known as the critical or 
 limiting angle. 
 
 With this same optical circle I am able to show you 
 another phenomenon, 
 that of total internal re- 
 flexion. If I send the 
 light upwards through the 
 glass hemisphere (Fig. 
 23), at an angle beyond 
 that of the critical angle, 
 none of it will come up 
 through the surface ; all 
 will be reflected inter- 
 nally at the under side, 
 the top surface acting as 
 a polished mirror. You can see the same effect with 
 a tumbler full of water with a spoon in it. 
 
 This same phenomenon of total reflexion can be 
 beautifully illustrated by the luminous cascade or fairy 
 fountain. I allow water to stream out of a nozzle, and 
 shine light in behind through a window into the cistern 
 from which the water flows. It falls in a parabolic curve, 
 the light following it internally down to the place where 
 the jet breaks (Fig. 24) into drops. 
 
 Total reflexion can also be illustrated by shining 
 light into one end of a solid glass rod, along whichp 
 
 Fig. 23. 
 
40 
 
 LIGHT 
 
 LECT. 
 
 though it is of a bent and crooked shape, the light 
 travels until it comes to the other end. 
 
 Returning now to the use of lenses to cause the 
 waves to converge and diverge, we will adjust our lan- 
 tern to send out a straight beam, and then interpose in 
 
 Fig. 24. 
 
 the path a lens made of glass thicker in the middle 
 than at the edges. At once it is observed — thanks to 
 the dust in the air — to make these waves converge to a 
 focus at F (Fig. 25). This is again a real focus. A lens 
 that is thus thicker in the middle than at the edges is 
 called a convex k?is. 
 
 Had we taken a piece of glass that is thinner in the 
 
1 LIGHTS AND SHADOWS 41 
 
 middle than at the edges — a concave lens — the effect 
 
 Fig. 25 
 
 would be the opposite. Since the thin middle retards 
 the mid parts of the wave-front less than the thick glass 
 
 Fig. 26. 
 
 edges retard the edge parts, the middle part of the 
 
42 LIGHT LECT. 
 
 wave-front will gain on the outlying parts, and the wave 
 will emerge as a bulging wave, and will therefore march 
 as if diverging from some virtual focus. 
 
 You will not have failed to note that this property of 
 lenses to converge or diverge light depends on the fact 
 that light travels slower in glass than in air ; and you will 
 perhaps wonder what would be the effect if there were 
 no change in the speed of travelling. Well, that is a 
 very simple matter to test. If the action of the lens 
 depends upon the difference of speed of light in the 
 glass and in the surrounding medium, what ought to be 
 the result of surrounding the glass lens with some other 
 medium than air ? Suppose we try water. The speed 
 of light in water is less than in air — it is more nearly 
 like that in glass. And if the action depends on differ- 
 ence of speed, then a glass lens immersed in water ought 
 to have a less action than the same glass lens in air. 
 Try it, and you see at once that when immersed in 
 water a magnifying glass does not magnify as much as 
 it does in air. A burning-glass does not converge the 
 rays so much when immersed in water ; its focus is 
 farther away. Nay, I have here a lens which you see 
 unquestionably magnifies. I immerse it in this bath of 
 oil — and behold it acts as a minifying lens — it makes 
 the beam diverge instead of converge ! Carry the 
 argument on to its logical conclusion. If the effect of 
 the medium is so important, what would be the effect 
 of taking a lens of air (enclosed between two thin walls 
 of glass) and surrounding it by a bath of water or oil ? 
 If the reasoning is right, a concave air lens in oil ought 
 to act like a convex glass lens in air, and a convex air 
 
LIGHTS AND SHADOWS 
 
 43 
 
 
 
 
 
 
 
 
 ^^^t£ 
 
 --^^- 
 
 
 
 ~_~_" 
 
 -——j^— 
 
 
 
 .__^™,^ 
 
 "^- 
 
 
 --- 
 
 
 ^ 
 
 
 
 
 
 ~-J- 
 
 
 
 g 
 
 -'--t. >r- 
 
 
 
 
 
 
 Ls^^gs^r^; -:=^ 
 
 h 
 
 Fig. 27. 
 
 lens in oil like a concave glass lens in air. Let us put 
 it to the test of experiment. Here is a concave air lens. 
 In air it neither converges nor diverges .the light — the 
 speed inside and outside the 
 lens is the same — therefore 
 there is no action. But plunge 
 it in oil (Fig. 27) and, see, it 
 
 brings the beam to a focus --_ i[-:-vJ-t".-:-."Ji-:::-i'"-^>p 
 
 (F) exactly as a convex glass 
 lens in air would do. 
 
 Let me sum up this part 
 of my subject by simply 
 saying that lenses and curved mirrors can change the 
 march of light-waves by imprinting new curvatures on 
 the wave-fronts. Indeed, speaking strictly, that is all 
 that any lens or mirror, or combination of lenses or of 
 mirrors, can do. 
 
 Now the human eye, that most wonderful of all 
 optical instruments, is a combination of lenses within a 
 cartilaginous ball, the back of which is covered on its 
 inner face with an exquisitely fine structure of sensitive 
 cells, through which are distributed ramifications of the 
 optic nerve. All that that nerve can do is to feel the 
 impressions that fall upon it and convey those impres- 
 sions to the brain. All else must be done on the one 
 hand by the lens-apparatus that focuses the waves of light 
 on the retina, or on the other hand by the brain that is 
 conscious of the impressions conveyed to it. With neither 
 the nerve-structures nor with the brain are these lectures 
 concerned. We have merely to treat of the eye as a 
 combination of lenses that focuses images on the retina. 
 
44 
 
 LIGHT 
 
 LECT. 
 
 Consider a diagram (Fig. 28) of the structures of the 
 human eyeball. The greater part of the refractive 
 effect is accomplished by a beautiful piece of trans- 
 parent horny substance known as the crystalline lens 
 (L^), which is situated just behind the iris or coloured 
 diaphragm of the eye. The pupil, or hole through the 
 iris, leads straight toward the middle of this crystalline 
 
 C the cornea. 
 
 R the retina. 
 
 N the optic nerve. 
 
 Li aqueous humour. 
 
 L2 the crystalline lens. 
 
 L3 vitreous humour. 
 
 i the iris diaphragm. 
 
 b the blind spot. 
 
 y the yellow spot, or 7na- 
 cula lutea. 
 
 Fig. 28. 
 
 lens. But it is immersed in a medium, or rather between 
 two media, a watery medium (L ) in front and a gelatin- 
 ous one (L ) behind ; the latter filling up the rest of 
 the globe of the eyeball. The crystalline lens has 
 therefore a less magnifying power than it would have 
 if it were immersed in air. It acts very much as a 
 lens in water. But the watery liquid in front of it 
 also acts as a lens, since it occupies the space in front 
 of the crystalline lens and between it and the trans- 
 
I LIGHTS AND SHADOWS 45 
 
 parent cornea^ the bulging window of the eye. Taken 
 together these form a lens - combination adapted to 
 form images upon that back-screen or retina^ R, where 
 are spread out the sensitive nerve structures. All 
 that the eye can do as an optical instrument can be 
 imitated by optical combinations of lenses. An ordinary 
 photographic camera may be regarded as a sort of 
 artificial eye. In front is a combination of lenses the 
 function of which is to focus images upon a back screen, 
 or upon a plate which is made chemically sensitive. 
 To make the analogy more complete one ought to 
 think of the eye as a kind of camera in which the 
 hollow body is filled up with a thin transparent watery 
 jelly, and in which also the space between the front 
 lens and the one behind it is full of water. 
 
 Apart from the complication introduced by the 
 watery and gelatinous media, it is very easy to imitate 
 the optical arrangements of the eye by lenses. Any 
 photographic camera will serve indeed for the purpose. 
 Its lens combination throws upon the screen at the 
 back real images of the objects placed in front. 
 
 As in the camera, so in the eyeball, the images 
 thrown on the back are inverted images. If you have 
 not thought of this before it seems hard to believe it : 
 nevertheless it is true. You have all your lives had the 
 images inverted. Your brains, while you were yet 
 babies learned to associate the impression received on 
 the lower part of the retina with objects high above 
 you. However you may explain or doubt, the facts 
 are simply what they are : the images are upside-down 
 at the back of your eyeball. 
 
46 
 
 LIGHT 
 
 I,ECT. 
 
 Beside the general proof afforded by camera-images, 
 there are two extremely simple proofs of this fact. The 
 first any of you can try at home ; all the apparatus it 
 needs being a common pin and a bit of card. It 
 depends upon the circumstance that if you hold a small 
 object close to a lens a shadow of it may be cast right 
 through the lens without being turned upside down. 
 Here is a lens — it will form inverted images of objects 
 if it focuses them on a screen. But hold a small object 
 close to the lens (Fig. 29) and shine light through it ; the 
 shadows are actually cast right side up on the screen. 
 
 Now take a visiting-card and prick 
 a pinhole through it with a large- 
 sized pin. Place this hole about 
 an inch from the eye and look 
 through it at a white cloud or a 
 white surface strongly illuminated. 
 Then hold the pin upright, as in 
 Fig. 30, between the eye and the 
 pinhole. It may require a little 
 patience to see it, as the pin must 
 be held exactly in the right place. 
 You know you are holding it with the head up, yet you 
 see it with its head down, looking as in Fig. 31. Now 
 if in the case where you know that its shadow is 
 being thrown upright on the back of your eye you 
 feel the shadow upside down, it follows that when you 
 feel any image right way up it must really be an in- 
 verted image that you are feeling. 
 
 The other proof has the merit of being direct and 
 objective, but does not succeed with every eye — some 
 
 Fig. 29. 
 
i LIGHTS AND SHADOWS 47 
 
 persons have the cartilaginous walls of the eyeballs too 
 
 thick. Stand in front of a mirror, close one eye — 
 
 say the right — and hold a candle in the hand on the 
 
 same side. Hold the candle about 
 
 at the level of the closed eye so that 
 
 its light just falls across the bridge 
 
 of the nose into the open eye. 
 
 Then if you look very carefully you 
 
 will see, right in the extreme corner 
 
 of the eye, shining dimly through 
 
 the cartilaginous white wall, a small ^^'^' 2^- 
 
 image of the candle flame — and it is inverted. If you 
 
48 LIGHT LECT. 
 
 raise the candle higher, the image goes down ; if you 
 lower the candle, the image rises. 
 
 Leaving lenses let me show you a couple of 
 interesting experiments depending on the property of 
 refraction that we have been discussing. In passing 
 through the earth's atmosphere obliquely, as they do 
 when the sun is low down near the horizon, the sun's 
 rays are refracted, and he seems to be a little higher up 
 in the sky than he really is. Indeed, under certain 
 circumstances, the sun can be seen above the horizon 
 at a time when it is absolutely certain that he has really 
 set ; his rays in that case come in a curved path over 
 the intervening portion of the globe. Now the circum- 
 stances in which this can occur are these — that the 
 successive strata of the air must be of different 
 densities ; the densest below, next the earth, and the 
 less dense above. To demonstrate this I will take a 
 glass tank into which there have been carefully poured 
 a number of solutions of chloride of calcium in water of 
 
 different densities — the 
 densest at the bottom. 
 You note that the beam 
 
 of light sent into the 
 ' ^ ' trough takes a curved 
 
 path (Fig. 32). In fact, the light turns round a corner. 
 The difference of refractivity that accompanies 
 difference of density is well shown by a very simple 
 experiment upon heated air. You all know that when 
 air is heated it rises, becoming less dense. You all 
 know that, when cooled, air becomes more dense, and 
 tends to fall. But did you ever see the hot air rising 
 
LIGHTS AND SHADOWS 
 
 49 
 
 from your hand, or even from a hot poker? Or did 
 you ever see the cold air descending below a lump of 
 ice? This is exceedingly easy to show you. All I 
 require is a very small luminous point. We will take 
 the light of an arc-lamp, shining through a small hole 
 in a metal diaphragm close to it, and let it shine on the 
 white wall. Now I let this hot poker cast its shadow 
 on the screen, and you see torrents of hot air, which 
 rising, cast their shadows also. Here is a lump of ice. 
 The cold air streaming down from it casts its shadow. 
 Even from my hand you see the hot air rising. A 
 candle flame casts quite a dense shadow, and when I 
 open a bottle of ether you see the ether vapour — which 
 is ordinarily quite invisible — streaming out of the neck 
 and falling down. Even a jet of escaping gas reveals 
 itself when examined by this method. 
 
 Another curious experiment consists in using as a 
 lens a piece of glass which has been ground so as to be 
 curved only one way — say right and left — but not 
 curved in the other way. If this 
 lens is thicker in the middle part 
 from top to bottom, as in Fig. 33, 
 than it is at the two edges, it will 
 magnify things from right to left, 
 but not from top to bottom; hence 
 
 • Fig 'X'x 
 
 It produces a distortion. I throw 
 
 upon the screen the portrait of a well-known old gentle- 
 man. Then if I interpose in front of him one of these 
 "cylindrical " lenses, his face will be distorted. And if I 
 then turn the lens round the distortion will alternately 
 elongate his features and broaden them. There are 
 
 E 
 
50 LIGHT LECT. 
 
 also cylindrical lenses of another kind, thinner in the 
 middle than at the edges. These produce a distortion 
 by minifying. 
 
 Finally, I return to the point which I endeavoured 
 to explain to you a few minutes ago, that all that any 
 lens or mirror can do is to impress a curvature upon the 
 wave-fronts of the waves. 
 
 The most striking proof of this is afforded by that 
 now rare curiosity the magic mirror of Japan. In old 
 Japan, before it was invaded and degraded by Western 
 customs, many things were different from what they 
 now are. The Japs never sat on chairs — there were 
 none to sit upon. They had no looking-glasses — their 
 mirrors were all of polished bronze ; and, indeed, those 
 interesting folk had carried the art of bronze-casting and 
 of mirror polishing to a pitch never reached in any 
 other nation before them. The young ladies in Japan 
 when they were going to do up their hair used to squat 
 down on a beautiful mat before a lovely mirror standing 
 on an elegant lacquered frame. Fig. 34 is photographed 
 from a fine Japanese drawing in my possession. You 
 may have seen pretty little Yum-yum in the "Mikado" 
 squat down exactly so before her toilet-table. Here (Fig. 
 35) is one of these beautiful Japanese mirrors, round, 
 heavy, and furnished with a metal handle. One face 
 has been polished with care and hard labour ; the other 
 has upon it in relief the ornament cast in the mould — in 
 this case the crest of the imperial family, the kiri leaf 
 (the leaf of the Paullonia imperialis) with the flower-buds 
 appearing over it. The polished face is very slightly 
 convex ; but on looking into it none of you young 
 
A 
 
 Fig. 34 —Japanese Girls with Mirrors- 
 
>. 
 
 
 ^ 
 
 
 ,_^ 
 
 
 
 
 a 
 
 , 
 
 is 
 
 
 <i) 
 
 nS 
 
 rC 
 
 
 
 
 S 
 
 C 
 O 
 
 O.'*^ 
 
 3 
 
 -r) 
 
 na 
 
 1) 
 
 < 
 
 O G 
 ^ O 
 
I LIGHTS AND SHADOWS 51 
 
 ladies would see anything but your own fair faces, or the 
 faces of your friends around you, or the things in the 
 room. Certainly you would see nothing of the orna- 
 ment on the back. It is merely — so far as you or the 
 former owner of the mirror is concerned — a mirror. 
 
 But now take this mirror and hold it in the light of 
 the siin, or in the beams of an electric lamp, and let it 
 reflect a patch of light upon the white wall, or upon a 
 screen. What do you see ? Why, in the patch of light 
 reflected from the front of the mirror, you see (Fig. 36) 
 the pattern that is on the back. This is the extra- 
 ordinary "magic" property that has made these mirrors 
 so celebrated. 
 
 Another mirror has at the back a circle in high relief, 
 with a fiery dragon in low relief sprawling around it. 
 The face is beautifully polished, and shows no trace of 
 the pattern at the back. But when placed in the beams 
 of the arc-lamp it throws a patch of light on the wall, in 
 which the circle stands out as a brilliant line, whilst the 
 dragon is invisible. It is quite usual .for the parts in 
 high relief to produce this " magical " effect, while those 
 in low relief produce none. 
 
 For many years it was supposed that these mirrors 
 were produced by some trick. But the extraordinary 
 fact was discovered by Professor Ayrton in Japan that 
 the Japanese themselves were unaware of the magic 
 property of the mirrors. It results, in fact, from an 
 accident of manufacture. Not all Japanese mirrors 
 show the property : those that show it best are generally 
 thin, and with a slightly convex face. It was demon- 
 strated by Professor Ayrton, and I have since accumu- 
 
52 
 
 LIGHT 
 
 LECT. 
 
 lated some other proofs,^ that the effect is due to 
 
 extremely slight inequalities of curvature of surface. 
 
 These arise accidentally in the process of polishing. 
 
 The mirrors are cast in moulds. To polish their faces 
 
 they are laid down on their backs by the workman, who 
 scrapes them violently with a blunt iron tool, using great 
 force. Fig. 37 is taken from a Japanese print in the 
 British Museum. During this process they become 
 slightly convex. The polishing is completed by scouring 
 
 ^ These differences of curvature of surface can be proved (i) by 
 actual measurement, in some cases by spherometer ; (2) by placing 
 a convex lens in front to correct the general convexity and then 
 observing directly, as in Foucault's method for testing figure of 
 mirrors; (3) by reflecting in the mirror the image of a number of 
 fine parallel lines, whose distortion reveals the inequalities of curva- 
 ture ; (4) by taking a mould in gutta-percha, and reproducing the 
 polished surface by electrotype, which is then silvered. The silvered 
 type will act as a magic mirror. In some cases the "silvering" 
 wears off the surface unequally, remaining last on the parts that are 
 slightly concave. The front then shows faintly to the eye the 
 pattern on the back. 
 
r LIGHTS AND SHADOWS 53 
 
 with charcoal and scrubbing with paper, after which 
 they are " silvered " by application of an amalgam of tin 
 and mercury. Now during the violent scraping with the 
 iron tool the mirror bends, but the thin parts yield more 
 under the pressure than the thick parts do ; hence the 
 thick parts get worn away rather more than the thin 
 parts, and remain relatively concave, or at least less 
 convex. 
 
 Amongst the proofs that these very slight inequalities 
 of curvature can thus reveal themselves by imprinting a 
 convergivity or a divergivity upon the reflected waves, 
 let me show you this glass mirror, silvered in front and 
 quite flat, but having a star engraved on its back. By 
 merely blowing air against the back to bend it, the star 
 becomes visible in the patch of light reflected from the 
 face. Here the thin parts yield more than the thick 
 ones. Again, simply heating a piece of looking-glass 
 locally, by applying a heated iron stamp to the back of 
 it, will cause the glass to expand in the heated region, 
 and exhibit the pattern of the stamp in the patch of 
 light reflected on the wall by the mirror. 
 
 Lastly, I have to exhibit some magic mirrors made by 
 a former pupil of mine, Mr. Kearton — English magic 
 mirrors — which have no pattern upon them, either back 
 or front, but yet show images in the light they reflect 
 upon the wall. Here is one that shows a serpent ; here 
 another with a spider in his web ; another with a man 
 blowing a horn. These are made by etching very slightly 
 upon the brass mirror with acid (an immersion of three 
 seconds only is ample), and then polishing away the 
 etched pattern. After polishing for twenty minutes the 
 
54 LIGHT LECT. I 
 
 pattern will have disappeared entirely from sight. But 
 you may go on polishing for an hour more, and still 
 the minute differences of curvature that remain will 
 suffice,' — though quite undiscoverable otherwise — ^to 
 produce a magic image in the patch of reflected light. 
 Though so excessively minute these differences of 
 curvature of the mirror print their form upon the wave- 
 fronts of the light, and alter the degree of convergency 
 or divergency of the beam. 
 
APPENDIX TO LECTURE I 
 
 General Method of Geometrical Optics 
 
 The method of teaching Geometrical Optics upon the hneg 
 of the wave-theory, which is the key-note to this Lecture, 
 has been followed systematically by the author for fifteen 
 years in his regular courses of instruction in Optics to 
 students attending his lectures in Physics. The treatment 
 of the subject before the audience attending the Christmas 
 course at the Royal Institution, many of whom were 
 juveniles, was necessarily simplified and popularised ; but 
 the essential features of the method remain. 
 
 The author also published a brief notice of this method 
 of teaching the subject in 1889 in a paper entitled "Notes 
 on Geometrical Optics," read before the Physical Society 
 of London, and printed in the Philosophical Magaziite 
 (October 1889, p. 232). 
 
 As the development of the method in the present 
 lecture is so slight, the author deems it expedient to add 
 as an Appendix a few further points showing the application 
 to the establishment of formulas for lenses and mirrors. 
 These are, in fact, established much more readily on this 
 basis than by the cumbrous methods that are consecrated 
 by their adoption in every text-book of Geometrical Optics. 
 
 Basis of the Method 
 
 In treating optics from the new standpoint, we 
 have to think about surfaces instead of thinking about 
 
56 LIGHT LECT. I 
 
 mere lines. Waves march always at right angles to 
 their surfaces ; a change in the form of the surface 
 alters the direction of march. The wave -surface is 
 to be considered instead of the " ray." The curvature 
 of the surface therefore becomes the all-important con- 
 sideration. All that any lens or mirror or any system 
 of le?ises or mirrors can do to a wave of light is to im- 
 print a curvature upon the surface of the wave. If the 
 wave is initially a plane-wave, then the curvature imprinted 
 upon it by the lens or mirror will result in making it either 
 march toward a point (a real focus) or march as from a 
 point (a virtual focus). If the wave possesses an initial 
 curvature, then all that the lens or mirror can do is to 
 imprint another curvature upon its surface, the resultant 
 curvature being simply the algebraic sum of the initial and 
 the impressed curvatures. As will be seen, in the new 
 method the essential thing to know about a lens or mirror 
 is the curvature which it can imprint on a plane wave : 
 this is, indeed, nothing else than what the opticians 
 call its " power " ; the focal power being inversely propor- 
 tional to the so-called focal length. Another but less vital 
 point in the method, is the advantage of using instead of 
 the so-called index of refraction a quantity reciprocally re- 
 lated to it, and here denominated the velocity-constant. 
 The use of the index of refraction dates from a time 
 anterior to the discovery that refraction was a mere 
 consequence of the difference of velocity of the 
 waves in different media. The index of refraction 
 is a mere ratio between the sines (or originally the 
 cosecants) of the observed angles of incidence and re- 
 fraction. The uselessness of clinging to it as a founda- 
 tion for lens formulas is shown by the simple fact that, in 
 order to accomplish the very first stage of reasoning in the 
 orthodox way of establishing the formulge, we abandon the 
 sines and write simply the corresponding angles, as Kepler 
 did before the law of Snell was discovered. The ele- 
 mentary formulae of lenses are, in fact, where Kepler left 
 them. It is now common knowledge that the speed of 
 light, on which refraction depends, is less in optically dense 
 
APP. METHOD OF RECKONING CURVATURE 57 
 
 media than in air. The speed of light in air is not 
 materially different from one thousand million feet per 
 second, or thirty thousand million centimetres per second. 
 If we take the speed of light in air as unity, then 
 the numeric expressing the speed in denser media, such as 
 glass or water, will be a quantity less than unity, and will 
 differ for light of different wave-lengths. It is here pre- 
 ferred to take the speed of light in air, rather than i?i vacuo^ 
 as unity, because lenses and optical instruments in general 
 are used in the air. The numeric expressing the relative 
 velocity in any medium is called its " velocity-constant " ; 
 it is the reciprocal of the index of refraction. The velocity- 
 constant, for mean (yellow) light, for water is about 075 ; 
 that of crown glass 0*65 ; that of flint glass from 0'6i 
 to 0-56, according to its density. 
 
 Method of Reckoni7tg Curvature 
 
 The Newtonian definition of curvature as the reciprocal of 
 the radius has a special significance in the present method of 
 treating optics : for some of the most important of lens and 
 mirror formulae consist simply of terms which are reciprocals 
 of lengths, that is to say of terms which are curvatures. The 
 more modern definition of curvature as rate of change of 
 angle per unit length of the curve (Thomson and Tait's 
 Natural Philosophy^ vol. i. p. 5) is equivalent to Newton's ; 
 for if in going along an arc of length 8^, the direction 
 changes by an amount S^, the curvature is W\^s. But the 
 angle W = ^s\r^ where r is the radius of curvature; hence 
 the curvature = SsjrSs = i jr. 
 
 There is, however, another way of measuring curvature, 
 which, though correct only as a first approximation, is 
 eminently useful in considering optical problems. This 
 way consists in measuring the bulge of the arc subtended 
 by a chord of given length. 
 
 Consider a circular arc AP, having O as its centre. 
 Across this arc draw a chord PP' of any desired length. 
 The diameter AB bisects it at right angles in M. The 
 
58 
 
 LIGHT 
 
 LECT. I 
 
 short line MA measures the depth of the curve from arc 
 to chord. If the radius is taken as unity the line MA is the 
 versed-sine of the angle subtended at B by the whole chord, 
 or is the versed-sine of the semi-angle subtended at the 
 
 centre. In Continental works 
 it is frequent to use the name 
 sagitta for the length of this line 
 MA ; and as this term is pre- 
 ferable to versed -sine, and can 
 be used generally irrespective of 
 the size of radius, it is here 
 adopted. The proposition is 
 that, for a given chord, the 
 sagitta is (to a first degree of approximation) proportional 
 to the curvature. For it follows from the construction 
 that 
 
 MA. MB=r(PM)' 
 
 assuming PM as unity, 
 
 MA = 
 
 MB 2r-AM 
 
 But, for small apertures, AM is small compared with 2r, 
 and may be neglected in the denominator, whence, to a 
 first approximation, 
 
 MA = - . - . 
 
 2 r 
 
 Twice ^ the sagitta represents numerically the curvature. 
 The error is less than one per cent when the semi-angle sub- 
 tended at the centre is io° ; less than two per cent when 
 it is 15°; less than five per cent when it is 25°. 
 
 If the method of reckoning curvatures by means of the 
 sagitta required justification, that is afforded by the fact 
 that the practical method of measuring the curvatures of 
 
 •^ Though the sagitta is numerically half the curvature, since all 
 the formuke of first approximation are homogeneous and of the first 
 degree as regards sagittse and curvatures, the numerical factor \ dis- 
 appears iii passing from sagittse to curvatures, or vice versa. 
 
APP, METHOD OF RECKONING CURVATURE 59 
 
 lenses and mirrors by the spJieromete?' consists essentially in 
 applying a micrometer-screw to measure the sagitta of the 
 arc subtended by a fixed chord, the diameter of the contact 
 circle drawn through the three feet of the instrument. In 
 this case, as. indeed in all cases where accuracy, not 
 approximation, is desired, the basis for calculation of the 
 correction exists in the actual size of the diameter of the 
 contact circle, which is a fixed parameter for all measure- 
 ments made with the instrument. The ^'' lens measurer'''' 
 used by opticians to test the curvatures of spectacle-lenses 
 is a very simple micrometer which reads off directly the 
 sagitta of the curve against which it is pressed, and indi- 
 cates on a dial the value in terms of formula [10] on p. 65. 
 
 The sign of the curvature remains to be defined. In the 
 case of actual waves of light, the sign adopted will be + 
 for the curvature of waves which are converging upon a 
 real focus ; — for those which are diverging either from a 
 luminous source or from a virtual focus. This agrees with 
 the practice of the ophthalmists and of the opticians, who 
 always describe a converging lens as positive. A positive 
 lens is one which imprints a positive ctirvature i{p07t a plane 
 wave which traverses it. 
 
 The unit of curvatu7'-e., whether of the wave-surface 
 itself or of the surface of any mirror or lens, will be taken 
 so as to accord with modern ophthalmic and optical 
 practice as the dioptric; that is to say, the curvature of a 
 circle of one metre radius will be taken as unity. The 
 dioptrie, originally proposed by Monoyer as the unit of focal 
 power of a lens, was formally adopted in 1875 by the 
 International Medical Congress at Brussels, and its great 
 convenience has led to its universal adoption for the 
 enumeration of the focal powers of lenses. That lens which 
 has a focal length of one metre is said to have a focal power 
 of one dioptrie. In other words, such a lens prints a 
 curvature of one dioptrie upon a plane wave which is 
 incident upon it. For the present proposal to extend the 
 use of the term from focal powers {i.e. imprinted wave- 
 curvatures) to the curvatures of curved surfaces in general, 
 the writer is responsible. 
 
6o 
 
 LIGHT 
 
 LECT. I 
 
 Notatio7i 
 
 In adopting a notation which embodies the new method 
 it is obviously advisable to choose one which lends itself 
 most readily to the existing and accepted notations. In 
 the great majority of books on optics, the recognised 
 symbol for focal length is/"; that for radius of curvature r. 
 And in the Cambridge text-books for many years the 
 distances from lens or mirror of the point-object and the 
 point-image have respectively been designated by the 
 letters u and v. Now it is the reciprocals of these which 
 occur in the expressions for the curvatures of surfaces or of 
 waves. The symbols adopted respectively for the four 
 reciprocals are accordingly F^ i?, U^ and V. The accepted 
 symbol for the index of refraction is the Greek letter /x ; for 
 the velocity-constant, which is its reciprocal, we take the 
 letter h. The following is a tabular statement of the 
 symbols and their meanings : — ■ 
 
 Symbol. 
 
 Meaning. 
 
 Equivalent In 
 Current 
 
 Notation. 
 
 F 
 
 R 
 U 
 
 V 
 h 
 
 Focal curvature, or P'ocal power of lens or 
 mirror ( = dioptrics, if metre is taken as 
 unit of length) 
 
 Curvature of Surface . . ' . 
 
 Curvature of Incident wave ; i.e. curva- 
 ture which it has acquired by having 
 travelled from point of origin (" incident 
 focus ") to incidence .... 
 
 Curvature of Resultant wave; i.e. curva- 
 ture with which wave emerges from 
 the lens 
 
 Velocity-constant of medium ; i.e. velo- 
 city of light in that medium compared 
 with velocity in air taken as unity 
 
 1 I 
 
 / -^ 
 
 I 
 r 
 
 I 
 
 \ . 
 / ^ 
 
 ) . 
 ) ^ 
 
APP. 
 
 REFRACTION FORMULA 
 
 6i 
 
 Expansion of Curvatures 
 
 If the curvature 7? of a wave at any point is known it is 
 easy to calculate the curvature at any other point at distance d 
 farther from or nearer to the centre, the formula for the new 
 curvature R being as follows : — 
 
 R' ^R 
 
 \±Rd 
 
 [I] 
 
 The + sign must be taken where the new point is farther 
 from the centre than the point for which the curvature R 
 is specified ; the — sign when it is nearer the centre. This 
 proposition is of use in dealing with thick lenses, and with 
 thin lenses at a given distance apart. 
 
 Refraction ForinulcE 
 
 As a preliminary to lens formulas, it is convenient to 
 consider certain cases of refraction. 
 
 Consider a retarding medium, such as glass, bounded 
 on the left (Fig. 39) by a plane surface SS'. Let P be a 
 
 Fig. 39 
 
 source of waves incident on the surface, PM being a line 
 perpendicular to SS.'. The wave-fronts, at successive small 
 intervals of time, are represented by arcs of circles. At a 
 
 F 
 
62 
 
 LIGHT 
 
 LECT. I 
 
 certain moment the wave, had it been going on in air, 
 would have had for its surface the position SAS' ; the 
 curvature being measured by the sagitta AM. The 
 medium, however, retards the wave, and it will only 
 have gone as far as B instead of penetrating to A ; B 
 being a point such that BM = /i. AM, where A is the 
 velocity-constant of the medium into which the wave enters. 
 The curvature of the wave is flattened as the result of the 
 retardation. Now draw a circle through SBS', and find its 
 centre O. To a first degree of approximation the arc SBS' 
 represents the retarded wave-front, the set of wave-fronts 
 
 Fig. 40, 
 
 from B onwards being represented by the series of arcs 
 drawn from Q as centre. An eye situated in the medium 
 on the right of SS' will perceive the waves as though 
 coming from O, the (virtual) point-image of P. Accurately 
 the wave-fronts should be hyperbohc arcs, but if SS' is 
 small relatively to PM the circular arcs are adequate. 
 Now AM = [/, and BM = F. Hence the action of the 
 plane surface upon the curvature (in this case a divergivity) 
 of the incident wave is given by the formula 
 
 T'^/iC ... [2] 
 
 In the above case, which should be compared with Fig. 
 ^9j P- 34? the wave had a negative curvature. If the 
 
APR " REFRACTION FORMULA 63 
 
 entrant wave has a positive curvature or convergence such 
 as would cause it to march to a point P to the right in the 
 air, a similar set of considerations will readily show that if 
 on entering the flat surface of a more retarding medium its 
 curvature is flattened, it will march to a focus farther to the 
 right, the ratio of the original and the acquired curvatures 
 being, as before, dependent sirnply on the relative velocities ; 
 and formula [2] above still holds good. 
 
 Consider next the wave emerging (Fig. 40) into air from 
 a point P, situated in the retarding medium whose velocity- 
 constant is h. Had the wave been going on wholly through 
 the denser medium, the wave-front would have been at 
 SAS' ; but, being accelerated on emergence into air, it 
 reaches B instead of A. The new curve SBS' has Q for its 
 centre ; that is to say, the wave emerges from Q as a virtual 
 focus, its curvature being augmented. The sagitta BM is 
 greater than AM in the ratio of i to h. Hence in this 
 case the formula is 
 
 v=\v ... [3] 
 
 The case of an emergent wave of positive curvature 
 leads to the same formula. In the case of either positive 
 or negative initial curvature, emergence from the retarding 
 medium through the plane surface into air augments the 
 curvature. 
 
 If a plane wave travelling in air meets a bulging surface 
 of a more retarding medium such as glass, the portion of 
 the advancing wave which first meets the surface is re- 
 tarded, so that the wave front receives a depression, and 
 hence on entering the second medium marches converging 
 toward a focus. The relation between the impressed focal 
 curvature and the curvature (R) of the surface is given by 
 the formula 
 
 F=R{i-h) . . [4] 
 
 It will be noted that if the curvature of the surface is 
 positive (z.e. bulging toward the source of light), the im- 
 pressed focal curvature is also positive. The formula, 
 therefore, is the same for entrant plane-waves whether the 
 
64 LIGHT LECT. I 
 
 surface be convex or concave, the sign of F following the 
 sign of R. For the case of any two media having respec- 
 tive velocity-constants /z^ and h^^ the formula becomes 
 
 A plane wave traversing a medium with velocity h and 
 emerging through a curved surface into air has a curvature 
 imprinted upon it that is of opposite sign to that of the 
 surface itself. If the wave travehing to the right emerges 
 through a (hollow) surface whose centre of curvature lies to 
 the right, the acquired focal curvature will have its centre 
 to the left, or will be negative ; and its relation to the 
 curvature (R) of the surface is given by the rule 
 
 F=R 
 
 (^) • • c. 
 
 As before, for any two media having respective velocity- 
 constants h-^ and h^^ the formula becomes 
 
 which, in the present case where h^<Ji,-^^ will give F of 
 opposite sign to R. 
 
 The cases in which a wave possessing initial curvature 
 passes through a curved surface and acquires a resultant 
 curvature may be dealt with, apart from any further geo- 
 metrical constructions, by applying the principle of super- 
 position of curvatures. Thus, take the case of a wave 
 possessing initial curvature U entering from air into a 
 medium having velocity-constant h^ and so curved that the 
 focal power of the curved surface is F. Then, as the wave 
 enters the surface of the medium two effects will occur : 
 its initial curvature will be altered in the ratio of the velo- 
 cities, and there will be superposed upon it the focal curva- 
 ture of the surface; or, in symbols, 
 
 V^^hU^F^ . . [7] 
 
APP. LENS FORMULA 65 
 
 For an emergent wave, possessing initial curvature U in the 
 medium, the formula will be 
 
 V^ = \U\F^ . . [8] 
 
 Or, for the case of a wave passing from a medium of 
 velocity-constant h^ to another of velocity-constant h^^ the 
 formula will be 
 
 v=hu-\-F . . [9] 
 
 It is easy, however, to prove any one of the several cases 
 that may arise, without in this way relying upon the 
 principle of superposition. 
 
 Lens FormulcB 
 
 In the case of a lens, the curvature F^ imprinted on 
 a plane wave by entrance at the first surface may be 
 regarded as an initial curvature of the wave which 
 emerges through the second surface. Emergence into air 
 will, as shown above, alter the curvature by augmenting it 
 in the ratio of i to /?, and superpose upon it the focal 
 curvature F^ due to the second surface. Hence the whole 
 resultant curvature F imprinted by a thin lens on the plane 
 wave will be 
 
 But 
 and 
 
 whence 
 
 or 
 
 f=\f,^f,. 
 
 F^ = R^[i-h\ 
 
 F^R^-j'-R^-j^, 
 
 F={R^-R^^-^ , .. [10] 
 
66 LIGHT LECT. I 
 
 This formula may be compared with that in the current 
 notation, 
 
 Fig. 20 (p. 36), gives an illustration, in which however 
 J^^ is zero, as the first face of the lens is flat. 
 
 In the case of a lens composed of a medium /i^, lying 
 between two other media A.^^ and /t.^, the formula becomes 
 
 F=^{R^{h^ - h^)h^ + R^h^ - h^)h^ . [II] 
 
 The general formula [10] for the power of any lens 
 consists of two factors — one depending solely on the shape 
 of the lens, the other upon its material. The latter factor, 
 
 — ^-, or /x - I, is a mere numeric; whilst the former, being 
 
 the difference of two curvatures, is itself a curvature. If 
 the curvature thus determined by shape solely is expressed 
 in dioptrics, then, on multiplying by the numeric which 
 depends on the nature of the material, the resultant power 
 of the lens will also be expressed directly in dioptrics. In 
 the optician's "lens-measurer" (p. 58) the dial readings are 
 already corrected by being multiplied by this numeric, thus 
 obviating calculation. 
 
 If the lens has thickness d^ the rule for expansion of 
 curvature at end of § 4 above at once gives 
 
 ^=^2+^-^iY±7j/ • 
 
 [12] 
 
 or 
 
 ^-{^^i±A(i-/'K-^4^' • ^'^^ 
 
 Universal Formula for Lenses 
 
 The principle of superposition at once gives the universal 
 formula for all lenses bounded by identical media on the 
 two sides : — 
 
 V=U+F; , , [14] 
 
APP. REFLEXION FORMULA d'] 
 
 or, in words, the resiiltaitt curvature is the algebraic sum of 
 the initial curvature and the impressed curvature. This 
 may again be compared with the formula in current 
 notation : 
 
 11 I 
 
 v~ f u' 
 
 The difference in sign attributed to the term - arises from 
 conventions adopted m the two systems. 
 
 Formula for Two Thin Lenses at a Distance Apart 
 
 The principle of expansion of curvature at once gives us 
 as the equivalent focal power, 
 
 I 
 
 ^=^^+^'rT7i 
 
 d 
 
 [15] 
 
 where F^ and F^ are the focal powers of the first and second 
 lenses, and d the distance between them. F will be in 
 dioptrics if F-^ and F^ are in dioptrics and d in metric units. 
 If the two thin lenses are close together, the resultant 
 power is simply the algebraic sum of the powers of the 
 separate lenses. One simply adds the dioptrics of the 
 separate lenses to find the resultant dioptrics. 
 
 Reflexion FormuicB 
 
 The plane mirror (Fig. 41) has surface SMS'. The 
 incident wave would have had front SAS' at a certain 
 instant had its path lain wholly in air. The central 
 portion of the wave, which would have reached A, travels 
 backwards to B, an equal distance, in the same time. 
 The sagitta BM of the resultant curvature is equal to 
 and of opposite sign to the sagitta AM of the initial 
 curvature ; or 
 
 V=-U . . . [16] 
 
 There are two cases, equally simple, of convex and con- 
 
68 
 
 LIGHT 
 
 LECT. I 
 
 cave mirrors. These are separately shown in Figs. 1 3 and 1 4 
 (pp. 25, 26), in both of which the incident waves are plane. 
 Consider (Fig. 42) a plane wave which at a certain instant 
 would have arrived at SAS' had its path lain wholly in air. 
 The central portion of the wave has, however, struck at M, 
 and marches backwards to B in same time as it would 
 have taken to reach A. Hence 
 
 or 
 
 BM = AM, 
 
 BA = 2AM. 
 But AM measures the curvature of the mirror, whilst BA 
 
 4. 
 
 Fig. 41. 
 
 Fig. 42. 
 
 measures the curvature impressed on the plane wave. 
 Hence 
 
 F^iR . . , [17] 
 
 In Cambridge notation 
 
 I_2 
 
 or 
 
 •^ 2 
 
 It is equally easy to establish the formula for the action 
 of a curved mirror on a curved wave. The principle of 
 
APP. REFLEXION FORMULAE 69 
 
 superposition at once leads to a general formula, expressing 
 the sum of the two actions of the mirror on the wave ; it 
 reverses its initial curvature, and then imprints a focal 
 curvature upon it. In symbols, 
 
 V=-U+F . . [18] 
 
 The application of wave principles to find the direction 
 of a refracted beam is best handled by Ampere's modifica- 
 tion of Huygens's construction, as in Fig. 43. In that 
 figure the dispersion produced by the difference between 
 the velocities of light of different colours is also illustrated. 
 
 Fig. 
 
 43- 
 
 During the time that light (of any colour) would travel I 
 foot in air, red light would travel about y^ inches in glass, 
 and violet light about 7 inches in glass. The velocity-con- 
 stant for glass of this kind is 0-625 for red waves, and 
 0-583 for violet waves. Let EF be the surface of glass 
 (Fig. 43) at which the wave is entering. It marches in the 
 direction AO. Consider the portion of wave -front that 
 arrives at O. If it is regarded as setting up a new set of 
 waves, these will spread in circles according to the velocity. 
 Therefore around O as centre draw a number of circles, one 
 with unit radius to show how the wave would have spread 
 in a given time had it spread all round in air, one (a semi- 
 
70 LIGHT LECT. I 
 
 circle only) with radius 0-625 to show how far the red wave 
 would have penetrated in glass, one (also a semicircle) with 
 radius 0-583 to show how far the violet wave would have 
 travelled in glass in the same time. Now produce OA to 
 B, and at B draw a tangent meeting the surface of the glass 
 at E. From E now draw as many tangents as you can to 
 the circles, to represent the wave-fronts. EC will be the 
 wave-front of that part of the light which is reflected back 
 in the direction OC ; ER will be the wave-front of the red 
 light refracted down along the direction OR ; and EV will 
 be the wave-front of the violet light refracted down the 
 direction OV. 
 
LECTURE II 
 
 THE VISIBLE SPECTRUM AND THE EYE 
 
 Colour and wave-length — Rainbow tints — The spectrum of visible 
 colours — Spectrum made by prism — Spectrum made by grating 
 — Composition of white light — Experiments on mixing colours 
 — Analysis of colours — Blue and yellow mixed make white, not 
 green — Complementary tints — Contrast tints produced by 
 fatigue of eye — Other effects of persistence of vision — Zoetrope 
 — Animatograph. 
 
 Waves of light are not all of the same wave-length. The 
 difference of size makes itself known to our eyes as 
 colour. Just as the sounds of different wave-lengths 
 produce in our ears perceptible differences in pitch, so 
 the lights of different wave-lengths produce in our eyes 
 different sensations, which we call colour. Any simple 
 kind of light — I am not speaking here of mixtures — can 
 be described in two ways; either (i) by stating the 
 colour-sensation which it produces on the eye, or (2) 
 more accurately, by stating what its wave-length or the 
 frequency of its vibrations is. To ascertain the wave- 
 length of any particular kind of simple light may not 
 be a very easy matter, but when once it has been 
 measured, the statement of the wave-length is an 
 accurate description. 
 
72 
 
 LIGHT 
 
 LECT. 
 
 To begin then, here is a table in which I have set 
 down, in their order according to wave-length, biggest 
 first, the various kinds of simple light that are visible to 
 the eye. 
 
 Table I. — Colours of the Spectrum 
 
 i 
 
 
 
 Name of Colour. 
 
 Wave-length 
 in milllonths of 
 
 Wave-length 
 in milHonths of a 
 
 
 an inch. 
 
 centimetre. 
 
 Extremest red 
 
 32-4 
 
 81 -o 
 
 Red . 
 
 
 26*0 
 
 6$-o 
 
 Orange 
 Yellow 
 
 
 
 23 '3 
 22 'O 
 
 58-3 
 55-1 
 
 Green . 
 
 
 
 20'5 
 
 51-2 
 
 Peacock 
 Blue . 
 Violet . 
 
 
 
 19-0 
 i8-o 
 i6-o 
 
 47-5 
 44-9 
 40-0 
 
 Extreme vio 
 
 let 
 
 
 14-4 
 
 36-0 
 
 You will note that the red waves are about twenty-six 
 millionths of an inch long (i.e. about 3-9^^^ of an inch), 
 while the violet waves are a little more than half as great, 
 namely, sixteen millionths of an inch in wave-length (i.e. 
 about qqIqq of an inch). All the other simple kinds 
 of light are of intermediate size. You will note the names 
 of the colours. In the list you will find neither white 
 nor black, for white (as I shall presently show you) is a 
 mixture of all these simple colours, and black is simply 
 the absence of all light — a mere darkness. 
 
 Now this set of colours can be produced naturally 
 in their proper order in several different ways. The 
 simplest way is to take some white light which contains 
 
II- VISIBLE SPECTRUM AND THE EYE 73 
 
 all these colours mixed up together, and sort them out. 
 But how ? That is what I want you to understand. In 
 nature we find them sorted out in the rainbow, where 
 these tints stand side by side. Can we make an artificial 
 rainbow ? How is a rainbow made ? Of the smiles of 
 Heaven commingled with the tears of Earth, if we 
 believe the poets.-^ Of sunlight and raindrops — (is it 
 not ?) — which refract the light, and in refracting it sort 
 out the different kinds of light, and display them in their 
 proper order. Perhaps that is a very incomplete de- 
 scription of the operation of building a rainbow, but it is 
 good enough to give us a hint towards experiments. 
 
 Here is an optical lantern, with an electric arc-lamp 
 inside, a sort of miniature sun to give us white beams of 
 light. We let the light pass Qut in a fine straight beam, 
 and in that beam we place — to serve as a sort of mag- 
 nified raindrop — this sphere of water contained in a thin 
 shell of glass. See the bow which it casts back upon 
 the whitened screen. You can recognise the usual tints, 
 though they are not so brilliant as in the natural bow. 
 
 But having got our clue to experiment, let us go on 
 farther. Try instead of the bulb a three-cornered bottle 
 full of water. We have now no bow. The beam of light 
 is abruptly turned upward into a new direction, and falls 
 upon the wall or ceiling. But, though we have lost the 
 shape of the arch we have gained in the development of 
 the rainbow hues. We have now a brilliantly coloured, 
 though rather nebulous, colour-patch. Try again, and 
 
 ^ "We are like Evening Rainbows, that at once shine and 
 Weep — things made up of reflected splendor and our own Tears." — 
 5". T. Coleridge. 
 
74 LIGHT LECT. 
 
 this time try the effect of varying the liquid. Here is a 
 three-cornered bottle full of turpentine. The angular 
 deviation of the colour-patch has become greater, but so 
 has the breadth of our set of tints. Try oil of cinnamon, 
 it is still better. Try bisulphide of carbon, still more 
 brilliant though still fuzzy at the edges. Naturally, one 
 begins to think that if a transparent three-cornered bottle 
 full of liquid will thus display rainbow effects, a three- 
 cornered piece of transparent glass ought to do the same. 
 So it does : and so we have arrived at the use of the 
 well-known glass prism to produce a spectrum of colours. 
 The word spectrutn means simply " an appearance " : in 
 this case an appearance of colours — the colours sorted 
 out in their order. To emphasise the fact that the 
 spectrum is in this case projduced by use of a prism, it is 
 sometimes called the "prismatic spectrum." In all 
 cases you will have noticed that the order of the colours 
 is the same, and that the red light is always refracted 
 least, and the violet light refracted most. If the refrac- 
 tions of these colours were equal, the prism would not 
 separate them. The difference of the refractions between 
 the most-refracted (violet) and least-refracted (red), of 
 the visible kinds of light, is sometimes called " the dis- 
 persion " of the prism. 
 
 We have now got to the stage of Newton's researches, 
 but there is one further improvement to make, which 
 was indeed tried by him. Let us try the effect of alter- 
 ing the arrangement of our beam of light. You see we 
 have been using a beam streaming out through a round 
 hole ; when nothing is interposed it falls in a round spot 
 against the wall. Newton used sunlight streaming 
 
IJ VISIBLE SPECTRUM AND THE EYE 75 
 
 through a hole in a shutter. Well, let us try the effect 
 of using holes of different sizes and shapes. And, while 
 we are about it, let us try the effect of focusing on the 
 wall the image of the aperture — by interposing a positive 
 lens — so as to work with a well-defined spot of light 
 instead of a fuzzy patch. 
 
 We begin by using round holes of different sizes, 
 which we can try one after the other. Now, interpose 
 the prism — the best one of those yet tried — in the path 
 of the light. You see that when the aperture used is a 
 large circular hole, the colours overlap much near the 
 middle and give a mixed effect. Whereas when we use 
 a smaller hole, though we have less total light, the 
 colours are more intense, simply because they overlap 
 less. Well, then, let us take the hint, and substitute for 
 the small round hole a narrow slit. By employing a slit 
 with movable jaws (like a parallel ruler) we can adjust 
 it to be as wide or as narrow as we like. Again, we find 
 that if the slit is too wide the colours overlap, while 
 with a narrow slit the tints are more intense. 
 
 Our successive improvements have then led us to 
 the following combination : a slit to limit our beam, a 
 lens to focus the image of the slit as a fine white line 
 on the screen, and a prism, which, in refracting the light 
 of the lamp, also splits it up (Fig. 44) into the various 
 colours of which it is compounded. 
 
 Perhaps you think I am assuming things not yet 
 proved to describe the action of the prism as splitting 
 up the light of the lamp into the colours of which it is 
 compounded. Well, I admit, the phrase "splitting it 
 up " is not the best that might be selected ; " sorting it 
 
LIGHT 
 
 LECT. 
 
 out " would be a better phrase. But each of these 
 phrases carries in its use the assumption that the white 
 light is a mixture that can be split up or sorted out into 
 simpler constituents. That is precisely Newton's great 
 discovery. White light, supposed down to that time 
 to be itself a simple thing, was found and proved by 
 him to be a mixture. The prism added nothing to the 
 white light, it simply spread out the constituents in their 
 
 <^ 
 
 \W 
 
 Fig. 44 
 
 natural order. More than a hundred years afterwards the 
 great poet and dramatist Goethe — " master of those who 
 know "• — fought against this idea, and threw the whole 
 weight' of his genius to demonstrate, in his Farhenlehre^ 
 the erroneous nature of Newton's views. According to 
 him the prism does not merely spread out the simple 
 constituents of white light : it takes simple white light 
 and adds something to it which gives it a tint of one 
 sort or another. But it was in vain. Beautiful as many 
 of Goethe's experimental researches were, his theory 
 
II VISIBLE SPECTRUM AND THE EYE 77 
 
 died of inanition. To-day not a single scientific man, 
 even in Germany, holds Goethe's theory of optics j 
 though his fame as a poet stands immortal. 
 
 Before we follow the quest of those other experi- 
 ments by which Newton's theory of the compound 
 nature of white light is established, let me show you a 
 second method of spreading out white light into a spec- 
 trum of colours. In this case I use no prism ; and the 
 effect will not be produced by refraction through any 
 transparent solid or liquid. Instead, I employ the little 
 instrument which I hold in my hand. It is called a 
 "diffraction grating." It is simply a polished mirror of 
 hard bronze, a little more than two inches wide, across the 
 surface of which there have been ruled with a diamond 
 a large number of parallel and equidistant scratches. 
 You may think it odd to call a scratched mirror a 
 "grating." But the fact is that the properties it pos- 
 sesses were originally discovered by the use of gratings 
 made of fine wires. It would be quite impossible, how- 
 ever, to make a grating with wires as fine as these 
 scratches. When you want to produce a perfect 
 diffraction grating there is nothing for it but to rule 
 diamond scratches ; and they must be ruled by machin- 
 ery of the utmost precision. Over the face of this little 
 mirror there have been ruled about 30,000 parallel 
 lines, and not one of them is a millionth of an inch out 
 of its proper place. It was ruled at Baltimore on Pro- 
 fessor Rowland's machine. The exact number of lines 
 is 14,400 side by side to the inch. 
 
 I set up the grating so that the light of my lantern, 
 issuing through the slit, falls upon it, and you see the 
 
78 LIGHT LECT. 
 
 spectrum that it casts upon the wall. This is not a 
 pris7natic spectrum^ for there is no prism. It is a diffrac- 
 tion spedi'iwi ; and that is not quite the same thing. 
 As a matter of fact the grating, as you see, casts on the 
 wall a whole series of spectra. It reflects back centrally 
 a white image of the slit. Right and left we have on 
 each side a bright spectrum with all the colours. Then, 
 still farther away on each side, a rather longer and 
 nearly equally brilliant spectrum of the second order; 
 while, more dimly, and slightly overlapping one another, 
 we have spectra of the third and fourth orders. We 
 will deal only, however, with the first bright spectrum. 
 There are our rainbow tints in their order as before. 
 But note that now it is the red light that seems to have 
 been turned most aside, and the violet light which is 
 least. Note, further, that while the order of the colours 
 between red and violet is the same as in the prismatic 
 spectrum, the spacing of them is not the same. In the 
 prismatic spectrum the orange is huddled up toward 
 the red, and the yellow toward the orange; while the 
 violet and blue are highly elongated. In the diffrac- 
 tion spectrum the red end is not squeezed together un- 
 duly, nor the violet end unduly drawn out. 
 
 Time will not allow me^ to dwell, on the reasons for 
 these differences. Suffice it to say that they depend 
 upon the wave-lengths of the different kinds of light, 
 and their relations on the one hand to the size of the 
 molecules of the refracting prism, and on the other hand 
 to the width of the bars of the grating. 
 
 Incidentally you maybe interested in. knowing that 
 
 ^ See Appendix, p. lOO. 
 
ir VISIBLE SPECTRUM AND THE EYE 79 
 
 this property of diffraction, which belongs to the surface 
 that has thus been covered with parallel scratches, can 
 be transferred from the grating to another surface by 
 merely taking a cast. Here is a cast made in gutta- 
 percha ^ from the grating ; it is itself a grating. Like the 
 bronze original it glitters with rainbow tints, and will 
 throw a set of spectra on the wall. Mother-of-pearl 
 glitters with rainbow-tints for precisely the same general 
 reason, it possesses naturally a structure of fine striations 
 or ridges which produce (rather irregularly) diffraction. 
 But as the ridges are not quite equidistant, the tints are 
 never pure. But, do you know, if you will take with 
 sealing wax — black wax is best — an impression from a 
 piece of mother-of-pearl, you will find it gUtter just as 
 the mother-of-pearl does. 
 
 Whichever of these two means we use — prism or 
 grating — of producing a spectrum, you will note that 
 what we do is to sort out the mixture into its con- 
 stituents ; we analyse the light. Presently we shall be 
 able to make use of this sorting process to discover what 
 some of the compound colours are made up of. But in 
 the meantime we will return to the prismatic method to 
 show some further experiments. 
 
 To produce a good arched rainbow artificially, but in 
 all the splendour of the natural colours, I have recourse 
 to a specially constructed compound conical prism. A 
 glass cone of light crown glass is mounted with its point 
 turned inward (Fig. 45), within a hollow truncated cone 
 of glass, the face of which is closed with a glass plate. 
 The annuJar space is filled with a highly refracting liquid, 
 ^ Made by Mr. E. Rousseau ; see footnote, p. 31 ante. 
 
8o 
 
 LIGHT 
 
 LECT. 
 
 cinnamic ether. -^ An annular slit in a plate of tin-foil is 
 fixed against the end of the cone ; and the whole prism 
 
 Fig. 45. 
 
 is placed in a nearly parallel beam of light issuing from 
 the lantern. Beyond it is a lens to focus the light. 
 
 1 This liquid is excellent for direct-vision prisms, for it has 
 exactly the same mean refractive index as one kind of the Hght 
 crown glasses made at Jena. Figs. 46 and 47 depict the direct 
 vision prism with parallel end-faces designed by the author in 1889, 
 and constructed by Messrs. R. and J. Beck. A prism A of this glass 
 
 ^f)- 
 
 Fig. 46. 
 
 Fig. 47. 
 
 with a refracting angle of 135° is immersed in a glass cell filled with 
 cinnamic ether. Yellow light, as shown in Fig. 47, goes straight 
 through, while red light is thrown to one side and violet to the other. 
 This prism is very suitable for projecting the spectrum on the screeUc 
 
II VISIBLE SPECTRUM AND THE EYE 81 
 
 Thus we project upon the white screen in almost exactly 
 true proportions a rainbow. Note the order of the 
 colours, red along the outer edge, then orange, a trace 
 of yellow, green, peacock, blue, and lastly violet along 
 the inner edge. This is the correct order as in the 
 natural rainbow. But you often see it incorrectly de- 
 picted by artists — they put the colours in the wrong 
 order, or with the red along the inner edge and the 
 violet along the outer. 
 
 My assistant will now give us upon the screen the 
 bright spectrum which we saw before, in order that we 
 may study the effects of the different kinds of light on 
 coloured stuffs. Here is a piece of blue drapery, and 
 here a piece of scarlet. What is the effect of putting 
 these into the spectrum, first into one kind of light, and 
 then into another? If we put the blue stuff into the 
 red or orange or yellow light it looks simply black. But 
 in the blue part of the spectrum it looks blue, in the 
 violet part it looks violet, in the green part it looks 
 green. Clearly the surface of it is incapable of reflect- 
 ing back either red, orange, or yellow, while it is cap- 
 able of reflecting back green, blue, and violet. In fact, 
 when ordinary daylight falls on it, it absorbs some of 
 the waves and destroys them, while it reflects back to 
 our eyes some others of the waves, and gives us on the 
 whole a blue effect from the green, blue and violet 
 waves mixed together and reflected back. The red 
 stuff, when placed in the red part of the spectrum, looks 
 red, and in the orange and yellow parts it looks dull 
 orange and dull yellow ; while in all the other parts of 
 the spectrum it looks black. This red stuff then absorbs 
 
82 
 
 LIGHT 
 
 LECT. 
 
 iJk 
 
 and suppresses all the violet, blue, peacock, and green 
 rays, and reflects back only those at the red end of the 
 spectrum. But, of course, it would only look red if 
 there was some red light present. And the blue stuff 
 would only look blue if there was some blue light pre- 
 sent. The colour is really not in the stuff, it is in the 
 light that the stuff reflects. 
 
 To prove this let us see how these red and blue stuffs 
 appear when we shine upon them 
 some light that has neither red, nor 
 blue, nor green, nor violet in it, but 
 has yellow only. The monochro- 
 matic lamp which Professor Tyndall 
 used to employ here (Fig. 48) has 
 been lit. It consists of an atmo- 
 spheric gas-burner, into the dim 
 flame of which salt is projected,^ 
 making a splendid yellow flame 
 devoid of every other kind of light. 
 I hold these blue and red stuffs in 
 the light of the yellow flame. The 
 one appears simply black, the other 
 a dull gray. A set of stripes of 
 gay colours painted upon a board 
 appear simply dull grays and blacks, except the yellow 
 stripe, which seems brighter than all the others. Even 
 gaily-coloured flowers seem merely black or gray ; while 
 
 ^ The salt is contained in an annular pan at the top of an external 
 tapering chimney of sheet iron. This annular pan has a gauze 
 bottom, through which on tapping the chimney the salt falls in fine 
 powder into the flame. 
 
II VISIBLE SPECTRUM AND THE EYE 83 
 
 the complexion of the human countenance appears 
 simply ghastly. 
 
 Now if Newton's view is correct that white light 
 consists of the lights of various colours mixed up 
 together, it ought to be possible to make white light 
 by taking lights of all the various colours and mixing 
 them together. Do not try to mix together pigments 
 out of your paint box — they won't make white paint 
 when mixed. That is because pigments are not lights 
 — they are darknesses rather than lights. Think for an 
 instant what you do when you want to paint a card 
 crimson. You take a piece of white card, and paint over 
 it a pigment which darkens it, so that it sends back to 
 your eyes crimson only, and absorbs the other parts of 
 the white light. No, you must not mix pigments — you 
 must mix lights, and mix them in the correct proportions. 
 
 Now there are several ways of doing this ; and first 
 of them we will take the spectrum colours and recom- 
 bine them to produce white light. We take the spec- 
 trum light as it issues obliquely from the prism, and 
 reflect it upon the screen with a piece of silvered mirror- 
 glass. By simply waggling the mirror -glass upon its 
 stand, we cause the spectrum to oscillate rapidly across 
 the screen. The colours then all blend by rapid super- 
 position, and we obtain a white band bordered by colour 
 only at the ends. 
 
 Another way to recombine the spectrum is to employ 
 a cylindrical lens (Fig. 33, p. 49), so placed in the path of 
 the diverging coloured rays that it collects them back 
 to a focus on the screen, and gives us back the image of 
 our slit as a white streak. 
 
84 
 
 LIGHT 
 
 LECT. 
 
 An independent plan, suggested by Newton, is to 
 
 paint upon a circular card ^ 
 (Fig. 49), in narrow sec- 
 tors, the various tints in 
 proportions ascertained by 
 experiment to give the best 
 result; and then, putting 
 this upon a small whirling- 
 table, spin it round so fast 
 that the colours all blend 
 in the eye, giving, when 
 well illuminated against a 
 black background, the 
 effect of white. A similar 
 arrangement can be made 
 for use in the lantern, the 
 sector-disk being painted 
 ^^^' "^9- in transparent tints, or 
 
 coloured by affixing narrow wedges 
 
 of coloured transparent gelatine. 
 This method of colour - mixing 
 
 by whirling round before the eye 
 
 surfaces tinted with the colours 
 
 desired to be mixed, is capable of 
 
 extension to other cases. Suppose 
 
 we wish, for example, to mix red and 
 
 green, or blue and orange together, we have only to paint 
 
 ^ The colour-whirler actually shown was lent by Messrs. Harvey 
 and Peak, who use strips of brilliantly tinted paper pasted upon a 
 card in such a way as to repeat the gamut of colours from red to 
 violet five times around the circle. If the colours are thus repeated 
 the card does not require to be whirled very fast to produce white. 
 
 Fig. so 
 
JI 
 
 VISIBLE SPECTRUM AND THE EYE 
 
 85 
 
 a round disk (Fig. 50) with the colours desired to be 
 mixed, either in semicircles, quadrants, or in any other 
 desired proportion, and place them upon a whirling 
 machine to see the effect. 
 
 The arrangement w^hich I now show offers an im- 
 provement in several respects. Upon the whirling- 
 table is fixed a light 
 cylinder of wood, 
 which is slightly 
 tapered toward the 
 top, so that over it 
 may easily be slipped 
 a paper sleeve or 
 tube upon which the 
 colours are painted. 
 I have here several 
 of these paper tubes. 
 The colours (in most 
 cases coloured paper 
 being cut to shape 
 and pasted on) to be 
 mixed are arranged 
 in two sets of narrow 
 triangles, as shown in the figure. When these are 
 whirled, one gets combinations in all proportions. 
 For instance, if red and green are the two colours 
 chosen, one end of the revolving surface is full red, 
 the other full green, and the colours gradually fade 
 one into the other. About the middle, one obtains 
 a curious gray, which if seen by daylight looks rather 
 greenish; but by gas-light or lamp-light looks rather 
 
 Fig. 51. 
 
86 LIGHT LECT. 
 
 reddish owing to the greater relative prevalence of red 
 waves in artificial light. This change of apparent tint 
 is similar to that observed in the rare gem called the 
 alexandrite/ which is green by day and deep red by night. 
 Returning from the operations of colour-mixing by 
 rotation, I return to the property of the prism to analyse 
 mixed lights by spreading out the constituent colours as 
 a spectrum. Newton tried an experiment to see whether 
 if you took light of one tint alone you could split it up 
 still further by passing it through a second prism. I 
 introduce across the path of the spectrum on its way to 
 the screen a diaphragm of cardboard, having a narrow 
 slit in it. I push it along so that the slit allows waves of 
 but one particular colour — say green — to pass. Now, if 
 I interpose beyond this slit a second prism, I find that 
 it turns the beam of green light round at an angle, and 
 widens it out a little more, but it does not split it up into 
 any other colours : it is still a green beam. So it would 
 be with any other. When once you have procured a 
 simple tint by dispersing away the other colours to right 
 and left, the prism effects no further analysis ^ of colour. 
 
 ^ A gem of the emerald species found in a mine in Siberia be- 
 longing to the Imperial Russian family. 
 
 ^ In this sense every pure spectrum tint is a primary tint, and the 
 number of such tints incapable of further analysis is infinite. Each 
 kind of light of a given wave-length is thus a simple tint. But the 
 eye possesses three different sensations of colour, each of which is 
 physiologically a primary sensation. These three primaries are, a 
 red, a rather yellowish green, and a blue-violet. Any other tint than 
 these excites more than one sensation. For instance, a pure spec- 
 trum yellow excites both the red and the green sensations ; there- 
 fore yellow cannot be called truly a primary. In the same way 
 peacock tint excites the green and the blue-violet sensations. 
 
II VISIBLE SPECTRUM AND THE EYE 87 
 
 Now let us try a few experiments In the analysis of 
 colours by the prism. There are many well-recognised 
 tints, known by familiar names, which are not to be seen 
 in the simple colours of the spectrum, for the simple 
 reason that they are compound colours. In the spec- 
 trum there is no purple ; for purple is a mixture of red 
 from one end of the spectrum with violet or blue from 
 the other end. Pink does not exist in the spectrum — 
 for pink is red, diluted by admixture with white, that is 
 to say, with a little of every other colour. Neither is 
 there any chocolate colour, which is red or orange 
 diluted with black, that is to say, a little red or orange 
 spread where there is no light of any other colour. 
 Buff, olive, russet, bistre, slate, and many other colours 
 are also compounds. Well, whatever they are, the prism 
 can analyse them. Here is a piece of gelatine, such as 
 you may get off a Christmas cracker, stained a beautiful 
 purple. Why does it look purple ? What kinds of 
 light does it actually allow to pass through it that it 
 should look purple? I. have merely to interpose it in 
 the path of the white light for you to see the beautiful 
 purple colour on the screen. Now placing the prism 
 in front of it you see the purple spread out into its 
 constitutents. There is red at one end ; there are 
 violet and blue at the other. But in between, where 
 orange, yellow, green, and peacock colours should come, 
 there is darkness. The purple stain in the gelatine cuts 
 off all these and lets the others go by. Here is another 
 piece of gelatine stained with magenta — you see it lets 
 more red and a little violet and blue go through. Here 
 is a small glass tank containing the pale purple liquid 
 
88 
 
 LIGHT 
 
 LECT. 
 
 known as Condy's fluid (permanganate of potash) ; on 
 interposing it across the beam of light through the prism 
 you see (Fig. 52) that it cuts off the yellow and greenish 
 yellow, but transmits red and orange at one end of the 
 spectrum, and at the other violet, blue, peacock, and 
 some green. Here are some coloured liquids in bottles 
 (Fig. 53) ; red liquid (amyl alcohol dyed with aniline- 
 red) floating on the top of a green liquid (cupric 
 
 Fig. 52. 
 
 Fig. 53. 
 
 chloride dissolved in dilute hydrochloric acid) without 
 mixing. The red — as . you see when I expose it to 
 analysis in the spectrum — is a good red — it cuts ofl" every 
 tint except red. The green is also a fairly good green — 
 it cuts off everything except green, peacock, and a trace 
 of blue. What will happen if I now shake up the two 
 solutions and mix them ? I obtain a mixed liquid ^ that 
 cuts off everything, and is simply black. Many other 
 experiments of an instructive kind may be tried with 
 
 ^ The liquids named possess the very convenient property of 
 separating from one another in a very few minutes. In preparing 
 the experiment a little trouble and care is required to get the solu- 
 tions to balance. By adding first a little of the red liquid and then 
 a little green as may be required, and trying the effect of shaking 
 up, the liquids may be adjusted. Various other colour-combinations 
 are possible in this v/ay. 
 
II VISIBLE SPECTRUM AND THE EYE 89 
 
 coloured liquids, their apparent tints depending on the 
 kinds of light they absorb and transmit respectively. 
 Any liquid which merely absorbs green will look red- 
 dish, since in the balance of colours it transmits, the 
 complementary red will preponderate. Similarly any 
 liquid (or glass) which merely absorbs the blue part of 
 the spectrum will look yellow. It is even possible to 
 find a liquid,^ which though it looks yellow to the eye 
 really transmits nothing but green and orange, which 
 when mixed have the same effect on the eye as yellow. 
 This proves that the sensation of yellow, though it may 
 be excited by a simple spectrum tint of a particular 
 wave-length, can also be excited by a mixture of other 
 tints, and is therefore not a primary colour-sensation as 
 red, green, and blue-violet are. 
 
 And this brings me to another point, viz. that while 
 yellow light can be thus made by mixing together 
 orange and green lights, it is found to be absolutely 
 impossible to produce green ^ by mixing together any 
 two other pure lights. Blue light and yellow light, as 
 remarked above, do not when mixed produce green, 
 but white. This is so fundamental- a matter that it is 
 worth while to illustrate it by further experiment. 
 
 My assistants have two lanterns. From each of 
 them there is now thrown upon the screen a round 
 white disk of light. In front of one lantern is interposed 
 a film of blue gelatine — and that disk turns unmistak- 
 
 ^ Mixed solutions of chromic chloride and potassium bichromate. 
 
 ^ Just as also it is impossible to produce red light by mixture of 
 any other two simple lights, or to produce blue- violet by admixture 
 of any other two simple lights. These three — red, green, and blue- 
 violet being the three primary colour sensations. 
 
90 
 
 LIGHT 
 
 LECT. 
 
 ably blue. In front of the other lantern is interposed 
 a film of yellow gelatine, and the second disk of light 
 on the screen becomes bright yellow. Now one of the 
 lanterns is turned a little aslant so as to make one of 
 the disks overlap the other (Fig. 54). Where they over- 
 
 FiG. 54 
 
 lap and the lights mix we have — not green — but white ! 
 I put in the lantern a colour-whirler, having a disk 
 
 covered over half with blue and half with yellow 
 
 gelatine, and on whirling it round the blue and yellow 
 
 mix, and make white. 
 
 Here is an experiment that any boy might make at 
 home. A cardboard disk is divided 
 into twelve sectors, six of which are 
 covered with blue paper, and the 
 alternate six with yellow (Fig. 55). 
 I put a pin through the centre, and 
 spin it round by hand — and behold 
 blue and yellow are mixed, and 
 make white. 
 
 Fig. 55. 
 
 We give the name of " complementary " tints to any 
 
II VISIBLE SPECTRUM AND THE EYE 91 
 
 pair of tints which thus mixed together make white. 
 That is to say, if any two tints are so related that each 
 contains the constituents that are wanting in the other, 
 then we describe them as the complement one of the 
 other. Here is a table of some tints which experience 
 shows to be complementary one to the other ; — 
 
 Table IL — Complementary Tints 
 
 Crimson is 
 Scarlet 
 
 complementary to 
 
 Moss green 
 Peacock 
 
 Orange 
 Yellow 
 
 
 5 J 
 
 Turquoise 
 Blue 
 
 Primrose 
 
 
 JJ 
 
 Violet 
 
 Green-yellow 
 
 55 
 
 Purple 
 
 There are other cases also not set down on the list. 
 Now seeing that the sensation of white is not excited 
 unless all three primary sensations ^ (red, green, violet), 
 are stimulated at the same time, it is clear that when 
 two colours are found that are complementary to one 
 another, by no possibility can both be primary colours. 
 One may be, but in that case the other will be a 
 mixture of the two other primaries. If primary red is 
 one of the two complementary tints the other will be a 
 bluish -green or peacock colour made up of primary 
 green and primary blue-violet mixed. 
 
 Probably most of you are aware of the subjective 
 colours that are seen on closing the eyes after looking 
 
 ^ The red that is primary is a full red. The green that is 
 primary a rather yellowish-green, the violet a rather bluish-violet. 
 
92 
 
 LIGHT 
 
 LECT. 
 
 at a bright light. These are connected with the fatigue 
 of the nerve, and with the residual nervous stimulation. 
 But closely connected with them are the "contrast" 
 colours that are seen on a gray background after the 
 eye has been fatigued by looking at any coloured object. 
 The tints of these " contrast " colours are approxi- 
 mately, though not accurately, the complementaries of 
 the respective colours that have excited them. Thus 
 
 after staring intently for 
 some time at a bright 
 green disk, the after-image 
 against a gray wall is of 
 a reddish tint. There are 
 many ways of showing 
 these tints. I will give 
 you as an example that 
 used here in this theatre 
 by the late Professor 
 Tyndall. Against the 
 white wall, half- lit with 
 daylight, I hold up on 
 the end of a stick a 
 cardboard disk about a foot in diameter covered 
 with bright blue paper (Fig. 56). The beams of an 
 electric lamp are directed upon it to make its tint more 
 brilliant. You must look at it fixedly while I count 
 thirty in a distinct voice. When I come to "thirty" I 
 will drop the disk ; but you must continue to look at 
 the same region of the wall, where you will see — now 
 that I drop the disk, — a yellow image or ghost, of the 
 same size as the blue disk. In like manner if I hold up 
 
 Fig. 56. 
 
II 
 
 VISIBLE SPECTRUM AND THE EYE 
 
 93 
 
 a red disk you will, when your eye is fatigued, see a green 
 or peacock coloured ghost. The reason of the contrast 
 tint is that if you have fatigued the eye, or any region 
 of the retina of the eye, with red waves, that region will 
 be less sensitive for red than it is for the other colours. 
 Hence if gray {i.e. diluted white) is presented in view, 
 the retina at the fatigued region is more sensitive to 
 all the other tints present than it is to red, and will 
 therefore on the whole receive an impression in which 
 green predominates. 
 
 Another way to see the contrast tints is to stretch 
 upon a ring of cardboard a sheet of semi-transparent 
 coloured tissue paper, and then 
 upon this as a background gum 
 a smaller ring of white card- 
 board. Diffuse daylight should 
 be allowed to fail from the front 
 upon the white card, making it 
 gray. By lights suitably placed 
 behind the coloured tissue is 
 lit up. The eye therefore sees 
 the gray ring between an inner 
 and an outer circle of colour. 
 
 And after looking for a very few seconds will pronounce 
 the gray card to have become of a complementary tint. 
 Thus if the tissue paper is orange in hue the gray card 
 ring takes a bluish-peacock colour by contrast. 
 
 But the persistence of impressions in the eye is not 
 limited to phenomena of colour. All ordinary visual 
 impressions last a perceptible time, the images of brightly 
 lighted objects, even when only viewed for a thousandth 
 
 H 
 
 Fig. 57. 
 
94 
 
 LIGHT 
 
 LECT. 
 
 of a second will take a whole tenth of a second to die 
 away. If, then, you can present to the eye a second 
 
 Fig. 58 
 
 impression before the first one has, died away, the effect 
 is the same as though both had been present at one 
 time. A familiar toy depending on this principle, and 
 called the thaumatrope, consists of a bit of white card 
 
II 
 
 VISIBLE SPECTRUM AND THE EYE 
 
 95 
 
 held between two strings on which it can be twirled. 
 On one side there is painted say a horse ; on the other 
 his rider. On blowing against the card to twirl it you 
 see the rider (Fig. 58) mounted on his horse. We may 
 try this experiment in a new way. On one side of a 
 vertical card is painted in outHne a birdcage. On the 
 
 other side, a bird. By a band and pulley we spin the 
 card rapidly ; and lo ! you see the bird within its cage. 
 
 I hold in the lantern a small disk of sheet metal 
 having a number of holes pierced in it ; giving on the 
 screen a lot of bright points of light. Then on making 
 this disk vibrate on the end of a spring, or rotate about 
 a pin, each little white hole is transformed apparently - 
 into a luminous line, moving about on the screen. 
 
 Another optical illusion depending chiefly upon the 
 persistence of vision is afforded by the strobic circles 
 which I devised in 1877. On giving these black and 
 
96 
 
 LIGHT 
 
 LECT. 
 
 white patterns a small " rinsing " motion, the circles and 
 toothed wheels seem to rotate on their axis. 
 
 The latest of optical illusions and one not easy to 
 explain,-^ is Benham's colour-top. A number of narrow 
 black-lines are drawn as arcs of circles of various lengths, 
 upon a white surface, half of which (Fig. 6i) is coloured 
 
 Fig. 6o. 
 
 Fig. 6i 
 
 black. On revolving this disk, and viewing it by a 
 
 sufficiently strong light, the arcs of some of the circles 
 
 appear coloured. The rotation must be neither too 
 
 slow nor too quick. On reversing the rotation the order 
 
 of the colours reverses. The effect appears to be due 
 
 to the intermittent stimulation. 
 
 ^ See recent paper in Proceedings of the Royal Society^ by Mr. 
 Shelford Bidwell, F.R. S., whose explanation is that when the 
 particular nerve-fibres which give the red sensation are excited at 
 any part of the retina, the immediately adjacent parts of the same 
 nerve-fibres are for a short period sympathetically affected, so that 
 a red border seems for an instant to grow around the image of a 
 white object suddenly seen. In the same way, when the image of 
 a white ol)ject is suddenly cut off there is a sympathetic reaction 
 giving a transient blue border around the disappearing image. 
 
II VISIBLE SPECTRUM AND THE EYE 97 
 
 Another example of effects produced by persistence of 
 the optical impressions in the eye is afforded by an old 
 toy, the zoetrope, or wheel of life ; in which the sem- 
 blance of motion is given to pictures by causing the eye 
 to catch sight, in rapid sequence, through moving slits, of 
 a series of designs in which each differs slightly from the 
 one preceding. Thus if you want to make the sails of 
 a windmill seem to go round, the successive pictures 
 must represent the sails as having turned round a little 
 during the brief moment that elapses between each 
 picture being glimpsed and the next being seen. These 
 intervals must be less than a tenth of a second, so that 
 the successive images may blend properly, and that the 
 movement between each picture and the next may be 
 small. Mr. Muybridge has very cleverly applied this 
 method to the study of the movements of animals. 
 Anschiitz's moving pictures, illuminated by intermittent 
 sparks, were the next improvement. And the latest 
 triumph in this development of the subject has been 
 reached in the animatograph^ which the inventor, Mr. 
 R. Paul, has kindly consented to exhibit. 
 
 The animatograph pictures are photographed upon 
 a travelling ribbon of transparent celluloid ; the time 
 which elapses between each picture being taken and 
 the next being about one -fiftieth of a second. A 
 scene lasting half a minute will, therefore, be repre- 
 sented by about 1500 pictures, all succeeding one 
 another on a long ribbon. If these pictures are then 
 passed in their proper order through a special lantern, 
 with mechanism that will bring each picture up to the 
 proper place between the lenses, hold it there an 
 
98 
 
 LIGHT 
 
 LECT. 
 
 Ijkiim 
 
 Fig. 62. 
 
 instant, then snatch it away and put 
 the next in its place, and so forth, 
 the photograph projected on the 
 screen will seem to move. You see 
 in a street scene, for example, the 
 carts and omnibuses going along ; 
 the horses lift their feet, the wheels 
 roll round, foot passengers and 
 policemen walk by. Everything 
 goes on exactly as it did in the 
 actual street. Or you see some 
 children toddling beside a garden 
 seat. A big dog comes up, and the 
 boy jumps astride of him, but falls 
 off (Fig, 62), and rises rubbing his 
 bumps. Or a passenger steamer 
 starts from Dover pier : you see her 
 paddles revolve, the crowd on the 
 pier wave farewells with handker- 
 chiefs or hats, the steamer wheels 
 round, you see the splash of foam, 
 you note the rolling clouds of black 
 smoke proceeding from her funnel, 
 then she goes out of sight round 
 the corner. The reality of the 
 motions is so great that you feel as 
 though you had veritably seen it all 
 with your own eyes. And so you 
 have. You have just as truly seen 
 the movements of the scene as when 
 you have listened to the phonograph 
 
II VISIBLE SPECTRUM AND THE EYE 99 
 
 you have heard the voice which once impressed the 
 record of its vibrations. Of all the animatograph 
 pictures those that appeal most to me are the natural 
 scenes, such as the waves rolling up into a sea-cave 
 and breaking on the rocks at its mouth, and dashing 
 foam and spray far up into its interior. Nothing is 
 wanting to complete the illusion, save the reverberat- 
 ing roar of the waves. 
 
 Note. — Since the delivery of these lectures, Mr. Shelford Bidwell, 
 F.R.S., has pursued the subject of the curious colour - effects 
 mentioned at the foot of p. 96, and has reached some extraordinary 
 results. A cardboard disk 8 inches in diameter is half-covered with 
 black velvet, the other half being left white or gray. A sector of 
 45° is cut away at the junction of the black and white portions, and 
 this disk, suitably balanced, is mounted upon a revolving apparatus 
 to rotate with a speed of about 6 to 8 turns per second. Behind 
 it, so as to be visible at each turn through the gap where the 
 sector has been cut away, is placed a coloured picture, and a bright 
 lamp is placed a few inches in front to illuminate it. The direction 
 of rotation is such that the open sector is preceded by black and 
 followed by white. On thus viewing a picture by intermittent vision, 
 each part appears of a pale colour complementary to its actual 
 tint. A red rose with green leaves appears a green rose with 
 reddish leaves. A blue star on a yellow ground appears as a yellow 
 star on a bluish ground. Black printing on a white paper appears 
 whitish printing on a grayish paper, and so forth. 
 
APPENDIX TO LECTURE II 
 
 ANOMALOUS REFRACTION AND DISPERSION 
 
 On p. 78 attention is drawn to the circumstance that the 
 spectrum as produced by a prism is irrational ; that is to 
 say, that the dispersion is such that the different waves are 
 not spaced out in proportion to their wave-length, the red 
 and orange waves being relatively crowded together at one 
 end of the spectrum, while the violet and blue waves are 
 unduly spread out. But the dispersion is different on 
 different substances. In fact, no two substances disperse 
 the light in exactly the same way, though in general the 
 order of the colours is the same, and the general trend of 
 the irrationality is to compress the red end. But there are 
 a few known substances in which this irrationality becomes 
 excessive, and develops into an entirely abnormal dispersion 
 in which the violet waves are less refracted than the red ! 
 This phenomenon of anomalous dispersion was first noticed ^ 
 in 1840 by Fox Talbot in some crystals of the double 
 oxalate of chromium and potassium. The colours of the 
 spectra of some of these crystals were so anomalous that he 
 could only explain them " by the supposition that the 
 spectrum, after proceeding for a certain distance, stopped 
 short and returned upon itself." In 186 1 Le Roux found 
 that vapour of iodine, which transmits only red and blue, 
 actually retards the red more than the blue, and gives an 
 inverted spectrum. Christiansen in 1870 noticed that an 
 alcoholic solution of magenta (rosaniline) has an ordinary 
 refraction for the waves from red to yellow, the yellow being 
 
 1 See Tail's Light, p. 156. 
 
APF. REFRACTION AND DISPERSION loi 
 
 refracted more than orange, and orange than red, but 
 it absorbs green powerfully, and all the rest of the colours 
 — commonly called more refrangible — are in this substance 
 refracted less than the red ! In this case, the spectrum 
 literally returns back upon itself. Other observations have 
 been added by Kundt and others. In particular, Kundt 
 discovered that some of the metals, when made up into 
 excessively thin prisms, possess an anomalous dispersion. 
 
 The first point to note in discussing this phenomenon of 
 anomalous dispersion is that it only occurs in highly 
 coloured substances. It is closely related to the circum- 
 stance that in these substances there is, by reason of their 
 molecular constitution, a strong absorption for waves of 
 some particular wave-length. Thus in rosaniline, which 
 has a strong absorption-band in the green, the fact that 
 green light is absorbed appears to exercise a perturbing 
 influence upon the waves of the shorter kinds, causing them 
 to be less refracted instead of more. These remarkable 
 phenomena obviously have something to do with the way 
 in which the molecules of ponderable matter are con- 
 nected with the ether. None of the dispersion formulae of 
 Cauchy, Ketteler or others gave a satisfactory account of 
 them. 
 
 In 1872 Lord Rayleigh considered the problerri of the 
 refraction of Hght by opaque bodies, and in the Philosophical 
 Magazine (vol. xHii. p. 322) gave the following exceedingly 
 suggestive comment : — 
 
 " On either side of an absorption-band there is an 
 abnormal change in the refrangibility (as determined by 
 prismatic deviation) of such a kind that the refraction is 
 increased below (that is on the red side of) the band, and 
 diminished above it. An analogy may be traced here with 
 the repulsion between two periods which frequently occurs 
 in vibrating systems. The effect of a pendulum, suspended 
 from a body subject to horizontal vibra;tion, is to increase or 
 diminish the virtual inertia of the mass according as the 
 natural period of the pendulum is shorter or longer than 
 that of its point of suspension. This may be expressed by 
 saying that if the point of support tends to vibrate more 
 
I02 LIGHT LECT. ir 
 
 rapidly than the pendulum it is made to go faster still, and 
 vice versa. Below the absorption-band the material vibra- 
 tion is naturally the higher, and hence the effect of the 
 associated matter is to increase (abnormally) the virtual 
 inertia of the ether and therefore the refrangibility. On 
 the other side the effect is the reverse." 
 
 In 1893 von Helmholtz published a remarkable study ^ 
 based on Maxwell's electromagnetic theory of light. The 
 essence of this theory is as follows : — 
 
 The electromagnetic waves passing through the ether 
 travel at a rate which is retarded by the presence of 
 material molecules, the ether being as it were loaded by 
 them. These heavy particles cannot be set into vibration 
 without taking up energy from the advancing wave ; and so 
 long as there is no absorption, they give up this kinetic 
 energy again to the wave as it passes on. In this way the 
 velocity of propagation of the train of waves is slightly less 
 than the velocity of propagation of the individual wave ; 
 the front wave of the train continually dying out in giving 
 its energy to the material particles in the medium. In such 
 a medium there will of course be ordinary refraction ; and 
 as the velocity of propagation of the wave-train will depend 
 on the frequency {i.e. on the wave-length) of the oscillations 
 (there being in general a greater retardation of the waves of 
 higher frequency, i.e. of shorter wave-length), there will be a 
 dispersion of the ordinary kind. All this applies to waves 
 the wave-length of which is large compared with the size 
 of the molecules. But if there were smaller waves, the 
 frequency of which coincided very nearly with the natural 
 oscillation-period of the molecules or atoms, such waves 
 would set up a violent sympathetic vibration of these 
 material particles, and would be strongly absorbed. 
 Suppose that there are waves of still smaller size and still 
 higher frequency. Their oscillations are too rapid to 
 affect the atoms ; they pass freely between the interstices of 
 
 ^ See Wiedemam^s Annalen, xlviii. p. 389. The fullest account 
 of this that has appeared in English is in The Electrician, xxxvii. 
 p. 404, and an abstract account by Professor Oliver Lodge, ib^ 
 
APP. REFRACTION AND DISPERSION 103 
 
 matter and are not retarded, therefore not refracted or dis- 
 persed, or only very slightly so. The medium would act as 
 if almost perfectly transparent to such waves ; and their 
 refraction might be either slightly negative or slightly 
 positive ; whilst for the minutest waves of all the refraction 
 would be simply zero. The formula which von Helmholtz 
 deduced is in its simplest form the following : — 
 
 ^ — 02 179' 
 
 where /x is the refractive index of the medium (supposed 
 quite transparent), 7i the frequency, and a and /? constants 
 depending on the material. To interpret the formula, con- 
 sider what it reduces to in the following cases (i) n much 
 smaller than a or /3 ; (2) n = ^; (3) ;? = a; and (4) n 
 much greater than a or /?. In the first case, n being small 
 we are dealing with long, slow -period waves such as 
 Hertzian waves or those of infra-red dimensions. Neglect- 
 ing 71^ compared with a? or /i^^ ^j^g formula reduces to /x 
 = a//?, being independent of wave-length. In the second 
 case if the frequency is such that 7t = (3 the medium cannot 
 possibly be transparent, as there would be violet absorp- 
 tion. The real meaning is that as ?t increases from case 
 ( I ) toward the value ;z == /? the refractive index increases, 
 and would become indefinitely great were it not for the 
 absorption that sets in. In the state of things between 
 cases (2) and (3), where ?t is larger than (3 but smaller than 
 a, the values of yu, calculated by the formula are imaginary ; 
 but owing to the absorption they would in reality diminish 
 down to near zero, that is to say, the refraction in these 
 conditions becomes negative. This corresponds to the state 
 of things observed by Kundt with their refracting prisms 
 of iron, nickel, and platinum, which refract the light toward 
 the refracting edge instead oi from it. As 7t increases from 
 case (3) when it equals a, the zero value of /a gradually 
 changes, and, when n becomes very great compared with 
 a and ^, it approaches to unity, so that for excessively 
 short waves there is no refraction at all. 
 
I04 
 
 LIGHT 
 
 LECT. II 
 
 Consider the particular value of n for which /x becomes 
 a maximum. This is the case in which the excessive 
 absorption makes the medium practically opaque. For 
 values of n a little less than this there will be practically 
 complete transparency and ordinary refraction and dis- 
 persion ; for values of 71 a little greater than this there will 
 again be practical transparency, but there will be a refraction 
 in the wrong direction (/x being less than unity), and the 
 dispersion will be anomalous. Fig. 63 illustrates this 
 dependence of /x upon n. In the case of rosaniline, the 
 frequency for which the absorption becomes excessive is 
 about 578 billions per second, corresponding to a wave-length 
 
 <s: ^ 
 
 
 
 
 ■ -2 
 
 
 
 
 
 
 5-i 
 
 
 
 £: 
 
 
 '•1 
 
 
 
 
 
 
 IP 
 
 
 
 
 i3 
 
 • 
 1 
 
 
 
 
 
 
 ^^ 
 
 
 
 / 
 
 1 
 
 t 
 1 
 
 
 
 
 
 
 Lino 
 
 ofzer 
 
 ) refri 
 
 'ction 
 
 
 r 
 
 
 
 
 
 
 
 
 
 
 
 
 Fr 
 
 equen 
 
 "U 
 
 Fig. 63. 
 
 of 2 1 millionths of an inch in air. E. F. Nichols has found 
 that quartz shows a similar change of properties for infra-red 
 waves of a frequency between 36 and 4 5 "4 billions per 
 second. For almost all ordinary transparent substances 
 the absorption-band occurs a long way down in the ultra- 
 violet ; in some it may possibly occur in the infra-red. It 
 may be possible, for example, for flint-glass, the refractive 
 index of which for ordinary light is between 1*5 and 17, to 
 have a refractive index as high as 2-6 for disturbances of 
 very low frequency such as Hertzian waves : that being the 
 theoretical value for long waves as required by Maxwell's 
 theory to correspond to the square-root of the observed 
 dielectric constant. Probably many substances have more 
 than one absorption-band, thus still further complicating the 
 anomalous dispersion. 
 
LECTURE III 
 
 POLARISATION OF LIGHT 
 
 Meaning ol polarisation — How to polarise waves of light — Illus- 
 trative models — Polarisers made of glass, of calc-spar, and of 
 slices of tourmaline — How any polariser will cut off polarised 
 light — Properties of crystals — Use of polarised light to detect 
 false gems — Rubies, sapphires, and amethysts — Polarisation by 
 double refraction — Curious coloured effects, in polarised light, 
 produced by colourless slices of thin crystals when placed 
 between polariser and analyser — Further study of comple- 
 mentary and supplementary tints — Exhibition of slides by 
 polarised light — Effects produced on glass by compression, and 
 by heating. 
 
 Scientific men often fall into the habit of using long 
 and difficult words to express very simple and easy 
 ideas. The natural consequence is, that people are 
 often led to think that there is something difficult about 
 a really easy subject, whereas the main difficulty is to 
 understand the meaning of the words selected to de- 
 scribe it. 
 
 The word "polarisation," used in optics, is one of 
 these terms. It sounds very learned and difficult, but 
 the idea it is intended to express is really very simple. 
 Let me try to give you the idea before we try to fit any 
 name to it. 
 
io6 LIGHT LECT. 
 
 In my first lecture I endeavoured to give you some 
 simple notions about waves and the way they travel. I 
 asked you particularly to distinguish between the oscil- 
 latory motions of the particles and the forward travelling 
 of the waves themselves. Let us return to the motions 
 of the particles. Suppose any particle or group of par- 
 ticles to have motion given to it, a rapid "to-and-fro " — 
 in other words, let it be supposed to vibrate. Then if 
 it is surrounded by a suitable medium, and its vibra- 
 tions occur with a sufficiently great frequency, it will set 
 up waves in the surrounding medium which will start 
 oif from it, travelling away at a definite speed depending 
 on the rigidity and density of the medium. In the case 
 of a compressible surrounding medium such as air, the 
 vibrating body (if vibrating between the limits of fre- 
 quency appropriate for sound — that is to say, between 
 about 30 per second and 38,000 per second) will com- 
 press the air in front of itself as it moves forward, and 
 rarefy the air behind it as it moves back, with the 
 
 - result that it sends off waves of condensation 
 
 t and rarefaction. If, as in Fig. 64, the oscillating 
 
 ^M body is a sphere moving rapidly up and down 
 
 y^ along the short path AB, it will tend alternately 
 
 ^ to condense and rarefy the air above and below 
 ^^' ^^' it, and these compressions and rarefactions will 
 travel off upwards and downwards, spreading a little 
 as they go. But hardly any waves of compression and 
 rarefaction will travel off sideways from the oscillating 
 sphere, because in oscillating up-and-down it does 
 not either condense or rarefy the air at its sides. 
 The wave in this case would be described as a longi- 
 
Ill POLARISATION OF LIGHT 107 
 
 tudinal wave, meaning one which is propagated along 
 the line in which the particular motion exists — in this 
 case vertical. If you want to know more about the 
 travelling of sound-waves, you must read Professor 
 Tyndall's delightful book On Sound ; or if you are deep 
 students you will study Lord Rayleigh's two mathe- 
 matical volumes on the Theory of Sound. 
 
 But now, suppose that you have to deal not with a 
 medium like air that is compressible, but with a medium 
 like jelly that is incompressible, and in which the 
 density is small compared with the rigidity that it 
 opposes to any rapid shear. If in this case you set up 
 
 an oscillation with a . 
 
 A 
 
 sufficiently great fre- \ 
 
 quency, waves will be .♦*'••. /'"•.. -£%, .•"•. .•'*•. 
 set up which will travel *"•''' '■••-•*' ^^''— '' 
 
 off at a high rate, but '•'b 
 
 not in the line of the ^^^- ^s- 
 
 motion. On the contrary, they will travel off sideways 
 in all directions in ripples. Let Fig. 65 represent a 
 sphere embedded in the middle of a great block of 
 surrounding jelly, and that it is made to oscillate up- 
 and-down as before, but with a great rapidity. It 
 cannot move up without tending to tear or shear the 
 jelly all around its girth, nor can it move down without 
 tending to tear or shear the jelly downwards ; and 
 these shearing stresses travel outward in all direc- 
 tions, so that a particle at a will, as these waves or 
 ripples in the solid jelly reach it, tend also to move up 
 and down. In any medium, whether a jelly or not, if 
 the particles are in such relation to one another that 
 
io8 LIGHT LECT. 
 
 the movement of any of them tends to set up a shearing 
 stress, then that medium will, like the jelly, propagate 
 the disturbances sideways. The waves in such cases 
 would be described as transverse waves — meaning 
 waves which are propagated in directions at right angles 
 to the direction in which the to-and-fro displacements 
 are executed. 
 
 Now the waves of light are of the transverse kind ; 
 and though they can pass through air, are not waves of 
 the air as sound-waves are. Waves of light can cross 
 the most perfect vacuum ; they travel thousands of 
 millions of miles in the vacuous space between the stars. 
 They are waves of another medium which, so far as we 
 know, exists all through space, and which we call, using 
 Sir Isaac Newton's term, the ether. If you ask me what 
 the ether is made of, let me frankly say I do not know. 
 But if light consists of waves, and if those waves can 
 travel across the millions of miles that separate the stars 
 from the earth, then it is clear that they must be waves 
 of so7nethi7ig ; they are not air- waves nor water-waves, 
 because interstellar space is devoid both of air and of 
 water. They are waves of a medium which, though 
 millions of times less dense than water or air, has yet a 
 property that resists being torn or sheared asunder ; ex- 
 ceeding the resistance to shear even of hard-tempered 
 steel. Though it is not a jelly, since things can move 
 through it more freely than you or I can move through 
 the air, yet it resembles the jelly in this property of 
 resistance to shear, and propagates vibrations transversely 
 to the direction of their displacements. Though we 
 know neither the density of the ether (though it must 
 
Ill POLARISATION OF LIGHT 109 
 
 be very small) nor its rigidity to shear (which must be 
 very great), we do know something which depends on 
 the ratio of these two properties, namely, the velocity of 
 propagation of those ether-waves which we call " light " 
 (see Appendix, p. 156). 
 
 Well, now having got this notion about transverse 
 waves, let us go back to the wave-motion model which 
 we used in the first lecture. It has, as you will 
 remember (Fig. 2, p. 8), a row of little white particles, 
 along w^hich row the wave is propagated from left to right, 
 though each little particle moves up and down. It is, 
 therefore, a model of a transverse wave ; the direction of 
 travel of the wave is at right angles to the direction of 
 the displacements. 
 
 But if a wave is to travel along a line of march, from 
 A to B, we may fulfil the condition of transverse vibra- 
 tion in other ways. The small to-and-fro motions must 
 be executed across the line of march ; and they may be 
 across the line of march without being vertical — they 
 may be horizontal, or oblique. If I turn my wave-motion 
 model on its side, the little white particles now move 
 horizontally toward you and from you, but the wave still 
 travels from left to right. 
 
 If I stretch across the room a long indiarubber cord, 
 holding one end of it in my hand, I can throw it into 
 transverse vibrations. If I move my hand rapidly up- 
 and-down, I produce up-and-down vibrations. If I 
 move my hand right-and-left, I get right-and-left vibra- 
 tions. If I move my hand obliquely to-and-fro, I produce 
 oblique vibrations ; and the cord transmits them all. 
 
 Now, all that the word polarisation means is that the 
 
 I 
 
no 
 
 LIGHT 
 
 LECT. 
 
 motions are being executed in some particular transverse 
 direction. If the vibrations are polarised vertically, that 
 means that they are up-and-down waves that are travel- 
 ling along. If I say that the light is polarised hori- 
 zontally, all I mean is that the motions are executed 
 
 Fig. 66 
 
 right-and-left across the hne of march. Can anything 
 be simpler? 
 
 Here is a lump of jelly. It will serve excellently to 
 show how a polarised vibration is propagated. I stick 
 into it horizontally two pins with silvered heads — one at 
 
 one side, the other at the 
 other. If I give a sudden 
 displacement to one pin it 
 quivers, and the jelly carries 
 on the motion to the other. 
 And note, if I strike one so 
 as to make it quiver up-and-down, the other quivers up- 
 and-down — here we have a vibration polarised vertically. 
 If I make one quiver right-and-left the other quivers 
 right-and-left — here we have a vibration polarised hori- 
 zontally. If I make one quiver circularly round and 
 round, the other quivers round and round also ; giving 
 an illustration of a circularly polarised vibration. 
 
 Fig. 67. 
 
Ill POLARISATION OF LIGHT in 
 
 Now let us go to the waves of light themselves. If 
 you look at a beam of white light you cannot by the 
 eye ^ tell whether it is polarised to move up-and-down, 
 or right-and-left. In fact you cannot tell whether it is 
 polarised at all. Naturally, if the waves are so ex- 
 cessively small, and vibrate so many millions of millions 
 of times a second, your eye cannot catch their motions. 
 
 The fact is that light of any natural kind, whether 
 from the sun, an electric lamp, a flame, or any other 
 source, is non-polarised ; that is to say, it consists of 
 vibrations which are not specially directed either up-and- 
 down or right-and-left, or in any other one direction. 
 Natural light, given out by hot bodies, is absolutely 
 miscellaneous. Not only does it consist, as we saw in 
 the last lecture, of a lot of different colours — that is, of 
 different wave-lengths, mixed up together — but it con- 
 sists of waves whose direction of transverse vibrations 
 are also all jumbled up. At one instant they may be up- 
 and-down; then they change to right-and-left, or to 
 oblique, or circular, or elliptical, or possibly to some- 
 thing still more complex. Just think how the light 
 starts from the white-hot tip of the carbon pencil in 
 my electric lamp. The particles of white-hot carbon 
 are in fierce vibration, jostling against one another, and 
 in jostling impart vibrations to the ether — setting up 
 
 ^ Not as thrown on the screen. But the eye can be trained to 
 detect the plane of polarisation, for example, of light from the blue 
 sky, which is naturally polarised in directions at right angles to the 
 position of the sun. The training consists in being able to recog- 
 nise certain appearances called " Haidinger's brushes" which result 
 from the feeble polarising properties of the refracting structures of 
 the eye. 
 
■ t.^^>X/***j*^ ' 
 
 112 LIGHT LECT. 
 
 ether-waves. When any one particle gets a sudden jolt 
 it quivers, and gives out a vibration, which we may 
 
 represent by the curve (Fig. 
 68), with a lot of little wave- 
 lets each like its fellow, per- 
 haps several thousands ^ of 
 them before they die away. 
 ^^°" ^^' Each such vibration would 
 
 die away like the note of a piano -string struck and 
 left to itself. But perhaps before the motion has died 
 away another jolt sets it off vibrating in a new direc- 
 tion, again to die away. Suppose millions of these 
 little particles, all jostling, and vibrating, and sending 
 out trains of wavelets. It is clear that one ought to 
 expect the utmost admixture of wave-sizes and directions 
 of vibration in the resultant light. 
 
 Then, you understand, that as natural light is not 
 polarised in any particular direction, if we want to get 
 polarised light we must do something to it to polarise it. 
 But how ? 
 
 ^ According to the researches of Fizeau, at least 50,000, on the 
 average, in ordinary light. Prof. Michelson's more recent experi- 
 ments, in which he has obtained interference between two waves 
 the paths of which differed by more than 20 cm. or 1,000,000 wave- 
 lengths, prove that the average number of wavelets in each train 
 must be reckoned in millions. 
 
 [Table 
 
in 
 
 POLARISATION OF LIGHT 
 
 113 
 
 Table III. — Polarisers 
 
 Principle. 
 
 
 Nature of Apparatus. 
 
 Reference. 
 
 By Reflexion . . -j 
 By Refraction . < 
 
 By Double Refraction < 
 
 By Double Refraction, \ 
 with Absorption . / 
 By Double Refraction, \ 
 with Internal Reflexion j 
 
 I. 
 IL 
 
 in. 
 
 IV. 
 
 V. 
 VI. 
 
 VII. 
 VIII. 
 
 Black glass at about 57° 
 Delezenne's Polariser . 
 Glass sheet at about 57° 
 Bundle of thin glass 
 
 sheets set obliquely 
 Rhomb of Iceland Spar 
 Double-image Prism . 
 
 Slice of Tourmaline . 
 
 Nicol's Prism and its 
 modei'n Varieties 
 
 (p. 153)- 
 (p. 123)- 
 (P- 154)- 
 
 (p. 154). 
 (p. 120). 
 (p. 125). 
 
 (p. 119). 
 
 (p. 121). 
 
 In Table IIL I have set down some eight different 
 ways of polarising, which we will presently consider in 
 their order. But before we deal with any of them, let 
 us go back to the vibrations of cords and see how they 
 can be polarised. 
 
 Here (Fig. 69) is an indiarubber cord passing through 
 a wooden box with vertical partitions. These partitions 
 limit the movements and only allow vertical vibrations 
 to pass through. If I vibrate the cord in any way, it is 
 only the vertical components of the vibration that suc- 
 ceed in getting through. The waves, after passing 
 through the box, come out polarised in a vertical plane. 
 If I turn the box over on its side (Fig. 70) it will now 
 transmit only horizontal components of vibration. What 
 will happen, then, if I pass the cord through a second 
 box, as in Fig, 70 ? That depends on the positions of 
 the boxes. If the first one P is set with its partitions 
 
114 
 
 LIGHT 
 
 LECT. 
 
 vertical, it will polarise the waves vertically, and as these 
 waves travel on they will come to the second box marked 
 
Ill POLARISATION OF LIGHT 115 
 
 A. If this also has its partitions vertical, the vertical 
 waves will get through it also. If both boxes are turned 
 over on their side, then the first one will polarise the 
 waves horizontally, and the horizontally polarised waves 
 will pass through both boxes. But if I have the first box 
 P set vertically and the second box A horizontally (Fig. 
 71), P will polarise the vibrations so that they will not 
 get through A, but will be cut off. However P is 
 placed it will polarise the waves ; if A is turned so as 
 to cross the waves they will be cut off. 
 
 Upon the lecture table is another model which illus- 
 trates the same set of facts more fully. If you under- 
 stand it you will have no difficulty in understanding the 
 optical apparatus that we are going to use. In this 
 apparatus the vibrations of a thin silk cord — best seen 
 by those in front of the table — are produced by attach- 
 ing one end to the prong of a tuning-fork, the vibrations 
 of which are maintained by an electromagnetic attach- 
 ment. To the distant end of the cord is attached a small 
 weight, which has been so adjusted that the cord is thrown 
 into stationary waves. In brief, the vibrations of the cord 
 are tuned to those of the fork. To polarise the vibrations, 
 the motions of the cord are confined by means of a pair 
 of glass plates mounted in wooden cylinders (Figs. 72, 
 73). At the first nodal point of the cord the first pair 
 of glass plates acts as a polariser, P ; the cord beyond 
 that point vibrating in the plane thus imposed upon it. 
 A pointer fixed upon the wooden cylinder shows the 
 direction of the plane of polarisation.^ The second 
 
 ^ Concerning the term, "plane of polarisation," see remarks in 
 Appendix to this Lecture, p. 158. 
 
ii6 
 
 LIGHT 
 
 LECT. 
 
 0."= 
 
 ^^m 
 
 pair of glass plates is set at the second nodal point 
 to act as an analyser^ A. The vibrations of the cord 
 
Ill POLARISATION OF LIGHT 117 
 
 are made vertical by the polariser P, and when the 
 plane of the analyser A is also vertical (as in Fig. 72) 
 the vibrations which pass through the polariser pass 
 through the analyser also. But, if (as in the previous 
 experiment with the boxes) the analyser is turned round 
 a quarter, so that the slit between the glass plates lies 
 across the vibrations (as in Fig. 73) the vibrations are 
 no longer transmitted. To recapitulate, the vibrations 
 are tra?ismitted when the polariser and analyser are 
 parallel to one another : but are cut off and extinguished 
 when polariser and analyser are crossed. Hence, by 
 turning round the analyser to such a position that it 
 cuts off the vibrations we can ascertain with accuracy ^ 
 the direction of the vibrations proceeding from the 
 polariser. 
 
 But why should we linger longer upon mere models 
 when we can operate with light - waves themselves ? 
 My assistant throws upon the screen a beam of white 
 light from the electric lamp within the optical lantern. 
 He now places in the path of the beam a large polariser, 
 P (Fig. 74). What this polariser is, I will presently 
 explain. He now sets it so that it polarises the light, 
 allowing to fall upon the screen those waves only whose 
 vibrations are executed in a vertical plane. The white 
 disk of light on the screen consists, in fact, of up-and- 
 down light only. Your eye would not tell you whether 
 the light was vibrating up and down, or even that it was 
 
 ^ The model will enable the orientation of the plane of the vibra- 
 tions to be determined to within about half a degree of angle. That 
 is, if the analyser is as much as half a degree out of the crossed 
 position, the vibrations are not completely extinguished. 
 
ii8 
 
 LIGHT 
 
 LECT. 
 
 polarised at all. To ascertain that the waves are really 
 polarised we must have recourse to an analyser. This 
 analyser, A, is itself simply a smaller polariser. In 
 order that you may see it the better it is mounted 
 (see Fig. 75) by thin strings upon a ring - support, 
 the shadow of which you see on the screen. If this 
 is also set in the proper position to transmit up-and- 
 down vibrations, the polarised light will come through 
 
 Fig. 74. 
 
 it, both polariser and analyser being clear as glass. If 
 now the analyser A is turned round one quarter it will, 
 though clear as glass, entirely cut off the up-and-down 
 vibrations, with the result (Fig. 76) that no light gets 
 through it. This cutting off of the light by turning 
 the analyser one quarter round proves that the light was 
 polarised. When the planes of polariser and analyser 
 are parallel to one another — both vertical, or both 
 horizontal, — then we have the "bright field" of trans- 
 mitted light. When the planes of polariser and analyser 
 
Ill 
 
 POLARISATION OF LIGHT 
 
 119 
 
 then 
 
 are crossed — one vertical, the other horizontal 
 the light is cut off, and we have the "dark field." 
 
 There is a gem called the tourmaline which, when 
 cut into thin slices, has the property of polarising light. 
 This gem^ is often found of a dark green colour, but 
 also of brown, dark blue, and even ruby tint. Into the 
 beam of ordinary white light now cast upon the screen 
 
 Fig. 75. 
 
 Fig. 76. 
 
 there is now introduced a thin slice of brown tourmaline 
 (Fig. 77). It looks dark, for it cuts off more than half 
 the light. But such light as succeeds in getting through 
 is polarised — the vibrations being parallel to the longer 
 dimension of the slice. A second thin slice of tourmaline 
 is now introduced, and superposed over the first. When 
 they are parallel to one another light comes through 
 both of them (Fig. 78). But if one of them is now 
 
 ^ The dark green tourmaline is also sometimes called the Brazilian 
 emerald, though it is of entirely different composition from an 
 emerald. The bishops of the South American Catholic churches 
 wear tourmalines in their episcopal rings, instead of emeralds. 
 
I20 
 
 LIGHT 
 
 LECT. 
 
 turned round, so that they are crossed, as in Fig. 79, no 
 light can get through the crossed crystals. The one cuts 
 off all horizontal vibrations and horizontal components 
 of vibration, the other cuts off all vertical vibrations and 
 vertical components of vibration. Hence, when crossed, 
 they produce a "dark field." One acts as polariser, the 
 other as analyser. 
 
 Let us return to the big polariser (Fig. 74) which we 
 used in the previous experiment, and which was as clear 
 as glass. It is made of Iceland spar, a natural crystal, 
 
 Fig. 
 
 n- 
 
 Fig. 78. 
 
 Fig. 79. 
 
 which once w^as common but now is rare and expensive. 
 As imported from the mine in Iceland this spar possesses 
 the peculiar property known as " double refraction " : 
 when you look through it you see everything double. 
 Here is a fine specimen mounted in a tube. Look at 
 your finger through it j you will see two fingers. It is a 
 substance which splits the waves of light into two parts, 
 giving two images ; and, moreover, polarises the light in 
 the act of splitting it, so that each part is polarised. 
 We do not, however, want both images ; .we want only 
 one. What do we do ? We adopt the method proposed 
 eighty years ago by William Nicol, a celebrated Scotch 
 
Ill 
 
 POLARISATION OF LIGHT 
 
 121 
 
 philosopher, and construct out of a crystal of the spar 
 a "polarising prism," or Nicol prism. Here are several 
 
 Pircction 
 
 vs>» > 
 
 of Light 
 
 Direction 
 
 »>» >/- 
 
 of Light 
 
 Fig. 8o. 
 
 Nicol prisms of various sizes ; and also several modern 
 modifications ^ of the Nicol prism. Here also is a large 
 wooden model to illustrate Nicol's method. 
 
 ^ In Foucault's modification, a film of air is interposed between 
 the two wedges of crystal. In Hartnack's prism a film of linseed 
 oil is interposed, and the ends of the wedges are squared off. I have 
 myself from time to time suggested several modifications which are 
 
 Fig. 8i 
 
 improvements upon the original Nicol prism. In one of these, the 
 natural end- faces of the prism are sliced away parallel to the 
 crystallographic axis so as to leave terminal faces that are " principal 
 planes" (Fig. 8i), and the crystal is then sliced with an oblique cut 
 
122 
 
 LIGHT 
 
 LECT. 
 
 Selecting a piece of Iceland spar of suitable propor- 
 tions we slice it across (with a piece of copper wire, 
 used as a saw, and some emery powder) in an oblique 
 direction from one of its two blunt corners to the other ; 
 polish the surfaces, thus dividing the prism into two 
 wedges. These are then cemented together again 
 with Canada balsam (a resinous cement) ; and the 
 polarising prism is complete. Its operation upon light 
 is as follows. When the waves enter through one end- 
 face they are split into two parts which take slightly 
 different directions, and strike at different angles upon 
 the film of balsam. As a consequence one of the two 
 beams when it meets the film of balsam is reflected 
 off sideways, as from an oblique mirror, while the 
 other goes through the prism and emerges at the other 
 end-face. Consequently only one of the two beams 
 gets through the prism, the other being suppressed or 
 reflected out of the way. Prisms made in Nicol's way 
 
 that is also a principal plane, and these wedges are then reunited 
 
 with Canada balsam or linseed oil. In a 
 cheaper modification — a "reversed Nicol" 
 — the natural end-faces are cut off (Fig. 
 82) so as to reverse the shape, and the 
 oblique cut is then made along a re- 
 versed diagonal and is nearly in a 
 "principal plane." In a third modifica- 
 tion the end-faces are first trimmed off 
 obliquely as principal planes of section 
 through one of the natural edges of the 
 end -face; an oblique cut is then given 
 (as in Fig. 83) between two of the 
 
 terminal arretes, from FM to GN, and the two pieces are then 
 
 transposed ; and they are finally reunited by balsam along two of 
 
 their natural faces. 
 
in 
 
 POLARISATION OF LIGHT 
 
 123 
 
 Fig. 84. 
 
 have usually oblique end-faces of diamond shape. The 
 
 vibrations which pass through are those 
 
 executed in the direction parallel to the 
 
 shorter diagonal (Fig. 84) ; while those 
 
 which are suppressed are those parallel to 
 
 the longer diagonal. The large polariser 
 
 used in front of the lantern (Fig. 74, p. 118) is simply 
 
 a large Nicol prism. ^ 
 
 ^ In consequence of the dearth of spar, large Nicol prisms can 
 only be procured at extravagant prices. In 1888 Mr. Ahrens con- 
 structed for me a large reflecting polariser, having a clear aperture 
 of 2| inches. For projection purposes it is quite equal to a Nicol 
 prism of equal aperture, and is much less costly. In this reflecting 
 polariser, which is constructed on a principle suggested by Delezenne, 
 the light is first turned to the proper polarising angle (about 57°) by 
 a large total-reflexion prism of glass cut to a special shape. It is 
 then reflected back parallel to its original path by impinging upon 
 a mirror of black glass covered by a single sheet of the thinnest 
 
 patent plate glass to increase 
 the intensity of the light. 
 Fig. 85 shows the design of 
 this prism. Compared with 
 a large Nicol prism it has 
 one disadvantage : it cannot 
 be conveniently rotated, so 
 that it polarises the light in 
 a fixed plane. To obviate 
 this defect, I devised an 
 "optical rotator" to place 
 on the end of the prism. 
 This consists simply of two 
 plates, </i and ^2) of ' ' quarter- 
 FiG. 85, wave " mica ; the first of 
 
 them bemg mounted with its axis fixed at 45° to the plane of polar- 
 isation ; the second ^o being mounted in a revolving frame which 
 can be turned to any desired position. The eff"ect of rotating this 
 plate is to rotate the plane of polarisation. 
 
124 
 
 LIGHT 
 
 LECT. 
 
 Let us devote a little further attention to this pheno- 
 menon of the double-refraction that thus yields us two 
 beams of light that are polarised in different planes. 
 Here is another wave-motion model (Fig. 86) constructed 
 to show two sets of waves which are polarised in planes 
 mutually at right angles to one another. Here are two 
 
 Fig. 86. 
 
 waves of silvered beads, both of which, when I turn the 
 handle of the model, will march along. Both have the 
 same wave-length, both march at the same pace and 
 toward the same end. But there is this difference 
 between them : in one the displacements are polarised 
 at 45° one way ; in the other the displacements are at 
 45° the other way. Of course there is some mechanism 
 inside the box to make them move thus ; but they illus- 
 
Ill POLARISATION OF LIGHT 125 
 
 trate what is meant by saying that there can be two 
 waves polarised at right angles to one another. 
 
 But the point still remains why should the spar so 
 act on the light as to split it into two oppositely polarised 
 beams ? Let us first prove that the spar really does 
 this. Here is a piece of spar mounted as a double- 
 image prism.^ In front of the lantern is placed first a 
 metal diaphragm having a round hole in it, the image of 
 which is focused on the screen, giving a circular white 
 spot. Now interposing the double-image prism we see 
 that it splits the light into two parts, diverting half the 
 light away from the original spot, 
 and producing a second one which 
 (with this size of aperture in the 
 diaphragm) overlaps the first. On 
 rotating the double -image prism 
 it is seen that the ordinary image 
 remains stationary, whilst the ex- 
 traordinary image revolves around 
 
 -NT 1 1 Fig- 87. 
 
 It. Now to prove that they are 
 
 each polarised, I interpose a Nicol prism. On rotating 
 it, it is observed to cut off first one of the two spots and 
 then the other. If the Nicol prism is set so as to 
 
 ^ That is a prism of spar mounted in such a way as to throw 
 both the images upon the screen. There are several modes in 
 which such may be constructed. The usual mode is to take a 
 wedge of spar cemented to a corresponding wedge of glass. The 
 " extraordinary " beam (that is to say, the beam whose vibrations 
 are executed in planes parallel to the crystallographic axis) goes 
 straight through the " ordinary " beam, and emerges obliquely, 
 giving a displaced image. If the prism is made of two wedges of 
 spar cut at different angles and crossed, the " ordinary " image is 
 the central one while the "extraordinary" image revolves. 
 
 K 
 
126 
 
 LIGHT 
 
 LECT. 
 
 transmit only vertical vibrations, while the double-image 
 prism is rotated it is observed that when, as in Fig. 
 88, the two images are in the position vertically above 
 one another, the ordinary is cut off, while the extra- 
 ordinary is transmitted. On turning round the double- 
 image prism the ordinary image gradually appears, while 
 the extraordinary fades away, until when the prism has 
 been turned one quarter round the ordinary image is 
 transmitted, and the extraordinary cut off, as is evident 
 from Fig. 89. If the prism is turned to 45° both images 
 
 Fig. 90. 
 
 are equally bright, the directions of the resolved vibra- 
 tions being as shown by the fine lines in Fig. 90. 
 
 Now, having proved that the spar does split the 
 light into two beams in which the vibrations are 
 moving in different ways, we have yet to consider how 
 it effects this. The resolution of oblique movements 
 into two components at right-angles to one another is an 
 important principle in mechanics, and one which is best 
 illustrated for our purpose by means of a model. Sup- 
 pose that there is a displacement taking place obliquely, 
 it can always be resolved into two parts — a part which 
 is up-and-down, and a part which is right-and-left in 
 
Ill 
 
 POLARISATION OF LIGHT 
 
 127 
 
 direction. The model (Fig. 91) is fixed upon a board, 
 on the corner of which is drawn a Uttle diagram. Sup- 
 pose the oblique motion to be from A to B, then you can 
 resolve that oblique motion into two parts — a vertical 
 part marked AV, and a horizontal part marked AH. 
 Every schoolboy knows the problem called the " paral- 
 lelogram of forces," according to which when two forces 
 act on one point at the same time, they combine into a 
 
 Fig. 91 
 
 single oblique resultant along the diagonal of the paral- 
 lelogram of which the two component forces are sides. 
 Well our present problem is simply the converse to that. 
 Here in the model are two wooden slides, with cross- 
 heads. One can slide up-and-down only : the other 
 right-and-left only. Running in grooves in the two 
 cross-heads is a roller fixed to the end of a third bar of 
 wood, which can be set in any oblique position, and 
 then slid along obliquely by hand. If I set this third 
 bar horizontal and slide it along, all the movement it 
 
128 LIGHT ' lECT. 
 
 gives is horizontal — there is no vertical component. If 
 I set it vertical and slide it up and down, it simply moves 
 the vertical slide up and down. But if I set it obliquely, 
 and slide it along, then part of its motion produces a 
 movement of the vertical slide, while the horizontal 
 component of its motion produces a movement of the 
 horizontal slide. How much of the motion will be 
 vertical, and how much horizontal, will depend obviously 
 on the angle at which the original motion is imparted. 
 Now there is one particular angle at which the resolved 
 portions would be equal to one another. What is that 
 angle ? If I set the bar which produces the displace- 
 ment exactly midway, at 45°, that displacement will be 
 resolved into two equal components. You will remember 
 that at 45° the double-image prism yielded two images of 
 equal brightness. 
 
 Such a resolution into two parts can be produced on 
 oblique waves of light by any of the crystals here, such 
 as Iceland spar, quartz, mica, or selenite, provided the 
 crystal possesses a particular kind of structure. The 
 condition is that the crystal shall possess a greater optical 
 rigidity — a greater stiffness that is — in one direction 
 than in another. You know what one means when one 
 talks about the grain in a piece of wood — how much 
 easier it is to split in one direction than in another. 
 But this grain, which depends on the fibrous structure, 
 manifests itself in other ways than by ease of cleavage. 
 A piece of wood is harder to bend along the grain than 
 across it. And so with some crystals : they possess an 
 invisible structure, a grain so fine that we cannot see 
 the fibres or lines of structure ; but that grain manifests 
 
HI POLARISATION OF LIGHT 129 
 
 itself in various ways. There are differences in ease of 
 cleavage, and also differences in rigidity ^ in different 
 directions. This particular crystal — Iceland spar — has 
 a greater rigidity in one direction than in another, and, 
 as a result, any wave of light passing obliquely into it 
 is split into two portions, one having vibrations parallel 
 to the axis of greatest rigidity, and another portion 
 having vibrations parallel to the axis of least rigidity ; 
 therefore at right angles to the former. That is why it 
 splits the light into two parts, and why those parts are 
 each polarised. Some crystals, namely, diamonds and 
 garnets, are equally rigid in all directions, and therefore 
 
 ^ The word rigidity is here preferred, though in many treatises 
 it would be spoken of as "elasticity." To say that the crystal 
 possesses different "elasticities" in different directions — which 
 is quite correct — would convey to many people an erroneous idea, 
 because of the incorrect way in which the word " elastic " is often 
 used. Elastic does not mean that a thing can be easily stretched. 
 The hardest hard steel is more perfectly elastic than a bit of india- 
 rubber, but it certainly is not so stretchable. In saying that it has 
 elasticity we mean that however little we may succeed in compress- 
 ing or stretching it, it returns back, when released, to its former 
 shape or size. In scientific treatises that substance is regarded as 
 having the highest co-efhcient of elasticity which requires the 
 greatest stress to produce a given deformation or strain. When we 
 are dealing, as in the case of transverse displacements, with motions 
 tending to shear (see p. 107 above) the medium, the particular 
 elasticity to be considered is the elasticity which resists shearing, 
 and for this the term rigidity is entirely appropriate. In all this 
 optical work we are of course dealing not with the mechanical 
 elasticity, but with the optical elasticity, that is to say, with the 
 elasticity of the ether within the substance. It is this which in the 
 crystals in question is regarded as greater in one direction than 
 in another. The greater the rigidity the higher is the velocity of 
 propagation of the wave whose displacements are in that direction. 
 See Appendix to Lecture III. p. 156. 
 
I30 . LIGHT LECT. 
 
 these do not show any double refraction, nor do they 
 polarise the light. 
 
 Here, again, is a model to illustrate the splitting of 
 the waves. Two thin flexible strips of ebonite, O and E 
 (Fig. 92), are inserted into saw-cuts in the end of a 
 cylinder of wood. In the other end I can fix a third 
 strip, A. Notice, if you please, that O is set with its 
 edge vertical, and is capable of vibrating right and left ; 
 while E is set with its edge horizontal, and can vibrate 
 up and down only. Holding the wooden cylinder in 
 my left hand, I apply my right hand to give a vibratory 
 
 movement to the strip A at the other end. If I waggle 
 A from right and left, then O vibrates from right and left. 
 If I waggle A up and down, then E vibrates up and 
 down, while O is quiet. If now I impart to A an 
 oblique ^ motion, both O and E vibrate, the oblique 
 motion being resolved into a vertical part and a hori- 
 zontal part. 
 
 Returning to the waves of light themselves, let us 
 now see some of the beautiful effects which result from 
 these operations of splitting the vibrations into two 
 parts, of recompounding them after passing through a 
 
 ^ The end of the cylinder into which A is fixed is capable of 
 being turned round on a joint at J, the cylinder being made in two 
 parts fitting on one another. 
 
Ill POLARISATION OF LIGHT 131 
 
 slice of crystal, and then of analysing — that is to say, 
 resolving — the light that falls upon the screen. 
 
 In front of the lantern there has been set the large 
 Nicol prism as polariser, and in front of that a smaller 
 Nicol prism as analyser. When the latter is turned to 
 cross the former we have the dark field (p. 1 1 9), all light 
 being cut off. The only light that passes through the 
 first Nicol consists of vertical vibrations, and as the 
 second Nicol is set to transmit horizontal vibrations only, 
 nothing comes through it. Now I take up a piece of 
 thin mica (that crystalline substance of which lamp- 
 shades are sometimes made, and often miscalled "talc"), 
 and hold it obliquely in the path of the polarised light 
 on its way from the polariser to the analyser — in fact, 
 between the two Nicols. See how it brings light into 
 the dark field. In Tyndall's expressive phrase it seems 
 to "scrape away the darkness." See, too, what beauti- 
 ful tints it shows ! Yet it itself is perfectly colourless. 
 Why, then, this light and these colours? Well, the 
 light is easily explained. The crystal possesses a 
 greater rigidity in one particular direction through 
 its substance — an "axis of maximum elasticity," as 
 it is called. If we set the slice with that axis 
 obliquely (Fig. 93) across the direction of the vertical 
 vibrations, then it will resolve those vertical vibrations 
 into two parts, one part parallel to the axis of maxi- 
 mum elasticity and the other at right angles to that 
 axis. These two sets of waves in the crystal will both 
 be oblique. They travel through the thickness of the 
 slice at unequal speeds, and when they emerge again at 
 the other side of the crystal one set of vibrations will 
 
132 
 
 LIGHT 
 
 LECT. 
 
 have got a little behind the other. Hence the two 
 components as they emerge and recombine will not 
 produce, as their resultant, vibrations that are vertical. 
 The resultant vibrations may be oblique, or even 
 elliptical ; their precise nature and orientation will 
 depend on the nature and thickness of the slice of 
 crystal used, and upon the wave-length of the light. 
 But the immediate point is that the resultant emerging 
 
 ANALYSER 
 
 Fig. 93. 
 
 waves will no longer be polarised vertically ; the crystal 
 slice will have so split up, retarded, and recombined 
 the vibrations that they emerge vibrating in other direc- 
 tions. Hence this emerging light when it falls upon the 
 analyser will possess some horizontal components, and 
 these will, as you see, be transmitted. The colours 
 depend upon the thickness of the slice of crystal ; 
 which in this case is very irregular, seeing that it is 
 merely a rough piece split off with a pocket knife. 
 
iif tOLARiSATlON OF LIGHT 133 
 
 This is mica ; but there is another kind of thin crystal 
 known as selenite (or gypsum), which is also readily 
 split with the knife, and produces similar effects. There 
 are plenty of other crystals that will produce similar 
 effects. 
 
 Here, mounted in a small glass cell, are two rubies. 
 Putting them in front of the polariser, and adjusting a 
 lens to focus them on the screen, you see how much 
 alike they are. One is a genuine ruby, though slightly 
 flawed j the other is a sham ruby of glass, also slightly 
 flawed. Which is which ? You may guess or choose ; 
 but I wish to know. That knowledge we can obtain 
 by putting in front of the lens the second Nicol as 
 analyser. Turning it to position we obtain the dark 
 field ; and in that field one of the two gems shines out 
 while the other is dark. The one which shines out is 
 the genuine ruby, for it alone possesses the axis of 
 maximum elasticity. If we slowly revolve the cell so 
 as to make the images of the two gems move around 
 one another, the one simply remains dark, while the 
 other goes through alternations of light and darkness. 
 It is dark in those positions in which its axis of maxi- 
 mum elasticity stands either vertical or horizontal, and 
 shines brightest in the two positions where that axis 
 stands at 45° of obliquity to right or to left, at which 
 angle the resolution into the two oblique components is 
 most complete. 
 
 Now we place in our apparatus, between polariser 
 and analyser, another cell containing several assorted 
 gems — a ruby, garnet, topaz, emerald, sapphire, chryso- 
 beryl, and a little diamond in the centre. Adjusting 
 
134 LIGHT LECT. 
 
 the analyser to give us the dark field, and then slowly 
 rotating the cell, note how each crystal, when its axis 
 of maximum elasticity comes to the oblique position of 
 45°, shines out. But there are two that show nothing — 
 the diamond and the garnet, crystals belonging to the 
 " cubic " system, whose elasticities in all directions are 
 equal. 
 
 Here are a few more objects for our polariscope, 
 slices of crystals and minerals. First of all a bit of 
 amethyst. Though by ordinary light it is quite clear 
 and of a pale purple tint, yet when examined by polar- 
 ised light it is at once evident that the gem consists of 
 a number of superposed separate layers, which show 
 alternately dark purple and white ; while some regions 
 of the crystal show strange masses of unexpected colour, 
 and these, if one turns the analyser round, change tint. 
 A second piece of amethyst, from Brazil, shows a more 
 perfect structure. 
 
 Here is a thin slice of gray Scotch granite. Its 
 natural mottlings are merely black and white, being 
 composed, as mineralogists will tell you, of small crystals 
 of transparent quartz and transparent felspar, mingled 
 with specks of black mica. But when viewed by 
 polarised light the whole slice shows wonderful gleams 
 of colour, and reveals new details of structure. 
 
 This next beautiful object is a thin transverse slice 
 of a stalactite, one of those natural deposits of calcare- 
 ous spar which hang like icicles from the roofs of caves. 
 Its deposit, layer by layer, almost concentrically, is 
 evident ; but in the polariscope it shows a mysterious 
 black cross, which, when the analyser is rotated, changes 
 
Ill 
 
 POLARISATION OF LIGHT 
 
 135 
 
 to a white cross. This black cross is visible in the dark 
 field. Such black crosses one obtains whenever the 
 object is one having different rigidities in the radial 
 and tangential directions. The next slide shows it even 
 better. This is an artificial crystal of a stuff called 
 salicine, which can be dissolved in alcohol. When the 
 solution is poured upon a warm piece of glass the 
 alcohol evaporates, and the salicine crystallises. The 
 
 Fig. 94. 
 
 operation of crystallising starts at a number of inde- 
 pendent centres, around which little groups of crystals 
 grow with a radial structure and a circular outline. As 
 their axes of maximum elasticity all point radially to 
 the centre around which they grew, the maximum 
 resolution of the wave light will occur in each little 
 circle at 45° to right and to left, while in the two direc- 
 tions, vertical and horizontal, there will be no resolu- 
 tion of the polarised light, and these (in the dark field) 
 
136 LIGHT LECT. 
 
 will therefore remain dark, giving the black crosses. 
 On revolving the analyser quickly, so that the black 
 crosses change to white crosses and then turn black 
 again in rapid succession, all the little crosses in the 
 separate groups of crystals appear to revolve like so 
 many little windmills. 
 
 Here is a thin slice of selenite, split off quite irregu- 
 larly, and of different thicknesses in different parts. 
 Without the analyser it shows on the screen nothing 
 worthy of attention, being nearly clear as glass and 
 quite colourless. But replacing the analyser to produce 
 the dark field, at once a gorgeous set of tints is pro- 
 duced. At one part the thickness is such that the tint 
 is a fine orange ; just above it where the crystal is a 
 shade thicker comes a patch of brilliant crimson. These 
 being the tints in the dark field, see how, when the 
 analyser is rotated a quarter so as to give the bright 
 field, each tint changes to its complementary, the 
 orange turning to azure blue, and the crimson turning 
 to vivid green. 
 
 Now I have yet to explain how these colours come 
 about. All I have said is that they depend upon the 
 thickness of the crystal film, and upon the wave-lengths 
 of the different kinds of light. If on its passage through 
 the crystal the vibrations are split up into two parts 
 that travel with unequal speeds, one set of vibrations 
 will gain on the other ; the one that is more retarded 
 will, when it emerges, have lost step with the other set. 
 This " difference of phase " due to the different speed of 
 travelling may in a thin slice of crystal be as little as 
 one quarter or one half only of a wave-length ; or it 
 
Ill POLARISATION OF LIGHT 137 
 
 may, if the crystal is thicker, be more than a whole 
 wave-length, or it may be several wave-lengths. The 
 question as to how the two sets of waves will recombine 
 when they emerge at the other face of the crystal slice, 
 will depend upon the question how much one wave has 
 got out of step with the other wave. Clearly that will 
 depend on the kind of light and on the thickness of the 
 slice. If, for example, a slice of mica -g-u-g- inch thick 
 caused the waves of yellow light to get exactly one 
 quarter of a wave-length out of step with one another, 
 then it is clear that a slice twice as thick would produce 
 twice as much retardation of phase, and make the two 
 sets of yellow waves get exactly half a wave-length out 
 of step. Further, if the thickness were such as to make 
 yellow waves get exactly half a wave out of step, it 
 would not produce an exact half-wave retardation upon 
 the larger red waves, or upon the shorter violet waves. 
 The consequence of all this is that in the recombina- 
 tions of the emerging waves after passing through any 
 given thickness of material, the angle at which the 
 vibrations recombine is different for different wave- 
 lengths. If, for example, the crystal thickness is such 
 that green waves in recombining come out vibrating 
 nearly vertically, then for that thickness of crystal green 
 light will be almost entirely cut off in the dark field, but 
 almost entirely transmitted in the bright field. So we 
 have this complementary relation between the tints in 
 the two positions of the analyser. 
 
 The succession of tints for a regularly increasing 
 thickness of crystal follows the order of the tints known 
 as Newton's "Colours of Thin Plates." Sir Isaac 
 
 <'--, 
 
138 
 
 LIGHT 
 
 LECT. 
 
 Newton examined the colours of soap-bubbles and 
 other thin films, and ascertained their relation to the 
 
 Fig. 95. 
 
 thickness of the film by ingeniously producing a film of 
 air between two pieces of glass, one flat, the other very 
 slightly curved. There is now projected upon the 
 screen the system of coloured rings ("Newton's rings") 
 
Ill POLARISATION OF LIGHT 139 
 
 so produced. Where the two pieces of glass touch 
 there is a central dark spot ; around this centre there 
 are coloured rings of light of the several "orders." 
 The tints are set forth in the accompanying Table. 
 Those of the first' order, beginning with black, are very 
 dull, and end with a dark purple or "transition tint," 
 after which the colours of the second order follow 
 almost those of the spectrum, except that there is no 
 good green. At the end of the second order comes 
 again a purple "transition tint," after which the third 
 order gives a sort of spectrum series with a good green 
 but with a poor yellow. To the third order succeeds a 
 fourth with paler tints, mostly green and red, then a 
 still paler fifth, followed by higher orders that are still 
 less distinct. The corresponding thicknesses of the film 
 are given in millionth parts of an inch. For producing 
 the kindred set of Newton's tints by means of thin films 
 of selenite in the dark field of the polariscope much 
 greater thicknesses must be used, because the pheno- 
 menon is due to the difference between the two velocities 
 of the two sets of vibrations. Thus to produce a re- 
 tardation of J wave, and therefore give the same whitish 
 tint as an air-film 5*5 millionths of an inch thick, a piece 
 of selenite must be used the thickness of which is about 
 T"oVo inch. The tints so produced — see p. 145 — are 
 not precisely identical with the air film tints because of 
 the modifying effect of dispersion in the selenite. 
 
 [Table 
 

 Table IV 
 
 Tints of 
 
 Newton's Colours of Thin Films. 
 
 Order. 
 
 Film Thickness. 
 
 Tint in Reflected Light. 
 
 I. 
 
 O 
 
 8 
 
 Black. 
 
 Gray. 
 
 Whitish. 
 
 Straw. 
 
 
 10 
 
 10-5 
 
 II 
 
 Orange. 
 Brick Red. 
 Dark Purple. 
 
 II. 
 
 II-5 
 13 
 15 
 18 
 
 Violet. 
 Blue. 
 Peacock. 
 Yellow. 
 
 
 19-5 
 
 21 
 
 22 
 
 Orange. 
 Red. 
 
 Violet. 
 
 III. 
 
 24 
 
 25-5 
 27 
 
 29-5 
 31 
 
 Blue. 
 Peacock. 
 Green. 
 
 Yellowish Green. 
 Rose. 
 
 
 32-5 
 33 
 
 Crimson. 
 Purple. 
 
 IV. 
 
 f 34-5 
 36 
 38 
 
 40 
 
 Violet. 
 Peacock. 
 Green. 
 Yellowish Green. 
 
 
 44 
 
 Rose. 
 
 V. 
 
 i 48 
 
 52 
 I 55 
 
 Pale Green. 
 Pale Rose. 
 Rose. 
 
 VI. 
 
 r 60 
 
 64 
 
 [ 66 
 
 Pale Peacock. 
 Pale Rose. 
 Rose. 
 
 VII. 
 
 r 71 
 
 1 74 
 
 Pale Green. 
 I'ale Rose. 
 
LECT. Ill POLARISATION OF LIGHT 141 
 
 Now the reason for this pecuUar succession of tints 
 arises from the overlapping of the successive orders for 
 the waves of different colours. When produced as 
 Newton made them, by the interference of light by re- 
 flexion from the upper and lower surfaces of a film of air, 
 there would be found, at a particular distance from the 
 centre a particular thickness of film such that the light 
 reflected at the second surface is exactly half a wave 
 out of step with the light reflected at the first surface. 
 At this place — the air film being here of a thickness 
 equal to a quarter of the wave-length of that kind of 
 light — that particular kind of light would be cut off by 
 self- interference. For example, yellow light having 
 waves 22 millionths of an inch long, would be cut off 
 by interference when the film is 5^^ millionths of an inch 
 thick. But as all the waves have, to begin with, lost 
 half a wave-length (as evidenced by the central spot 
 being black), by reason of the second reflexion being an 
 external one, the result is that all round the centre, at 
 such a distance that the film is 5I millionths of an inch 
 thick, yellow light is reinforced, and there is seen a bright 
 ring — the first order for yellow light. As red waves are 
 27 millionths of an inch long the ring for the first order 
 for red light will be at a place where the film is about 
 6| millionths of an inch thick. Newton's rings then 
 will seem of different sizes in different kinds of light ; 
 and since white light consists of all different colours 
 mixed up, the Newton tints will be produced by the 
 overlappings of all the different tints. This may be made 
 plainer by considering a diagram (Fig. 96) in which the 
 various sizes of waves are represented to scale. Let the 
 
 L 
 
142 
 
 LIGHT 
 
 LECT. 
 
 distances measured horizontally from the left side repre- 
 sent the distances from the centre of the system of 
 Newton's rings, the air-gap being supposed to widen in 
 
 RED 
 
 ORANGE 
 
 YELLOW 
 
 GREEN 
 
 PEACOCK 
 
 BLUE 
 
 VIOLET 
 
 Resultant Tint 
 
 Relative 
 
 Thickness of 
 film 
 
 Millionths of 
 Inch 
 
 AAIIIAAJA 
 
 44 
 
 5D 
 
 proportion to the radius. Then if red light was the 
 only kind falling on the apparatus it would show a 
 system of red rings with a black centre, the distance of 
 each successive red ring from the centre corresponding 
 to the places where the crests of the red waves occur 
 
Ill POLARISATION OF LIGHT 143 
 
 in the highest row. Similarly, if yellow light alone were 
 present, there would be a rather smaller system of 
 yellow rings seen, spaced out as are the crests of the 
 yellow waves in the third line. And so forth for other 
 colours. Now since the rings in the self-produced light 
 of any one colour are of a different size from the rings 
 in the set produced by light of another colour, it follows 
 that when white light is used, the sets of rings of the 
 different colours of which white light is compounded 
 will overlap one another. At any given distance from 
 the centre the resultant light will be the sum of all the 
 various amounts of coloured lights at that distance. 
 Take the rows of waves in Fig. 96 and treat them as if 
 Fig. 96 were an addition sum of which we had to write 
 down the total from left to right at the bottom. At the 
 very beginning, on the left, there is nothing to add up, 
 because the waves have not yet more than begun to 
 rise. A little farther along all the waves are rising. 
 Consider a distance such that the yellow wave is at its 
 highest point. Imagine a vertical line drawn through 
 the top of the first yellow wave. How much of the 
 other kinds of light are present ? There is a great deal 
 of orange and some red, a great deal of green and pea- 
 cock, some blue and some violet. Now all these added 
 together will make a nearly white light, but rather 
 yellowish owing to the preponderance of yellow. The 
 result is that at the corresponding distance from the 
 centre there will be a yellowish white ring : and the air 
 film at this place is about 5 J millionths of an inch thick. 
 Now consider a distance twice as great from the centre 
 of the rings, or at the end of the first yellow wave, where 
 
144 LIGJIT LECT. 
 
 the film is about 1 1 millionths of an inch thick. There 
 is no yellow, but there will be a very little orange and 
 some red ; there will be next to no green or peacock, 
 but there will be a little blue and much violet. These 
 colours add up to a dark purple tint, which in the 
 coloured diagram on the screen is set down as the total 
 in the bottom line. It may be new to you to think of 
 adding up colours and putting down the totals, but that 
 is the way to reckon out the resultant tint. Referring 
 back to the table of Newton's tints we now see that they 
 range themselves in regular orders with the purple 
 transition tint at the end of the first order where the 
 air film is 1 1 millionths of an inch thick, another purple 
 tint at the end of the second order where the film is 22 
 millionths thick, and a third at the end of the third 
 order, where the film is 33 millionths thick. In fact 
 these darkest tints in the series correspond to the 
 thickness at which interference occurs for yellow light. 
 
 Now these Newton's tints — produced by interference 
 and overlapping — are in general the same as the tints 
 which result from the introduction of our thin slices of 
 crystals into the polariscope. And the reason why the 
 thin slices of crystal when examined by polarised light 
 give the same general series of tints is as follows. 
 
 Suppose the polarised light to fall on a thin slice of 
 crystal that has its axis set obliquely across the beam. 
 Then the vibrations in going through the crystal are 
 split, as I have explained, into two parts, one vibrating 
 parallel to the axis, the other part at right angles : and 
 they do not take the same time to traverse the crystal 
 film because of the difference between the rigidity in the 
 
Ill POLARISATION OF LIGHT 145 
 
 two directions. And as the wave-lengths of different 
 colours are different, the waves of various colours, though 
 they traverse the same actual thickness, emerge in 
 different states. When the two components of a wave 
 of any given colour recombine on emergence, they will 
 recombine to form a vibration in some new direction, 
 and that resultant direction — whether oblique or ellip- 
 tical — will be different for different colours. Hence it 
 follows that the analyser will cut off more of one colour 
 than of another; and the light which comes through 
 the analyser will be the total of all the resolved parts 
 of each kind of light. If, for instance, the thickness of 
 the crystal is such that the yellow light on emerging 
 recombines to form a nearly vertical vibration, then the 
 analyser when horizontal will cut off the yellow, and the 
 resultant light that comes through — the total of all the 
 other parts — will be of a dark violet hue. Just as the 
 colours in the Newton's rings depend on the thickness 
 of air film between the glasses, so do these colours of 
 the film of crystal in the polariscope depend on the 
 thickness of the film (see p. 139). 
 
 The next object to be shown you is a slice of selenite 
 that is exceedingly thin at one end, and thick at the 
 other, being tapered as a very thin wedge. It exhibits 
 most magnificently the Newton's tints up to the end of 
 the third order. It will serve as a standard of com- 
 parison for other slices. For instance, there is now 
 placed in the apparatus a uniform piece of crystal. It 
 shows in the dark field the red of the second order. I 
 therefore know that it is precisely of the same thickness 
 as the wedge is at the place where the wedge shows 
 
146 LIGHT LECT. 
 
 the same red tint This red turns to a vivid green 
 when the analyser is rotated so as to give the bright 
 field. So again a slice which in the dark field gives a 
 violet of the second order changes in the bright field to 
 the complementary primrose tint, 
 
 I now take two prepared pieces of mica, which will 
 be exhibited to you first separately and then together. 
 One of them shows the blue of the second order, a tint 
 which by reference to the table is the same as that pro- 
 duced by an air film 13 millionths of an inch thick. 
 The other shows a yellow of the second order, corre- 
 sponding to an air film 18 millionths of an inch thick. 
 Now guess what will happen if they are both put in to- 
 gether. Will blue and yellow make green ? Not by 
 any means. If superposed (with their axes both at 45° 
 to the right) they will have the same effect as a piece of 
 mica would have if its thickness was equal to that of 
 the two added together : or it will act as a film of air in 
 the Newton's rings 31 millionths of an inch thick, 
 giving a tint which, by the table, you see to be a rose 
 
 red. My assistant slides one 
 crystal over the other (Fig. 
 97) and you observe in this 
 case the unexpected, though 
 predicted, result that blue and 
 yellow added in this way make 
 pink. Let one of the crystals 
 ^^' ^^* now be turned about so as to 
 
 put its axis 45° to the left, so that it will act negatively, 
 giving-the same result as if we had subtracted one thick- 
 ness from the other. What tint ought it to give ? Sub- 
 
Ill POLARISATION OF LIGHT 147 
 
 tracting 13 millionths (blue) from 18 millionths (yellow), 
 we obtain the answer that it ought to give the same tint 
 as an air film 5 millionths of an inch thick, which tint is 
 a grayish white. Look for yourselves and see how on 
 the screen where the blue (reversed) crystal overlaps the 
 yellow crystal, the resultant tint is a grayish white. 
 
 The next object is a wedge combination made of 
 twenty-four very thin pieces of mica set to overlap one 
 another, so as to form a wedge in steps. It, like the 
 smooth wedge of selenite, gives the Newton tints of the 
 first three orders ; in this case, however, not gradating 
 finely into one another, but presenting sudden changes 
 from the tint of one thickness to the tint of the next. 
 Where the crystal shows the nearest approach to white, 
 namely, at a point half way along the first order, it cor- 
 responds to an air film having a thickness of one-half of 
 the length of the wave of yellow light. Hence such a 
 crystal is called a half-wave plate. If it is placed (with 
 axis also at 45°) upon one of the other slices of crystal 
 in the polariscope, it is observed to raise all the tints by 
 an amount corresponding to an addition of 1 1 millionths 
 of an inch to the corresponding thickness of air film, 
 and changes each to almost exactly ^ its complementary 
 tint. Whitish in the dark field, it is nearly black — a 
 very dark purple — in the light field. The next slide to 
 be put in the polariscope is an illustration of this 
 principle. In the dark field you see a white swan which, 
 
 ^ Not precisely exactly, as it cannot be an exact half- wave plate 
 for all different colours. It is selected so as to be an exact half- wave 
 plate for that tint that is brightest to the eye, namely, yellow, or 
 yellow-^reen. 
 
148 LIGHT LECT. 
 
 when I turn the analyser to give us the bright field, changes 
 to a black swan. The space, in the slide, within the 
 outline of the swan, is covered with a piece of half-wave 
 selenite. I possess another slide in which a baker's boy 
 attired in white clothes, with a sack of flour on his back, 
 changes to a chimney-sweep bearing a bag of soot. 
 
 A slice of crystal half as thick as the half-wave plate 
 is called a quarter-wave plate. It produces a retardation 
 of one quarter of a wave ^ (for yellow light) between the 
 two components of vibration that traverse the slice. 
 
 Here is a beautiful object. A thin slice of selenite 
 has been ground so as to be hollow on one face like a 
 concave lens, thin in the middle, thicker at the edges. 
 As a result it shows Newton's rings in a far more splendid 
 manner than Newton's delicate air film ever showed 
 them. Along with this concave selenite T now introduce 
 a slide made up of twelve sectors of quarter-wave crystal, 
 set with their axes alternately at 45° to the right and 
 45° to the left. They seem to dislocate the Newton's 
 rings, pushing the alternate segments in or out by one 
 quarter of a whole "order" of tints. Beyond these 
 objects, and next the analyser, I now introduce upright ^ 
 
 ^ The quarter- wave, ifsetwithaxisat 45°, to produce as mentioned 
 a difference of phase of a quarter between- the two components, pro- 
 duces circularly polarised light. In all positions of the analyser 
 light still comes through, nearly equally ; there is no dark field. 
 Also, if a quarter-wave is placed, along with other polarising objects 
 (such for example as the concave selenite next described), but is set 
 with its axis vertical, instead of at 45°, and is inserted between the 
 polarising object and the analyser, it produces great varieties of tint, 
 each tint in changing to its complementary going, while the analyser 
 is turned, through an intermediate series of tints. 
 
 ^ .See preceding note. 
 
Ill POLARISATION OF LIGHT 149 
 
 a quarter-wave plate. And now, on rotating the analyser 
 we have the strange appearance of all these dislocated 
 rings of colour marching inwards to disappear at the 
 centre, though succeeded in turn by other rings. Re- 
 volving the analyser in the opposite sense causes the 
 rings to seem to grow at the centre and march outwards. 
 
 Here is another object of great beauty, a butterfly in 
 form, cut out of selenite ; and here also is a heart's-ease ; 
 and here some daisies which, though pale yellow in the 
 dark field, turn to purple Michaelmas daisies in the 
 bright field. To pretty devices like these there is no 
 end. 
 
 We may now apply our knowledge to a further study 
 of complementary and supplementary tints. A few 
 minutes ago I showed you (Fig. 87, p. 125) how the 
 double-image prism as analyser gives us two polarised 
 images, which, when the polarised light passes through 
 a circular aperture of suitable size, overlap one another. 
 If with the same arrangement I cover the aperture with 
 a thin slice of mica or selenite, and superpose a vertical 
 quarter-wave plate, our two overlapping disks on the screen 
 are seen to give us two complementary colours, as though 
 we had two analysers, one of which had been turned 
 through 90°. As we turn the double-image analyser the 
 two images revolve around one another exactly as they 
 did previously. But as they revolve they change their 
 colour in regular successions. And in every position, 
 whatever tint one image shows, the other shows the com- 
 plementary tint ; while in every position the patch of 
 light where they overlap is white. But that is because 
 we take the white light of the electric lamp and are 
 
I50 LIGHT LECT. 
 
 resolving it into two complementary constituents. Now 
 if I interpose a piece of coloured glass to colour the 
 beam, we shall resolve that coloured light into two con- 
 stituents which we may describe as " supplementary " to 
 one another. Blue glass, as we found last lecture, 
 lets some green and violet pass as well as blue ; and 
 here again you see the fact revealed. Red glass on the 
 contrary is fairly monochromatic ; for though we split 
 its light into two supplementary beams, both are red, 
 scarcely differing in hue from one another. 
 
 Natural crystal patterns, produced on glass by pour- 
 ing upon it some crystallisable solution which is then 
 allowed to dry, form objects of great beauty. Here, for 
 example, is a plate prepared from a solution of anti- 
 pyrin. It produces an effect like frost on the window- 
 pane. But the delicate traceries and plumes, when 
 placed in the polariscope, show the most gorgeous play 
 of colours, as you see. And here are some crystals of 
 sulphate of copper, and some of pyrogallic acid, which 
 are equally curious. 
 
 Now there are in nature sundry substances besides 
 crystals which possess different rigidities in different 
 directions. Thin slices of wood, for example, and bone, 
 and horn, and many other animal and vegetable structures. 
 Here is a thin slice of horn. It is nearly transparent 
 and colourless. But if put into the polariscope, with its 
 grain inclined at 45° to the vertical, you see at once the 
 remarkable streak of colour which it produces. Here 
 again is a quill-pen flattened out and mounted as a 
 polariscope object. It is really quite gorgeous in its 
 hues. 
 
Ill POLARISATION OF LIGHT 151 
 
 Here is a very interesting object, the natural lens 
 from the eye of a codfish. Having a fibrous and radial 
 structure it shows a black cross in the dark field. 
 
 Glass, under ordinary circumstances, is devoid of any 
 difference in rigidity between one direction and another. 
 Nevertheless, if it is suddenly heated or suddenly cooled, 
 the unequal expansion of its parts produces differences in 
 rigidity which make themselves visible in the polariscope. 
 Here, for example, is a small square piece of glass, which 
 at present shows no colour or any other effect when 
 placed in the polariscope. But if it is dropped into a 
 heated brass frame which will quickly warm its edges 
 before the central part of it has time to expand, its 
 structure will be put under unequal stresses, and the 
 resulting strains will show themselves in the strange 
 patterns of colours which you now see growing into sight 
 upon the screen. 
 
 If a hot piece of glass is suddenly chilled, so that the 
 
 outer part cools and contracts before the inner part has 
 
 time to cool, the piece may acquire and remain in a 
 
 state of permanent strain. Such glass, usually described 
 
 as "unannealed," is very liable to break ^ with almost 
 
 explosive violence. Here is a square piece of glass, of 
 
 no colour in itself, but which has been suddenly chilled. 
 
 Its state of permanent strain is at once revealed by the 
 
 peculiar pattern and the dull tints that seem to form 
 
 around a distorted cross radiating from its centre. A 
 
 ^ The extreme case is presented by "Rupert's drops," which 
 are drops of melted glass suddenly cooled by dropping them into 
 water. When examined in polarised light (best when immersed in 
 a small glass tank filled with oil to obviate surface reflexions) they 
 show fine colours. 
 
152 LIGHT LEcT. 
 
 second square of glass which has been still more suddenly 
 cooled shows the same black cross (Fig. 98), but the tints 
 
 in the corners of the square run 
 up into the second order. Here 
 again is a short cylinder of glass 
 which was suddenly chilled all 
 round its outside. The peripheral 
 surface has contracted upon the 
 inner part and compressed it with 
 an enormous force. As a result 
 ' ^ ' you see not only the black cross 
 
 indicative of a radial disposition of the axes of elasticity, 
 but a number of concentric rings coloured with the now 
 familiar succession of Newton's tints right up to the 
 fourth order. 
 
 And now I am going to squeeze a piece of glass 
 mechanically, by gripping it in a strong brass frame and 
 then forcirig a point against its side by turning a strong 
 screw. In the dark field the glass itself shows neither 
 light nor colour, until I put on the screw. But so soon 
 as compression is applied a luminous pattern at once 
 seems to grow, stretching off in two patches at about 45° 
 on each side of the point where the screw point has been 
 forced against the glass. Tightening the screw makes 
 the internal strain greater, and the pattern more brilliant. 
 Loosening the screw releases the strain, and the glass 
 resumes its ordinary colourless state. So, you see, you 
 can use polarised light not only to detect false gems from 
 real, not only to tell glass from crystal, but also to 
 ascertain whether any piece of glass is likely to break ot 
 not. Any piece of glass that has been too suddenly 
 
Ill 
 
 POLARISATION OF LIGHT 
 
 153 
 
 cooled, that is, has not been properly annealed by slow 
 cooling down- from the furnace heat, can always be 
 detected by the colours it shows when placed in the 
 polariscope between polariser and analyser. 
 
 For such purposes a very simple polariscope, such as 
 any ingenious boy might construct for himself, is quite 
 sufficient. Here (Fig. 99) is such a polariscope, made 
 entirely of glass. The polariser is simply a flat piece of 
 window-glass, 9 inches long by 5 inches wide, blackened 
 
 
 H 
 
 with black varnish on its under side, and laid down on a 
 simple frame of wood. Two other pieces of glass are 
 cut of the size 8| by 5 inches. One of these is of clear 
 window-glass, the other of ground glass. Across the 
 lower part of the clear piece, and at if inch from its 
 edge, is cemented a strip of glass 5 inches long by 
 I inch broad, to serve as a ledge on which to support 
 the objects when being looked at. These two pieces of 
 glass are joined at the top by a hinge of paper or cloth 
 cemented to them ; and they stand up, like a roof, over 
 
154 LIGHT lECt. 
 
 the piece of blackened glass, being kept in their places 
 on the baseboard by two strips of wood which are 
 fastened to the board 9J inches from one another. 
 The baseboard should be 17 inches long by 5 inches 
 wide. Daylight or lamp-light is allowed to strike upon 
 the ground glass, and thence passes down to the 
 blackened glass, is reflected at an incidence of about 
 57° to its surface, and so passes as a polarised beam 
 through the piece of clear glass on its way to the eye. 
 As analyser, seeing that Nicol prisms are expensive, a 
 cheap substitute must be found. One that is quite good 
 enough for many purposes, may be made by taking a 
 bundle made of eight or ten very thin slips of glass, ^ 
 each about ij inch long and | inch wide, and fixing 
 them with sealing wax obliquely across a small wooden„ 
 tube or box with open ends. They should be fixed in 
 the wooden tube so that the glass slips are inclined at 
 about 33° to the direction in which the light is to pass 
 through the tube. 
 
 With quite simple apparatus you can verify and repeat 
 many of the experiments that have now been shown 
 before you. There are many others, equally beautiful, 
 that I have not shown ; for in a single lecture one can 
 only deal very incompletely with this fascinating and com- 
 plicated subject of polarisation. I have not shown you 
 how quartz crystals possess a special property of rotating 
 the polarised light, nor have I told you how solutions of 
 sugar and sundry other liquids are found also to produce 
 
 ^ The very thin glass used for "covers" for microscopic objects 
 is suitable. It is usually supplied only in round cover-disks. But 
 any good optician could procure rectangular slips of the size named. 
 
Ill POLARISATION OF LIGHT 155 
 
 an optical rotation. Indeed, the regular way adopted 
 in sugar factories to measure the amount of sugar in a 
 watery syrup is to put some of it into a polariscope and 
 measure how much it turns the direction of the vibra- 
 tions. Lastly, I have said nothing about the remarkable 
 discovery with which Faraday crowned his researches in 
 this place, namely, that the polarised waves of light can be 
 rotated by a magnet. Let me hope that some day you 
 may learn of these marvellous discoveries, to which the 
 things you have seen to-day constitute a first step. 
 
APPENDIX TO LECTURE III 
 
 THE ELASTIC-SOLID THEORY OF LIGHT 
 
 On p. 34 it is remarked that light-waves travel slower in 
 denser media; and on p. 129 it is explained how in a 
 double-refracting crystal the waves are split into two sets 
 which travel with different velocities. It is expedient to 
 enter further into the question of the velocity of propagation 
 of light-waves. If it is assumed as a fundamental point 
 that the velocity of propagation of a wave is equal to the 
 square-root of the elasticity of the medium divided by its 
 density (or, as expressed in symbols, v= ^/ E-^D, which 
 is Newton's law), then it is only possible to account for the 
 co-existence of two different velocities by supposing that 
 displacements in different directions either evoke a different 
 elasticity or call into operation a different density. But, 
 since the medium of which the waves constitute light is the 
 ether, one has to deal, in the case of the transmission of 
 light through crystals, with the ether as it exists in the 
 crystal. If we assume that the ether acts as an in- 
 compressible homogeneous elastic solid, then the ordinary 
 theory of elasticity suffices as a theory of the ether. For 
 long this " elastic-solid theory " of the ether has held sway, 
 and has received elaborate mathematical treatment at the 
 hands of Green, Fresnel, MacCullagh, Neumann, Cauchy, 
 and others. On this view the ether particles within crystals 
 are arranged differently in different directions, symmetrically 
 with respect to three rectangular axes, and therefore the 
 properties of the ether as a medium for transmitting waves 
 will be modified by the presence of the crystalline matter. 
 
APP. ELASTIC-SOLID THEORY OF LIGHT 157 
 
 But here a difference of view may arise ; for it may be held 
 (with Fresnel) that the presence of the crystalhne matter 
 modifies the density of the ether without altering its 
 elasticity ; or it may be supposed (with MacCullagh and 
 Neumann) that the presence of the crystalline matter 
 modifies the elasticity in different directions without 
 affecting its density. In either case the assumptions lead 
 to equations that fit the fundamental facts of double-refrac- 
 tion and polarisation. But there arises this difference that 
 whereas the theory of Fresnel supposes the displacements 
 to occur at right angles to the so-called "plane of polarisa- 
 tion," that of MacCullagh treats them as executed in that 
 plane. As to the actual direction in which the displace- 
 ments are executed, the properties of tourmaline suffice 
 (apart from other proofs) to determine the fact that in the 
 extraordinary wave, which is transmitted, the displacements 
 are executed parallel to the principal axis of the crystal, 
 while in the ordinary wave, which is absorbed, the displace- 
 ments are at right angles to that axis. The simple proof 
 being (see Philosophical Magazine^ August 1881) that 
 tourmaline is opaque (at least in thick sHces) to all light 
 travelling along the principal axis of crystallisation ; hence 
 it absorbs those vibrations which are transverse to that 
 axis. (Compare p. 119 above.) 
 
 But the elastic-solid theory is not the only possible 
 theory of light. Instead of supposing the ether to be itself 
 modified in arrangement or properties by the presence of 
 crystalline matter we might suppose it to be itself isotropic, 
 having equal elasticity and density in every direction, but 
 that in its motions it communicates some of its energy to 
 the particles of matter through which the wave is travelling. 
 If the particles of gross matter thus load the ether their 
 vibration will during the passage of the wave take up some 
 of the energy and retard the rate at which the group of 
 waves can travel. We should then have a difference 
 between the velocity of propagation of the individual waves 
 themselves and the velocity of propagation of the group of 
 waves ; and in that case the velocity of propagation of the 
 group — the apparent velocity of light — would be slower 
 
 M 
 
158 LIGHT LECT. Ill 
 
 than the velocity of the waves themselves, and would be 
 different for waves of different frequency. This is in fact 
 the phenomenon of dispersion. In the case of crystalline 
 media the retardation and the dispersion would be different 
 in different directions, and would depend upon the direction 
 of the displacements with respect to the axes of the crystal. 
 But as to the connection between the molecules of matter 
 and the ethereal medium involved in such theories, very 
 little is known, and there is room for many different 
 hypotheses as to the nature of such connection. Helmholtz, 
 Kelvin, Lommel, Sellmeier and others have made various 
 suggestions of which an admirable account is to be found 
 in Glazebrook's " Report on Optical Theories," British 
 Association Report^ 1885. 
 
 The electromagnetic theory of light which Maxwell 
 founded upon the basis of the experimental work of Faraday 
 has now definitely superseded all the purely mechanical 
 theories. Some account of this theory is given in the 
 Appendix to Lecture V. (p. 230). 
 
 The only other point that need claim attention here is the 
 use of the term "plane of polarisation." This term, which 
 is variously defined by different writers, is used to denote a 
 plane with respect to which the polar properties of the 
 wave can be described. It must necessarily contain the 
 line along which the wave is being propagated {i.e. the 
 "ray" lies in this plane); but, so far as the orientation of 
 this plane around the ray is concerned, its definition with 
 respect to the polar properties is purely a matter of conven- 
 tion. The following is Herschel's definition (^Encyclopedia 
 Metropolitana^ article "Light," p. 506) — "The plane of 
 polarisation of a polarised ray is the plane in which it must 
 have undergone reflexion, to have acquired its character of 
 polarisation ; or that plane passing through the course of 
 the ray perpendicular to which it cannot be reflected at the 
 polarising angle from a transparent medium ; or, again, 
 that plane in which, if the axis of a tourmaline plate exposed 
 perpendicularly to the ray be situated, no portion of the ray 
 will be transmitted." If we refer to the Nicol prism (Fig. 
 84, p. 123) we shall see that, according to the convention 
 
APP. ELASTIC-SOLID THEORY OF LIGHT 159 
 
 thus laid down by definition, the plane of polarisation of the 
 light that emerges is parallel to the longer diagonal of the 
 end-face ; and the vibrations are executed at right angles to 
 this. To avoid periphrasis in these Lectures the author 
 speaks of the plane in which the vibrations are executed as 
 the plane in which the wave is polarised (see descriptions 
 of Figs. 69-73, pp. 1 1 3-1 17). 
 
LECTURE IV 
 
 THE INVISIBLE SPECTRUM (ULTRA-VIOLET PARt) 
 
 The spectrum stretches invisibly in both directions beyond the visible 
 part — Below the red end are the invisible longer waves that 
 will warm bodies instead of illuminating them — These are 
 called the calorific or infra-red waves. Beyond the violet end 
 of the visible spectrum are the invisible shorter waves that 
 produce chemical effects — These are called actinic or ultra-violet 
 waves — How to sift out the invisible ultra-violet light from the 
 visible light — How to make the invisible ultra-violet light 
 visible — Use of fluorescent screens — Reflexion, refraction, and 
 polarisation of the invisible ultra-violet light — Luminescence : 
 the temporary kind called Fluorescence, and the persistent 
 kind called Phosphorescence — How to make " luminous paint" 
 — Experiments with phosphorescent bodies — Other properties 
 of invisible ultra-violet light — Its power to diselectrify electri- 
 fied bodies — Photographic action of visible and of invisible light 
 — The photography of colours — Lippmann's discovery of true 
 colour-photography — The reproduction of the colours of natural 
 objects by trichroic photography — Ives's photochromoscope. 
 
 All kinds of light in the visible spectrum are comprised 
 between the extreme red at one end and the extreme 
 violet at the other. Their wave-lengths vary between 
 about 32 millionths of an inch (extreme red) and 15 
 millionths of an inch (extreme violet). But besides the 
 waves of various colours, between those limits, which 
 
 1 
 
LECT. IV THE INVISIBLE SPECTRUM i6i 
 
 are visible, there are other waves that bring no sensa- 
 tion to our eyes, which are invisible, and yet are light- 
 waves. In brief, the spectrum extends in both directions 
 invisibly, both below the extreme red and beyond the 
 extreme violet. 
 
 Perhaps you raise the objection that if such waves 
 are invisible they cannot be waves of light. Well, if 
 you were to lay down as a definition beforehand that 
 the term " light " must be applied only to the waves 
 that are visible to the human eye, there is nothing more 
 to be said. But what if there are other eyes, or other 
 processes that will enable these waves to be observed ? 
 Further, if it is found that these invisible waves agree 
 with the visible waves in other important respects, if, in 
 fact, it is found that they can be reflected, refracted, 
 polarised, and diffracted, then we are bound to regard 
 them as light. They may have wave-lengths that are 
 larger than that of the red waves, or smaller than that 
 of the violet waves, and so our eyes, with their limited 
 range of perception, may fail to be sensitive to them. 
 Nevertheless if in their physical properties they agree 
 with the visible kinds, then the fact that to us they 
 are invisible simply demonstrates the imperfection of 
 our eyes. Had we lived all our lives behind screens of 
 red glass we should never have known anything of green 
 or blue waves : we should have been blind to waves of 
 these particular kinds. But though we should never 
 have seen them that would not prove that they were 
 not waves of light. 
 
 Now that part of the invisible spectrum which con- 
 sists of waves of too large a size — of too great a wave- 
 
1 62 LIGHT LECT. 
 
 length — to affect our eyes possesses another property, 
 namely, that of warming the things upon which it falls. 
 Some of the visible waves, particularly those toward the 
 red end of the spectrum, share the same property, but 
 to a less extent. The longer invisible waves are called 
 variously the calorific or infra-red waves. We shall deal 
 with these in the next lecture. At the other end of the 
 spectrum, beyond the violet, we have again waves which 
 are invisible by reason of being of too small a size to 
 affect our sense of sight ; and these possess several 
 remarkable properties. They are active in producing 
 certain chemical effects, notably those known as photo- 
 chemical or photographic. They produce certain 
 physiological effects also on animal and vegetable 
 tissues. They actively provoke in certain bodies the 
 property of shining in the dark, or phosphorescence. 
 Lastly, they have certain electrical properties. These 
 short waves are known by the various names of actinic, 
 photographic^ or ultra-violet waves. The last of these 
 terms is much to be preferred. Some of these chemical 
 effects are also produced by visible light, especially by 
 the blue and violet waves. Fig. loo is a diagram which 
 gives a general idea ^ of the distribution of these effects 
 for waves of different lengths. The greatest luminosity 
 to the eye is possessed by waves having a wave-length 
 of about 22 millionths of an inch or o"ooo55 of a 
 millimetre. The greatest heating effect occurs with 
 waves of about 40 millionths of an inch, or 0*00 1 of 
 
 ^ A table of wave-lengths and frequencies of all kinds of light 
 from the lowest infra-red up to the highest ultra-violet has been 
 added as an Appendix to this Lecture. 
 
IV THE INVISIBLE SPECTRUM 163 
 
 a millimetre. The greatest chemical^ effect occurs 
 with waves of about 16^ millionths of an inch, or about 
 o'ooo4i of a millimetre. 
 
 Now, it is desirable for purposes of experiment to 
 separate the waves which can produce one of these 
 effects from those which produce another. If we desire 
 to sift out the ultra-violet waves from all other kinds, 
 there are several courses open to us. Firstly, we may 
 
 Fig. 100 
 
 use prisms which will disperse the waves and sort them 
 out into a spectrum according to their sizes. Secondly, 
 we may accomplish the same thing by using a diffraction 
 grating to produce a spectrum. Or, thirdly, we may 
 employ, as a means of sifting, sheets of different sub- 
 stances that have the power of absorbing waves of one 
 sort while transmitting those of another. This last 
 process we found excellent when applied to visible light, 
 
 ^ This is on the assumption that the effect is measured by a 
 particular chemical reaction, viz. the darkening of chloride of silver. 
 If a different reaction, say, for example, the darkening of ferro- 
 prussiate salts (" blue-prints ") were taken as a basis of measurement, 
 the maximum effect would be found to occur at some other point in 
 the spectrum. 
 
1 64 LIGHT LECT. 
 
 for by using a red-coloured glass we were able to cut off 
 all the other colours and leave only the red. Unfortu^ 
 nately no perfect filter-screen exists that will cut off all 
 the visible light and yet transmit the ultra-violet waves. 
 Glass tinted a deep violet colour with manganese, or with 
 manganese and cobalt, may serve to cut off most of the 
 visible light while transmitting a fair proportion of ultra- 
 violet waves, mixed with some violet light. For many 
 purposes this is good enough. But, unfortunately, every 
 kind of glass cuts off the extreme part of the ultra-violet 
 light. Even the lightest crown glass, though moderately 
 transparent to waves from 15 millionths to 11 millionths 
 of an inch long is totally opaque to all weaves smaller than 
 1 1 millionths ; while dense flint glass (containing lead) 
 is opaque to everything beyond the wave-length of 13 "3 
 millionths of an inch. Hence, for experiments on ultra- 
 violet light it is expedient not to use glass lenses or 
 prisms, provided some more transparent medium can be 
 found. Happily both quartz and fluor-spar are much 
 more transparent to ultra-violet waves than glass is. 
 Quartz transmits them down to about 8*i millionths 
 of an inch, and fluor-spar down to 8 millionths. My 
 lantern is on this occasion provided with condensing 
 lenses of quartz. When we want a spectrum we will 
 use a quartz prism and a focusing-lens also cut from 
 quartz crystal. 
 
 Let me now proceed to demonstrate some of the 
 photographic properties of light-waves. Here is a piece 
 of ordinary " printing-out " paper, that is paper which 
 has been covered with a sensitive film impregnated with 
 chloride or bromide of silver, which, when exposed for 
 
IV THE INVISIBLE SPECTRUM 165 
 
 a sufficient time to light, turns nearly black. Over this 
 sheet of sensitised paper I place some stencil-plates cut 
 out in sheet-zinc ; and then expose it to the white light 
 that comes from an electric arc-lamp on the table. In 
 half a minute the paper will have darkened sufficiently 
 for you to see that where the light-waves have fallen 
 upon the exposed parts they have produced the chemical 
 action, and have printed the patterns of the stencils. 
 In this experiment all kinds of rays — calorific, visible, 
 and actinic — have been allowed to fall on the paper; 
 but which of them were the agents in producing the 
 effect ? That is easily tested. We turn on the light in 
 the optical lantern, using the quartz lenses and prism to 
 produce a spectrum for us. Then along the whole 
 length of the visible spectrum, and to a distance into 
 the invisible spectrum at both ends, we stretch out a 
 long strip of sensitised photographic paper. It must be 
 left there for several minutes, during which time we 
 may investigate another point. 
 
 Here is another sheet of sensitised paper. Over it 
 I lay a sheet of opaque tin-foil, through which there have 
 been cut a number of holes. Over these holes are 
 laid a number of thin slices of various materials: (i) 
 window glass ; (2) flint glass ; (3) red glass ; (4) green 
 glass ; (5) blue glass ; (6) quartz ; (7) fluor-spar ; (8) rock- 
 salt ; (9) ebonite. I now slide the whole arrangement 
 under the beams of the arc-lamp, which is set to throw 
 its whole light downwards. If any of these materials 
 cuts off the active waves we shall find it out at once, 
 for the paper will be darkened only under those sub- 
 stances that are transparent to the photographic rays. 
 
1 66 LIGHT LECT. 
 
 Two minutes' exposure suffices for our simple test. On 
 bringing out the sheet you will note that ebonite (which 
 is black) and red glass have alike stopped off the whole 
 of the photographic rays. Green glass has stopped off 
 the greater part of them, and the flint glass has evi- 
 dently not transmitted them all. But under the blue 
 glass, the quartz, the rock-salt, the fluor-spar, and the 
 window glass the paper seems to have darkened about 
 equally. With a more refined test we should discover 
 differences between these also. One fact we have 
 proved, which is of practical importance, namely, that 
 red light does not affect a photographic film though it 
 affects our eyes. Every photographer knows this ; for 
 he takes advantage of it in using ruby glass or red- 
 coloured tissue to cover the windows of his "dark-room," 
 or to screen the lamp by whose light he works in pre- 
 paring and developing his plates. 
 
 Meantime our long strip of sensitised paper has been 
 exposed to the spectrum, and now, examining it, we see 
 that it has sensibly darkened at the violet end and 
 beyond the end of the visible violet to some distance 
 into the region where our eyes see nothing ; in short, 
 the photographic spectrum lies mostly beyond the violet, 
 the most active waves being shorter than any that are 
 visible. But we must not forget that with other chemical 
 preparations the range of sensitiveness can be changed. 
 To Captain Abney science owes the introduction of 
 emulsions of bromide of silver in films of gelatine, pre- 
 pared in such a way as to be sensitive not only to violet 
 light or ultra-violet, but also to green, to yellow, and 
 even to red waves. 
 
IV 
 
 THE INVISIBLE SPECTRUM 
 
 167 
 
 Another chemical effect which light-waves can pro- 
 duce is to cause mixed hydrogen and chlorine gases to 
 enter into combination. These gases (prepared by 
 electrolysis of hydrochloric acid) may be kept mixed, 
 but not chemically combined with one another, in glass 
 bulbs for any length of time, provided they are kept in 
 the dark. If exposed to the diffused light of a room 
 they slowly combine. But if exposed to direct sunlight 
 or to the light of the arc -lamp they combine with extra- 
 ordinary violence and explode the bulb. Again, the 
 question arises : which part of the light is it that produces 
 the effect ? Certainly not the red waves, for these bulbs 
 of mixed gas may be exposed freely if protected by red 
 glass and will not explode. The active kind of waves is 
 in this case also the ultra-violet kind. 
 
 A thin glass bulb containing the mixed gases is now 
 taken by my assistant 
 from a tin box, where 
 it has been kept in the 
 dark. To prevent acci- 
 dents he places it in an 
 empty lantern (Fig. loi), 
 into the nozzle of which 
 we will direct, from out- 
 side, the beams of an 
 electric arc -lamp. To 
 cut off the bulk of the 
 ordinary light I inter- 
 pose first a sheet of violet glass, which allows only violet 
 and ultra-violet to pass. Then, interposing a quartz 
 lens, I concentrate the beam upon the bulb, when — bang 
 
 Fig. ioi. 
 
1 68 LIGHT LECT. 
 
 — it explodes, demonstrating the activity of waves of this 
 sort. 
 
 Perhaps it may have struck some of you that if so 
 great a photographic activity is possessed by waves that 
 are invisible to our eyes, it ought to be within the limits 
 of possibility to photograph things that are invisible. 
 And so it is. It is now some twenty years since a 
 lecture was delivered in this theatre on the photography 
 of the invisible by the veteran chemist, Dr. J. Hall 
 Gladstone, who succeeded in photographing images of 
 things quite invisible to the eye. Behind me, against 
 the wall, stands a drawing-board covered with a white 
 sheet of cartridge paper. The light of the arc-lamp 
 shines on it. You see merely a white surface. The 
 photographer, Mr. Norris, has brought his camera here 
 and he will now take a photograph of it. When he 
 develops the photograph you will find that the photo- 
 graph will reveal the fact that an inscription has been 
 written upon the sheet, which, though invisible to you, 
 can be photographed by the camera.^ 
 
 Since photographic action serves to detect these 
 ultra-violet waves, even in the absence of all kinds of 
 visible light, it may be used in the further exploration of 
 the properties of these invisible waves. We might apply 
 photographic plates to prove the possibility of the re- 
 
 ^ The inscription was written on the sheet with colourless sulphate 
 of quinine dissolved in a solution of citric acid. This substance 
 fluoresces, and in the act of fluorescing destroys the ultra-violet light, 
 which would otherwise be reflected from the parts of the paper so 
 treated. The parts where the sulphate of quinine has been applied 
 consequently come out in the photograph darker than the untouched 
 surface of the paper. 
 
IV THE INVISIBLE SPECTRUM 169 
 
 flexion and refraction of these waves, as well as of their 
 interference and polarisation. There exists, however, 
 another and more ready method of investigation, to 
 which we will now proceed. 
 
 Instead of photographing the invisible we may make 
 it visible to the eye by applying the discoveries of 
 Herschel, Brewster, and Stokes. There are a number 
 of solid substances, such as fluor-spar, uranium glass, and 
 also of liquids, such as petroleum, solutions of quinine, 
 and of many of the dye-stuffs derived from coal-tar, which 
 present the appearance of a surface -colour different 
 from that of the interior. Thus quinine is colourless, 
 but shows a fine blue tint on the surface exposed to the 
 light. Uranium glass is itself yellow, but has a splendid 
 green surface-tint. The fact is that these substances 
 have the property of absorbing the very short waves of 
 ultra-violet light and transforming them into waves of 
 longer length that are visible to our eyes. To this 
 phenomenon Stokes gave the name of fluorescence. Let 
 us see a few of the principal cases. 
 
 From the optical lantern, provided for the present 
 experiments with quartz lenses, a beam of light streams 
 forth. Over the nozzle of the lamp is now placed a 
 cap of dark violet glass to cut off all the visible light 
 except a little violet that unavoidably accompanies the 
 invisible ultra-violet waves. This beam is directed 
 upon a cube of uranium glass ; which transmutes the 
 invisible waves into a brilliant green. And you see the 
 glass cube standing out vividly in the darkness. I hold 
 in the beam a bottle of paraffin oil — it seems brilliantly 
 blue. A green decoction of spinach leaves (boiled first 
 
lyo 
 
 LIGHT 
 
 LECT. 
 
 in water, then dried, and lastly extracted with ether) 
 exhibits a strange blood-red fluorescence on its surface. 
 
 Here is a row of specimen bottles containing 
 fluorescent liquids. Yellow fluorescein gives a splendid 
 green fluorescence ; pale pink eosin (made by diluting 
 red ink) gives an orange fluorescence. A crimson 
 solution of magdala-red gives a scarlet fluorescence; 
 and colourless quinine gives its characteristic surface- 
 blue. 
 
 These things may be even more strikingly shown 
 by reflecting the ultra-violet beam down into a tall glass 
 
 cylinder filled with 
 fluorescent liquid. 
 A quartz lens placed 
 just above the jar 
 (Fig. 102) serves to 
 concentrate the beam 
 into a sharp cone of 
 colour. I take a 
 second jar filled 
 simply with dilute 
 ammonia-water, and 
 project the beam 
 down it. Then I 
 sprinkle into the 
 ^'^- ^°^' water a few grains 
 
 of dry fluorescein. As they dissolve there descend to 
 the bottom curling wreaths of bright green hue and 
 indescribable beauty. A few chips of horse-chestnut 
 bark, or of ash bark, would yield similar effects. 
 
 And now, perhaps, you will appreciate the secret 
 
IV THE INVISIBLE SPECTRUM 171 
 
 of the photography of the invisible. The inscription 
 painted on the white sheet was painted with a solution 
 of quinine. You shall see for yourselves the invisible 
 inscription ; for I have only to cast upon it a beam of 
 ultra-violet light to cause the parts painted with quinine 
 to shine out in pale blue amid the darkness. 
 
 Here are some other sheets of card on M^hich 
 fluorescent patterns have been painted. On one of 
 these, side by side, are two fleurs-de-lys., which in day- 
 light appear to be equally yellow. One is painted in 
 common gamboge, the other in fluorescein. But when 
 I turn upon them the beam of the lamp filtered through 
 dark blue glass, the whole card looks deep violet, one 
 of the lilies seeming black, the other luminous and 
 greenish. Another card, when viewed in ordinary white 
 light seems to be merely yellow all over : but as part 
 of the yellow surface is painted in gamboge, and the 
 other fluorescein, the effect, when examined in the 
 ultra-violet beam is to give a black pattern on a bright 
 ground. 
 
 Twenty years ago when the late Professor Tyndall 
 was delivering in the United States his famous series of 
 lectures on light, he received from President Morton, 
 of the Stevens Institute at Hoboken, some samples 
 of a new fluorescent hydrocarbon, "thallene," prepared 
 from petroleum residues. Some large sheet diagrams 
 of flowers, painted in parts with thallene and other 
 fluorescent materials, were amongst the objects which 
 Professor Tyndall brought back to London. These 
 have been carefully preserved in the Royal Institution, 
 and I am able to show you them in all their beauty. 
 
172 LIGHT LECT. 
 
 One of them represents a wild mallow, the leaves being 
 coloured with some substance which fluoresces green, 
 whilst the flowers have a pale purple fluorescence. 
 The efl'ect of throwing on this object light that has 
 passed through a dark blue or dark violet glass is very 
 striking. 
 
 Of all substances, however, that are known to me, 
 the most highly fluorescent is a rather expensive 
 crystalline product called by chemists the platino- 
 cyanide of barium. In ordinary light it looks like a 
 pale yellow or greenish yellow powder, closely resembling 
 powdered brimstone. When a piece of paper covered 
 with this substance is held in the ultra-violet beam it 
 emits a yellowish-green light far surpassing in brilliance 
 that emitted by uranium glass or by fluorescein. Here 
 is a small fluorescent screen of platino - cyanide of-^ 
 barium that has been in my possession some sixteen 
 years. 
 
 Now, having so excellent a means of making ultra- 
 violet waves visible, let us apply the fluorescent screen, as 
 Stokes did in 1851, to explore the ultra-violet spectrum. 
 My assistant puts up the quartz prism in front of the 
 slit to give us once more the spectrum. Taking a long 
 sheet of cardboard that has been painted over with 
 quinine, I hold it so that the spectrum falls upon the 
 middle of the prepared surface. And now you see that 
 the spectrum stretches visibly away beyond the violet 
 end, for here, crossed by several transverse patches of 
 brighter light, the ultra-violet spectrum comes into view 
 as a pale -blue extension. Substituting a sheet of 
 uranium glass we note a similar extension visible into 
 
IV 
 
 THE INVISIBLE SPECTRUM 
 
 173 
 
 the ultra-violet to a distance that makes 
 this part of the spectrum seem quite 
 twice as long as the whole visible part. 
 Here, best of all, is a fluorescent 
 screen covered with platino-cyanide of 
 barium. And now we see the "long 
 spectrum," stretching away to three 
 or four times the length of the visible 
 part from red to violet. If placed 
 at the other end, below the red, 
 these fluorescent screens show no- 
 thing whatever. They are excited 
 into luminous activity not by the 
 long waves, but by the very short 
 ones. 
 
 Let us then avail ourselves of the 
 luminous quality of the fluorescent 
 screen to examine afresh the differ- 
 ent degrees in which transparent 
 substances transmit or absorb these 
 ultra-violet waves. The ultra-violet 
 part of the spectrum now falls upon 
 the screen, the surface of which is 
 thereby stimulated into emitting its 
 fine greenish light. Across the path 
 of the invisible beam I interpose a 
 piece of window glass. The light is 
 dimmed but not extinguished. A 
 piece of flint glass cuts it off alto- 
 gether; a piece of blue glass dims 
 it, but does not cut it off; while a 
 
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174 LIGHT LECT. 
 
 piece of red glass proves to be absolutely opaque. 
 A slice of quartz crystal is fully transparent ; one 
 of calc-spar- rather less so, while a thin film of yellow 
 gelatine is quite opaque. These experiments con- 
 firm those we made by the use of photographic 
 paper. 
 
 And now in a very few moments we can demonstrate 
 reflexion and refraction of the ultra-violet waves. I 
 place my fluorescent screen in a position where none of 
 the waves fall upon it. Then holding a mirror in the 
 invisible beam I reflect ultra-violet waves upon the 
 screen, which at once shines with its characteristic 
 greenish tint. To prove refraction I interpose in the 
 invisible beam a quartz prism, which deviates ultra-violet 
 waves upon the fluorescent screen, and again it shines. 
 Polarisation may be proved by using two Nicol prisms 
 precisely as was done in my last lecture for ordinary 
 light. 
 
 This phenomenon of fluorescence is only one of a 
 number of kindred phenomena, now generally classified 
 together under the name of Luminescence. This name 
 was given by Professor E. Wiedemann to all those cases 
 in which a body is caused to give out light without 
 having been raised to the high temperature that would 
 correspond to the ordinary emission of light. To make 
 ordinary solids red-hot they must be raised to between 
 400° and 500° of the centigrade scale of temperature. 
 To make them white-hot — that is to say, to cause them 
 to omit not only red, orange, and yellow, but also green, 
 blue, and violet light, they must be raised to 800° or 
 1000° of temperature. At red-heat a body emits few or 
 
IV 
 
 THE INVISIBLE SPECTRUM 
 
 175 
 
 no green, blue, or violet waves. But as we have seen in 
 the examples of fluorescence some substances while quite 
 cold can be stimulated into giving out light by letting 
 invisible ultra-violet waves fall upon them. So we may 
 well inquire what other cases there are of the emission 
 of cold light. Accordingly, on the table before you 
 there are enumerated the various cases of lumin- 
 escence. 
 
 Phenomenon. 
 
 Substance in which it occurs. 
 
 1. Chemi-luminescence 
 
 2. Photo-luminescence : 
 
 {a) ^ranszejit — Fluorescence 
 
 « 
 
 {d) persistent — Phosphores- 
 cence .... 
 
 3. Thermo-kuninescence . 
 
 4. Tribo-luminescence 
 
 5. Electro-luminescence : 
 
 [a) Effluvio-luminescence . 
 
 [b] Kathodo-luminescence . 
 
 6. Crystallo-Iuminescence . 
 
 7. Lyo-luminescence 
 
 8. X-luminescence . 
 
 Phosphorus oxidising in moist 
 air ; decaying wood ; decay- 
 ing fish ; glow-worm ; fire- 
 fly; marine organisms, etc. 
 
 Fluor-spar ; uranium - glass ; 
 quinine ; scheelite ; platino- 
 cyanides of various bases ; 
 eosin and many coal-tar pro- 
 ducts. 
 
 Bologna-stone ; Canton's phos- 
 phorus and other sulphides 
 of alkaline earths ; some 
 diamonds, etc. 
 
 Fluor-spar ; scheelite. 
 
 Diamonds ; sugar ; quartz ; 
 uranyl nitrate ; pentadecyl- 
 paratolylketone. 
 
 Many rarefied gases ; many 
 of the fluorescent and 
 phosphorescent bodies. 
 
 Rubies ; glass ; diamonds ; 
 many gems and minerals. 
 
 Arsenious acid. 
 
 Sub - chlorides of alkali- 
 metals. 
 
 Platino-cyanides ; scheelite, etc. 
 
176 LIGHT LECT. 
 
 The Chemi- luminescence which heads the Ust in- 
 cludes those cases in which the emission of cold light 
 is accompanied by chemical changes. The best known 
 instance is the shining in the dark of phosphorus when 
 slowly oxidising in moist air. Lucifer matches, if 
 damped and then gently rubbed, shine in the dark. 
 The best way to show this is to take a sheet of ground 
 glass, dip it into warm water, and then write upon its 
 roughened surface with a stick of phosphorus, which, 
 for the sake of safety, is held in a wet cloth. See how, 
 on lowering the lights in the theatre, the inscription I 
 have scribbled upon the glass shines with a pale blue 
 glimmer. In a few minutes the film of phosphorus will 
 have oxidised itself completely, and the emission of 
 light will be at an end. Curiously enough, this light 
 itself consists not only of the blue waves that you can 
 see, but of some invisible waves also, which have photo- 
 graphic properties, and can, like Rontgen's rays, affect 
 a photographic plate that is enclosed behind an opaque 
 screen of black paper. It is now known that the 
 emission of light by glow-worms and fire-flies, and by 
 the innumerable species of marine creatures and deep- 
 sea fishes that shine in the dark, belongs to the class 
 of chemi-luminescence. So does the emission of light 
 by the microbes that are developed in decaying fish and 
 in rotten wood. In all these cases there is chemical 
 decomposition at work. 
 
 Under the next heading — Photo-luminescence — are 
 included those cases in which bodies give out cold light 
 under the stimulation of light -waves. Of this phe- 
 nomenon there are two cases. Fluorescence is one. 
 
IV THE INVISIBLE SPECTRUM 177 
 
 and in that case the emission of light is temporary, 
 lasting only while the stimulation lasts. The other 
 case is that known as Phosphorescence, a term applied 
 to those instances in which the emission of light persists 
 after the stimulation has ceased. The earliest known 
 example of phosphorescence is that of the celebrated 
 Bologna stone. A shoemaker in the city of Bologna, 
 Casciarolo by name, about the year 1602 discovered 
 a way of preparing a species of stone which, after 
 having been exposed to sunlight, would shine in 
 the dark. This was done ^ by the partial calcination 
 of "heavy-spar" — the sulphate of barium — found near 
 that city. Here is a small sample on the table. Since 
 that time many other substances have been found to 
 possess the same property. Some diamonds, as Robert 
 Boyle observed, have this property. And amongst 
 artificial substances the sulphide of calcium (Canton's 
 "phosphorus") and sulphide of strontium possess the 
 property to a very high degree. Sulphide of calcium 
 can be prepared by pounding up oyster-shells and 
 heating them to redness, mixed with a little brimstone, 
 in a closed crucible. The addition of small quantities 
 of other materials — a little bismuth, or manganese, or 
 copper — has a remarkable influence in aiding the 
 production and in changing the colour of the light 
 emitted. The substance sold as Balmain's luminous 
 paint is a preparation mainly of sulphide of calcium 
 with a trace of bismuth. Of all these artificial phos- 
 
 ^ See a singular little volume published in Rome in 1680, by 
 Marc' Antonio Cellio, with the title // Fosforo, 0' vero la pietra 
 Bolognese, preparata per rilucere frh- fombra. 
 
tjB LIGHT LECT. 
 
 phori the most powerful by far is a new luminescent 
 paint prepared by Mr. Horne.^ 
 
 Behind me an electric lamp is arranged to throw a 
 beam of light down a tube. At the bottom of this 
 tube I expose to the stimulation of its beams a few of 
 these phosphorescent stuffs. Here is the bit of Bologna 
 stone. On removing it from the beam it shines in the 
 dark, but not nearly so brightly as the bit of Home's 
 new material, the light of which is equal to about one- 
 tenth of a candle for each square inch of surface exposed. 
 One can see to read print by the light of a bit of this 
 stuff. I have heard of people using a glow-worm in 
 the same way in order to read at night. Here is a 
 diamond ring having five fine diamonds. On exposing 
 it for a minute to the light, and then bringing it out 
 into the darkness, two of the diamonds are seen to 
 shine like little glow-worms. 
 
 Here is a box containing a row of glass tubes, in 
 each of which is a white powder. These powders are 
 phosphorescent. But first they must be stimulated, 
 for at present they emit no light. Let us expose them 
 for thirty seconds to the beams of the arc -lamp. On 
 then bringing them into the darkness of the theatre it 
 will be seen that they glow brightly in all the colours 
 of the rainbow. 
 
 Here, again, is a large sheet of glass which has been 
 painted over with luminous paint. I lay my hand 
 against it, and expose it for a minute to the beams of 
 the arc -lamp. Extinguishing the light, you see the 
 whole sheet splendidly luminous, save where the shadow 
 1 Of 97 Bromley Road, Catford, S.E. 
 
IV THE INVISIBLE SPECTRUM 179 
 
 of my hand appears as a black silhouette. In this case 
 the luminescence is at first of a fine blue tint. In a 
 few minutes as it fades out it becomes whiter ; but it 
 will go on all night giving out a faint light, and even 
 then will not have yielded up its whole store of luminous 
 energy. Even after having been kept six weeks in 
 darkness a sheet of luminous paint will still emit waves 
 that will fog a photographic plate. If one takes a 
 sheet of luminous paint that has been exposed to light, 
 and of which the phosphorescence has already died 
 away, one finds that merely warming it will cause it to 
 shine more brightly, though afterwards it is darker. 
 Here is such a sheet. I place my hand against the 
 back, and you note that where my hand has warmed it 
 it shines more brightly. If one makes the converse 
 experiment of chilling a sheet of luminous paint while 
 it is phosphorescing, one finds its light dimmed, but it 
 grows brighter while being warmed. Professor Dewar 
 has made the curious discovery, that when cooled to a 
 temperature of about 200° below zero in liquid air, 
 many substances become phosphorescent that are not 
 so at ordinary temperatures. Thus he has shown in 
 this theatre how such things as feathers, ivory, and 
 paper become highly phosphorescent on being cooled 
 to these low temperatures and then illuminated. They 
 seem when chilled to acquire the power of absorb- 
 ing luminous energy and storing it for subsequent 
 emission when warmed. The analogies between these 
 properties and those of luminous paint are most 
 suggestive. A sheet of luminous paint which has 
 been exposed and cooled becomes a veritable lamp 
 
i8o LIGHT LECT. 
 
 of Aladdin. One has but to warm it by the hand and 
 it shines. 
 
 Here we touch upon the third sort of luminescence 
 named in our list (p. 175), namely Thermo-luminescence. 
 This term is applied to the property possessed by 
 various minerals, particularly by the green sorts of fluor- 
 spar, to shine in the dark on being heated. Over a 
 large atmospheric gas-burner a square of sheet-iron has 
 been heated to near redness. Upon this hot surface, 
 invisible in the darkness, I scatter out of a pepper-box 
 some fine fragments of crushed fluor-spar. As they 
 heat up they shine like little glow-worms. They shine 
 brightly for a few minutes, then fade, but w^ould continue 
 for several hours to emit a faint glow. After having 
 been once thus heated they seem to have lost their 
 store of luminous energy, for on a second heating they 
 do not again luminesce.-^ 
 
 The term Tribo-luminescence, which stands next on 
 the list, relates to the production of luminescence by 
 friction. There is a very simple experiment that can 
 be tried at home without any apparatus. Crush a lump 
 of sugar in a perfectly dark room. In the act of being 
 crushed it emits a pale luminescence. So do crystals of 
 uranium nitrate if shaken up in a bottle, or triturated in 
 a mortar. Let me show it to you on a larger scale. 
 
 ^ Many other minerals have similar powers. In some cases the 
 power of thermo-luminescing can be revivified by fresh exposure to 
 light, or by stimulation by an electric spark or by kathode rays. 
 Wiedemann has found artificial substances that are thermo- 
 luminescent, and in particular a preparation of sulphate of calcium 
 having intermingled as a "solid solution" a small percentage of 
 sulphate of manganese. 
 
IV THE INVISIBLE SPECTRUM i8i 
 
 Here is a large specimen of quartz crystal weighing 
 nearly a hundred pounds. One of its faces is almost 
 flat. Taking a smaller crystal of quartz in my hand I 
 rub it to and fro upon the larger crystal. You can all 
 see the brilliant flashes that are emitted in the operation. 
 
 Reserving for discussion in my final lecture the use 
 of electric discharges to produce luminescence, we will 
 return to the properties of the ultra-violet light. One 
 effect which they possess above all other kinds of light 
 is that of producing diselectrification of electrified 
 bodies, a phenomenon discovered by the late Professor 
 Hertz. But there is this peculiar limitation. If the 
 electrified surface is that of a metal surrounded by air, 
 then when ultra-violet light falls upon it it will produce 
 diselectrification if the surface is negatively electrified, 
 but not if the electrification is of positive sign.^ 
 
 The fundamental point is easily shown. Here is 
 an electroscope made with two leaves 
 of aluminium mounted on either side 
 of a central blade of aluminium, and 
 enclosed (Fig. 104) in a thin glass 
 jar. To the top of the stem is affixed 
 a disk of sheet zinc which has been 
 freshly cleaned with a little sodium- 
 amalgam. It is bent back at 45°, 
 at which incidence the results are FiG,io4r 
 
 most favourable. I hold near the 
 zinc disk a rubbed glass rod, and touch the disk while 
 
 ••^ I have, however, found that a surface of peroxide of lead sur- 
 rounded by an atmosphere of hydrogen is diselectrified if the 
 electrification is positive. 
 
1 82 . LIGHT LECT. 
 
 it is under the influence of the positive charge of the 
 glass. The electroscope thus acquires by influence a 
 negative electrification, and the aluminium leaves stand 
 out at a sharp angle. Now throwing a beam of ultra- 
 violet light upon the disk, the leaves are seen to collapse 
 rapidly. If the electroscope is positively electrified, the 
 leaves do not fall down when the beam of ultra-violet 
 light is directed upon the disk. Even the longer waves 
 of visible light are active on a clean surface of sodium 
 or potassium. The different kinds of light-waves have 
 different photo-electric powers as well as different photo- 
 chemical powers. 
 
 At the beginning of this lecture I dwelt upon the 
 photographic actions of light-waves, and I return now 
 to this topic in order to speak of the problem which 
 has of late aroused such keen interest amongst scientific 
 photographers, namely, the photography of colours. 
 Many have been the attempts to produce true photo- 
 graphs of things in their natural colours. All hope of 
 this was vain so long as photographers worked with 
 chemically prepared plates that were more sensitive to 
 the invisible light than to the visible kinds. Further, 
 in the old collodion processes the greater sensitiveness 
 of the chemicals to bliie and violet waves, and their 
 relative insensitiveness to orange and red light, caused 
 all photographs to represent coloured objects untruly 
 in their relative luminosity. It was an old complaint 
 that blues photographed like white, and reds came out 
 like black. The first steps towards remedying this arose 
 in the discoveries of Vogel and of Abney that by staining 
 the film or by giving to it in its preparation as an 
 
IV THE INVISIBLE SPECTRUM 183 
 
 emulsion a fine granulation, its sensitiveness toward the 
 longer visible waves might be increased. Thus were 
 introduced the orthochromatic plates which gave as 
 photographs a more accurate representation in black 
 and white of the relative luminosities of objects ; the 
 ideal orthochromatic plate being one which should have 
 the same relative sensitiveness tow^ard the light of each 
 part of the visible spectrum as our eyes have. Even 
 before these discoveries the theory of the trichroic 
 method of reproducing colour by photography had been 
 enunciated by Clerk Maxwell.^ In the theory of colour- 
 vision originated by Thomas Young, all colour-sensations 
 are referred to three simple or primary colour-sensations, 
 and it can be shown that no more than three ^ are 
 needed to account for the various phenomena of colour- 
 vision. These three primary sensations are the sensation 
 of red^ the sensation of green (a full green inclining to 
 yellowish-green), and the sensation of blue-violet (a violet 
 inclining toward blue). A red light stimulates but one 
 of these sensations in the nerves of the eye. A yellow 
 light stimulates two, namely, red and green,. and is not 
 therefore itself a primary sensation. Now if we could 
 take three photographs of an object, each photograph 
 corresponding only to the light of each primary sort, 
 and if we could then illuminate each photograph with 
 its own kind of light and superpose them, we ought to 
 get a reproduction of the natural colours of the object. 
 That, briefly, is the three-colour method. 
 
 ^ Discourse at Royal Institution, 17th May 1861. 
 ^ The reader should consult Captain Abney's treatise on Colour- 
 vision. 
 
1 84 LIGHT LECT. 
 
 The true photography of colours was only discovered 
 a year or two ago by Professor Lippmann, whose 
 exceedingly precious and beautiful results are individual 
 pictures, incapable of being multiplied or reproduced. 
 By placing at the back of the transparent sensitive film 
 a mirror of mercury, each train of waves is reflected 
 back during the exposure; and where the reflected 
 waves meet the advancing waves of the train they set 
 up stationary nodes that are spaced out through the 
 thickness of the film at distances apart corresponding 
 to the exact wave-lengths of the various lights. At 
 these nodes the chemical action takes place, and pro- 
 duces a permanent picture which, when viewed by 
 reflected light, shows all the natural colours of the object 
 that has been photographed. I am, by the kindness of 
 my colleague. Professor Meldola, able to show here, and 
 to project on the screen, a Lippmann photograph of the 
 spectrum in which all the colours show in their natural 
 tints. More recently Professor Lippmann has shown in 
 this theatre the remarkable colour-pictures which he has 
 produced of landscapes, still-life subjects, and even of 
 the human figure.^ 
 
 Returning to the three-colour method of registering 
 and reproducing by photography the natural colours of 
 
 ^ Since the delivery of these lectures several new processes of 
 colour-photography have been announced. The most important of 
 these is the process of the Brothers Lumiere, of Lyon, in which the 
 sensitized plate is covered with a layer of mixed starch grains which 
 have been dyed with transparent red, transparent green, and trans- 
 parent blue. These grains act as minute local colour filters for the 
 three primary tints, as explained on p. 185. The results are very 
 surprising. 
 
IV THE INVISIBLE SPECTRUM 185 
 
 objects, I am happy in conclusion to be able by the 
 kindness of Mr. Ives to show you the remarkable results 
 which he has attained with his photochromoscope* 
 Starting from Young's theory of the three primary 
 sensations, Mr. Ives sought to construct colour filters 
 which should transmit for each of the three primaries 
 all those waves of the spectrum which excite that 
 sensation, and in proportion to their power of exciting 
 that sensation in the eye. Thus the filter for red should 
 transmit not simply red light, but should transmit all 
 those waves of whatever colour that are competent to 
 excite the red sensation, but transmit them only in 
 proportion as they are competent to excite the red 
 sensation. To select the proper tints as colour filters 
 is a matter of no small skill and experience. Through 
 three such screens — one for red, one for green, one for 
 blue -violet — three photographs (negatives) are taken 
 simultaneously side by side upon a single orthochro- 
 matic plate. From these three negatives (which are of 
 course colourless themselves) three positives are printed. 
 These also are colourless, but they show differences 
 according to the colours of the different parts of the 
 object photographed. Fig. 105 is a triple chromogram 
 of a butterfly. Its wings beside, having definite patches 
 of red and white on a black background, have all over 
 them a beautiful sheen of brilliant blue. The upper- 
 most image of the three is that which is to be placed in 
 the blue-violet light. The second figure is that for the 
 green light, while the lowest is for the red light. Those 
 parts which are to show as white, when combined, are 
 white in all three images. Each image is itself, like the 
 
1 86 
 
 LIGHT 
 
 LECT. 
 
 printed cut, colourless ; a mere black and white tran- 
 script on glass. 
 
 Now let these three colourless pictures be placed in an 
 instrument so arranged that blue-violet light falls through 
 
 Fig. 105. 
 
 I. To be illuminated by Blue-Violet Light. 
 2. To be illuminated by Green Light. 3. To be illuminated by Red Light. 
 
 the first, green through the second, and red through 
 the third of these separate photographs, and let them 
 then be recombined by suitable mirrors so that the eye 
 shall view them simultaneously, the primary colours 
 
IV 
 
 THE INVISIBLE SPECTRUM 
 
 187 
 
 Fig. 106. 
 
 will recombine and give the object in all the glory of 
 the natural tints. 
 
 The instrument (Photochromoscope or Kromskop) 
 which Mr. Ives has designed for recombining these triple 
 photographs stands upon 
 the table. Fig. 106 gives 
 a diagram ^ of its construc- 
 tion. Mr. Ives has also 
 brought a lantern photo- 
 chromoscope, by means of 
 which he will now project 
 on the screen a few of those 
 beautiful photographs. 
 The lantern itself has three 
 nozzles, through which the 
 
 red, the green, and the blue-violet pictures are separately 
 projected on the screen, and by their overlapping give 
 the colour-combinations. He first shows us separately 
 the three-coloured disks or circles of light, red, green, 
 and blue-violet. Then he moves the nozzles so that 
 
 ^ A, B, and C are red, blue-violet, and green glasses against 
 which the" three corresponding transparent photograms are respect- 
 ively placed. Two of the pictures at A and B are illuminated 
 directly by light from above, the third C is illuminated by an oblique 
 reflector. The red picture is viewed by rays which are reflected at 
 the front surface of D an oblique transparent glass sheet. The 
 blue-violet picture at B sends its light down upon another oblique 
 transparent sheet at E, which reflects it through the sheet D 
 to the eye. The green picture at C is viewed through both the 
 transparent reflectors D and E. The lens F collects the rays for 
 the eye, which thus views the three pictures as if they were super- 
 posed and at equal distance away. The instrument is made 
 binocular, so the eyes see as it were a single image in its natural 
 colours, and in solid relief. 
 
LIGHT 
 
 LECT. 
 
 Fig, 107. 
 
 the three disks partially overlap as in Fig. 107. Where 
 red and green mix they give us yellow; where green 
 
 and blue - violet overlap 
 they give us a peacock 
 blue ; where blue - violet 
 and red overlap they give 
 us purple ; where all three 
 overlap at the centre they 
 give us white. Note how 
 the tint produced by the 
 overlap of two gives us the 
 complementary to the 
 third. Thus the yellow 
 is the complementary to 
 the blue-violet ; the peacock is complementary to the 
 red ; and the purple is of a tint complementary to the 
 green. 
 
 Those three photographs of the butterfly are now 
 introduced into the chromoscope -lantern, and are 
 brought to accurate superposition. The blue shimmer 
 on the insect's wings is shown with marvellous fidelity. 
 No painter could hope to produce by pigments such a 
 natural picture. Here is a photograph of a basket of fruit. 
 Note the yellow lemon. On examining the separate 
 colour-pictures one sees that this yellow is made up of the 
 red and green lights mixed. Here is a gorgeous bouquet 
 of flowers. The colours are superb. Here is a cigar- 
 box showing the natural brown tint of the wood ; and 
 beside it a piece of doisonnk enamel, with all the delicate 
 shades of dull tints in their due relations. And lastly, 
 here is a box of sweetmeats, so naturally photographed 
 
IV THE INVISIBLE SPECTRUM 189 
 
 that one feels them to be really edible. After that we 
 need no further proof that by the proper selection of 
 the primary tints the dream has at last been realised of 
 registering and reproducing by photography the colours 
 of natural objects. 
 
APPENDIX TO LECTURE IV 
 
 TABLE OF WAVE-LENGTHS AND FREQUENCIES 
 
 
 
 Wave- 
 
 ength. 
 
 
 Name of Line. 
 
 Element. 
 
 
 
 frequency 
 (billions 
 
 Micro-cen- 
 
 INIillionths 
 
 
 
 timetres. 
 
 of inch. 
 
 per second). 
 
 (Rubens and Nichols' longest 
 
 2400 
 
 944 
 
 i2-5(xioi2) 
 
 waves) 
 
 
 
 
 (Langley's longest waves) 
 
 1500 
 
 592 
 
 20 
 
 (Paschen's longest waves) 
 
 945 
 
 370 
 
 317 
 
 ^2 \ 
 ^1 J 
 
 
 
 270 
 
 106-24 
 
 III 
 
 ^2 
 
 
 
 124 
 
 4873 
 
 242 
 
 *1 
 
 
 
 120 
 
 47-25 
 
 250 
 
 - { 
 
 
 
 89-904 
 
 35*36 
 
 333-7 
 
 
 
 89-865 
 
 35-35 
 
 334 -o 
 
 X4 
 
 
 
 88 -06 1 
 
 34 '64 
 
 340-8 
 
 X3 
 
 
 
 86-614 
 
 34-1 
 
 346-2 
 
 X2 
 
 
 
 85-418 
 
 33-63 
 
 351-3 
 
 Xi 
 
 
 
 84-97 
 
 33*44 
 
 353-3 
 
 z 
 
 
 
 82-264 
 
 32-34 
 
 364-5 
 
 A 
 
 
 
 75 "94 
 
 29-28 
 
 395-2 
 
 B 
 
 
 
 68-674 
 
 27-03 
 
 436-5 
 
 C 
 
 H 
 
 65-630 
 
 25-83 
 
 457-2 
 
 1^1 
 
 Na 
 
 58-961 
 
 23-21 
 
 508-8 
 
 D2 
 
 Na 
 
 58-902 
 
 23-18 
 
 509-1 
 
 D3 
 
 He 
 
 58-760 
 
 23-13 
 
 510-5 
 
 E ' 
 
 Fe 
 
 52705 
 
 20-78 
 
 569-2 
 
 Ca 
 
 52-704 
 
 20-78 
 
 569-2 
 
 Eo 
 
 Fe 
 
 52-697 
 
 20-74 
 
 569-3 
 
 ^ 
 
 Mg 
 
 51-838 
 
 20-40 
 
 578-9 
 
 h 
 
 Mg 
 
 51-729 
 
 20-36 
 
 580-0 
 
 ^ [ 
 
 Fe 
 
 5 1 '692 
 
 20-351 
 
 580-4 
 
 Fe 
 
 51-691 
 
 20-350 
 
 580-4 
 
APP. 
 
 TABLE OF WAVE-LENGTHS, ETC. 
 
 191 
 
 TABLE OF WAVE-LENGTHS AND FREQUENCIES — Continued 
 
 Name of Line. 
 
 Element. 
 
 Wave-length. 
 
 Frequency 
 (billions 
 
 Micro-cen- 
 
 Millionths 
 
 
 
 timetres. 
 
 of inch. 
 
 per second). 
 
 '- { 
 
 Fe 
 
 51-677 
 
 20*306 
 
 580-5 
 
 Mg 
 
 51-675 
 
 20-305 
 
 580-5 
 
 F 
 
 H 
 
 48-615 
 
 19-14 
 
 617-I 
 
 « { 
 
 Fe 
 
 43-081 
 
 16-96 
 
 696-3 
 
 Ca 
 
 43-079 
 
 16-95 
 
 696-4 
 
 h 
 
 H 
 
 41-018 
 
 16-17 
 
 731-3 
 
 H 
 
 Ca 
 
 39-686 
 
 15-63 
 
 756-0 
 
 K 
 
 Ca 
 
 39-338 
 
 15-48 
 
 762-7 
 
 L 
 
 Fe 
 
 38-206 
 
 15-04 
 
 785-1 
 
 M 1 
 
 Fe 
 
 37-278 
 
 14-676 
 
 804-6 
 
 Fe 
 
 37-271 
 
 14-673 
 
 804-9 
 
 N 
 
 Fe 
 
 35-813 
 
 14-09 
 
 837-7 
 
 
 
 Fe 
 
 34-411 
 
 13-55 
 
 871-8 
 
 P 
 
 Fe 
 
 33-613 
 
 13-23 
 
 892-6 
 
 Q . 
 
 Fe 
 
 32-869 
 
 12-94 
 
 912-6 
 
 R { 
 
 Ca 
 
 31-814 
 
 12-52 
 
 942-9 
 
 ^" 1 
 
 Ca 
 
 31-794 
 
 12-51 
 
 943-5 
 
 r 
 
 Fe 
 
 31-446 
 
 12-38 
 
 954-1 
 
 Si _' 
 S2 \ 
 
 Fe 
 
 3 1 -008 
 
 12-207 
 
 967-4 
 
 Fe 
 
 3 1 -004 
 
 1 2 -206 
 
 967-6 
 
 Fe 
 
 3i'ooi 
 
 12-205 
 
 967-7 
 
 s 
 
 Fe 
 
 30-477 
 
 11-99 
 
 984-5 
 
 r { 
 
 Fe 
 
 30-212 
 
 1 1 -894 
 
 993-0 
 
 Fe 
 
 30-207 
 
 1 1 -892 
 
 993-3 
 
 t 
 
 Fe 
 
 29-945 
 
 11-79 
 
 1002-0 
 
 u 
 
 Fe 
 
 29-480 
 
 II '60 
 
 1017-6 
 
 (Miller's limit 
 
 photographic) 
 
 20-2 
 
 7-95 
 
 1485-1 
 
 (Stokes' limit, 
 
 fluorescent) 
 
 18-5 
 
 7-28 
 
 1621-6 
 
 (Schumann's ' 
 
 lighest 
 
 10 
 
 3-93 
 
 3000 
 
 frequency) 
 
 
 
 
 
LECTURE V 
 
 THE INVISIBLE SPECTRUM (iNFRA-RED PARt) 
 
 How to sift out the invisible infra-red light from the visible light — ■ 
 Experiments on the absorption and transmission of invisible 
 infra-red. light — It is cut off by transparent glass, but trans- 
 mitted by opaque ebonite — Use of radiometer — Use of thermo- 
 pile and bolometer — ^" Heat-indicating " paint — Experiments 
 on the reflexion, refraction and polarisation of invisible infra- 
 red light — Discovery by Hertz of propagation of electric waves 
 • — Hertzian waves are really gigantic waves of invisible 
 light — Experiments on the properties of Hertzian waves ; their 
 reflexion, refraction and polarisation — Inference that all 
 light-waves, visible and invisible, are really electric waves of 
 different sizes. 
 
 To-day we deal with those waves of invisible light 
 which lie beyond the red end of the spectrum. They 
 are invisible to us because their wave-lengths are longer 
 than any to which the nerve-structures of our eyes are 
 sensitive ; or, to put it in the inverse way, because their 
 vibrations are of a frequency lower than any within our 
 range of optical perception. 
 
 Just as the ultra - violet waves have a shorter 
 wave-length and a higher frequency than the visible 
 waves, and have to be detected by their chemical, 
 luminescent, and diselectrifying effects, so the infra-red 
 
LECT. V THE INVISIBLE SPECTRUM 193 
 
 waves of larger wave-length and lower frequency have to 
 be detected and investigated by other physical effects 
 than that of sight. The chief physical effect produced 
 by these long infra-red waves is that of warming the 
 things upon which they fall. For this reason they are 
 sometimes called the calorific waves ; and the invisible 
 light of this kind is sometimes ^ called "radiant heat." 
 
 But if, as I shall have to show you, this so-called 
 radiant heat possesses (save in respect of visibility) all 
 the physical properties of light ; if we can reflect it, and 
 refract it, disperse it, diffract it, and polarise it, then we 
 are logically compelled to admit that it is really a kind 
 of light. 
 
 In brief, the spectrum extends both ways beyond the 
 visible part ; beyond the violet are the chemical waves, 
 and below the red are the heat waves. 
 
 If you find, as every one finds, that the light from the 
 sun or from a flame warms the things on which it 
 shines, it is natural to ask which of all the waves mixed 
 up together in the beam give the warmth. To answer 
 that question let us have recourse to the test of a 
 carefully considered experiment. First let us spread 
 out the rays into a spectrum, and then explore which 
 part has the greatest warming effect. 
 
 The first explorations of the spectrum were made by 
 
 putting into the different parts of the spectrum the 
 
 bulb (blackened) of a thermometer. This showed 
 
 ^ Another term used by some writers is "the radiation." This 
 use of the term is to be deprecated ; for the word radiation ought 
 not to be used in two senses. If it is rightly used to mean the act 
 of radiating^ then some other term ought to be used to denote that 
 which is radiated, namely, the waves. 
 
194 LIGHT LECT. 
 
 that the heating effect is mainly at and beyond the red 
 end of the spectrum. 
 
 The spectrum which is once again thrown on the 
 screen (Fig, io8) is produced as on previous occasions 
 by employing in the lantern an electric arc-lamp, in 
 front of which is placed a slit, a lens to focus an image 
 of the slit, and a prism to disperse the mixed waves 
 into their proper places according to their wave-length. 
 
 Our spectrum to-day is neither so brilliant nor so 
 extended as you have seen it on former occasions ; the 
 cause for this circumstance being that (for reasons you 
 will presently appreciate) we are obliged to abandon the 
 use of glass lenses and glass prisms, and substitute lens 
 and prism of rock-salt. This material is less refractive 
 and less dispersive, hence the narrowness of the rainbow- 
 coloured band. 
 
 And now we have to make good by experiment the 
 proposition that I have advanced that the heating effect 
 is due to the longest waves — those at the red end and 
 beyond the red end of the spectrum. 
 
 But as an ordinary thermometer would not be con- 
 venient I adopt another method, using instead a sort of 
 electric thermometer — the thermopile. If you want to 
 know all about this instrument, you must refer to 
 treatises on electricity. All I need say now about it is 
 that it is an apparatus. Fig. 109, which is exceedingly 
 sensitive to heat, and which, when the face of it is 
 warmed, generates an electric current. The electric 
 current is led into a galvanometer which reflects a spot 
 of light upon the scale against the wall. So, you may 
 take it that that spot of light will indicate by its position 
 
THE INVISIBLE SPECTRUM 
 
 195 
 
 whether the face of the thermopile is warmer or colder 
 
196 
 
 LIGHT 
 
 LECT, 
 
 than the air of the room. If it is warmer the spot 
 will move to the right ; if colder, to the left. 
 
 The spectrum now falls upon a small brass screen 
 with a slit in it, behind which is the thermopile ; and at 
 present the part of the spectrum that enters through the 
 slit and falls on the face of the pile is the ultra-violet 
 part. The spot of light is still at zero, showing that the 
 ultra-violet light does not appreciably warm the face of 
 
 Fig. 109. 
 
 the pile. I now explore the spectrum by pushing the 
 thermopile gently along. The slit now lies in the violet 
 — yet there is no heating effect. The blue, the peacock, 
 the green, and the yellow are successively explored — 
 yet the spot of light remains at the zero. These waves 
 do not produce appreciable heating. Another move 
 forward, and the orange waves enter the slit and fall on 
 the face of the pile — the spot begins to move. The 
 orange waves warm slightly. I push on into the red, 
 
V THE INVISIBLE SPECTRUM 197 
 
 and the spot moves gently across about a quarter of 
 the scale. Red waves heat more than orange ones. 
 Pushing on beyond the end of the visible red (Fig. 108) 
 the effect increases. At a point about as far beyond 
 the end of the red as the red is beyond the green of the 
 spectrum, the heating effect is much greater — the spot 
 flies across the scale. Clearly the waves which have the 
 greatest calorific power are those some little way in the 
 invisible infra-red region : or in other words the waves 
 that heat most are waves having a wave-length somewhat 
 greater than that of the largest waves of the visible 
 spectrum. Taking the size of the extreme red waves at 
 32 millionths of an inch, we may put down these more 
 powerful invisible waves as about 40 to 45 millionths 
 of an inch in length. 
 
 The invisible infra-red spectrum has often been 
 explored, and by many explorers. Langley, using a 
 different electric instrument of his own invention, 
 termed a bolometer^ has succeeded in observing waves 
 whose length was 592 millionths of an inch, or which 
 have a wave-length twenty times as great as those 
 of red light. Professor Rubens has independently 
 measured infra-red waves as large as 0*002400 centi- 
 metre, or about 944 millionths of an inch in length. 
 Hence, if set down in a scale of wave-lengths, the 
 infra-red spectrum stretches out to about fifty times 
 the extent of the visible spectrum. In the language 
 adopted for describing musical intervals, while the 
 range of visible light is about one octave (the extreme 
 violet having about double the frequency of the ex- 
 treme red), the infra-red waves are known to go down 
 
198 LIGHT LECT. 
 
 more than five octaves below, and the ultra-violet waves 
 ascend to about two octaves above the visible kinds of 
 waves (see the Table on pp. 190, 191). 
 
 Our thermopile has been placed at that part of the 
 spectrum, a little beyond the end of the visible red, 
 where we found the greatest heating effect. To 
 increase the effect somewhat, I will open the slit a 
 little, thus permitting a larger amount of these longer 
 waves to fall upon the face of the instrument. Having 
 thus adjusted our arrangements so as to be sensitive to 
 the heat-waves, we will try an experiment or two to find 
 whether these longer waves which produce the heating 
 effect are able to penetrate through the various materials 
 which we have tried for ordinary light. In the first 
 place, take a piece of window-glass, and try whether the 
 heat-waves will pass through it. On interposing it in' 
 front of the slit we notice, by the indication given by 
 the galvanometer and thermopile, that though it cuts 
 off much of the heat it does not cut it off entirely. 
 Substituting a piece of red glass, we find that it also 
 cuts off some of the effect, but a blue glass cuts it off 
 much more. Now I take a piece of flint glass, which 
 contains lead : you note that it cuts off the waves much 
 more. Here is a slice of quartz crystal ; it does not cut 
 off the effect as much as the glass did. Again, here is 
 a slice of calc-spar of the same thickness. It cuts off 
 the heat-waves more completely than any of the mate- 
 rials I have yet tried. Lastly, here is a slice, also of 
 the same thickness, of rock-salt ; that is to say, a slice 
 of a big crystal of common salt sawn off and polished. 
 The rock-salt hardly cuts off the heat-waves at all. 
 
THE INVISIBLE SPECTRUM 
 
 199 
 
 Here then are four substances — glass, quartz, calc-spar, 
 and rock-salt — all transparent alike to ordinary light. 
 Quartz, as we saw in the last lecture, is exceedingly 
 transparent to the ultra-violet kind of invisible light ; 
 that is to say, to the shortest waves. But to-day we 
 prove that rock-salt is the one that is most transparent 
 to the infra-red kind of invisible light. Naturally, seeing 
 that this fact was discovered half a century ago, we now 
 apply the discovery in the construction of our apparatus. 
 The lenses and the prism I am using to-day for these 
 experhnents are made neither of glass nor of quartz, 
 but of rock-salt. And as rock-salt possesses a very 
 poor dispersive power for the visible kinds of waves, it 
 produces, as you have seen, but 
 a poor spectrum of colours as 
 compared with the spectrum that 
 would be produced by the use 
 of a prism of glass or quartz of 
 the same size. 
 
 We might have used as an 
 exploring apparatus, instead of 
 our thermopile, another instru- 
 ment, the radiometer (Fig. no). 
 Rather more than twenty years 
 ago the celebrated chemist 
 Crookes made the discovery that 
 when light falls on movable 
 things, such as the vanes of a 
 lightly -poised mill, in a glass 
 bulb from which the air has 
 been mostly exhausted, so as to leave a fairly perfect 
 
 Fig. 1 10. 
 
2O0 LIGHT LECT. 
 
 vacuum, the vanes of the mill are driven round. 
 Apparently the blackened vane of the mill tends to 
 retreat from the light. Why, we must presently con- 
 sider. My present point is that it is possible to use 
 this apparent repulsion to measure the intensity of the 
 radiation that falls upon the instrument. Place the 
 radiometer in one part of the spectrum ; it turns round 
 slowly. Move it on into the red end ; it spins more 
 quickly. But the effect is found to depend not merely 
 upon the kind of waves, but also to some extent upon 
 the nature of the surface of the vanes, and upon the 
 degree of vacuum in the bulb. Some radiometers 
 revolve most rapidly in the bright part of the spectrum 
 to which our eyes are sensitive. 
 
 Whether we explore the spectrum with a thermo- 
 meter, as Sir William Herschel did, or with a thermo- 
 pile, or a bolometer, or a radiometer, we find that it 
 consists of waves spread out in different directions, and 
 that the different waves have different heating powers. 
 And yet all these different waves, with their different 
 powers, are emitted at one and the same time from the 
 same source. If the thing that is heated is insuffi- 
 ciently heated it will not shine — that we all know. 
 But even if not hot enough to shine visibly it will still 
 emit some invisible waves. When you begin to warm 
 a substance, at first it gives out only a few waves of 
 very long wave-length. As you heat it more it gives 
 out more of these heat-waves, and along with these 
 heat-waves it also gives out -some visible waves of shorter 
 wave-length. If heated still hotter, so as to be white- 
 hot, it gives out not only heat-waves of all sorts, but 
 
V THE INVISIBLE SPECTRUM 201 
 
 visible waves from red to violet, and also with ultra- 
 violet waves, all mixed up together. 
 
 In the next experiment we will employ a dark source 
 of waves. In short, we will test the waves that are 
 emitted from a vessel of hot water. Here is a beaker- 
 glass which I fill with boiling water from the kettle. 
 You would not see that in the dark, would you ? In a 
 perfectly dark room it would not give out any of those 
 waves to which your eyes are sensitive. But if you were 
 to hold your hand a few inches away from it you would 
 feel a gentle warmth radiating from it. You can feel 
 with the nerves of your hand that which the nerves of 
 your eyes do not perceive, namely, the long waves or 
 calorific radiations. But these long waves warm all 
 things on which they fall. They do not, however, warm 
 them all equally. The fact can be established in many 
 ways. Black and dark substances absorb the waves 
 that fall upon them. Bright and shiny bodies reflect 
 most of the waves. What becomes of the waves that 
 fall on black and dark bodies ? Their energy is not lost ; 
 it is transmuted into sensible heat. Instead of wave- 
 motions in the free space, we have molecular vibrations 
 in the substance. The bright surface, such as polished 
 metal, upon which the waves may fall, is not warmed 
 by them, for the waves as they meet the surface are 
 not broken up, but simply start off again in some new 
 direction. Whenever waves break on a surface, and 
 are destroyed or absorbed in the action of breaking, the 
 result is heat. The so-called heat-waves, or infra-red 
 waves, are not themselves hot. They do not heat the 
 medium through which they travel as waves. But they 
 
202 
 
 LIGHT 
 
 LECT. 
 
 are readily absorbed by the things they fall upon, and, 
 being absorbed, they warm that on which they fall. A 
 black surface is one which absorbs both the invisible 
 and the visible waves. It heats more readily than a 
 white or a bright surface. 
 
 Now here is a peculiar thermometer (Fig. iii), 
 having two bulbs, full of air, joined together by a bent 
 
 tube, containing a little coloured 
 liquid to serve as an index : it 
 
 fl I^H r^ stands up to the height marked 
 
 ^^^^L If a. If I put my hand on either 
 
 \kllij;J^j'j<jl' J 
 -^^•^■^ bulb, and so warm the air in- 
 
 side it, the expansion of the air 
 will depress the liquid in the 
 tube below the bulb that is 
 warmed. Were both warmed 
 equally the liquid would not 
 move. So this apparatus will 
 ^^^' indicate a difference of tempera- 
 
 ture, and is therefore called a differential thermometer. 
 Next, note that one of the bulbs, B, has been painted dull 
 black, the other, G, has been gilt with gold-leaf. I am 
 going to put the beaker of hot water exactly in the middle 
 between the two bulbs, where it can radiate equally to 
 both of them. The gilt bulb, having a bright surface, 
 reflects the waves, and is scarcely warmed at all by 
 them. But the black bulb, having a more absorptive 
 surface, will be warmed more than the bright gilt one ; 
 and you will see the indicating liquid fall in the tube 
 below B and rise in the tube below G. Had we 
 employed a beaker half blackened and half gilt, we 
 
THE INVISIBLE SPECTRUM 
 
 203 
 
 could have readily demonstrated another point, namely, 
 that a hot black surface radiates out the heat-waves 
 more readily than a hot bright surface does. 
 
 Now let us pass on to another experiment in which 
 we again employ the thermopile. Here is a thermopile 
 connected by wires to the galvanometer, with its re- 
 flected spot of light on the wall. It is arranged so that 
 if the face of the thermopile is warmed the spot will 
 move to the right and indicate to you the circumstance. 
 In front of the conical mouth-piece of the thermopile 
 
 Fig. 112. 
 
 stands an ordinary Bunsen burner, which, as you know, 
 is a gas jet having openings at the foot to let atmospheric 
 air mix with the gas. It gives a smokeless blue flame 
 very different from the ordinary bright flame of gas. 
 Though very hot, this flame radiates out but little light, 
 neither does it, as a matter of fact, radiate off much heat. 
 True, a column of hot air ascends from it straight up. 
 But that is not what I am thinking of. The question 
 is. Is it sending out heat sideways ? Well, we can try. 
 Opening the metallic shutter that closes the mouth- 
 piece of the thermopile I let the light and heat, such as 
 they are, radiate from the flame upon the face of the 
 
204 LIGHT LECT. 
 
 pile. At once the spot of light on the wall moves off 
 to the right, showing that there are at any rate, some 
 waves present that can heat the pile. If I interpose for 
 a moment a sheet of glass between the burner and the 
 thermopile the spot of light comes back almost to its 
 zero, showing that glass screens off nearly all the waves. 
 I remove the screen, and the spot goes back to the 
 right, showing that the heating effect has recommenced. 
 Now comes the particular point of the experiment. If 
 I stop up the holes at the foot of the burner where the 
 air has been entering, the flame will at once burn 
 brightly as an ordinary gas flame. The combustion 
 will, as a matter of fact, be less perfect, for the flame 
 will be sooty, and the total amount of heat produced in 
 a given time will be less, because of the imperfection of 
 the combustion. But also because of the imperfection- 
 of the combustion there are innumerable solid particles 
 formed in the flame which get brilliantly heated, and 
 emit light. They are better radiators of waves than 
 the gaseous particles of the pale blue flame, and they 
 radiate long waves better. To make the flame shine 
 thus, I have but to stop up the air-holes with my finger 
 and thumb ; and instantly the spot of light on the wall 
 rushes to the right, even beyond the end of the scale, 
 proving that the bright flame radiates more heat-waves 
 to the pile. I take away my fingers, air is readmitted, the 
 flame relapses to its former pale state, and the spot of light 
 settles back to its former position. Every time I let the 
 flame burn brightly it radiates more waves sideways. 
 
 You may use a radiometer instead of a thermopile 
 to demonstrate the facts. The vanes of the mill turn 
 
V THE INVISIBLE SPECTRUM 205 
 
 fast when the flame is bright, and more slowly when, 
 by admitting air to the flame, you improve the com- 
 bustion. 
 
 The next experiment I have to show you is with the 
 same thermopile, only, instead of shining upon it with a 
 flame, I will put a lump of ice in front of it. The spot 
 of light on the wall now retreats, right beyond the zero 
 mark, to the left, indicating that the face of the thermo- 
 pile has been chilled. Perhaps you will say that this 
 proves that the ice is radiating out cold. It may seem 
 so. But that which is really occurring is this. " Cold " 
 is a relative term meaning really "less hot." All things 
 that are not in that unattainable state of absolute zero 
 of temperature are more or less hot ; hot things more, 
 cold things less. And everything tends to radiate its 
 heat away — the hotter it is the greater its tendency. 
 Ice is less hot than the other things in this room. The 
 ice is colder than the thermopile. The thermopile 
 itself is radiating out heat, some of which goes to the 
 ice. The ice is also, though to a lesser degree, radiating 
 out heat. Here then we have two things, a thermopile 
 which is warm, and ice which is colder, radiating to one 
 another unequally. The result of this unequal exchange 
 is that the thermopile parts with more heat than the ice 
 parts with, and therefore is cooled. But the effect is 
 the same as if the cold were being radiated. 
 
 Now let us go to an experiment that I believe took 
 its origin nearly a century ago in this Royal Institution. 
 In the Royal Institution the emission of heat and the 
 properties of heat-waves have ever been favourite topics 
 of study. The founder of the Royal Institution, Count 
 
2o6 
 
 LIGHT 
 
 LECT. 
 
 Rumford, himself originated many experiments on the 
 radiation of heat. It was he who discovered that heat 
 could be radiated across a vacuum. Sir Humphry 
 Davy, while Professor here, showed a most beautiful 
 experiment in which heat-waves were reflected from one 
 point of space to another by means of two paraboloidal 
 mirrors of silvered metal. That experiment I propose 
 to repeat, using the two mirrors which are believed to 
 be the actual pair used by Sir Humphry, and often 
 since used by Professor Tyndall. 
 
 One of the two curved mirrors is hung mouth- 
 downwards at a height of some fifteen feet above the 
 
 lecture-table. The other stands 
 mouth -upwards on the table 
 exactly beneath the first. The 
 upper mirror is lowered for a 
 moment. A red-hot iron ball 
 is slung by a hook in the focus 
 of the mirror, and it is hoisted 
 up again into its position above 
 the table. The ball being at 
 the focus, the mirror collects 
 the diverging waves and reflects 
 them straight down in a parallel 
 beam. It is a beam of invisible 
 infra-red waves, accompanied 
 by a few waves of visible red. 
 This beam falls upon the second 
 mirror (Fig. 113), which once more collects them and 
 converges them to a focus, F. I put my hand at 
 the place toward which the waves converge ; it is 
 
 Fig. II- 
 
 1 
 
V THE INVISIBLE SPECTRUM 207 
 
 intolerably hot. I hold in the focus a bit of black 
 paper, at once it smokes, and kindles into visible com- 
 bustion. Other things can be lighted. Here is a 
 cigar. Holding it in the focus it absorbs enough waves 
 to warm it up ; and the ascending wreath of smoke 
 proves that it has been kindled by the reflected and 
 concentrated waves. 
 
 Our experiment has proved that heat-waves can be 
 reflected. Our earlier experiments with the rock-salt 
 prism proved that they can be refracted. Let us con- 
 firm these points by other experiments. 
 
 The red-hot ball, fast fading into dulness as it parts 
 with its store of energy, has now been placed upon a 
 stand on the table. Taking up the thermopile, which 
 did such useful service just now, we will see what it can 
 tell us about that red-hot ball. The distance between 
 the two is some seven or eight feet. I have, however, 
 only to turn the conical mouth-piece of the thermopile 
 straight toward the ball, and at once the spot of light 
 on the wall indicates that the thermopile has received 
 some of the radiation. Waves that our eyes cannot see, 
 this thermopile can see if only we turn it so as to look 
 straight at the source of the waves. It acts as a kind 
 of eye that is sensitive, not to visible light, but to infra- 
 red waves of invisible light. I turn the aperture of the 
 pile on one side so that none of the heat-waves can 
 enter it ; the spot of light on the wall settles down to 
 its zero. Then, taking up a simple piece of tin-plate 
 to serve as a mirror I reflect some of the heat-waves 
 from the iron ball back into the mouth of the thermo- 
 pile. As soon as the mirror is set at the proper angle 
 
208 LIGHT LECT. 
 
 the spot moves to the right, showing that we have 
 reflected some of the heat-waves into the pile. 
 
 While we have a hot ball — best if we had one that 
 was brightly red-hot — I can show some interesting experi- 
 ments which turn upon the employment of certain heat- 
 indicating paints.^ Here is a specimen of heat-indicating 
 paint of a scarlet colour that turns black when heated. 
 Here is another of pale yellow tint which turns red even 
 when quite gently warmed. Here is a paper screen 
 mounted in a convenient frame. The front is painted 
 over with yellow heat-indicating paint : the back has been 
 blackened that it may the better absorb the heat-waves. 
 
 ^ These heat-indicating paints are double iodides of mercury with 
 other metals. They were discovered nearly thirty years ago by Dr. 
 Meusel. The scarlet paint that turns almost black at about 87° C. is 
 the double iodide of mercury and copper. The yellow paint which- 
 turns red at about 45° C. is the double iodide of mercury and silver. 
 To prepare the former, a solution of potassium iodide is added to a 
 solution of copper sulphate until the precipitate is redissolved, when 
 a concentrated solution of mercuric chloride is added precipitating 
 the red double-iodide. To prepare the more sensitive yellow paint, 
 add to a solution of silver nitrate a solution of potassium iodide 
 until the precipitate (silver iodide) redissolves. To this solution add 
 a concentrated solution of mercuric chloride until a bright yellow 
 precipitate is formed. The precipitates are collected on filter paper, 
 and should be washed with cold water. They may be mixed 
 with very dilute gum-water to enable them to be used as paint. 
 With these paints many interesting experiments can be performed in 
 illustration of the propagation of heat by conduction and convexion 
 as well as by radiation. One very simple experiment is worthy of 
 mention. It is to show how hot water will float on cold water. A 
 strip of paper painted with the yellow paint is pasted vertically 
 against the outside of a tall glass beaker. This is half filled with 
 cold water. A floating disk of wood is introduced to prevent undue 
 agitation, and then the beaker is filled up by pouring in boiling water 
 out of a kettle. The top half only of the strip of paper turns red. 
 
V THE INVISIBLE SPECTRUM 209 
 
 Holding it a little way from the hot ball — an ordinary 
 coal fire answers even better — I place my hand between 
 the ball and the screen, against the back of it. In a 
 few seconds the screen turns red all over except where 
 it is protected by my hand, of which a shadow — a sort 
 of heat-shadow — in yellow is temporarily photographed, 
 or rather thermographed, upon the screen. As the 
 screen cools it returns to its former yellow tint. 
 
 Here is another screen made of paper painted with 
 scarlet heat-indicating paint. The back has been gilt 
 all over, and then on the gilt surface a big letter S has 
 been painted. I hold this with its gilt back to the hot 
 ball ; and the gilt surface reflects away most of the heat- 
 waves. Now you might suppose that the part where 
 black paint has been put on over the top of the gold 
 would be doubly protected against heat. But, no ! It 
 absorbs the waves and grows warm ; and the heat being 
 conducted through the gold film causes the scarlet paint 
 on the front of the screen to turn black. The letter 
 painted on the back is visible on the front of the 
 screen. 
 
 Here is a variation upon one of Professor Tyndall's 
 observations. You will find it recorded in his book^ 
 on heat how, on one occasion when a fire broke out in 
 a street, the heat radiated across the street from the 
 burning house, charred the window-frames and burned 
 and blistered the paint on the sign-boards. But where 
 the number of the house stood in gilt letters on the 
 sign-board the mere film of metal had reflected away 
 the waves, protecting paint and wood behind it from 
 
 ■^ Heat a Mode of Motion, p. 263. 
 
2IO LIGHT LECT. 
 
 being charred. In illustration, here is a blackened 
 sheet of paper upon which a triangle of gold - leaf 
 has been pasted. The other surface of the paper has 
 been coated with the scarlet heat -indicating paint. 
 Exposing it to the radiation of the hot ball you see how 
 the triangular space protected by the gold-leaf remains 
 cool, while the rest absorbs heat, turning the scarlet to 
 black. 
 
 You will probably admit that we have now plenty of 
 proofs that these invisible heat-waves are really a kind 
 of invisible light : that the difference is one of degree 
 rather than of kind. Consider yet again the process of 
 incandescence in which such waves are emitted. Heat a 
 body, beginning by gently warming it. At first its particles 
 vibrate but moderately ; the waves they send out into the 
 surrounding ether are few and of relatively great wave-' 
 length. As you warm the body more and more its 
 particles vibrate more actively, they jostle together ; it 
 gives out more waves and waves of shorter length and 
 higher frequency. There are still the long waves, in fact 
 there are more long v/aves than before, but there are some 
 shorter waves in addition. Heat it still hotter. The 
 lower kinds of waves still continue to be emitted, nay, are 
 emitted more copiously, but some waves of a still higher 
 kind now accompany them. Here is a thin platinum 
 wire stretched between two supports. By leading into it 
 through a thicker copper wire an electric current I can 
 heat it as little or as much as I choose. It is now warm, 
 giving out a few dull waves. Increasing the current its 
 temperature is raised, and now it gives out much more 
 heat, and with the heat a few waves of the visible red 
 
V THE INVISIBLE SPECTRUM 211 
 
 sort. Every solid body when heated shows red as its 
 first colour on heating. Never is the first glow^ of a 
 blue or yellow hue. Increasing the temperature of the 
 wire it emits orange light as well as red, and looks there- 
 fore bright red. The next increase brings in yellow 
 along with orange and red : then green comes in to join 
 the yellow, orange, and red. So soon as the wire is 
 heated so hot as to give out all the different visible 
 kinds, so soon we call the state a white heat. But 
 no solid ever gets blue hot, because in all cases the 
 emission begins at the bottom of the spectrum with red, 
 the other colours chiming in until white is attained. 
 Nor is white ^ attained until a certain proportion of the 
 still higher ultra-violet waves are being also emitted. So 
 then it appears that the process by which visible light- 
 waves are emitted is only a continuation of the process 
 by which the invisible infra-red waves are emitted. What 
 further proofs do you require as to the essentially kindred 
 nature of the visible and invisible waves ? I have shown 
 you that these infra-red waves behave as visible light- 
 
 ^ Captain Abney has shown, however, that owing to the want of 
 sensitiveness of the eye for red light, and its greater sensitiveness for 
 green light, the tint of minimum visible luminosity of any hot body 
 or indeed of any feebly illuminated body in a perfectly dark room is 
 greenish. This is true even of a light seen through ruby glass if the 
 eye has been kept some time in darkness. 
 
 2 The whitest known artificial light is that of the arc-lamp ; it is 
 the light of carbon incandescent at about 3500° C. This is a 
 temperature considerably lower than that of the sun's surface, which 
 emits a light having a relatively higher proportion of blue and violet 
 and of ultra-violet waves. In fact, when seen in full sunlight the 
 light of the arc-lamp is decidedly dull and reddish. No accurate 
 definition of any standard of whiteness has ever been given. 
 
212 LIGHT LECT. 
 
 waves do in a number of respects. You have seen that 
 we can refract them with a lens, disperse them with a 
 prism, reflect them with a mirror, and absorb them with 
 a black surface. Further, they travel at the same rate 
 across space as the visible waves do. This we know 
 from that which happens at the time of a total solar 
 eclipse. At the moment when the sun's light ceases to 
 be visible, his heat ceases also to reach us. When the 
 light reappears the heat-waves are also restored. This 
 one fact proves these heat-waves to be simply light of 
 an invisible kind. But if you are not satisfied I will give 
 you yet one further proof. You shall see that they can 
 be polarised. 
 
 Here, as in my third lecture (Fig. 93, p. 132), stand a 
 pair of Nicol prisms, one to serve as polariser and the 
 other as analyser. The lantern sends its beams through 
 them. Receiving the visible light on a paper screen we 
 note that when the analyser is set with its principal 
 plane parallel to that of the polariser light is transmitted : 
 but on rotating the analyser through a right angle all 
 light is cut off. That is a purely optical experiment. 
 Now let me take my thermopile — which I described to 
 you as a sort of eye which is sensitive to the invisible 
 heat-waves — and put it in the place where the paper 
 screen was. At present the polariser and analyser are 
 crossed, giving the "dark field" (p. 119). No light falls 
 on the thermopile, nor any heat-waves, for, see, the spot 
 of light from the galvanometer that indicates the state of 
 the thermopile is at its zero point. Now I turn back 
 the analyser and restore the bright field. At once the 
 spot of light on the scale swings over to the right, telling 
 
 I 
 
V THE INVISIBLE SPECTRUM 213 
 
 us that the polarised heat, as well as the polarised light, 
 is coming through the analyser. Turn the analyser 
 back again, the visible waves are cut off, and so are the 
 invisible ones, for the spot of light has returned. Now 
 let me clinch the proof by working entirely with invisible 
 waves to the exclusion of visible ones. Here is a sheet 
 of opaque hard black indiarubber, of ebonite in fact. No 
 visible light will come through it. But yet, you observe, 
 when I have thus filtered out the invisible waves, and 
 stopped off the visible ones, still there come through the 
 polariser some waves which can warm the thermopile, 
 and which can be cut off by turning the analyser to the 
 position at right angles. 
 
 This material ebonite is a most interesting one from 
 the circumstance that it can thus act as a wave -filter 
 transmitting only the longer waves. Wave -filters (or 
 ray -filters) were extensively used by Professor Tyndall 
 in his lectures on radiant heat : but I do not think he 
 was acquainted with the properties of ebonite. Here is 
 one of the Crookes radiometers (Fig. no, p. 199). I 
 place it in front of the lantern but screen it at first by a 
 thick sheet of metal so that it is all but at rest. The 
 diffused light in the room suffices to make it turn slowly. 
 I substitute for the metal sheet a sheet of ebonite which 
 is equally opaque to ordinary light. Yet the little vanes 
 now run merrily round. 
 
 Tyndall's filter for heat-waves consisted of a cell con- 
 taining a dark solution of iodine in bisulphide of carbon. 
 Here is one of the cells, kept cool by an outer jacket in 
 which cold water circulates. Behind the wall at the back 
 of the theatre is a powerful electric arc-lamp, the beams 
 
214 LIGHT LECT. 
 
 of which pass into the theatre through an aperture. 
 This beam I propose to concentrate by a rock-salt 
 lens, bringing it to a focus, after it has passed through 
 the cell that filters out the invisible waves and stops the 
 visible ones. First we make the experiment without the 
 cell. All the waves visible and invisible come to a focus. 
 Holding at the focus a bit of black paper it smoulders 
 and then takes fire. Now^ interpose the cell. The 
 visible light is cut off; but holding the bit of paper in 
 the invisible focus it again begins to smoulder and finally 
 breaks into visible burning. 
 
 And now I have to pass to the most important recent 
 discoveries — discoveries dating only from 1888 — of some 
 larger waves which are exactly like light -waves in the 
 following respects : they can be reflected, refracted, 
 absorbed, polarised, and diffracted. Yet they differ in 
 the most striking way from any of the waves of light 
 that we have hitherto considered. Their wave-lengths, 
 instead of being measured by a few millionths of an 
 inch, may be several inches, several yards, or even several 
 hundreds of yards long. I refer to the electric zvaves pre- 
 dicted in 1864 by the late Professor Clerk Maxwell, and 
 discovered experimentally in 1888 by the late Professor 
 Hertz. 
 
 Hertz was occupied with researches upon electric 
 sparks, which, under certain circumstances, were known 
 to be oscillatory. That is to say, each spark might, 
 under certain conditions, consist of a series of sparks 
 flying backwards and forwards along the same path with 
 great regularity and excessive rapidity. If, for instance, 
 there were twenty successive oscillations, each lasting 
 
V THE INVISIBLE SPECTRUM 215 
 
 only one one hundred-millionth part of a second/ the 
 whole series would only last one five-millionth part of a 
 second, and would, of course, seem to the eye as siniply 
 an instantaneous spark. In working with these oscilla- 
 tory sparks .Hertz was led to investigate the disturbances 
 which they set up in the surrounding medium, and 
 
 Fig. 114. 
 
 which are propagated as waves. To illustrate Hertz's 
 work I must have recourse to a few diagrams. Fig. 114 
 illustrates the apparatus which is set up on the table. 
 To produce the sparks we employ an induction-coil. 
 The electric discharges produced by the coil are sent 
 into the simple apparatus called by Hertz an oscillator. 
 As you see it consists of two square sheets of metal, 
 affixed upon two metal rods that nearly meet and are 
 
 •^ It may be useful to note that since the velocity of propagation 
 of electric waves in air (or vacuum) is identical with that of light 
 (186,400 miles per second, or 30,000,000,000 centimetres per second), 
 the wave-length can be deduced from the frequency by the rule that 
 the product of frequency and wave-length is equal to that velocity. In 
 tiie above example, if the period is one one hundred -millionth of a 
 second, the frequency is one hundred million a second ; dividing 
 30,000,000,000 by 100,000,000 we get as the wave-length 300 centi- 
 metres, or about ten feet as the wave-length. 
 
2i6 LIGHT LECT. 
 
 ^provided with two well-polished metal balls as terminals. 
 There is a small gap between which the sparks are seen 
 to pass. But each such spark is really a series of oscil- 
 lations ; the electric discharge oscillating backwards and 
 forwards, not simply across the gap where you see the 
 spark, but from one end to the other of the apparatus. 
 Suppose the coil to make one of these metal wings (say 
 A in Fig. 114) positive, while the other wing (B in Fig. 
 114) is negative. When the electric state has risen 
 sufficiently high, the air in the gap is pierced by a spark. 
 
 OSCILLATOR. RESONATOR. 
 
 Fig. 115. 
 
 The charge rushes from A to B, and in so doing over- 
 charges B, making it positive, while leaving A negative. 
 At once the charge surges back again from B to A, and 
 again back to B, each oscillation lasting only about the 
 one hundred-millionth part of a second. The frequency 
 of the oscillations depends on the size of the apparatus. 
 At every oscillation an electric wave is sent off from the 
 apparatus into the surrounding space, and is propagated 
 with the velocity of light. The wave is propagated with 
 the greatest intensity in the directions at right angles to 
 the metal rods along which the electricity is oscillating, 
 and at right angles to the plane of the metal wings. 
 Fig. 115 gives a front view of the oscillator, and also of 
 the apparatus called the resonator used by Hertz for 
 
V THE INVISIBLE SPECTRUM 217 
 
 detecting the waves. The model on the table is made 
 of the same size as one of Hertz's smaller pieces of 
 apparatus, the two metal wings being each 40 centi- 
 metres square, and the distance between them 60 
 centimetres. The wave-length of the waves emitted is 
 rather less than 300 centimetres, or nearly 10 feet. 
 The resonator or detector is a simple wire, bent into a 
 ring so that its two ends nearly meet. Hertz demon- 
 strated the fact that waves pass from the oscillator by 
 holding the resonator some distance away from it, and 
 observing the minute electric sparks which they set up 
 in the small gap between the ends of the wire. But it is 
 necessary that the resonator ring should be of the proper 
 size, and that it should be held in the right position. 
 The size (in this example 70 centimetres diameter) should 
 be such that the natural period of oscillations of an elec- 
 tric current around the ring should agree with the period 
 of the waves emitted by the oscillator. The position 
 should be such that as the waves from the oscillator 
 reach the resonator they set up secondary oscillations 
 in the ring. If the resonator is set up vertically edge- 
 ways to the oscillator, no sparks are produced : the 
 waves simply stream past the resonator. If, however, 
 the resonator is held horizontally, and in the base-line 
 shown in Fig. 114, sparks may be detected in the gap. 
 Hertz put at the far end of the room where he was 
 .working a great sheet of metal to reflect back the waves, 
 and then went about to different positions in the room 
 exploring the space to find at what points sparks were 
 produced. He found that when the waves are thus re- 
 flected back on themselves there are nodal points, just 
 
2l8 
 
 LIGHT 
 
 LECT. 
 
 as there are nodal points in sound-waves and in light- 
 waves when reflected back. These nodal points were 
 spaced out at distances apart exactly equal to half the 
 wave-length, which thus could be precisely measured. 
 
 Before Hertz's time it was indeed known that there 
 were oscillating sparks. Fig. ii6 illustrates some experi- 
 ments which I myself made -^ in the year 1876 on this 
 subject. I had an induction coil connected to send sparks 
 
 Detector 
 
 Fig. 116. 
 
 across a small air-gap, A, to a condenser made of a dielec- 
 tric, D, between two metal plates, P and Q. I found that 
 if there was this spark-gap in the circuit of the coil I could 
 draw secondary sparks at B from the outer plate of the 
 condenser ; and by means of a small vacuum-tube and a 
 rotating mirror I proved that these sparks were oscilla- 
 tory in character. When these arrangements were made 
 I was able to get sparks from insulated metal objects in 
 the room. These sparks could be traced all about the 
 room. I had but to hold a knife or pencil-case to the 
 
 ^ Philosophical Magazine, September 1876. 
 
Fig. 117.— Professor Heinrich Hertz. 
 
THE INVISIBLE SPECTRUM 
 
 219 
 
 door-knob or other piece of metal to draw sparks. I 
 even did this : I took two door-keys and tied them on to 
 a piece of wood, so as almost to touch one another, and 
 with this detector I could get sparks while walking about 
 to different parts of the room. But it never dawned 
 upon me that these sparks were the evidence of electric 
 waves crossing the space. That was Hertz's discovery. 
 He did not go idly about the room noticing the sparks, 
 but explored the positions where the sparks were to be 
 detected, and holding his apparatus in the right position 
 to detect them. 
 
 A word more about the electric oscillations them- 
 selves. Each sudden discharge of the induction coil — and 
 to make them sudden 
 the discharge balls 
 must be well polished 
 — sets up a set of 
 oscillations, diagram- 
 matically represented 
 in Fig. 118 by the 
 upper curve, which die 
 away as time goes on. 
 A mechanical analogy 
 may be found in the 
 vibrations of a spring 
 denly release it 
 
 Fig. 118. 
 
 Bend it on one side and sud- 
 It flies backward and forward, the 
 motion dying out after a certain number of swings. 
 So trains of waves are set up, which also die away as 
 they travel across space. But suppose they fall upon a 
 proper resonator or detector, then they will set up, by 
 their timed impulses, a sympathetic electrical vibration 
 
 Q 
 
22 O 
 
 LIGHT 
 
 LECT. 
 
 in that resonator, the oscillations thus set up beginning 
 and increasing in strength as wave after wave arrives. 
 This is represented graphically by the lower curve in 
 Fig. 1 1 8. This corresponds, in fact, to the way in which 
 the sound-waves from a tuning-fork, when they fall upon 
 another tuning-fork, will set it into sympathetic vibration, 
 provided it is tuned to the same note. 
 
 In Fig. 119 are represented the parabolic mirrors, each 
 about 6 feet high, with which Hertz demonstrated the 
 
 Fig. 119. 
 
 reflexion of electric waves. At the focus of one of these 
 mirrors there was placed vertically an oscillator, an 
 arrangement to produce sparks vibrating up-and-down. 
 The waves which resulted were, of course, waves of up- 
 and-down motion — polarised in a vertical plane — which 
 were reflected in a beam straight across the room to the 
 second mirror, which collected them and reflected them 
 to a focus upon a detector, which in this case was 
 straight, not circular, with a small spark-gap at its middle, 
 where the minute sparks could be detected. 
 
 Many forms of oscillator or vibrator have been used 
 
 I 
 
THE INVISIBLE SPECTRUM 
 
 221 
 
 by different experimenters to produce electric waves. 
 Some of these are shown in Fig. 120. The first is one 
 of those used by Hertz himself. Instead of flat wings of 
 metal he used in this case cylindrical metal conductors. 
 In another form, described as a dumb-bell oscillator, 
 there were two large metal balls. In every case the 
 spark-gap was arranged between two small highly- 
 polished metal balls, midway along the length of the 
 oscillator. The third shape is one devised by Professor 
 
 Hertz 
 
 Hertz 
 
 Righi for making short waves. Here there are three 
 gaps, the central one being between two balls immersed 
 in an oil vessel to prevent premature discharges. The 
 lowest form in Fig. 1 20 is also of Professor Righi's devis- 
 ing. It represents his apparatus for producing exceed- 
 ingly short waves — less, in fact, than an inch long — by 
 the oscillations set up between two spheres to which 
 sparks were communicated from two smaller terminal 
 balls outside them. 
 
 The next diagram (Fig. 121) depicts two forms of 
 
222 
 
 LIGHT 
 
 LECT. 
 
 oscillator used by Professor Oliver Lodge, of Liverpool. 
 Here is a well-polished metal ball supported between 
 two smaller balls that nearly touch it, one on each side. 
 When a discharge is made through this central ball, an 
 electric charge surges from side to side in it with great 
 vigour, but the motion dies out after only about three or 
 four such surgings, since it readily radiates its energy 
 into space. It does not emit a long train of waves ; the 
 
 Fig. 121. 
 
 Fig. 122. 
 
 eifect dying out after about i| or 2 complete oscillations. 
 The wave-length of the emitted waves is about i^- times 
 the diameter of the ball. The other form. Fig. i2\b^ 
 shows a metal cylinder which is sparked into at opposite 
 ends of a diameter by two interior balls. This oscillator 
 emits its energy less rapidly, and the oscillations last 
 longer. It gives rise to a train of waves which are pro- 
 pagated chiefly straight out of the mouth of the cylinder. 
 Fig. 122 depicts one of the simplest ways of detecting such 
 
V THE INVISIBLE SPECTRUM 223 
 
 electric waves, and at the same time makes them evident 
 to a whole audience. An ordinary gold-leaf electroscope 
 is provided with a by-pass of wire arranged with a minute 
 gap (adjustable by a^ screw) to break the continuity. If 
 the gold leaves are carefully charged they will remain 
 diverging, because their electric potential has not been 
 raised sufficiently to cause a discharge across the gap. 
 But if now an electric wave from a Hertz oscillator — 
 especially from an oscillator set vertically to produce 
 vertically polarised waves — falls on the electroscope, it 
 sets up in the wire by-pass an electric surging that will 
 overleap the gap with a minute spark. And, during the 
 
 Fig 123. 
 
 time that the spark bridges the gap the gold leaves dis- 
 charge themselves and fall together. 
 
 A still more sensitive detector used by Lodge consists 
 of a bit of glass tube filled loosely with iron filings 
 (Fig. 123) and joined along with a weak voltaic cell in 
 circuit with a galvanometer. Loose, metal filings or 
 powders form a very bad and incoherent conductor: 
 hardly any current passes through them. But let an 
 electric wave fall on the tube, instantly the filings become 
 — as discovered by M. Branly — an excellent conductor. 
 So there results a movement of the galvanometer, and of 
 the spot of light reflected by it, proving that an electric 
 wave has been detected by the tube. 
 
 Fig. 124 shows a set of the apparatus with which 
 
224 
 
 LIGHT 
 
 LECT. 
 
 Professor Lodge repeated and verified the observations of 
 Hertz as to the optical properties of these electric waves. 
 Hertz had reflected them with parabolic mirrors 6 feet 
 high, and refracted them with huge prisms of pitch. He 
 found they could penetrate through wooden floors and 
 stone walls. He polarised them and diff'racted them. 
 Lodge was able to repeat these results, but with an 
 apparatus of less heroic dimensions. The sender con- 
 
 FlG. 124. 
 
 sists of an oscillator, like Fig. 121^:, having a 5-inch ball 
 emitting 7-inch waves, enclosed in a copper box furnished 
 in front with a diaphragm perforated with apertures of 
 various sizes to moderate the radiation to any desired 
 degree. As detector, D, there is used a tube full of 
 coarse iron filings put at the back of a copper hat, whose 
 open end is turned in any direction in which waves are 
 to be received. Wires pass from the detector to the 
 galvanometer, G, and are enclosed in a metal tube to 
 shield them from stray radiations. If the receiver is set 
 
THE INVISIBLE SPECTRUM 
 
 225 
 
 obliquely to the sender so that no waves from the sender 
 enter the receiver, the galvanometer will give no indica- 
 tions. But if the waves from the sender are reflected 
 into the mouth of the receiver by holding in front, at the 
 proper angle, a sheet of metal, at once the detector is 
 affected, and the galvanometer reveals the fact. Simi- 
 larly, as shown in the diagram, a prism of paraffin wax 
 may be used to refract the electric waves into the mouth 
 
 Fig. 125 
 
 of the receiver. The picture also shows a grid of metal 
 wires which can be used to polarise the electric waves. 
 
 More recent than the researches of Professor Lodge 
 are those of Professor Righi, whose apparatus is shown 
 in Fig. 125. It consists of two parts, a sender and a 
 receiver. The sender, on the left, consists of a small 
 oscillator (that shown at the bottom of Fig. 120), with 
 three spark-gaps, the central gap being capable of fine 
 adjustment. In Fig. 125 this gap is between the ball 
 marked V, and one below it (not shown), enclosed in a 
 small leather capsule filled with vaseline. This oscil- 
 lator is set at the focus of a parabolic mirror, M, to reflect 
 
226 LIGHT LECT. 
 
 the waves out straight to the right across the central 
 table C. The detector, D, which also is furnished with a 
 parabolic mirror, is an optical one. It is made of a film 
 of silver upon a slip of glass, the film being divided in 
 two across the middle with a diamond cut. Across this 
 narrow gap minute sparks pass, and are viewed through 
 an eye-piece at D. The apparatus is quite small enough 
 to be put upon an ordinary table, and presents quite the 
 appearance of a piece of optical apparatus. Upon the 
 central table can be mounted reflectors, prisms, lenses, 
 grids, or any other apparatus. With these devices 
 Professor Righi has tracked down the optical properties 
 of the electric waves varying from 8 inches down to 
 I inch in length. He has demonstrated interference 
 fringes by Fresnel's mirrors, and with the biprism, with 
 thin plates, and by diffraction. He has verified the 
 laws of refraction, reflexion, total reflexion, polarisation, 
 and of elliptical and circular oscillations. He also 
 investigated the transparency of media and the selective 
 transparency of wood according to its grain, a property 
 which makes it polarise the electric waves just as tour- 
 maline polarises light. In short, he has completed the 
 proofs that these waves possess all the known properties 
 of ordinary light. Other workers have occupied them- 
 selves in the same field. We are shortly to hear a 
 discourse here ^ by Professor J. Chunder Bose, of Calcutta, 
 
 ^ Given Friday, January 29, 1897. Professor Bose's oscillator 
 is depicted in Fig. 126 ; it is made of a small ball of platinum 
 between two smaller balls. Single sparks are given to this from a 
 small induction-coil. A cylindrical lens of ebonite in front of this 
 oscillator renders parallel the emitted waves. The complete 
 apparatus is shown in Fig. 127, The oscillator or sender S enclosed 
 
THE INVISIBLE SPECTRUM 
 
 •227 
 
 upon the polarisation of the electric wave as studied by 
 him, with an exceedingly elegant apparatus producing 
 still shorter waves. 
 
 But before I close I must show you at least some of 
 
 in a metal tube projects from a box A, doubly cased in metal ; 
 which contains the induction-coil and battery. The detector D 
 consists of a number of 
 small metal springs 
 lightly pressing against 
 one another, traversed 
 by a current from a 
 single cell C in the 
 circuit of which is in- 
 cluded the galvanometer G. The detector D is set up on a 
 movable arm on an optical circle, so that the optical properties 
 of the electric beam may be studied. M is a plane mirror, N 
 a curved mirror for studying the laws of reflexion, P is a prism 
 of ebonite, T a special apparatus for observing the total reflexion 
 
 Fig. 126. 
 
 Fig. 127. 
 
 at films of liquid enclosed between two semi-cylinders of ebonite, 
 H is a holder in which pieces of minerals can be clamped for 
 observation. Professor Bose has found that many crystalline and 
 fibrous minerals, such as epidote and asbestos, polarise the electric 
 
228 LIGHT LECT. 
 
 these effects in actual experiment. Here, in a metal 
 
 box, is a small induction coil actuated by a couple of 
 
 battery cells. The spark which this makes is carried to 
 
 a small oscillator, closely resembling Lodge's (Fig. 121). 
 
 It is, in fact, a short piece of polished platinum tube, 4 
 
 millimetres in diameter, between two small beads of 
 
 platinum. It emits waves about J inch long. I surround 
 
 , it with a metal bonnet or tube to 
 
 direct the waves straight out. The 
 
 detector is simply a tube loosely 
 
 ^, ^^ . i filled with iron filings in circuit with 
 
 Elevation • ° 
 
 I a galvanometer, and a cell made 
 
 with a bit of iron and a bit of 
 
 copper dipping into salt water. It 
 also is furnished with an outer metal 
 tube to screen it from stray radia- 
 Plan i tions. If I set the sender and 
 
 Fig. 128. receiver opposite one another, the 
 
 receiver will respond whenever a spark is passed through 
 the sender. After each such response, I have to give a 
 gentle tap to the detector to shake up the filings and 
 send the galvanometer index back to zero. 
 
 Now I set the receiver askew so that none of the 
 waves from the sender shall get to the detector. Spark 
 after spark is discharged across the oscillator, but there 
 is no response. Then I hold a plate of metal as a mirror 
 
 beam, since (like the tourmaline for short waves) these materials 
 absorb the electric vibrations in directions parallel to certain axes of 
 their structure. Even an ordinary book possesses for these waves 
 a polarising structure ; the waveis that vibrate parallel to the leaves 
 being absorbed more than those that vibrate in a direction trans- 
 verse to them. 
 
V THE INVISIBLE SPECTRUM 229 
 
 to reflect the waves, or I interpose at the proper angle a 
 small prism of paraffin wax. At once the detector 
 responds, proving that the waves have been turned 
 round the corner, refracted by the prism. 
 
 If then it can be proved that these electric waves, 
 though invisible, can be reflected, refracted, polarised, 
 and absorbed, exactly as the visible waves of ordinary 
 light, have we not good reason to regard them as one 
 and the same phenomenon? By every test, in every 
 physical property, save only the accident that our eye is 
 not sensitive to them, they are nothing else than waves 
 of light. But if that is so, are we not entitled logically 
 to draw the converse inference that if light, ordinary 
 light, behaves in the same way and has all the same pro- 
 perties on the small scale as these electric waves on the 
 larger scale, then the little waves of ordinary light are 
 also electric waves ? That, indeed, was the brilliant 
 speculation, the daring theory propounded in 1864 by 
 the late Professor Clerk Maxwell. Basing his ideas upon 
 the investigations pursued in this institution by Faraday, 
 who himself ventured first into this enchanted domain of 
 electro-optics. Maxwell predicted the properties of electric 
 waves in that famous memoir wherein he set forth the 
 doctrine that light consists of electric vibrations in space. 
 And the brilliant success of Hertz and those who have 
 followed him in demonstrating by experiment the optical 
 properties of these waves, is the abundant justification of 
 Maxwell's prediction. 
 
APPENDIX TO LECTURE V 
 
 The Electromag7ietic Theo?y of Light 
 
 During the first quarter of the present century the wave- 
 theory of light successfully displaced the older corpuscular 
 theory. Young, Fresnel, Arago, Biot, and Airy established 
 the laws of physical optics upon an unimpregnable basis of 
 undulatory theory, leaving Brewster the sole surviving 
 exponent of the material nature of light. But none of those 
 who thus contributed to establish the wave-theory of light 
 could do much to elucidate the nature of that wave-motion 
 itself. If light consist of waves they must be waves in or 
 of something : that something being provisionally called the 
 ether. But as to the nature of the ether itself, or as to the 
 particular motions of it that were propagated as waves, 
 scarce anything was to be learned save that the ether itself 
 behaved rather like an incompressible liquid or solid of 
 extreme tenuity but great rigidity, and that the waves were 
 of the kind classed as transversal (see p. io8). 
 
 In 1845 Faraday discovered the singular fact that the 
 magnet exercises a peculiar action on light ; the plane of 
 polarisation of a polarised beam being rotated when the 
 beam passes along a magnetic field. 
 
 The existence of a relation between light and magnetism 
 being thus established, Faraday proceeded to look for other 
 relations, including the action of an electrostatic strain on 
 polarised light, and the effect of reflecting polarised light 
 at the polished pole of a magnet, neither of which, how- 
 ever, he succeeded in observing. 
 
 In 1846 he sent to the Philosophical Magaziiie some 
 
APP. ELECTROMAGNETIC THEORY 231 
 
 "Thoughts on Ray Vibrations" in which he suggested that 
 radiation of all kinds, including light, was a high species of 
 vibration in the lines of force. " Suppose," he says, " two 
 bodies A B, distant from each other and under mutual 
 action, and therefore connected by lines of force, and let us 
 fix our attention upon one resultant of force having an 
 invariable direction as regards space ; if one of the bodies 
 move in the least degree right or left, or if its power be 
 shifted for a moment within the mass (neither of these 
 cases being difficult to realise if A and B be either electric 
 or magnetic bodies), then an effect equivalent to a lateral 
 disturbance will take place in the resultant upon which we 
 are fixing our attention ; for either it will increase in force 
 whilst the neighbouring resultants are diminishing, or it will 
 fall in force as they are increasing. . . . The propagation 
 of light, and therefore probably of all radigmt action, occupies 
 time J and, that a vibration of the line of force should 
 account for the phaenomena of radiation, it is necessary that 
 such vibration should occupy time also. . . . As to that 
 condition of the lines of force which represents the assumed 
 high elasticity of the aether, it cannot in this respect be 
 deficient : the question here seems rather to be, whether 
 the lines are sluggish enough in their action to render them 
 equivalent to the aether in respect of the time known 
 experimentally to be occupied in the transmission of radiant 
 force." 
 
 In 1864 Clerk Maxwell, in a paper in the PJiilosophical 
 Transactions on " A Dynamical Theory of the Electro- 
 magnetic Field," wrote : — " The conception of the propaga- 
 tion of transverse magnetic disturbances to the exclusion of 
 normal ones is distinctly set forth by Professor Faraday in 
 his ' Thoughts on Ray Vibrations.' The electromagnetic 
 theory of light, as proposed by him, is the same in substance 
 as that which I have begun to develop in this paper, except 
 that in 1846 there were no data to calculate the velocity of 
 propagation." Maxwell then sets out new equations to 
 express the relations between the electric and magnetic 
 displacements in the medium and the forces to which they 
 give rise. He not only accepts the idea of Faraday that a 
 
232 LIGHT LECT. V 
 
 moving electric charge (as on a charged body in motion) 
 acts magnetically as an electric current, — a proposition at 
 that time unsupported by any experimental demonstration 
 — but goes further and maintains that there is also a 
 magnetic action produced during the production or release 
 of an electric displacement in a dielectric medium : in 
 fact, that displacement-currents in non-conductors produce, 
 while they last, exactly the same magnetic action as the 
 equivalent conduction -current would produce. He finds 
 that if magnetic methods of measurement are adopted, 
 the unit of electricity arrived at has a certain value, while 
 if purely electrical methods are used the unit has a different 
 value. The relation between these two units was found 
 to depend on the " electric elasticity " of the medium, 
 and to be a velocity ; namely, that velocity with which an 
 electromagnetic disturbance is propagated in space. This 
 velocity had already been determined as a ratio of units by 
 Weber and Kohlrausch, who found it to be 3-19x1 o^^ 
 centims. per second. The velocity of apparent propagation 
 of an electric disturbance along a wire had previously been 
 roughly determined by Wheatstone at a somewhat higher 
 figure. Commenting on Weber's result Maxwell proceeds : — 
 " This velocity is so nearly that of light, that it seems we have 
 strong reason to conclude that light itself (including radiant 
 heat, and other radiations, if any) is an electromagnetic 
 disturbance in the form of waves propagated through the 
 electromagnetic field according to electromagnetic laws. If 
 so, the agreement between the elasticity of the medium as 
 calculated from the rapid alternations of luminous vibra- 
 tions, and as found by the slow processes of electrical 
 experiments, shows how perfect and regular the elastic 
 properties of the medium must be when not encumbered 
 with any matter denser than air. If the same character of 
 the elasticity is retained in dense transparent bodies, it 
 appears that the square of the index of refraction is equal to 
 the product of the specific dielectric capacity and the 
 specific magnetic capacity. Conducting media are shown 
 to absorb such radiations rapidly, and therefore to be 
 generally opaque." These two conclusions Maxwell himself 
 
APR ELECTROMAGNETIC THEORY 233 
 
 attempted to verify, and pointed out an apparent exception 
 in the case of electrolytes, which conduct and yet are 
 transparent. In Maxwell's theory every electromagnetic 
 wave must consist of two kinds of displacements both trans- 
 verse to the direction of propagation, and at right angles to 
 one another, one being an electrostatic displacement, the 
 other a magnetic displacement. In this feature Maxwell's 
 theory reconciles the conflicting views of Fresnel and 
 MacCullagh respecting the relation of the displacements to 
 the " plane of polarisation." It is now known that the 
 electric displacements are at right angles to that plane and 
 agree with the Fresnel vibrations ; whilst the magnetic dis- 
 placements are in the plane of polarisation as required by 
 the theory of MacCullagh. When, in 1874, Maxwell 
 published his Treatise on Magnetism and Electricity, he 
 had already attempted a further verification of the theory 
 by means of a new determination of the ratio of the units. 
 During the next ten years British physicists were busy 
 following out the applications of the theory, and testing 
 its truth in particular instances. Lord Rayleigh showed 
 that it led much more readily than the old elastic-solid 
 theory of light to the equations for double refraction, and to 
 the explanation of the scattering of light (as in the blue of 
 the sky) by small particles. FitzGerald applied it to the 
 problems of the reflexion and refraction of light. J. J. 
 Thomson undertook a new determination of the ratio of 
 the units. Ayrton and Perry pursued a similar investiga- 
 tion by a new method. The same two observers also 
 verified the relation of the optical and dielectric properties 
 in the case of gases as required by the theory, and ex- 
 amined the anomalies presented by ice and ebonite. 
 Poynting and Heaviside independently deduced from 
 Maxwell's theory the proposition that the energy of an 
 electric current travels by the medium and not by the 
 wire. Hopkinson investigated the relation between the 
 refractive index of a number of substances and their 
 dielectric inductivity ; and found some notable deviations 
 from the values required by theory. Lodge added a number 
 of important considerations, and produced mechanical 
 
234 LIGHT LECT. V 
 
 models in illustration of Maxwell's ideas. The present 
 author investigated the opacity of tourmaline in relation to 
 its conductivity ; and found also, in accordance with 
 Maxwell's views, that the conductivity of the double iodide 
 of mercury and copper increases when it is raised to the 
 temperature at which its opacity to light is suddenly 
 augmented. Even more important, because independent 
 of Maxwell's theory, Dr. Kerr in 1876 and 1877 discovered 
 by direct experiment new relations between light and 
 magnetism and between light and electrostatic strain, 
 effects which Faraday had suspected, but sought in vain to 
 discover. Lastly, FitzGerald had, in 1879 and 1883, 
 suggested means of starting electromagnetic waves in the 
 ether. 
 
 By the year 1884 all British physicists, except perhaps 
 Lord Kelvin, who had just then been elaborating an inde- 
 pendent spring-shell theory of the ether as an improvement 
 on the elastic-solid theory, had accepted Maxwell's theory. 
 Three years later Lord Kelvin gave his adhesion. On 
 the Continent it was, however, barely recognised. In- 
 France it was quite ignored until Mascart and Joubert 
 gave some account of it in their treatise on electricity. 
 In Germany it was not quite so entirely neglected. Von 
 Helmholtz appears to have been early drawn to study it, 
 and himself evolved a new theory of dielectric action on 
 similar lines. Later (in 1893) ^^ applied the electro- 
 magnetic theory to explain anomalous refraction and 
 dispersion (see Appendix to Lecture III., p. 100, above). 
 It was von Helmholtz who first drew the attention of Hertz 
 to the possibility of establishing a relation between electro- 
 magnetic forces and dielectric polarisation. Boltzmann 
 had also attempted to verify Maxwell's theory with respect 
 to the relation between the optical and dielectric properties 
 of transparent substances. But, for the rest. Maxwell's 
 theory was practically ignored. Boltzmann himself wrote 
 in 1891 : "The theory of Maxwell is so diametrically 
 opposed to the ideas which have become customary to us 
 that we must first cast behind us all our previous views of 
 the nature and operation of electric forces before we can 
 
APP. ELECTROMAGNETIC THEORY 235 
 
 enter into its portals," Wiedemann appears to have 
 deemed the discrepancies observed by Hopkinson and 
 Boltzmann as sufficient to call in question the validity of 
 the theory, of which little notice is taken in the volumes of 
 the 1885 edition of Die Lehre von der Elekfridtcit. (Pleming 
 has in 1897 shown that these discrepancies disappear when 
 the substances are cooled in liquid oxygen to about - 180° 
 
 In 1886 Lodge, investigating the theory of lightning 
 conductors, carried out a long series of experiments on the 
 discharge of small condensers, leading him to the observa- 
 tion of electric oscillations and of the travelling of electric 
 waves as guided by wires. Hertz taking up the problem 
 put to him by von Helmholtz, threw himself into investi- 
 gating the influence of non-conducting media on the 
 propagation of electric sparks. By March 1888 he had 
 succeeded not only in producing electric oscillations and 
 electric waves by the apparatus described above (Fig. 114, 
 p. 215), but in demonstrating that these waves could be 
 reflected and refracted like ordinary light. 
 
 The result of the publication of Hertz's work was 
 immediate and widespread. To those continental physi- 
 cists who had hitherto ignored Maxwell's theory, or who 
 were unaware of the proofs accumulated by British 
 physicists, Hertz's work was nothing short of a revelation. 
 Scientific Europe precipitated itself upon the production of 
 electric oscillations, as if eager to make up .lost headway. 
 The revelation was the more significant, since for those who 
 had not accepted the ideas of Faraday and Maxwell as to 
 action in the medium, it meant the abandonment of all the 
 other electrical theories then extant which were based on 
 the now untenable principle of action at a distance. None 
 the less heartily was Hertz's work welcomed in England by 
 those who were already disciples of Maxwell. They saw 
 in it the crowning proofs of a theory which on other 
 grounds they had already accepted as true. To adopt 
 Oliver Lodge's words, by the end of 1888 the science of 
 electricity had definitely annexed to itself the domain of 
 optics, and had become an imperial science. 
 
 R 
 
APR A HERTZ- WAVE MODEL 237 
 
 Since 1888 much has been done to complete the 
 experimental verification of the complete analogy between 
 electric waves and waves of light. Of these the more 
 important are briefly noticed on pp. 221-229 above. Suffice 
 it here to say that there is no known physical property 
 possessed by waves of ordinary light that has not been 
 found to be also correspondingly a property of the longer 
 invisible waves produced by purely electric means. By 
 reason of their greater wave-length they will pass through 
 many substances opaque to ordinary light, such as stone 
 and brick walls, and through fogs and mists. 
 
 A Hertz-wave Model 
 
 Subsequently to the delivery of these lectures, and while 
 this volume was preparing for the press, the author devised 
 a wave-motion model to illustrate mechanically the propa- 
 gation of a wave from a Hertz oscillator to a Hertz 
 resonator. This apparatus, which is depicted in Fig. 129, 
 should be compared with Fig. 114, p. 215. In this 
 model the " oscillator " is a heavy mass of brass hung by 
 cords, and having a definite time of swing. The 
 "resonator" is a brass circle hung at the other end of the 
 apparatus by a trifilar suspension. They are adjusted by 
 lengthening or shortening the cords so as to have identical 
 periods of oscillation. Between them, to represent the 
 intervening medium and transmit the energy in waves, is a 
 row of inter-connected pendulums (on a plan somewhat 
 similar to one suggested in 1877 by Osborne Reynolds) 
 consisting each of a lead bullet hung by a V thread, the 
 separate Vs overlapping one another so that no bullet can 
 swing without communicating some of its motion to its 
 next neighbour. On drawing the oscillator aside and 
 letting it go it sets up a transverse wave which is propa- 
 gated along the row of balls in a manner easily followed by 
 the eye, and which, on reaching the resonator at the 
 other end, sets it into vibration. 
 
 I 
 
LECTURE VI 
 
 RONTGEN LIGHT 
 
 Rontgen's Discovery — Production of light in vacuum tubes by 
 electric discharges — Exhaustion of air from a tube — Geissler- 
 tube phenomena — The mercurial pump — Crookes's-tube phe- 
 nomena — Properties of Kathode light — Crookes's shadows — 
 Deflection of Kathode light by a magnet — Luminescent and 
 mechanical effects — Lenard's researches on Kathode rays in 
 air — Rontgen's researches — The discovery of X-rays by the lu- 
 minescent effect — Shadows on the luminescent screen — Trans- 
 parency of aluminium — Opacity of heavy metals — Transpar- 
 ency of flesh and leather — Opacity of bones — Absence of 
 reflexion, refraction, and polarisation — Diselectrifying effects 
 of Rontgen rays — Improvements in Rontgen tubes — Specula- 
 tions on the nature of Rontgen light — Seeing the invisible. 
 
 So many erroneous accounts have appeared, chiefly in 
 photographic journals, written by persons unacquainted 
 with physical science, that it seems worth while in 
 beginning a lecture on the subject of Rontgen's rays to 
 state precisely how Rontgen's discovery was made, in 
 the language in which he himself has stated it. 
 
 ''Will you tell me," asked Mr. H. J. W. Dam in 
 an interview' with Prof. Rontgen in his laboratory at 
 Wiirzburg, '' the history of the discovery ? " 
 
 * McClure's Magazine^ vol. vi., p. 413. 
 
LECT. VI RONTGEN LIGHT 239 
 
 ''There is no history," he said. '' I had been for a 
 long time interested in the problem of the kathode rays 
 from a vacuum tube as studied by Hertz and Lenard. 
 I had followed theirs and other researches with great 
 interest, and determined, as soon as I had the time, 
 to make some researches of my own. This time I 
 found at the close of last October [1895]. I had been 
 at work for some days when I discovered something 
 new. ' ' 
 
 ''What was the date ?" 
 
 "The 8th of November." 
 
 " And what was the discovery ? " 
 
 " I was working with a Crookes's tube covered by a 
 shield of black cardboard. A piece of barium platino- 
 cyanide paper lay on the bench there. I had been 
 passing a current through the tube, and I noticed a 
 peculiar black line across the paper." 
 
 "What of that ?" 
 
 "The effect was one which could only be produced, 
 in ordinary parlance, by the passage of light. No light 
 could come from the tube because the shield which 
 covered it was impervious to any light known, even that 
 of the electric arc." 
 
 " And what did you think ? " 
 
 "I did not think; I investigated. I assumed that 
 the effect must have come from the tube, since its 
 character indicated that it could come from nowhere 
 else. I tested it. In a few minutes there was no doubt 
 about it. Rays were coming from the tube, which had 
 a luminescent effect upon the paper. I tried it success- 
 fully at greater and greater distances, even at two metres. 
 
240 LIGHT LECT. 
 
 It seemed at first a new kind of light. It was clearly 
 something new, something unrecorded." 
 
 *as it light?" 
 
 *' No." [It can neither be reflected nor refracted.] 
 
 *' Is it electricity ? " 
 
 ^' Not in any known form." 
 
 'MVhat is it ?" 
 
 *' I do not know. Having discovered the existence 
 of a new kind of rays, I of course began to investigate 
 what they would do. It soon appeared from tests that 
 the rays had penetrative power to a degree hitherto un- 
 known. They penetrated paper, wood, and cloth with 
 ease, and the thickness of the substance made no per- 
 ceptible difference, within reasonable limits. The rays 
 passed through all the metals tested, with a facility 
 varying, roughly speaking [inversely], with the density of 
 the metal. These phenomena I have discussed carefully 
 in my report ^ to the Wiirzburg Society, and you will 
 find all the technical results therein stated." 
 
 Such was Rontgen's own account given by word of 
 mouth. It is entirely borne out by the fuller document, 
 in which in quiet and measured terms Rontgen described 
 to the Wiirzburg Society his discovery under the title 
 "On a new kind of Rays," and which was the first 
 announcement to the scientific world. 
 
 Now you will note that in the whole passage I have 
 read describing the discovery, there is not a word about 
 photography from beginning to end. Photography 
 
 ^ Ueber eine neue Art von Strahlen (Vorlaufige Mittheilung), 
 von Dr. Wilhelm Konrad Rontgen. (Sitzungsberichte der Wurz- 
 burger physik-medic. Gesellschaft, 1895.) 
 
VI RONTGEN LIGHT 241 
 
 played no part in the original observation. No photo- 
 graphic plate or sensitised paper was employed. The 
 discovery was made by the use of the Imiiinescent screen, 
 the acquaintance of which you made (if you did not know 
 of it before) at my fourth lecture, when we were dealing 
 with ultra-violet light. On that occasion I showed you 
 a card partly covered with platino-cyanide of barium 
 which has been in my possession since 1876. When 
 exposed to invisible ultra-violet light it shone in the 
 dark. No one who has ever used such a luminescent 
 screen can blunder into mistaking it for a photographic 
 plate. Such a screen — a piece of paper covered with the 
 luminescent stuff ^ — was Rontgen using in his investi- 
 gations. And as luminescent screens are not things to 
 be found lying about by accident, it is evident that 
 its presence on the bench in Rontgen' s laboratory on 
 8th November, 1895, when he was deliberately investi- 
 gating the phenomena observed by Lenard, was in no 
 sense accidental. That you may the better understand 
 the precise nature of Rontgen' s discovery, we will 
 repeat the observation with the appliances now at our 
 disposal. 
 
 Before you stands a Crookes's tube, which I can at 
 any moment stimulate into activity by passing through 
 it an electric spark from a suitable induction-coil. It 
 shines with visible light, the glass glowing with a 
 beautiful greenish-gold fluorescence. To stop off all 
 
 ^ It is interesting to note that Lenard's investigations of 1894 
 were conducted by the aid of a luminescent screen composed of 
 paper impregnated with the wax-like chemical called pentadecyl- 
 paratolylketone. 
 
242 LIGHT LECT. 
 
 visible light, I place over the Crookes's tube this case 
 made of black cardboard, which cuts off not only the 
 visible rays of every sort, but also cuts off the invisible 
 rays of the infra-red and ultra-violet sorts. On the table, 
 just below the tube, lies a sheet of paper covered with 
 pi ati no -cyanide of barium — in fact, a luminescent screen. 
 And, on passing the electric discharge through the shielded 
 Crookes's tube you will all see that this luminescent sheet 
 at once shines in the dark; while across it — as those 
 who are near may observe — there falls obliquely a dark 
 line which is simply a shadow of a small support that 
 stands between the tube and the screen. Something 
 evidently is causing that sheet of luminescent paper to 
 light up. Can the effect come from anywhere else than 
 from the tube ? Try by interposing things, and see 
 whether they cast shadows on the paper. The nearest 
 thing at hand is^a wooden bobbin, on which wire is 
 wound. If I interpose it, it casts a shadow on the paper. 
 But looking at the shadow one notices, curiously enough, 
 that while the wire casts a decided shadow, the wood 
 casts scarcely .any. I hold up the screen that you may 
 see the shadow more plainly. Yes ! there is something 
 coming from that tube which causes the screen to light 
 up, and which casts on the screen shadows of things 
 held between tube and screen. This light — if light 
 it be — comes from the tube. But is it light ? Light, as 
 we know it, cannot pass through black cardboard. If it 
 be light it is light of some wholly new and more pene- 
 trative kind. I move away, still holding the screen in 
 my hand, to greater distances. Here, two metres away, 
 the screen still shines, though less brilliantly. And, 
 
VI RONTGEN light 243 
 
 note, it shines whether its face or its back be pre- 
 sented toward the tube. The rays, having penetrated 
 the shield of black cardboard that encloses the tube, can 
 also penetrate the paper screen from the back, and make 
 the chemically-prepared face shine. Let us follow 
 Rontgen farther as he investigated the penetrative power 
 of the rays. I interpose a block of wood against which 
 a pair of scissors has been fixed by nails. You can see 
 on the screen the shadow of the scissors ; the light 
 passes through the wood, though not so brightly, for the 
 wood intercepts some of the rays. Paper, cardboard, 
 and cloth are easily penetrated by them. The metals 
 generally are more opaque than any organic substance, 
 and they differ widely amongst one another in their 
 transparency. Thin metal foil of all kinds is more or 
 less transparent ; but when one tries thicker pieces they 
 are of different degrees of opacity. Ordinary coins are 
 opaque. A golden sovereign, a silver shilling, and a 
 copper farthing are all opaque, but the lighter metals 
 such as tin, magnesium, and aluminium, notably the latter, 
 are fairly transparent. Here is my purse of leather with 
 a metal frame. I have but to hold it between the tube 
 and the screen to see its contents — two coins and a 
 ring — for leather is transparent to these rays. A sheet 
 of aluminium about the twentieth of an inch thick, 
 though opaque to every other previously-known kind of 
 light is for this kind of light practically transparent. On 
 the other hand lead is very opaque. Rontgen found 
 opacity to go approximately in proportion to density. 
 It is now found that those metals which are of the 
 greatest atomic weight are the most opaque to Rontgen 
 
244 LIGHT LECT. 
 
 light. Uranium^ the atomic weight of which is 240, is 
 the most opaque ; whilst lithimii, whose atomic weight is 
 only 7, and which will readily float on water, is exceed- 
 ingly transparent. In fact I have never yet got a good 
 shadow from lithimn. This relation extends not only to 
 the metals themselves but to their compounds. Thus 
 the chloride of lithium is more transparent than the 
 chloride of zinc or than the chloride of silver. Finding 
 that the denser constituents were the more opaque, and 
 that while glass and stone are tolerably opaque such 
 substances as gelatine and leather were comparatively 
 transparent, it occurred to Rontgen that bone would be 
 more opaque than flesh — and so it proved : for inter- 
 posing the hand between the tube and the screen we 
 find that while the flesh casts a faint shadow the bones 
 cast a much darker one, and so we are able to see upon 
 the luminescent screen, in the darkness, the shadow of 
 the bones of the hand, and of the arm. This is truly 
 seeing the invisible. 
 
 But now the investigation took another turn. So far 
 there has been no mention of photography. But the 
 peculiar penetrative light having been discovered, and 
 the shadows having been seen on the luminescent screen, 
 it was a pretty obvious step to register these shadows 
 photographically. For, as was already well known in 
 the case of ultra-violet light, the rays that stimulate 
 fluorescence and phosphorescence are just those rays 
 which are most active chemically and photographically. 
 Hence it was to be expected that these new rays would 
 affect a photographic plate. This Rontgen proceeded 
 to verify. He obtained a photograph of a set of metal 
 
VI RONTGEN LIGHT 245 
 
 weights that were shut up in a wooden box. Also of a 
 compass, showing the needle and dial through the thin 
 brass cover. He then put his tube under a wooden- 
 topped table ; and laying his hand on the table above 
 it, and poising over it a photographic dry-plate, face 
 downwards, he threw upon the plate, by light which 
 passed upwards through the table top, a shadow of his 
 hand. So for the first time he succeeded in photograph- 
 ing the bones of a living hand. It was the photography 
 of the invisible. But, note, even here there is no '' new 
 photography." The only photography in the matter is 
 the well-known old photography of the dry-plate, which 
 must first be exposed and afterwards developed in the 
 dark-room. 
 
 And now, though it anticipates somewhat the course 
 of this lecture, since the process of photographic develop- 
 ment in the dark-room requires a little time, I will pro- 
 ceed to take a few photographs which will then be taken 
 to the dark-room to be developed, and will afterwards 
 be brought back and shown you upon the screen by 
 means of the lantern. 
 
 [In the experiments which followed photographs were 
 taken of the hands of a boy and of a girl, also shadows 
 cast by sundry gems, including a fine Burmese ruby, a 
 sham ruby, a Cape diamond, and an Indian diamond.] 
 
 Retracing our steps in the order of discovery I must 
 at once take you back, nearly two hundred years, to 
 the time of Francis Hauksbee, when, with the newly 
 invented electric machine, and the newly perfected air- 
 pump, the first experiments were made on the peculiar 
 light produced by passing an electric spark into a partial 
 
246 
 
 LIGHT 
 
 l.ECT. 
 
 vacuum. About that time Europe was nearly as much 
 excited — considering the state of knowledge and the 
 slow means of communication — over the " mercurial 
 phosphorus," as it was last year over the Rontgen rays. 
 This " mercurial phosphorus" was simply a little glass 
 tube, such as that (Fig. 130) which I hold in my hand. 
 It contains a few drops of quicksilver; and the air that 
 otherwise would fill the tube has been mostly pumped 
 out by an air-pump, leaving a partial vacuum. I have 
 but to shake the tube and it flashes brightly with a 
 greenish light. The friction of the mercury against the 
 
 Fig. 130, 
 
 glass walls sets up electric discharges, which flash through 
 the residual air, illuminating it at every motion. 
 
 While I have been talking to you an air-pump in the 
 basement, driven by a gas-engine, has been at work 
 exhausting a large oval-shaped glass tube. Only per- 
 haps 3-^ part of the air originally in it remains. On 
 sending through it from top to bottom the sparks 
 from an induction coil, it is filled with a lovely pale 
 crimson glow, which changes at the lower end to a 
 violet-coloured tint. On reversing the connections so 
 as to send the discharge upwards the violet -coloured 
 part is seen at the top. It shows you, in fact, the end 
 
VI RONTGEN light 247 
 
 at which the electric discharge is leaving the tube. The 
 pale glow of this primitive vacuum tube is rich in light 
 of the ultra-violet kind, which, as you know, readily 
 excites fluorescence. I have but to hold near it my 
 platino-cyanide screen for you to observe the rich green 
 fluorescence. My hand will cast a shadow on the screen 
 if I interpose it, but there are no bones to be seen in 
 the shadow. For here there is none of the penetrative 
 Rontgen light : the fluorescence is due to ordinary ultra- 
 violet waves, to which flesh and cardboard are quite 
 opaque. If the tap is turned on to readmit the air you 
 see how the rosy glow contracts first into a narrowing 
 band, then into a mere line, which finally changes into 
 a flickering forked spark of miniature lightning ; and all 
 is over until and unless we pump out the air again. 
 i\nother beautiful effect is shown by use of an exhausted 
 glass jar, within which is placed a cup of uranium glass, 
 as described fifty years ago by Gassiot. The discharge 
 overflows the cup in lovely streams of violet colour, 
 while the cup itself glows with vivid green fluorescence. 
 Some thirty years ago vacuum tubes became an article 
 of commerce, and were made in many complex and 
 beautiful shapes by the skill of Dr. Geissler of Bonn, 
 who devised a form of mercurial air-pump ^ for the pur- 
 pose of extracting the air more perfectly; though the 
 degree of vacuum, which sufficed to display the most 
 brilliant colours when stimulated by an electric discharge, 
 is far short of that which is requisite in the modern 
 
 ^ See the author's monograph on The Development of the Mer- 
 curial Air-Pump, published in 1888, by Messrs. E. and F. N, 
 Spon. 
 
248 LIGHT LECT. 
 
 vacuum tubes of which I must presently speak. Here 
 is a Geissler's tube showing wondrous effects when the 
 spark discharge is passed into it. Strange flickering 
 striations palpitate along the windings of the glass tubes 
 which themselves glow with characteristic fluorescence. 
 Soda-glass fluoresces with the golden-green tint, lead 
 glass with a fine blue, and uranium glass with a brilliant 
 green. The violet glow which appears in the bulb at 
 one end of the tube surrounds the metal terminal by 
 which the current leaves the tube, and is itself due to 
 nitrogen in the residual air. Each kind of gas gives its 
 own characteristic tint. And with any kind of gas within 
 
 ? ^ ■' / 
 
 Fig. 131. 
 
 the tube the luminous phenomena are different at different 
 degrees of exhaustion. 
 
 I have here a set of eight tubes, all of the same simple 
 shape (Fig. 131), but they differ in respect of the degree 
 of vacuum within them. Platinum wires have been sealed 
 through the ends of each, one wire a to serve as the anode 
 or place where the electric current enters, another wire k 
 to serve as kathode or place where the current makes its 
 exit from the tube. Both anode and kathode are tipped 
 with aluminium, as this metal does not volatilise so 
 readily under the electric discharge. The small side- 
 tube s by which the tube was attached to the pump 
 during exhaustion is hermetically sealed to prevent air 
 from re-entering. The first tube of the set is full of air 
 ' at ordinary pressure, and does not light up at all. The 
 
 A 
 
VI RONTGEN LIGHT ^ 249 
 
 length between anode and kathode (about 12 inches) is 
 so great that no spark will jump between them. In the 
 second tube the air has been so far pumped away that 
 only about ^ of the original air remains. Across this 
 imperfect vacuum forked brush-like bluish sparks dart. 
 The third tube has been exhausted to about ^'-g- part ; 
 that is to say, ^ of the air have been removed. It 
 shows, instead of the darting sparks, a single thin red 
 line, which is flexible like a luminous thread. In the 
 fourth tube the residual air is reduced to -^^ or -^ 
 part; and you note that the red line has widened out 
 into a luminous band from pole to pole, while a violet 
 mantle makes its appearance at each end, though brighter 
 at the kathode. In the fifth tube, where the exhaustion 
 has been carried to about ^^q, the luminous column, 
 which fills the tube from side to side, has broken up 
 into a number of transverse striations which flicker and 
 dance ; the violet mantle around the kathode has grown 
 larger and more distinct. It has separated itself by a 
 dark space from the flickering red column, and is itself 
 separated from the metal kathode by a narrow dark 
 space. The degree of exhaustion has been carried in 
 the sixth tube to about j-q^-^q : and now the flickering 
 striations have changed both shape and colour. They 
 are fewer, and whiter. The light at the anode has 
 dwindled to a mere star; whilst the violet glow around 
 the kathode has expanded, and now fills the whole of 
 that end of the tube. The dark space between it and 
 the metal kathode has grown wider, and now the kathode 
 itself exhibits an inner mantle of a foxy colour, making 
 it seem to be dull and hot. The glass, also, of the tube 
 
250 LIGHT LECT. 
 
 shows a tendency to emit a green fluorescent light at the 
 kathode end. In the seventh tube the exhaustion has 
 been pushed still farther, only about q^^-q-q part of the 
 original air being left. The luminous column has sub- 
 sided into a few greyish-white nebulous patches. The 
 dark space around the kathode has much expanded, 
 and the glass of the tube exhibits a yellow-green fluor- 
 escence. In the eighth tube only one or two millionths 
 of the original air are present ; and it is now found much 
 more difficult to pass a spark through the tube. All the 
 internal flickering clouds and striations in the residual 
 gas have disappeared. The tube looks as if it were quite 
 empty : but the glass walls shine brightly with the fine 
 golden-green fluorescence, particularly all around the 
 kathode. If we had pushed the exhaustion still farther, 
 the internal resistance would have increased so much 
 that the spark from the induction coil would have been 
 unable to penetrate across the space from anode to 
 kathode. 
 
 To attain such high degrees of exhaustion as those of 
 the latter few tubes recourse must be had to mercurial 
 air-pumps ; no mechanical pump being adequate to pro- 
 duce sufficiently perfect vacua. The Sprengel pump, 
 invented in 1865 by Dr. Hermann Sprengel, is an admir- 
 able instrument for the purpose. But it was modified 
 and greatly improved' about 1874 by Mr. Crookes, 
 
 ' These improvements comprised the following : — A method of 
 lowering- the supply- vessel to refill it vi^ith the mercury that had run 
 through the pump ; the use of taps made wholly of platinum to 
 ensure tightness ; the use of a spark-gauge to test the perfection 
 of the vacuum by observing the nature of an electric spark in it ; 
 the use of an air-trap in the tube leading up to the pump-head ; the 
 
VI 
 
 RONTGEN LIGHT 
 
 251 
 
 whose form of pump is shown in Fig. 132. 
 placed in a supply- 
 vessel, which can be 
 raised to drive the 
 mercury through the 
 pump, and lowered, 
 when empty, to be 
 refilled. This vessel 
 is connected by a 
 flexible indiarubber 
 tube to the pump, 
 which consists of 
 glass-tubes fused to- 
 gether. From the 
 pump-head the mer- 
 cury falls in drops 
 down a narrower 
 tube, called the fall- 
 tube, and each drop 
 as it falls acts as a 
 little piston to push 
 the air in front of it, 
 and so gradually to 
 empty the space in 
 the farther part of the tube. 
 
 Mercury is 
 
 Fig. 132 
 
 A drying-tube, filled with 
 
 method of connecting the pump with the object to be exhausted, 
 by means of a thin, flexible, spiral glass tube ; the method of 
 cleansing the fall-tube by letting in a little strong sulphuric acid 
 through a stoppered valve in the head of the pump. In carrying 
 out these developments Mr. Crookes was assisted by the late Mr. 
 C. Gimingham, whose later contributions to the subject are de- 
 scribed in the author's monograph on the Mercurial Air-pump. 
 
 S 
 
252 LIGHT LECT. 
 
 phosphoric acid to absorb moisture, is interposed between 
 the pump and the vacuum-tube that is to be exhausted. 
 It is usual to add a barometric gauge to show the degree 
 of vacuum that has been reached. 
 
 Before you, fixed against the wall, is a mercury-pump 
 substantially like Fig. 132, but having three fall-tubes 
 instead of one, so as to work more rapidly. Through 
 these fall-tubes mercury is dropping freely ; the pump 
 being at the present moment employed in the exhaustion 
 of a Crookes's tube, which has been sealed to it by a 
 narrow glass tube. When the exhaustion has been 
 carried far enough, this narrow pipe will be melted with 
 a blow-pipe, so as to seal up the tube and enable it to 
 be removed from the pump. 
 
 It was with such a pump as this that Crookes was 
 working from 1874 to 1875 i^^ ^^e memorable researches, 
 on the repulsion caused by radiation, which culminated 
 in the invention of that exceedingly beautiful apparatus 
 the radiometer, or light-mill, which we were using in my 
 last lecture. From that series of researches Mr. Crookes 
 was led on to another upon the phenomena of electric 
 discharge in high vacua. Professor Hi ttorf of Miinster 
 had already done some excellent work in this direction. 
 He had noted the golden-green fluorescence around the 
 kathode when the exhaustion was pushed to a high 
 degree ; and he had found that this golden glow, unlike 
 the luminous column which at a lower exhaustion fills 
 the vacuous tube, refuses to go round a corner. He 
 had even found that it could cast shadows, owing to its 
 propagation in straight lines. 
 
 Starting at this point on his famous research, Crookes 
 
Fig. 133.— Sir William Crookes, F.R.S. 
 
VI 
 
 RONTGEN LIGHT 
 
 253 
 
 investigated the properties of this kathode light, and 
 found it to differ entirely from any known kind of radia- 
 tion. It appeared to start off from the surface of the 
 kathode and to move in straight lines, penetrating to 
 a definite distance, the limit of which was marked by 
 the termination of the " dsiik space," according to the 
 degree of exhaustion, and causing the bright fluorescence 
 when the exhaustion was carried so far that the dark 
 space expanded to touch the walls. Acting on this hint 
 he proceeded to construct tubes in which the kathode, 
 instead of being as previously a simple wire, was 
 
 4^ 
 
 (r^r< 
 
 Figs. 
 
 134, 135- 
 
 shaped as a flat disk, or as a cup (Figs. 134, 135). From 
 the flat disk the kathode rays streamed backwards in a 
 parallel beam. Crookes regarded these kathode streams 
 as flights of negatively-electrified molecules shot back- 
 wards from the metal surface. Doubtless such flying 
 molecules of residual gas there are ; and they take part 
 in the phenomenon of discharge, bombarding against the 
 opposite wall of the tube. There are, however, strong 
 reasons for thinking that the kathode rays are not 
 merely flights of ''radiant matter," but that the flying 
 molecules are accompanied by ether-waves or ether- 
 motions which cause the fluorescence on the walls of the 
 tube. Be that as it may, Crookes found the kathode 
 
254 LIGHT LECT. 
 
 rays to be possessed of several remarkable properties. 
 Not only could they excite fluorescence and phosphor- 
 escence to a degree previously unknown, but they 
 exercised a mechanical force against the surfaces on 
 which they impinged. They cast shadows of objects 
 interposed in their path ; and were capable of being 
 drawn aside by the influence of a magnet, just as if they 
 were electric currents. 
 
 Here are some Crookes's tubes which display the 
 luminescent effects. At the top of the first is a small 
 fiat disk of aluminium to serve as kathode. From it 
 shoots downward a kathode-beam upon a few Burmese 
 rubies fixed below. They glow with a crimson tint 
 more intense than if they had been red-hot. In a 
 similar tube is a beautiful phenakite,^ looking like a 
 large diamond. When exposed to the kathode rays^ 
 it luminesces with a lovely pale blue tint. In the 
 third is placed a common whelk shell, which has 
 been lightly calcined. As the kathode rays stream 
 down upon it it lights up brilliantly. And, after the 
 electric discharge has been switched off, the shell 
 continues for some minutes to phosphoresce with a 
 persistent glow. 
 
 In the next tube, which contains a sheet of mica 
 painted with a coat of sulphate of lime so that you may 
 better see the bright trace of its luminescence, a narrow 
 kathode ray is admitted through a slit at the bottom, 
 and extends in a fine bright line upwards. Holding a 
 
 ^ A species of white emerald found in the Siberian emerald 
 mines, and often sold in Russia as a Siberian diamond. It is not 
 so brilliant as a diamond, though much more rarely met with. 
 
VI 
 
 RONTGEN LIGHT 
 
 255 
 
 magnet near it, I draw the kathode ray on one side, 
 illustrating its deflectibility. 
 
 To illustrate the mechanical effect of the kathode 
 rays I take a Crookes's tube, having at its ends flat 
 disks of metal as electrodes. Between them is a nicely- 
 balanced paddle-wheel, the axle of which runs upon a 
 sort of little railway. • On sending the spark from the 
 induction-coil through the tube the little wheel is driven 
 round and runs along the rails. Its paddles are driven 
 as if a blast issued from the disk which serves as kathode. 
 On reversing the current its motion is reversed. 
 
 Here (Fig. 136) is a Crookes's tube of a pear shape, 
 having a piece of sheet- 
 metal in the form of a 
 Maltese cross set in the 
 path of the kathode 
 rays. See what a fine 
 shadow the cross casts 
 against the broad end 
 of the tube ; for the 
 whole end of the tube Fig. 136. 
 
 glows with the characteristic golden-green luminescence, 
 except where it is shielded from the rays by the metal 
 cross. 
 
 With this tube I am able to show you a most interest- 
 ing and novel experiment discovered only a few days 
 ago by Professor Fleming. If you surround the tube 
 with a magnetising coil through which an electric current 
 is passed, the magnetic field produces a remarkable 
 effect on the shadow. Instead of pulling it on one side 
 (as a horse-shoe magnet would do), the magnetising 
 
256 LIGHT LECT. 
 
 coil causes the cross to rotate on itself, and at the same 
 time to grow smaller. To show the effect more con- 
 veniently I have put the magnetising coil not around 
 the tube itself, but around an iron core beyond the end 
 of the tube. So I am able to diminish or augment the 
 effect by simply moving the tube away from the iron 
 core, or by bringing it nearer. As I move it up, the 
 shadow of the cross contracts, and grows smaller but 
 brighter. It also twists round and turns completely over 
 top for bottom as it vanishes into a mere point. But 
 just as it vanishes you see its place taken by a second 
 large shadow, which, as I push the tube still closer to 
 the magnetised core, grows brighter and also turns 
 round and contracts as its predecessor did. Its arms 
 are more curved than those of the first cross. At the 
 same moment when the second shadow-cross appears a_ 
 third shadow makes its appearance as a distorted 
 annular form against the walls of the tube between the 
 metal cross and the kathode. Its position is such that 
 the shadow seems to have been cast as by rays diverging 
 from the other end of the tube. As yet we know not 
 the explanation of these remarkable facts. 
 
 The last tube of this set that illustrates Crookes's 
 researches has as kathode a large hollow cup of 
 aluminium at the bottom (Fig. 137). This concave 
 kathode focuses the kathode rays by converging them 
 to a point in space a little above the centre of the tube. 
 Crookes found that if the kathode rays were in this way 
 focused upon anything, they produced great heat. 
 Glass was melted, diamonds charred, platinum foil 
 heated red-hot and even fused by the impact of the 
 
VI 
 
 RONTGEN LIGHT 
 
 257 
 
 concentrated kathode stream. In the focusing-tube 
 now before you — an old one, made more than ten years 
 ago — ^there is a piece of thin platinmn foil hung in the 
 tube to be heated by the rays. But it has become dis- 
 placed and no longer hangs in the focus. Yet by hold- 
 ing a small horse-shoe magnet outside the tube to 
 deflect the rays a little, I can displace the focus until it 
 
 Fig. 137, 
 
 falls upon the surface of the platinum foil, which you 
 now see is raised to a bright red heat. 
 
 Since the date, now nearly twenty years ago, when 
 these most beautiful and astonishing observations were 
 made by Crookes, there has been much speculation as 
 to the nature of these interior kathode rays ; their prop- 
 erties were so extraordinarily different from anything 
 in the nature of ordinary light that even the name " ray " 
 
258 
 
 LIGHT 
 
 LECT. 
 
 as applied to them seemed out of place. Crookes's own 
 term, '^ radiant matter," was objected to as necessarily 
 implying their material nature ; and yet no other ex- 
 planation of them seemed reasonable than Crookes's 
 own suggestion that they consisted of flights of electri- 
 fied particles. It was supposed that they could only 
 exist in a vacuum tube under an exceedingly high con- 
 dition of exhaustion. 
 
 However in 1894 Dr. Philipp Lenard, acting on a 
 hint afforded by an observation of Professor Hertz ^ 
 
 Fig. 138. 
 
 succeeded in bringing out the kathode rays into the 
 air at ordinary pressure. For this purpose he fitted up 
 a tube with a small window of thin aluminium foil 
 opposite the kathode, as shown in Fig. 138. The 
 general form of tube was the same as that previously 
 used by Hertz, namely, cylindrical, with a small kathode 
 disk on the end of a central wire, protected by an inner 
 glass tube. The anode was a cylindrical metal tube 
 surrounding the kathode. Upon the further end of the 
 
 ^ Hertz noticed that when a very thin metal fihn was interposed 
 inside the Crookes tube, the glass still fluoresced under the kathode 
 discharge. He found this still to be the case when the film was 
 replaced by a piece of thin aluminium foil which was quite opaque 
 to light. 
 
VI RONTGEN LIGHT 259 
 
 tube was cemented a brass cap, having at its middle a 
 small hole covered with aluminium foil x^^oo ii^ch 
 thick. Through this "window," when the tube was 
 highly exhausted, there came out into the open air rays 
 which, if not actual prolongations of the kathode rays, 
 are closely identified with them. They can be deflected 
 by a magnet — though in varying degrees depending on 
 the internal vacuum. They can excite luminescence. 
 Lenard explored them by using a small luminescent 
 screen of paper covered with a chemical called penta- 
 decylparatolylketone. He found them to be capable of 
 affecting a photographic dry-plate ; and studied both by 
 the luminescent screen and by the photographic plate 
 their power of penetrating materials. He found that 
 air at ordinary pressure was not very transparent, acting 
 toward them as a turbid medium. He found them to 
 pass through thin sheets of aluminium and even of 
 copper. He also caused them to affect a photographic 
 plate that was completely enclosed in an aluminium 
 case, and to discharge an electroscope enclosed in a 
 metal box. All this work was done in 1894 and 1895 
 and duly published. Though it excited no public notice, 
 it was regarded by physicists as of very great importance. 
 As you were told at the beginning in Rontgen's own 
 account of the matter, his research began with the 
 deliberate aim of reinvestigating the problem of the 
 emission of kathode rays from the vacuum tube as 
 studied by Hertz and Lenard. So as Lenard had done, 
 he employed a luminescent screen to explore the rays, 
 and used a Crookes tube (Fig. 139) of a form closely 
 resembling Lenard's, and indeed identical with that 
 
26o 
 
 LIGHT 
 
 LECT. 
 
 previously employed by Hertz. The end opposite the 
 kathode was simply of glass, without any brass cap or 
 aluminium window. Thus prepared he found what I 
 have already described, those mysterious rays which 
 with characteristic modesty he described as ''X-rays," 
 but which will always be best known as Rontgen's rays. 
 They are not kathode rays, though caused by them. 
 Kathode rays will not pass through glass, and are de- 
 flected by a magnet. Rontgen rays will pass through 
 glass and are not deflected by a magnet. They seem 
 
 Fig. 139 
 
 indeed to be formed by the destruction of the kathode 
 rays, having for their origin the spot where the kathode 
 rays strike against any solid object, best of all against 
 some heavy metal such as platinum or uranium. 
 Neither are they ordinary light of either the infra-red 
 or of the ultra-violet kind, though they resemble the 
 latter in their chemical activity and in so freely exciting 
 luminescence. But ultra-violet light can, as we have seen 
 in previous lectures, be reflected, refracted, and polarised, 
 while Rontgen light cannot.' Nor are Rontgen's rays 
 
 ^ Reflexion there is, but not of a regular kind ; the supposed 
 cases of true reflexion announced by Lord Blythswood and others 
 belong to the category of myths. There is diffuse reflexion of 
 
VI RONTGEN LIGHT 261 
 
 the same thing as Lenard's rays ; for the latter are in 
 various degrees deflectible by the magnet; and air is 
 toward them relatively much more opaque than it is for 
 Rontgen's rays. Rontgen seems to have been fortunate 
 in having the means of producing the most perfect 
 exhaustion by his vacuum pump : for on the perfection 
 of the vacuum more than on any other detail does the 
 successful production of the Rontgen rays depend. The 
 vacuum, which is abundantly good enough to evoke 
 luminescence, or to show the shadow of the cross, or to 
 produce the heating at the focus, or to drive the ^' mole- 
 cule mill," does not suffice to generate the Rontgen rays. 
 For this last purpose the exhaustion must be carried 
 to a higher point — to a point so high indeed that the 
 tube is on the verge of becoming non-conductive. 
 
 Rontgen rays from polished metals, particularly from zinc, just as 
 there is diffuse reflexion of ordinary light from white paper. As 
 to refraction, Perrin in Paris, and Winkelmann in Jena, have inde- 
 pendently found what they think evidence of feeble refraction 
 through aluminium prisms. But the deviation (which is towards 
 the refracting edge) is so excessively small as to be scarcely dis- 
 tinguishable from mere instrumental errors. Polarisation has 
 been looked for by many skilled observers, using many materials 
 including tourmalines. Only one success has been alleged, by 
 MM. Galitzine and Karnojitzky, using tourmaline ; but their 
 result has not been confirmed and is probably erroneous. Neither 
 has interference of Rontgen light yet been shown to be possible. 
 Several observers have professed that they have obtained diffrac- 
 tion fringes from which the wave-length of the Rontgen rays 
 could be measured. But some of these measurements show a 
 wave-length greater than that of red light, and others less than 
 that of ordinaiy ultra-violet : they are probably all due to some 
 unnoticed source of error. None of them can be accepted without 
 subsequent confirmation by other experimenters, and this is not 
 yet forthcoming. 
 
262 LIGHT LECT. 
 
 Here let me say a word about the man himself and 
 his material surroundings. Still in the prime of life, at 
 the age of fifty-one, Professor Wilhelm Konrad Rontgen 
 had already made himself a name among physicists by his 
 work in optics and electricity before the date of the bril- 
 liant discovery that gave him wider fame. He occupies 
 the chair of Physics in the University of Wiirzburg in 
 Bavaria, and lives and works in the physical laboratory 
 of the University. The little town of Wiirzburg, of 
 61,000 inhabitants, boasts a university frequented by 
 1,490 students, and supported with an income of ;£"4i,ooo 
 a year, of which more than half is contributed by the 
 State. There are 53 professors and 40 assistants. Its 
 buildings comprise a group of laboratories and insti- 
 tutes devoted to chemistry, physiology, pathology, 
 mineralogy, and the like. Its physical laboratory, a 
 neat detached block of buildings, wherein also the pro- 
 fessor has his residence, is of modern design. Its 
 equipment for the purpose of research is infinitely 
 better than that of the University of London ; ' and it is 
 
 ^ From a Report recently presented to the Convocation of the 
 University of London, it appears that the physical and chemical 
 laboratories of the University are practically non-existent. " There 
 are three rooms at Burlington House v^^hich are occasionally used 
 as laboratories during examinations, and for examinational purposes 
 only. The largest of these is a large hall lit from the top. When 
 used as a chemical laboratory, it is fitted up with working benches 
 down the middle and along the two sides, the benches being 
 divided into separate stalls to isolate candidates in their work. It 
 was stated that the middle stalls and benches are taken down when 
 the hall is used for written examinations, and are re-erected when 
 a chemical examination is to be held. In a second hall, also lighted 
 from above, where frequent written examinations are held, tem- 
 porary arrangements are made whenever an examination in practical 
 
Fig. 140.— Professor W. K. Rontgen. 
 
VI RONTGEN LIGHT 263 
 
 expected of the professor that he shall contribute to the 
 advancement of science by original investigations. With 
 such material and intellectual encouragements to research 
 as surround the university professor in even the smallest 
 of universities in Germany, what wonder that advances 
 are made in science ? Would that a like stimulus were 
 existent in England. The Professor of Physics in the 
 University of London has made no discovery like that 
 of Professor Rontgen, for the very good reason that the 
 University of London has neither appointed any Pro- 
 fessor of Physics, nor built any physical laboratory where 
 he might work. Neither the State nor the municipality 
 has provided it with the necessary funds. Its charter 
 
 physics is to be held. A curtain of black cloth slung across one end 
 of the room gave partial obscurity over the tables where photo- 
 metric and spectroscopic apparatus was placed. The third room, 
 sometimes called the galvanometer room, is a smaller room in the 
 basement, artificially lighted, and used chiefly for printing, except 
 at the times of examinations in practical physics." Such is the 
 melancholy state of things in a University where everything is sacri- 
 ficed on the altar of competitive examinations. 
 
 Bavaria has a population of 6,000,000. It supports the three 
 Universities of Munich, Erlangen, and Wtirzburg, with a total of 
 over 6,000 students, at a cost of ^150,000 a year, of which 
 ;^93,ooo is provided by the State. London, with a population of 
 5,000,000, has the University of London, a mere Examining 
 Board, to which come up for intermediate and degree examina- 
 tions about 2,000 students yearly, of whom a large proportion are 
 from the provinces. It has no professors. Its laboratories are in 
 the deplorable position above mentioned. So far from being 
 endowed by the State, it pays in to the State about ;^i6,5oo a 
 year, and nominally receives back about ^16,280 as a parliamen- 
 tary grant. It receives no subvention from the municipality. Its 
 library is closed for a large portion of the year, the room being 
 used for examination purposes almost every day. 
 
 T 
 
264 LIGHT LECT. 
 
 precludes it from doing anything for science except hold 
 examinations ! Perhaps some day London may have a 
 university worthy of being mentioned beside that of 
 Wiirzburg, which is eleventh only in size amongst the 
 universities of Germany. 
 
 Rontgen had so thoroughly explored the properties 
 of the new rays by the time when his discovery was 
 announced, that there remained little for others to 
 do beyond elaborating his work. One point deserves 
 notice ; namely, the improvement of the tubes. Rontgen 
 held the view that his rays originated at the fluorescent 
 spot where the kathode rays struck the glass. This led 
 some persons to the idea that fluorescence was advan- 
 tageous. Several workers, however, discovered about 
 the same time that if the kathode rays were focused 
 upon a piece of metal the emission of Rontgen light 
 became more copious. When studying early last year 
 the conditions under which the rays were produced, I 
 found that the best radiators are substances which do 
 not fluoresce — namely, metals. I found zinc, magnesium, 
 aluminium, copper, and iron to answer; but platinum 
 was better than these, and uranium best of all. Directing 
 the kathode discharge against a target or '^ antikathode " 
 of platinum fixed in the middle of the tube, I carefully 
 watched, by aid of a luminescent screen, the emissive 
 activity of the surface during the process of exhaustion. 
 After the stage of exhaustion has been reached at which 
 Crookes's shadows are produced, one must go on further 
 exhausting before any trace of Rontgen rays appear. 
 The first luminosity seems to come (as in Fig. 141) from 
 both front and back of the target at once ; an oblique 
 
VI RONTGEN LIGHT 265 
 
 line, corresponding to the plane of the " antikathode** 
 or target of metal, being seen on the screen between 
 two partially luminescent regions. On continuing the 
 exhaustion, the light behind dies out while that in 
 front increases, as in Fig. 142, the rays being emitted 
 copiously right up to the plane of the antikathode. This 
 
 Fig. 141. Fig. 142. 
 
 lateral emission is quite unlike anything in the emission 
 or reflexion of ordinary light, and has to be accounted 
 for in any theory of the Rontgen rays. I have myself 
 observed' that within the tube there are some other 
 rays given off in a similar way, along with the Rontgen 
 rays, but which are not Rontgen rays, for they can be 
 
 ^ See Electrician. 
 
266 
 
 LIGHT 
 
 LECT. 
 
 deflected by a magnet, and more nearly resemble the 
 kathode rays. It is these that produce on the glass 
 wall of the tube a well-marked fluorescence delimited 
 (as in Fig. 142) by an oblique plane corresponding to 
 the delimitation of Rontgen rays seen in the fluorescent 
 screen. The tube which I used at the beginning of 
 this lecture, and which we will use again at the close 
 of the lecture to show you your own bones, is of the 
 focus type (Fig- 143)- It is of the pattern devised by 
 Mr. Herbert Jackson, of King's Col- 
 lege. The concentration of the kath- 
 ode rays upon the little target of 
 platinum (which often becomes red-hot) 
 has the advantage not only of allowing 
 a more copious emission of Rontgen 
 rays than would be possible if the anti- 
 kathodal surface were the glass wall, 
 but also of causing the Rontgen rays 
 to issue from a small and definite 
 source so that the shadows cast by 
 cbjects are more sharply defined. 
 Here are two still more recent tubes 
 (Figs. 144, 145) constructed for me 
 by Mr. Bohm, in which the focus prin- 
 ciple is preserved ; but in which there 
 is the improvement that the anti- 
 kathode T is not used also as an anode. It is an 
 insulated target of platinum, while the anodes are 
 aluminium rings through which the cone of kathode 
 rays passes. These tubes are not liable to blacken, as 
 is the case with tubes in which the antikathode is alco 
 
VI 
 
 RONTGEN LIGHT 
 
 267 
 
 used as anode. The tube (Fig. 145) has two concave 
 electrodes, either or both of which may be used as 
 
 Fig. 144. 
 
 Fig. 145. 
 
 kathode ; it is a convenient form for those cases in 
 which an alternating current is employed. 
 
 In another direction many efforts have been made 
 at improvement of the luminescent screen. At first 
 good barium plati no-cyanide was not to be procured, 
 and hydrated potassium platino-cyanide was found far 
 superior. But the good barium salt now procurable is 
 
265 LIGHT LECT. 
 
 quite as luminescent, and is less troublesome to manage. 
 One result of the ignorance which at first prevailed as 
 to the real origin of Rontgen's discovery was that 
 various experimenters up and down the world supposed 
 themselves to have invented something when they took 
 to using fluorescent screens. One man puts a fluor- 
 escent screen at the bottom of a pasteboard tube, with 
 a peep-hole lens at the top, and calls it a *^ cryptoscope." 
 Another, in another part of the globe, puts a fluorescent 
 screen at the bottom of a nice cardboard box furnished 
 with a handle and a flexible aperture to fit to the eyes, 
 and styles it a ^* fluoroscope." Both are useful; but 
 the only invention in the whole thing is Rontgen's. 
 
 Within a few days of the publication of Rontgen's 
 discovery another effect, however, which had escaped 
 Rontgen's scrutiny, was observed by several independ- 
 ent observers. It had been known for several years 
 that when ultra-violet light falls upon an electrically- 
 charged surface it will cause a diselectrification, but 
 only if the surface is negatively charged. Ultra-violet 
 light will not diselectrify a positive charge.' But 
 Rontgen rays are found to produce a diselectrification 
 of a metal surface (in air) whether the charge be posi- 
 tive or negative. Here is a convenient arrangement for 
 exhibiting the experiment. An electroscope made on 
 Exner's plan with three leaves — the central one a stiff 
 plate of metal — is charged, and then exposed to Ront- 
 
 ^ Ultra-violet light will not diselectrify a metal surface in air 
 unless that surface is negatively charged. I have observed a case, 
 however, in which a positively-electrified body was discharged by 
 ultra-violet light, but it was not a metal surface, nor in ain 
 
VI 
 
 RONTGEN LIGHT 
 
 269 
 
 Fig. 146. 
 
 gen light. The three leaves are made of aluminium, 
 aluminium foil being better than leaf -gold for electro- 
 scopes. They are supported within a thin 
 flask of Bohemian glass entirely enclosed, 
 except at the top, in a mantle of trans- 
 parent metallic gauze. After the leaves 
 have been charged — either positively by a 
 rod of rubbed glass, or negatively by a rod 
 of rubbed celluloid — a metal cap is placed 
 over the top (Fig. 146). 
 
 The leaves, being thus completely sur- 
 rounded by metal, are effectually screened 
 from all external electrical influences. My 
 electroscope is now charged. To enable you to see 
 the effect better, a beam of light is directed upon it, 
 throwing a magnified shadow of the leaves upon the 
 white screen. Then, exposing the electroscope to 
 Rontgen light from a focus tube situated some 18 
 inches away, you see the leaves at once closing together, 
 proving the diselectrification. It succeeds whether the 
 charge be positive or negative in sign. 
 
 It now only remains for me to exhibit to you the 
 photographs which were taken at the beginning of this 
 lecture, and a number of others _prepared as lantern- 
 slides. In Figs. 147, 148 we have the hand of a poor 
 child aged thirteen, a patient in St. Bartholomew's 
 Hospital. She was brought to my laboratory that the 
 deformities of her hands might be examined. The first 
 of the two plates was insufficiently exposed, with the 
 result that the bones scarcely show through the flesh at 
 all. The second plate was over-exposed, and the rays 
 
270 LIGHT LECTc 
 
 have penetrated the flesh so thoroughly that only the 
 bones appear. 
 
 Fig. 149 is the hand of a child of eleven years old. 
 In a child's hand the bones are not yet completely 
 ossified, their ends being still gelatinous and trans- 
 parent, so that there seem to be gaps between them. 
 Compare this with the hand of a full-grown man, and 
 you will see how age changes the aspect of the bones. 
 
 Fig. 150 is the hand of a full-grown woman. You 
 will observe in the case of the lady's rings that the 
 diamonds are transparent, while the metal portion 
 casts a shadow even through the bones. These two 
 photographs were taken by Mr. J. W. Gifford, of Chard, 
 an early and most successful worker with Rontgen rays. 
 
 Fig. 151 is the hand of Lord Kelvin, and shows 
 traces of age, and of a tendency to rheumatic deposits. 
 
 Fig. 152 is the hand of Mr. Crookes, and though 
 a knottier hand, shows some points of resemblance 
 with that of Lord Kelvin. 
 
 Fig. 153 is the hand of Sir Richard Webster. The 
 shadow is interesting as showing not only an athletic 
 development, but as revealing, embedded in the flesh 
 between the thumb and the first finger, two small 
 shot, the result of a gunshot wound received many years 
 previously. This photograph and the two preceding 
 are from the series taken by Mr. Campbell-Swinton, 
 who was first in England to put into practice this newest 
 of the black arts. 
 
 By the courtesy of Mr. Campbell-Swinton I am also 
 able to show you a number of other slides — the hand 
 of Lord Rayleigh ; the hand of a lady with a needle 
 
Fig. 149. — Hand of Child, aged eleven years. 
 (Photo, by Mr. J. VV, GiffordJ. 
 
Fig. 150. — Hand of full-grown Woman. 
 (Photo, by Mr. J. IF. Gifford). 
 
Fig. 151.— Hand of Professor Rt. Hon. Lord Kelvin. 
 
Fig. 152.— Hand of Sir W. Crookes, F.R.S. 
 
 U 
 
Fig. 153. — Hand of Rt. Hon. Lord Alverstone. 
 
 I 
 
Fig. 154. 
 
 Fig. 155. 
 
VI RONGTEN LIGHT 271 
 
 embedded in the palm ; a hand terribly swollen with the 
 gout; a foot, showing the heel-bone and the smaller 
 bones down to the toes, as well as the bones in the 
 ankle ; a view through the left shoulder of a young lady, 
 showing her ribs, shoulder-blade, and collar-bone; the 
 torso of a young man, showing his ribs, and, dimly, his 
 heart, like a central dark shadow with a triangular apex 
 pointing down toward the right, that is, to his left side ; 
 lastly, the shadow of a living head, showing all the 
 vertebrae of the neck. 
 
 Here, again, is the shadow of a newly-born child, 
 taken by Mr. Sydney Rowland. Note the imperfect 
 state of the bones in the hands. 
 
 Passing from human objects, we will look at the 
 shadows of a few animals. These are a chameleon, 
 giving a clear view not only of its skeleton but of the 
 internal organs ; a mouse ; a frog ; and some fishes. 
 The next slide was taken from an Egyptian mummy in 
 its wrappings. Before this photograph was taken there 
 was some dispute as to whether it was the mummy of a 
 cat or of a girl. The photograph sets the question 
 entirely at rest. 
 
 Earlier in my lecture I mentioned that glass is tolerably 
 opaque to these rays. Of this you have a proof in the 
 next photograph (Fig. 154), which represents a pair of 
 spectacles photographed while lying in their case, the 
 covering of which, in shagreen, shows all the markings 
 peculiar to the shark's skin, with which the case was 
 covered. The next photograph by Mr. Campbell- 
 Swinton enables you to read the contents of a sealed 
 letter which he received. His also is the next picture 
 
272 
 
 LIGHT 
 
 LECT. 
 
 (Fig. 155), which is the photographed shadow of an 
 aluminium cigar-case, containing two cigars. And 
 
 lastly (Fig. 156), I exhibit to 
 you a photograph of two ruby 
 rings. By gaslight the gems 
 of one are not distinguishable 
 from those of the other; and 
 in broad daylight it would take 
 an expert to pronounce between 
 them. Bat when viewed or 
 photographed by Rontgen light 
 there remains no manner of 
 doubt. The rubies of one ring 
 are true Burmese rubies, and 
 they appear transparent. The 
 ^^^- ^56- others are imitation rubies 
 
 made of ruby-coloured glass, and appear quite opaque. 
 
 You will have noticed that I have spoken of these 
 rays as " Rontgen light." But are we really justified in 
 calling it light ? It is invisible to our eyes ; but then so 
 also is ordinary ultra-violet light, and so is infra-red light, 
 and Hertzian light. And there are other kinds of light 
 too, amongst them one discovered during last year by 
 M. Becquerel ' and myself, which are invisible. But if 
 
 ^ M. Henri Becquerel (see Comptes Rendus. cxxii. pp. 559, 790, 
 etc.) and I myself (see Philosophical Magazine, July, 1896, p. 103) 
 quite independently discovered some invisible radiations that are 
 emitted by uranium salts, and by the metal uranium, which can 
 affect photographic plates, and will pass through a sheet of alu- 
 minium or of cardboard. They resemble Rontgen's rays in 
 possessing the negative property that they cannot be reflected, 
 refracted, and polarised. They also produce diselectrification. 
 
VI RONTGEN LIGHT 273 
 
 the Rontgen light can be neither reflected nor refracted, 
 neither diffracted nor polarised, what reason have we 
 for calling it light at all ? In fact, direct proof that it 
 consists of transverse waves is wanting. Many conjec- 
 tures have been formed respecting its nature. Rontgen 
 himself suggested that it might consist of longitudinal 
 vibrations. Others have suggested ether streams, ether 
 vortices, or even streams of minute corpuscles. At one 
 time the notion that it might be simply an extreme kind 
 of ultra-violet light of excessively minute wave-length 
 was favoured by physicists, who were disposed to explain 
 the absence of refraction, and the high penetrative power 
 of the rays upon von Helmholtz's theory of anomalous 
 dispersion, according to which the ultra-violet spectrum 
 at the extreme end ought to double back on itself. 
 
 The most probable suggestion yet made, and the only 
 one that seems to account for the strange lateral emission 
 of the rays right up to the plane of the antikathode (see 
 Fig. 142, p. 265), is that of Sir George Stokes. Stokes's 
 view is that while all ordinary light consists of trains ^ 
 of waves (Fig. 6S, p. 112), in which each ripple is one of a 
 series that gradually dies away, the Rontgen light con- 
 sists of solitary ripples, each of not more than one or 
 
 It was at first thought that these rays were due to a sort of invisible 
 phosphorescence, and were ultra-violet light of a very high order ; 
 but it is now certain that this view was erroneous. 
 
 ^ It has long been known from the experiments of Fizeau, 
 that in ordinary light each train consists on the average of at 
 least 50,000 successive vibrations ; for it is possible to produce 
 interference of light between two parts of a beam which have 
 traversed lengths differing by more than 50,000 wave-lengths. 
 Michelson has gone far beyond that number. See the footnote 
 on p. 112. 
 
274 LIGHT LECT. 
 
 one and a half waves. According to Stokes the Rdntgen 
 light is generated at the antikathode by impact of the 
 flying negatively-electrified molecules (or atoms) which 
 constitute the kathode stream. At the moment when 
 each of these flying molecules strikes against the target 
 and rebounds, there will be a quiver of its electric 
 charge ; in other words, the charge on the molecule will 
 perform an oscillation. Now that electric oscillation 
 will be executed across the molecule in a direction gen- 
 erally normal to the plane of the target, and will give rise 
 to an electro-magnetic disturbance which will be propa- 
 gated as a wave in all directions, except where stopped 
 by the metal of the target. And this oscillation being 
 of excessively short period, and dying 
 out after about one or two (Fig. 157) 
 complete periods, will generate a wave, 
 which, though of a frequency as high as,^ 
 or even higher than, that of ordinary 
 ultra-violet light, and therefore capable 
 Fig. 157. q£ producing kindred effects, will not be 
 
 capable of being made to interfere, nor to undergo 
 regular refraction or reflexion, because it does not 
 consist of a complete train of waves. Here is a model 
 intended roughly to illustrate the theory. An iron hoop 
 (Fig. 158) which can be thrown or swung against the 
 wall represents the flying molecule. The electric charge 
 which it carries is typified in the model by a lump of 
 lead capable of sliding on a transverse wire, and held 
 centrally by a pair of spiral springs. When this model 
 molecule is caused to strike against the wall and rebound, 
 the leaden mass is disturbed, and executes an oscillation 
 
VI RONTGEN LIGHT 275 
 
 to and fro along the wire. The oscillation dies out after 
 about Iy periods. Now, suppose this oscillation to set 
 up a transverse wave in surrounding 
 space. Though it consists of but 
 i^ ripples, they would be propagated 
 outward just as trains of waves are. 
 And if there were millions of such 
 flying molecules in operation, these 
 solitary ripples might come in mil- 
 lions one after the other, but not 
 regularly spaced out behind one 
 another like the trains of waves 
 
 T T 1 rr>l • • Fl*^' ^5^- 
 
 constituting ordinary light ims is 
 but a gross and rough illustration of Stokes's hypo- 
 thesis ; but it must suffice for the present. 
 
 But I cannot close this course of lectures without 
 one word as to the possibilities which this amazing dis- 
 covery of the Rontgen light has opened out to science. 
 It is clear that there are more things in heaven and 
 earth than are sometimes admitted to exist. There are 
 sounds that our ears have never heard : there is light 
 that our eyes will never see. And yet of these inaudible, 
 invisible things discoveries are made from time to time 
 by the patient labours of the pioneers in science. You 
 have seen how no scientific discovery ever stands alone : 
 it is based on those that went before. Behind Rontgen 
 stands Lenard; behind Lenard, Crookes; behind 
 Crookes the line of explorers from Boyle and Hauksbee 
 and Otto von Guericke downwards. We have had 
 Crookes 's tubes in use since 1878, and therefore for 
 nearly twenty years Rontgen' s rays have been in exist- 
 
276 LIGHT LECT. VI 
 
 ence, though no one, until Rontgen observed them on 
 8th November, 1895, even suspected^ their presence 
 or surmised their qualities. And just as these rays 
 remained for twenty years undiscovered, so even now 
 there exist, beyond doubt, in the universe, other rays, 
 other vibrations, of which we have as yet no cognisance. 
 Yet, as year after year rolls by, one discovery leads to 
 another. The seemingly useless or trivial observation 
 made by one worker leads on to a useful observation by 
 another; and so science advances, ^'creeping on from 
 point to point." And so steadily year by year the sum 
 total of our knowledge increases, and our ignorance is 
 rolled a little further and further back ; and where now 
 there is darkness, there will be light. 
 
 ^ It is but fair to Professor Eilhard Wiedemann to mention that 
 in August i8g5 he described some " discharge-rays " (Entladungs- 
 strahlen) inside a vacuum tube, which, though photographically 
 active, refused to pass through fluor-spar, and were incapable of 
 being deflected by a magnet. But their properties differ from 
 Rontgen rays in some other respects. 
 
APPENDIX TO LECTURE VI 
 
 OTHER KINDS OF INVISIBLE LIGHT 
 
 Upon the discovery by Rontgen of the rays that bear his 
 name it was natural that the inquiry should be raised whether 
 there exist any other rays having penetrative properties in 
 any degree similar, Lenard's rays, discovered in 1894, to 
 which some reference is made on p. 258 above, have the 
 power of penetrating thin sheets of metal and of producing 
 photographic action as well as of discharging electrified 
 bodies. But they differ from Rontgen's rays in their pene- 
 trative power, for air is relatively opaque to them. Also 
 they are deflected in varying degrees by the magnet. 
 Wiedemann's " discharge-rays," briefly mentioned above, 
 are further described on p. 281. 
 
 Many persons have supposed Rontgen's rays to be 
 produced by electric sparks in the open air, simply because 
 such sparks will fog photographic plates and cause images 
 of coins and other metal objects in contact with the plates 
 to impress images upon them. These images are, however, 
 due to direct electric action. They are not produced when 
 a sheet of aluminium is so interposed as to screen off all 
 direct electrical action. 
 
 In sunlight there do not appear to be any Rontgen rays, 
 nor yet in the hght of the electric arc ; for neither of these 
 sources contains any rays that will affect a photographic 
 plate that is protected by an aluminium sheet. 
 
 There are, however, some kinds of light that, like Ront- 
 gen's rays, will pass through aluminium or through black 
 
278 LIGHT LECT. VI 
 
 cardboard, and produce photographic effects. These are 
 worthy of some notice. 
 
 BecquereVs Rays. — Early in 1896 M. Henri Becquerel, 
 as mentioned on p. 272, and the author of this book in- 
 dependently, made the observation that some invisible radia- 
 tions are emitted from some of the salts of the metal 
 uranium, as, for example, the nitrate of uranyl and the 
 fluoride of uranium and ammonium. These and other salts 
 of uranium, whether in the dark or in the light, emit a sort 
 of invisible light, which can pass through aluminium and 
 produce on a photographic plate shadows of interposed 
 metal objects. 
 
 PhospJwrus Light. — The author has examined the pene- 
 trative effect of some other kinds of light. The pale light 
 emitted by phosphorus when oxidising in moist air is 
 accompanied by some invisible rays which will penetrate 
 through black paper or celluloid, but will not pass through 
 aluminium. So will some invisible rays that are emitted by 
 the flame of bisulphide of carbon. 
 
 Light of Glow-worms and Fireflies. — Dr. Dawson 
 Turner has found that the light emitted by glow-worms 
 contains photographic rays which will pass through alu- 
 minium. 
 
 In Japan, Dr. Muraoka has examined the rays emitted 
 by a firefly (" Johanniskafer "). He found that they emitted 
 rays which, after filtration through card or through copper 
 plates, would act photographically. These rays can be 
 reflected, and probably refracted and polarised. He used 
 about 1000 fireflies shut up in a shallow box over the 
 screened photographic plate. 
 
 IViedejnami's Rays. — Professor E. Wiedemann in 1891 
 described some rays (named by him Discharge-rays, or 
 E7itladiiiigsstrahle?i) which are produced in vacuum-tubes 
 by the influence of a rapidly-alternating electric discharge. ' 
 They have the property of exciting in certain chemically 
 prepared substances, notably in calcium sulphate containing 
 a small percentage of manganese sulphate, the power of 
 thermo-luminescence. In other words, the substance after 
 exposure to these rays will emit light when subsequently 
 
APP. OTHER KINDS OF INVISIBLE LIGHT 279 
 
 warmed. They are emitted at lower degrees of rarefaction 
 than are necessary for producing the kathode rays. They 
 are emitted from all parts of the path of the spark-discharge, 
 but more strongly near the kathode. They are propagated 
 in straight lines, but no reflexion of them by solid bodies 
 has yet been observed. They are readily absorbed by 
 certain gases, oxygen and carbonic dioxide, but their 
 production is promoted by hydrogen and nitrogen. Those 
 produced in hydrogen are partially transmitted by quartz 
 and fluor-spar. They are apparently not present in the 
 glow discharge. In vacuo these rays are produced by all 
 parts of the discharge. Under the influence of electric 
 oscillations they are emitted, even in some cases at half an 
 atmosphere of pressure, at the boundary of the rarefied gas 
 and the glass wall, even before any visible light is seen. 
 No deviation of them by the magnet has yet been observ- 
 able/ Those produced at relatively great pressures have in 
 general the power of penetrating bodies according to the 
 inverse ratio of their densities. 
 
 New kinds of Kathode Rays. — The author in 1896 
 found three new kinds of kathode rays. One of these, 
 termed parakathodic rays, is produced when ordinary 
 kathode rays strike upon an anti-kathode, as in the " focus " 
 tubes. If the vacuum is low, there are emitted from the anti- 
 kathode, in nearly equal intensity in all directions, some 
 rays that closely resemble ordinary kathode rays. They can 
 be deflected electrostatically and magnetically, and can cast 
 shadows of objects on the glass walls. If the vacuum is 
 high enough for the production of Rontgen's rays, some 
 parakathodic rays are also produced at the same time. 
 They cause the glass bulb to fluoresce over an obliquely 
 limited region as in Fig. 142, p. 265. 
 
 The second kind, termed diakathodic rays, is produced 
 by directing the ordinary kathode rays full upon a piece of 
 wire-gauze, or upon a spiral of wire which is itself negatively 
 electrifled. The ordinary kathode rays refuse to pass through 
 the meshes of the gauze, but instead there passes through 
 a beam of bluish rays, which differ from kathode rays in that 
 they are not directly affected by a magnet. These diakathodic 
 
28o LIGHT LECT. VI 
 
 rays can also produce fluorescence of the glass where they 
 meet the walls of the tube, and can cast shadows of inter- 
 venmg objects ; but the fluorescence is of a different kind, 
 for ordinary soda glass gives a dark orange fluorescence 
 instead of its usual golden green tint. This orange fluor- 
 escence when examined by the spectroscope shows the 
 D-lines characteristic of sodium. 
 
 A third kind, termed isokathodic rays, are formed by 
 passing ordinary kathode rays along a vacuum tube 
 in which the discharge travels successively through a 
 number of small glass funnels, and is subjected at the same 
 time to a transverse magnetic field. After passing through 
 several of these the rays change their character so that 
 they no longer cause fluorescence of the glass wall of the 
 tube, and are no longer ordinary kathode rays. 
 
 Goldstein^ s Rays. — ^Herr Goldstein has also described 
 some rays apparently closely akin to the diakathodic rays. 
 If a perforated disk is used as a kathode there are produced 
 some blue rays which stream back behind the kathode 
 opposite the apertures. He calls these Canal-rays, 
 
LECTURE VII 
 
 RADIUM AND ITS RAYS 
 
 Emission by certain substances of radiations that will penetrate 
 opaque screens — Properties of uranium salts — The Becquerel 
 rays — Radio-activity — Examination by electroscope — Researches 
 of the Curies — Madame Curie discovers polonhim and radium 
 in pitchblende — Experiments with radium — Separation by 
 magnetic field of the three kinds of rays emitted by radium — 
 Strutt's radium clock — Crookes's spinthariscope — Researches 
 of P. Curie on heat emitted by radium, and of Rutherford on 
 disintegration of radium atom. 
 
 Early in the year 1896, when all the scientific world 
 was astir over the then newly discovered Rontgen rays, 
 and the omniscient journalists were writing rubbish about 
 the "new photography," many a quiet worker was trying 
 over again the wonderful experiments by which Rontgen 
 had enabled us to see, by their shadows cast on a 
 fluorescent screen, the forms of hidden things. Let me 
 recapitulate briefly the sum and substance of Rontgen's 
 discovery. It had been known for many years that the 
 substances which are fluorescent — in particular the 
 crystalline powder called barium platinocyanide — shine 
 in the dark when there fall upon them the invisible waves 
 of ultra-violet light. Rontgen, using a Crookes' tube 
 
 X 
 
282 LIGHT LECT. 
 
 excited by internal electric discharges from an induction 
 coil, had found that from the antikathode of the tube 
 there was emitted an invisible radiation — a new kind of 
 rays — which resembled ultra-violet light in possessing the 
 power of exciting fluorescence, but which differed from 
 ultra-violet light, and indeed from every known kind of 
 radiation, in being able to penetrate through black card- 
 board, wood, and even through thin sheets of metals 
 that are quite opaque to everything else. He was thus 
 able to cast upon a fluorescent screen the shadows of 
 the bones within the hand, or of the coins inside a purse. 
 
 Now every student of physics knows of the principle 
 of reversibility ; the principle which has led to so many 
 discoveries of converse phenomena. Chemical combina- 
 tion can create an electric current : the electric current 
 can in turn produce chemical decomposition. An electric 
 current can be used to magnetize a magnet : therefore,^ 
 argued Faraday, it ought to be possible to generate an 
 electric current by means of a magnet — and the idea led 
 him to discover the principle of the dynamo. The cir- 
 cumstance that invisible rays when falling on a fluorescent 
 substance can make it shine in the dark naturally raised 
 the speculation whether it were not possible to make 
 a fluorescent body emit these invisible rays. The 
 possibility of reversing Rontgen's discovery must have 
 occurred to many minds. To two scientific workers, one 
 in London, one in Paris, this thought came with sufficient 
 force to cause them to make experiments to try whether 
 this possibility could be realized. 
 
 On February i6, 1896, I covered up a photographic 
 dry-plate in an opaque envelope of thin black paper, and 
 
VII RADIUM AND ITS RAYS 283 
 
 laying it face upwards on a window-sill, I laid upon it a 
 number of patches of substances known to be fluorescent 
 or phosphorescent, fluor spar, sulphides of the alkaline 
 earths, nitrate of uranium, bits of uranium glass, quinine, 
 and some platinocyanides. Other plates were prepared, 
 some of them having metal foil above the sensitive plate, 
 and different materials were placed above them in various 
 dispositions. After they had been given time to act, the 
 photographic plates were to be developed in the dark- 
 room. If after development they showed any markings 
 in the parts where the fluorescent substances had been 
 laid, this would have been prima facie evidence that the 
 fluorescent body did emit some sort of radiation akin to 
 the Rontgen rays. 
 
 On the 27 th of February the plates were developed. 
 The plate on which the miscellaneous collection of sub- 
 stances had been exposed through black paper showed, 
 to my great joy, a number of darkened patches, proving 
 that some of them had indeed emitted a radiation of a 
 highly penetrating character. 
 
 The scientific consequences of a discovery of this kind 
 are so important that they cannot be published without 
 further corroboration or criticism. The result seemed 
 to contradict the law laid down by Sir George Stokes for 
 fluorescence many years before, that in any transformation 
 of rays there is always a degradation of the wave-length 
 to a slower frequency, whereas this seemed to be a 
 transformation to a higher kind. So at once I wrote to 
 Sir George Stokes to apprise him of my observations, 
 and to ask his opinion. Meantime I developed some of 
 the other plates and found that some of them showed 
 
284 LIGHT LECT. 
 
 traces of action ; others none. None of the sulphides of 
 alkaUne earths or the platinpcyanides showed anything 
 through metal foil. In fact the only one that showed 
 anything through metal foil was a plate on which there 
 had been placed a number of crystalline fragments of 
 nitrate of uranium arranged in a circle over a sheet of 
 aluminium foil. Fig 159 is a reproduction of the identical 
 photograph obtained on February 27, 1896. 
 
 Then came Stokes's reply followed by a second letter. 
 He was most encouraging in saying that for some years 
 he had known observations that were exceptions to the law 
 he had laid down. But his second letter contained the 
 ominous remark: "I fear you have already been anticipated. 
 See Becquerel, Comptes rendus for Feb. 24, p. 420." And, 
 sure enough, there was announced in black and white the 
 discovery of the very same phenomenon by M. Henri 
 Becquerel, the third of the famous scientific dynasty of 
 Becquerels whose names are associated imperishably with 
 electricity and optics. He had found that crystals of the 
 double sulphate of uranium and potassium would act on 
 a photographic plate wrapped in black paper, and would 
 even traverse thin sheets of glass, aluminium, or copper. 
 
 The Becquerel rays — for, by the wholesome rule 
 established by Faraday, priority falls to him who first 
 publishes a discovery — are then a species of ray or emana- 
 tion which like the Rontgen rays can act on a photographic 
 plate, and can pass through opaque substances. During 
 the next few weeks I sought, as I had sought in the case 
 of Rontgen rays, to ascertain by experiment whether these 
 rays from uranium compounds could be polarized, or 
 refracted. To avoid all dogmatizing as to their nature 
 
Fig. 159.— Photographic Plate acted on by Fragments of 
 Uranium Nitrate. Obtained by the Author, February 
 27, 1896. 
 
Fig. i6o. — Henrt Becquerei., Discoverer of the 
 Becquerel Rays. 
 
VII RADIUM AND ITS RAYS 285 
 
 I spoke of the phenomenon as hyperfiuorescence. M. 
 Becquerel, who had apparently set out from much the 
 same standpoint of searching for a possible inverse rela- 
 tion between fluorescence and radiation, announced that 
 the rays discovered by him could not only penetrate 
 opaque substances, but could be reflected and refracted, 
 whilst I could not find any such effects. In the course 
 of the next few months he had pushed his investigations 
 much farther, and had established several facts. These 
 rays were independent of any fluorescence, and were 
 emitted by all the various salts of uranium. They were 
 continuously emitted, without appreciable diminution, 
 month after month. The emitting substance required no 
 stimulus such as subjection to light, or to heat : indeed 
 its emission of the rays appeared to be altogether inde- 
 pendent of temperature or any other physical conditions. 
 Further, and of utmost importance, it was observed that 
 these new rays possessed the power of causing the dis- 
 charge of electrified bodies, situated at a distance, across 
 the intervening air. Brought near to a charged gold leaf 
 electroscope the leaves gradually collapsed, the rate at 
 which the discharge proceeded being a measure of the 
 efficiency of the specimen in emitting these rays. This 
 furnished a second and quantitative method of study, 
 which proved in the sequel most invaluable. In the first 
 place it enabled M. Becquerel to ascertain that metallic 
 uranium was about two and a half times as active as the 
 double sulphate of uranium and potassium at first used. 
 Then it was found that the air plays a distinct part in 
 the effect, and that a sphere of electrified uranium though 
 it spontaneously discharges itself in the air does not 
 
286 
 
 LIGHT 
 
 LECT. 
 
 discharge itself in vacuo. Also that the air acted -upon 
 by uranium or its salts behaves just like air which has 
 been exposed to Rontgen rays, being more or less 
 ionized thereby. These results led M. Becquerel to 
 regard this property of emitting these radiations as a 
 specific atomic property of the metal uranium, and he 
 described the property itself by the name of radio-activity. 
 As soon as the radio-activity of uranium and its salts 
 
 became an established fact there 
 arose a search for other materials 
 which might possibly be radio- 
 active. 
 
 The fundamental experi- 
 ment is very easily demon- 
 strated. A convenient form of 
 electroscope is that depicted 
 in Fig 1 6 1. From a highly- 
 insulating support of fused 
 quartz or of amber there hangs, 
 Fig. i6i.— Electroscope suitable on a short metal stcui, a single 
 
 for Observation of Radio- . . , . . 
 
 Activity. gold leaf or a leai of alummmm, 
 
 beside a stiff metal strip. To 
 charge the apparatus a short crooked brass wire which 
 passes through the top of the apparatus is turned until 
 its lower end touches the stem of the electroscope, and 
 so a charge given to the crooked wire is conveyed to 
 the gold leaf, which instantly stands out at an angle 
 from the stiff strip by mutual repulsion. The crooked 
 wire is then turned away out of contact with the strip, 
 leaving the electroscope charged. Now if one brings 
 near to the electroscope a bottle containing a few frag- 
 
VII 
 
 RADIUM AND ITS RAYS 
 
 287 
 
 ments of metallic uranium, the gold leaf is seen gently to 
 fall down toward the vertical position. The time 
 required for the deflexion of the leaf to be reduced to 
 half its initial value is, caeteris paribus^ a measure of the 
 discharging influence of the active substance. 
 
 Another and more accurate method of procedure is 
 
 to the 
 
 Electrometer 
 
 Bt 
 
 Layer of Radioactive Stuff 
 
 r\\ 
 
 Battery_ 
 'of Cell's 
 
 ^\W^W^ 
 
 » Switch 
 
 ^ 
 
 m 
 
 Fig. 162. — Madame Curie's Apparatus for detecting Radio- Activity. 
 
 shown by the arrangement of apparatus depicted in Fig. 
 162. 
 
 In this apparatus two metal plates are arrayed 
 parallel to one another. One of them can be highly 
 electrified by means of a battery consisting of a large 
 number of small accumulators, while the other is joined 
 by a wire to an electrometer, and by a switch to earth. 
 There is, therefore, an electric field between A and B, 
 the intensity of which can be varied by varying the 
 number of cells in the battery. If now the lower plate 
 is covered with a layer of some uranium compound (or 
 other active substance), the radiations which it emits 
 
288 
 
 LIGHT 
 
 LECT. 
 
 cause the air above it to become conductive, and an 
 actual electric current, weak indeed, but sufficient to be 
 measured, passes across from the lower to the upper 
 plate ; the strength of this current depending on the 
 electromotive force of the battery, the amount of surface 
 of the plates, and the intensity of the activity of the 
 substance laid on the lower plate. So long as the switch 
 is closed so that plate A and the electrometer are put to 
 earth, nothing is observed. But on opening the switch 
 so as to insulate the electrometer, it is observed to 
 become charged, and the rate at which its index is 
 deflected is proportional to the current to be observed. 
 With this delicate means of observation, made still more 
 accurate by a method of balancing the deflexion, devised 
 by the late M. Pierre Curie, Madame Curie investigated 
 the various compounds of uranium, and minerals con- 
 taining uranium and thorium. The relative results were 
 as follow : — 
 
 Metallic uranium 
 
 . 2-3 
 
 Green oxide of uranium . 
 
 . 1-8 
 
 Nitrate of uranium . 
 
 . .07 
 
 Oxide of thorium 
 
 o-i to 1-4 
 
 Pitchblende from Joachimsthal . 
 
 . 7-0 
 
 Pitchblende from Cornwall 
 
 . 1-6 
 
 Orangite .... 
 
 2-0 
 
 Monazite . . . . 
 
 • 0-5 
 
 Carnotite .... 
 
 . 6-2 
 
 Chalcolite . , c . . 
 
 . 5-2 
 
 All the minerals which showed themselves active 
 contained either uranium or thorium ; but the surprising 
 fact appeared that some of them were more active than 
 pure uranium itself To clear up this anomaly Madame 
 
VII RADIUM AND ITS RAYS 289 
 
 Curie prepared from pure nitrate of uranium and acid 
 phosphate of copper an artificial chalcolite which, how- 
 ever, showed only about 0*92, a figure about proportional 
 to the quantity of uranium in it. Thence it became 
 probable that since pitchblende and natural chalcoHte 
 showed so great an activity, it must be that they contain 
 also a small quantity of some much more highly active 
 substance different from either uranium or thorium. 
 Madame Curie and her husband therefore set to work 
 to extract, if possible, by processes of chemical analysis, 
 from the mineral pitchblende, the more active constituent, 
 the existence of which she had thus been led to suspect. 
 The research was extremely laborious ; for a large 
 quantity of pitchblende ore had to be first dissolved, and 
 all the various known constituents separated out by 
 precipitation and each result tested to find the presence 
 of the active substance. Two such substances were in 
 fact discovered. One which closely resembled bismuth 
 was found by Monsieur and Madame Curie and was 
 called polonium in honour of Madame Curie's native 
 land; the other, which was precipitated along with 
 barium, was separated by Madame Curie in collaboration 
 with M. Bremont and was called radium. Chemically 
 it resembled barium, from which it was finally separated 
 as chloride of radium by fractional crystallization, the 
 radium salt being slightly less soluble than that of 
 barium. A third active body was afterwards obtained 
 from pitchblende by M. Debierne and called actinium ; 
 it resembles thorium chemically. 
 
 Radium is now extracted from pitchblende, and 
 chiefly from the uranium residues of the pitchblende 
 
290 LIGHT LECTo 
 
 mines at Joachimsthal in Bohemia. This mine has 
 been long worked for uranium, which is used in the 
 manufacture of canary-coloured glass. The mineral is 
 roasted with carbonate of soda, and the resulting mass is 
 treated with warm water and then with dilute sulphuric 
 acid. The solution contains the uranium. The insoluble 
 residues which contain the radio-active bodies used to bs 
 thrown away. Madame Curie obtained some tons of 
 these residues. They consist chiefly of sulphate of 
 lead, sulphate of lime, silica, alumina, and oxide of iron, 
 accompanied by small quantities of many other metals. 
 The process of extraction of the radium is tedious and 
 costly. One ton of residue yields from forty to fifty 
 pounds of crude sulphates, the activity of which is from 
 thirty to sixty times as great as that of metallic uranium. 
 Then begins the long process of fractionating the chlorides 
 or bromides to concentrate the least soluble part by ' 
 crystallizing and redissolving many times. Each operation 
 reduces the quantity of material till at last a few grains 
 only remain, which may have, however, an activity a 
 million times greater than that of the original residue. 
 Radium is consequently very costly — in fact the most 
 costly substance known on earth. The price in 19 lo for 
 the purest radium bromide is about ;£i6 for one 
 milligramme. That is ^16,000 per gramme, or 
 ;£"5, 25 7,600 per pound ! 
 
 Pitchblende is also found in Cornwall. The Cornish 
 samples are less rich in radium than those from Bohemia, 
 but at the present price of the precious product the 
 Cornish ore should be well worth extracting. Several 
 mineral springs, such as the waters of Bath and those of 
 
Fig. 163. — Madame Curie. 
 
Fig. 164. — Righi's Skiagram. 
 Obtained by exposure to Radium Bromide. 
 
VII RADIUM AND ITS RAYS 291 
 
 Buxton, are found to contain traces of radium. The 
 Hon. R. J. Strutt has indeed found that many soils and 
 rocks contain radio-active matter in small traces. 
 
 A small quantity of radium bromide, as large only as 
 a mustard seed, will suffice to show the characteristic 
 properties, so marvellously active it is. If a photographic 
 plate is covered with a sheet of aluminium foil, and 
 opaque metal objects are laid over it, then an exposure 
 for a few minutes to the radiations of radium will suffice 
 to produce on it the shadows of these objects. The 
 accompanying plate, Fig. 164, was thus produced by 
 Professor Righi, of Bologna, during a lecture. It is 
 easy similarly to show the radio-active properties of 
 thorium salts. If a piece of a common Welsbach mantle 
 (see p. 345 below) is flattened out and dried, and is then 
 laid down (in the dark) on an ordinary photographic 
 dry-plate and left there in the dark for a few days and 
 developed, a print will be found showing the structure of 
 the mantle. 
 
 The power of radium to ionize the air in its neigh- 
 bourhood is shown by its rendering the air conductive 
 as in the experiment illustrated in Fig. 162 ; but another 
 electrical experiment shows how it may facilitate the 
 passing of a spark between two metal conductors in air. 
 
 Let an ordinary small spark-coil be arranged with 
 wires from its secondary terminals SS to two spark-gaps 
 A and B, in parallel to one another. By adjusting the 
 brass balls at each of these gaps to equality it can be 
 arranged that the sparks shall pass equally frequently at 
 A or B. Now bring a small specimen of radium bromide 
 near either one. At once the sparks will disappear at 
 
292 
 
 LIGHT 
 
 LECT. 
 
 Induction Coil 
 
 the other spark-gap, and will be redoubled at the spark- 
 gap that is near the radium. 
 
 When the Becquerel rays were first discovered the 
 question was keenly discussed whether they were the 
 
 same as Rontgen's rays or 
 not. Becquerel himself at 
 first thought that he had been 
 able to reflect, refract, and 
 polarize them, so that in spite 
 of their great penetrative power 
 they were essentially different. 
 But when other experimenters 
 Fig. 165. -Spark-gap Experiment, totally failed to find any trace 
 
 of these actions, the question 
 of similarity once more became important. Like the 
 Rontgen rays the Becquerel rays could be stopped, 
 absorbed, by using thick sheets of lead, though they" 
 would penetrate lighter metals, and thin sheets even 
 of lead. But the Rontgen rays differed amongst them- 
 selves. ■ Those generated in tubes that had been 
 carried to the highest degree of exhaustion (or "hard" 
 tubes) were more highly penetrative in their action 
 than those generated in less highly exhausted (or 
 " soft ") tubes. They were not deflected by a magnet as 
 the kathode streams were. Would the Becquerel rays 
 show similar peculiarities ? So soon as the isolation of 
 radium furnished a more powerful source of radio-activity 
 it became possible to answer such questions. The 
 Curies made careful experiments and showed that they 
 were not homogeneous, but consisted of several sorts with 
 distinct properties. A small quantity of radium salt was 
 
VII 
 
 RADIUM AND ITS RAYS 
 
 293 
 
 placed in a hole bored into a small thick cylinder of 
 lead. From the mouth of the hole the rays of all sorts 
 emerged. Across the line of their path was directed a 
 very intense magnetic field. If the radium rays were 
 negatively electrified like the kathode rays they would 
 be deflected sideways. If they were positively electrified 
 like the parakathodic (or " canal ") rays they would be 
 
 Fig. 166. — Deflexion by a Magnetic Field of the Rays emitted by Radium. 
 
 deflected to the other side. If they were like Rontgen's 
 rays they would not be deflected at all 
 
 To test any such deflexion a photographic dry-plate 
 was placed parallel to, and just below, the emerging 
 stream of rays, and in a plane at right angles to the 
 magnetic field. 
 
 The experiment revealed a surprising fact. All three 
 kinds of rays were present. Fig. 166 shows the result. 
 Some rays were deflected slightly to the left at A and 
 
 Y 
 
294 LIGHT LECT. 
 
 apparently unable to penetrate far into the air, and 
 the direction of their deflexion proved them to carry 
 positive charges of electricity. Others deflected to the 
 right, at B, showed beautifully curved trajectories, and 
 carried negative charges. A third series were shot out 
 almost straight, and carried no electric charges. Later 
 these three kinds of " rays " were denominated by 
 Professor Rutherford as a, ^, and 7 rays respectively. 
 
 CL. The aipha-r2iys resemble ca?tal-r^.ys, and are positive. 
 13. The beta-rsiys resemble kathode-xdcys^ and are negative. 
 y. The gamma-rdcy?> resemble Rontgen's rays or X-rays. 
 
 The a-rays have little penetrative power ; a sheet of 
 aluminium a few thousandths of an inch thick stops 
 them : they appear to consist of flights of single atoms 
 positively charged, moving at a high speed. 
 
 The ^-rays also behave like charged bodies ; but 
 they are negatively charged ; and their mass is much less 
 than that of ordinary atoms, being minute corpuscles — 
 — " electrons " — certainly not more than -y^^-q P^^^t as 
 heavy as hydrogen atoms. 
 
 The <y-rays are not deflectable : they form but a 
 small part of the total radiation, but their penetrative 
 power is extraordinary. They act exactly like Rontgen 
 rays of the most penetrating kind. 
 
 Further to test the deflexion question the following 
 apparatus was devised. A small quantity of radium salt 
 in a little lead capsule was placed within a very thick 
 tube of lead. The rays would be shot out along the 
 tube. A charged electroscope placed opposite the 
 other end of the lead tube was at once discharged by 
 
VII 
 
 RADIUM AND ITS RAYS 
 
 295 
 
 the rays. Let a powerful electromagnet be placed to 
 produce a magnetic field at right-angles to the line of 
 the rays. On exciting the electromagnet the ^-rays are 
 deflected laterally against the inside of the lead tube, 
 and if the electroscope is now charged, it will remain so, 
 or will discharge itself only very slowly. On switching 
 off the exciting current from the electromagnet the /3-rays 
 
 Electroscope 
 
 ij 
 
 Fig. 167. — Deflexion Experiment. 
 
 once more strike the electroscope, causing the gold 
 leaves to drop down. 
 
 Another most informing experiment is the following, 
 which proves that the ^-rays carry negative charges. A 
 block of lead M (Fig. 168) is insulated and placed 
 within a closed metallic chamber C, the bottom of which 
 is made of thin sheet aluminium. This enclosing 
 chamber is earthed, and the inner block is connected 
 by a wire to an electrometer. A thick lead tray PP, in 
 
296 
 
 LIGHT 
 
 LECT. 
 
 which rests a layer R of radium salt, is now brought 
 below the chamber. The a-rays are stopped by the 
 
 ^ <^ to Electrometer 
 
 Fig. 168. — Experiment to prove that Beta-rays carry a Negative 
 Electric Charge. 
 
 metal sheet and the thin layer of paraffin wax between 
 it and M. But the /5-rays penetrate through until 
 absorbed in the block M, which becomes negatively 
 electrified as ascertained by the electrometers. 
 
 Fig. 169. — Experiment on electrification by Alpha- rays. 
 
 To show the production of positive electrification by 
 the a-rays the disposition is varied, as in Fig. 169. The 
 lead tray is now itself enclosed in the earthed metal 
 
VII 
 
 RADIUM AND ITS RAYS 
 
 297 
 
 chamber, and is connected to the electrometer. The 
 i8-rays can penetrate through the paraffin wax and the 
 metal of the enclosure, but the a-rays are stopped, 
 consequently the lead tray acquires a positive charge, 
 which (in the same time) is 
 equal in amount, as shown 
 by the electrometer, to the 
 negative charge of the previous 
 experiment. 
 
 This explains a curious cir- 
 cumstance which was observed 
 by an experimenter who opened, 
 with a file, a sealed tube of glass 
 in which some salts of radium 
 had been kept for a time. On 
 scratching the glass with the 
 file a spark pierced it at the 
 place where the file was applied, 
 and the observer received a 
 small electric shock in his hand. 
 The tube had acted as a spon- 
 taneously-charged Leyden jar. 
 
 This self- charging property 
 of the radium salts is the ex- 
 planation of the ingenious radium-clock of the Hon. 
 R. J. Strutt. (Fig. 170.) A morsel of radium salts is 
 enclosed in a thin- walled glass tube ^, coated with a film 
 of conducting substance, and ending at the bottom in a 
 brass cap from which hang a pair of gold leaves. This 
 system is suspended by a glass stem from a stopper that 
 fits hermetically tight into an outer glass tube from which 
 
 Fig. 170. — Strutt's "Radium 
 Clock." 
 
298 LIGHT LECT. 
 
 the air is exhausted. Inside the outer tube are fixed 
 two tinfoil strips tt which are connected to the earth by 
 a wire k sealed through the glass. The action is as 
 follows. The y8-rays carry negative electricity away from 
 the central system, which is left positively charged, 
 causing the gold leaves to diverge gradually. When 
 they diverge enough they touch the tinfoil of the 
 containing envelope, and are discharged and fall down. 
 Then the process begins again. The frequency with 
 which the operation is repeated depends on the quantity 
 of radiurft salt used — with a single milligramme each 
 operation may last several minutes. But it goes on 
 night and day without ceasing, since radium appears to 
 be practically unending in its activity. It is the nearest 
 approach yet to realizing a perpetual motion. 
 
 The radiations thrown off from radium can excite 
 brilliant fluorescence and phosphorescence. A single- 
 milligramme of radium salt placed near a piece of the 
 mineral willei7iite (a sulphide of zinc) evokes a fine 
 green light, which continues as long as the radium is 
 near the mineral. Sir William Crookes observed that 
 the minutest specks of radium compounds, if placed on 
 the surface of a fluorescible body such as the platino- 
 cyanide of barium or willemite, produce in their 
 immediate neighbourhood a scintillating luminescence. 
 He accordingly constructed the little apparatus called 
 the Spinthariscope^ in- which a minute particle of radium 
 salt is held above a fluorescent screen and observed in 
 the dark through a simple microscope. A perpetual and 
 silent display of minute sparks is seen to radiate out, like 
 microscopic fireworks, from the central speck of radium. 
 
VII RADIUM AND ITS RAYS 299 
 
 This effect is due to the a-rays, and can be produced 
 also by using polonium instead of radium. 
 
 When this property of uranium and radium to emit 
 rays spontaneously was discovered, much question arose 
 as to the source of the energy so manifested. Uranium 
 would go on month after month showing radio-activity, 
 apparently without any diminution, without requiring 
 to be heated or to receive any stimulus of sunshine. 
 After the far more powerful radium had been discovered, 
 the source of its energies was still more puzzling. It 
 was found, amongst other effects, that the discharges 
 from it were dangerous to the person who handled it, 
 and that if held near the body — in the waistcoat pocket 
 — or against the arm, even if enclosed in a glass tube, it 
 was liable to set up sores. 
 
 Most surprising of all was the discovery by Pierre 
 Curie that the radium salt was itself always at a slightly 
 higher temperature than the surrounding objects. 
 Radium is itself always and continually giving out heat 
 spontaneously. To demonstrate this he took two ordinary 
 mercury thermometers, and placed them each one in 
 a Dewar's vacuum-jacketed glass flask. Beside the 
 thermometer in one flask A (Fig. 171) was placed a 
 little glass tube a containing seven decigrammes of pure 
 bromide of radium. In the other was placed a similar 
 tube with an equal weight of bromide of barium (or other 
 inactive salt). The openings ot the flasks were closed 
 with cotton-wool. In these conditions the thermometer 
 in the flask beside the radium persistently showed a 
 temperature three degrees higher than the other. A 
 gramme of radium will in one hour evolve no less than 
 
300 
 
 LIGHT 
 
 LECT. 
 
 eighty calories of heat — enough to melt its own weight 
 
 of ice. 
 
 Whence comes this inexhaustible supply of energy? 
 
 The radium does not seem to lose weight, though the 
 
 minuteness of the quantities to be experimented upon 
 
 makes certainty on this head difficult. 
 
 It w^ould lead us too far away from the subject of the 
 
 radiations to follow the 
 amazing views which the 
 chemists have been com- 
 ry^. A^r/^ pelled to frame in order 
 to explain the mystery. 
 Radium, though it acts as 
 an element, forming regu- 
 lar salts and compounds, 
 
 /~Mi 
 
 
 
 \jy 
 
 h 
 
 B 
 
 Fig. 171. — P. Curie's Experiment on 
 Heat evolved from Radium. 
 
 having a characteristic 
 spectrum of its own, a 
 specific heat of its own, 
 and an atomic weight (225 
 according to Madame 
 Curie) of its own, neverthe- 
 less appears to be an ele- 
 ment capable of an evolution, and to be itself evolved 
 from the element uranium. If we are to accept the 
 conclusions of Rutherford and other workers, in fact of 
 those best entitled to speak, and if we can swallow our 
 objection to such a contradiction in language, and accept 
 as possible the division of an " atom " into constituent 
 parts, then it appears that the radium atom disintegrates. 
 In disintegrating it gives off a chemically inert gas called 
 the " emanation," which can be collected, and which in 
 
VII RADIUM AND ITS RAYS 301 
 
 turn deposits a solid called " Radium A," and this in a 
 very short time gives rise to another body " Radium C," 
 and this changes (while giving off a, ^, and y rays) into 
 " Radium D," a comparatively stable body, which slowly 
 changes into "Radium E," and this into another body, 
 "Radium F." During some of these changes rays are 
 given off; during others no rays are given off; and 
 apparently at every stage an atom of the inert gas helium 
 is also emitted. The last stage appears to be a degenera- 
 tion into an inert material like lead. Other radio-active 
 bodies undergo similar changes. Uranium itself produces 
 a substance called "Uranium X," and thorium also 
 undergoes stages of transformation, in one of which 
 there is an emanation produced. 
 
 All this is most mysterious, not to say perplexing. 
 It suggests that locked up in the atom itself is a store of 
 intra-atomic energy, which is observable only in the 
 atoms of such bodies as are in constitution unstable. 
 The stable atoms — by far the greater majority of the 
 elements — may equally have stores of energy locked up 
 in them ; but, being stable, they undergo no changes or 
 disintegrations and we look upon them as inert and 
 unchangeable, even though we can no longer deny that 
 they may be capable of being changed. The transmuta- 
 tion of the metals in the old sense of the alchemists may 
 be still as hopeless as ever, but a new kind of trans- 
 mutation has been discovered. It is the beginning of a 
 new departure in science. 
 
 ^^^■fc 
 
LECTURE VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 Primitive sources of light — Invention of gas-lighting — Invention of 
 electric -lighting — Cause of incandescence — Incandescence by 
 electricity — Luminescence — Luminous efficiency — Photometry 
 — The Photometer — Inequality of distribution of light from 
 lamps — Inequality of composition of lights — The teaching of the 
 spectrum — Spectra of incandescent solids and vapours — Sensi- 
 tiveness of the eye to radiations of particular v^'ave-lengths — 
 Absorption and emission — Measurement of emission — Bad 
 economy of ordinary sources of light — Light of the fire-fly^ 
 Temperature, and quality of radiation — Emissivity of the rare 
 earths — High-pressure incandescent gas-lighting — Efficiency of 
 glow-lamps — New kinds of glow-lamps — New kinds of arc- 
 lamps — Electric vapour-lamps — Cost of manufacture of light — 
 Cheapest form of light — Future progress — Sunlight after all. 
 
 Primitive Sources of Light. — From the earliest dawn 
 of primitive civilization one of the needs of man- 
 kind has been that of artificial light wherewith to lighten 
 the darkness that fell upon him when the sun withdrew 
 his beams. The savage who invented the use of the 
 fire-drill, and by the friction of two pieces of wood pro- 
 cured for himself a spark of fire, discovered thereby not 
 merely the means of warming himself and of cooking his 
 food, but also of providing himself with a luminous flame. 
 Primitive man, while yet a mere cave-dweller, discovered 
 
 302 
 
 ^^ 
 
LECT. VIII THE MANUFACTURE OF LIGHT 303 
 
 that animal fats and oils would burn, and putting a bit 
 of dry wood or a wisp of grass into a shell or a hollow 
 skull filled with oil, constructed a simple lamp. Amongst 
 the relics of Egyptian tombs or the excavations of 
 Pompeii are found domestic lamps of earthenware or 
 of bronze. Rudely hollowed stones were used by the 
 Northmen. In the museums of Roman remains are to 
 be seen the rude lamps of burned clay which served to 
 hold the oil. Fish oils and animal oils were the sole 
 supply. The blubber of the sperm whale was sought for 
 in an industry which flourished for several centuries, and 
 made prosperous the seaports of Whitby and Hull. 
 Vegetable oils pressed out from seeds, though known to 
 the apothecary, were scarcely used for illuminating pur- 
 poses till the eighteenth century ; while mineral oils pro- 
 cured from shale, or collected from petroleum wells, were 
 unknown till about eighty years ago ; and the paraffin 
 oil-lamp is an invention within the lifetime of most of us. 
 Primitive man also discovered that a piece of dry 
 wood, or a rope of grass dipped into melted fat, would 
 make an excellent torch ; and by dipping a dried rush 
 into molten tallow he procured for himself in small 
 portable form a candle. To substitute a woven wick, 
 and to devise means of casting tallow or wax around the 
 wick in a mould, were improvements devised a little 
 more than a hundred years ago. If we were to go back 
 to the days of good Queen Bess we should find that the 
 means of lighting either hovel or palace were primitive 
 in the extreme. In the guttering of the rushlights and 
 the splutter and smell of the lamps fed with animal oil 
 we should scarcely rejoice. 
 
304 LIGHT LECT. 
 
 Invention of Gas -lighting. — Early in the nine- 
 teenth century came a great scientific stride forward in 
 the invention of coal gas. In 1802 Murdoch lit the Soho 
 Works with gas distilled in iron retorts ; and in 1803 and 
 1804 Winsor exhibited at the Lyceum in London a system 
 of gas-distribution, and patented a method of purifying 
 the gas and saving the residual products tar and ammonia. 
 In 1808 Murdoch received the Rumford medal of the 
 Royal Society for this application of science to industry. 
 In 18 TO the Gas-light and Coke Co. was incorporated-^ 
 to light the streets of London. 
 
 It was several years later that gas-lighting was begun 
 on the continent of Europe. 
 
 Invention of Electric-lighting. — If the application 
 of science to industry in the manufacture of illuminat- 
 ing gas was of British origin, no less truly British was 
 the origin of lighting by electricity. Humphry Davy 
 in 1 80 1 or 1802 showed the arc light between carbon 
 
 ^ In view of recent legislation as to electric-supply companies, it 
 is of interest to note the conditions imposed in 1 8 10 on the pioneer 
 Gas Company. They are stated as follows in Accum's Treatise on 
 Gas-light, fourth edition, 1818, p. 44 : — 
 
 "The power and authorities granted to this corporate body are 
 very restricted and moderate. The individuals composing it have 
 no exclusive privilege ; their charter does not prevent other persons 
 from entering into competition with them. Their operations are 
 confined to the metropolis, where they are bound to furnish not only 
 a stronger and better light to such streets and parishes as chuse to 
 be lighted with gas, but also at a cheaper price than shall be paid 
 for lighting the said streets with oil in the usual manner. The 
 corporation is not permitted to traffic in machinery for manufacturing 
 or conveying the gas into private houses, their capital or joint stock is 
 limited to ;^200,ooo, and his Majesty has the power of declaring the 
 gas-light charter void if the company fail to fulfil the terms of it." 
 
 A^ 
 
VIII THE MANUFACTURE OF LIGHT 305 
 
 poles in the theatre of the Royal Institution, using a 
 battery of 150 pairs of plates; and thirty years later 
 Faraday invented the principle of the dynamos by which 
 the requisite currents could be generated mechanically.^ 
 As for electric glow-lamps, Grove lit the London Institu- 
 tion in 1847 with little lamps having platinum -wire 
 filaments ; and Swan's experimental carbon wire lamp of 
 1879 was the precursor of all the glow-lamps of to-day. 
 
 That an electric spark or discharge could produce 
 light goes back far beyond Davy's time, since electric 
 sparks were first observed about 1670 by Otto Guericke, 
 and Hawkesbee in 1706 observed that an electric dis- 
 charge sent through a vacuous globe caused it to be 
 filled with a soft, luminous glow. But Bishop Watson, 
 using an inverted U-shaped tube on the plan devised by 
 Lord Charles Cavendish for a double barometer, pro- 
 duced a most beautiful arch of lambent flame. The 
 source was, of course, an old-fashioned friction-machine, 
 for the date was 1746 {Phil. Trans, xlvii.) ; but if we now 
 repeat the experiment and apply a current from a modern 
 electric-supply we get a fine vacuum lamp. It was not 
 until the middle of the nineteenth century that Dr. Geissler 
 popularised the vacuum tube, in which the residual gases 
 became luminous under the discharge of the induction coik 
 
 In this very brief enumeration we have gone over 
 
 ^ It is true that improvements in both carbons and in generating 
 machines were made in France as well as in England. But the 
 first mechanical arc-lamp was that of Staite in 1847 ; and while 
 batteries were used for the electric-lighting of London Bridge in 
 1863, on the occasion of the marriage of King Edward VII., then 
 Prince of Wales, dynamos were employed when electric-lighting was 
 introduced in the 'seventies into lighthouses and into the navy. 
 
3o6 LIGHT LECT. 
 
 the early attempts to manufacture light by flames or by 
 electric-currents. 
 
 Let me point out that in all these attempts (save the 
 vacuum tube or vapour lamps) the evolution of the light 
 is the result of incandescence. The process that is common 
 to all of them is that something is made very hot, and 
 shines only because it is very hot. That is to say, 
 whether by flames or by electric-currents, that which is 
 primarily manufactured is heat, and the body on which 
 the heat is concentrated becomes luminous, so that as a 
 secondary effect light is produced. 
 
 Incandescence in G-eneral. — Now, as a great part 
 of this lecture will deal with incandescence^ it is worth 
 while to try to understand thoroughly all we can about 
 the matter. Incandescence is the shining of hot bodies 
 because they are hot. We all know that if we take any 
 solid thing, such as an iron rod, or a brick, and heat it 
 enough it will become red hot, that is, it will emit or 
 radiate out a red light. If you heat it hotter it shines 
 more brightly, and gives out not only more light, but 
 light of a whiter quality. If it is less heated, the light 
 is duller and redder ; and if heated insufficiently to make 
 it red hot, it still emits radiations, but they are of an 
 invisible kind, though they can be felt as radiant heat 
 by holding one's hand near the hot object. All hot 
 bodies emit into the space round them these invisible 
 heat-rays, and the hotter they are the more they emit. 
 So that in all cases of lighting by incandescence the 
 incandescent substance sends out a lot of heat as well 
 as some light As we shall see, one of the scientific 
 problems of the day is to find a sort of lamp which shall 
 
VIII THE MANUFACTURE OF LIGHT 307 
 
 give light without heat. All our sources of artificial light 
 are deplorably wasteful. They burn up a lot of gas or oil, 
 or use up an electric-current, and waste the greater part of 
 it in emitting heat-rays tliat we don't want, and utilise very 
 little of it in generating the light-rays that we do want. 
 
 Solid Particles in Incandescence. — We have seen that 
 all solid bodies when heated hot enough become in- 
 candescent. The brightness of ordinary flames such as 
 those of coal gas, paraffin, tallow, etc., is, as we shall see, 
 due to there being solid particles in them. If you can 
 burn these substances so that no solid particles are 
 present in their flames we get very little light indeed, but 
 we can introduce into them solid refractory substances, 
 such as lime or magnesia, or a wire of some difficultly 
 fusible metal (such as platinum), or scatter into them fine 
 solid particles, and then the solids so introduced shine 
 brightly though they are no hotter than the flame itself. 
 You all know the pale blue flame of the atmospheric 
 gas-burner, called the Bunsen-burner, after the famous 
 chemist who introduced its use. It gives very little light, 
 but it is very hot. If you stop up the holes where the 
 air is admitted the flame at once becomes bright, but it 
 also becomes larger, so that it is in reality less hot ; the 
 smaller the space into which we can concentrate the 
 burning of a given quantity of gas, the hotter we should 
 expect the flame to be. 
 
 Everybody will have observed that the flame of an 
 ordinary candle or of an ordinary gas-jet is not all of 
 uniform brightness : there is always a bluish or nearly 
 non-luminous part at the bottom below the bright part, 
 and this blue part really extends round the flame. It is 
 
3o8 
 
 LIGHT 
 
 LECT. 
 
 here that the coal gas is burning with plenty of air as 
 in the Bunsen-burner, and the heat which it gives out 
 decomposes the gas within the flame and causes the 
 separation of a sheet of carbon particles which glow partly 
 because they are in a very hot region and partly because 
 they burn away like so many bits of charcoal. Professor 
 Smithells of Leeds, the principal authority on flames, has 
 
 Interior 
 
 Exterior 
 
 -1613° 
 
 1613° 
 
 --1100° 
 
 1613° 
 
 --1110° 
 
 1576° 
 
 --947° 
 
 1547° 
 
 -641° 
 
 1560° 
 
 --382° 
 
 1564° 
 
 -196° 
 
 1478° 
 
 Fig. 172. — Temperatures in Gas Flame (Smithells). 
 
 shown that an ordinary flat flame really consists of two 
 flat burning surfaces with two sheets of glowing particles 
 and a cooler layer of unburned gas between. The 
 temperature varies from point to point and it has been 
 mapped out ; in some places it is sufficient to melt a very 
 fine platinum wire, and in that neighbourhood of course 
 the glow of the carbon particles is very bright. Fig. 172 
 represents a batswing gas-flame; at the side are marked the 
 temperatures on an arbitrary scale, lower than true centi- 
 grade, which were found to exist at the various points indi- 
 
will THE MANUFACTURE OF LIGHT 309 
 
 cated. The brightness of the upper part of the gas-flame 
 is due to the particles of carbon formed in the lower part. 
 
 When once it was recognised that the brightness of 
 flames was due to radiation from solid matter, it was 
 an obvious suggestion to try to get more light from gas 
 by burning it with the hottest sort of flame — the non- 
 luminous one, and then inserting in the flame a network of 
 fine wire. In fact thirty years ago mantles of wire-gauze 
 were tried. The town of Nantes was lit with such burners. 
 But they were not successful, being too perishable. 
 
 In the delightful lectures of Faraday on the Chemistry 
 of the Candle a number of simple experiments are 
 described about the production of flame : how the 
 melted wax or tallow rising in the wick is distilled into 
 gas which burns and forms the flame. Every flame is, 
 in fact, a gas-flame- — the lower part of the flame and 
 the wick acting as a sort of miniature gas-factory wherein 
 tallow or wax is distilled in place of coal. We may now 
 add to this that every luminous flame also resembles 
 the incandescent gas - burner, for though it has no 
 " mantle " hung in it to be heated, it has solid particles in 
 it to emit the light. The lime-light is an example of the 
 principle of incandescence ; for the oxyhydrogen blow- 
 pipe flame used in it is non-luminous, while the piece of 
 lime against which it plays becomes brilliantly white-hot. 
 
 Incandescence by Electricity. — Turning to incan- 
 descence by electricity, we may remember that when- 
 ever an electric - current is forced to flow through a 
 resistant conductor — one made of a resistant material, 
 or a wire of a relatively thin section — the energy expended 
 on forcing the current through is transformed into heat, 
 
3IO LIGHT LECT. 
 
 and this heat is evolved at the place where the current 
 meets with the resistance. If a chain be made of alter- 
 nate links (well joined together) of iron and copper wire, 
 and an electric-current is sent through it, the iron links 
 will become red hot, because they resist more than the 
 copper links. Combustion has nothing to do with this 
 electric - heating. The thin carbon wires inside our 
 ordinary glow-lamps are not consumed. They are simply 
 heated from within by the current that is being forced 
 through the highly -resistant carbon thread; and they 
 shine simply because they are hot. In order that they 
 shall not burn away, the air is pumped out of the glass 
 globes in which they are enclosed. In the arc-lamp two 
 stout pencils of carbon, joined by wires to the two electric- 
 supply mains, are made to touch one another for an 
 instant, and then are parted asunder to about -|-irich. 
 The current forms a sort of flame — called the arc—^ 
 between the carbon tips^ and the tips, particularly the 
 tip of the positive pencil, become white hot. The arc is 
 an intensely hot flame ; but it is not a flame that will 
 burn of itself. It only burns so long as electric-energy 
 is pumped into it. If the supply of electric-current is 
 cut off the arc goes out at once. The tips of the carbons 
 give light by incandescence, that is to say they shine 
 because they are hot. In one sense the arc-lamp and 
 the glow-lamp are not really electric-lights, but heat- 
 lights. In them we produce electrically an intense heat, 
 and then the carbons shine simply because they are hot. 
 The arc can be formed in air, or in an atmosphere of inert 
 gas, or even under water, though then it is very unstable. 
 When formed in the air, as in ordinary open arc-lamps, 
 
viir THE MANUFACTURE OF LIGHT 311 
 
 there is some combustion ; for the carbon tips slowly burn 
 away. But this combustion adds nothing to the light. 
 
 Luminescence. — So far we have been dealing with 
 the manufacture of light by incandescence, in which the 
 light is a consequence of heating. The question arises, 
 Is there no other way of manufacturing light without 
 heat ? There are certainly some examples to be found 
 in nature in which light is produced without heat. They 
 are mostly feeble, but they exist. There is first the 
 class of phenomena called by the name oi Phosphorescence. 
 Decaying timber and putrescent fish are often observed 
 to shine faintly in the dark. The sea in summer is often 
 phosphorescent at night, every stroke of the oar sending 
 out ripples that seem alive with innumerable points of 
 pale blue light. There is the glow-worm with its exquisite 
 little pale blue gleam, and, less frequently, the fire-fly 
 with its tiny star of light. These are all cold lights. 
 Then there is the artificial pale light emitted in the dark 
 by a stick of damp phosphorus — an example of a slow 
 chemical reaction. To these we must add the class of 
 artificial preparations — of which Balmain's luminous paint 
 is the best known — that have the property of shining in 
 the dark for hours, after having been shone upon by 
 daylight. This category of cold lights has been vastly 
 increased in recent years by the discoveries of Sir 
 WiUiam Crookes and others, of the possibility of making 
 substances shine in the dark by exposing them to the 
 special kinds of radiation known as kathode-rays and 
 their allies, excited by electric discharges in vacuous 
 tubes. Thus these kathode-rays falling on rubies make 
 them shine with an intense crimson light as if red-hot — 
 
312 LIGHT LECT. 
 
 yet they are quite cold. In the kathode beam a diamond 
 shines as a brilliant white star ; and the earthy oxides, 
 alumina, yttria, etc., glow with strange gleams. We 
 require a name to include all these cases of light without 
 heat. The name that has been given is Luminescence. 
 
 The problem before us is the manufacture of light ; 
 and we now realize that there are two different ways of 
 manufacturing it : one direct, the other indirect. In 
 the direct process, Luminescence^ energy of some other 
 kind is directly transformed into light. In the indirect 
 process, Lncandescence, we use either the energy of fuel 
 or that of the electric-current (itself generated from fuel 
 indirectly) to manufacture heat ; and the hot substance 
 gives out in reality most of its energy as invisible radiant 
 heat, and very little of its energy as visible light. It has 
 been shown that all our ordinary lamps waste in invisible 
 radiations — that is, in heat — over 99 per cent of the"^ 
 energy supplied to them, and turn less than i per cent of 
 their energy into light. Think, then, of the importance 
 of the problem before us. That man is said to be a 
 benefactor to his race who will make two blades of grass 
 grow where but one grew before. But in the manufac- 
 ture of light there is a vastly wider margin. He who 
 will invent a lamp giving light without heat will make 
 all his lamps a hundred times brighter than lamps now 
 are, or will make them as bright as they now are while 
 saving 99 per cent of the energy that they now require. 
 
 Luminous Efficiency. — You may well seem in- 
 credulous of such statements as that all our ordinary 
 lamps, electric as well as gas and oil, waste over 99 per 
 cent of their energy on making heat. You did not 
 
VIII THE MANUFACTURE OF LIGHT 313 
 
 imagine ^ that their efficiency was so low as i per cent. 
 But there can be no doubt about it. If science be 
 measurement, let us try to understand how such measure- 
 ments are made. 
 
 As a beginning let us get some simple notions as to 
 how the brightness of lights is measured. 
 
 Photometry. — Though the eye may be able to say of 
 two lights that one is brighter than the other, yet it is 
 quite incapable gf precision in judging how many times 
 one light is brighter than another. But a very simple 
 apparatus will enable us to form a notion of the prin- 
 ciples that are used in measuring the relative quantities 
 of light sent out by different lamps. Firstly, we must 
 
 ^ The term Iwjiinous efficiency is used to denote the percentage 
 borne by the energy of such rays as fall within the luminous range, 
 in the spectrum, to the total energy emitted. There is some dispute 
 about this, probably arising from the different apparatus used by 
 different observers. A good many determinations have been 
 published formerly in which various lamps are credited with much 
 higher efficiencies. Thus in Professor Fleming's work on Electric 
 Lamps the efficiencies of several lamps are given at about 3 per cent ; 
 while one is credited with an efficiency of 15 per cent. In this 
 lecture the more recent figures of Professor W. Wedding of Berlin 
 have been followed. He used a special bolometer provided with 
 means for discriminating between the energy of the visible and that 
 of the invisible rays. He finds no source of light except the flaming 
 arc to have a luminous efficiency as high as t per cent. On the 
 other hand, C. E. Mendenhall finds the luminous efficiency of an 
 ordinary glow-lamp to be about 2"6 per cent, which, according to . 
 him, would correspond to a temperature of 2150° (absolute) if the 
 filament acts as a black body. The term needs more exact definition, 
 since it is conceivable that two different sources might spend equal 
 fractions of their energy upon luminous rays, and yet, if one of them 
 radiated mainly light in the middle of the visible spectrum, whilst 
 the other radiated much red and blue as well as yellow and green, 
 the former would have a much higher luminosity. 
 
314 LIGHT LECT. 
 
 have some standard to go by. For many years the legal 
 standard in this country wsls the so-called standard cand/e, 
 being a candle of spermaceti wax, six to the pound, 
 burning 120 grains per hour. But the Metropolitan Gas 
 referees have instead adopted as their official standard a 
 light ten times as bright and much more constant, called 
 the Vernon Harcourt 10 candle-power Pentane Lamp. 
 All flame standards ^ are open to certain objections ; and 
 it is quite probable that shortly a special electric glow- 
 lamp^ of 10 candle-power will be adopted instead. 
 
 The oldest plan of comparing the light of two candles 
 or lamps was to set them to shine upon two pieces of 
 white paper side by side, with an opaque partition between, 
 so that each light shone on one of the white surfaces 
 only; and then the candles were moved until, judging 
 by eye, the brightness of the illuminated surfaces was 
 equal. To get such equality the brighter source must be 
 put farther away from the white surface. Thus if a 
 
 ^ Flames are unsuitable as standards because the emission by 
 them of light is dependent on so many different circumstances, the 
 temperature and pressure of the surrounding air, and the amount of 
 water-vapour present, as well as the purity of the substance burnt, 
 and the conditions under which the substance is supplied. In 
 Germany the standard flame is the Hefner Lamp in which amyl 
 acetate is burned, the flame being regulated to a precise height. 
 The light of this standard flame is called " one Hefner candle" and 
 is equal to 0*9 standard British candle. A serious objection to 
 the Hefner candle is its decidedly red colour. 
 
 2 See Professor Fleming, F.R.S.," On the Photometry of Electric- 
 Lamps," Jou7'nal of Proceedings of the Itisiitution of Electrical 
 Efigineers, xxxii. p. 119, Dec. 1903. Fleming's lamp derives its 
 standard quality by being always used at a rather low incandescence, 
 and by being enclosed in a rather large bulb. But its light is also of 
 a rather unduly red quality. 
 
VIII THE MANUFACTURE OF LIGHT 315 
 
 candle and a more powerful oil-lamp were being com- 
 pared, if the candle is set i foot away from the white 
 surface, the oil-lamp may need to be set at a distance of 
 3, 4, or 5 feet away to give an equal illumination. Now 
 if we may regard the candle and the lamp as being points 
 of light, the well-known law of point-action holds good, 
 namely, that the effect of that which emanates from the 
 point varies inversely as the square of the distance. So 
 after adjusting to equality we make the calculation by 
 squaring the distance. Thus if the candle at i foot away 
 is balanced by the oil-lamp at 3 feet away, we know that 
 the oil-lamp will be 3 X 3 times, that is, 9 times as bright as 
 the candle, that is, it is of 9 candle-power. If we had 
 had to put it 4 feet away to get equality of illumination, we 
 should know that it was of 4 X 4, that is, 16 candle-power. 
 
 Another simple way of comparing together the bright- 
 ness of two sources was to set up a rod in front of a 
 white surface, and let the two lights cast two shadows. 
 The stronger light casts the darker shadow. So again the 
 stronger source must be moved away until the shadows 
 are equally dark, and the calculation made as before. 
 
 Tlie Photometer. — Any such apparatus for measur- 
 ing the relative brightness of two lights is called a 
 Photometer^ and most modern photometers are pieces of 
 apparatus ^ of great precision. 
 
 In order to demonstrate the measurement of lights to 
 
 a large audience I have recourse to a novel sort of photo- 
 
 ^ The principal sorts of photometers are those of Foucault, Bunsen, 
 Lethaby, L. Weber, Joly, Swan, and Lummer-Brodhun. The last- 
 named is an instrument of precision superseding the Bunsen grease- 
 spot photometer. For comparison of lights of different colours the 
 *' flicker" photometers of Rood or of Simmance- Abady are preferred. 
 
3i6 
 
 LIGHT 
 
 LECT. 
 
 meter (Figs. 173 and 174). Along the top^ of the table 
 before me lies a narrow bench of wood about 30 feet in 
 length, along which I can slide the lights to be compared. 
 The standard light S^ is placed on one end of this bench ; 
 the light Sg, to be measured, is set on the beam toward 
 the other end. Between them stand two pieces of mirror 
 
 ©«*---:---r-.W. 
 
 
 Fig. 173 — Diagram of Simple Photometer. 
 
 glass, M and M , each set at 45°, so that the beams of 
 1 ght which come along the bench from the right and 
 from the left are both reflected forward toward my 
 audience. After being thus reflected forward they are 
 received on a piece of ground glass (or other dull trans- 
 lucent surface, such as a piece of tracing paper), where 
 
 Fig. 174. — General View of Simple Photometer. 
 
 you see, therefore, two rectangular patches of light side 
 by side. One of these is due to the beam from the 
 standard source, the other from the lamp that is to be 
 compared with it. Now as this is rather a large hall we 
 must work with a fairly large unit of light, so I take 10 
 candles mounted on a board and place them on the left 
 side, and set them at such a distance that they would 
 balance i candle placed at i foot on the- right side. 
 
VIII THE MANUFACTURE OF LIGHT 317 
 
 The distance at which tliey have to be placed is marked 
 " 10 " on the scale along the table. 
 
 Keeping these 10 candles in their place 1 take an 
 ordinary paraffin oil-lamp and put it on the other side. 
 If it happened to be exactly of 10 candle-power, we 
 should get balance when it was at an equal distance on 
 the other side. But it happens to be a stronger light, so 
 to get balance I push it along the bench until balance 
 is obtained, and then reading off the scale we find it to 
 be about 16 candles. The scale divisions do not, you 
 will notice, come at equal distances apart. For, accord- 
 ing to the law of squares, a light which balances at double 
 distance is 2 X 2 (that is, 4) times as bright, so that the 
 place for 40 on the scale is just twice as far along as the 
 place for 10, and the place for 90 will be 3 times as far 
 along, and so forth. The number of feet along will be 
 proportional to the square root of the candle-power. So 
 to balance i candle at i foot we may have 4 candles at 
 2 feet, 9 candles at 3 feet, 10 candles at 3 "16 feet, or 
 going to higher numbers, 36 candles at 6 feet, 100 
 candles at 10 feet, 400 candles at 20 feet. 
 
 Inequality of Distribution. — When once we have 
 got a means of measuring the brightness of lights we 
 soon discover that the amount of light shed by a lamp 
 is not equal in different directions. Some lamps send 
 more light downwards than upwards, others more upward 
 than downward. A flat flame like that of a paraffin-lamp 
 with a flat wick sends out rather less light in the edge- 
 ways direction than in any other. For factory lighting 
 the lamps are fixed overhead and are required to send 
 the light chiefly downwards. For street lighting a lamp 
 
3i8 
 
 LIGHT 
 
 LECT. 
 
 is desired that sends out most of its light sideways ; for 
 we do not want the pavement immediately beneath to 
 be brilliantly lit, while the spaces between the lamps are 
 in comparative darkness. Some of the results of 
 measurements of lamps by specially arranged photo- 
 meters are shown in the diagrams which follow and 
 
 130 
 
 120 
 
 no 
 
 loo 
 
 140 
 
 150 
 
 160 170 180 170 160 
 
 150 
 
 130 
 
 >< 
 
 
 \)<X\ 
 
 Tyx?\x 
 
 / 
 
 /\ 
 
 m 
 
 9^ 
 
 k 
 
 ^ 
 
 WM 
 
 Kg? 
 
 60 SO 40 30 20 10 10 20 30 40 50 60 
 
 Fig. 175. — Distribution of Luminous Rays of Paraffin- Lamp (Wedding). 
 
 110 
 
 100 
 
 so 
 
 70 
 
 which are due to Professor Wedding. In these the 
 amount of light at different angles above and below the 
 horizontal are plotted out in radial directions, giving a 
 curve of distribution. 
 
 Fig. 175 shows the unequal distribution of a paraffin- 
 lamp flame. It gave about 13 candles horizontally, but 
 at 45° downwards it gave only about 7 ; while at 45" 
 above horizontal, or 135° from the downward direction, 
 
VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 319 
 
 it gave nearly 22 candle-power. Straight down it gave 
 no light, owing to the shadow of the oil vessel. The 
 mean of the candle-power in every direction was 1 1 '6. 
 
 Fig. 176 gives the distribution of light from an inverted 
 gas-mantle. It gave very nearly 40 candle-power in 
 
 HK-50 
 
 50 HK- 
 
 Fig. 176. — Distribution of Luminous Rays of Inverted Gas-Mantle 
 (Wedding). 
 
 every direction below the horizontal, and threw practi- 
 cally no light above its own level. 
 
 Fig. 177 is the curve given by an ordinary Welsbach 
 mantle gas-light. This particular lamp gave 65 candle- 
 power horizontally, 80 candle-power at 20° above hori- 
 zontal, 43 at 20° below horizontal, about 21 straight up, 
 and none straight down. 
 
 Fig. 178 shows the curve of a high - pressure 
 
320 LIGHT LECT. 
 
 130 140 150 160 170 180 170 160 150 140 130 
 
 120 
 
 
 
 110 
 
 
 
 100 
 
 90 
 80 
 
 llTrr^^r 
 
 r r 1 ^ 
 
 M 
 
 
 'X yC\~"~^2r~ 7 1_ y 
 
 ^ 
 
 70 
 
 
 
 60 50 40 30 20 10 O 10 20 30 40 50 60 
 
 Fig. 177. — Distribution of Luminous Rays of Welsbach Mantle (Wedding). 
 
 120 
 
 100 
 
 130 140 150 160 170 180 170 160 150 140 130 
 
 120 
 
 100 
 
 30 20 1 
 
 Fig. 178. — Distribution of Luminous Rays of " Millennium ' Light : 
 High-Pressure Gas, with Long Mantle (Wedding). 
 
 120 
 
VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 321 
 
 110 
 
 100 
 
 "Millennium" incandescent gas-light of 1320 candle- 
 power horizontally, about 700 at 45° above the horizon- 
 tal, and about 560 at 45° below the horizontal. Straight 
 up and straight down it gave very little. 
 
 Fig. 179 gives the distribution curve of an ordinary 
 glow-lamp, the horizontal power of which was i6'i 
 
 130 
 
 140 
 
 150 160 170 180 170 160 150 140 
 
 Fig. 179. — Distribution of Luminous Rays of Electric Glow-Lamp (Wedding) ; 
 Watts, 59"i ; Mean Spherical Candle-Power, ii'26; Watts per Candle, 5-24. 
 
 120 
 
 100 
 
 candles. It gave 5 candles vertically downward, and 
 none vertically upward because of the socket from which 
 it hung. 
 
 Fig. 180 is the curve of a Nernst-lamp, giving 162 
 candle-power horizontally. Vertically up and down its 
 power was zero. 
 
 From a study of these diagrams of distribution it is 
 clear that a very imperfect, not to say inaccurate, state- 
 ment of the power of a lamp would be afforded if we 
 
322 
 
 LIGHT 
 
 LECT. 
 
 were merely to measure the candle-power in a horizontal 
 direction. Take, for example, a good gas-jet giving a 
 light equal to i6 candles in the horizontal direction. 
 Upward and downward such a flame sends out much 
 less light than i6 candles, so that the mean of the values 
 in all directions would be less than i6. In fact, the 
 
 120 
 
 no 
 
 130 140 150 160170 180 170 160 150 140 130 
 
 120 
 
 110 
 
 Fig. i8o. — Distribution of Luminous Rays of Nernst-Lamp with Vertical 
 Filament (Wedding) ; Watts, 213 ; Mean Spherical Candle-Power, 99*4 ; 
 Watts per Candle, 2*13. 
 
 mean spherical candle-power of such a gas-jet would only- 
 he about 13I candles. In any fair comparison, there- 
 fore, between lamps of different kinds, one ought to 
 consider, not the light sent out horizontally only, but the 
 total light in all directions, which is proportional to the 
 mean spherical candle-power;^ and this can either be 
 measured by a special spherical photometer or " lumen- 
 
 ^ See a valuable discussion by Mr. A. Russell in the Journal of 
 the Institution of Electrical Engweers, vol. xxxii. p. 631, 1903. 
 
viii THE MANUFACTURE OF LIGHT 323 
 
 meter," or must be calculated from a number of measure- 
 ments made at different angles. There is one exception, 
 that of arc-lamps and high-power gas-lamps intended for 
 lighting streets or large rooms. In such cases, as the 
 light thrown upwards is for the most part wasted and 
 lost, it is usual to calculate out the mean effect only of the 
 lower half, that is, the mean hemispherical candle-power. 
 
 Inequality of Composition. — An equally serious 
 consideration is the composition of the light. Every 
 schoolboy is familiar with the great discovery of Sir Isaac 
 Newton that white light consists in reality of a mixture 
 of lights of all different colours — red, orange, yellow, 
 green, blue, and violet. All the ordinary sources of 
 light, natural and artificial, send out these mixtures of 
 colours. To sort them out from one another we have 
 only to look at the light through a three-cornered glass- 
 prism, or to let a beam of the light pass through such a 
 prism and fall on the ceiling or on the wall. The prism 
 refracts the lights of different colours through different 
 angles, and presents the result to us as a rainbow-coloured 
 patch or spectrum. Fig. 181 depicts Newton's funda- 
 mental experiment. We now know the reason of all 
 this. Light consists of innumerable little wavelets of 
 extraordinary minuteness. Light of each colour 
 possesses its own particular wave-length ; or rather, light 
 of each particular wave-length has its own particular 
 colour. These wave-lengths (the widths of the ripples) are 
 so small that they must be measured in millionths of an 
 inch. Thus light that has ripples having a wave-length 
 of 27 to 30 millionths of an inch (70 to 75 millionths of a 
 centimetre) produces a red sensation in the eye, and we 
 
324 LIGHT LECT. 
 
 call it red light. Wavelets having a wave-length of about 
 15 to 16 millionths of an inch (t,6 to 40 millionths of 
 a centimetre) the sensation of violet. Those of 20 
 millionths of an inch (50 millionths of a centimetre) the 
 sensation of green. And since all incandescent bodies 
 give out waves of all sorts and sizes, they shine with all 
 the different colours at once, and give out w/ii'fe light. 
 We can demonstrate the composition of white light from 
 colours in another way, namely, by taking a glass disk 
 
 VIOLET 
 BLUE 
 PEACOCK 
 GREEN 
 .YELLOW? 
 
 ■^, -—•■:, ORANGE 
 
 ' - — -RED 
 
 Fig. 181. — Newton's Fundamental Experiment of Decomposition of White Light 
 
 by a Prism. 
 
 tinted with transparent coloured films, and causing the 
 light of the lantern to shine through it, so that the colours 
 are thrown on the screen before you. Then whirl the disk 
 round so as to mix the colours, and at once we get an 
 artificially mixed white light. 
 
 But all our lamps are not equally white in the light 
 they give. That is because a true white is not pro- 
 duced unless a proper proportion is preserved between 
 the component colours. If in the mixture there is too 
 much red, the resulting light will be a reddish white. If 
 there is too much green the light will be greenish, and 
 so forth. 
 
VIII THE MANUFACTURE OF LIGHT 325 
 
 For a complete study, then, of the light emitted by 
 any lamp we ought not only to measure its mean or 
 total candle-power, but also to analyse the light to 
 observe its composition. So we must have recourse to 
 Newton's prism and study the spectrum of the light of 
 the lamp. 
 
 The Teaching of the Spectrum. — Now, what the 
 prism does is this : it throws the largest waves, the red 
 ones, to one end of the spectrum, and the shortest 
 waves, the violet ones, to the other end of the spectrum 
 — the intermediate colours being ranged between in the 
 order of their respective wave-lengths. To produce the 
 spectrum we first pass the light through a narrow sHt, 
 focus the beam with a lens, and then interpose the 
 prism to spread out the coloured components. 
 
 In this way we now produce on the screen not a 
 picture, but the actual spectrum itself of the white light 
 of an ordinary arc-lamp. To one not familiar with the 
 subject it may seem incredible that these gorgeous 
 colours are actually present in the white light. We are 
 not conscious of them until we thus analyse or sort them 
 out with the prism. 
 
 Let me draw your attention to several features : (i) the 
 order of the colours — red at one end, violet at the other ; 
 (2) the circumstance that they merge gradually into one 
 another with infinite gradations of tone ; (3) that to our 
 eyes the middle part of the spectrum, in the yellowish- 
 green region, is the brightest part ; (4) that at the ends 
 the colours fade off into darkness. Now that darkness 
 is not due to absence of rays. There is plenty of radiation 
 in the dark spaces at both ends — only our eyes cannot 
 
 2 A 
 
326 LIGHT LECT. 
 
 see it ; we are blind to the wavelets that are either shorter 
 than the violet ones or longer than the red ones. The 
 proofs that there are these invisible radiations are very 
 simple. A thermometer placed beyond the red end will 
 show that there are dark radiations — which we may call 
 heat-waves — in that region j and a bit of photographic 
 printing-out paper placed in the region beyond the violet 
 will be darkened, showing that at that end there are 
 some invisible photographic rays. 
 
 Spectra of Incandescent Solids and Vapours. — 
 Now I come to an exceedingly important point. Any 
 ordinary solid body, when heated hot enough to become 
 incandescent, will give a spectrum just like that I have 
 shown, in that it will show all the colours from red to 
 violet in a continuous band. All ordinary solids yield, 
 when they shine, a continuous spectrum. But there are 
 some substances which when heated to incandescence' 
 give out a spectrum of a different kind, consisting, not of 
 all the colours, but of only a few particular ones. Think 
 of some musical instrument such as a piano. What 
 would result if we were to strike all the keys at once ? 
 A horrid crash — a mere noise — with all the different notes 
 jumbled up together. How different if we strike but one 
 key and produce a pure tone, or only two or three keys 
 and produce a chord ! Now, as I say, there are certain 
 substances which instead of giving us white light — a 
 crash of all different colours jumbled up together — emit 
 a few single pure rays. The coloured lights that are 
 used in fireworks are crude examples. Now remember 
 this, the substances which will thus emit a few selected 
 rays are, for the most part, not solids. Gases and 
 
VIII THE MANUFACTURE OF LIGHT 327 
 
 vapours, if heated hot enough to shine, give out only 
 selected rays. The gas helium^ discovered by Sir 
 William Ramsay, shines with a pure yellow light ; the 
 vapour of sodium gives out two rich yellow rays ; 
 hydrogen shines with four rays, a red, a sea-green, a blue, 
 and a violet ray ; the vapour of silver with three beautiful 
 green rays ; the vapour of fuercury with some red and 
 some green rays ; the vapour of iron with several 
 hundreds of rays. 
 
 I want, while the prism is in its place, to show you 
 the spectrum of some of these. Inside the lantern there 
 is an arc-lamp, and in the lamp I place a small carbon 
 crucible. In this crucible we will put a pellet of silver, 
 and then cause the arc to play on it. In a moment, 
 such is the intense heat of the arc, the silver will be 
 melted, then it will boil, giving off vapour of boiling 
 silver ; and this vapour will be heated to incandescence 
 and will glow with a green light. The prism will analyse 
 the light, and there on the screen you see the charac- 
 teristic rays in the green part of the spectrum. Into the 
 crucible we will now drop a scrap of zinc, and forthwith 
 you observe the spectrum of the vapour of zinc, with two 
 blue rays and a fine red one. Yet once more we will 
 drop into the crucible a bit of a salt of calcium, — fluor 
 spar, — and behold the characteristic orange light of the 
 vapour of calcium. We shall come back to the import- 
 ance of these things presently, but must not forget 
 the difference between the continuous spectrum of 
 ordinary solids and these discontinuous spectra that 
 consist of bands of selected rays, and are the spectra of 
 vapours. 
 
328 LIGHT LECT. 
 
 Sensitiveness of the Eye to Radiations of Particu- 
 lar Wave-lengths. — I have already drawn your attention 
 to the circumstance that the ordinary spectrum is 
 brightest in the middle region. The spectrum was 
 made of the white light of the arc ; and the light 
 of the arc is manufactured by passing an electric- 
 current between the white-hot tips of the carbons, thus 
 pouring in electric-energy which makes them radiate. If 
 you did not know otherwise you might suppose — and 
 the supposition would be wrong — that the greater 
 luminosity of the yellow -green region was due to a 
 larger amount of energy being expended on manufactur- 
 ing rays of that colour. But this is not so. The 
 yellow-green in the middle looks indeed brighter than 
 the red at the end of the spectrum, but there is far more 
 of the energy being spent on producing the red rays than 
 in producing the yellow - green. Why then does the'^ 
 yellow-green look brighter ? The answer is to be found 
 in the properties of the eye. I have already remarked 
 on the fact that the eye does not see the invisible dark 
 rays that lie beyond the end of the spectrum — it is blind 
 to those rays. And the eye is not so sensitive to those 
 kinds of light that lie near either end of the spectrum as 
 to those in the middle region. Careful experiment has 
 shown that the eye is most sensitive to light that has a 
 wave-length of about 50 millionths of a centimetre (20 
 millionths of an inch), and is of a green colour. The 
 diagram, Fig. 182, gives the result of the measurements 
 of the late Professor Langley. He found the apparent 
 luminosity, for equal amounts of energy, to be a maxi- 
 mum for light of a wave-length of 53 milHonths of a 
 
VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 329 
 
 centimetre. And, as the table shows, if we take the 
 luminosity of this green light as 100, then that of blue is 
 only 62 per cent as great, that of yellow 28, that of 
 orange 14, that of violet i'6, and that of red 1*2. But 
 you will say the red in the spectrum looks more than 
 I '2 per cent as bright as the green part. Yes, but re- 
 member that these figures are for equal amounts of 
 energy j and as in reality the lamp throws three or four 
 
 IOOt 6, 
 
 Fig. 182. — Langley's Curve of Relative Luminosity for equal amounts of 
 Radiation of different Wave-lengths. 
 
 times as much energy into the red as it does into the 
 green, the relative luminosity as we see it in the spectrum 
 does not fall for red so low as i "2 per cent of the green. 
 
 While we are on this question of the composition of 
 lights, let us give a moment's attention to some of the 
 results that have been obtained by applying the principles 
 of photometry to the measurement of the amounts of 
 different colours that enter into the composition of the 
 light from different sources. We all know that gas- 
 lights look yellow compared with daylight. What is 
 
330 
 
 LIGHT 
 
 LECT. 
 
 there in the composition of these lights to account for 
 this ? We do not need to make a separate measurement 
 for every kind of ray all along the spectrum ; that would 
 be too endless a task. It suffices to measure the relative 
 brightness for three particular colours, namely, the colours 
 which are respectively nearest to the three primary colour- 
 sensations of the eye. Now the three colour-sensations 
 which are primary ^ to the normal eye are red, green, and 
 blue (a blue inclining toward violet). Hence if we 
 measure the relative amounts for these three colours 
 we can infer the composition for any light of the kind 
 that has a continuous spectrum. The following are 
 some of Sir William Abney's measurements : — 
 
 Relative Composition of Lights, 
 recalculated from Abney's COLOUR Measurement, p. 121, 
 
 
 Red (C). 
 
 Green (E). 
 
 Blue (G). 
 
 Gas-light 
 Arc-light 
 Sunlight 
 Sky-light 
 
 45 
 18 
 19*2 
 9 
 
 367 
 
 37 
 23 
 
 12 
 
 45-3 
 
 43-8 
 68 
 
 ^ If any colour excites but one set of nerve-sensations in the eye 
 it will be called a primary. If it excites two or three sensations it 
 cannot itself correspond to a single primary sensation. Yellow, for 
 instance, excites two sensations, the red and the green : therefore it 
 is not physiologically a primary. Red (of a wave-length 65 "2 
 millionths of a centimetre, and of a quality near the spectrum-line C) 
 is a primary. Green (a rather yellowish green of a wave-length 53*5, 
 and of a quality near the spectrum-line E) is another primary. The 
 third primary is blue-violet (of wave-length 45 near the spectrum- 
 line G). 
 
VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 331 
 
 Gas-light contains less blue than sunlight, while it has 
 a relatively larger percentage of green, and a much 
 larger percentage of red. 
 
 Another point of significance in the study of light 
 and its absorption by passing through translucent media, 
 is afforded by Abney's measurements on the absorbing 
 effect of the atmosphere at sunset. Why does the sun 
 look red as he goes down ? The atmosphere absorbs 
 
 Absorption Effect on Sunlight when Overhead, 
 calculated from Abney. 
 
 Eleva- 
 tion. 
 
 90° 
 
 ii°-3 
 
 7°'Z 
 
 Through 
 Atmospheres. 
 
 Red (C). 
 
 Green (E). 
 
 Blue (G). 
 
 
 I 
 
 5 
 8 
 
 I9"2 
 
 12-5 
 
 1*0 
 
 37 
 12 I 
 
 43 "8 
 
 4*4 
 
 = 100% white 
 = 29 % yellow 
 = i%red 
 
 a lot of the light, and absorbs the short waves — blue and 
 violet — more than the long ones. A greater thickness 
 of atmosphere as the sun nears the horizon absorbs off 
 not only the violet and blue, but also the green and the 
 yellow. When he is still seven degrees above the 
 horizon — a quarter of an hour before final disappear- 
 ance — all is absorbed off except i per cent, and that 
 unabsorbed residue is red. 
 
 Absorption and Emission. — We have not done 
 with the spectrum yet ; but the mention of absorption 
 leads us at once to another all-important question, the 
 relation between absorption and emission. Consider 
 what may happen if any radiation, whether of the visible 
 sort or of the invisible heat-waves, falls upon any sub- 
 
332 LIGHT LECT. 
 
 stance. There are three things that may befall those 
 rays, (i) If the substance is a polished metal like silver 
 the rays will be reflected — thrown off — as from a mirror; 
 you all know that a concave polished metal surface can 
 act as a burning mirror and reflect heat-waves as well as 
 light-waves. (2) If the substance be clear, as glass or 
 water, the rays will be transmitted^ and come out at the 
 other side. (3) If the substance is dark, opaque, rough, 
 like black cloth or slate, the rays will be absorbed) that 
 is, they will neither be turned back nor allowed to go 
 through, but will be stopped, destroyed, killed, absorbed 
 into the substance, and the substance will thereby be 
 warmed. Any black, unpolished, opaque substance 
 exposed to sunlight gets hot, much hotter than either 
 a polished metal substance or a transparent substance. 
 A black hat is much hotter in sunshine than a polished 
 brass helmet. All black bodies absorb. The black 
 soot on the bottom of a kettle helps the kettle to absorb 
 the heat radiating up from the fire. 
 
 Now what has this to do with emission of light or 
 heat ? Simply this : those substances which are good 
 absorbers are found to be also good emitters. Make a 
 substance hot, then if it has ^ a black, opaque, rough 
 surface it will radiate out its heat (and light) into the 
 surrounding space quicker than if it were either polished 
 like silver or clear like glass. Take three glass flasks of 
 equal size ; let one be silvered over like a mirror on its 
 outside, let the second be left clear glass, let the third be 
 blackened on the outside. Fill all three with hot water. 
 The one with black surface will be found to cool fastest ; 
 because its surface is black it can emit heat-waves better 
 
VIII THE MANUFACTURE OF LIGHT 333 
 
 than either of the others. Now it has been known for 
 many years that the absorbing and emitting powers 
 of bodies are precisely proportional to one another. If 
 we could procure an absolutely black body it would 
 not only absorb all radiation that might fall on it, but it 
 would, at any given temperature, emit all sorts of rays 
 better than any other sort of body black or bright could 
 do. Bright polished bodies are bad emitters. A silver 
 teapot for this reason ought to be kept bright, in order 
 that it shall not emit the heat that you want to keep 
 inside it. The pot ought to be able to call the kettle 
 black. Kirchhoff showed that this reciprocity between 
 radiating and absorbing power was true for particular 
 rays of the spectrum. Any substance — didymium, for 
 example — which absorbs special rays, will emit special 
 rays of that same wave-length, when raised to in- 
 candescence. 
 
 Measurement of Emission. — Again, to win a 
 knowledge of the facts we must have recourse to 
 measurement. We must know to measure the amount, 
 not of light, but of heat — that is to say, of energy, 
 whether visible or invisible — that is radiated from hot 
 bodies ; and we must further be able to find out the sort 
 of waves — that is, the wave-length of the waves — that are 
 being emitted. Sir John Herschel did this roughly in 
 1802 when he explored the spectrum of sunlight with a 
 thermometer having its bulb blackened. He found, as 
 you already know, that the red waves are hotter than the 
 yellow, green, or blue, and that the hottest waves were 
 invisible in the dark part of the spectrum beyond the 
 end of the red. In the progress of science finer measur- 
 
334 
 
 LIGHT 
 
 LECT. 
 
 ing instruments have been invented — the thermopile, the 
 radio-micrometer^ and, lastly, the bolometer. Time fails me 
 to describe these. Suffice it to say, that the bolometer, 
 the invention of the deceased physicist Langley, is a 
 beautiful electrical instrument which measures with 
 utmost precision the amount of energy that may fall 
 upon it. It makes no discrimination between visible 
 
 100- 
 
 75 
 
 50- 
 
 25 
 
 o 
 S 
 
 to 
 
 
 
 
 A 
 
 ■s: 
 
 . ^^^L 
 
 -a 
 
 ^^^^^^ 
 
 2 
 
 / 
 
 ^^L 
 
 .2 
 
 A 
 
 ^^^^ll^ 
 
 V 
 
 Luminous 
 
 R Non-Aluminous radjations 
 
 Fig. 183. — Tyndall's Curve of Amounts of Energy radiated in the Spectrum of 
 
 Electric Arc. 
 
 and invisible radiations, but impartially absorbs them all, 
 and measures the quantity of energy they bring. 
 
 Langley, producing first a spectrum and then explor- 
 ing the spectrum with his bolometer, was able to show 
 the essential correctness of the earlier experiments of 
 Tyndall, who, using a thermopile, had shown how in the 
 spectrum of the arc-lamp (Fig. 183) the radiation beyond 
 the end of the visible red increases, to quote his own 
 inimitable phrase, into a perfect " Matterhorn of heat." 
 Langley's diagram reproduces in general Tyndall's; but he 
 
vrii 
 
 THE MANUFACTURE OF LIGHT 
 
 335 
 
 also added for comparison a diagram of the distribution 
 of energy in the spectrum of the sun (Fig. 184), from 
 which two things become evident: (i.) that there is 
 
 Langley's Curves for One Unit of Heat. 
 
 Energy Curve 
 
 Electric Arc Spectrum 
 
 Maximum at 1^16. 
 
 Energy Curve 
 
 Solar Spectrum 
 Maximum at 0^62. 
 
 0;4. 0-6 0-8 1-0 1-2 1-4 1-6 V8 2-0 2-2 2-4. 2-6 2-8 30 
 
 I Limits 
 I of 
 
 I Visible 
 '^Spectrum\ 
 
 400 -^ 
 300— { 
 200—1 
 100—1 
 
 Energy Curve 
 Fire- Fly Spectrum 
 Maximum at 0^57. 
 
 Wave-lengths in thousandtlis of Millimetre 
 
 0-4 0-6 0-8 l-O a-2 1-4 1-6 1-8 2-0 2-2 2-4 2-6 2-8 3-0 
 
 Fig. 184. — Langley's Curves of Energy in the Spectrum for one unit of Heat ; 
 a, Electric Arc ; 3, Sunlight ; c, Light of Fire-fly. 
 
 irregular absorption in the sun's heat spectrum, indicated 
 by the jagged outline of the curve ; (ii.) that while the 
 peak of the curve for the arc is a considerable distance 
 to the right in the dark rays beyond the red, the peak 
 for the sun's energy is within the range of the rays that 
 
336 LIGHT lect. 
 
 are luminous. In other words, while the wave-length 
 for which the energy of the electric carbon arc is a maxi- 
 mum is about 1 20 millionths of a centimetre, in the sun's 
 spectrum the wave-length for maximum energy is about 
 55 (or, according to Abbott, just under 50) millionths of a 
 centimetre. 
 
 Bad Economy of Ordinary Sources. — When we look 
 at these diagrams, whether Tyndall's or Langley's, we 
 cannot but be struck with the disproportion between 
 the energy wasted on producing dark heat, and that use- 
 fully expended on producing light that we can see. In 
 the case of the arc spectrum of Langley the energy wasted 
 on heat is over a hundred times as great as that utilised 
 in giving light. In the sun's spectrum- the proportion 
 utilised is much greater ; the energy wasted on heat is 
 roughly five times as great as that employed in bringing 
 light. The luminous efficiency of the arc is under i 
 per cent, while that of the sun is nearly 20 per cent. 
 The most recent researches on this subject by Professor 
 Wedding^ have shown the luminous efficiency of ordinary 
 oil-lamps, gas-burners, and electric glow-lamps to be all 
 under one per cent. The process of incandescence, as 
 carried out in all the ordinary sources of light, whether 
 flames or electric-lamps, appears extraordinarily wasteful 
 When we make a hot body hot enough to shine it gives 
 out far more heat than light. By no process of pure 
 incandescence can we realize the ideal case of getting 
 light without heat. Nature does not work that way, at 
 
 ^ W. Wedding, On the Efficiency and Practical Value of the 
 most usual Sources of Light [in German], Berlin, 1905. See table 
 on p. 364. 
 
VIII THE MANUFACTURE OF LIGHT 337 
 
 any rate in the emission of radiation from solids. When 
 by heating them we set their molecules vibrating, they 
 emit the longer wave-lengths first, and not until they 
 have been raised to so high a temperature as to emit a 
 large proportion of the longer waves will they emit a 
 measurable, amount of the shorter waves that are 
 luminous. 
 
 Light of the Fire-fly. — But Langley, if he thus 
 showed how uneconomical are all our ordinary sources 
 of light, also told us in a very remarkable paper ^ pub- 
 lished in 1890, that there is in nature another process by 
 which it is possible to produce light without heat other 
 than that of the luminous rays themselves. My col- 
 league. Professor Meldola, F.R.S., who is a distinguished 
 naturalist as well as an eminent chemist, once examined 
 with the prism the light of the glow-worm, and found it 
 to be a narrow region principally in the middle part of 
 the spectrum, without any red rays. Professor Young, 
 an American astronomer, similarly examined the light 
 of the American fire-fly, and found it to give a spectrum 
 from the orange end of the red to about half-way down 
 the blue — that is, of rays which, while they are just those 
 that stimulate vision, produce relatively little thermal or 
 actinic effect ; " in other words, very little of the energy 
 expended in the flash of the fire-fly is wasted." This 
 interesting suggestion Langley set himself to test with 
 his delicate bolometer. The result is given graphically 
 in the lowest diagram of Fig. 184, where it appears that 
 the fire-fly emits no measurable amount of dark heat, 
 
 ^ '^ On the Ckeapest Form of Light," by S. P. Langley and F. 
 W. Very, American Journal of Science, xl. p. 97. August 1890. 
 
338 
 
 LIGHT 
 
 LECT. 
 
 but sends out radiations only within the hmits of the 
 visible spectrum. The top of the fire-fly curve could not 
 
 Photometric Curves 
 
 for equal total amounts of LigJit 
 
 A— Sun-light 
 B— Fire-fly light 
 
 Abscissae. — Wave Lengths 
 Ordinates. —Luminous Intensities 
 
 0^40 o'ia.5 o'^so o'^ss o^eoo^ei" 0*^70 
 V B G Y O ^R 
 
 B. Fire-fly Spectrum 
 
 Fig. 185. — Comparative Spectra of Energy for equal amounts of Visible 
 Radiation of Sun and Fire-fly. 
 
 be included in the diagram for want of height. Not only 
 so, but the distribution of the rays within the visible 
 spectrum is extraordinarily favourable. Fig. 185 shows 
 
VIII THE MANUFACTURE OF LIGHT 339 
 
 the comparative luminous spectra of sunlight and fire-fly 
 light. Both spectra are bright in the middle, and fade 
 off towards the ends ; but the fire-fly spectrum is more 
 concentrated than the sunlight spectrum about the green 
 rays — precisely those to which the eye is most sensitive. 
 The light of.the insect is accompanied by approximately 
 only 4Joth part of the heat that would ordinarily 
 be associated with an equal amount of light if pro- 
 duced by ordinary flames. If only we could expend 
 the energy of our gas, oil, or electricity as economically 
 as the fire-fly expends its energy, we ought to get for 
 the same expenditure four hundred times as much light 
 as we do in fact now get. Clearly the glow-worm and 
 the fire-fly manufacture their light by some quite different 
 process from our crude and inefficient methods. They 
 certainly do not work by incandescence. 
 
 The fact that there exists in nature a process so much 
 more economical, impels us to reconsider the • method 
 of incandescence to learn more closely how the emission 
 of light depends upon the temperature. 
 
 Temperature and Quality of Radiation. — We owe 
 chiefly to German physicists — Wien, Paschen, Lummer, 
 Pringsheim — the careful investigation of the relation 
 between temperature and radiation. As already stated, 
 the best emitter ought to be an absolutely black body. 
 Even lamp-black is not absolutely black, but is slightly 
 a reflector. But Lummer has ingeniously realized an 
 apparatus equivalent to an absolute black body, by using 
 an enclosed cavity blackened on the inside, with a small 
 hole opening into it. If this is heated, the radiations 
 which come out of the hole will be practically the same 
 
340 
 
 LIGHT 
 
 LECT. 
 
 as those of an absolutely black body of the same tem- 
 perature. With this apparatus he has verified a law 
 enunciated by Wien and Paschen, that the wave-length 
 of the dominant radiation is inversely proportional to 
 the absolute temperature. For any given absolute 
 temperature one may calculate the dominant wave-length 
 (in micro-centimetres) by dividing 294,000 by that tem- 
 perature. This gives us the following relations : — ■ 
 
 Absolute Temperature. 
 
 Dominant Wave-length. 
 
 1000° 
 
 294 
 
 2000° 
 
 147 
 
 3000° 
 
 98 
 
 4000° 
 
 73 
 
 5000° 
 
 59 
 
 6000° 
 
 49 
 
 Fig. 186 gives in a diagram, in which no attempt is 
 made, however, to present the amounts of energy to scale, 
 the way in which, as the temperature is raised, the black 
 body emits more and more radiant energy, and in which 
 while the emission of energy thus rises ^ with the tempera- 
 ture, the position of the peak of the curve shifts from the 
 region of invisible heat-rays toward the region of visible 
 luminous rays. In reality the upper curves ought to 
 
 ^ The law was discovered by Stefan that the amount of energy 
 radiated (from a black body) is proportional to the fourth power of 
 the absolute temperature. For the absolute black body, according 
 to the measurements of Kurlbaum, the coefficient of emissivity is 
 5*32 X io~^^ watts per square centimetre, so that any black body at 
 an absolute temperature of 6 radiates to its surroundings with a 
 power of 5*32 xd^x 10-'^^ watts. 
 
VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 341 
 
 rise much higher; for the total energy radiated by a 
 black body at 4000° is sixteen times as great as that 
 radiated at 2000°. Looking at this diagram we see that 
 at 2000° (absolute) the dominant wave-length is about 
 147 millionths of a centimetre, and that by far the greater 
 
 40 80 12Q }GQ Waue-Jengths' 
 
 Fig. 186. — Energy Spectra of Radiation of Black Body at different 
 Temperatures. 
 
 part of the radiation is dark. When the temperature is 
 raised to 3000° there is more dark radiation than before ; 
 but the peak has crept to the left, so that the dominant 
 wave-length is now at 98, and the proportion of visible 
 radiations to invisible has greatly increased. Every 
 increase of temperature raises the relative amount of the 
 visible radiation, 
 
 2 B 
 
342 
 
 LIGHT 
 
 LECT. 
 
 In the following Table are given the temperatures and 
 the dominant wave-lengths for a number of sources of 
 radiant energy, on the supposition that they all radiate 
 as an absolute black body would do. The temperature 
 of the Welsbach mantle is probably estimated too high, 
 being inferred from its dominant wave-length. 
 
 Temperatures and Dominant Wave-Length of 
 Different Sources of Radiation 
 
 
 Centigrade. 
 
 Absolute. 
 
 '^dom: 
 
 Sun .... 
 
 5880° 
 
 6153 
 
 50 
 
 Carbon Arc . 
 
 3800° 
 
 4073 
 
 72 
 
 Welsbach Mantle . 
 
 (?)2027° 
 
 2300 
 
 128 
 
 Nernst Lamp 
 
 2027° 
 
 2300 
 
 128 
 
 Bunsen Flame 
 
 1871° 
 
 2144 
 
 138 
 
 Platinum melts 
 
 1775° 
 
 2048 
 
 143 
 
 Carbon Glow-lamp 
 
 1727° 
 
 2000 
 
 147 
 
 Gas Flame 
 
 1712° 
 
 1985 
 
 148 
 
 Candle Flame 
 
 1577° 
 
 1850 
 
 159 
 
 Gold melts 
 
 1250° 
 
 1523 
 
 195 
 
 Silver meUs . 
 
 1000° 
 
 1273 
 
 230 
 
 Water boils . 
 
 100° 
 
 373 
 
 790 
 
 Ice melts 
 
 0° 
 
 273 
 
 1080 
 
 Note : Wave-lengths are given in micro-centimetres, 
 radiations lie between X = 8i and X = 36. 
 
 Visible 
 
 But no actual substance is so good a radiator as the 
 ideal black body, and such a metal as polished platinum, 
 which remains highly reflective even when white hot, 
 ought to be of lower emissive power. And so it was 
 found to be by Lummer. Fig. 187 gives, in correct scale, 
 the energy spectra as measured by Lummer both for the 
 black body and for bright platinum. The highest of -the 
 
VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 343 
 
 curves drawn with a full line represents the radiation of 
 the black body at 1646° (absolute), and its dominant 
 
 Waue-lengttis in millionths of a Centimetre 
 
 Fig. 187.— Lummer's Energy Spectra at different Temperatures of a Black Body 
 and of Bright Platinum. 
 
 wave-length is 180 millionths of a centimetre. There is 
 given — in dotted line — a curve for platinum at nearly 
 the same temperature, viz. 1689°. On comparing these 
 two curves we see that the radiation from the platinum 
 
344 LIGHT lect, 
 
 is much less ; the areas under the respective curves 
 represent the totals of energy emitted. But the peak of 
 the platinu7?i curve is nearer the region of visible rays, the 
 dominant wave-length being 156 instead of 180. The 
 platinum, though it radiates less energy, will have a larger 
 proportion of visible rays in its radiation ; hence bright 
 platinum is a more economical radiator than the black 
 body. It may be far from realizing the ideal of light 
 without heat, but it emits less heat and relatively more 
 light. 
 
 Emissivity of the Rare Earths. — The question 
 naturally arises, If pohshed platinum, which, because of 
 its reflective power, is a bad emitter of heat, is thus a 
 more efficient producer of light than the black body, are 
 there any other bodies which show any special power of 
 emitting rays of the luminous quality ? Well, it has long 
 been known — was known even in Tyndall's time — that 
 certain rare earths exhibit when incandescent a remark- 
 able spectrum different from that of ordinary solids. 
 Amongst these earths are the materials known to the 
 chemists as erbia, yttria, didymia, zirconia, thoria, ceria. 
 They are all white substances resembling hme; but 
 whereas lime, when heated to incandescence, gives an 
 ordinary continuous spectrum, these rare earths are 
 observed to give in addition certain bright bands in 
 particular regions of the spectrum. Their spectrum, in 
 fact, is of a mixed type, partly like that of a solid, partly 
 like that of a vapour. Further, it is found that some of 
 these have this property to an exceptional degree when 
 admixed in small quantities with other bodies. For 
 instance, zirconia is very like lime, but if admixed with 
 
VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 345 
 
 a small percentage of yttria gives out far more light than 
 when used aloilfe ; the yttria seems to make it emit 
 relatively more yellow and green rays. 
 Also thoria, if mixed with only i per cent 
 of ceria, gives out far more light than 
 pure thoria, and the mixture radiates an 
 unusually high proportion of green and 
 yellow in its spectrum. Whatever be 
 the physical explanation ^ of these phe- 
 nomena, the facts are of utmost import- 
 ance in their practical bearings. 
 
 Incandescent Gas-lights. — An enor- 
 mous stride in gas-lighting was made in 
 1883, when Dr. Auer von Welsbach 
 introduced the well-known Welsbach 
 mantle^ which he hung in the non- 
 luminous flame of a Bunsen burner. The 
 merit of his invention consisted mainly 
 iG. I . — incan- -^^ ^ practical realization of a means 
 
 descent Gas ^ 
 
 Mantle of Auer for raising to incandcscence the rare 
 
 von Welsbach. , , . , ,. . 
 
 earths, the special radiating properties 
 of which have just claimed our attention. The modern 
 
 ^ Recent research has removed any doubt that might have 
 existed as to the fact that particular solids possess specific emis- 
 sivities. The similar circumstance that a mixture of 99 per cent of 
 thoria with i per cent of ceria emits at the same temperature a 
 brighter light than either pure thoria or pure ceria, has been ex- 
 plained by Professor Rubens. The action does not appear to be 
 chemical, nor is it luminescent : for though in some of the phenomena 
 of luminescence (fluorescence for example), the action appears to 
 depend on the presence of small quantities in dilute liquid or solid 
 solution, the percentages in such cases are far less than of the order 
 of I per cent. 
 
346 
 
 LIGHT 
 
 LECT. 
 
 process of making the mantle is as follows : — A light 
 fabric of cotton, or, better still, of ramite fibre, is woven 
 or knitted with an open mesh. It is soaked in a chemi- 
 cal solution of the rare earths, dried, and then burned. 
 This leaves a skeleton of white ash, of the same shape 
 as before, but shrunken in size. When hung in the hot 
 flame of an atmospheric burner (Fig. 188) it glows with 
 great brilliancy. The mantles are temporarily hardened 
 for transportation by being treated with collodion. The 
 composition of the rare earths left in the ash is stated 
 to be 99 per cent of thoria, with i per cent of ceria. 
 How great an improvement is effected in gas4ighting by 
 this invention is readily understood by reference to the 
 following table : — 
 
 Light emitted per Cubic Foot of Normal Gas 
 PER Hour 
 
 'Union flat-flame jet No. i 
 
 :3 D 
 o ■-; 
 rS i= 
 
 No. 
 
 No. 
 No. 
 No. 
 
 No. 
 No. 
 
 
 ^Standard Argand burner 
 r Ordinary Welsbach 
 < Kern burner . 
 (^High-pressure burner 
 
 Candle-power 
 per Cubic Foot. 
 0-85 
 I '22 
 1-63 
 174 
 1-87 
 2-15 
 2-44 
 3-20 
 
 II to 19 
 20 to 25 
 30 to 35 
 
 Comparing the case of an ordinary No. 4 flat-flame gas- 
 jet with an ordinary Welsbach burner and mantle, we see 
 that using precisely the same amount of gas the amount of 
 light is increased from 6 to 1 1 fold. That is to say, the 
 cost of gas-lighting has been reduced to J or ^r of its 
 
VIII THE MANUFACTURE OF LIGHT 347 
 
 former cost. There is, of course, the cost of mantles to 
 be set off against the gain ; and it must not be forgotten 
 that the light of the mantles diminishes after they have 
 been for some time in use. By use of Kern burners, which 
 produce a more complete admixture of air and gas, a 
 hotter flame is produced, increasing the output of light. 
 The cause of this gain in brilliancy will be clear from 
 what we have already learned. In the atmospheric 
 burner a higher temperature is reached (namely, about 
 2144° absolute) than can be attained in the flat gas 
 flame (about 1712° C. or 1985° absolute). This higher 
 temperature of itself accounts for something; but over 
 and above this we have the use of a mantle composed of 
 these rare earths of specific emissivity which radiate out 
 particular groups of rays of the kind to which the eye is 
 sensitive. Do we object to the greenness of the Welsbach 
 lights ? It is that very greenness which gives them their 
 high illuminating quality. 
 
 Researches of Rubens. — Professor Rubens has shown^ 
 that the emissive power of the Welsbach mantle is in 
 general very low. For the longer wave-lengths of invisible 
 quality it has not so much as i per cent of the emissivity 
 of an absolute black body. It has, for these rays, an 
 emissivity even less than that of the Bunsen flame itself. 
 But for light of the wave-length of 100 micro-centimetres 
 or less, its emissivity, though less than that of an absolute 
 black body, is far greater than that of the Bunsen flame. 
 In the energy - spectrum of a Welsbach burner with 
 mantles of different composition (Fig. 189) various 
 maxima occurred. If pure oxide of iron or pure ceria 
 
 ^ See Drude's Annalett, vol. xx. p. 593, 
 
348 
 
 LIGHT 
 
 LECT. 
 
 were used, there was a maximum at about 200 micro- 
 centimetres. The flame also itself gave two maxima, one 
 at about 260 due to water- vapour, and another at 430 
 due to the emissivity of carbonic acid. The energy- 
 
 Energy Spectrum of Welsbach 
 mantle and flame. 
 
 Energy Spectrum of flame only. 
 
 Fig. 
 
 -Rubens' Curves, Energy-Spectra of Radiations from Incandescent 
 Gas-flames with various Mantles. 
 
 spectrum of the ordinary mantle itself (Fig. 190) showed 
 two maxima, one for a wave-length of 120 micro- 
 centimetres, the other at about 900. The former showed 
 that though the temperature was only about 1590° C. (/.<?. 
 
 ]Q0 200 300 400 500 600 700 800 SOO jgQO 
 
 Fig. 190. — Rubens' Curve for Emissivity of Weisbach Mantle. 
 
 1863° absolute), the quality of the light more nearly 
 corresponded to that from a black body at 2450° 
 absolute. 
 
 High - pressure Incandescent Gas - lighting. — A 
 still further advance has been made of late in the use 
 
VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 349 
 
 of high-pressure gas-burners. The usual pressure at 
 which gas is supplied to our houses is quite low — about 
 equal to the pressure of a column of water 2 to 2 J 
 inches high. But if the gas is compressed to a pressure 
 of 40 or 50 inches of water, and supplied under that 
 pressure to appropriate atmospheric burners, it mixes 
 
 Fig. 191. — Colonia Gas Compressor for High-pressure Incandescent 
 Gas-lighting. 
 
 still more completely with the air, the temperature of 
 combustion is raised still higher, and a still greater 
 luminosity of the mantle results. It is this kind of 
 incandescent gas-light which is rapidly coming into favour 
 for lighting of squares and streets. And no wonder. Pro- 
 fessor Wedding's tests on the high-pressure "Millennium 
 light" show 1320 candle-power horizontally, or 933 
 candle-power spherically, with a consumption of 25 cubic 
 
350 LIGHT LECT. 
 
 feet per hour of normal gas. If this had been burned 
 in batswing burners it would have given only about 75 
 candle-power in total. 
 
 One objection to high -pressure gas -lighting is the 
 trouble of the compressor to compress the gas mechani- 
 cally. This can be done by any small motor. The 
 "Colonia" compressor, Fig. 191, kindly exhibited by Mr. 
 Blakey, is an example. The motor in this case is a 
 small electric motor of J horse-power. Mr. Blakey has 
 also brought for exhibition a new automatic hydraulic 
 compressor-^ of simple and ingenious design. It does 
 not matter whether the gas is compressed, or whether 
 we compress air and blow it into the gas-burner. Cer- 
 tainly the latter course seems preferable. 
 
 Mr. D. Anderson has kindly supplied an example of 
 the Scott-Snell burner in which the compression of the 
 air is effected by a small hot-air engine cunningly placed 
 in the top of the lantern, and worked by the waste heat, 
 so that each lamp is self-contained. Fig. 192 shows a 
 section of the arrangement. 
 
 Great as has been the advance in gas-lighting, it is 
 certain that the last word has not been said, and that 
 abundant room exists for further improvements. The 
 great economies effected by the use of high pressures 
 apply only to burners of high candle-power. No one 
 has yet invented an equally economic gas-burner for a 
 small light of, say, 20 candle-power. For lights of 5 to 
 
 ^ The details of construction of this new compressor I am not at 
 liberty to disclose. It is remarkably small and inexpensive, and 
 the consumption of water, taken from the ordinary town mains, is 
 very small. 
 
VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 551 
 
 20 candle-power the old, uneconomical flat-flame jet 
 still remains. And against gas -lighting in domestic 
 use there still remains the hygienic objection that all 
 
 gas-lighting poisons the air 
 with carbonic acid gas, the 
 product of combustion. 
 With the economic signi- 
 ficance of the modern 
 gas-lights we shall deal 
 presently. 
 
 Efl5.ciency of Glow - 
 lamps. — We are now in 
 a position to discuss the 
 case of electric glow-lamps. 
 The ordinary glow-lamp 
 of commerce, with its 
 filament of thin black 
 carbon, as generally used, 
 takes approximately 3 to 
 3J watts (spherical) per 
 candle - power, and has 
 a life of 1000 hours 
 on the average, during 
 which, however, owing 
 to blackening of the 
 bulb, its efficiency goes 
 down by some 20 to 40 per cent, the consumption in- 
 creasing to 4 or 4 J watts per candle. Fig. 193 gives the 
 result of a test on twenty-four lamps at the National 
 Physical Laboratory. The temperature of the filament 
 may be taken as about 2000° (absolute). As is well 
 
 Fig. 192. — Scott-Snell self-contained 
 High-pressure Gas Lamp. 
 
352 
 
 LIGHT 
 
 LECT. 
 
 known, if the lamp is subjected to a higher vohage than 
 that for which it is designed it will give out much more 
 light, but its life will be short. Increasing the voltage 
 causes more current to go through it ; and when more 
 
 Watts per candle 
 
 Fig. 193. — Test-chart of Twenty-four Electric Glow-lamps. 
 
 current is forced through it it gets hotter, and with the 
 rise in temperature it emits more light and a whiter 
 light. 
 
 A simple experiment will point the moral. Here is a 
 glow-lamp designed to give 50 candle-power at 50 volts. 
 By raising the electric pressure its light goes up; we 
 
VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 353 
 
 reach 70 candles, 100 candles, 150 candles, 200 candles. 
 If we could only double the temperature before the fila- 
 ment breaks down we might theoretically reach 10,000 
 candle-power. But long before this the end comes, 
 for at this high temperature the carbon in the filament 
 
 120 
 
 2000 
 
 
 
 
 
 
 
 
 1800 
 
 
 
 
 
 
 y 
 
 
 1600 
 
 
 
 
 
 
 / 
 
 
 ■1^00 CO 
 
 
 
 
 
 J'k 
 
 / 
 
 
 120dq 
 
 
 
 
 rt<' 
 
 K/ 
 
 
 ^ 110 
 
 1000.= 
 
 N. 
 
 
 
 w 
 
 
 
 ^in:> 
 
 800 ^K 
 
 '^ 
 
 '• ^ 
 
 ^^\ 
 
 y 
 
 
 
 
 -J 
 600 
 
 
 ^. 
 
 y. 
 
 , • 
 
 
 
 104 
 
 4.00 
 
 
 ./ 
 
 ^ c< 
 
 ^-^.. 
 
 
 
 
 200 
 
 ^ 
 
 r 
 
 
 ^-'i 
 
 ^"^o 
 
 
 100 
 
 ^ 
 
 
 
 
 
 ^^--J 
 
 3-75 3-5 3-25 S'O 2-73 2-5 2-25 2-0 
 
 Efficiency in Watts per candle power 
 
 Fig. 194. — Curve of Relation between Efficiency and Life of Carbon 
 Filament Glow-lamps. 
 
 quickly disintegrates, and it breaks at the weakest point. 
 Fig. 194 gives, from the experience of the Robertson 
 Lamp Company, a diagram of the way in which, as we 
 raise the volts, we raise the efficiency, but also shorten 
 the life of the lamp. The life-factor may be conveniently 
 stated in the following form : — 
 
 
 
 Per Cent of Normal Voltage. 
 
 Percentage of Normal Life. 
 
 I03 
 
 100 
 
 JOI 
 
 80 -8 
 
 102 
 
 68-1 
 
 103 
 
 56-2 
 
 104 
 
 45-2 
 
 105 
 
 37-4 
 
 106 
 
 31-0 
 
354 LTGHT lect. 
 
 Raising the voltage only 6 per cent reduces the average 
 life to less than one-third of the normal value. 
 
 New Kinds of Glow-lamps. — Several newer kinds of 
 glow-lamps are now in the market. A radical departure 
 was made some seven years ago, when Nernst proposed to 
 use a filament that looks like a thread of pipe-clay, but is 
 in reality made of zirconia and yttria, or similar materials 
 of special emissivity. Such a thread does not conduct 
 the electric-current unless first heated ; so the Nernst- 
 lamps. Fig. 195, contain a special heating device warmed 
 by the current itself, so that the filament lights up as soon 
 as it becomes conductive. Partly because of the specific 
 emissivity of the materials, also probably, in part, because 
 of the attainment of a higher temperature, the Nernst 
 filament works with a higher efficiency than the carbon 
 filament, requiring only about 2 to 2 J watts per candle. 
 The dominant wave-length of its light is 128 millionths-^ 
 of a centimetre, which would correspond to a temperature 
 of 2300° (absolute), if it radiates as a black body does. 
 If the effect is due to a specific emissivity the actual 
 temperature may be lower. 
 
 More recently glow-lamps have been proposed having 
 metallic filaments. Platinum will not do for this purpose ; 
 its melting point (1775° C.) is too low. But the rare 
 metal osmium has been proposed by Auer von Welsbach, 
 tantalum by von Bolton and Feuerlein, zirconium by 
 Zerning, and tungsten by Kusel. The difficulty in pre- 
 paring fine wires, about -gJo inch thick, of these hard 
 and almost infusible metals is great but not insur- 
 mountable. Osmium-lamps have been on the market 
 since 1904, tantalum-lamps since the year 1905 only. 
 
VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 355 
 
 Tungsten -lamps came in only in 1906. Yet the 
 results have been most promising, and the tantalum- 
 lamp (Fig. 196) is already largely in use. With the 
 osmium-lamp the consumption of energy goes down to 
 176 watt per candle; with the tantalum-lamp to 1-5 
 
 Fig. 195. — Nernst Glow-lamp. 
 
 Fig. 196. — Tantalum-lamp. 
 
 watt per candle. The melting point of tantalum is 
 about 2520° or 2570° (absolute) ; hence the light is very 
 white. For the tungsten-lamp an efficiency of i candle 
 per watt is claimed. If this can be realized, the cost of 
 electric-lighting will be reduced to one-third of that of 
 our present carbon glow-lamps. Here, at least, is attain- 
 able a considerable economy in the manufacture of light. 
 
356 
 
 LIGHT 
 
 LECT. 
 
 Since the delivery of this lecture tungsten-lamps have 
 been much developed. The leading sort is that put on 
 the market under the name of the " osram " lamp. The 
 
 metal tungsten is so ex- 
 cessively hard that it can- 
 not be drawn into wire 
 in the ordinary way. The 
 ingenuity of inventors has 
 therefore been exercised 
 in devising methods for 
 handling it, so that it can 
 be made into thin wires, 
 otherwise than by drawing 
 through dies. Fig. 197 
 represents an " osram " 
 lamp giving about 16 
 candle-power, and con-- 
 suming only 17 watts. If 
 supplied at an electric pres- 
 sure of 105 volts, it takes 
 only about J of an ampere 
 of current, whereas a car- 
 bon glow-lamp of equal 
 brightness would take h an 
 ampere. Besides this, the 
 light is whiter than that of 
 a carbon glow-lamp, as the temperature of the tungsten 
 filament may be made higher. Moreover, metallic filament 
 lamps are less sensitive than carbon filament lamps to varia- 
 tions in the electric pressure. They are, however, rather 
 more fragile, owing to the extreme tenuity of the filament 
 
 Fig. 197. — An "Osram" Lamp with 
 Tungsten Filament. 
 
VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 357 
 
 New kinds of Arc -lamps. — Improvements in 
 electric arc-lamps are also to be noted. The ordinary 
 arc-lamp sheds its light mainly from the white-hot end 
 of the upper carbon rod ; but as the lower carbon comes 
 
 Fig. 198. — Curve of Distribution of Light of Ordinary Arc-lamp. 
 
 into the way, the maximum illumination is cast obliquely 
 downward, as the curve of distribution of light. Fig. 198, 
 shows. x\bout twelve years ago the fashion began of 
 enclosing the arc in a nearly air-tight inner globe. By 
 this device the rate of consumption of the carbon rods 
 was greatly reduced, thereby saving much of the cost 
 
 2 c 
 
358 
 
 LIGHT 
 
 LECT. 
 
 and labour of renewals. But the loss in light by absorp- 
 tion due to the double globe was very considerable, and 
 the efficiency of the lamp reduced. More recently an 
 advance has been made in the introduction of impreg- 
 nated carbons. Salts of potash have long been known 
 to improve the quality of the light emitted ; and, more- 
 over, their introduction permits a wider separation of the 
 
 Fig. 199. — Curves of Distribution of Light of Arc-lamps. 
 
 carbons, so that the downward light is less intercepted. 
 Salts of strontium and calcium, particularly the fluoride 
 of calcium, are effective in increasing the quantity of light 
 emitted for a given consumption of energy. In these 
 cases the arc becomes a veritable flame of light, the 
 luminosity being mainly in the arc itself and no longer 
 in the incandescent tips. By using two inclined carbons 
 with arc deflected downward, an enormous increase in 
 light is obtained. The curves of Fig. 199 (due to Wed- 
 
VIII THE MANUFACTURE OF LIGHT 359 
 
 ding) are instructive. That marked A is the distribution 
 curve of an ordinary "open" arc -lamp. When sur- 
 rounded by an interior globe as an " enclosed " arc, the 
 output is diminished to the value shown by B ; while, 
 when a " flaming " arc was produced, using only the same 
 amount of energy, the output of light was increased more 
 than fourfold, and the distribution curve takes the form 
 delineated in C. The introduction of salts of calcium 
 gives to the arc a fine orange hue, which appears to 
 possess special penetrative powers in a foggy atmosphere. 
 By the kindness of the Union Electric Company of 
 London, one of their " Excello " flaming arc-lamps is 
 here exhibited. 
 
 The Magnetite Arc-lamp. — The newest species of 
 arc-lamp is that of Dr. C. P. Steinmetz of Schenectady. 
 After careful study of incandescent materials, he selected 
 the oxide of iron, called juagnetife, for making the negative 
 electrode of the lamp. This material, mixed with the 
 oxide of chromium or of titanium, rammed into an iron 
 cartridge, is supported at the bottom of the lamp. The 
 upper or positive pole is a piece of solid copper. The 
 arc thus produced is an intensely white column of light 
 about I inch long. The copper pole is not consumed, 
 and the cartridge of magnetite is only slowly used up. 
 One feature of this lamp is that the maximum of the 
 light is thrown almost horizontally, so that it is admirably 
 adapted for the lighting of streets. This lamp, not being 
 yet in the market in this country, I am indebted to Dr. 
 Steinmetz for the specimen now shown. It is highly 
 efficient, giving about twice as much light as the ordinary 
 arc-lamp for equal consumption of energy. The spec- 
 
36o LIGHT LECT. 
 
 trum of the light of the magnetite arc reveals the cause 
 of this high efficiency. It consists largely of brilliant 
 bands of light in the green and red regions ; in fact, it is 
 largely a gaseous spectrum. 
 
 The Electric Vapour-lamp. — The vapour of mer- 
 cury, traversed by an electric current, emits a brilliant 
 bluish-green light. Various lamps have been designed to 
 bring this into practical use. Of these the best known 
 is the Cooper-Hewitt. The British Westinghouse Com- 
 pany has kindly supplied two of these for this lecture. 
 
 -Posit we had 
 
 Negative lead- 
 Seal off 
 / Condensing chamber 
 
 VaOLium tube 
 
 Platinum \ \/ron electrode '"^'""''y electrode 
 
 mire \ Platinum wire 
 
 Protective porcelain tip a j. 4- , ■ x- 
 
 '^ Protective porcelain tip 
 
 Fig. 200. — Cooper- Hewitt Mercury Vapour-lamp. 
 
 A glass tube, about i inch in diameter, and 3 or 4 feet 
 in length, according to the voltage of supply, is arranged 
 with suitable electrodes at the ends (Fig. 200), and con- 
 tains nothing else except mercury and mercury vapour. 
 To cause the current to flow it is sufficient to tilt the 
 lamp, causing the thread of mercury that is formed along 
 the bottom of the tube when horizontal to part. At once 
 the tube is filled with a soft but brilliant flood of green 
 light. It is found to be about the same efficiency as the 
 ordinary arc-lamp, giving about i'66 candle per watt, 
 and is therefore far above any of the glow-lamps in its 
 economy. 
 
VIII 
 
 THE MANUFACTURE OF LIGHT 
 
 361 
 
 As already pointed out, it is the property of a vapour, 
 when incandescent, to throw its energy into a few 
 briUiant rays, producing in this case a predominance of 
 green and blue. The heat-rays are not absent : but there 
 is a higher proportion of luminous rays than would be 
 the case if the shining body were a solid. Vapour-lamps 
 may therefore be regarded as a step towards the lumin- 
 escence lamp of the future. If only one could devise a 
 plan of setting the atoms or electrons into vibration 
 without exciting the grosser vibrations of the molecules 
 the end would be attained, and the very freedom of the 
 molecules in the gaseous state seems to favour this 
 possibility. Yet in the phosphorescence of the fire-fly, 
 and in the luminescence produced by cathode discharges, 
 there appears to be a possibiHty of touching the atom 
 within the molecule, even in substances that are not 
 vapours. 
 
 Comparison of Electric - lamps. — The following 
 table exhibits in comparative form the efficiencies of the 
 
 Efficiencies of Electric-Lamps 
 
 
 Watts per 
 
 Candle 
 (horizontal). 
 
 Candles per 
 Watt. 
 
 Candles per 
 H.P. 
 
 Glow-lamp. 
 
 3' 3 
 
 0-3 
 
 246 
 
 Nernst-lamp 
 
 1-5 
 
 0-67 
 
 495 
 
 Osmium-lamp 
 
 1-5 
 
 o'6j 
 
 495 
 
 Tantalum-lamp . 
 
 I '4 
 
 07 
 
 532 
 
 Tungsten-lamp . 
 
 I'O 
 
 I"0 
 
 746 
 
 Arc-lamp . 
 
 0-67 
 
 1-5 
 
 IIIO 
 
 Vapour-lamp 
 
 0-6 
 
 1-66 
 
 1240 
 
 Magnetite-lamp . 
 
 0*25 
 
 4-0 
 
 2984 
 
 Flame Arc-lamp 
 
 0-17 
 
 5-8 
 
 4300 
 
62 LIGHT LECT. 
 
 various kinds of electric lamps, and shows how great is 
 the advance made by the recent inventions. The great 
 economy of the flame arc is, however, not sustained 
 except for arcs of enormous power ; and a small lamp, 
 that is, one of from 5 to 20 candle-power, giving more 
 than I candle-power per watt, is a thing still awaiting 
 invention. 
 
 Cost of Manufacture of Light. — We come now to 
 the all- important question of the cost of the light as 
 manufactured in these different kinds of lamps. To 
 deal with this question we must adopt some figures for 
 the cost of the gas, the oil, and the electric-energy which 
 are respectively the supplies from which the light is 
 manufactured. Prices differ in different districts. Those 
 taken here for convenience are — 
 
 Gas (normal quality i6-candle gas at 5 cubic feet per 
 hour) taken at 2s. per 1000 cubic feet. 
 
 Paraffiit Oil (American kerosene, with flash-point at 
 110° F.) taken at 8d. per gallon. 
 
 Electric -Energy taken at 2-4d. per "unit" {i.e. per 
 kilowatt-hour). 
 
 One must also adopt a unit for quantity of light, 
 and for this we take the candle-hour^ meaning the total 
 quantity of light given out during i hour by a light of 
 I candle-power. 
 
 The following table gives a resume, according to 
 the measurements of Professor Wedding, translated into 
 British values, of a number of different sources of light 
 as measured by him. For these the figures of cost 
 given are calculated down into pence per candle-hour 
 
VIII THE MANUFACTURE OF LIGFIT 363 
 
 on the foregoing basis. In a city like York, where gas 
 costs IS. lod. instead of 2s. per 1000 cubic feet, the 
 cost of gas-lights will be reduced correspondingly. 
 In London, where the price is 2s. 9d., they will be 
 correspondingly raised. 
 
 [Table 
 

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 t,-J= 
 
 li-) 
 
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 CU 
 
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LECT. viii THE MANUFACTURE OF LIGHT 365 
 
 If we cast our eye upon the column of figures headed 
 British Thermal Units per Candle -hour we find there 
 how vastly the different sorts of lamps differ in the 
 amounts of heat-energy which has to be supplied to 
 them to generate equal total amounts of light. While 
 the batswing gas-jet needs 310 British thermal units ^ 
 to give T candle-hour, an electric flaming arc needs 
 less than i B.T.U. per candle-hour. Again, an in- 
 candescent gas-light of ordinary Welsbach mantle type 
 requires 43 B.T.U. per candle-hour, whilst an ordinary 
 glow-lamp only requires from 10 to 15 B.T.U. per 
 candle-hour. Yet it is notorious that the incandescent 
 gas is cheaper than the glow-lamp for equal amounts of 
 Hght. The explanation lies in the difference in the cost 
 of the heat ; for the incandescent gas-light gets its heat 
 by burning gas, whereas the glow-lamp gets its heat 
 from the electric-energy supplied to- it. And (at the 
 prices taken) a B.T.U., if manufactured by burning gas, 
 costs 0*000042 pence, whilst if manufactured by expendi- 
 ture of electric-energy costs o "00064. That is to say, so 
 far as production of the mere heat in the lamp is con- 
 cerned, electricity (at the prices taken) costs fifteen times 
 as much as gas. But heat is precisely what we do not 
 want. And because the gas-lamps waste so much of 
 their energy in mere non-luminous heat, they are not, 
 when we come to examine the column of figures of costs 
 per candle-hour, so superior. How much light can we 
 
 ^ The British thermal tinit is that amount of heat which would 
 warm i pound of water I degree of the Fahrenheit scale. It is 
 equal to 251 '98 gramme-calories^ or to 1048 joules, or to 0'000296 
 kilowatt-hour. One kilowatt-hour equals 3435 British thermal 
 units- 
 
366 LIGHT lect. 
 
 buy for a florin? That question is answered in the 
 figures of the table. The dearest source, except a bad 
 glow-lamp (No. 2, which was obviously very inefficient 
 even for a glow-lamp), is the batswing gas-jet, which 
 gives for a florin only 2720 candle-hours. The best of 
 the gas-lights is the high-pressure incandescent, re- 
 presented in the Table by the Millennium, which gives 
 25,494, but it is surpassed by the Flame Arc, which 
 gives 47.619. 
 
 During the last two years much progress has been 
 made with inverted incandescent gas lamps. In this 
 pattern the Bunsen burner is turned upside down, and 
 the flame is directed into a mantle hanging like a bag 
 beneath it. Though this form is not more economical 
 in itself, it is effectively so, because more of its light is 
 advantageously downwards. 
 
 The Cheapest Form of Light. — Earlier our atten- 
 tion was drawn to the circumstance that the process 
 of manufacturing light by incandescence was indirect, 
 while the process of manufacture by liwiinescence was 
 direct, the energy being turned into light without the 
 production of extraneous heat. This is the secret of 
 the glow-worm and the fire-fly ; but it is also the secret 
 of the more brilliant phosphorescence of the cathode 
 rays, as shown you in Crookes' vacuum tubes. Somehow 
 these insects have found out the way, which man also 
 has found in the case of the Crookes' tubes, how to 
 excite the delicate vibrations of the atoms, or of the 
 electrons associated with them, without having to resort 
 to the coarser process of setting all the molecules of the 
 mass dancing with heat. In the possibility of chemical 
 
VIII THE MANUFACTURE OF LIGHT 367 
 
 or cathodic means of exciting luminescence lie the 
 immense opportunities of the future. We in Great 
 Britain spend annually a gigantic sum, estimated at 
 from ;2^i 0,000,000 to ;^2o,ooo,ooo in manufacturing 
 for ourselves such artificial lights as our civilization 
 demands. Ninety-nine per cent at least of this colossal 
 sum is thrown away on mere heat. What a future 
 awaits the man who will invent a practicable luminescence 
 lamp giving light without heat ! 
 
 Future Progress. — It is abundantly evident that there 
 is room for future developments. Progress comes about 
 in two ways. We may take the existing things and by 
 careful experiment and attention to detail improve them 
 bit by bit : that is one way. But every now and then 
 it happens that a man of genius working in the quiet 
 of his laboratory discovers some new fact, which is at 
 first apparently obscure and of no importance. He 
 publishes the observation by reading a paper to some 
 learned society : it is printed in its journal of proceedings 
 and promptly forgotten. Years afterwards, it may be, 
 some practical man comes along, gets hold of the obscure 
 fact, and works it up into a shape that has commercial 
 value. He gets hold of a financier who puts it on the 
 market, and the world hears of a new invention. Some- 
 body makes a fortune, but very seldom does it benefit the 
 original discoverer. The special incandescence of erbia 
 and thoria was known to the chemists forty years ago ; 
 but no one heard of incandescent gas-lighting till Auer 
 von Welsbach devised the mantle to utilize this remark- 
 able property. I have shown you the remarkable 
 luminescence of rubies and of willemite when stimulated 
 
368 LIGHT lect, 
 
 by cathode discharges in a Crookes' tube ; but lumin- 
 escent-lamps on that plan are not yet practical. 
 
 The great economies effected by high-pressure gas 
 and by the flame arc are as yet only attained in big lamps. 
 The immediate want is the production of small lamps of 
 equal economy. Perhaps we shall have small electric 
 vapour-lamps before long. One step toward improve- 
 ment will be the cheapening of the sources of supply, 
 both of gas and of electric-energy. Gas ought now to 
 be evaluated not by its supposed candle-power, but by 
 its calorific power. A gas equal in heating-power to 
 that now supplied could be made for tenpence per 
 looo cubic feet if we did not require it to burn with 
 a bright flame -of its own, and were to use mantles to 
 get the light. -And electric-energy instead of costing 
 2*4 pence per unit can be manufactured at far less 
 than a halfpenny per unit, if manufactured on a large 
 enough scale. There are tremendous possibilities before 
 us : but the possibilities before us in the domain of 
 luminescence are far greater than those in the domain of 
 incandescence. I have no fear as to the ultimate 
 solution of the problem of the manufacture of light. 
 The lamp of the future giving light without extraneous 
 heat will be a luminescence lamp. It will therefore be 
 an electric-lamp, but not an incandescent one. 
 
 A Radium - lamp. — To the possibilities already 
 named, science has lately added a new one in the dis- 
 covery of radium. This surprising and perplexing metal 
 acts as though it were an inexhaustible source of invisible 
 radiations of singular power. A few milligrammes of 
 radium placed near a piece of phosphorescent material 
 
VIII THE MANUFACTURE OF LIGHT 369 
 
 such as willemite, cause it to shine in the dark, making 
 thus a perpetual lamp. You may think that here we 
 have the promise of the very cheapest source of light. 
 Alas for such wishes, the laws of economy are not yet to 
 be over-ridden. Radium is excessively rare and expen- 
 sive. To produce by its phosphoric stimulus on willemite 
 a lamp of even i candle-power requires a few milligrammes 
 of radium, and those few milligrammes will cost at least 
 £']o. For a capital cost of ^70 one may perhaps get a 
 perpetual light of i candle-power ! And the mere interest 
 on the capital will run to something like one farthing 
 per hour for all the hours that the light would be of 
 service. Why, a tallow candle would be cheaper. The 
 dearest of all our sources of light by incandescence does 
 not run to more than y^o^th of a penny per candle-hour. 
 So that which seemed to be the cheapest source of 
 light, costing nothing but interest on capital, turns out 
 to be the dearest. 
 
 Sunlight after all. — No, the cheapest source of light 
 still remains to be the commonest and most universal, 
 the light of the sun, which shines alike on rich and poor, 
 and gives us — such is the admirable economy — a light of 
 which the dominant wave-length is 50 millionths of a 
 centimetre, just that wave-length to which our eyes have 
 become, in the long evolution of the ages, the most 
 sensitive. By no artificial process can we manufacture 
 light so cheaply that it would not be still cheaper to 
 adjust our social habits to the hours of sunlight, and do 
 our day's work while it is yet day. 
 
INDEX 
 
 Abney, Sir William, on emulsion 
 films, i66 
 on colour- vision, 1^2 fi^^'^'^^^ 
 on minimum visible luminosity, 
 
 211 
 his measurements of the relative 
 
 composition of light, 330 
 measurements on the absorbing 
 effect of the atmosphere, 331 
 Absorption of light by coloured 
 media, 88 
 and anomalous dispersion, 104 
 by black surfaces, 201, 332 
 of Rontgen rays, 243 
 effect on sunlight, 331 
 and emission, 331 
 Acetylene lamp, 20 
 Actinic waves, 162, 182 
 Actinium, 289 
 Ahrens's polariser, 123 
 Air-pump, the mercurial, 247, 251 
 thermometer, experiment with, 
 202 
 Aladdin, lamp of, 180 
 Alexandrite, colours of, 86 
 Aluminium, transparency of, to 
 Rontgen rays, 243 
 leaf, use of, in electroscopes, 
 269 
 Amethyst, optical properties of, 
 
 134 
 Ampere's construction, 69 
 Amplitude of wave-motion, 9 
 Analyser, 117 
 
 Analysis of light by prism or 
 
 grating, 79, 86 
 Animatograph, 97 
 Anomalous refraction, 100 
 Ansc^iitz's moving pictures, 97 
 Antikathode, the, 265 
 Antipyrin, optical properties of, 
 
 150 
 Arc-lamp, emission of light by, 
 III, 310 
 whiteness of light of, 211 
 magnetite, 359, 361 
 new kinds of, 357, 361 
 Artificial rainbow, 79, 80 
 Ayrton and Perry, on ratio of the 
 electric units, 233 
 
 Becquerel, Professor Henri, his 
 discovery of radiations from 
 uranium salts, 272, 278, 284 
 Becquerel rays, 278, 284 
 
 power of causing the discharge 
 
 of electrified bodies, 285 
 compared with Rontgen rays, 
 292 
 Benham's colour-top, 96 
 Bid well, Shelford, on fatigue of 
 retina, (^6 footnote 
 strange colour-effects, 99 foot- 
 note 
 Black, is mere absence of light, 72 
 cross, in polariscope, 134, 152 
 surfaces absorb waves and grow 
 warm, 202 
 
 371 
 
372 
 
 LIGHT 
 
 Black surfaces radiate better than 
 bright, 203 
 radiation of black bodies at 
 different temperatures, 339 
 
 Blue and yellow make white, 89, 
 146, 188 
 
 Blue of the sky, theory of, 233 
 
 Bologna stone, 177 
 
 Bolometer, use of, 197, 334 
 
 Boltzmann, Professor Ludwig von, 
 on electro-optics, 234 
 
 Bose, Professor J. Chunder, his 
 apparatus for optical study of 
 electric waves, 226 
 on polarisation of electric 
 waves, 227 
 
 Boyle, Hon. Robert, on phosphor- 
 escence of diamonds, 177 
 
 Brdmont, M., 289 
 
 Bright field, 118, 212 
 
 Brightness of lights, 14 
 
 Brodhun and Lummer Photo- 
 meter, 1 9 
 
 Burning-glass, 37 
 mirror, 206 
 
 Calc-spar : see Iceland spar 
 Calorific waves, 162, 193, 197 
 Campbell -Swinton, A. A., his 
 photographs by Rontgen 
 rays, 270 
 Candle, standard, 14, 20, 314 
 Canton's phosphorus, 175 
 Casciarolo of Bologna, 177 
 Chalcolite, 289 
 
 Chemical effects of waves, 163, 166 
 Chemi-luminescence, 175, 176 
 Christiansen dn anomalous dis- 
 persion, 100 
 Cold, apparent radiation of, 205 
 "Colonia" gas compressor, 349, 
 
 350 
 Colour and wave-length, 71, 72 
 sensations, primarj', 183 
 top, Benham's, 97 
 vision, 183 
 Coloured stuffs viewed in coloured 
 light, 82 
 
 Colours that are not in the spec- 
 trum, 87 
 complementary, 91, 11 1, 136, 
 
 188 
 of polarised light, 136 
 of thin plates, 137 
 of soap bubbles, 138 
 supplementary, 149 
 Combination of colours, 84, 85, 
 142 
 to produce white light, 83, 146 
 Complementary colours, 91, 188 
 tints, 91, 93, III, 188 
 in bright and dark field, 136 
 in double-image prism, 149 
 Concave lens, diverges light, 41 
 
 mirror, reflexion by, 27 
 Contrast tints, 92, 93, 99 
 Convergence of light to focus by 
 reflexion, 26 
 by refraction, 35, 37, 41 
 Convex lens, converges the light, 
 40 
 mirror, reflexion by, 24, 28 
 Cooper-Hewitt mercury vapour 
 
 lamp, 360 
 Cornea of eye, 44 
 Corpuscular theory of light, 230 
 Critical angle, 39. 
 Crookes, Sir William, his radio- 
 meter, 199, 213, 252 
 tube used by Rontgen, 239, 
 
 241 
 improvements in vacuum pump, 
 
 250 
 on repulsion due to radiation, 
 
 252 
 on properties of kathode rays, 
 
 253 
 views on radiant matter, 253, 
 
 258 
 tube with shadow of cross, 255 
 invented focus tube, 256 
 his spinthariscope, 298 
 vacuum tubes, 366 
 Cryptoscope, 268 
 Crystalline lens of the eye, 44 
 Crystallo-luminescence, 175 
 
INDEX 
 
 373 
 
 Crystals, elasticity of, 129 
 Curie, M. Pierre, 288, 289 
 experiments with radium, 292 
 on heat emitted by radium, 299 
 Curie, Madame, her apparatus 
 for detecting radio-activity, 
 287 
 results of her investigations of 
 the various compounds of 
 uranium, 288 
 discovers polonium and radium, 
 
 289 
 experiments with radium, 292 
 Curvature, printed on the wave- 
 front, 24 
 imprinted by lens or curved 
 
 mirror, 43, 56, 67 
 expansion of, 61 
 Cylindrical lens, 49, 83 
 
 Dark field, 119, 120 
 
 for polarised heat-waves, 212 
 
 Davy, Sir Humphry, on reflexion 
 of heat, 206 
 
 Debierne, M. , obtains actinium 
 from pitchblende, 289 
 
 Delezenne's polariser, 12,2) footnote 
 
 Den,ser medium, 35 
 
 Density and refractivity, 49 
 
 Detection of false gems by polar- 
 ised light, 133 
 by Rontgen rays, 272 
 
 Detectors of electric waves, 217, 
 223, 226 
 
 Dewar, Professor James, on phos- 
 phorescence of bodies cooled 
 in liquid air, 179 
 
 Diakathodic rays, 279 
 
 Diamond does not polarise light, 
 
 134 
 phosphorescence of, 177, 178 
 transparency of, to Rontgen 
 rays, 270 
 Dielectrics, 104, 232 
 Difference of phase, 136 
 Diffraction-grating, 31, -jj, 79 
 
 spectrum, 78 
 Diffuse reflexion, 30 
 
 Dioptrie, definition of, 59 
 Direction of the vibrations in 
 
 polarised light, 233 
 Diselectrification by ultra -viole, 
 light, 181 
 by Rontgen rays, 268 
 Dispersion of light by prism, 74 
 anomalous, 100 
 and frequency, 158 
 Divergence of light from focus by 
 reflexion, 24 
 by refraction, 41 
 Divergivity, 62 
 Double-image prism, 125 
 Double refraction, 120 
 
 refraction. Lord Rayleigh on 
 theory of, 233 
 
 Ebonite, transparency of, for 
 heat-waves, 213 
 optical and dielectric properties 
 of> 233 
 Effluvio-luminescence, 175 
 Elasticity in crystals, \2.<^ foot7iote 
 
 axes of, 131 
 Elastic-solid theory of light, 156 
 Electric oscillations, 219 
 waves, 214 
 waves, prediction of, by Clerk 
 
 Maxwell, 229 
 sparks in vacuo, luminosity due 
 
 to, 247 
 sparks, oscillatory, 218 
 lamps, table of efflciencies of, 
 
 361 
 lighting, invention of, 304 
 vapour lamp, 360, 361 
 Electricity discharged by ultra 
 violet light, 181 
 by Rontgen rays, 268 
 incandescence by, 309 
 Electro-luminescence, 175 
 Electromagnetic theory of anomal- 
 ous dispersion, 102 
 theory of light, 230 
 ' ' Electrons," 294 
 Electroscope, used for photo-elec- 
 tric experiments, 181 
 
 D 
 
374 
 
 LIGHT 
 
 Electroscope, as detector of electric 
 waves, 223 
 with aluminium leaves, 181, 269 
 use of, by Lenard, 259 
 use in observing diselectrifica- 
 
 tion, 269 
 for observation of radio-activity, 
 286, 287 
 Emission of light at different tem- 
 peratures, 174, 331 
 measurement of, 333 
 Emissivity of the rare earths, 344 
 Emulsion films in photography, 
 
 166 
 Energy curves, 334, 335 
 
 spectra of black body, 341, 343 
 of platinum, 343 
 Ether, the, 108, 230 
 Eye, the, sensitiveness of, 14, 328 
 as optical instrument, 43 
 images are inverted in, 45 
 unable to detect polarisation, 
 
 III 
 of codfish, in polarised light, 151 
 
 Fairy Fountain, 39 
 
 Faraday, Professor Michael, first 
 experiments in electro -optics, 
 229, 230 
 his electromagnetic theory of 
 
 light, 231 
 on the production of flame, 309 
 
 Fatigue, effects of, x-\foot7iote, 93, 
 96 
 
 Fechner's law of magnitude of sen- 
 sation, -L^ footnote 
 
 Filter-screens for invisible light, 
 164, 213 
 
 Fireflies, luminescence of, 176, 
 
 311. 337 
 rays emitted by, pass through 
 
 copper, 278 
 light of the American, 337 
 FitzGerald, Professor George F. , 
 
 on electromagnetic theor}' of 
 
 reflexion and refraction, 233 
 on starting waves in the ether, 
 
 234 
 
 Fizeau, on number of waves in 
 
 train, 112, o.-j ^ foot?iote 
 Flame standards, 314 
 Flames, radiation of heat by, 203 
 Fleming, Professor J. A. , magnetic 
 action on kathode rays, 255 
 his electric lamp, 2,'^^ footnote 
 "Flicker" photometers, 2>'^i, foot- 
 note 
 Fluorescence, phenomejion of, 169 
 experiments in, 170, 283 
 table of, 175 
 Fluorescent screens, 268 
 
 use of, in ultra-violet light, 172 
 Fluoroscope, 268 
 Fluor-spar, transparency to ultra- 
 violet light, 164, 166 
 photo - luminescence (fluores - 
 
 cence) of, 169 
 thermo-luminescence of, 180 
 Focus, real, 26, 37 
 virtual, 26 
 
 tubes, Crookes's, 256 
 , , the author's, 265 
 ,, Jackson's, 266 
 ,, Bohm's, 267 
 Formulae for refraction, 61 
 for lenses, 65, 67 
 for reflexion, 67 
 Foucault's modification of Nicol's 
 
 prism, 121 
 Frequencies and wave-lengths, 
 
 table of, 190 
 Frequency of sound waves, 106 
 
 of different colours, 190 
 Fresnel's theory of light, 157 
 views as to direction of vibra- 
 tions, 233 
 
 GaUtzine, Prince, on alleged 
 polarisation of Rontgen rays, 
 2.61 footnote 
 Gases, optical and dielectric pro- 
 perties of, 233 
 glow in vacuum tubes, 248 
 Gas flame, temperatures in, 308 
 Gas-lighting, invention of, 304 
 high-pressure incandescent, 349 
 
INDEX 
 
 375 
 
 Gas-lights, incandescent, 345 
 mantle, inverted, distribution 
 of luminous rays, 321 
 Geissler's tubes, 247 
 Gems, optical properties of, 86, 
 119, 120, 133, 134, 178, 254 
 Geometrical optics, methods of, 55 
 Germany, research in, 263 
 Gifford, J. W. , his photographs by 
 
 Rontgen rays, 270 
 Gladstone, Dr. J. Hall, on photo- 
 graphing the invisible, 168 
 Glass, velocity of light in, 35 
 polarising properties of un- 
 
 annealed, 151 
 strain in, 152 
 absorption of ultra-violet light 
 
 by, 164 
 opacity of, to ultra-violet light, 
 
 166, 174 
 opacity of, to heat-waves, 198 
 opacity of, to Rontgen rays, 
 271, 272 
 Glazebrook's Report on Optical 
 
 Theories, 158 
 Glovv^ lamps, 310, 361 
 
 electric, distribution of luminous 
 
 rays, 321 
 electric efficiency of, 351 
 test chart of twenty-four, 352 
 new kinds of, 354 
 Glow-worm light will pass through 
 
 aluminium, 278 
 Glow-worms, luminescence of, 176, 
 
 178, 311, 337 
 Goethe, his theory of colours, 76 
 Goldstein, Dr. Eugen, on rays 
 
 behind the kathode, 280 
 
 Granite, in polarised light, 134 
 
 Grating, diffraction produced by, 31 
 
 Green cannot be made by mixing 
 
 pure yellow and blue, 89 
 
 Haidinger's brushes, iii footjiote 
 Hartnack's modification of Nicol's 
 
 prism, 121 
 Hauksbee, Francis, on electric 
 
 luminosity, 245, 246 
 
 Heat-indicating point, 206 
 waves reflected to focus, 206 
 shadows, 209 
 spectrum, 162, 197 
 Heating effect of waves, 162 
 
 by absorption of waves, 201 
 Heaviside, Oliver, on propagation 
 
 of energy, 233 
 Hefner's standard lamp, 20, 314 
 
 footnote 
 Helium, 301, 327 
 Helmholtz, Hermann von, on 
 anomalous refraction, 102 
 on electromagnetic theory, 234 
 Herschel, Sir J. W. F. , on plane 
 of polarisation, 158 
 Sir William, on heat spectrum, 
 200 
 Hertz, Professor Heinrich, on dis- 
 electrification, 181 
 discovery of electric waves, 
 
 214 
 on oscillatory sparks, 214 
 his oscillators, 215, 221 
 on reflexion of waves, 217, 220 
 effect of his discoveries, 234 
 waves, model illustrating pro- 
 pagation of, 237 
 on transparency of metal films 
 to kathode rays, 258 
 Hittorf, Professor W. , on kathode 
 
 phenomena, 252 
 Hopkinson, Professor John, on 
 optical and electric proper- 
 ties, 233 
 Horn, optical properties of, 150 
 Home's luminescent stuff, 178 
 Huygens's principle of wave pro- 
 pagation, 9 
 construction, 69 
 Hydraulic compressor, automatic, 
 
 350 
 Hyperfluorescence, 285 
 
 Ice, apparent radiation of cold 
 by, 205 
 optical and dielectric properties 
 of, 233 
 
376 
 
 LIGHT 
 
 Iceland spar, 120, 129, 174, 198 
 Illumination of a surface, 15 
 Image of a luminous point, 22 
 
 in mirror, position of, 23 
 Images, formation of, 28 
 
 inverted, 30 
 
 in eye are inverted, 45, 47 
 Incandescence, the process of, 
 210, 306 
 
 solid particles in, 307 
 
 by electricity, 309 
 Incandescent gas-lights, 345 
 
 high-pressure, 348 
 Incandescent solids and vapours, 
 
 spectra of, 326 
 Infra-red waves, 192 
 Interference of waves, 10 
 
 of light produces colours, 141 
 Internal reflexion, 39 
 Inverse squares, law of, 16 
 Invisible, the photography of the, 
 
 168 
 Invisible spectrum, ultra-violet 
 part, 160 
 
 infra-red part, 192 
 Iodine vapour, anomalous refrac- 
 tion of, 100 
 Irrationahty of dispersion, 78 
 Isokathodic rays, 280 
 Ives's method of registering colour 
 by photography, 185 
 
 photochromoscope, 187 
 
 Jackson, Professor Herbert, his 
 
 focus tube, 266 
 Japanese mirrors, 50 
 Jelly, vibrations transmitted by. 
 
 Kaleidoscope, principle of, 33 
 Karnojitzky on alleged polarisa- 
 tion of Rontgen rays, 261 
 footnote 
 Kathode irays, 253 
 focusing of, 256 
 name ihappropriate, 258 
 new varieties of, 266, 279 
 
 Kathode, phenomena at, 249 
 shadows, 252, 254 
 
 , , magnetic deflexion of, 
 
 255. 257 
 streams, 253, 256 
 Kathodo-luminescence, 175 
 Kearton, J. W. , his magic mirrors, 
 
 53 
 Kelvin, Lord, theory of the ether, 
 
 234 
 
 Kern burners, 347 
 
 Kerr, Dr. John, magneto -optic 
 discoveries, 234 
 
 Kromskop, 187 
 
 Kundt, August, on anomalous re- 
 fraction, loi, 103 
 
 Lamp, arc-, images of carbons 
 in, 29 
 Hefner's standard, 20 
 monochromatic, 82 
 Langley, Professor S. P. , on 
 longest waves, 190, 197 
 his bolometer, 197 
 his curves for one unit of heat, 
 
 335 
 on the production of light 
 without heat, 337 
 Law of Fechner, 1^ footnote 
 
 of inverse squares, 16 
 Le Roux on anomalous dispersion, 
 
 100 
 Lenard, Professor Philipp, his 
 researches, 258 
 diselectrifying effect of kathode 
 rays, 259 
 Length of wave : see wave-length 
 Lens, crystalline, principle of, 36, 
 40 
 of eye, 44 
 Lens, cylindrical, 49 
 
 measurer, 59 
 Light, velocity of, 2, 33, 129, 156 
 emission of, 332 
 manufacture of, 302-569 
 primitive sources of, 302 
 cold lights, 311 
 
INDEX 
 
 377 
 
 Light, two different Ways of 
 manufacturing, 312 
 bad economy of ordinary sources , 
 
 336 
 cost of, 362 
 cheapest form of, 366 
 future progress, 367 
 Lights, measurement of, 315 
 inequaUty of distribution, 317 
 inequality of composition, 323 
 relative composition of, 330 
 Limelight, 309 
 Lippmann, Professor Gabriel, on 
 
 photography in colour, 184 
 Lodge, Professor Oliver Joseph, 
 his oscillators, 222 
 his detector, 223 
 his apparatus for optical study 
 
 of electric waves, 224 
 illustrations of Maxwell's theory, 
 
 234 
 on electric oscillations, 235 
 London, University of, contrasted 
 
 with that of Wlirzburg, 262 
 Longest waves of infra-red, 190 
 Lumiere, the Brothers, their pro- 
 cess of colour-photography, 
 iS^ footnote 
 Luminescence, 174, 311 
 
 of radium compounds, 298 
 Luminescent screen in ultra-violet 
 light, 172, 241 
 used by Rontgen, 239, 241 
 best kind of, 268 
 Luminous efficiency, 312 
 Luminous paint, 177, 311 
 Lummer, Prof. Otto, his photo- 
 meter, 0.0 footnote 
 on the energy spectra of black 
 body and of platinum, 342 
 Lyo-luminescence, 175 
 
 MacCullagh's theory of light, 
 
 .157 
 views as to direction of vibra- 
 tions, 233 
 Magenta, absorption spectrum of, 
 87. 104 
 
 Magenta, anomalous refraction 
 
 of, 100, 104 
 Magnetite arc-lamp, 359, 361 
 Magneto -optic discoveries, 230, 
 
 234 
 Manufacture of light : see Light 
 Maxwell, Professor James Clerk, 
 on colour- vision, 183 
 predicted electric waves, 214 
 electromagnetic theory of light.. 
 
 229 
 on Faraday's electromagnetic 
 theory, 231 
 Meldola, Professor, on the light 
 
 of the glow-worm, 337 
 Mendenhall, C. E. , on luminous 
 
 efficiency, ■^i'^^ footnote 
 Mercurial air-pump, 247, 251 
 
 phosphorus, 246 
 Mica, optical properties 01, 137, 
 
 146 
 Michelson, Professor, on number 
 of waves in train, wo. footnote 
 Millennium light, 349 
 
 distribution of luminous rsiys, 
 321 
 Miller's limit of shortest waves, 191 
 Mirage, experiment illustrating, 48 
 Mirror, magic, reflexion of lij ht 
 by, 21 
 of Japan, 50 
 English, 53 
 Mirrors, concave and convex, 24 
 paraboloidal, for reflecting heat- 
 waves, 206 
 parabolic, used by Hertz, 220 
 Model illustrating Stokes's theory 
 of Rontgen rays, 275 
 illustrating propagation of Hertz- 
 wave, 237 
 Models of wave-motions, 8, 109, 
 
 115, 124, 130, 236 
 Molecular bombardment, 253 
 Monochromatic lamp, 82 
 Monoyer on definition of dioptric, 
 
 59 
 Morton, President Henry, his 
 
 fluorescent dyes, 171 
 
378 
 
 LIGHT 
 
 Mother-of-pearl, colours of, 79 
 
 Moving pictures, 97 
 
 Muraoka, Dr., on fire-fly light, 
 
 278 
 Muybridge, on movements of 
 
 animals, 97 
 
 Nernst lamp, 354, 361 
 
 distribution of luminous rays, 
 321 
 New kinds of kathode rays, 279 
 Newton's colour-whirler, 84 
 theory of nature of white light, 
 
 n, 83, 86, 323 
 tints, 137 
 rings, 138 
 table of, 140 
 explanation of, 142 
 Nichols, Edward Fox, on anomal- 
 ous refraction, 104 
 and Rubens's longest waves, 
 190 
 Nicol, William, his polarising 
 prism, 121 
 prism, modern varieties of, 121 
 Nodal points in reflected waves, 
 217 
 
 Opacity and electric conductivity, 
 relation between, 232, 234 
 of electric conductors, 232 
 Opacity of glass to invisible hght, 
 166, 174, 198, 271 
 of metals to Rontgen rays, 
 
 243 
 Optical circle, for demonstrating 
 
 refraction, 38 
 
 illusions, 92, 94, 96, 99 
 
 rotator, zt.-^ footnote 
 
 Orders of Newton's tints, 139 
 
 Ordinary and extraordinary waves, 
 
 125 
 Orthochromatic photography, 183 
 Oscillating sparks, 214, 218 
 Oscillator, Hertz's, 215, 221 
 Osmium lamp, 354, 355, 361 
 Osram lamp, 356 
 
 Paint, luminous, 177, 178, 311 
 
 heat-indicating, 208 
 Parabolic mirrors. Hertz's, 220 
 Paraboloidal mirrors for reflecting 
 
 heat, 206 
 Paraffin oil, fluorescence of, 169 
 lamp, unequal distribution of 
 flame, 318 
 Parakathodic rays, 279 
 Paschen's longest waves, 190 
 Pentadecylparatolylketone, 241 
 
 footnote 
 Permanganate of potash, absorp- 
 tion by, 88 
 Perrin, Jean, on refraction of 
 
 Rontgen rays, 2.6x footnote 
 Persistence of vision, 93, 97 
 Petroleum, fluorescence of, 169 
 Phase, difference of, 136 
 Phenakite, kathodo-luminescence 
 
 of, 254 
 Phosphorescence, 175, 311 
 Phosphorus, luminescence of, 176 
 Canton's artificial, 177 
 the mercurial, 246 
 nature of rays emitted by, 278 
 Photochemical effects of light, 163, 
 
 166 
 Photochromoscope, 187 
 Photo-electric effects of Hght, 181, 
 
 182, 268 
 Photographic waves, 162, 182 
 spectrum, 166 
 registration of Rontgen shadows, 
 
 245 
 Photography of the invisible, 168 
 
 in natural colours, 183, 184 
 
 the " new," 245 
 Photo-luminescence, 175, 176 
 Photometer, 14, 315 
 
 Thompson and Starling's, 18 
 
 Trotter's, 18 
 
 Brodhun and Lummer's, 19-20 
 footnote, '^ic^ footnote 
 
 Bunsen's, ig footnote 
 
 Joly's, 19 
 
 Rood's, -J,! c^ footnote 
 
 Simmance-Abady's, 315 
 
INDEX 
 
 379 
 
 Photometry, 313 
 
 Pigments darken the light, 83 
 
 Pink not a spectrum colour, 87 
 
 Pitchblende, 288-290 
 
 Plane- waves, 7 
 
 Platinocyanides, their optical pro- 
 perties, 172, 281 
 
 Polarisation, 105 
 plane of, 158 
 
 of electric waves, 225, 227 
 of Rontgen rays, attempts at, 
 261 
 
 Polariscope, simple, 153 
 
 Polarisers, different kinds of, 113 
 
 Polonium, 289 
 
 Positive curvature, definition of, 
 
 59 
 
 lens, definition of, 59 
 Poynting, Professor J. H. , on 
 ripple-tanks, 6 
 
 on energy paths, 233 
 Primary colour sensations, 86, 89, 
 91, 183 
 
 tints, 86, 91 
 Prism, refracting, 74 
 
 direct-vision, 80 
 
 Foucault's, 121 
 
 Hartnack's, 121 
 
 Nicol's, 121 
 
 double-image, 125 
 Prismatic spectrum, 74 
 
 irrationality of, 78 
 Propagation of waves, 9 
 
 of light in glass, 35 
 
 of waves longitudinally, 106 
 
 of waves transversely, 107, 237 
 Purple not a spectrum colour, 87 
 
 analysis of, 87 
 
 Quarter- wave plate, 148 
 use of, 12.2, footnote, 148 
 
 Quartz, anomalous dispersion of, 
 104 
 rotatory optical properties of, 
 
 154 
 transparency of, to ultra-violet 
 light, 164 
 
 Quartz lenses and prisms, use of, 
 164, 170, 174 
 tribo-luminescence of, 181 
 transparency of, to heat-waves, 
 198 
 Quinine, fluorescent property of, 
 169, 171, 172 
 
 Radiant heat, 193, 299 
 matter, 253, 258 
 
 Radiation, t^O) faotjiote 
 from radium, 291, 298 
 temperature and quality of, 339 
 wave-length of the dominant, 
 340 _ 
 
 Radio-activity, electroscope suit- 
 able for observation of, 286 
 
 Radiometer, Crookes's, 199, 213, 
 
 Radio-micrometer, 334 
 Radium, discovery of, 289 
 its cost, 290, 369 
 power to ionise the air, 291 
 experiments with, 292 
 deflexion of the rays, 293 
 the alpha, beta, and gamma 
 
 rays, 294-298 
 continually evolves heat, 299 
 disintegration of the radium 
 atom, 300 
 Radium clock, Hon. R. J. Strutt's, 
 297 
 lamp, 368 
 
 salt, temperature of, 299 
 Rainbow due to refraction, 73 
 
 artificial, 79, 80 
 Ray-filters for infra-red light, 213 
 
 for ultra-violet light, 164 
 Rayleigh, Lord, on shadows of 
 sounds, 5 
 on anomalous refraction, loi 
 Theory of Sound, 107 
 on electromagnetic theory of 
 
 light, 233 
 on blue of the sky, 233 
 on double-refraction, 233 
 Rays, non-existence of, 12 foot- 
 note 
 
38o 
 
 LIGHT 
 
 Rays, kathode, use of term, 257 
 
 uranium, 278, 284 
 
 radium, 292 
 Real focus, 26 
 Reflexion by plane mirror, 21 
 
 by convex mirror, 24 
 
 by concave mirror, 26, 27 
 
 irregular or diffuse, 30 
 
 by multiple mirrors, 33 
 
 of heat-waves, 206, 209 
 
 alleged, of Rontgen rays, 260 
 Refraction of light, 35 
 
 anomalous, 100 
 
 double, 120 
 
 feeble, of Rontgen rays, 261 
 Resolution of vibrations, 126 
 Resonator, Hertz's, 216 
 Retina of the eye, 45 
 
 fatigue of, 93, 96, 99 
 Reversibility, the principle of, 282 
 Righi, Professor Augusto, his os- 
 cillators, 221 
 
 his apparatus for optical study 
 of electric waves, 225 
 Ripples, on water, 6 
 
 convergence and divergence of, 
 12 
 Ripple-tank, 6 
 
 Rock-salt lenses and prisms, use 
 of, 194 
 
 transparency to ultra-violet light, 
 166 
 
 transparency to infra-red light, 
 198 
 Rontgen, Professor Wilhelm Kon- 
 rad, 262 
 
 account of his discovery, 238 
 
 his form of tube, 260 
 
 his theory of the rays, 273 
 Rontgen rays, properties of, 240 
 
 penetrative power of, 243, 292 
 
 not deflected by magnet, 260, 
 292 
 
 are not kathode rays, 260 
 
 are not ordinary ultra-violet 
 light, 260 
 
 are not reflected, 260 
 
 point of origin of, 264 
 
 Rontgen rays, curious lateral 
 emission of, 265 
 shadows of bones made by, 269 
 speculations as to nature of, 273 
 compared with Becquerel rays, 
 292 
 Rosaniline, absorption spectrum 
 of, 87, 104 
 anomalous refraction of, 100, 
 104 
 Rotation of polarised light by 
 quartz and sugar, 154 
 in magnetic field, 230, 234 
 by reflexion at magnet pole, 234 
 Rotator, optical, yq.-^ footnote 
 Rowland's ruling machine, 77 
 Rubens, Professor, on the emis- 
 sivity of solid particles, 345 
 on the emissive power of the 
 Welsbach mantle, 347 
 Rubens's and Nichol's longest 
 
 waves, 190 
 Rubies, real and sham, 133 
 glow in kathode stream, 254 
 transparency of, to Rontgen 
 
 rays, 272 
 luminescence of, 367 
 Rumford, Count, on radiation of 
 
 heat, 206 
 Rupert's drops, optical properties 
 
 of, xt^x footnote 
 Rutherford, Professor, on the 
 disintegration of the radium 
 atom, 300 
 
 Salicine, optical properties of, 135 
 Schumann'slimit of shortest waves, 
 
 191 
 Scott-Snell burner, 350 
 Selenite, crystal films of, 133, 136, 
 
 145, 148 
 Sensitiveness of eye, 14 
 Shadows, light penetrates into, 4, 
 12 
 of sounds, 4 
 of heat, 209 
 
 cast by Rontgen rays, 242 
 kathodic, in Crookes's tubes, 255 
 
INDEX 
 
 381 
 
 191 
 
 Shortest waves in ultra-violet, 
 Silk, shot, reflexion by, 31 
 Smithells, Professor, on tempera- 
 tures in gas flame, 308 
 Smoothness, optical definition of, 
 
 21 
 Soap-bubbles, colours of, 138 
 Sound-waves, size of, 3 
 
 frequency of, 106 
 Spar, calc : see spar, Iceland 
 Spar, Iceland, 120, 129, 174, 
 
 198 
 Spectrimi analysis, vii, 87 
 
 of colours, 74 
 
 produced by prism, 75, 100 
 
 produced by diffraction-grating, 
 
 77 
 visible part of, 161 
 invisible parts of, 161 
 the photographic, 166 
 the long, 173 
 teaching of the, 325 
 Speed of light : see Velocity 
 Spherometer, 59 
 Spinthariscope, Sir William 
 
 Crookes's, 298 
 Sprengel's vacuum-pump, 250 
 
 Crookes's improvements in, 250 
 Standard candle, 14, 20, 314 
 
 lamp, 20, 2^i\foot7iote 
 Stationary waves, 184, 217 
 Steinmetz, Dr. C. P. , his magnetite 
 
 arc-lamp, 359 
 Stokes, Sir George Gabriel, his 
 discoveries in fluorescence, 
 169, 172, 283 
 his theory of Rontgen rays, 
 
 273 
 shortest waves observed by, 
 191 
 Strain in imperfectly annealed 
 glass, 151 
 in compressed glass, 152 
 Strobic circles, 96 
 Strutt, Hon. R. J. , on radio-active 
 matter, 291 
 his radium clock, 297 
 Subjective colours, 92 
 
 Sugar, luminescence of, 180 
 rotatory properties of, 155 
 Supplementary tints, 149 
 
 Talbot on anomalous refraction, 
 100 
 
 Tantalum lamp, 354, 355, 361 
 
 Temperature in relation to emis- 
 sion of light, 174, 211 
 
 Thaumatrope, 94 
 
 Thermo-luminescence, 175, 180 
 
 Thermometer, to explore infra-red 
 spectrum, 193, 200 
 air, experiment with, 202 
 
 Thermopile, use of, 194, 196, 203, 
 207, 212, 334 
 
 Thomson, Professor Joseph John, 
 on ratio of units, 233 
 
 Thorium salts, radio-active proper- 
 ties of, 291 
 
 Three-colour method of photo- 
 graphy, 183 
 
 Tints of spectrum, 72 
 
 complementary, 91, 93, in, 
 
 136, 149, 188 
 Newton's, 137, 142 
 supplementary, 149 
 
 Total internal reflexion, 39 
 
 Tourmaline, optical properties of, 
 119, 120 
 opacity and conductivity of, 234 
 
 Trains of waves, 112, 219, 222, 273 
 
 Transition-tintj 139 
 
 Transparency of flesh, leather, and 
 paper, 243 
 
 Transverse waves, 108 
 
 Tribo-luminescence, 175, 180 
 
 Trichromic theory of colour-vision, 
 183 
 
 Tungsten lamp, 355, 361 
 
 Turner, Dr. Dawson, on glow- 
 worm hght, 278 
 
 Tyndall, Professor John, his use 
 of colour disks, 92 
 experiments on reflexion of 
 
 heat-waves, 209 
 On Sound, 107 
 lectures on light, 171 
 
382 
 
 LIGHT 
 
 Tyndall, Professor John, his wave- 
 filter for infra-red, 213 
 on the spectrum of the arc- 
 lamp, 334 
 
 Ultra-violet light, 160, 162 
 chemical effects of, 167 
 reflexion, etc., of, 174 
 light, diselectrification by, 181 
 Universities of London and Wiirz- 
 
 burg, 262 
 Uranium, great density and opacity 
 of, 244 
 its radio-activity, 299, 301 
 Uranium glass, fluorescence of, 169 
 nitrate, tribo-luminescence of, 
 
 180 
 rays, 278, 284, 285 
 " Uranium X," 301 
 
 Vacuum-pump, the mercurial, 
 
 247, 251 
 Vapour lamp, electric, 360, 361 
 Velocity of light in air, 2, 33 
 
 in water, 33 
 
 in glass, 33 
 
 of heat-waves, 212 
 
 of propagation of electric dis- 
 turbances, 232 
 Velocity-constant, definition of, 60 
 Vernon Harcourt pentane lamp, 
 
 314 
 Virtual focus, 26, 27 
 
 Wave-length, 4, 7, 72 
 
 motion models, 8, 124, 236 
 
 front, motion of, 9 
 
 tables of, 72, 190 
 
 filters for infra-red waves, 213 
 Wave-length of electric waves, 214 
 Wave-lengths of different sources 
 
 of radiation, 340, 342 
 Wave-theory of hght, 230 
 Waves, travelling of, 7, 106, 237 
 
 propagation of, 9, 106, 156, 
 157. 233 
 
 Waves, trains of, 112, 219, 222, 
 
 273 
 
 Weber, Professor Wilhelm, on 
 ratio of electric units, 232 
 
 Webster, Rt. Hon. Sir Richard, 
 his hand, 270 
 
 Wedding, Professor W. , on 
 luminous efficiency, -^i-^ foot- 
 note, 336 
 his comparison of sources of 
 light, 362, 364 
 
 Welsbach mantle, 342, 345 
 its emissive power, 347 
 distribution of luminous rays, 
 319. 320 
 
 Wheatstone, Sir Charles, on 
 velocity of electric disturb- 
 ances, 232 
 
 Wheel of life, 97 
 
 White light, analysed, 79, 149 
 synthesis of, 83, 84, 90 
 
 Whiteness, no standard of, known, 
 211 foot7iote 
 
 Wiedemann, Professor Eilhard, on 
 luminescence, 174, 180 foot- 
 note 
 his "discharge-rays," 276, 278 
 
 Willemite, 298 
 
 luminescence of, 367 
 
 Winkelmann, Professor A., on re- 
 fraction of Rontgen rays, 261 
 footnote 
 
 Wiirzburg, University of, 262 
 
 X-luminescence, 175, 260 
 X-rays : see Rontgen rays 
 
 Yellow not a primary colour- 
 sensation, 89 
 
 Young, Dr. Thomas, on colour- 
 sensation, 183 
 
 Young, Professor, on the light of 
 the American fire-fly, 337 
 
 Zoetrope, 97 
 
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