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 The wonder book of light, 
 
 3 1924 002 931 909 
 
Library 
 
 The original of tliis book is in 
 tine Cornell University Library. 
 
 There are no known copyright restrictions in 
 the United States on the use of the text. 
 
 http://www.archive.org/details/cu31924002931909 
 
THE WONDER BOOK OF LIGHT 
 
Large Light-house Lejhs 
 
 Exhibited at the Chicago World's Fair, in 1893, and was employed in connection with a 
 nine thousand candle power arc Hght. 
 
 By courtesy of ' 7'he Scientific American'' 
 
THE WONDER BOOK 
 OF LIGHT 
 
 BY 
 
 EDWIN J. HOUSTON, Ph.D. 
 
 Author of "The Wonder Book of Volcanoes and Earthquakes," 
 
 " The Wonder Book of the Atmosphere," " The Wonder 
 
 Book of Magnetism," etc. 
 
 WITH IIB ILLUSTRATIONS 
 
 NEW YORK 
 
 FREDERICK A. STOKES COMPANY 
 
 PUBLISHERS 
 
Copyright, 1908, by 
 
 Frederick A. Stokes Company 
 
 November, 1908 
 
 All rights reserved 
 
Dedicated to my Friend 
 
 L. WEBSTER FOX, M.D., LL.D., 
 
 Professor of Ophthalmology in the MedicO' 
 
 Chirurgical College of Philadelphia, 
 
 in appreciation of the encouragement given me in 
 
 the new field of juvenile literature 
 
PREFACE 
 
 Light, though one of the most difficult of all the natural 
 forces to explain thoroughly, contains within it so many 
 wonders that it can be made attractive to young and old 
 alike. 
 
 The author has not hesitated, in either this or the other 
 Wonder Books, to employ freely a few of the old, old fairy 
 stories for illustrating some especially difficult point. 
 It is his experience in the teaching of the natural sciences 
 to the young, that the more unusual or out-of-the-way 
 the illustrations employed, the greater the chance of their 
 producing a marked effect on the mind. 
 
 At the same time, however, too elementary a treatment 
 has been carefully avoided. Especial care has been taken 
 to employ the optical terms in general use; for there is 
 no more difficulty in learning the correct scientific names 
 than in remembering a number of fictitious names. In 
 this way the reader has laid up in his or her memory, 
 along with the elementary principles of the science of 
 optics, information that cannot fail to be of great value in 
 after years. 
 
 The author acknowledges his indebtedness to his friend, 
 Dr. L. Webster Fox, Professor of Ophthalmology in the 
 Medico-Chirurgical College of Philadelphia, for reading 
 the proof sheets. 
 
 Acknowledgment is gratefully made to Messrs. William 
 
PREFACE 
 
 Wood and Company, Messrs. Henry Holt and Company, 
 The McGraw Publishing Company and to Messrs. Hinds, 
 Noble and Eldredge for permission to reproduce various 
 illustrations in this volume. 
 
 B. J. H. 
 
 Phuadelfbia, 1908. 
 
CONTENTS 
 
 CHAPTEB PAGE 
 
 I. INTEODUCTOBT ...... 
 
 II. Peopeeties of Light .... 
 
 III. When Light Falls on Mattbe 
 
 IV. Looking Glasses 
 
 V. Bukning Glasses and Shaving Glasses 
 
 VI. The Bending oe Refeaotion of Light . 
 
 VII. Lenses 
 
 VIII. The Mioeoscope 
 
 IX. The Telescope 
 
 X. The Cameea Obscuea, the Photogeaphic 
 Cameea, and the Magic Lanteen 
 
 XI. The Human Eye 
 
 XII. Spectacles and Ete-Glasses. Some PecuL' 
 iaeities of Eyesight .... 
 
 XIII. How We See Solid Bodies 
 
 XIV. The Natuee of Light .... 
 XV. The Light Mill 
 
 XVI. The Magic Wand of Peince Peecinet . 
 
 XVII. COLOUE 
 
 XVIII. Invisible Light ..... 
 XIX. Specteum Analysis .... 
 
 XX. Diseases ob Abeeeations of Lenses 
 XXI. Fluobescbnce and Phosphoeescence 
 
 1 
 8 
 22 
 35 
 51 
 63 
 75 
 85 
 97 
 
 112 
 127 
 
 140 
 152 
 160 
 170 
 180 
 192 
 203 
 217 
 231 
 240 
 
COISTTEKTS 
 
 CHAPTEB PAGH 
 
 XXII. X-Eats and Radio-Activity .... 248 
 
 XXIII. Il-LTJMINATION. THE SALE OF LiGHT . . 257 
 
 XXIV. Lighthouse Illumination and Seabchlights. 
 
 Illumination of the Stage . . . 270 
 
 XXV. Seeing is Not Always Believing . . . 279 
 
 XXVI. Effects Caused by Persistence of Vision . 287 
 
 XXVII. Ghosts 294 
 
 XXVIII. Sunshine and Gladness — Moonshine and 
 
 Madness 307 
 
 XXIX. Photography 315 
 
 XXX. SoAP-BuBBLE Colours 326 
 
 XXXI. Rainbows and Opalescence .... 334 
 
 XXXII. Polarised Light 343 
 
ILLUSTRATIONS 
 
 FULL-PAGE 
 
 Large Light-house Lens Frontispiece 
 
 Facing Page 
 
 Terkes Observatory, Williams Bay, Wisconsin . . . 104 
 
 An X-Ray Photograph . . 248 
 
 Thirty-inch Projector on Mount Washington .... 276 
 
 ILLUSTRATIONS IN THE TEXT 
 FiGTJEE Page 
 
 1 Some Common Forms of Light Rays 23 
 
 2 Reflection of Light from Plane Surfaces .... 24 
 
 3 Reflection of Beam of Light .... ... 26 
 
 4 Diffusion or Scattering of Beam of Light .... 27 
 
 5 Path of a Ray seen by Diffusion from Dust Particles 27 
 
 6 Image formed in Dark Chamber by Aperture . . 28 
 
 7 Shadow of Sphere Produced by Luminous Point — 
 
 Umbra only Produced 30 
 
 8 Shadow of Sphere Produced by Luminous Surface — 
 
 Umbra and Penumbra 31 
 
 9 Profiles of Faces Produced by Shadows .... 33 
 
 10 Why Looking Glasses make Images " Go the Other 
 
 Way" 36 
 
 11 A Looking Glass Image 38 
 
 12 Looking Glass Writing 39 
 
 13 Apparatus for Looking Through a Millstone ... 42 
 
 14 Arrangement of Mirrors In Pig. 13 44 
 
 15 Multiple Images of 'Parallel Mirrors 48 
 
 16 Differently Shaped Bodies that can be Employed as 
 
 Mirrors 51 
 
 17 Sections of Concave and Convex Mirrors .... 52 
 
 18 Position of Image formed by Plane Mirrors ... 53 
 
ILLFSTEATIONS 
 
 Figure Page 
 
 19 An Image formed by a Concave Mirror .... 54 
 
 20 Another Image of Concave Mirrors 55 
 
 21 The Foci of Concave Mirrors 56 
 
 22 Image formed by Convex Mirrors 57 
 
 23 Burning Mirror 59 
 
 24 Curious Effect of Cylindrical Mirror 60 
 
 25 Curious Double Curve formed by Concave Cylin- 
 
 drical Mirror 61 
 
 26 An Effect of Refraction 65 
 
 27 Refraction of Light 66 
 
 28 EfCect of Refraction or Apparent Depth of Water . 70 
 
 29 Critical Angle and Total Internal Reflection ... 73 
 
 30 Convex Lens Employed in Burning Glass .... 75 
 
 31 Passage of Light Through Plate- of Glass .... 76 
 
 32 Refraction of Light at Surfaces of Prism .... 77 
 
 33 The Principle of the Lens 78 
 
 34 Foci of Lens 79 
 
 35 Converging and Diverging Lenses 80 
 
 36 Focus of Parallel Rays of Convex and Concave 
 
 Lenses 81 
 
 37 Some Other Foci of Convex Lenses 82 
 
 38 Use of Convex Lens as a Magnifier 83 
 
 39 Formation of Image by Convex Lens 84 
 
 40 Simple Microscope 86 
 
 41 Magnifying Lenses of Short Focal Lengths ... 90 
 
 42 Measurement of Magnifying Power 92 
 
 43 Compound Microscope 94 
 
 44 Compound Achromatic Microscope 95 
 
 45 Astronomical Telescope 97 
 
 46 Lehses in Astronomical Telescope 98 
 
 47 Terrestrial Eye-Piece 103 
 
 48 Construction of the Galilean Telescope .... 104 
 
 49 Newtonian Telescope 106 
 
 50 Combined Image forming Lens and Mirror . . . 113 
 
 51 Photographic Camera with Ground Glass Plate In 
 
 Position 113 
 
 52 Cross-Section of Photographic Camera 119 
 
 53 Magic Lantern 120 
 
ILLUSTEATIOXS 
 
 FiGUBE Page 
 
 54 Arrangement of Lenses and Reflector in Magic 
 
 Lantern 121 
 
 55 Solar Microscope 12!i 
 
 56 Direction of Light through Solar Microscope . . 123 
 
 57 Photo-Electric Microscope 125 
 
 58 Longitudinal Section of the Human Eye .... 127 
 
 59 Action of Eye on Pencils of Light 134 
 
 60 Visual Angle 135 
 
 61 Action of Concave Spectacle Lens 142 
 
 62 Bi-Focal Spectacle Glasses 144 
 
 63 Astigmatic Dial 146 
 
 64 Helmholtz's Ophthalmoscope 150 
 
 65 Irradiation 151 
 
 66 Stereoscope 157 
 
 67 Action of Stereoscope 158 
 
 68 A Serious Objection to the Corpuscular Theory of 
 
 Light 163 
 
 69 Light Mill or Radiometer . .... 171 
 
 70 Action of Equal and Unequal and Opposite Directed 
 
 Jets 176 
 
 71 Phosphorescence by Electrical Bombardment . . 178 
 
 72 A Prism Sorting Out the Different Coloured Rays 
 
 of Sunlight 182 
 
 73 Newton's Experiment with Violet Light .... 186 
 
 74 The Recomposition pf Light 189 
 
 75 Colour Disc Employed for the Recomposition of 
 
 Light 190 
 
 76 Thermal, Luminous and Actinic Portions of Solar 
 
 Spectrum 206 
 
 77 Distribution of Heat and Light in Diffraction 
 
 Spectrum 209 
 
 78 A Spectroscope 228 
 
 79 Passage of Light Through Spectroscope .... 230 
 
 80 One of the Diseases of Lenses 232 
 
 81 Equal and Opposite Refraction and Dispersion . . 237 
 
 82 Principle of Achromatism 238 
 
 83 Achromatic Lens 238 
 
 84 Caustic Curve, a Common Disease of Mirrors . . 239 
 
 85 Fluorescent Screen 253 
 
ILLrSTEATIONS 
 
 FiGXiBE Page 
 
 86 The Principle of the Inverse Squares .... 266 
 
 87 The Bunsen Photometer 267 
 
 88 Light-house Lens 272 
 
 89 Fresnel Fixed Light 274 
 
 90 Flashing Light 275 
 
 91 Arc Headlight for Locomotive 276 
 
 92 Thirty-inch Projector on Mount Washington . 0pp. 277 
 
 93 Stage Lamp 278 
 
 94 An Optical Illusion 280 
 
 95 Another Optical Illusion 281 
 
 96 Other Optical Illusions 281 
 
 97 A Strange Optical Illusion 282 
 
 98 Optical Illusion Causing Circles to Appear as Six- 
 
 Sided Figures 284 
 
 99 Sylvanus P. Thompson's Optical Illusion . . . 285 
 
 100 Optical Illusion — Apparent Rotation of Cog-Wheel 285 
 
 101 The Thaumatrope or Wonder Turner 287 
 
 102 Mixing Black and White in the Eye 291 
 
 103 Paper Disc for Colour Top 292 
 
 104 A Scientifically Raised Ghost 303 
 
 105 The Phantom Bouquet 304 
 
 106 An Easily Raised Devil 305 
 
 107 An Exceedingly Venerable Soap-Bubble .... 328 
 
 108 Soap-Bubble Film 332 
 
 109 Action of Rain Drop on Beam of Sunlight ... 336 
 
 110 Cause of the Primary and Secondary Rainbows . 338 
 
 111 Tourmaline Polarisers and Analysers . . . . 346 
 
 112 Nicol's Prism 347 
 
THE WONDER BOOK OF LIGHT 
 
The Wonder Book of Light 
 
 CHAPTEK I 
 INTRODUCTORY 
 
 In his inspired account of the creation of the world, 
 Moses hegins with the statement that when, in the begin- 
 ning, God created the heaven and the earth, the earth was 
 without form and void and darkness was upon the face of 
 the deep. The creative spirit of God moved upon the face 
 of the waters: "And God said. Let there be light; and 
 there was light. And God saw the light that it was good." 
 Then the light was divided from the darkness, and the 
 light called day and the darkness night. The first great 
 act in the creation of the world was completed. For the 
 first time night, following day, falls on the earth. 
 
 During the second day, or the second great creative 
 period of time, as the word day is now generally under- 
 stood, God made the firmament, and divided the waters 
 that were under the firmament from those that were above. 
 Fight again falls on the earth, and the second day's work 
 of creation is completed. 
 
 On the third day, at the divine command, the waters 
 under the heaven are gathered together into one place 
 and the dry land appears, and the earth brings forth 
 grass, the herb yielding seed after his kind, and the fruit 
 tree yielding fruit after his kind, whose seed is in itself, 
 and darkness again falls on the face of the earth, and the 
 third day's work is completed. 
 
2 THE WONDEE BOOK OF LIGHT 
 
 Had it not been for the creation of light on the first 
 day, God's great gift of the herb yielding seed and the fruit 
 tree yielding fruit would have been futile ; for it was only 
 under the awakening influence of light that the seeds 
 germinated. It is only because of the action of light that 
 plants can take from the carbonic acid of the atmosphere 
 the food they need for the formation of their woody 
 fibre. 
 
 The fourth great creative period or day now dawns on 
 the earth. At the divine command, lights appear in the 
 firmament of the heaven to divide the day from the night, 
 the sun to rule the day, the moon to rule the night; and 
 night again falls on the earth. 
 
 We must not forget that in the account Moses gives of 
 the creation of the world, he is telling how things looked 
 to him as God caused a vision of the creation to pass, 
 as it were, in panoramic view before him. We know that 
 the sun and the stars were created long before the earth, 
 but by reason of great clouds of steam and vapor, their 
 light was completely hidden from the observer. They are, 
 therefore, spoken of as being brought into existence only 
 so far as the earth itself is concerned. 
 
 On the fifth day came the great creation of marine life. 
 The waters brought forth abundantly moving creatures 
 that have life; fowls that fly in the air above the earth, 
 and great whales, and every living creature that moves in 
 the water. Night again falls on the earth and another 
 great creative day has passed. 
 
 On the sixth day comes the creation of the land ani- 
 mals, cattle and creeping things, and beasts of the earth, 
 and then followed the greatest of all creative acts, when 
 
INTRODUCTOEY 3 
 
 God made man in his own image and breathed into hia 
 nostrils the breath of life. 
 
 I do not think I could begin The Wonder Book of Light 
 in any better manner than by thus describing briefly to 
 you the wonderful account that Moses gives iu the Bible 
 of the creation of the world. When properly read and 
 understood, I feel, as a man who has given considerable 
 attention to the manner in which the earth has been 
 gradually developed or created by God, that this account 
 is wonderful in its general accuracy as to the order in 
 which the different creative stages followed one another. 
 Considering the early date at which Moses wrote it would 
 clearly have been impossible for him to have obtained 
 such knowledge as is found in his account in any other 
 way than as an inspiration from God. 
 
 I hope by the time you have finished reading The Won- 
 der Book of Light, and have thoroughly grasped the differ- 
 ent matters that will be brought to your attention, you will 
 be better able to appreciate how very, very good was the 
 light that God called into existence at the beginning of 
 earth's first creative day. 
 
 It is customary, in undertaking the description of any 
 natural phenomenon, to define the meaning generally 
 given it. This, we must agree, is an excellent method. 
 You will probably wish to ask me the question : " What is 
 light?" 
 
 Now it happens this was the question that Sir Oliver 
 Lodge, one of the most celebrated and ablest of English 
 philosophers, put to himself while deliveriug a popular 
 lecture at the London Institution, December 15, 1880, 
 on the relations existing between electricity and light. 
 
4 THE WONDEE BOOK OF LIGHT 
 
 When Lodge put to himself the question, " What is 
 electricity ? " he very properly answered, " Nobody knows." 
 At the same time, however, he suggested that the inability 
 to answer this question need discourage no one, since in a 
 certain sense we do not know any more about either matter 
 or energy. 
 
 When Lodge came to answer the second question, " What 
 is light?" he gave the following answer — I quote it here 
 in his own words, for to my mind it is expressed in charm- 
 ing language : 
 
 " Now we will pass to the second question ; What do 
 you mean by light? And the first and obvious answer 
 is, everybody knows. And everybody that is not blind 
 does know to a certain extent. We have a special sense 
 organ for appreciating light, whereas we have none for elec- 
 tricity. Nevertheless, we must admit that we really know 
 very little about the intimate nature of light — very little 
 more than about electricity. But we do know this, that 
 light is a form of energy; and, moreover, that it is energy 
 rapidly alternating between the static and the kinetic 
 forms — that it is, in fact, a special kind of energy of 
 vibration. We are absolutely certain that light is a peri- 
 odic disturbance in some medium, periodic both in space 
 and time; that is to say, the same appearances regularly 
 recur at certain equal intervals of distance at the same 
 time, and also present themselves at equal intervals of time 
 at the same place; that in fact it belongs to the class of 
 motions called by mathematicians undulatory or wave 
 motions." 
 
 When, therefore, you ask me what is light, I can answer 
 with Lodge that you all know what light is. You know 
 
INTEODUCTOKY 5 
 
 it by its many effects. It is the great agency that not only 
 drives away the darkness which covers the face of the earth 
 during night, but that also lights up surrounding objects 
 and enables us to distinguish readily their colours, forms, 
 and appearances. It is the potent form of energy that 
 awakens the vegetable germs in the earth, so that the grass 
 may grow, the herb may yield its seed and the fruit tree 
 may yield its fruit after its kind. Moreover, after these 
 have been started into life, it is the power by means of 
 which they are supplied with food from the atmosphere 
 for the production of their woody tissues, through the 
 decomposition of the carbonic acid gas the plants breathe 
 in from the air. 
 
 It is a wonderful thought, that the delicate structure 
 of the leaf of the living plant is not only able to act as the 
 lungs, by which it can take in carbonic gas from the air, 
 but also as the laboratory in which, under the decomposing 
 power of light, the combination between the oxygen and 
 the carbon is broken up, the oxygen thrown out into the 
 air, and its carbon retained for the formation of its woody 
 tissues. 
 
 Should light disappear entirely and forever, our beau- 
 tiful earth would soon become a vast eharnel house. Dark- 
 ness would then again brood over the face of the deep, and 
 silence and death would reign supreme. 
 
 You will recall the fact that when God created male 
 and female, he blessed them, commanding them among 
 other things to subdue the earth; that is, to have power 
 over the natural forces he had created. 
 
 I sometimes think that one of the most important direc- 
 tions in which man has subdued the earth is in his ability 
 
6 THE WONDER BOOK OF LIGHT 
 
 to produce light artificially^ so as to be able to extend the 
 day far into the night. Man is no longer obliged, as were 
 his ancestprs, to cease from his labor and seek rest in sleep, 
 on the setting of the sun, when, in the absence of moon- 
 light, there is no light by which he can work. 
 
 Man's first invention for turning night to day, was 
 crude indeed — a piere burning fagot, snatched hurriedly 
 from the fire. It was but a poor means of illumination, 
 yet it permitted him to go some little distance from his 
 dwelling, and, to a certain extent, to prolong the time dur- 
 ing which he could work. 
 
 The next improvement was the invention of the rush 
 light, practically the candle. Then followed the invention 
 of the oil lamp. These were great inventions, although, 
 when compared with the more complete devices of to-day, 
 they seem poor and crude. 
 
 Then, after a long while, the invention was made that 
 introduced illuminating gas into our houses, and the gas 
 burner took its place as one of the marvels of that time. 
 In its turn the gas light was to a great extent displaced 
 by the are and incandescent electric lamps. At a some- 
 what later date, there was such a marked improvement 
 either by the invention of the Welsbach incandescent man- 
 tles, or by the replacing of ordinary illuminating gas by 
 acetylene gas, that the use of gas for illumination greatly 
 increased. These, and other improvements, have left the 
 world in a wonderfully advanced condition, so that man is 
 now able to prolong the day by extending it far into the 
 night. 
 
 While the inventions which have been made for the pro- 
 duction of artificial light, may properly be regarded as 
 
INTEODUCTOEY 7 
 
 among the most noted of man's achievements, yet I am 
 not quite sure that they have produced results entirely 
 devoid of evil consequences. One of the conditions nature 
 has imposed on man, in order that he shall retain his 
 health, is that he shall rest or sleep during a certain 
 number of hours each day. If he foolishly fail to take 
 this rest he must, sooner or later, pay a dear price for 
 his disregard of nature's laws. I would, by no means, 
 advocate the re-establishment of the curfew bell, on the 
 ringing of which, as you know, all lights were to be ex- 
 tinguished and everyone was to go to bed. The world 
 has gone too far for that; the curfew is a thing of the past. 
 Night work is now essential. Take the case of any large 
 city, such as London, New York, or Philadelphia. It is 
 during the small hours of night, while most people are 
 asleep, that the bread is made and baked, the foodi 
 products prepared and brought into the city, and the great 
 newspapers printed, so that they can be placed on the 
 table the next morning, at the meal which marks the 
 break of man's long fast. Should artificial illumination 
 be no longer employed, the world would be set back in its 
 civilisation to an extent that would seem almost in- 
 credible. 
 
 The immensity of this night work was thoroughly ap- 
 preciated during the siege of Paris by the Germans, when 
 the government had to provide for the feeding of the in- 
 habitants. The same thing has doubtless been experi- 
 enced during other sieges which to a great extent have 
 shut cities off from the outside world. 
 
CHAPTEE II 
 PROPERTIES OF LIGHT 
 
 "While I believe that most of the interest attached 
 to those fairy stories which have been handed down from 
 generation to generation, arises from the apparent im- 
 probability of their fulfilment in actual life, yet what 
 gives them an added charm, to the more mature mind, is 
 the thought that in nearly all these stories there are sug- 
 gestions of some great truth in physical science. Indeed, 
 I feel that it is due to this fact that I am as fond now of 
 fairy stories as I was in my childhood. 
 
 Perhaps some older readers may feel disposed to 
 look on the use of fairy stories in books like The Wonder 
 Books, as bad taste, or out of place. They are well enough, 
 they may say, in nursery stories, but have no proper place 
 in books of this character. Now I want to say here that 
 I do not at all agree with this statement. Science is so 
 full of the most wonderful fairy stories that I may, some 
 of these days, write an entirely new series of these stories, 
 in which I will describe the wonderful forces of the 
 physical world in the guise of fairies. It will suffice here, 
 however, to say that, even if no other advantage were 
 gained by their use in The Wonder Books than that of 
 directing the attention of young readers to the strange 
 manner in which some important fact in nature has been 
 explained, their use would be more than justified. Al- 
 
 8 
 
PEOPERTIES OP LIGHT 9 
 
 though fairy stories are literally untrue, yet they cannot 
 fail to impress on our minds any great principle in science 
 they have been employed to illustrate. 
 
 And now, as we come to a matter in light that is hard 
 to understand, and is, moreover, generally stated in a dry 
 and uninteresting manner, I am reminded of the beauti- 
 ful story of Graeiosa and Percinet. 
 
 Like all fairy stories it begins with the phrase, " Once 
 upon a time," as if it were endeavouring to establish its 
 claim to belief on the pretense of limiting the occurrences 
 described to a definite time. " Once upon a time " there 
 was a King and Queen who had a daughter named Graei- 
 osa, a charming princess, worthy the name they had given 
 her. The Eang, Queen and Princess lived happUy to- 
 gether until the Queen died. Now, unfortunately, the 
 King was a great lover of riches, and, still more unfor- 
 tunately, there lived in his kingdom a rich Duchess so 
 deformed and ugly, both in body, mind and disposition, 
 that she was known among her neighbours as Grumbly, 
 a name that suited her extremely well. 
 
 The Duchess was jealous of the Princess because the 
 Princess possessed qualities or properties of which she was 
 so deficient. Taking the first opportunity that presented 
 itself, Grumbly, who was enormously wealthy, permitted 
 the King to examine her treasures, and promised to give 
 them all to him if he would only marry her. She made it 
 a condition, however, that Graeiosa should be placed en- 
 tirely in her power. 
 
 N"ow, although the King dearly loved his daughter, yet 
 he loved riches more. He, therefore, made a miserable bar- 
 gain and sold himself to the Duchess for her wealth, thus 
 
10 THE WONDEE BOOK OP LIGHT 
 
 not only cheating his daughter out of a father's love that 
 was hers by right, but also cheating the Duchess, who 
 received for her money so poor a specimen of a man that 
 he did not begin to be worth the price she paid for him. 
 
 Great preparations were made for the marriage. The 
 Princess had no difficulty in dressing so as to look exceed- 
 ingly charming. With the Duchess, however, it was quite 
 a different thing. However, she tried her best. By mak- 
 ing the heels of one of her shoes half an inch higher than 
 the other, she somewhat decreased her limping gait. By 
 padding her clothes, she hid some of the crookedness of 
 her body. An artificial glass eye, made to replace one 
 she had lost, caused her face to look somewhat less ugly. 
 Then she dyed her red hair black, put a blush on her 
 yellow face by rouge, and hid her wrinkles under enamel. 
 Indeed, when thus adorned, she might have been fairly 
 good looking at a distance if she had not displayed such 
 awful taste in dressing. But the colours of the materials 
 she employed were so abominable in their contrast that 
 there was probably never a more awful looking dress, 
 despite the fact that it cost so much money. 
 
 There lived in the same kingdom an extremely agree- 
 able and beautiful young prince named Percinet, who 
 possessed immense wealth and at the same time had certain 
 fairy gifts that made him very powerful. He had for a 
 long time been in love with Graciosa. Hearing of the 
 approaching marriage and knowing the danger that threat- 
 ened Graciosa, he dressed himself as a page and went to her, 
 offering his protection. 
 
 As soon as the Duchess saw the lovely Princess seated 
 on a spirited horse that Percinet had brought her, and 
 
PEOPEETIES OF LIGHT 11 
 
 which he was leading as her page, she became jealous, 
 and told the King that unless the Princess' horse was given 
 to her, as well as the page who was leading it, she would 
 not marry him. The King agreed to do as she demanded. 
 Somehow or other, I think you and I can guess why, the 
 horse ran off, the Duchess was dragged through the mud, 
 and her clothing was torn. But the marriage went on, 
 nevertheless, and Graciosa was delivered into the power of 
 the Duchess, who was now her stepmother, the Queen. 
 
 Although unable actually to prove it, the Queen felt 
 sure that her troubles had come from Graciosa. She, 
 therefore, commanded her servants to carry her daughter- 
 in-law to a great forest, one hundred leagues from the 
 palace, and leave her there alone, believing that the wild 
 beasts would soon devour her. Graciosa, however, was res- 
 cued by Percinet and taken to his palace, where she was 
 welcomed by his mother. After a happy eight days, Gra- 
 ciosa persuaded Percinet, much to his regret, to take her 
 back to her father. As soon as she entered the palace she 
 was again placed in the power of her stepmother, the 
 Queen, who determined to kill her. In order, however, to 
 pretend that she did this only because of the Princess' 
 disobedience, the Queen summoned some wicked fairies, 
 and commanded them to set some impossible tasks for the 
 Princess. 
 
 A number of tasks were set, none of which Graciosa 
 was able to perform. In each case, however, at the last 
 moment. Prince Percinet appeared, and by a mere touch 
 of his magic wand, the task was instantly completed. 
 
 Furious that she has found nothing Graciosa was unable 
 to perform, the Queen, after consulting with a wicked 
 
13 THE WONDER BOOK OP LIGHT 
 
 fairy, at last set her a task that certainly seemed to be 
 impossible. 
 
 Ifow as this is a part of the story I intend to use, in 
 order to illustrate a difiScult point in light, a point that is 
 all the more difficult because at first sight it seems easy, I 
 will quote somewhat in full: 
 
 " The next day the Fairy appeared with a huge barrel 
 full of the feathers of all sorts of birds. There were night- 
 ingales, canaries, gold finches, linnets, tomtits, parrots, 
 owls, sparrows, doves, ostriches, bustards, peacocks, larks, 
 partridges, and everything else that you can think of. 
 These feathers were all mixed up in such confusion that 
 the birds themselves could not have chosen out their own. 
 
 " ' Here,' said the Fairy, ' is a task which it will take 
 all your prisoner's skill and patience to accomplish. Tell 
 her to pick out and lay in a separate heap the feathers of 
 each bird. She would need to be a fairy to do it.' 
 
 " The Queen was more than delighted at the thought 
 of the despair this task would cause the Princess. She 
 sent for her, and, with the same threats as before, locked 
 her up, ordering that all the feathers should be sorted by 
 sunset. Graciosa set to work at once, but before she had 
 taken out a dozen feathers she found that it was perfectly 
 impossible to know one from another. 
 
 " ' Ah ! well,' she sighed, ' the Queen wishes to kill me, 
 and if I must die I must. I cannot ask Percinet to help me 
 again, for if he really loved me he would not wait till I 
 called him, he would come without that. 
 
 " ' I am here, my Graciosa,' cried Percinet, springing 
 out of the barrel where he had been hiding. ' How can 
 you still doubt that I love you with all my heart.' 
 
PEOPEKTIES OP LIGHT 13 
 
 "Then he gave a stroke of his wand upon the barrel, 
 and all the feathers flew out in a cloud and settled down in 
 little separate heaps all round the room. 
 
 " ' What should I do without you, Pereinet? ' said 
 Graciosa gratefully. But stUl she could not quite make 
 up her mind to go with him and leave her father's kingdom 
 forever ; so she begged him to give her more time to think 
 of it, and he had to go away disappointed once more." 
 
 I imagine I see some of you after looking at the first 
 part of the chapter heading, inquire, " What possible con- 
 nection can the remarkable fairy story just told have with 
 such an unattractive and dry subject as ' The Properties 
 of Light,' whatever that may be? Perhaps that word 
 " properties " is perplexing you as I remember once it did 
 me. Let me first tell you briefly its exact meaning, and 
 I will then tell you what this fairy story has to do with 
 this chapter. 
 
 Strictly speaking, a property is a belonging or a pos- 
 session. If someone should make you a present of a 
 fine four-story brick house, he would be giving you some- 
 thing that could be correctly spoken of as your property, 
 and, if you had several houses, they could be spoken of as 
 your properties. 
 
 Now the word property is employed in a somewhat sim- 
 ilar sense in physical science. But here it means more 
 than this. A property is not only a belonging; it is a 
 belonging, suflBciently different from the belongings of 
 other things, to enable one readily to recognise this par- 
 ticular thing. It is in this sense that we have used the 
 words " properties of light." 
 
 By the properties of light therefore, we mean those be- 
 
14 THE WONDEE BOOK OF LIGHT 
 
 longings or peculiarities that enable ns to distinguish the 
 peculiar force called light from other forces. Let me, 
 therefore, tell you some of these properties of light, re- 
 membering all the time that you are to try to find one par- 
 ticular set of properties that will remind you of the 
 barrel filled with feathers in the fairy story of Graciosa 
 and Percinet, as well as the wonderful manner in which 
 by a touch of his wand all these feathers instantly arranged 
 themselves in separate lots. In order the better to do this, 
 let me ask you carefully to examine a patch of bright sun- 
 light, such, for example, as the patch that may fall 
 on a sheet of white paper lying on the floor, or on the 
 table. 
 
 There certainly seems to be nothing magical about a 
 mere patch of sunlight. It seems to be only an uniformly 
 white space, wonderfully bright, wonderfully steady, pleas- 
 antly warm in winter, and sometimes disagreeably warm 
 in summer. You therefore say, and I am not surprised 
 to hear you say it : "I see nothing here to remind me of 
 any fairy story." 
 
 I can only say : wait a while. I don't expect you to un- 
 derstand this matter at first sight. It took many ex- 
 ceedingly bright men to observe, experiment and examine 
 that patch of sunlight before they saw anything curious 
 about it. Give me your close attention while I name some 
 of its properties. 
 
 In the first place, that patch of sunlight does not con- 
 sist of any kind of matter. It is a variety of force. It 
 is not a material thing but a peculiar condition in which 
 energy is capable of existing, and, indeed, in which any 
 other form of energy can be readily made to show itself, 
 
PEOPEETIES OF LIGHT 15 
 
 if only one learns how to make it do so. In other words, 
 no matter from what source it may have been derived, 
 whether from a steam engine, from the force of the wind, 
 from a falling body, from electricity, magnetism, heat, 
 or from the elasticity of a bent-spring, energy can be 
 changed into light; that is, can be made to produce light. 
 Being a force, light, like all other forces, is weightless. 
 Therefore, while you are looking at that patch of sunlight 
 I wish you to bear this in mind; it possesses no weight. 
 This property of light cannot remind you of the feathers 
 because feathers do possess weight. 
 
 The next property or peculiarity of light is that it is 
 by no means as still as it appears to be as it lies so quietly 
 on that patch on the sheet of paper or on the floor. It pos- 
 sesses the power of moving from place to place in perfectly 
 straight lines. 
 
 You can easily convince yourself of this as follows. 
 Place a small piece of looking glass on a piece of paper, 
 so that some of the light falls on it. As you well know, 
 the light will be thrown off from the glass and will appear 
 on the ceiling or somewhere else. You can still see, on 
 the part of the paper not covered by the glass, the light 
 that is coming from the sun. You can also see it on the 
 ceiling. If you wish to follow the path of the light, raise 
 a smoke in the air by burning paper. The path of the 
 light can then be easily seen. 
 
 Some of the light, thrown off in all directions from the 
 smoke particles, enters your eyes, and enables you to see 
 the line of particles that have been lighted up in the 
 direction of its path. 
 
 There is another way in which it can be proved that 
 
16 THE WONDER BOOK OP LIGHT 
 
 light moves or travels in straight lines. Objects become 
 visible by means of the light they throw oflE in all direc- 
 tions. Now carefully looking at any object, hold a sheet 
 of paper, a book, or any opaqne body, between yonr eyes 
 and these objects, and slowly raising it higher and higher, 
 notice how the objects instantly disappear as the opaque 
 body comes between them. Notice, moreover, that the 
 portions of the objects that are thus removed from view 
 will take the outlines of the edges of the book or other 
 object. This is because the rays of light, coming in 
 straight lines from the objects, fall on the book, and, 
 being unable to pass through it, fail to enter your eyes. 
 If you use a circular or triangular piece of paper, the 
 portions of the objects cut out from view will possess the 
 outlines of the paper. The same is true if holes of any 
 shape be cut in the paper, for the oi^tlines of portions of 
 the objects seen will be strictly those of the holes. 
 
 There is still another proof that light travels in a straight 
 line. When a man wishes to shoot a bird, with a ball 
 from a gun, he is safe in assuming that the light by which 
 he sees the bird passes between it and his eye in a per- 
 fectly straight line; for, aiming his gun straight in this 
 direction, allowing, of course, a trifle for the fall of the 
 ball while passing from the gun to the bird, if. his aim is 
 correct, and his gun shoots true, the ball will undoubtedly 
 strike the bird. 
 
 The facts that sunlight can move and that it moves 
 in straight lines are two other properties of light. But 
 neither of these can possibly remind you of the big barrel 
 crammed full of all kinds of birds' feathers. 
 
 Another property of light is that it moves rapidly 
 
PEOPEETIES OP LIGHT 17 
 
 from place to place. Indeed, tMs motion is so swift that 
 you cannot detect it. Light travels much more rapidly 
 than sound. If you are standing where you can see a dis- 
 tant man strike a blow with a sledge hammer, you can 
 see the hammer strike long before you can hear the sound. 
 As you know, the sound instantly follows the blow, but 
 if the man is say half a mUe distant, quite a little time 
 will pass from the time you see the hammer strike before 
 the sound reaches you. This is because the light travels 
 so much more rapidly than sound. 
 
 If you wish to convince yourself of this, take a piece 
 of looking glass off the table, and see how many you can 
 count while holding it in different positions in the sunlight 
 before the light flying off the mirror strikes the ceiling 
 or a distant wall of a room. If you try this you may come 
 to the conclusion that it does not take any time to pass 
 through this distance. It's there before you see it. I answer 
 of course it is; for if it were not there you would not be 
 able to see it. But to get there it must have passed from 
 the mirror to that part of the wall, and to enable you to see 
 it, it must have come from that part of the wall to teU you 
 that it was there, so that it must have passed twice through 
 this distance. 
 
 You would have the same trouble, if, taking the look- 
 ing glass to the window, you permitted the patch of light 
 to move across the street to one of the opposite houses, or 
 even to a considerable distance up and down the street. 
 The light moves so rapidly from the mirror that it seems 
 to you to take no time to move from place to place, or, 
 in other words, its motion seems to be instantaneous. But 
 you must not get the idea that light requires no time 
 
18 THE WONDEE BOOK OF LIGHT 
 
 to travel from place to place. It does require time, al- 
 though this time is exceedingly small; for, as we shall 
 show in a subsequent chapter, the speed with which light 
 moves, or its velocity, is so great that it far exceeds the 
 speed of the swiftest locomotive. Indeed, the best locomo- 
 tive moves at but a snail's pace when compared to the 
 motion of light; for, while the locomotive can move at 
 say about 90 miles an hour, light can travel with a speed 
 greater, in round numbers, than 186,000 miles per second 
 or 669,600,000 miles per hour. 
 
 Ifow this enormous speed is another property of light. 
 But it can certainly in no manner suggest to you the big 
 barrel of birds' feathers. 
 
 But there is still another peculiarity of light that you 
 certainly would never be able to detect by merely watch- 
 ing the patch of sunlight on the sheet of paper. Light 
 does not consist of a single thing. It consists of a great 
 number of different things ; or, as will be shown in another 
 chapter, of waves set up in an imponderable medium, 
 called the ether, that exists not only throughout all por- 
 tions of space but even in between the small particles of 
 which matter is composed. 
 
 Like sound, light is the result of a wave motion, and 
 that patch of sunlight you are looking at on the piece of 
 white paper consists of an enormous number of waves of 
 different lengths. 
 
 Perhaps you will be able to form a clearer idea of this 
 fact if we take the somewhat simpler idea of a musical 
 sound. Suppose a sound is produced by striking all the 
 strings of a piano, at the same time. Now while a trained 
 ear can distinguish the separate notes that are thus 
 
PROPEKTIES OF LIGHT 19 
 
 sounded, yet to most ears they seem to consist of a single 
 noisy tone only. 
 
 That patch of sunlight ■which lights up the sheet of 
 paper is more complex and contains a greater number 
 of separate kinds of light waves than would the sound 
 produced not only by striking at the same time all the 
 strings of a single piano, but even all the strings of many 
 pianos differently tuned. 
 
 But the patch of sunlight is even more complex than 
 this. Besides the light waves it is giving off, which are 
 able to produce different effects on the eye, there are other 
 ether waves far greater in number than even the visible 
 waves that are unable to produce any effect on the eyes. 
 These are the waves called heat waves, or, as they are some- 
 times called, waves of ' dark light.' They are much 
 longer than the light waves. Besides these there are still 
 other sets of waves, shorter than light waves, that are 
 too small to affect the eye, but produce curious effects in 
 matter called actinic effects. In other words, the white 
 patch of sunlight is extremely complex, consisting as it 
 does of many kinds of waves that can affect the eye as 
 light; and of a still greater number of heat waves that 
 are too long to affect the eye, as weU as a great number 
 of waves that can produce photographic effects, but are 
 far too short to influence the eye. Moreover, these in- 
 visible waves are very much greater in number than the 
 visible waves. 
 
 I am sure all of you can now see that in this last prop- 
 erty of light — its wonderful complexity — is to be found 
 the suggestion of the fairy story of Graciosa and Pereinet. 
 In that patch of light, which seems so simple as it shines 
 
20 THE WONDER BOOK OF LIGHT 
 
 on the sheet of paper, there are far more kinds of light 
 waves than there were feathers that the wicked Queen 
 had set for poor Graciosa to sort out before sunset. 
 
 Possibly some of you may say, " Are you not stretching 
 a point here? Is it possible that in so common a thing 
 as sunlight any proper comparison can exist between the 
 different kinds of waves and the actual number of differ- 
 ently coloured feathers that were crammed in that great 
 barrel ? " 
 
 I am glad you asked this question. I can assure you I 
 have not forgotten what you have suggested, and when we 
 come to study light more carefully, I am sure you wiU 
 agree with me that, if there is any absurdity whatever in 
 the comparison, it lies in comparing a barrel filled with the 
 feathers with the sunlight itself. It will, of course, take 
 some little time for you to get an adequate idea of the 
 complexity of sunlight. I will merely tell you now that 
 in the part of the spectrum capable of affecting the eye, 
 there are no less than 2,000,000 different kinds of waves, 
 and that, besides these, there is a still greater number of 
 wave lengths that are unable to affect the eye. 
 
 I will not, however, keep you in suspense until we have 
 gone so far through The Wonder Book, but will simply say 
 that, if you can get one of the prisms from a glass chande- 
 lier, or from some other place, you will have in your 
 possession the magic wand of Prince Percinet by means of 
 which in that very short space of time that is called by 
 the Germans " ein Augenblick " or " a wink of the eye," 
 you can make the separation yourself. For, if you now 
 examine that patch of white light, through the prism, you 
 will be able to see its separate constituents as wonderful 
 
PEOPERTIES OP LIGHT 21 
 
 groups of coloured lights of almost innumerable tints of 
 reds, oranges, yellows, blues, greens, violets, and indigoes. 
 If, therefore, you were in the place of poor Graciosa and 
 heard the wicked Queen coming to have you killed because 
 your task had not been completed, it woidd not be too 
 late even if you heard her footsteps coming up the stairs. 
 It would not be too late even if you heard the key turning 
 in the door lock, for you need only permit the light to 
 pass through the prism and in less than the twinkling of 
 an eye, all the many feathers — ^that is, all the different 
 coloured light waves — will have been sorted out in separate 
 patches that will have collected on the floor before the 
 door opens. 
 
CHAPTBE III 
 WHEN LIGHT FALLS ON MATTER 
 
 "When light falls on matter, it is either thrown off from 
 its surface, or enters it. In either case, except where 
 the light strikes at right angles to its surface, a change 
 takes place in the direction in which the light has been 
 moving. 
 
 When light is thrown off directly from the surface of 
 matter it is said to be reflected. Light is thrown off or 
 reflected just like a ball bounds off or is reflected from a 
 wall. 
 
 The reflection of light may be regular or irregular. 
 When light is regularly reflected it is generally said to be 
 reflected. When irregularly reflected, it is said to be dif- 
 fused. The light that enters matter is either quenched as 
 light and changed into heat, or it passes completely through 
 the matter and comes out on some other side. 
 
 If a body permits the light to pass through it so readily 
 that objects can be seen through it, as, for example, a plate 
 of clear window glass, it is said to be transparent. If no 
 light passes through the body, the body is said to be 
 opaque, and the light that partially enters it is said to be 
 absorbed. Sometimes a part of the light passes through 
 the body and a part is absorbed. In such cases the light 
 that passes through is coloured, and the body is said to be 
 a coloured transparent body. Sometimes all the light which 
 
WHEN LIGHT FALLS OX MATTEE 33 
 
 falls on a body is thrown off from its surface so that the 
 body shines with the same white light as that of the sun. 
 Generally, however, only some of the coloured light is 
 thrown off, and the body is then said to be coloured. 
 
 All these peculiar properties of bodies as regards the 
 light that falls on matter will be explained in due course. 
 
 A single liae of light is called a ray. A number of par- 
 allel rays are called a beam, while a number of separate 
 rays, that either collect together at a single point, or pro- 
 
 A B 
 
 Eay of Light. Beam of Light. 
 
 Diverging Pencil of Light. Converging Pencil of Light 
 
 Fig. 1. — Some Common Foems of Light Rays. 
 
 ceed outward from a single point, form what is called a 
 pencil. 
 
 In Fig. 1, at A, is the usual conventionalised representa- 
 tion of a single ray of light ( the arrow drawn alongside the 
 line indicates the direction in which the light is moving) . 
 This is not a correct representation of a ray. That would 
 require a much more complex drawing. A beam of light 
 consists of a number of parallel rays, is represented at B. 
 
 A pencil of light consists of a number of rays, that are 
 either diverging as shown at C, or converging as at D. 
 
24 
 
 THE WONDEE BOOK OP LIGHT 
 
 As you can see, in a diverging pencil the rays are repre- 
 sented as originating or starting from a single point C, 
 and moving outwards away from one another in the di- 
 rection indicated by the arrow. In the converging pencil, 
 at D, however, the separate rays are represented as all 
 moving towards a single point D, at which they meet. 
 
 Let us now take up the subject of reflection. When a 
 single ray of light falls on a highly polished surface such 
 as metal, glass, or on the piece of looking glass with which 
 we were experimenting in the preceding chapter, the light 
 is thrown off from the surface something like a rubber 
 ball is flung off from the wall against which it is thrown. 
 
 Fig. 2. — Reflection of Light fbom Plane Suefaces. 
 
 If the light strikes the mirror at right angles to its surface, 
 as at Y Fig. 2, it is simply thrown off from the surface so 
 as to move directly back again in the same path in which 
 it was advancing. But if it is inclined to the surface, as 
 at Z, so as to strike it at any other angle, then the 
 light is flung off from the mirror at an equal and adjacent 
 angle to that at which it struck. An examination of this 
 figure will show that if XX represents the surface of the 
 mirror and B the point where the light strikes, then, if 
 we imagine a line DB to be drawn perpendicular to the 
 point B, the angle ABD, or the space included between the 
 
"WHEN LIGHT FALLS ON MATTEE 25 
 
 direction AB of the ray, and BD, the perpendicular at the 
 point B, where the ray strikes the mirror, will be of ex- 
 actly the same value as the angle DEC, at which the light 
 is flung off from the surface. The angle ABD is called 
 the angle of incidence, and the angle DBC is called the 
 angle of reflection. These two angles are always equal and 
 adjacent to each other. In other words, as you can see from 
 the figure, the ray AB, striking the mirror at any angle 
 whatever, is flung off from it at an angle that is exactly 
 equal to that at which it strikes. 
 
 The reflection of light is so important a matter that I 
 will ask you to make yourself thoroughly familiar with 
 what I have been talking about by an experiment with a 
 piece of looking glass. 
 
 Place a piece of looking glass on the surface of a table 
 in a darkened room, and let a ray of sunlight come in 
 through an opening in the shutter, so as to fall on its sur- 
 face and be reflected or thrown to the wall or ceiling. 
 
 In order to be able to see the path taken by the light, 
 you should permit some smoke to fill the air of the room. 
 Since in many of the experiments described in this Won- 
 der Book you will need smoke, I wiU. tell you how you can 
 easily make smoke paper. 
 
 Take any kind of paper, but preferably a paper con- 
 taining but little sizing material, such as a sheet of blot- 
 ting paper, and dip it in a solution of nitre, or saltpetre, 
 and kt it dry. Such paper, when dried, is very inflam- 
 mable. When touched with a lighted match, iastead of 
 burning as a bright flame, it rapidly glows with a red light 
 and produces much smoke. If you ignite a small piece of 
 this smoked paper, loosely wrapped in the shape of a paper 
 
»6 THE WONDER BOOK OF LIGHT 
 
 lighter, and hold it near the mirror, the space will be 
 filled with finely divided smoke, and a beam of light can 
 be seen both where it enters the room and where it is flung 
 off from the surface of the mirror as shown in Fig. 3. If 
 now yoli hold a plumb-line and bob C, as shown, so that 
 the point of the plummet points in the direction of the 
 
 centre of the place where 
 
 c the beam strikes the mirror, 
 
 but above this place, you 
 
 ^^^ V \/^ can see distinctly the path 
 
 ^^^\ i /^//// taken by the beam both be- 
 
 ^^^L^o/ fore it strikes the mirror 
 
 E ^iwf^ E and after it strikes it, and 
 
 Fig. 3.. — Eefleotion of Beam of you can see that the beam 
 ^'^^^' is inclined equally and op- 
 
 positely to the vertical marked by the plumb-line, so that 
 the angle BAG, the angle of incidence or striking, of one of 
 the rays, is equal to the angle CAD, the angle of reflection, 
 and adjoins it. 
 
 If, instead of employing the smooth and polished sur- 
 face EB of such a mirror, a rough surface, such as a plate 
 of ground glass BB, Fig. 4, is used, and a beam of light 
 AC be again permitted to fall on the glass at in the dark- 
 ened room, then, instead of the light being thrown off in a 
 single direction, it will be thrown off as a great num- 
 ber of separate rays in every direction as shown in the 
 figure. In other words, the beam will no longer be regu- 
 larly reflected, but will be irregularly reflected; that is, 
 will be diffused or scattered. In order to avoid confu- 
 sion, only one of the rays of the beam ; i. e., that which is 
 incident at C, is shown as being diffused. All the other 
 
WHEX LIGHT PALLS ON MATTER 
 
 27 
 
 rays are similarly diffused. You can call this either 
 diffused or scattered light, but please do not say that the 
 light will be dispersed as is some times done, since, as I 
 will afterwards show 
 you, this word is prop- 
 erly used to describe 
 an entirely different 
 phenomenon. 
 
 You will remember 
 that you can only see 
 an object when the light coming from it enters your eye 
 directly. If, therefore, in any of the preceding experi- 
 ments you have been able to see a ray, or a beam of light, 
 it was because that ray, while passing through the fine 
 particles of dust in its path, so lighted each of these par- 
 ticles that they diffused or scattered the light in all direc- 
 tions, as represented in Eig. 5. And it was by means of 
 the diverging pencils of light thrown in all directions by 
 
 -Diffusion or Scattering of 
 Beam of Light. 
 
 Fig. 5.- 
 
 -Path of a Eat seen by Diffusion fbom Dust 
 Pabticles. 
 
 the illumined dust particles that you saw the direction in 
 which the light was passing before it fell either on the 
 reflecting or diffusing surfaces and was afterwards flung 
 off from such surfaces. 
 
 If light coming from a small illumined object, such as 
 that represented in Eig. 6, be permitted to enter a dark 
 chamber or box through a small opening in one of its 
 
28 THE WONDEE BOOK OF LIGHT 
 
 sides, there will be produced, on the side of the box oppo- 
 site the opening, a small image of the object from which 
 the light has come. Instead, however, of this object being 
 seen in its erect or upright position, it will appear to be 
 upside-down or inverted. In this case, the size of the 
 image will be smaller than that of the object itself, because 
 the object is situated at a greater distance from the hole 
 in the box than is the wall of the box on which the image 
 is received. 
 
 I think you can understand why the object should be 
 inverted as well as its difference in size; for, as is shown 
 
 Fig. 6. — Image formed in Dask Chambee by Apebtuee. 
 
 in the figure, the diverging pencils of light, coming from 
 different parts of the object, passing through the opening 
 in the box, fall on the wall of the box where they form the 
 image. Since the rays from the top of the object form the 
 lower part of the image, and the rays from the bottom of 
 the object form the top of the image, the image will ap- 
 pear inverted as shown. If the box be brought nearer to 
 the object, the size of the image would be increased, and 
 if it be moved away from it the size of the image would 
 be decreased. 
 
 You will see later that the clearness of an image pro- 
 
WHEN LIGHT FALLS ON MATTEE 29 
 
 duced in this way will be greatly increased if instead of 
 employing a hole in the dark box, a lens is placed in this 
 hole, thus producing what is known as a camera obscura. 
 
 If you make such a box for yourself, which I woidd ad- 
 vise you to do, arrange matters so you can sit inside it 
 without getting in the way of the light as it passes through 
 the aperture at the end, and so you can see the picture 
 that is thus formed on the side of the box opposite the 
 opening, on a sheet of white paper placed so as to re- 
 ceive it. 
 
 An ordinary soap box wOl answer this purpose. In 
 order that you can see the picture that is formed, remove 
 one of the sides of the box and replace it by a long dark 
 cloth, so that while sitting at one side of the box, you 
 can throw the cloth over your head and shoulders and thus 
 keep the inside of the box dark. To succeed, however, 
 with this experiment, it is necessary that no light enter 
 the box except through the opening in its side. 
 
 The smaller the opening in the side of the box the 
 sharper will be the pictures of outside objects. A very 
 small auger hole, bored through the end of the box, would 
 be too large and rough for this purpose. The best 
 results will be obtained if you cover an auger hole with 
 a piece of smooth tin foil, through the centre of which 
 you have pierced a small hole with a large darning needle. 
 You can have no end of fun if you place such a box at 
 the open window of a room, for then you can see on the 
 side of the box opposite the screen an actual moving pic- 
 ture or panorama of everything that is taking place in the 
 street. 
 
 If a single luminous point be brought before a white 
 
3d THE WOMDER BOOK OV LIGHT 
 
 screen, the diverging pencil of light it gives off will uni- 
 formly illumine or light it. If, now, a small opaque globe 
 M, Pig. 7, be placed between the screen and the luminous 
 points, since the rays of light that fall on the globe are 
 unable to pass through it, they will be unable to fall on 
 the screen, so that the portions illumined will have the 
 outlines of the outside of the globe, or will form a round 
 or circular shadow. The cause of this shadow is evident. 
 It occupies those parts of the screen from which the light 
 has been cut off by the intervention of the opaque body. 
 
 Fig. 7.. — Shadow of Sphere PEODtrcED bt LuMiNorrs Point 
 Umbea only Pboduced. 
 
 If you examine this shadow carefully you will see that, 
 practically speaking, all portions of it are equally dark. 
 
 This, however, will not be the case, if, instead of em- 
 ploying a luminous point, a luminous surface is used, as, 
 for example, that shown in Pig. 8. Here A has a sensible 
 surface. Instead of the space back of the opaque body 
 being devoid of light over all parts of its surface, there 
 will be seen a central space, where no light whatever enters, 
 that is surrounded by concentric circular spaces growing 
 less and less dark as we pass from the centre towards the 
 outside: that is, the shadow increases in depth from the 
 outside to the centre where no light penetrates. 
 
 Probably many of you know what funny shadow pic- 
 
WHEX LIGHT FALLS ON MATTEE 31 
 
 tures can be produced by holding the fingers of two hands 
 in different positions in front of a candle flame. In order 
 to get the best results in shadow pictures it is necessary 
 to keep the light that is held near the hands in a fixed 
 position. 
 
 It is very easy to cut out pieces of paper in irregular 
 
 Shadow Produced by Luminous 
 Surface. 
 
 Cross Section of 
 
 Shadow Produced 
 
 by Luminous 
 
 Surface. 
 
 Fig. 8. — Shadow of Sphebe Produced ey Luminous Sueface 
 Umbba and Penumbba. 
 
 outlines with proper apertures left here and there, so as 
 to produce exceedingly curious results in the shapes of 
 shadow pictures on the wall. 
 
 It is possible for pieces of paper or leather to be .so cut 
 into different shapes as to produce what are known as Chi- 
 
32 THE WONDER BOOK OP LIGHT 
 
 nese Fantoccini shadows. By causing these shadows to 
 follow one another, in proper order, it is possible, espe- 
 cially when aided by conversation, and by suitable move- 
 ments of the shadows, to tell a story that will be readily 
 understood by the audience. The movements necessary 
 for this purpose are obtained by the movements of the 
 pieces of paper casting the shadows, by the movements of 
 the light, or lights, or by movements of the joints in the 
 pieces of paper. 
 
 Interesting forms of shadow pictures are sometimes ob- 
 tained by means of a suitably turned wooden handle. In 
 Fig. 9 I have shown a piece of wood that, when not in use, 
 was concealed in the top of a cane employed during 1817 
 for spreading seditious ideas in the city of Paris. I will, 
 however, permit M. Marion, from whose book I have 
 taken the figure, to tell the story of one of these : 
 
 "In 1817, one winter's night we were all sitting round 
 the table listening to my father, who was reading aloud 
 an interesting book of the period, when a friend of our 
 family, who had been formerly an officer of the Empire, 
 entered the room. He was a serious, upright, soldierly 
 man, and wore his coat buttoned up to his chin. He had 
 hardly replied to our salutations, when' he drew a chair 
 up to the table, and made a sign with his hands and eyes 
 that plainly indicated silence and discretion. There was 
 something in the expression of his countenance that seemed 
 to show that he had something mysterious in store for us, 
 and we fully expected to hear some extraordinary news, 
 or to see him bring out a Bonapartist pamphlet of more 
 than usual importance. Our surprise was consequently 
 great when we saw him slowly unscrew the top of his cane. 
 
WHEN LIGHT PALLS ON MATTEE 
 
 33 
 
 which was turned out of boxwood, and presented nothing 
 very remarkable either in form or material. He, however, 
 took up a copybook which was lying on the table, placed it 
 at a certain distance from the lamp, and then laid upon it 
 
 ■Pbofiles of Faces Pboduced by Shadows. 
 
 the little piece of turned boxwood. At first we noticed 
 nothing at all extraordinary, and he smiled at our want 
 of intelligence, until at last my younger brother cried out 
 suddenly, ' Look ! there's the head of Napoleon ! ' and truly 
 
34 THE WONDER BOOK OF LIGHT 
 
 enough, we found, on looking more attentively at the 
 shadows of the turned knob of the cane, that their profile 
 was that of the great exile, most correctly and clearly por- 
 trayed. The old Captain's face lighted up at the sight, 
 and the tears came into his eyes. * We shall see him again,' 
 he murmured in a low voice, and he hummed the burden 
 of a Bonapartist song then in vogue. During the rest of 
 the evening he was very lively, and proved to us most con- 
 clusively that before six months the "' Grande Armee ' would 
 be revenged for their defeat at Waterloo. Some weeks 
 after, there was hardly anyone in the town who did not 
 possess a stick or a tobacco-pipe stopper turned in this 
 fashion, but one day a panic seized everybody, and the 
 canes and pipe stoppers were all burnt." 
 
CHAPTEE IV 
 LOOKING GLASSES 
 
 In that diarming book by Lewis Carroll, " Through the 
 Looking Glass," there are told the odd adventures of a 
 little girl named Alice, who had often seen, back of the 
 looking glass in the drawing-room at her home, another 
 room that looked exactly like the one she was in, except 
 that in the looking-glass room, as Alice said, "things go 
 the other way." Even the books in the looking-glass room 
 had this peculiar appearance, since the letters of the words 
 all went the wrong way. 
 
 One day Alice fell asleep, and dreamed that, climbing 
 up on the mantelpiece, she had been able to pass through 
 the looking glass and thus entered the room in which she 
 had so often wished to be. The first thing she did was to 
 examine the little bit of the room, which, being situated 
 immediately under the mantelpiece, she had never been 
 able to see from the other side. Suppose you and I, though 
 much older than Alice, take a peep through a real looking 
 glass and see whether we cannot find out what actually 
 takes place in mirrors that make the images formed in 
 them "go the other way," and especially to see if we can 
 solve the problem of why it is that looking-glass writing 
 has so queer an appearance. 
 
 In the first place, let me see if I can explain to you how 
 a piece of looking glass actually produces a change in the 
 
 35 
 
36 
 
 THE WONDER BOOK OP LIGHT 
 
 direction of the light it throws ofE from its surface. We 
 will begin with the simple case of a beam of light. Sup- 
 pose, as at X in Pig. 10, that a beam of light of which 1 
 and 2 form respectively the lowermost and the uppermost 
 rays, be thrown off or reflected from the surface of a mir- 
 ror MN. Then^ as you can see from carefully examining 
 the figure, owing to the fact that the beam is inclined, after 
 reflection the lowermost ray 1 becomes the uppermost ray 
 of the reflected beam, while the uppermost ray 2 becomes 
 its lowermost ray. Parallel rays of light, reflected from a 
 
 Tig. 10. — Why Looking Glasses make Images " Go the Otheb 
 
 Wat." 
 
 looking glass, therefore, continue parallel after reflection, 
 but are turned from left to right or from right to left ; or, 
 as Alice expressed it, " they go the wrong way." 
 
 The same is true of a pencil of light diverging from the 
 point A, as shown at Y in the same figure. Here, only two 
 of the diverging rays are shown ; i.e., 3 and 4. If this pen- 
 cil of light be permitted to fall on the surface of a mirror 
 MN, it will be reflected and will continue to diverge after 
 it has been thrown from the surface, the rays taking 
 the direction they would have if, instead of coming from a 
 luminous point A, they came from the luminous point G, 
 
LOOKING GLASSES 37 
 
 situated the same distance back of the mirror that A is in 
 front of it. 
 
 If you examine this part of the preceding figure care- 
 fully, you will see that here, as in the other case, the rays 
 of light after their reflection from the mirror still keep on 
 diverging, and that 3, the lower ray of the pencil, before 
 reflection becomes the upper ray of the reflected pencil, 
 and that 4, the upper ray, becomes the lower ray after re- 
 flection. 
 
 Fow it is a matter of experience that the position in 
 which an object is seen depends on the direction in which 
 the rays of light, by means of which it is seen, enter the 
 eyes. 'No matter why this is, it is enough to know that it 
 is so; that is, that you have found it to be so. For ex- 
 ample, in the diverging pencil of light, when the rays, 
 after reflection, have entered the eyes of an observer, that 
 is, your eyes or mine placed in front of the mirror, we 
 will not see the luminous point A by the use of the re- 
 flected rays, but the image of this point will be situated 
 at G, as far back of the mirror as A is in front of it. It 
 is true that it is possible to see A and G at the same time, 
 but when you see A, it is by means of the rays of light that 
 have come directly from it without having been reflected 
 from the mirror, and when you see the image G, it is by 
 means of the rays that have first fallen against the mirror 
 and then enter your eyes after reflection from its surface. 
 
 The same thing is true of the parallel rays, but it will 
 be best to confine our attention now to the case of a diverg- 
 ing pencil of light. 
 
 We have taken above the case of a single luminous 
 point. The same thing would be true if we take an illu- 
 
38 THE WONDEE BOOK OF LIGHT 
 
 mined object, such, for example, as the arrow liown in 
 Fig. 11. Here, from all points of the arrow, small di- 
 verging pencils of light fall on the mirror and enter your 
 eye. After reflection the diverging pencil from the point 
 A, of the arrow, entering the eye as shown, appears to you 
 to be situated at a, while the pencil from B, the 
 
 feathered end of the arrow, 
 
 ^ffl^s:?. after reflection, enters your 
 
 n y^^w ®^'®' producing the image 6. 
 
 Aj^^lS^ y^^'^/ As you see, the image ah 
 
 ] ^*L ^s^^y^v^ appears to be situated back 
 
 m \^^^^^^^ ' ' H of the mirror at exactly the 
 
 i „'-'•'"! ,,'-'''^ same distance that the ob- 
 
 *'*--~^-''' ject AB is in front of it. 
 
 Fig. ll.-A Looking Glass ^°^ ^^ ^^^^^^^ ^^^^ ^^^S^' 
 Image. it is evident: 
 
 (1). That it occupies an entirely different position from 
 that of the object; for, it is back of the mirror. 
 
 (2). There is something odd about its appearance for, 
 being situated back of the mirror, it seems to go "the 
 wrong way " ; for, now, all the rays from the point of the 
 object which before reflection were uppermost, as shown at 
 1, are now lowermost, while the lowermost ray 3 is upper- 
 most after reflection. Therefore, the image is caused to 
 assume the curious appearance that images always do in 
 looking glasses, and this is the reason why everything 
 looked so odd to Alice in the looking glass room. 
 
 Can you read the writing on the slip of paper repre- 
 sented in Fig 12 ? N"o, it is not a foreign language ; it is 
 simply English written in looking-glass style. I wrote a 
 certain phrase on a slip of paper in ink, and then, while 
 
LOOKING GLASSES 39 
 
 tlie ink was still wet, I laid a piece of blotting paper on it, 
 and in this manner took off an impression of the wet ink 
 on the blotting paper. It is this impression you are look- 
 ing at in the above figure. It is true neither the words nor 
 the letters present the same appearance they ordinarily 
 do, but that is because they have been made to go the 
 " wrong way," just as they would have gone had you taken 
 them off, not on a sheet of paper, but on the surface of a 
 looking glass by holding it in front of the writing. 
 
 If you have difficulty in deciphering the writing in Fig. 
 13, hold the paper containing it in front of a looking glass. 
 
 Fig. 12. — Looking Glass Weiting. 
 
 and see how easy it is to read from the image in the glass, 
 the words " The Wonder Book of Light." The looking 
 glass completely undoes what the method of transferring 
 the image to the paper did. 
 
 If you think carefully over what I have just said, re- 
 membering all the time that the sheet of blotting paper 
 has produced the same effect on the appearance of the 
 writing that is produced by a looking glass, the difiBculties 
 of understanding the way ia which a looking glass acts on 
 images will, I think, disappear. Such a matter would be 
 much less difiBcult for a printer to understand, since, as 
 he knows very well, it is necessary to set type for printing 
 in this same backward, looking-glass style, so that, when 
 the surfaces of the type are inked, and an impression is 
 taken off from them, the characters will appear in the 
 
40 THE WONDEE BOOK OF LIGHT 
 
 everyday manner with which we are so familiar. The 
 printer, however, accustomed as he is to read this kind of 
 writing, has ho difiBculty with it. 
 
 A looking glass produces no change in the size of the 
 image over that of the object. This you can convince your- 
 self of by standing. in front of the glass, say in your bureau 
 or wardrobe, and looking at your own image. As you can 
 see, its size is exactly the size of your body. 
 
 Instead of looking at your own image in the glass, 
 look at the images of the fixed objects in the room, say 
 the image of a chair or a bedstead. You can see these im- 
 ages from different parts of the room, but no matter how 
 you may move about from place to place, their position 
 remains fixed. They wUl always be seen in the mirror at 
 a distance back of it exactly the same as their distance in 
 front of it. 
 
 Now again look at your own image in the mirror and 
 notice what happens as you walk towards or from the mir- 
 ror. Your image does the same thing. It walks further 
 away from the mirror, or nearer to it, as you do. 
 
 Of course all these images are inverted in looking-glass 
 fashion. I will only say one thing more about this pecu- 
 liarity and then I am through with it. The difficulty is 
 that we have become so accustomed to looking-glass images, 
 that, in most cases, we fail to note any peculiarity about 
 them unless we hold up a printed page and then we cannot 
 fail to see the curious change in the appearance of the 
 letters. As far as our own image is concerned, you have 
 seen it so often in a looking glass that it seems all right, but 
 if you will think a moment, you will see that the image of 
 your right eye appears directly in front of this eye, and the 
 
LOOKING GLASSES 41 
 
 same is true of other parts of the body, a thing that could 
 not possibly be true were not the images of these parts 
 inverted in looking-glass style. 
 
 Of course you know, when a number of people are in a 
 room, their images can all be distiuctly seen by every 
 person in the room, though, of course, by entirely different 
 rays of light ; for, it is only the rays of light that enter the 
 observer's eye that permit the objects to be seen, and these 
 rays of light are completely used up ia producing impres- 
 sions on the brains connected with the eyes through which 
 they have passed. 
 
 In " The Wonder Book of the Atmosphere," I have 
 briefly explained the cause of echoes. An echo is a repeti- 
 tion of a sound produced by reflection from an object at a 
 sufficient distance to permit the direct sound to die away 
 before the reflected rays entering the observer's ear again 
 produce the sound. Echoes closely resemble the sounds 
 that produce them. They are, however, fainter than such 
 soimds. 
 
 Now an echo can be regarded as the image of a sound. 
 The reflecting wall or surface, sometimes a huge perpen- 
 dicular cliflf, acts as a kind of acoustic looking glass, that, 
 by flinging the soxmd waves back to the observer, enables 
 his ear to see the image of the sound as far back of the cliff 
 as the true sound was in front of it. 
 
 In a similar manner, a looking-glass image may be re- 
 garded as a kind of luminous echo, closely resembling the 
 illumined object, but being much less bright. 
 
 The cause of the decreased loudness of an echo and the 
 decreased brightness of an image are practically the same; 
 i.e., the loss of light or sound by reflection. 
 
43 
 
 THE WONDEK BOOK OF LIGHT 
 
 While, as we have seen, light always travels in straight 
 lines, so that visible objects are cut ofE from view by the 
 interposition of a book or other opaque object between the 
 eye and the object, yet since it is easy to change the direc- 
 tion of light by reflection from mirrors, it is not difiScult in 
 this manner to be able to look not only around corners, but 
 even underneath an object. In this way objects can be 
 easily seen that would otherwise be invisible. For exam- 
 
 FiG. 13. — ^Appaeatus foe Looking Through a Millstone. 
 
 pie, in Fig. 13, there is represented a divided form of tele- 
 scope. 
 
 Now no one would be surprised if, looking through 
 the proper end of the telescope S, an object is seen in the 
 direction G in which the telescope is pointed, for this is 
 what a telescope is made for. Of course you can see 
 through it if you look in the proper direction. 
 
 I am sure, however, you would be considerably surprised 
 if, when some unquestionably opaque object, such as a 
 brickbat, be placed in the space between the two halves of 
 the telescope, you could still see the distant object quite as 
 
LOOKING GLASSES 43 
 
 distinctly as before; for here you are apparently looking 
 directly through a brick. 
 
 I have heard people say when asked to look at something 
 which it was clearly impossible to see, " Do you think I 
 can see through a millstone ? " WeU, I won't answer that 
 question directly. I will simply say that if a millstone in- 
 stead of a brickbat were placed between the two halves of 
 the telescope, or, indeed, any opaque object were so placed, 
 no effect whatever would be produced on your ability to 
 see a distant object at which the glass has been directly 
 pointed. 
 
 Of course the explanation is simple. You are not look- 
 ing through a brick, or through a millstone, or any other 
 opaque object. You are simply peeping down the side of 
 the brickbat nearest you, underneath it and up the further 
 side, so that there is nothing odd about it unless it is in 
 the ingenious manner in which you have been able to cause 
 your eye thus to dodge the obstacle. 
 
 Let me explain how very simple this contrivance really 
 is. All that is needed are four pieces of mirror or looking 
 glass placed as shown in Fig. 14. One of these. A, is placed 
 as shown on the side farthest from the eye. A ray of 
 light coming from the object at which the telescope is 
 pointing, falling on the mirror A, is reflected from it. 
 After having passed downwards to a point below the brick- 
 bat or millstone M, it falls on the surface of the mirror B, 
 and is reflected in a horizontal direction to the surface of 
 the mirror C, from which it is thrown upwards only to be 
 received by the fourth mirror D, which enables it to pass 
 through the other half of the telescope and so enter the eye 
 of the observer. 
 
44 
 
 THE WONDEE BOOK OP LIGHT 
 
 It is astonishing how many things that at first sight ap- 
 pear to be wonders or marvels are explained by the ease 
 with which a mirror may change the direction in which 
 light passes. Suppose, for example, people inside of a 
 besieged garrison wished to obtain a better view of the 
 movements of the enemy outside than can be obtained by 
 looking through a loophole. Of course, they could go on 
 top of the ramparts where they could see everything, but 
 since there would probably not be enough left of the ob- 
 server to take any valuable information to those inside if 
 the enemy's gunners were good marksmen, such an expo- 
 
 V 
 
 A 
 
 i 
 
 D/ 
 
 — > 
 
 \ 
 
 / 
 
 > 
 
 i 
 
 
 • 
 
 
 T 
 
 \ 
 
 > 
 
 / 
 
 Fig. 14. — Abeangement op Mieeors in Fig. 13. 
 
 sure would be foolhardy in the extreme. It would be silly, 
 because a looking glass couFd do this work quite as well 
 as, if not better than, a man. It would only be necessary 
 to support a mirror suitably above the ramparts and per- 
 mit the light reflected from it to fall on the face of a sec- 
 ond mirror placed below it inside them. Thus a man in- 
 side could see whatever is going on on the outside. 
 
 At the theatre opera glasses are levelled at the stage, 
 without arousing surprise, for the players expect to be 
 looked at. Indeed, people in the audience do not greatly 
 object to this. But excitement and disagreeable develop- 
 ments might result if one should persist for a half hour or 
 
LOOKING GLASSES 45 
 
 so in deliberately levelling his glasses at one particular 
 box. By simply placing a mirror in a pair of opera glasses, 
 matters can readily be arranged so that they, when appar- 
 ently pointed at the stage, are in reality pointed at some 
 particular part of the house that the persons who are using 
 the glasses desire especially to observe. 
 
 In a similar manner a person using coloured glasses to 
 protect the eyes from the sun can easily so adjust the 
 glasses as to be able to see what is going on in back of them, 
 so that the very common expression, " Do you think I have 
 eyes in the back of my head ? " might be answered : " No, 
 but I strongly suspect you have what amounts to practically 
 the same thing, mirrors in your eye glasses that enable you 
 to see what is going on back of you as well as in front." 
 
 There is a curious variety of plane mirror known as the 
 Japanese magic mirror. While ornamented on the back 
 with various Japanese characters, hieroglyphics, flowers, 
 birds, or other things, its reflecting surface consists appar- 
 ently of an entirely smooth mirror-like surface. It seems 
 to be continuous, even, indeed, when examined through a 
 powerful magnifying glass. 
 
 This even surface of the mirror, however, is only ap- 
 parent, for, if the mirror is held in a darkened room so 
 that a diverging pencil of light from an electric lantern 
 can fall on it, and is thence reflected to a distant screen, 
 instead of an evenly illumined disc that would have been 
 produced were its surface absolutely uniform, there will 
 be seen over its surface the same Japanese characters, hier- 
 oglyphics, etc., that ornament its back. 
 
 I understand that the cause of this curious phenomenon 
 is not entirely understood. In some manner the designs 
 
46 THE WONDEE BOOK OF LIGHT 
 
 placed on the back of the mirror have been permitted to 
 extend through from the back to the front without, how- 
 ever, leaving any traces of their existence. While the front 
 of the mirror appears to the eye to be absolutely uniform, 
 yet this uniformity is unreal, for when the light falls on it, 
 the structure is revealed in the image that appears on the 
 screen. 
 
 Sir David Brewster suggested that the explanation may 
 possibly be as follows : That the design has been carefully 
 engraved on the surface of the mirror, and this surface 
 then carefully ground and polished in such a way as to 
 leave the engraving invisible, but still to leave sufficient 
 traces of it to permit them to act on the light emitted 
 from its surface. 
 
 A more probable explanation is that the images are 
 caused by molecular changes in the surface due to ham- 
 mering or to a peculiar method of casting. 
 
 Another explanation suggested by Mr. Ives, of Phila- 
 delphia, as the result of an investigation of somewhat 
 similar figures produced by the reflection of light from a 
 brass button, attributes the cause of the images to slight 
 irregularities in the surface of the mirror; for he found 
 that, if a button that gave a good image was ground and 
 polished, all its magical images instantly disappeared, 
 being replaced by a plain bright disc. Mr. Ives expresses 
 the belief that the curious properties of the Japanese 
 mirror were in the first instance produced accidentally, 
 so that " our former estimate of the Japanese metal- 
 workers' ingenuity may be taken cum grano salis." 
 
 When instead of a single mirror two or more mirrors 
 are employed, there will be produced more than a single 
 
LOOKING GLASSES 47 
 
 image of any object before them. Suppose, for example, 
 two mirrors are placed at right angles to each other. 
 Then three images of any object, such as a lighted candle 
 and candlestick, placed before such a mirror will be pro- 
 duced. Moreover, the number of images seen in two plane 
 mirrors will vary with the inclination between them. If, 
 for example, the mirrors are inclined to each other at an 
 angle of sixty degrees, five images will be produced. If 
 inclined to each other at an angle of forty-five degrees, 
 seven images ivill be produced. 
 
 When the mirrors are parallel to each other, a great 
 number of images are produced. Indeed, it can be shown, 
 theoretically, that if the mirrors were absolutely parallel 
 to each other, and no light were lost by reflection, the 
 number of images that ought to be seen would be infinitely 
 great. In point of fact, however, so much light is lost 
 by reflection that after a certain number of images -have 
 been produced no more are seen owing to their feeble il- 
 lumination. Then again, it is a very difficult matter to 
 place mirrors so that they are exactly parallel to each 
 other. The slightest deviation 'from parallelism will re- 
 sult in the images being formed a trifle out of the perpen- 
 dicular, so that at last the curvature becomes too great to 
 permit the images to be seen. In such cases, the images 
 disappear long before they would on account of lack of 
 illumination. 
 
 You have probably seen the effects produced by reflec- 
 tion from parallel mirrors in the case of mirrors placed 
 on opposite sides of a room, or, especially, on the opposite 
 side of an entry or hall. Here, by looking into a mirror 
 on either side, you can see a great number of rooms open- 
 
48 THE WONDEE BOOK OE LIGHT 
 
 ing out from one other, in the dim distance, until they 
 become too faint to be seen any further. 
 
 Some idea of the numerous images formed by parallel 
 mirrors can be had from an inspection of Fig. 15. Here 
 an object is placed between two mirrors, the reflecting 
 surfaces of which face each other as shown. The first re- 
 
 Fig. 15. — Multiple Images of Parallel Miekoes. 
 
 flections form the images a^ o^. The second reflections 
 form the images a^ o^. These are the images of the 
 images. The third reflections form the images a^ O3. 
 No attempt has been made in this figure to show the 
 paths of the reflected rays except for the third image. 
 
 In " The Wonder Book of the Atmosphere," in speaking 
 of echoes, attention was called to what were called multiple 
 echoes. Here, the source of sound was situatecL h^tween 
 
LOOKING GLASSES 49 
 
 the two parallel reflecting surfaces, so that when the sound 
 waves were thrown alternately between the two surfaces, 
 they produced multiple echoes. As you now see, multiple 
 images are formed in the same way. 
 
 A beautiful piece of optical apparatus called the kaleido- 
 scope depends for its operation on the images produced 
 by two mirrors that are inclined to each other. The 
 kaleidoscope was invented by Sir David Brewster. He 
 called the instrument a kaleidoscope. The word is formed 
 of three Greek words meaning " to see beautiful forms." 
 
 Li the kaleidoscope, two mirrors are inclined, with their 
 reflecting surfaces facing each other, at an angle of ex- 
 actly 60°, and the objects whose images are to be reflected 
 are placed in the space between the two plates. These 
 objects consist for the greater part of coloured glasses or 
 other conspicuous small objects. 
 
 The kaleidoscope may be constructed as follows: Two 
 glass mirrors, in the shape of strips, are placed inside 
 a pasteboard tube of the same length as the glass, with 
 their reflecting surfaces facing and inclined to each other 
 at an angle of exactly 60°. One end of the tube, the end 
 at which the eye is to be placed, is closed by means of a 
 metallic plate provided at its centre with a small circular 
 opening for the eye. The other end of the tube is provided 
 with two plates. That on the outside is formed of ground 
 glass, and the other of clear glass. Small pieces of glass, 
 coloured beads, rods, or cylinders, are placed in between 
 the two glasses. If now the tube is pointed towards a 
 source of light, the coloured objects will be illumiaed, and 
 if the eye is placed at the other end of the tube, the images 
 will be s.^n arranged in symmetrical groupings. When 
 
60 THE WONDEE BOOK OP LIGHT 
 
 the tube is slowly turned, so as to produce different group- 
 ings of the objects, a succession of symmetrically grouped 
 images of wonderful and beautiful patterns will be seen. 
 
 In order to insure the symmetrical arrangement of the 
 images, the angle of inclination between the mirrors must 
 be such a number of degrees as will exactly divide an entire 
 circle, 360°, such, for an example, as 60°, 45°, etc. 60°^ 
 however, is almost always employed for kaleidoscopes. 
 
 The more usual form of kaleidoscope is made with three 
 glass plates extending the length of the tube instead of 
 the two above referred to. Here each pair of plates acts 
 as a kaleidoscope, so that the combined effect is the same 
 as if three separate kaleidoscopes were employed. 
 
 The kaleidoscope is sometimes employed by designers 
 in order to obtain satisfactory combinations of colors, 
 symmetrically grouped, as designs for wall papers, floor 
 coverings, and other similar objects. 
 
CHAPTEE V 
 
 BURNING GLASSES AND SHAVING GLASSES 
 
 Besides mirrors that are straight or flat, like ordinary 
 looMng glasses, there are curved mirrors of various outlines, 
 such as portions of spheres, egg-shaped bodies, cylinders, 
 cones, etc. In fact any curved body, whatever its shape. 
 
 2phe 
 
 Cy2inde.r 
 
 Cone 
 Fig. 16. — Diffebently Shaped Bodies that can be Employed 
 
 AS MlBBOBS. 
 
 if made of a suitable reflecting material, is capable of act- 
 ing as a curved mirror. 
 
 In curved mirrors reflection may take place from either 
 the outside or the inside surfaces. Mirrors can, therefore, 
 be divided into two classes ; i. e., convex mirrors, in which 
 the reflection is from the outer curved surfaces, and con- 
 cave mirrors, in which the reflection is from the inner 
 curved surfaces. 
 
 SI 
 
B3 
 
 THE WOFDBE BOOK OP LIGHT 
 
 But before going any further, let me see if I cannot 
 make clear to you just what the words concave and convex 
 mean. When a solid is arched or vaulted, its upper sur- 
 face is said to be convex. In other words, when an object 
 has a convex shape it bulges outward. For example, in 
 Fig. 16, you will find represented a sphere, a cylinder, a 
 cone and an egg. 
 
 Limiting our attention first to the sphere and the egg, 
 you can see that the outer surfaces of both of these bodies 
 are arched, vaulted or bulged outwards, that is, their 
 
 Fig. 17. — Sections of Concave and Convex Mirrobs. 
 
 outer surfaces are convex surfaces. So, too, is the outer 
 surface of the rounded portion of the cylinder or stove- 
 pipe. 
 
 A concave surface, on the contrary, has a curved or 
 rounded inner surface, such as the inside of a hollow 
 sphere, or of the shell of an egg cut in two and emptied of 
 its contents, or as is the inner surface of a cylinder or 
 stovepipe. 
 
 Suppose, now, the outside of the sphere, cylinder, cone or 
 egg, represented in Fig. 16, be covered uniformly with some 
 smooth and highly polished reflecting material, say, silver. 
 Then such surfaces would act as convex mirrors. And, in 
 
BUEFIISTG- AND SHAVING GLASSES 53 
 
 the same way, if these bodies were so cut in pieces as to 
 be able to get at the inside, and the inside surfaces were 
 coated with a smooth, reflectiag material they would form 
 concave mirrors. 
 
 In actual practice, instead of using one half ACB, of 
 either the inside or outside surface of the sphere or egg- 
 
 Fro. 18. — ^Position of Image foemed by Plane Mibboes. 
 
 shaped body shown in sections in Fig. 17, only a small por- 
 tion, or segment, of this surface is employed, as DOE. 
 
 Such segments or pieces, when covered with reflecting 
 material on the outside, would form convex spherical 
 mirrors, and with this material on the inside, would form 
 concave spherical mirrors. 
 
 Curved mirrors, whether convex or concave, are capable 
 of forming images that differ from the images formed by 
 plane mirrors, in that although they are sometimes the 
 same size as the object, yet are generally of a different 
 size; i. e., either larger or smaller. Moreover, unless care 
 be taken in the construction of the mirror, especially in 
 seeing that its surface has been formed by employing a 
 
54 THE WONDEE BOOK OP LIGHT 
 
 small portion only of the sphere or the egg-shaped body, 
 the images do not, as is always the case with plane mirrors, 
 closely resemble the shapes of the objects, but present 
 curious contortions, producing images of very odd or 
 
 ^- M 
 
 M 
 
 ■ 
 
 
 Hr 
 
 ' ^ hI 
 
 
 S^ i'' ^^& 
 
 1 
 
 
 
 p^ 
 
 Fig. 19. — ^An Image fokmed by a Concave Mibeob. 
 
 grotesque forms that in the case of the images of people 
 are often extremely ludicrous. 
 
 There is another respect in which the images produced 
 by curved mirrors differ markedly from those of plane 
 mirrors. In plane mirrors the images are always formed 
 at a distance back of the mirror exactly equal to the 
 distance the object is in front of it, as for example, in Fig. 
 18, which represents the image of a candle placed in front 
 of a plane mirror. As will be seen, the rays of light enter 
 the eye of an observer in front of the mirror, as though 
 
BUENING AND SHAVING GLASSES 55 
 
 they came from another candle placed back of the mirror 
 at the same distance the real candle is in front of it. You 
 will notice here that the image is erect, although, as I have 
 shown you, it is inverted from one side to another. It is 
 
 Fig. 20. — ^Anothee Image of Concave IIibboes. 
 
 also much fainter than the object, owing to the light lost 
 by reflection. 
 
 Xow, in the case of a curved mirror, the image may faU 
 either back of the mirror, as represented in Fig. 19, or 
 it may be formed on the same side of the mirror as the 
 object is situated, as shown iu Fig. 20. 
 
 You wUl observe, however, by carefully examining the 
 above figures, that in the first, where a lighted candle is 
 placed in front of a concave mirror, while the image ap- 
 pears both in its upright or true position on the other side 
 of the mirror as it would if it had been a plane mirror, 
 that the image is not the same size as the object, but is 
 
56 THE WOISTDBE BOOK OP LIGHT 
 
 larger, and that, moreover, it seems to be situated at a 
 shorter distance back of the mirror than the object is in 
 front of it. 
 
 In the case of the second figure, where the lighted candle 
 is placed in front of a concave mirror, its image, as re- 
 ceived on the screen shown at the right-hand side, is 
 magnified and is seen upside down, or in an inverted 
 position. 
 
 Before going any further, I wish to explain the meaning 
 of the word focus, as it is used in optics in connection 
 
 Fig. 21. — The Foci of Concave Mibboes. 
 
 with mirrors and lenses. If a beam of light, by which 
 you will remember is meant a collection of parallel rays, 
 such, for example, as exist in sunlight, be permitted to 
 fall on the surface of a concave mirror, as shown in Fig. 
 21 at A, then after reflection the rays will collect at a 
 single point or focus F, called the principal focus, as being 
 the focus of the principal source of light; i. e., sunlight. 
 
 If the luminous point be placed, as at B, at any other 
 point C, at a greater distance than the centre of the sphere 
 of which the mirror is a part, that is the centre of curva- 
 ture, then its focus will be found at C, at a distance from 
 the mirror less than the centre of curvation. These points 
 are interchangeable. If C is made the luminous point 
 
BURNING AND SHAVING GLASSES 57 
 
 then C becomes the focus. Such foci are therefore called 
 conjugate foci. 
 
 If, however, the luminous point is placed as at C, at 
 S, much nearer to the mirror, then the rays after reflec- 
 tion, will not come to any point in front of the mirror, 
 but will seem to come from a poiat V, back of the mirror. 
 This point is called a virtual focus, because it is not real, 
 but is only apparent. 
 
 I shall not attempt to go any further into the difficult 
 subject of the foci of mirrors. It is sufficient to know that 
 all mirrors, whatever 
 their shape may be, 
 possess foci that are 
 either real or virtual. 
 
 A very common use 
 of a concave mirror is 
 for a purpose my boy 
 readers will appreci- Fig. 22. — Image fobmed bt Convex 
 ate more than the Mibbobs. 
 
 girls; that is, they are used for shaving glasses. To use 
 such a glass, it is only necessary to bring the face near its 
 silvered concave surface, so as to produce an erect mag- 
 nified image of the face and whatever, perchance, may be 
 on it. Of course, I can imagine I hear you laughing and 
 saying that it will be some time before you wUl have use 
 for such a glass, which, of course, may or may not be true. 
 
 Convex mirrors generally produce images such as shown 
 in Fig. 23. Here, you can see the manner in which the 
 image is formed by the rays of light as they are reflected 
 or thrown off from its surface. The image is erect and 
 smaller than the object. 
 #- 
 
58 THE WONDEE BOOK OF LIGHT 
 
 If you are unable to obtain a well-formed concave or 
 convex mirror you can get fairly good results from the 
 outside or the inside of any highly polished silver dessert 
 or table spoon; for, if you employ the hollowed bowl of 
 the spoon, you will find it acts capitally as a concave mir- 
 ror, and will produce a fairly clear magnified image of 
 your face. Since, as you know, the bowl is not of the 
 same curvature in all parts, you must expect the image to 
 be twisted or contorted. This, however, will only result in 
 giving the image of your face a ludicrous appearance. 
 In a similar manner, you can obtain an image of your face 
 by holding the outside of the bowl of the spoon in front 
 of you. A soap bubble can also be used as a convex mirror. 
 Here, so large a portion of the surface is employed that 
 the images will be greatly distorted. 
 
 Brightly polished ice pitchers, soup ladles, soup tureens, 
 butter dishes, and other pieces of silverware, often give 
 grotesque and amusing images of the face or other parts 
 of the body seen in them. 
 
 Get a silvered glass ball like the ones that grow on 
 Christmas trees, and show your friends how comical they 
 look when seen by the light reflected from its convex sur- 
 face. As I told you, in order to obtain correct outlines in 
 spherical mirrors, it is necessary to employ only a small 
 portion of the sphere. 
 
 The focus or place where the sun's rays collect in front 
 of a concave mirror is a point of considerable heat, es- 
 pecially if the mirror is a large one. 
 
 Pig. 33 represents a huge concave mirror, employed as 
 a burning mirror, supported in the required position on 
 a platform mounted on wheels. When such a mirror is 
 
BUEXING AXD SHAVING GLASSES 
 
 59 
 
 turned towards the sun, on a day when the sky is clear 
 or free from clouds, the rays after reflection, when col- 
 lected at the focus of the mirror, will immediately set 
 fire to a pile of wood or other eombustihles, provided, of 
 course, the mirror has been placed so as to make its focus 
 fall directly on the pile. 
 
 BufEon, the French naturalist, constructed a huge eon- 
 cave mirror that was capable of producing some very re- 
 markable results when used as a burning glass. Owing to 
 
 Fig. 23. — BtiBNiNG Mibboe. 
 
 the large size of the mirror, the amount of heat collected 
 at the focus was so great that it would almost instantly 
 raisfe refractory substances to brilliant incandescence ; wMle 
 heavy logs of wood, which, as you know, are very difficult 
 to start burning, almost instantly burst into flame when 
 brought into the focus. 
 
 Archimedes, the Grecian philosopher, employed a burn- 
 ing mirror for setting on fire the ships of the Roman 
 
60 THE WONDBE BOOK OF LIGHT 
 
 fleet, when, having besieged Syracuse, they approached 
 within bowshot of the fortifications. 
 
 Some very curious effects are obtained by the use of 
 cylindrical mirrors. When the convex surface of the cylin- 
 der is employed, the image of a person can be seen in cer- 
 tain positions in correct dimensions in one direction, 
 but very much contorted in the opposite direction. The 
 result is that a curious change in appearance is produced. 
 
 Fig. 24. — CuBiorrs Effect of Cylindbical Mieeob. 
 
 In one direction, a person appears to be lengthened out in 
 a remarkable manner. In another direction, he appears 
 to be greatly increased in girth and decreased in height. 
 If you ever have an opportunity of obtaining a mirror of 
 this character, you can show your friends how odd they 
 will look in after years when they have taken on weight 
 and become round, and fat, and jolly, or how they might 
 look should they grow too thin. 
 
 You can form some idea of the deformations produced 
 
BTJEXIXG AND SHAYIXG GLASSES 61 
 
 by cylindrical mirrors by examinuig the exceedingly odd 
 looking figure represented on the sheet of paper on which 
 the cyliadrical mirror shown in Fig. 24 is resting; for this 
 figure when examined ia the cyliadrical mirror produces 
 the image you see represented in the mirror. 
 
 The inner surface of a teacup with fairly vertical walls 
 is capable of acting as a concave cylindrical mirror not un- 
 like the convex cylindrical mirror of the preceding figure. 
 And, moreover, the surface _ of coffee or tea in the cup 
 
 Fig. 25. — Cubiotjs Double CijE\-e formed by Concave Ctundbi- 
 
 CAl illBROE. 
 
 forms an admirable screen on which to receive the image. 
 If, therefore, some time when your tea or coffee cup is 
 only half filled, you so place it that the light from a distant 
 burner ia a chandelier is reflected from one side of the 
 cup you wiU see, thrown on the surface of the coffee or 
 tea, a curious double curve produced by reflection from the 
 inside of the cup. 
 
 If you employ a strip of the zinc such as is used for 
 placing under stoves, you can obtaia some of the effects 
 
63 THE WONDEE BOOK OP LIGHT 
 
 produced by cylindrical mirrors. For example, if you 
 place it, as shown in Eig. 25, on a sheet of white paper, 
 you will obtain the appearance represented, which will 
 be the same as that seen in the tea or coffee cup before 
 referred to from the reflected light of a gas burner or lamp 
 placed anywhere in front of it. 
 
CHAPTEE YI 
 THE BEXDIXG OR REFRACTION OF LIGHT 
 
 As we have already seen, -n-hen a beam of light falls on 
 a transparent medium, a portion of the light is thrown off 
 or reflected from the surface, and the remainder entering 
 the medium undergoes a change in the direction of its 
 path known as refraction. In the case of such transparent 
 substances as water, by far the greatest proportion of the 
 light enters them and is refracted. It can be shown that 
 the smallest proportion of the light is reflected, and the 
 greatest amount is refracted or passed into the water, 
 when the light falls directly on the surface, that is, strikes 
 it perpendicularly. 
 
 In this case only eighteen rays out of every thousand are 
 flung off or reflected, the remaining 983 rays passing 
 through the water. 
 
 If the beam pf light ig ppmnittef} t tO fgll.ni'lLt "^"^ more 
 obliquely on the surface oithe jsratex (that is at a greater 
 slant) the amquiit_xtfJighiL jceflsctei from jts_surf ace will 
 
 be increased, and, mnspgiipritly^ Jhp arnnurit JJTat._PTitPrg 
 
 the water, or undjrgoeg refracjipn, wlU be^deereased^ _ 
 
 TyndaU has shown that out of every 1,000 rays the 
 amount of light that is reflected and refracted for different 
 angles of incidence is as follows : 
 
 63 
 
64 THE WONDEE BOOK OF LIGHT 
 
 Angle of 
 striking. 
 
 No. of rays 
 reflected. 
 
 No. of rays 
 refracted. 
 
 0° 
 
 18 
 
 983 
 
 40° 
 
 %% 
 
 978 
 
 60° 
 
 65 
 
 935 
 
 80° 
 
 333 
 
 667 
 
 89° 30' 731 379 
 
 In other words, as the rays fall more and more obliquely 
 on the surface, the amount of light reflected becomes 
 greater and the amount refracted becomes less. 
 
 During mid-day, when the sun is almost directly over- 
 head, it is possible to look directly at its image in a body 
 of water acting as a huge mirror, for then, the rays strik- 
 ing the surface almost perpendicularly, the number of 
 rays that are reflected is comparatively small. If, how- 
 ever, you should attempt to look at the sun either near 
 sunrise or sunset, when the rays fall obliquely on the water, 
 the amount of light flung off from the surface is so great 
 that your eyes would be dazzled. 
 
 When a beam of light is permitted to fall on an opaque 
 substance, such, for example, as on the surface of a sheet 
 of polished metal, a small quantity of light enters the sub- 
 stance and undergoes refraction, while the remainder is 
 reflected or thrown off from the surface. The quantity 
 of light that can enter an opaque substance is quite 
 small. 
 
 In Fig. 36, a beam of light is permitted to fall ob- 
 liquely on the surface of a glass vessel nearly filled with 
 water. As you see, this vessel is placed on a table in the 
 path of a beam of light entering a darkened room through 
 
BENDING OE EEPEACTION OF LIGHT 65 
 
 a hole in the shutter. Generally, the path of the beam 
 before it strikes the surface of the water can be 
 easily seen by the beam lighting up or illumining 
 particles of dust that are scattered through the air. 
 It might be well, how- 
 ever, to ensure the beam 
 being seen by burning a 
 piece of smoked paper, 
 so as to make the air of 
 the room smoky. 
 
 You will notice that 
 the beam continues to 
 move in a straight line 
 until it strikes the sur- 
 face of the water, when 
 it is bent out of its 
 course as can be seen 
 while it passes through 
 the water. 
 
 You will also notice 
 ,, , ,, , . ,, Fig. 26. — An Effect of Retbaction 
 
 that the change m the 
 
 direction of the rays takes place only at the surface of the 
 water where the air comes in contact with it. While the 
 beam is passing through the water, it continues its straight ■ 
 course just as it did while passing through the air, but its 
 path through the air and through the water does not con- 
 tinue in one and the same straight line, being bent as 
 already mentioned, where the water surface touches the 
 air. Now it is this bending that is called refraction. 
 
 Eefraction, like reflection, is an important property of 
 light, and on it depend many of the wonderful things that 
 
66 
 
 THE WOKDEE BOOK OP LIGHT 
 
 ■we shall see when we come to study the phenomena of 
 
 light. 
 
 Let ns now examine more carefully what takes place 
 
 when light falls obliquely on a water surface. For the sake 
 
 of simplicity we will take the case of a single ray of light 
 
 in place of a beam. Suppose 
 then that EI represents a 
 single ray of light passing 
 from E to I, where it falls on 
 the water surface represented 
 in Eig. 27. Had the water 
 not been present, the ray EI 
 would have continued on in 
 a straight course EIE'; but, 
 meeting the water, it under- 
 goes refraction, and takes the 
 
 Fig. 27.— Refraction OF Light, p^th IS through the water, 
 
 being turned out of its course as shown. 
 
 It is not my intention here to enter into a full explana- 
 tion of all the peculiarities of the refraction of light. The 
 subject is very dif&cult. If, however, you examine the pre- 
 ceding figure you can easily see that the angle included be- 
 tween the direction of the refracted ray IS and of the 
 perpendicular at the point of incidence is smaller than 
 the angle between the same perpendicular and the direc- 
 tion the ray EI had when it struck the surface of the 
 water. In other words, when light passes from air to 
 water, it is so refracted or bent that its inclination to the 
 perpendicular is less than it was before refraction oc- 
 curred. 
 
 Now the proportionate amount a ray of light is bent out 
 
BENDING OR EEFEACTION OF LIGHT 67 
 
 of its course by refraction, while passing between any 
 two media, such as air and water, always remains the 
 same for these media. If, therefore, we know the direc- 
 tion in which light falls on a water surface, it is possible 
 to calculate the path it will take after refraction. 
 
 I would say here for the older reader that the constant 
 ratio existing between the angle at which the light falls 
 on the surface and the angle at which it is refracted 
 (or, more correctly, between the sines of these angles, 
 represented in the preceding figure by dotted lines) is 
 known in optics as the index of refraction. In other words, 
 the index of refraction is equal to the sine of the angle 
 of incidence divided by the sine of the angle of refrac- 
 tion. 
 
 Now the proportionate amount that a ray of light is 
 bent out of its course by refraction, while passing between 
 any two media, such as air and water, always remains the 
 same for these media. Therefore, knowing the direction 
 in which light falls on a water surface, it would be possible 
 to calculate the path it would take after it underwent 
 refraction ; for, there is always a constant relation existing 
 between the angles at which the light falls on the surface 
 and the angle at which it is refracted, this ratio being 
 known as the index of refraction. 
 
 I would say here for the older reader that the ratio 
 above referred to is not that of the angles of incidence 
 and refraction themselves, but of certain quantities known 
 in trigonometry as the signs of these angles, and repre- 
 sented in the preceding figure by the dotted lines E'P' and 
 SP. The index of refraction, therefore, is equal to the 
 sine of the angle of incidence divided by the sine of the 
 
68 THE WONDER BOOK OF LIGHT 
 
 angle of refraction. The further development of the 
 equation is beyond the scope of this book. 
 
 If a ray of light is passed from water into air it will 
 also undergo refraction as soon as it reaches the bound- 
 ing surface between the water and the air. If, for example, 
 SI in the preceding iigure, represents the direction of a 
 ray of light passing through the water from S to I, then 
 when it reaches I, at the surface between the water and the 
 air, it will be refracted, and will take the path IE. In 
 this case the ray will be bent away^ from the perpendicular. 
 When, therefore, light passes from a medium like air to 
 . a denser medium like water, it is refracted in such a 
 direction that it is bent towards the perpendicular, so 
 that the angle of refraction is less than the angle of inci- 
 dence, but when light passes from a dense to a rarer nie- 
 dium, as from water to air, the refracted ray is then bent 
 from the perpendicular. 
 
 It can be shown that the reason for the bending or re- 
 fraction of a ray of light when passing from one medium 
 to another is the difference in the velocity of the light in 
 passing through the two media. The extent of bending 
 depends on the difference of this velocity in the 
 media. In other words, the refracted ray is bent towards 
 the perpendicular when the light passes from air to water, 
 and from the perpendicular when the light passes from 
 water to air, because the light travels less rapidly through 
 a dense medium like water than it does through a rare 
 medium like air. 
 
 I have already called your attention to the fact that the 
 position of an object depends on the direction in which 
 the rays of light coming from the object enter the 
 
BENDING OE EEFEACTION OF LIGHT 69 
 
 eye of an observer. Now, since refraction alters the direc- 
 tion in which the rays of light coming from an object 
 in a medium such, for example, as water enter the eye of 
 an observer placed in the air over the water, it is evi- 
 dent that the apparent position of the objects will be 
 changed by refraction. 
 
 There is only one direction in which a person can see 
 an object in water in its true position; this is when the 
 rays of light coming from the object do not undergo re- 
 fraction, or when they strike perpendicularly on the 
 surface. But this is the same thing as saying that in order 
 to see the true position of an object in water the eye of 
 the observer must be situated directly over the surface 
 of the water ; for if the eye is placed to one side, when the 
 light coming from the object under the water is refracted 
 while passing through the air and enters the eye of the 
 observer, it makes the object seem to be elevated or brought 
 nearer the surface. 
 
 Suppose, for example, that A, Fig. 28, at X, represents 
 an object placed on the bottom of a stream of water whose 
 surface is situated at FF. Then if 1, 3, represent a diverg- 
 ing pencil of light coming from A, its rays, on passing 
 into the air will undergo refraction, and take the path 
 represented by 3, 4. These rays entering the eye of an ob- 
 server, would seem to come not from the true position of 
 the object A on the bottom, but as if both it and the bottom 
 had been raised to occupy the position B, so that the water 
 appears shallower. If, therefore, A were a fish and not a 
 stone, an air-rifle aimed at the apparent position of the ob- 
 ject at B would fail to strike it (that is, if aimed correctly), 
 for it would strike considerably above the true position of 
 
70 
 
 THE WONDEE BOOK OE LIGHT 
 
 the object. It is for this reason that due allowance must be 
 made in attempting to strike an object when viewed under 
 water for change in position caused by refraction. 
 
 You can easily try an amusing experiment in refraction 
 in the following manner. Place a quarter of a dollar at 
 m, n, Pig. 28, at Y, on the bottom of a vessel that contains 
 no water, and then stand with the eye at 0, so that the 
 light coming from the coin is just cut off by the upper edge 
 A of the basin, and the coin is, therefore, invisible. Now, 
 
 ZZ' 
 
 ^^ 
 
 ,jj^ 
 
 ^Z 
 
 1" ,->W'^' / 
 
 ^ p. 
 
 m< f/y 
 
 
 mK" 
 
 
 Fig. 2'8. — Effect of Eefeaction ok Apparent Depth of Water. 
 
 holding the head in a fixed position, get some one to pour 
 water in the basin and you will find that as soon as the 
 water reaches about the level PP', the coin will become 
 visible, without your having had to move your head. 
 
 The explanation is evident. The rays of light coming 
 from a point of the coin at m, although they continue in a 
 straight path while passing through the water are re- 
 fracted or bent out of their direction as soon as they enter 
 the air, and, being bent towards the eye as shown in 
 the above figure, appear to come from the coin as if it 
 were situated not at m, but at m'. In other words, re- 
 
BEXDIXG OE EEFEACTIOX OF LIGHT 71 
 
 fraction has not only made the vessel of water appear to 
 be less deep, but apparently raised the coin from m to 
 m', and has so enabled you to see it. 
 
 There are a number of common phenomena that are due 
 to the bending of a ray of light at the bounding surface 
 between media of different densities. 
 
 If you rest a straight stick in the air, against the 
 ground or the pavement, it will loook straight all the way 
 from the top to the bottom, for there is nothing to change 
 the direction of the rays of light as they come from its 
 difEerent parts between the top and the bottom and enter 
 3'our eye. If, however, the stick is plunged into a stream 
 of clear water, so as to rest against the bottom, with a 
 portion in the air, it wUl appear to have been broken at 
 the point where it enters the water. Its lower end wiU 
 seem to have been raised so as to make the water appear 
 less deep than it really is; and, since the portion of the 
 stick that is out of the water has not had its appearance 
 changed, the portion under water thus appears to be bent 
 at the point where it enters the water surface. I think 
 you will have no trouble in understanding the cause of its 
 broken appearance by carefully examining the direction of 
 the rays as they pass from the water at the lower part of 
 the stick into the air. 
 
 Some very curious effects are produced by refraction of 
 the light from a heavenly body, such as the sun or the 
 stars, as it passes from the regions outside the earth into 
 the earth's atmosphere. All rays of light from heavenly 
 bodies that enter the earth's atmosphere obliquely are 
 bent downwards by refraction. For example, a star does 
 not set or disappear as soon as it passes below the horizon. 
 
73 THE WONDEK BOOK OF LIGHT 
 
 but continues in view after it has passed for a considerable 
 distance below the horizon. In a similar manner, to an 
 observer on the earth, a star may be seen above the 
 horizon, as if it had just risen, long before it has actually 
 risen, because the ray of light is bent out of its course by 
 refraction. 
 
 The duration of twilight long after sunset is due not 
 only to the change resulting from refraction in the direc- 
 tion of the sun's light during its passage through the at- 
 mosphere, but also from the fact that, long after the sun 
 has set, its rays pass over the head, and enter the eye by 
 reflection from the higher portions of the earth's atmos- 
 phere. 
 
 It is, indeed, almost entirely by reflection and not by 
 refraction only, that twilight is due. Young has shown 
 that the length of time required by the sun, after setting, 
 to reach a point about 18° below the horizon when 
 the twilight may be considered as no longer contin- 
 uing (since stars of the sixth magnitude then become 
 visible directly overhead), varies with the season as well as 
 with the part of the world in which the observer may be 
 situated. At latitude 40° N". on the 1st of March and on 
 the 13th of October, the sun does not reach this point until 
 an hour and a half after its actual setting. At the time of 
 the summer solstice, or the 30th of June, more than two 
 hours are required before the sun, after passing below the 
 horizon, ceases to produce twilight effects. Indeed, in 
 latitudes further north than 50°, at the time of the year 
 when the days are longest, twilight never entirely disap- 
 pears even at midnight. On the mountains of Peru, how- 
 ever, twilight never lasts more than half an hour. 
 
BENDING OE REFEACTION OF LIGHT 73 
 
 If a luminous point S be situated below a water surface 
 AE, Fig. 29, its rays wUl undergo refraction at all points 
 except at B, where they strike perpendicularly against the 
 bounding upper surface. Those which strike obliquely 
 as SC, undergo refraction, continuing their way through 
 
 Fig. 29. — Cbitioal Angle and Totai, Inteenal Eeflection. 
 
 the air in the direction of the straight line CC. Since now 
 the light is passing from water to air the angle of refrac- 
 tion is greater than the angle of incidence, so that a 
 point will soon be found, situated say at D, where the angle 
 of incidence has such a value that the refracted ray will 
 emerge from the water parallel to its surface, represented 
 in the drawing as above the surface so as to be visible. 
 The angle of incidence at which this occurs is called the 
 critical angle. When the angle of incidence slightly ex- 
 ceeds the value of the critical angle, as at H, the light 
 will no longer emerge from the water and be refracted 
 but will undergo what is known as total internal reflec- 
 tion. 
 
 There is an interesting experiment in total internal 
 reflection that I would advise you to try as it is quite sim- 
 ple and yet very beautiful. 
 
74 THE WONDEE BOOK OF LIGHT 
 
 Partially filling a clear glass tumbler with filtered 
 water, rest a spoon in it; then, if you hold the tumbler of 
 water above the level of your eye, the upper surface of 
 the water will be seen to shine like a brilliant mirror, 
 while the image of a portion of the spoon will be seen as 
 reflected from it. 
 
 If, while standing near an aquarium, you place your eye 
 in various positions outside, below the level of its water 
 you will at last find a position at which its upper surface, 
 acting as a mirror, will give you by total internal reflection 
 some very beautiful effects of images inside the aquarium. 
 
CHAPTEE VII 
 LENSES 
 
 Just as there are variously shaped opaque bodies with 
 convex and concave surfaces, capable of causing pencils of 
 light after reflection to collect at places called foci, so 
 there are variously shaped transparent bodies that are like- 
 wise capable of causing rays of light passing through them 
 to collect at similar foci. Such transparent bodies form 
 what are called lenses. 
 
 The name lens has been given to these bodies by reason 
 of their resemblance to the flat, rounded seeds known as 
 
 B 
 Fig. 30. — Convex Lens Employed in Bubnino Glass. 
 
 lentils, found in certain vegetable pods, and at one time 
 commonly employed for food. Lentils are not so much 
 employed now, so that it is possible you have never seen 
 them. If this is so, I can only say that the shape of a 
 lentil resembles that of the ordinary burning glass shown 
 at AB, Pig. 30, which I am sure you have all seen. This 
 particular lens is called a burning glass because it brings 
 
 75 
 
76 
 
 THE WONDEE BOOK OF LIGHT 
 
 a beam of light A' to a focus F, which, besides being a 
 focus of light rays, is also a focus of heat rays. 
 
 As we have seen, mirrors can be divided into two classes : 
 convex and concave mirrors. In a similar manner, lenses 
 are divided into two classes called converging and diverg- 
 ing lenses, according to whether they cause the rays of light 
 passing through them to converge towards, and thus 
 collect at a single point, or caiise them to diverge, or 
 
 pass from a single point. 
 When a ray of light is 
 caused to pass through a 
 plate of glass the opposite 
 surfaces of which are par- 
 allel, as represented in 
 Fig. 31, the ray under- 
 goes a change in its direc- 
 tion, or is refracted, both 
 on entering and on leav- 
 ing the plate. If, for ex- 
 ample, a ray of light falls 
 on the plate in the direction S'E, it is bent in the direction 
 EI on entering the plate as well as in the direction IS on 
 leaving it, and passes out of the plate parallel to its origi- 
 nal direction. It does not, however, continue in the same 
 straight line it had when it fell on the upper surface of the 
 plate; for if the straight line S'E be prolonged it will be 
 seen that the emerged ray IS has been turned out of its 
 course. 
 
 It is for the above reason that when an object is viewed 
 obliquely through a glass plate bounded by parallel faces, 
 it is seen out of its true position. If, for example, a 
 
 s s 
 
 Fig. 31. — Passage of Light 
 Theoijgh Plate of Glass. 
 
LENSES 77 
 
 luminous point is viewed by an eye placed above the glass 
 plate the rays of light, when they finally enter the 
 eye of an observer, appear to come not from its true posi- 
 tion, but from a position much nearer the glass. If, there- 
 fore, an object be situated on the other side of the glass 
 screen, and is viewed obliquely by an eye, it will be 
 seen out of its true position. 
 
 If now instead of employing a plate of glass boiinded 
 by parallel sides, a piece of glass in the shape of a prism 
 ABC, Eig. 32, is used, and a luminous point L is placed 
 as shown on one side of the prism, a ray of 
 
 L A c L' u" 
 
 Fig. 32. — Refraction of Light at Suefaces or Pbism. 
 
 light LD, falling on the face AB, at D, undergoes re- 
 fraction, both on passing through the prism at D and 
 again on entering the air and taking the course EL'. This 
 ray of light after passing through the prism, cuts the line 
 XX, corresponding to the base AC, of the prism at the 
 point L'. 
 
 If, however, a second ray of light LD', falls on the face 
 of the prism AB, at some other point as D', after passing 
 through the prism, it would cut the line XX, not at the 
 point L', but at some other point, such as L", nor would 
 there be any other point than D at which a ray of light 
 from the luminous point L, after passing through the 
 
78 THE WONDEE BOOK OF LIGHT 
 
 prism would meet at the same point L' on the other side 
 of the prism. 
 
 If, however, the piece of glass ABC, Fig. 33, instead of 
 having a single inclined face AB, is provided with three 
 differently inclined faces as shown, the inclination of 
 these faces might be so arranged that the rays of light 
 LD, LD', and LD" might, after passage through the ir- 
 regularly shaped piece of glass, all meet at a single point or 
 focus L', on the other side of the glass. 
 
 But it would be only the rays that fall on the prism at 
 D", D', and D", that would collect at L'. Those striking 
 
 ■''■ U A c I.' 
 
 Fig. 33. — The Principle of the Lens. 
 
 at any other portions of the surface between A and B 
 would cut XX' at some other point. 
 
 Now I think you should have no difficulty in seeing that 
 if the opposite faces of ABC, instead of being flat or 
 plane, were curved, or in other words, constantly changed 
 their directions as curved surfaces, it should be possible 
 to cause all the rays of light diverging from a point placed 
 in front of such a surface to come to a single point or 
 focus on the other side. And this would produce what is 
 known as a lens. For it would be only necessary to place 
 a second such surface with its base AC resting on a 
 similar surface as shown in Fig. 34 to have a piece of 
 
LENSES 79 
 
 glass that possesses the power of causing all rays of light 
 diverging from a point L on one side to meet or collect at 
 a point L' on the other side. 
 
 I am now, therefore, able to tell you how a lens is made. 
 It consists of a medium formed of glass, or some other 
 transparent substance denser than air, bounded on its 
 
 D 
 
 Fig. 34. — Foci of Lens. 
 
 opposite faces either by curved surfaces, or by a curved 
 and a plane surface, that like convex and concave mirrors, 
 possesses the power of causing the rays of light that pass 
 through it to collect at points called foci. 
 
 As I have already told you, lenses can be divided into 
 converging and diverging lenses. Converging lenses can 
 be divided further into three classes as represented in 
 Fig. 35. I would advise you to memorize the names of 
 these three classes of lenses. That shown at A is called 
 a double convex lens because its opposite faces are convex 
 in shape. That at B is called a plano-convex lens because 
 its opposite faces are respectively convex and plane. That 
 shown at C is called a concavo-convex lens because its 
 opposite faces are respectively convex and concave. This 
 lens is sometimes called a meniscus, a name given to it 
 because of its moon shape. All these classes of converg- 
 ing lenses are sometimes called convex lenses. 
 
80 THE WONDEE BOOK OP LIGHT 
 
 If you examine these lenses carefully you will see that 
 they all agree in one respect ; they are thicker at the middle 
 than at the edges. This is important since, in all cases, 
 rays of light, when passing through any lenses, are bent 
 from the thinner portions towards the thicker portions. 
 You can see, therefore, that such lenses must converge the 
 light passing through them. 
 
 There are also three kinds of diverging lenses; or, as 
 they are sometimes called concave lenses. These are 
 named as follows: That shown at D is called a double 
 concave lens because its two opposite surfaces are concave. 
 
 CONVERGING DIVERGING 
 
 LENSES LENSES 
 
 ABC DBF 
 
 A, Double Convex — B, Piano-Convex — C, Concavo-Convex — D, 
 
 Double Concave — E, Piano-Concave — P, Diverging Meniscus 
 
 Fig. 35. — Converging and Diverging Lenses. 
 
 It is sometimes merely called a concave lens. That at E 
 
 is called a plano-concave lens because its opposite faces 
 
 are respectively plane and concave. And that shown at 
 
 P is called a convexo-concave lens because its opposite 
 
 surfaces are respectively convex and concave. This lens 
 
 is more frequently called a diverging meniscus. 
 
 If you examine the different concave lenses you will 
 
 find that they all agree in that their central portions are 
 
 thinner than the portions near the edges, and since the 
 
 rays of light after passing through lenses are always 
 
 turned from the thinner portions towards the thicker 
 
 portions, these lenses must necessarily be diverging lenses. 
 
LEXSES 81 
 
 Having now obtained some idea of what is meant by 
 lenses, let ns see how a lens brings to a focus the rays of 
 light that pass through it. 
 
 Suppose we begin with the commonest kind of light; 
 i. e., daylight or sunlight. The sun is so large a body 
 and is so far off from us that its rays may be regarded as 
 being practically parallel. How then will a beam of sun- 
 light or parallel rays be affected by its passage through 
 either a double convex or a double concave lens. As you 
 will see by an inspection of Fig. 36, at X, when such rays 
 pass through a double convex lens they are all brought to 
 a single focus on the other side of the lens at F. Since 
 
 X Y 
 
 Fig. 36. — Focus of Pabahel Rats of Convks and Concave 
 Lenses. 
 
 the rays actually meet at this point the principal focus of 
 a double convex lens is a real focus. 
 
 If, however, a beam of sunlight falls on a double con- 
 cave lens as at Y, in the same figure, the rays of light, 
 after emerging, diverge as though they came from a focus 
 F on the same side of the lens as that on which the parallel 
 rays fell. This is also called the principal focus of the con- 
 cave lens. But since these rays of light only appear to 
 meet at this focus, it is called a virtual focus. 
 
 You can readily see from the above figures that the dou- 
 ble convex lens as shown at X, has acted as a converging 
 lens, since it has bent all the rays of light that have passed 
 
82 
 
 THE WONDEE BOOK OF LIGHT 
 
 through it, so as to cause them to converge or be brought 
 together into a single focus at P. 
 
 N"ot so, however, with the parallel rays of light that have 
 passed through the double concave lens shown in the same 
 figure at Y, for here, diverging, they appear to come from 
 the virtual focus at P. 
 
 But convex lenses possess other foci. If, as at X, Eig. 
 37, a luminous point, C, is placed in front of a double 
 convex lens, at any point greater than twice the principal 
 
 C 
 
 Fig. 37. — Some Othee Foci of Convex Lenses. 
 
 focus, then after their passage through the lens the rays 
 will be converged or brought to a real focus on the other 
 side of the lens at C, nearer to the lens than at C, at some 
 point between the principal focus and twice the principal 
 focus. These points C and C are called conjoined or con- 
 jugate foci because they may replace each other; for, if 
 the luminous point be placed at C its focus will be at C. 
 These two foci are called respectively the longer and the 
 shorter conjugate focus. 
 
 When the luminous point is placed at as at Y, Fig. 37 
 
LENSES 83 
 
 at a point nearer to the lens than its principal focus, 
 then although the rays are converged yet they are not suffi- 
 ciently converged to meet in a focus at the other side of the 
 lens, but after passing through the lens would appear to 
 come from a focus V, on the same side of the lens called a 
 virtual focus. 
 
 As to the effects produced by lenses in the formation of 
 images, we shall understand them better when we discuss 
 some of the simple pieces of optical apparatus. It wlQ 
 suffice here to say that when an object placed quite near 
 
 Use of Convex Lens as a Magnhteb. 
 
 a double-convex lens is examined by the eye placed on the 
 opposite side of the lens as shown in Fig. 38, the lens is 
 employed in a very common manner as a magnifying lens, 
 and produces, as shown, an erect and largely magnified 
 image of the object that is being examined. 
 
 If, however, an object is examined in a somewhat similar 
 way through a concave lens, an erect and greatly dimin- 
 ished image of the object wUl be seen. 
 
 There are other images, formed at the longer and 
 shorter conjugate foci of the convex lenses, that are em- 
 ployed in optical apparatus. If, for example, as in Fig. 
 39, a flower is placed at the longer conjugate foci of a 
 
84 
 
 THE WONDEE BOOK OP LIGHT 
 
 double convex lens, an inverted and decreased image Avill 
 be seen at the focus, and, since it is a real image, it could 
 be received on a screen instead of on the eye of an ob- 
 server. Indeed, if a highly illumined object were placed 
 at the shorter conjugate focus of a double convex lens, 
 an inverted and greatly magnified image would be throv^n 
 on a screen placed at its longer conjugata foci. As we 
 shall see in a subsequent chapter, this is practically the 
 manner in which the magic lantern operates. For it is 
 only necessary to place a highly illumined inverted picture 
 at the shorter conjugate focus of a double convex lens and 
 
 Fig. 39. — Foemation of Image by Convex Lens. 
 
 receive on a screen a highly magnified image which would 
 now appear in its correct position. 
 
 Later you will see how highly important the images at 
 the longer and shorter conjugate foci of converging lenses 
 are, as well as the manner in which they are used in 
 such well-known pieces of optical apparatus as the camera 
 obscura, the photographic camera, the compound micro- 
 scope, the telescope, etc. 
 
CHAPTER VIII 
 
 THE MICROSCOPE 
 
 There are various instruments, consisting either of 
 mirrors or lenses, or of combinations of mirrors and lenses, 
 known as optical instruments, some of wHch are capable 
 of producing extremely wonderful results. 
 
 One of the most important of these is the microscope. 
 As its name indicates, the microscope is a device suitable 
 for the examination of objects that are too small to be 
 seen by the unaided eye. 
 
 There are two kinds of microscopes; i. e., the simple 
 microscope and the compound microscope. The simple 
 microscope consists essentially of a single converging lens; 
 the compound microscope consists of two converging lenses. 
 
 In the simple microscope, the object to be magniiied is 
 placed between the eve and a convex lens, nearer to the 
 lens than its principal focus, as represented in the last 
 chapter in Fig. 38. As the rays of light pass through the 
 lens, they are converged and, entering the eye of the ob- 
 server, appear to come from a greatly enlarged image of 
 the object that is being examined. 
 
 But, while the simple microscope consists essentially of 
 a single converging lens it is generally provided with sev- 
 eral additional parts that are necessary to permit the 
 instrument to be easily operated. In the first place, a 
 suitable support or stand must be provided for holding- 
 
86 
 
 THE WONDEE BOOK OP LIGHT 
 
 the object. If the magnification is great, and the object 
 is small, it would be too awkward to attempt to hold the 
 object in the hand while magnifying it. Moreover, such a 
 method would prevent the object from receiving on all 
 sides the light necessary to permit it to be greatly magni- 
 fied. Then, too, the object is often so small it can not be 
 held in the hand. With larger objects, such as watches, 
 plates of metal that are being engraved, or similar 
 
 articles, this objection is not 
 so serious. 
 
 In the form of simple mi- 
 croscope represented in Fig. 
 40, the object to be exam- 
 ined in this case (a very 
 minute one) is placed be- 
 tween two glass plates in 
 what is called a glass slide, 
 and supported on a stage 6. 
 ^M_]Sro attempt is made to move 
 M^ ^^r the object itself towards or 
 from the magnifying lens, 
 but the lens itself, mounted 
 in a brass tube, K, is arranged 
 so that it can readily be moved towards or from the object, 
 by turning the brass screw D, provided with a milled 
 edge. 
 
 An important part of a simple microscope consists of 
 means for properly illumining the object. In any optical 
 instrument in which the image of an object is greatly 
 magnified, the illumination of the image is necessarily 
 greatly decreased. Unless, therefore, the object is strongly 
 
 Fig. 40. — Simple Micboscope. 
 
THE MICEOSCOPE 87 
 
 illumined, its image will receive so little light, that its 
 details cannot be distinctly seen. 
 
 The means for properly illumining the obieets that are 
 to be examined by a microscope is a very important part 
 of the apparatus. An abundance of light thrown on the 
 object, is especially necessary where the amount of magni- 
 fication is large. 
 
 It is a virtual focus of the lens that is employed, when 
 the lens is used as a magnifying glass. Xow there are a 
 number of different virtual foci for every convex lens. 
 Indeed, you would have no trouble in proving this, for 
 you would find, by moving the lens to-or-from the object, 
 that its magnified image can be seen in a number of 
 different positions in some of which it appears much 
 larger than in others. It would not be long, however, be- 
 fore you discovered that there is only one of these posi- 
 tions at which you can see the image distinctly. In other 
 words, there is a certain distance, at which the object 
 must be held from the eye, in order that its minute struc- 
 ture shall be distinctly seen. By moving the magnifying 
 lens towards and from the object, there is obtained the 
 particular distance of the object from the lens that pro- 
 duces the most distinct vision in the eye of an observer. 
 This is known as focusing, or adjusting the lens. Sroce 
 the eyes of one person differ from those of another person 
 it is generally found necessary for each observer to focus 
 the lens. 
 
 There are a number of occupations ia which the work- 
 men are obliged to use a magnifjing glass. Where the 
 object to be examined is so fairly large that it can easily 
 be held ia the hand, it is customary to place the magni- 
 
88 THE WONDER BOOK OF LIGHT 
 
 fying lens in a cylindrical tube that the workman holds 
 against his eye by the contraction of the surrounding 
 muscles. You have probably often seen a watchmaker at 
 work holding a magnifying glass mounted in a small brass 
 tube firmly in place before one of his eyes. Engravers, 
 jewellers, and others, who work on similar objects that 
 can be supported on a stand, employ magnifying glasses 
 that are held in a similar manner. 
 
 The magnifying power of a simple microscope depends 
 on two quantities, one of which is fixed and the other is 
 capable of being varied. The fixed quantity, which varies 
 with different people, is called the limit of distinct vision, 
 or the distance an object must be situated from the eye in 
 order to ensure the most distinct vision. For most people, 
 the limit of distinct vision is about ten inches. The quan- 
 tity that is capable of being varied is the length of the 
 principal focus of the magnifying lens, or the distance on 
 the other side of the lens at which a beam of sunlight is 
 brought to a focus. The magnifying power of a lens is 
 obtained by dividing the limit of distinct vision by the 
 length of the principal focus, so you can easily calculate 
 the magnifying power in the case of any particular lens. 
 
 Magnifying lenses can be purchased at almost any op- 
 ticians for a small price. I would advise you to get one 
 and make experiments with it. If you hold it in a beam 
 of sunlight, in front of a sheet of paper, you can get a 
 bright spot of light just as you would with the lens as 
 a burning glass. The distance between the lens and the 
 paper is the principal focus of the lens. Now let us sup- 
 pose that this distance is equal to one inch. Since your 
 limit of distinct vision is gay ten inches, and the magni- 
 
THE MICEOSCOPE 89 
 
 fying power of the lens is equal to the limit of distinct 
 vision divided by the length of its principal focus, and you 
 have just proved this to be one inch, then the magnifying 
 power of that particular lens is ten divided by one; that 
 is, ten. This means that an object when examined by 
 such a lens looks ten times longer and ten times broader 
 than it actually is. 
 
 It is evident that if you wish to obtain a high magni- 
 fyiag power you must get a lens of a short focal length; 
 for, the shorter the focus is, the greater will be the mag- 
 nifying power. Suppose, for example, you obtaia a lens 
 whose focal length is half an inch, then its magnifying 
 power would be twice as great as if its focal length were 
 one inch; for ten divided by one-half equals twenty. In 
 the same way, if the focal length of a magnifying lens be 
 one-tenth of an inch, then its magnifying power would 
 be ten divided by one-tenth or 100. 
 
 Up to a certain limit it is not diflBcult to obtain 
 convex lenses with comparatively small focal lengths. But 
 as soon as this limit is passed, it becomes exceedingly 
 difficult to get satisfactory lenses of this character; for, 
 when an attempt is made to get a lens with a fairly short 
 focal length, certain imperfections are almost sure to be 
 introduced into the lens. These will be described more 
 fully in a subsequent chapter. These imperfections or 
 diseases are certain troubles or difficulties that result in 
 the failure of lenses correctly to reproduce, in their highly 
 magnified images, either the correct outlines or the correct 
 colours of the objects they have magnified. Consequently, 
 while one may obtain lenses of short focal length, and 
 therefore of great magnifying powers, yet these lenses may 
 
90 THE WONDEE BOOK OF LIGHT 
 
 produce images with their outlines so contorted, and their 
 colours so false that they are of very little use as magni- 
 fying lenses. 
 
 Another difficulty is found in the fact that lenses with 
 very short foci must be comparatively small, and conse- 
 quently will have a small width. This will necessarily 
 limit the amount of light that can pass through the lens 
 into the eye of the observer, and this will stand in the way 
 of great magnification, unless the amount of light thrown 
 on the object to be examined is quite large. 
 
 The method generally employed for obtaining lenses of 
 short focal length, without too greatly decreasing their 
 
 FiQ. 41. — Magnifying Lenses or Shokt Focal Lengths. 
 
 width, is to employ more than a single lens. If, for ex- 
 ample, two separate lenses are placed together as repre- 
 sented in Pig. 41, and mounted as shown, the focal length 
 of the combination will be much smaller than that of 
 either of the lenses separately. Moreover, as we shall 
 afterwards see, by the proper use of such a combination 
 of lenses, known generally as an eye piece, images are 
 obtained in which false outlines and false colours are to a 
 great extent avoided. 
 
 In the above figure there is employed a combination of 
 two plano-convex lenses placed with their flat sides to- 
 wards the object to be examined. 
 
 Sometimes, in order to decrease still further the 
 
THE MICEOSCOPE 91 
 
 amount of false coloiiration and false outline, a device 
 called a diaphragm is used. This consists of a flat plate 
 of brass, or other metal, covered with some dull black 
 material like lampblack mixed with turpentine, that pos- 
 sesses little or no power of either reflecting or diffusing 
 light. It is the edges of lenses that cause the greatest 
 amount of false colouration, and false outlines of objects. 
 Therefore, by placing a diaphragm with its central opening 
 immediately over the centre of a lens, the light is pre- 
 vented from passing through the edges. In this way the 
 images formed are comparatively free from false coloura- 
 tion or from false outline. 
 
 There are two ways of expressing the magnifying power 
 of a lens. One, called the linear magnifying power, is to 
 give it in terms of the width or breadth of the object. A 
 lens whose focal length is one-tenth of an inch, as we have 
 explained above, would have a magnifying power of 100; 
 that is, such a lens would make an object appear 100 times 
 longer as well as 100 times broader. Now sometimes the 
 magnifying power of such a lens is given in what is called 
 its superficial magnifying power, where both the length 
 and breadth of the object are considered, the magnifying 
 power being expressed as the increase in the area. Since 
 areas are obtained by multiplying the length by the 
 breadth, the superficial magnifying power of a lens is as 
 the square of its linear magnifying power, a lens that has 
 a linear magnifying power of 100 would have a super- 
 ficial magnifying power of 100 multiplied by 100, or 
 10,000. 
 
 A simple way of measuring the magnifying power of 
 a lens is shown in Fig. 43. Here a plane reflector, con- 
 
92 
 
 THE WONDEE BOOK OP LIGHT 
 
 sisting of a piece of glass provided with a hole in its 
 centre, is placed as shown in front of the eye-piece of a 
 microscope inclined at an angle of forty-five degrees. An- 
 other reflector, in the shape of a prism, is placed parallel 
 to the microscope at a distance of an inch or two, so that 
 the eye, looking down on the first mirror, is able to see 
 the reflected light that comes from a graduated scale 
 placed at A, eight or ten inches below the prism. In 
 making an observation, a similar scale is placed in the 
 microscope instead of the object to be magnified. The 
 
 Fig. 42. — ^Measubement of Magnifting Poweb. 
 
 observer, whose eye is placed as shown, is able to see both 
 scales simultaneously, and can, therefore, directly de- 
 termine the magnifying power by noting how many times 
 the magnified scale is larger than the unmagnified scale. 
 In the compound microscope there are two image-form- 
 ing lenses, each of which magnifies the object that is being 
 examined. The first produces a magnified image of the 
 object, and the second a magnified image of the magnified 
 image of the object. In the compound microscope, there- 
 fore, the amount of magnification is much greater than in 
 the simple microscope. 
 
THE MICROSCOPE 93 
 
 The two lenses employed in the compound microscope 
 are called respectively the object lens and the eye lens. 
 The object lens is the lens that lies next to the object that 
 is being examined. It is sometimes called the objective. 
 The eye lens 0' is the lens that lies nearest the eye of the 
 observer. 
 
 Although, in the above description of the compound 
 microscope, I speak of both the object lens and the eye 
 lens as being single lenses, yet, in point of fact, each gen- 
 erally consists of a combination of several lenses. Since, 
 however, these combinations act as single lenses, it is 
 simpler, in giving the general explanation of the instru- 
 ment, to regard them as single lenses only. 
 
 Xow in the compound microscope the object to be ex- 
 amined is placed in front of the object glass at a shorter 
 conjugate focus in such a position that a greatly magni- 
 fied and inverted image of the object is formed at the 
 longer conjugate focus, and this already magnified image, 
 being examined through the eye lens, is still more greatly 
 magnified. In the compound microscope, therefore, the 
 magnification of the object is very much greater than in 
 the simple microscope. 
 
 The compound microscope is generally provided with 
 a variety of object glasses and eye-pieces and with devices 
 for focusing and for illumining the object. This makes 
 the compound microscope, when of a high grade, an ex- 
 ceedingly expensive piece of optical apparatus. 
 
 A form of compound microscope is represented in Fig. 
 43. The object lens, or, as it is generally called, the ob- 
 jective, is seen at 0, the eye-lens, or, as it is generally 
 called, the eye-piece at 0'. 
 
94 
 
 THE WONDEE BOOK OF LIGHT 
 
 The object ab to be examined is placed in front of the 
 object glass at a shorter conjugate focus. A highly mag- 
 nified inverted image of this object is formed at the longer 
 conjugate focus of the lens at a' b'. The distance between 
 the objective and the eye-piece is 
 such that the image falls at such 
 a distance from the objective 0, 
 as to permit the eye to view it as a 
 highly magnified image at its 
 limit of distinct vision. 
 
 In the compound microscope, 
 both the objective and the eye- 
 piece consist of a number of 
 lenses so arranged as to act as a 
 single lens. By means of these 
 combinations of lenses a greatly 
 magnified image of the object is 
 seen at AB, both in its true or 
 natural colours, and free from 
 false outlines. 
 
 The ocular, or eye-piece, as 
 well as the compound objective, 
 are placed inside a series of brass 
 tubes DD', I and H, Pig. 44. The eye-piece at 0, is 
 placed in the tube H, and the object glass o, in the lower 
 part of the tube DD'. By means of suitable mechanism 
 the tube I can be moved to-and-fro with gentle friction in 
 the tube DD', and this tube can also, in its turn, be moved 
 in the larger tube fixed in the ring E. This latter ring is 
 also so fixed to the piece BB, that by turning a fine screw 
 T, by means of its milled head, it can be raised or lowered 
 
 4 
 
 //y 
 
 
 1 
 
 > 
 
 *' 
 
 / 
 
 B 
 
 Pig. 43. — Compound 
 
 MiCEOSCOPE. 
 
THE MICEOSCOPE 
 
 95 
 
 very gradually. This forms what is called the fine adjust- 
 ment. Besides this, the entire body of the microscope can 
 
 V\ / 
 
 Fig. 44. — Compound Acheomatio Miceoscope. 
 
 be raised or lowered by suitable means, thus forming what 
 is known as the coarse adjustment. 
 
 The object to be examined is placed between two glass 
 plates V, on a stage E, provided with a large central aper- 
 ture or opening, through which light can be thrown up- 
 
96 THE WONDEE BOOK OP LIGHT 
 
 wards on the object from a large concave mirror M. This 
 mirror is moved on an ingeniously arranged jointed sup- 
 port, so that it can be placed in any position that may be 
 necessary to throw the diffused daylight of the room from 
 a plane mirror on one side, or the light of a gas burner, 
 or lamp, from a concave mirror on the opposite side. A 
 convex lens, not shown in the figure, is sometimes ar- 
 ranged to throw a strong pencil of light on the object from 
 a lamp. 
 
 A diaphragm K, provided with openings of different 
 sizes, placed between the reflector and the stage, can be 
 so turned as to place any of these openings directly under 
 the stage so that the amount of light thrown upwards on 
 the object can be varied at pleasure. 
 
 On the right-hand side of Fig. 44 you can see the man- 
 ner in which a pencil of light is thrown upwards from 
 the reflecting mirror M, and entering the objective passes 
 through the different lenses of a compound microscope. 
 As here shown, the object glass consists of three small 
 compound lenses, each of which is made of two different 
 kinds of glass, the use of which in preventing false colour- 
 ation will be afterwards explained. 
 
 In a high-class compound microscope, the focal lengths 
 of some of the object glasses are so short that, in order to 
 bring the objects sufficiently near the objective, it is neces- 
 sary to cause them to cling to the objective by moistening 
 them with a drop of liquid. Such lenses are called im- 
 mersion lenses, and constitute no inconsiderable part of 
 the cost of the apparatus. 
 
CHAPTER IX 
 
 THE TELESCOPE 
 
 As its name indicates, the telescope is an optical instru- 
 ment suitable for the observation of objects at a distance. 
 The word is of Greek origin. 
 
 Telescopes can be divided, according to the purposes for 
 
 Tig. 45. — Astkonomical Telescope. 
 
 which they are intended, into astronomical telescopes and 
 terrestrial telescopes. 
 
 The general appearance of an astronomical telescope is 
 shown in Kg. 45. As you will see, this instrument is large 
 
 97 
 
98 
 
 THE WONDEE BOOK OP LIGHT 
 
 at one end and small at the other. The large end 0, is 
 pointed towards the star or other distant body to be ex- 
 amined, while the eye of the observer is placed at the 
 small end 0'. A convex lens is placed at each of these 
 ends, the one at the larger being called the object glass, 
 because it is pointed towards the object, and the smaller 
 one, the eye-piece, because it is placed near the eye. A 
 smaller telescope a, placed on the top of the instrument, 
 is known as the finder. Its purpose will be explained 
 shortly. 
 
 The arrangement of the lenses in an astronomical tele- 
 scope will be understood from an examination of Fig. 46. 
 
 Fig. 46. — Lenses in Asteonomicai, Telescope. 
 
 The object glass 0, in some instruments, is more than 
 twelve inches in diameter, and is always of a much greater 
 local length than the eye-piece 0'. 
 
 Since the telescope is always employed for the exami- 
 nation of a distant object such as the sun, a fixed star, 
 or something on the landscape, this object is always situ- 
 ated at a longer conjugate focus of the object lens. An 
 inverted image of this object is, therefore, always formed 
 at a shorter conjugate focus. And since the image of 
 an object at the longer conjugate focus of a converging 
 lens is always smaller than the object, the first thing a 
 
THE TELESCOPE 99 
 
 telescope does is to make tte image of tte distant object 
 appear smaller than it would if looked at directly. This, 
 however, strongly illumines the image, and, therefore, 
 permits it to be highly magnified, when examined by the 
 eye-piece or eye-lens 0'. 
 
 So far as the two lenses of a telescope are concerned, the 
 object glass and the eye-piece are exactly like the object 
 glass and the eye-piece of a microscope, with, however, 
 this difference: in the telescope the object examined is 
 necessarily so far from the object lens that it is situated 
 at a longer conjugate focus, and its image is, therefore, 
 smaller than the object ; while in the microscope the object 
 is so near to the object glass that it is always situated at 
 a shorter conjugate focus. Its image is, therefore, magni- 
 fied, and, as a rule, largely magnified. In both cases these 
 images are subsequently magnified by the eye-piece 
 through which they are seen. 
 
 I think you will have no difficulty in understanding 
 that, since the first action of a telescope is to make the 
 object examined appear smaller than it would to the naked 
 eye, the image finally seen through the eye-piece is magni- 
 fied to a much smaller extent than in the microscope. 
 
 You may ask, and quite naturally, why is the object 
 glass of a telescope so large. The answer to this question 
 would be the wolf's answer to little Eed Eiding Hood : 
 
 "To see the better, my dear." 
 
 The object glass of the telescope is enlarged in order that 
 it may see distant objects better. By the use of a large 
 object glass, one is able distinctly to see clearly objects so 
 far off that the amount of light that would enter the eye, 
 when looking directly at them, would be too small to per- 
 
100 THE WONDER BOOK OP LIGHT 
 
 mit their being seen at all, miich less to be magnified. 
 Although a much smaller image of the distant object is 
 thus formed than would be if examined directly by the 
 eye, yet the use of a large object glass produces an image 
 so strongly illumined that it can be magnified, to any 
 reasonable extent, without becoming too faint to be dis- 
 tinctly seen. 
 
 In order to give you some idea of the extent to which 
 the object glass of a telescope is sometimes able to in- 
 crease the amount of light that is swept into the eye, I 
 will mention the case of a large telescope constructed as 
 early as February, 1845, for the English astronomer. Lord 
 Eosse. The objective of this telescope, as was commonly 
 the case with large astronomical telescopes at this time, 
 instead of consisting of a glass lens, was a huge concave 
 mirror, some six feet in diameter. Since the amount of 
 light that falls on a surface is proportional to the square 
 of its diameter, and since the pupil of the human eye, the 
 name given to that portion of the eye which permits the 
 light from the outside to enter it, is only one-tenth of an 
 inch in diameter, then the reflecting mirror employed in 
 this huge telescope, which was six feet, or seventy-two 
 inches, in diameter, was 730 times wider than the pupil of 
 the eye. Squaring this, or multiplying 730 by 730 we have 
 518,400 times. In other words, the area of Lord Eosse's 
 telescope was roughly 518,400 times greater than the pupil 
 of the eye. Kow, since the telescope was arranged so as to 
 bring all this light to a focus, thus permitting it to pass 
 through the pupil of the eye of the observer, you can under- 
 stand that it was able to collect in this way from a distant 
 object and transmit to the eye for the purposes of visions 
 
 Bu^rav Vf7f7fre Sfudy^ 
 
THE TELESCOPE 101 
 
 an amount of light enormously greater than that which 
 the naked or unassisted eye could. Indeed, even supposing 
 that one-half of the light was lost by reflection from the 
 mirror, yet the amount collected would still be more than 
 250,000 times greater that what the eye alone could collect. 
 Such a telescope, therefore, would possess a marveUous 
 penetrating power, enabling objects to be seen at far 
 greater distances than could the eye itself. 
 
 I am anxious that you should understand this matter 
 thoroughly, since this ability to collect light and transmit 
 it to the eye is one of the principal advantages possessed 
 by the telescope. It is true that by the use of a proper 
 eye-piece we can magnify the bright image produced by 
 the object glass so as to make it appear larger than it 
 would appear when examined without a telescope. But 
 this is not the principal function of a telescope. Its func- 
 tion is rather to see " far " and " more distinctly " than 
 it is to see " large." 
 
 Generally speaking, a feeling of surprise and disap- 
 pointment is experienced when one looks for the first time 
 through a powerful astronomical telescope at a distant ob- 
 ject, such as the moon. The image is by no means as 
 large as the observer expected. He has likely associated 
 the size of the telescope with the size of some microscope 
 through which he has looked, and expects to see a highly 
 magnified moon, having doubtless argued to himself that 
 if a little microscope can make a tiny object look as big as 
 his head, then how immense must be the magnification of 
 a large body like the moon when seen through so large an 
 instrimient as the telescope. He should remember, how- 
 ever, that in the case of very distant stars he is looking at 
 
102 THE WONDBE BOOK OF LIGHT 
 
 objects that he would be unable to see at all without the 
 aid of the telescope. The telescope, although it usually 
 has a magnifying power of about ten diameters, has swept 
 BO great a quantity of light into his eyes that he can see 
 these objects not only distinctly, but also somewhat larger 
 than they would appear had they been directly visible. 
 
 An extremely important part of an astronomical tele- 
 scope is the equatorial mounting. Even with the com- 
 paratively small magnifying powers employed on telescopes, 
 the rotation of the earth on its axis would soon result in 
 causing a celestial object to pass out of the field of view, 
 thus making any long continued observations impracti- 
 cable. In order to avoid this difSculty, a very ingenious 
 mechanism is used. 
 
 The arrangements are such that the telescope can be 
 vertically raised or depressed on its second axis and can 
 also be moved at right angles to these directions on the 
 first axis so that it can be made to point at some particu- 
 lar part of the heavens, say to a star. When it is firmly 
 clamped in this position clockwork motion keeps it pointing 
 to this part of the heavens although by the rotation of the 
 earth on its axis the star is apparently moving across the 
 telescope from east to west; for the clockwork moves the 
 telescope just as fast in the opposite direction, and, there- 
 fore, keeps it pointed to the same part of the sky. 
 
 The terrestrial telescope differs from the astronomical 
 telescope in that it produces erect images. In the astro- 
 nomical telescope the fact that its images are inverted, or 
 turned upside-down, does not make any great difference, 
 but would of course be objectionable where objects on the 
 earth's surface are to be viewed. It is necessary, there- 
 
THE TELESCOPE 
 
 103 
 
 fore, in the terrestrial telescope to re-invert the image, so 
 as to make it appear in its ordinary position. This is 
 usually done by the introduction of two additional lenses 
 at 0" and 0" ', Eig. 47, between the image a' V, and the 
 eye lens 0'. How these lenses make the inverted image 
 erect can be understood by tracing the path of the rays of 
 light through the lenses. 
 
 An eye-piece provided with these additional lenses is 
 known as the terrestrial eye-piece. While it produces an 
 
 Fig. 47. — Tbbeesteial Eye-Piece. 
 
 erect image, yet the terrestrial eye-piece possesses the dis- 
 advantage of causing a considerable loss of light during 
 its passage through the two additional lenses. It is also 
 objectionable by reason of the increased length it gives the 
 telescope. 
 
 It is interesting to note here that in one of the first tele- 
 scopes ever produced, namely, that invented by Galileo, a 
 concave lens was employed for the eye-piece. This iq- 
 verted the image produced by the object glass and thus 
 produced an erect image. The construction of the Gali- 
 lean telescope, and the manner in which it operates to 
 
104 
 
 THE WONDER BOOK OP LIGHT 
 
 ensure an erect image can be seen from an examination 
 of Pig. 48. 
 
 You are probably all familiar with the opera glasses that 
 are used at the opera, theatre, or other crowded places, for 
 viewing people on the stage or in different parts of the 
 house. These glasses consist of two small Galilean tele- 
 scopes, suitably mounted in two tubes. The object glasses 
 generally consist of combinations of lenses that act as a 
 
 Fig. 48. — Constkuction of the Galilean Telescope. 
 
 single convex lens, and of eye-pieces that act practically 
 as a concave lens. 
 
 Somewhat similar instruments, mounted like opera 
 glasses, but generally provided with much larger object 
 glasses so as to sweep a greater quantity of light into the 
 eye, are employed by navigators at sea for the purpose of 
 viewing distant objects. These are generally known as 
 marine glasses. 
 
 Sometimes the object glass of a telescope, instead of 
 consisting of a convex lens, consists of a reflecting mirror. 
 For this purpose a concave mirror, known as a speculum, 
 is employed. Since a reflector necessarily forms its image 
 on the same side of the object as itself, various methods 
 
A. — Yebkes Oeservatoet, Williams Bat, Wiscoxsi:^^ 
 
 B. — The Forty-ixch Refractor of the Yeekes Observatory : the 
 Largest ix the World 
 
THE TELESCOPE 105 
 
 are employed for examining through the eye-piece the 
 image formed by reflection from the mirror. In some 
 forms of reflecting telescopes, a circular opening is cut in 
 the centre of the mirror and a second mirror is placed in 
 front of it in order to throw the image onto the eye-piece. 
 Since this method prevents the best — that is, the central 
 ■ — portion of the- reflector from being used, another method 
 is generally employed, such, for example, as that used in 
 Newton's telescope, where the mirror is placed inside a 
 tube with its reflecting surface turned towards the distant 
 object; and the image formed by the mirror is magnified 
 by means of an eye-piece placed at right angles to the 
 reflected rays. The mirror is placed inside a tube with its 
 reflecting surface pointed towards the distant object. The 
 image formed in this mirror is magnified by means of the 
 eye-piece placed at right angles to the reflecting surface, 
 the rays being passed into the eye-piece by a rectangular 
 prism. The general appearance of the Newtonian tele- 
 scope when mounted in its tube and provided with its eye- 
 piece is shown in Fig. 49. 
 
 Owing to the difficulties of making large reflectors of 
 the required shape and of keeping them polished, reflect- 
 ing telescopes are now but seldom employed, being almost 
 entirely replaced by some form of refracting telescope. 
 
 It is also extremely difficult to mount a large reflector 
 so that it can be safely turned on its edge into the vertical 
 or inclined position it must take when pointed towards a 
 star or other heavenly body that is not immediately over- 
 head. Unless great care is taken to stiffen the huge piece 
 of metal or glass that forms the mirror, it will bend 
 slightly. Although this bending is so small that you could 
 
106 
 
 THE WONDEE BOOK OF LIGHT 
 
 by no means detect it with the unaided eye, it is, however, 
 sufficiently great to distort the image formed by reflection 
 from the mirror and render it useless. 
 
 Then again another difficulty arises from the necessity 
 for keeping the surface of the speculum polished, so as 
 
 Fig. 49. — ^Newtonian Telescope. 
 
 to make the amount of light lost by reflection as small as 
 possible. This polishing requires great care, since, unless 
 it is done properly, the mirror will lose the true curved 
 surface it must retain in order to form correct images. 
 
THE TELESCOPE 107 
 
 Moreover, if during the polishing process, a portion of the 
 surface be thoughtlessly touched for a moment by the fin- 
 ger, it will become heated, and will bulge out by local 
 expansion. If, now, the polishing is continued before the 
 entire surface of the mirror cools, a small portion of the 
 swelled surface will be removed, thus leaving a little de- 
 pression or pit that will of course injure the distinctness 
 of the image. The same difficulty is especially apt to occur 
 during the final grinding of the surface. 
 
 Although large lenses are subject to similar objections, 
 yet they are never made of the great size of reflectors, and, 
 therefore, are more rigid. 
 
 The magnifying power of a telescope is the enlargement 
 of its image as seen in the telescope as compared with the 
 apparent dimensions of the object as seen by the unas- 
 sisted eye. 
 
 The illuminating power of a telescope is the amount of 
 light the telescope is able to collect from a distant object 
 and transmit to the eye for the purpose of vision as com- 
 pared to the amount of light the naked eye is able to collect. 
 
 It is on the magnifying and illuminating power that 
 the penetrating power of a telescope depends. This pene- 
 trating power is the ratio of the distances at which the eye 
 and the telescope would collect, for the purposes of vision, 
 an equal amount of light. 
 
 The magnifying power is the "how big" the telescope 
 makes the distant object appear as compared with what 
 the eye could do. 
 
 The illuminating power is the "how bright" the tele- 
 scope makes the distant object appear as compared with 
 what the eye could do. 
 
108 THE WONDER BOOK OF LIGHT 
 
 And the penetrating power is the " how far " a distant 
 object will be and still permit the telescope to examine it 
 as compared with what the naked eye could do. 
 
 To the above definitions might be added another known 
 as the visual power of a telescope. This may be defined as 
 the " how well " the telescope will enable one to view a 
 distant object as compared with what the unassisted eye 
 could do. 
 
 It may interest you if I tell you something about the 
 invention of the telescope. - According to some, the tele- 
 scope was accidentally invented by two boys while playing 
 with the lenses their father, Zachary Jansen, a maker of 
 spectacles, employed in his business. Jansen lived in Hol- 
 land and the discovery referred to is said to have taken 
 place during the year 1608, so you see the telescope has 
 been known for a very long time. 
 
 It is said that the two Jansen boys had placed the spec- 
 tacle lenses, with which they were playing, at the proper 
 distances apart and were looking through them at the 
 weathercock on the top of a distant church steeple. They 
 were surprised at discovering two things; first, that the 
 weathercock appeared upside down; and, second, it could 
 be seen much more distinctly through the glasses than with 
 the naked eye. Of course, they called the attention of 
 their father to this curious discovery. Jansen, who was an 
 intelligent man, and well acquainted with the properties 
 of lenses as they were known at that early time, constructed 
 a telescope based on the discovery of his sons. 
 
 The story about the Jansen boys is, I believe, generally 
 discredited. It appears there are two other claimants for 
 the honor of the invention; a man named Joseph Andri- 
 
THE TELESCOPE 109 
 
 ansz, and a man named Lippershey. Lippershey is gen- 
 erally credited with this invention. He was a spectacle 
 maker, who, in 1608, by arranging two convex glasses in 
 a tube, produced a telescope by which he was able to see 
 objects at a distance. 
 
 It sometimes happens, in the history of science, that a 
 great discovery is born so much out of date, or so far in 
 advance of a time when it could be appreciated that, like 
 immature fruit, it drops from the tree that bore it without 
 reaching maturity. Fortunately, this was not true with 
 the first telescope; for, it was made during the lifetime of 
 one of the most celebrated Italian philosophers, the great 
 Galileo Galilei. 
 
 Galileo, who happened to be in the town where Lipper- 
 shey had produced his first telescope, called on the inven- 
 tor and examined the instrument with great care. Now, 
 Galileo was probably the foremost astronomer of his time. 
 Eecognising the great aid that would be given to astrono- 
 mers in examining the heavens by the use of the curious 
 instrument that Lippershey had shown him, Galileo made 
 a careful study of its construction, and from his knowledge 
 of optics was able to produce an instrument with two 
 lenses that enabled him to obtain a much better view of a 
 distant body, in an erect instead of an inverted position, 
 than in the Lippershey telescope. The instrument Galileo 
 constructed was the Galilean telescope before referred to 
 that employs a convex lens as an object glass, and a concave 
 lens as an eye-piece. 
 
 Galileo built his first telescope in 1609. It was not 
 much of an instrument, consisting as it did of two lenses 
 placed at the opposite ends of an organ pipe. But this 
 
110 THE WONDEE BOOK OP LIGHT 
 
 little instrument, known as Galileo's tube, produced the 
 most astounding effects over all civilised Europe. 
 
 Being an astronomer, the first thing Galileo did was to 
 turn his telescope to the heavens. Small and clumsy as it 
 was, its penetrating power was much greater than that of 
 the eye, its illuminating power was vastly greater, and 
 even its magnifying power was fairly good. By its use, 
 therefore, he entered a hitherto unknown world in the 
 heavens. Better still, he not only saw the wonderful mar- 
 vels the glass revealed to him, but he understood them, 
 was able to spread the knowledge of these things in lan- 
 guage so simple that a child could understand it. 
 
 You must know that when Galileo was making these 
 wonderful discoveries with the telescope, the knowledge the 
 world had of the solar system, that is, of the sun and the 
 bodies that revolve around the sun and obtain their light 
 and heat from it, was exceedingly vague. The earth was 
 by most regarded as the centre of the solar system. The 
 sun and all the heavenly bodies were believed to revolve 
 around the earth, instead, as of course you know is the 
 case, of the earth revolving around the sun. 
 
 By means of his little tube, Galileo succeeded in showing 
 that these views were entirely erroneous; that the sun is 
 the centre of our system; that the earth and the other 
 planets revolve around the sun. This, after considerable 
 opposition, he succeeded in proving to all fair-minded men, 
 although it got him into considerable trouble. 
 
 Among other objects to which Galileo turned his tele- 
 scope was the moon. Instead of the apparently smooth 
 surface the moon presents to the eye, the glass showed that 
 it was even more diversified with mountains and valleys, 
 
THE TELESCOPE 111 
 
 hills and plains, than is the surface of our earth. He 
 proved that in certain positions, the planet Venus pre- 
 sented crescent shapes like the moon; that the planet Jupi- 
 ter has a number of moons revolving around it, somewhat 
 resembling the single moon our earth possesses. But 
 the great wonder the Galilean telescope disclosed, was the 
 fact that the stars constituting the hosts of heaven, great 
 as they appear in numbers to the unassisted eye, were mar- 
 vellously multiplied when examined with the telescope. 
 This was especially true when he turned his telescope to a 
 part of the heavens known as the Milky Way. As you 
 probably know, that is a part of the sky that on a clear 
 night presents a hazy, misty appearance, as though some- 
 one has spilled milk over the blue vault of heaven. When 
 examined by Galileo's tube, the milkiness disappeared and 
 thousands upon thousands of stars were seen, so far off 
 as only to produce a hazy uncertain light to the naked eye, 
 but seen as separate stars through the telescope. There 
 were, however, here and there in other portions of the sky, 
 dim misty masses known to astronomers as nebulae. 
 
 Galileo now determined to build a larger telescope which 
 by collecting a greater quantity of light would enable him 
 to examine the distant nebulae. This examination was one 
 of the first uses to which he put his new and enlarged 
 instrument. Its penetrating power was sufficient to reveal, 
 instead of the misty, hazy, nebulous light, thousands upon 
 thousands of stars. In still more remote regions there were 
 other nebulous masses that his telescope was unable to 
 resolve. 
 
CHAPTEE X 
 
 THE CAMERA OBSCTJRA, THE PHOTOGRAPHIC CAMERA, 
 AND THE MAGIC LANTERN 
 
 While describing the formation of images by the pas- 
 sage of light into a dark room through a small opening in 
 a shutter, I remarked that, when you had studied the prin- 
 ciples of lenses, you could understand how much better 
 and more distinctly images could be formed with a lens 
 than an aperture. If the dark room is of sufficient size 
 to contain a chair and a table at which one can sit, and if 
 a lens, provided with a mirror for changing the direction 
 of the rays of light, is placed in a suitable opening, the 
 image may be received on the surface of a sheet of paper 
 placed on the table, so that a person, sitting on the chair, 
 can readily sketch or draw the image. 
 
 Since the lens employed is a converging lens, and the 
 illumined object is situated at its longer conjugate focus, ' 
 the image on the paper will be inverted and smaller than 
 the object. Its illumination, however, will be brighter than 
 it would be if the object were viewed directly by the eye. 
 
 Of course, you will understand that, besides a lens, it is 
 necessary to employ a mirror, or some other device, in 
 order to change the direction of the rays of light coming 
 from the distant object, so as to cause them to pass through 
 the image-forming lens. It is not difficult, however, to 
 place an inclined mirror so that this may be done. 
 
 It is possible, however, by employing a piece of glass, 
 
 112 
 
THE CAMERA OBSCtTEA 
 
 113 
 
 shaped as shown in Fig. 50, to combine both a mirror and 
 lens in the same device; for, as shown, the convex surface 
 of the image-forming device acts as a converging lens, while 
 the rays of light, on striking the back of the lens, undergo 
 internal reflection, and thus pass through the lower por- 
 tion of the lens, and emerging, pro- 
 duce an image on a sheet of paper 
 below. 
 
 Of course, the room in which the 
 camera obscura is placed may be 
 large enough to permit a number of 
 people to examine the picture at the 
 same time. I remember a camera 
 obscura that was erected at Asbury 
 Park, ISTew Jersey, a well-known 
 bathing resort on the Atlantic. At 
 the bathing hour, it was beautifid 
 to examine in the dark room the 
 lifelike pictures received on the 
 smooth, whitened surface of a table 
 placed in the centre of the room. Not only could 
 the movements of the breakers be seen, as com- 
 ing in from the ocean they curled over and broke and 
 finally dashed against the beach, but the images of thou- 
 sands of bathers, in all kinds of postures, made an ani- 
 mated and at the same time exceedingly interesting pic- 
 ture. 
 
 So much pleasure can be obtained from a camera ob- 
 scura that I would earnestly advise you to make one. If 
 you do this, try to locate it as near as possible to some 
 place where at times there are apt to be crowds of people. 
 
 Fig. 50. — Combined 
 Image foeming Lens 
 
 AND illEEOB. 
 
114 THE WONDEE BOOK OP LIGHT 
 
 since the pictures thus obtained will have a lifelike air 
 about them that will produce very satisfactory results. 
 
 There is another kind of camera obscura (or as the 
 phrase means, darkened chamber), in which pictures of 
 distant objects are received, and in which such pictures 
 may be copied, not by drawing with lead pencils, etc., as 
 represented in the above figure, but by photography. In 
 this case a sensitive paper prepared in a certain way, and 
 covered with various salts that possess the power of being 
 darkened when light falls on their surface, can be used to 
 make the sunlight do the drawing, in a far more accurate 
 manner than could the most skilled artist. Suppose you 
 are in the darkened chamber of a camera obscura. Instead 
 of permitting the image of distant objects to fall on the 
 table on a sheet of common paper, let it fall on the sur- 
 face of a sheet of photographic paper. Then, of course, 
 you would be able to obtain a photograph of the image re- 
 ceived on the table; for that image consists of various 
 patches of light of varying degrees of intensity. Where 
 the greater quantity of light falls, there is produced a 
 more decided blackening of the. paper. The dark portions 
 are hardly affected at all, while the portions of intermedi- 
 ate brilliancy are darkened to different degress. Conse- 
 quently, there will be produced on the paper a series of 
 light and dark patches, arranged in the reverse order of the 
 light and dark portions of the objects; for all the light 
 spots have darkened the paper, while the dark spots have 
 left it unaffected, and, therefore, white. Such a picture 
 is called a photographic negative, because the lights and 
 shadows are the reverse of what they are in nature. 
 
 If you use a camera obscura as a photographic camera. 
 
THE CAMERA OBSCUEA 115 
 
 in which the picture is printed " while you wait," the walls, 
 ceiling and floor of the room must be blackened, so as not 
 to reflect or diffuse light on the picture. Indeed, this 
 should always be done in order to obtain the most satis- 
 factory results in a camera obscura. Moreover, you must 
 afterwards subject the sheet of photographic paper to a 
 process known as fixing, the nature of which will be 
 explained in a subsequent chapter on photography. Unless 
 the picture is "fixed" the entire surface of the paper 
 would be blackened when it is taken out of the darkened 
 room and exposed to the light. 
 
 But a camera obscura would be too awkward for use as 
 a photographic camera. It is more convenient to carry 
 the photographic camera to the object than to permanently 
 fix it in one place, when of course you could only take 
 photographs of outside objects at that place. Nor is it 
 necessary to make the darkened room large enough to hold 
 a table for the support of the photographic plate, or big 
 enough to permit people to enter the rooin, while the pic- 
 ture is being taken. On the contrary, the photographic 
 camera is generally so small that it can be packed in an 
 exceedingly limited space. 
 
 The photographic camera is a ridfeulously simple piece of 
 optical apparatus when you one« thoroughly understand its 
 construction. It consists merely of a light-tight box with 
 a lens so placed in an opening in One of its sides directly 
 opposite a place provided for the reception of a photo- 
 graphic plate, as to have the image fall sharply on its sur- 
 face. There are a Dumber of devices provided in order 
 so to adjust the iffiage-fonning lenses that a sharply fo- 
 cused image shall fall directly on the sensitive surface of 
 
116 
 
 THE WONDEE BOOK OP LIGHT 
 
 the plate. Moreover, means must be provided for examin- 
 ing the image, before the sensitive plate is introduced into 
 the camera, in order to see that the focusing has been prop- 
 erly done. Then, too, there must be means for the intro- 
 duction into and the subsequent removal of the photo- 
 graphic plate from the camera, so that no light can fall on 
 its surface except what it receives from the image-forming 
 lens. 
 
 Fig. 51 represents a photographic camera mounted on 
 the usual tripod. The apparatus consists of a darkened 
 
 
 ^- i 
 
 Fig. 51. — Photogbaphic Camera with Geound Glass Plate in 
 Position. 
 
 box or chamber C, provided at one end with a double con- 
 vex lens A, and at the other end with a readily removable 
 frame or box B, placed immediately opposite the lens. 
 This frame is provided with a plate of ground glass E, in- 
 tended to receive the image of the lens A, when sharply 
 focused on it by a focusing screw D, placed as shown. 
 
 The inside of the box of the photographic camera must 
 be made absolutely light-tight. The only light that is per- 
 
THE CAMEEA OBSCUEA 117 
 
 mitted to enter is that which passes through the image- 
 forming lens A, for the purpose of forming the image. 
 Should only a small quantity of light enter from any other 
 part of the box, it will act on the photographic plate and 
 completely ruin the picture. 
 
 It will be observed that in all eases the objects that 
 are being photographed are situated outside the camera 
 at the longer conjugate focus of the image-forming lens, 
 so that the image is always much smaller than the object 
 and is received in an inverted position on the photographic 
 plate. 
 
 In order to see the image clearly, whUe focusing the 
 instrument, it is necessary to cover the back of the box 
 with a dark cloth. You who have probably often seen a 
 photographer focussing the image in a photographic 
 camera, will remember how he stood facing the ground 
 glass screen, with a dark cloth so thrown over his shoulders 
 and head as to prevent any light from falling on it from 
 the back of the ground glass screen. In this position he is 
 able clearly to see the inverted image formed on the 
 plate of ground glass, and can so adjust or focus the lens 
 A by moving it towards or from the object, as to produce 
 a sharply defined inverted image. 
 
 The image being sharply focused on the ground glass 
 plate, it is then necessary to introduce into the camera 
 a sensitive photographic plate. This is done by means of 
 a device called a plate-holder, consisting of a light wooden 
 frame, generally containing two sensitive photographic 
 plates, one at the back and one in the front, covered by a 
 light-tight sliding plate of thin hardened black rubber. 
 The plate-holder is slipped into the camera in such a 
 
118 THE WONDER BOOK OE LIGHT 
 
 manner that, when the hard rubber slide is drawn up, the 
 sensitised surface of the plate thus uncovered occupies 
 exactly the same position the ground glass plate did. The 
 image of the lens, being then received on the sensitised 
 plate, produces a photographic picture by means of 
 chemical changes that will be explained in the subsequent 
 chapter on photography. 
 
 As soon as the plate has been properly exposed, a cap is 
 slipped over the image-forming lens, so as to prevent the 
 rays of light from continuing to enter the camera, and 
 the hard rubber plate is slid down, on the plate holder 
 while it is still in the camera, thus permitting it to be safely 
 removed. 
 
 I need not tell you that you must be careful to remem- 
 ber which side of the plate-holder you have already ex- 
 posed. Should you forget and expose the same plate 
 twice, when you come to develop the plates, you will learn 
 two things, neither of which is calculated to please you. 
 One of these is that of the two plates in the plate-holder, 
 one contains no photographic picture whatever, while the 
 other plate contains two different pictures, each of which 
 can be distinctly seen, though each mars the other. To 
 make this the more provoking, it too frequently happens 
 that these double pictures are the best exposures that you 
 have made; like the famous fish, the biggest and best, 
 you would have caught had it not unfortunately escaped 
 from your hook just as jou had almost landed it. 
 
 Photographic plates are now made so sensitive as to 
 produce excellent pictures by an almost instantaneous ex- 
 posure. It would, therefore, be impossible properly to 
 uncover 4he-. photographic lens by slipping a cap off its 
 
THE CAMEEA OBSCTJEA 
 
 119 
 
 face and then covering it again by replacing the cap. 
 You could not do this quickly enough. This would result 
 in an overexposure and therefore an unsatisfactory nega- 
 tive. All first-class cameras are now provided with a device 
 operated by compressed air, that, on the pressing of a 
 small India rubber ball filled with air, automatically 
 opens and closes the diaphragm or shutter placed in front 
 of the lens in a very small fraction of a second. The 
 click you hear on exposing a Kodak, or other camera, is 
 
 Fig. 52. — Ceoss Section of Photogbaphic Camera. 
 
 caused by the opening and closing of this pneumatic 
 shutter. 
 
 A cross section of a photographic camera is shown in 
 Fig. 52. Here, the image-forming lens, instead of consist- 
 ing of a single converging lens, is formed of two lenses 
 L and L', each of which is formed of two difEerent kinds 
 of glass, and is known as an achromatic lens. A camera 
 with such lenses is capable of producing a much more 
 satisfactory photograph than one employing a single lens 
 only. 
 
 Another form of optical apparatus that requires but a 
 single lens for its operation is a magic lantern. This is 
 
130 
 
 THE WONDBE BOOK OF LIGHT 
 
 an apparatus with which I imagine many, if not most, of 
 my readers are familiar. It consists of means for thor- 
 oughly illuminingi an object, generally a photograph, 
 that is, preferably, coloured with transparent colours and 
 placed before a convex lens at its shorter conjugate focus. 
 The image of this object is received on a distant screen 
 in a darkened room where it can be seen at the same time 
 by many people. Since the object is always placed before 
 the image-forming lens in an inverted position, the image 
 
 Fig. 53. — Magic Lantebn. 
 
 received on the screen, at its longer conjugate focus, is 
 both magnified and erect. 
 
 In order that the object shall be thoroughly illumined, 
 a convex lens, known as a condenser, is placed between 
 the source of light and the object in order to throw a large 
 quantity of light on it. 
 
 A common form given to the magic lantern is repre- 
 sented in Pig. 53. The image-forming lens is placed in a 
 horizontal tube attached to the front of the concave 
 reflector A so as to throw a strong light on an object 
 
THE CAMEEA OBSCUEA 131 
 
 placed on a slide at V, at the shorter conjugate focus of 
 the image-forming lens. 
 
 Fig. 54 represents the arrangement of lenses and the 
 concave reflector of this apparatus. The image-forming 
 lens is placed at C, and the condensing lens at B, in front 
 of the reflector A, while the picture slide, generally a 
 photograph, is placed at V so as to receive light on its 
 surface. 
 
 The magnifying power of a magic lantern is equal to 
 the distance of the image-forming lens from the screen 
 divided by the distance of the lens from the object. If, 
 for example, the screen is 100 times further from the 
 object than the image-forming lens, the magic lantern 
 
 ^Cl||v^ 
 
 Fig. 54. — Abbangement of Lenses and Reflectoe in Magic 
 Lantebn. 
 
 will have a magnifying power of 100 diameters. It is 
 evident, therefore, that in order to obtain a great magnify- 
 ing power in magic lanterns the focal length of the image- 
 forming lens should be small, and the screen fairly distant 
 from the lens. 
 
 Although I have spoken of the image-forming lens of 
 the magic lantern as consisting of a single lens, yet in fine 
 instruments this lens is formed of a number of lenses, 
 since in this manner it is not only possible greatly to de- 
 crease the focal length, and by obtaining a wide lens 
 to permit the passage of a considerable quantity of light, 
 but it is also possible to obtain images of objects that are 
 practically devoid of false colourations and false outlines. 
 
122 
 
 THE WONDER BOOK OP LIGHT 
 
 Where a considerable magnification of an exceedingly 
 small object is desired, a form of magic lantern known 
 as a solar microscope is employed. This is called a solar 
 microscope because the source of light it employs consists 
 of a beam of sunlight reflected from the surface of a mir- 
 ror. This instrument, however, does not differ in its 
 image-forming lens from an ordinary magic lantern, ex- 
 cept that this lens, possessing a very small focal length, 
 
 Fig. 55. — Solae Microscope. 
 
 causes a great magnification of the object that is being 
 examined. 
 
 Eig. 55 represents a well-known form of solar micro- 
 scope. Here, the microscope is attached as shown to a 
 plate placed over an opening in a shutter of a room. The 
 light employed is a beam of sunlight reflected from the 
 surface of a mirror M, placed on the outside of the room 
 and inclined so as to throw a beam of light directly into 
 the tube containing the condenser and the image-forming 
 lens. 
 
THE CAMEEA OBSCUBA 
 
 123 
 
 The direction taken by the beam of light as it is reflected 
 from the mirror and passes through the apparatus is repre- 
 sented in Fig. 56. As the beam is reflected from the 
 mirror M it enters the condensing lens, from which it 
 is thrown on the surface of the small object that is to 
 be magnified, and its image is received on the distant 
 screen. You will imderstand that the condenser em- 
 ployed in this instrument does not consist of a single 
 lens 1, but an additional lens placed as shown near the 
 object. The object to be magnified is placed between two 
 
 Fig. 56. — ^DiEECTioN or Light Through Solas Micboscope. 
 
 parallel plates of glass at m^ and is held firmly between 
 them in position in any suitable manner. The image-form- 
 ing lens employed consists of a number of lenses that form 
 the magnified image of the object on the distant screen. 
 Focusing screws C and D, are provided so as readily to 
 obtain a sharply magnified image on the screen. 
 
 The use of a beam of sunlight, for the illumination of 
 the solar microscope, affords a strong and steady light, 
 free from false colouration and, therefore, far more sat- 
 isfactory than any artificial light. In order that sunlight 
 
134 THE WONDER BOOK OF LIGHT 
 
 may be employed for this purpose, it is necessary that 
 means be provided to prevent the motion of the earth on 
 its axis, from changing the direction of the beam of light, 
 otherwise the beam would fail continually to pass through 
 the apparatus, and would require a constant and annoying 
 necessity for adjustment. This is accomplished by means 
 of a device that so alters the inclination of the mirror 
 as to maintain constant the direction of the ray reflected 
 from its surface, so that it will always pass directly through 
 the instrument. A clockwork is employed to alter the 
 inclination of the mirror, in a manner similar to that 
 already described in the equatorial mounting of the astro- 
 nomical telescope. In this way the mirror is always pointed 
 so as to cause the sunlight continually to pass through the 
 lenses, notwithstanding the rotation of the earth on its 
 axis. 
 
 There is another difficulty that must be overcome in 
 the use of sunlight for illumining the objects observed 
 in the solar microscopes. The sunlight is accompanied by 
 heat, and this heat is sufficient in amount possibly to ruin 
 the object that is examined. This difBculty, however, 
 can be removed by causing the beam of light to pass 
 through a strong solution of alum in clear water. In this 
 way the heat is absorbed by the solution and the light 
 permitted to pass through without any appreciable loss in 
 intensity. 
 
 The solar microscope, or, as it is also called, the pro- 
 jecting microscope, possesses the advantage of being able 
 to exhibit the microscopic details of a small object to a 
 large audience. It is able, for example, to throw on a 
 large screen the highly magnified images of the animal- 
 
THE CAMEEA OBSCUEA 
 
 135 
 
 culse that may exist in a drop of water. You may prob- 
 ably have seen at popular lectures a drop of water, highly 
 magnified, in which appear all sorts of horrible monsters. 
 The impression created that this is a drop of ordinary 
 water is far from correct. Our actual drinking water, 
 
 Fig. 57. — Photo-Electbic Miceoscope. 
 
 goodness knows, is at times bad enough without making 
 it any worse, but by no means does it begin to come near 
 this " faked " drop of water that is thus masquerading 
 as ordinary water. It has often been secured only after 
 great effort on the part of the so-called " scientist " who 
 usually exhibits it. 
 
126 THE WOKDEE BOOK OP LIGHT 
 
 Since, of course, the solar microscope is of no use dur- 
 ing cloudy weather, or at night, some other powerful 
 source of light must then be employed, when minute objects 
 are to be greatly magnified. Eor this purpose the photo- 
 electric microscope is employed. This differs in no respect 
 from the solar microscope except that, instead of using 
 a beam of sunlight, a powerful light known as the electric 
 arc light is employed. 
 
 A photo-electric microscope is represented in Fig. 57. 
 Here the source of light employed consists of a brilliant 
 bow or arc of light formed by passing a powerful electric 
 current between two vertical carbon rods placed at c, a, as 
 shown. The light from the carbon are, which by the way 
 is the same as the arc lights that are employed for the 
 illumination of streets, not only falls directly on the ob- 
 ject that is to be magnified, but by passing through the 
 condensing lens placed in the tube at D, is also reflected 
 onto the surface of the condenser from a reflecting mirror 
 m, placed back of the object. In this manner, the pencils 
 of light passing through the tube of the instrument DB 
 strongly illumine the object and permit its highly mag- 
 nified image to be thrown on the distant screen by means 
 of the combination of image-forming lenses shown at A. 
 
CHAPTEE XI 
 THE HUMAN EYE 
 
 By far the most important optical instrument to us is 
 the eye, since by it we are enabled to see the world of matter 
 that lies outside us. As an optical instrument, the human 
 eye resembles a camera obscura. It consists of a dark 
 
 ti yl" .t ctL . 
 
 Fig. 58. — Longitudinal Section of the Human Eye. 
 
 chamber provided with a converging lens placed in an 
 openiag in the front immediately opposite a screen that 
 receives the images of objects situated outside the eye. 
 In Fig. 58 the general structure of the human eye is 
 127 
 
138 THE WONDER BOOK OF LIGHT 
 
 represented as it would appear if cut in two vertically. 
 The eye consists of a nearly spherical ball, somewhat less 
 than an inch in diameter, that is able to turn in any 
 direction in its socket. Although the eyes of some people 
 appear much larger than those of others, yet this is not 
 so much due to difference in size, as to the differences in 
 the amount of the ball that can be seen projecting beyond 
 the lids. 
 
 With the exception of the opening in front, the ball of 
 the eye is opaque and consists of a coat known as the white 
 of the eye, or the sclerotic coat. It is represented in the 
 above figure at H. The front portion of the sclerotic coat 
 is transparent, and forms what is called the cornea. This 
 is shown in the above figure at A. The cornea has the 
 shape of an unusually convex watch crystal. 
 
 Immediately back of the cornea is an opaque coloured 
 portion called the iris. The iris is circular in shape and is 
 provided at its centre with a circular opening called the 
 pupil, that acts as a diaphragm. Instead of being clum- 
 sily operated by the hand, as is the diaphragm of most opti- 
 cal instruments, the iris is automatically operated by the 
 amount of light that falls on it. When it receives more 
 light than usual, the iris automatically decreases the size 
 of the pupil ; or, in other words, the pupil contracts, shut- 
 ting out the light to such an extent as to ensure the proper 
 illumination of the image. When, however, the amount 
 of light falling on the iris is smaller than usual, the pupil 
 automatically dilates and thus increases the amount of 
 light that is permitted to enter the eye. These movements 
 are beyond the will of the person, and are automatically 
 determined by the amount of light that falls on the iris. 
 
THE HUMAN EYE 139 
 
 They are of great value, since an object cannot be seen 
 distinctly unless its image is illumined by the proper quan- 
 tity of light. If the amount of light is either too large, 
 or too small, the image will appear blurred and indistinct. 
 
 The iris takes its name from the fact that it forms 
 what is called the coloured portion of the eye. In some 
 cases, the iris is black, in others, brown, gray, or various 
 depths and shades of blue. That you may surely know the 
 portion of the eye called the iris, I suggest you try the 
 following experiment that will enable you not only to ex- 
 amine the iris of one of your eyes, but also both to see 
 the position of the pupil, and at the same time to observe 
 the wonderful manner in which it is able automatically to 
 dilate and contract, as the amount of light falling on the 
 iris varies. 
 
 The name iris, by the way, has been given to this part 
 of the eye from Iris, the swift messenger of the gods, who 
 was afterwards personified as the rainbow. 
 
 Hang a small looking glass on the wall immediately be- 
 low a gas bracket. Carefully examine the colored portion 
 of either of your eyes by looking at the image formed in 
 the glass, and note particularly the extent of the pupil's 
 opening. Now, turning the light down to the smallest 
 amount that will still permit you to see the pupil, note the 
 wonderful manner in which the pupil dilates or increases 
 in diameter. Then turn the light up and observe how the 
 pupil contracts; and then remember the wonderful optical 
 instruments you possess and be careful you do not abuse 
 them, for they are the only eyes you will ever get. 
 
 Immediately back of the pupil is a lens E, known as 
 the crystalline lens. As you can see from the figure, this 
 
130 THE WONDER BOOK OF LIGHT 
 
 lens is more curved or conTex on the back than in front. 
 It consists of a number of concentric layers of transparent 
 material, like the successive coatings of an onion. These 
 layers gradually increase in density from the outside to- 
 wards the centre, and thus decrease the tendency of the 
 lens to produce false outlines in the images it forms; 
 otherwise, the rays that fall on its outside portions would 
 converge too much. 
 
 Between the cornea and the crystalline lens is a cavity 
 divided by the iris into two portions or chambers that are 
 filled with a watery liquid called the aqueous humour. 
 These chambers communicate through the opening of the 
 pupil. A larger chamber or cavity L, back of the crys- 
 talline lens, is filled with a transparent jelly-like material 
 known as the vitreous humour. This chamber is enclosed, 
 except at the front, by an opaque coating I, called the 
 choroid coat, that is rendered intensely black by means of 
 a black colouring matter. 
 
 On the inside of the choroid coat, on a small space op- 
 posite the crystalline lens, is an exceedingly sensitive por- 
 tion of the eye called the retina. The retina acts as a 
 delicate curtain or screen that is provided for the recep- 
 tion of the images formed by the lenses of the eye. It 
 consists of a network of the fibres forming a continuation 
 of the nerve of vision, known as the optic nerve, which is 
 represented at M as entering the eye in the position shown. 
 The nerve spreads over that portion of the eye immedi- 
 ately above it, as a delicate network of terminals that 
 receive the images formed by the eye and then transmit 
 them to the brain. This screen has received its name of 
 retina from a Latin word rete, meaning a net. 
 
THE HUMAX EYE 131 
 
 Although as I have described the human eye it may seem 
 to be a somewhat complex apparatus, as indeed it truly is, 
 yet it consists practically of a darkened box or chamber, 
 provided in front with an opening for the insertion of 
 the image-forming lenses, and, nearly opposite the open- 
 ing, of a screen for receiving the images thus formed. 
 Although in reality all the media of the eye — the cornea, 
 the aqueous humour, the crystalline lens, and the vitreous 
 humour — act as a succession of lenses that refract or bend 
 the rays of light while passing through the eye so as to 
 bring them to a sharp focus on the retina, yet they prac- 
 tically act together as a single converging lens. The 
 crystalline lens may be regarded as the principal image- 
 forming lens of the eye; so that, shorn of all its com- 
 plexity, the eye is merely a darkened chamber, or camera 
 obscura, with a lens in front and a screen at the rear, to 
 receive the images of external objects. 
 
 As you know, in most optical instruments, instead of 
 employing single lenses a number of separate lenses are 
 employed. This is exactly what is done in the ease of the 
 eye. Besides the crystalline lens with its concentric layers 
 of materials of varying density, we have the cornea, the 
 aqueous humour, and the vitreous humour, all of which act 
 together as a single lens capable of producing those images, 
 on which our knowledge of the world of matter outside of 
 us is so dependent. 
 
 I need hardly tell you that, since the ball of the eye 
 is only about an inch in diameter, and the principal lens 
 that produces images on the retina is situated inside the 
 eye, all objects seen by the eye are necessarily situated 
 much further from the crystalline lens than the retina is. 
 
133 THE WONDEE BOOK OF LIGHT 
 
 In other words, the objects seen by the eye are situated at 
 its longer conjugate focus. Consequently, the images re- 
 ceived on the retina must be situated at its shorter conju- 
 gate focus, and must, therefore, be much smaller than the 
 objects themselves. They must, moreover, be inverted, and 
 must be formed on the retina in an upside-down position. 
 
 Certain conditions are necessary in order that the image 
 formed on the retina shall be able to produce an impression 
 on the brain so as to be distinctly seen. Unless this image 
 is sharply focused on the retina, the object will either be 
 invisible, or will be seen so indistinctly as practically to be 
 unrecognisable. Now, since the eye can see objects when 
 situated at distances varying from many miles to a few 
 inches only, it is evident that, unless the eye were provided 
 with some kind of focusing apparatus, there would be but 
 a single distance in front of it where objects could 
 be distinctly seen. Either beyond or nearer than this 
 distance, all objects would be indistinct. A person would 
 not know what was before him until he had come within 
 a certain distance of the object. You can readily under- 
 stand how such an incomplete and insufficient optical in- 
 strument would limit a man's ability to see what was going 
 on around him and thus necessarily _ seriously retard his 
 development. 
 
 Fortunately, man has been provided with so wonderful 
 an optical apparatus that he is able literally in the twink- 
 ling of his eyes (for it is by twinkling his eyes that he 
 focuses them)- distinctly to see either distant or near ob- 
 jects with almost equal distinctness. Let me explain to you 
 now how this wonderful change is brought about. 
 
 If, when you have been looking at some distant object) 
 
THE H^JMA^T EYE 133 
 
 you suddenly direct your eyes to a nearer object, you will 
 become conscious of making an effort to see that object 
 distinctly, and in a very short time you will find yourself 
 able to see it distinctly. Now, what you are doing is this. 
 You are doing something with your eyes that wiU permit 
 the image of the nearer objects to fall sharply on the retina. 
 In ordinary optical instruments there is only one way in 
 which it is possible to do this; i. e., by focusing the lenses 
 and so varying their position until the image of the object 
 is brought into the desired position. It was at one time 
 believed that what the eyes do, when making changes that 
 enable them to see a nearer object very distinctly, is actually 
 to push the crystalline lens to-and-fro through more short 
 distances, and so enable it to bring its image sharply on the 
 retina. It is now, however, known positively that what is 
 done, when an effort is made to accommodate the eyes for 
 different distances, is actually to change the amount of cur- 
 vature or convexity of the crystalline lens. This power 
 of " accommodation," as it is called, is very wonderful, and 
 is far beyond anything that man has been able to contrive. 
 Let us now see how the human eye acts on the diverging 
 pencils of light coming from distant objects, so as sharply 
 to focus their images on the retina. Suppose that AB, 
 Fig. 59, represents a distant object, and that diverging 
 pencils of light, coming from the extremities of this ob- 
 ject, enter the eye by passing through the cornea, which, as 
 you remember, is set like a watch crystal in an opening in 
 the front of the ball of the 63^0. This pencil of light tinder- 
 goes refraction not only while passing through the cornea, 
 but also while passing through the aqueous humour, the 
 srystalline lens, and the vitreous humour. Now, if the eye 
 
134 THE WONDEE BOOK OF LIGHT 
 
 lias been properly focused by accommodation, for the dis- 
 tance of the object AB, there will be formed a sharply 
 focused image la, on the retina. This image, as we have 
 seen, must be both smaller than the object and inverted. 
 It probably seems odd that all images are received in an 
 inverted position on the retina. That this is actually so 
 can be proved by taking the eye of a recently slaughtered 
 ox, cutting away the sclerotic and choroid coats, so as to 
 expose the retina. On placing this eye before an opening 
 
 Fig. 59. — ^Action of Ete on Pencils of Light. 
 
 in the shutter of a darkened room, an inverted image of 
 the distant object can then be distinctly seen on its retina. 
 I will not attempt to explain to you just why it is that 
 we do not see images of external objects upside-down. In- 
 deed, I doubt very much whether any one could give a 
 satisfactory explanation of this curious fact. With many 
 others, I am disposed to believe that the real explanation is 
 that we actually see objects upside-down, but that, having 
 been accustomed to doing this all our lives, we do not 
 perceive anything strange about such images. Should we 
 see an object right side up, which can be done by means of 
 a simple experiment with a lens, we would probably declare 
 it to be upside-down. 
 
THE HUMAX EYE 
 
 135 
 
 I know that phj-siologists generally believe that it is by 
 means of a number of branchings and inversions of the 
 optic nerve that the brain is enabled to receive the impres- 
 sion of an erect image. But for my part I gravely doubt 
 the correctness of this explanation, and am by no means 
 alone in this opinion. 
 
 The explanation as to the branchings of the optic nerve 
 regards the brain as another eye examining the image. I 
 believe that the image on the retina so acts on the. brain 
 as to give it the impression of external objects in the 
 same position as regards our bodies, as that our sense of 
 touch informs us of. Having thus become acquainted by 
 
 Fig. 60. — Visuai, Angle. 
 
 touch with a certain position of the object seen, we are 
 afterwards enabled to determine this by the retinal images 
 alone. 
 
 But the human eye not only distinguishes or sees distant 
 objects. It is also capable of forming an idea of their 
 relative size and distance. Let us endeavour to understand 
 how this is possible. The angle under which an object is 
 seen, or the angle formed by straight lines drawn from the 
 centre of the crystalline lens to the extremities of the ob- 
 ject, is called the visual angle. This angle, as represented 
 in Fig. 60, is the angle AOB. As the visual angle de- 
 creases, the apparent size of the image decreases. In the 
 above figure, as the distance of the object increases, the 
 
136 THE WONDEE BOOK OF LIGHT 
 
 visual angle becomes smaller so that the image A' B' is 
 smaller than that at AB, 
 
 But we look at objects, not with a single eye but with two 
 eyes. There is, therefore, another angle that must be taken 
 into account; namely an angle known as the optic angle. 
 This angle is the angle formed by a straight line passing 
 through the centre of the pupil and the crystalline lens 
 in each eye to the object itself. It is evident that the 
 greater the distance of the object from the eye the smaller 
 will be the value of this angle. 
 
 Our ability to estimate the distance and size of objects 
 depends on a variety of circumstances, the principal of 
 which are the values of the visual angle and the optic 
 angle, the comparison of objects of known size with the 
 objects examined, as well as the differences in the distinct- 
 ness of the image, as influenced by the details of the ob- 
 jects that can be seen after the light has passed through a 
 greater or iless depth of air due to the distance of the 
 objects. 
 
 Suppose, for example, you are looking at a distant ob- 
 ject, the relative dimensions of. which are unknown to 
 you. If, by chance, you happen to see in the image some 
 object such as a man or a horse, whose usual size is known, 
 your mind immediately makes an estimate of the size of 
 surrounding objects by comparison with the dimensions 
 of the man or the horse. You say unconsciously to your- 
 self that the object at which you are looking is so many 
 times the height and breadth of the man or the horse. 
 
 Another way in which the mind estimates the distance 
 and size of an object is by the amount of conscious effort 
 that is made in so directing the two eyes to the object 
 
THE HUMAX EYE 137 
 
 as to be able to see it distinctly. In other words, the 
 estimation of size and distance is by means of the value 
 of the optic angle. 
 
 Again, the distinctness of the image as regards the de- 
 tailed parts that can be clearly seen, forms a very com- 
 mon method by ■which the eje estimates the distance of 
 objects. If one vrho has been accustomed nearly aU his 
 life to viewing objects through the comparatively dusty air 
 generally existing near the level of the sea, views them 
 through the clearer air of the higher mountain regions, or 
 through the wonderfully clear air that so frequently char- 
 acterises the Arctic and mountainous regions generally, 
 the details of the visual pictures are so wonderfully clear 
 that an erroneous estimate is apt to be made of the dis- 
 tance of such objects. Indeed, they are frequently Judged 
 to be but a few miles distant when their actual distance 
 may be thirty or forty mUes. 
 
 It is important to note in this connection the case of 
 people who have been blind from birth by reason of an 
 opaque crystalline lens or cataract. Skilled surgeons are 
 now able in almost every case to restore such persons to 
 sight by the removal of this lens. When the eye has healed 
 after the operation, by employing convex spectacles the 
 loss of the crystalline lens is compensated for, and normal 
 vision is secured. Although the images are then properly 
 formed on the retina, yet such people are iavariably unable 
 to estimate size and distance until they have had some 
 experience in learning how to use their restored sight. 
 
 In discussing the magnifjdng power of lenses, attention 
 was called to the fact that in order to obtain the most 
 distinct vision, the object examined must be situated at 
 
138 THE WONDEE BOOK OF LIGHT 
 
 a certain distance from the eye, and that this distance, 
 though varying in different people from ten to twelve 
 inches, could, perhaps, be taken on the average at a dis- 
 tance of ten inches from the eyes. It will be interesting 
 here to note some other conditions that are necessary in 
 order to ensure the most distinct vision. 
 
 In the first place, in order to see an object distinctly, 
 it is necessary that the object be situated at such a dis- 
 tance from the eye as to produce a sharply focused image 
 of some appreciable size on the retina. The image formed 
 on the retina is always very small, but, unless it is of suf- 
 ficient size to cover an appreciable portion of the nerve 
 fibres, it can not be transmitted to the brain as a distinct 
 image. In other words, it is necessary that a certain area 
 of the retina shall have the light fall on it; otherwise no 
 image will be seen. 
 
 In the next place, the image on the retina must be 
 properly illumined. If the amount of light is insuflBcient 
 in quantity, the image is not transmitted to the brain. 
 If the amount of light is too great, the image transmitted 
 will appear blurred and indistinct. 
 
 The distance at which an object can be seen depends 
 not only on the amount of illumination but also on the 
 colour of the object. When illumined by full sunlight, a 
 white object can be seen by normal eyes at a distance of 
 about 17,350 times its own diameter. If lighted by diffused 
 daylight, it can be seen at only about half this distance. 
 A blue coloured object is visible at a still smaller distance. 
 
 The distance an object can be distinguished is greatly 
 influenced by its colour as well as by the colour of the 
 background against which it is seen, the object being 
 
THE HUMAX EYE 139 
 
 seen at the greatest distance when the contrast between 
 its colour and the colour of the background is the most 
 marked. It is for this reason that different colours are 
 emploji^ed for signal flags for use at sea and for use on the 
 land. Eed, yellow, and blue flags may be employed at sea, 
 according to whether the background is the blue of the 
 sky or water, or whether it is the dull grey of the clouds 
 or of the rafBed water surface. In the same way, the 
 colours employed for railroad signals are frequently red, 
 green, and white. These colours can be seen with different 
 degrees of distinctness against the green background of 
 the foliage during summer, the white covering of the snow 
 in winter; or, in places, the dull yellow colour of the un- 
 cultiYated ground. 
 
CHAPTEE XII 
 
 SPECTACLES AND EYE-GLASSES. SOME 
 PECULIARITIES OF EYESIGHT ' 
 
 Although, as I have pointed out on several occasions 
 in the preceding chapters, the human eye is a wonderful 
 instrument, yet, in some respects, it possesses errors or 
 faults even in what would be regarded as perfectly normal 
 eyes. At the most, however, these defects are trifling when 
 one considers the uses to which the human eye is put, and 
 the fact that it is an instrument possessing life, and there- 
 fore must, like all our organs, have a condition correspond- 
 ing to the condition of bodily health. 
 
 I have no patience with certain scientific men who are so 
 impressed with their own knowledge and importance that 
 they do not hesitate to cast a slur on the All-Wise God, 
 who not only created man, but also planned the human 
 eye and gave it to him for the acquisition of knowledge 
 from the physical world that exists outside of him. I 
 especially remember reading a statement made by a very 
 distinguished scientist who declared that if a workman had 
 sent him so imperfectly constructed a piece of optical 
 apparatus as the human eye he would have sent it back 
 with a rebuke for the clumsiness of its construction! 
 
 Passing by the almost insufferable conceit of such a 
 statement, I would say that the eye, like all other portions 
 of the human body, represents a structure that requires, 
 
 HO 
 
SPECTACLES AND EYE-GLASSES 141 
 
 for its continued efficient use, both the maintenance of 
 physical health, as well as the utmost care in its use. 
 If, therefore, people, through errors in their own living, 
 or even as the result of errors committed by their ancestors, 
 possess abnormal or diseased eyes, they should accept them, 
 not as the result of bad workmanship, but as the punish- 
 ment that properly results from the transgression of physi- 
 cal laws. In the case of the eye, as with other organs of 
 the human body, it is only by its long-continued and proper 
 use that its functions can be preserved. The criticism, 
 to possess any weight whatever, should not have ignored 
 the eyes of the earlier people, as they more nearly came 
 from the hands of their Creator ; for I have no doubt that, 
 like the birds and other creatures of the woods and air, 
 our early ancestors possessed eyes which, not being injured 
 by incorrect habits of living, were far more nearly perfect 
 optically than are the eyes of most people to-day. 
 
 The images formed by the lenses of normal eyes are 
 sharply focused on the retina. In the abnormal eye, the 
 images fail to be brought to a sharp focus and therefore 
 appear blurred or indistinct. 
 
 There are three principal defects of human vision. In 
 one of these, objects cannot be distinctly seen unless they 
 are brought within six or eight inches of the eyes or nearer. 
 Such eyes are said to be myopic or near-sighted, and are 
 unable to see ordinary objects distinctly across the street, 
 or, indeed, in different portions of a large room. 
 
 The cause of myopia, or near-sightedness, is the too 
 great converging power of the eye, that thus brings parallel 
 or slightly diverging rays to a focus before they fall on 
 the retina. This defect is also due to the excessive length 
 
142 THE WONDER BOOK OF LIGHT 
 
 of the eyeball in the direction of a straight line passing 
 through the centre of th? cornea, the crystalline lens and 
 the retina. In other words, near-sightedness is due to 
 the ball of the eye being longer than that of the normal eye. 
 In order to ensure distinct vision for a myopic or near- 
 sighted eye, a concave lens is employed as a spectacle glass. 
 "Without the use of such lenses it would be necessary to 
 bring the object examined so near that its rays could enter 
 the eye under a considerable divergence, and thus permit 
 the otherwise too greatly converging lens of the eye to bring 
 the image to a sharp focus on the retina. Of course, such 
 
 Fig. 61. — Action of Concave Spectacle Lens. 
 
 a method would be impracticable. It is easy, however, for 
 a myopic eye to be made practically normal by the em- 
 ployment of concave lenses placed in suitable spectacle 
 frames. The diverging powers of concave lenses will 
 cause the rays to diverge, and thus, as shown in the above 
 figure, cause the image to be formed on the retina. To do 
 this, care must be taken to employ concave glasses of suf- 
 ficient diverging power to overcome exactly the too great 
 convergence of the myopic or near-sighted eye. 
 
 Some idea of the manner in which concave spectacle 
 glasses act to overcome near-sightedness may be had from 
 the examination of Fig. 61. Here A represents the position 
 
SPECTACLES AXD EYE-GLASSES 143 
 
 of a small object, and A' the position of its image when 
 seen through a concave spectacle lens placed as shown. 
 
 Another defect of the eye is presbyopia or long-sighted- 
 ness. This defect is common with old people, whose eyes 
 become flattened by reason of a hardening of the crystal- 
 line lens, due to structural changes that decrease the quan- 
 tity of fluid it contains and make its converging power so 
 feeble that the images of objects are brought beyond the 
 retina and are, therefore, indistinctly seen. Such an eye 
 is known as a presbyoptic or far sighted eye; or, since, as 
 above remarked, this defect is common in old people, it is 
 sometimes known as old-sight. 
 
 Far-sightedness, besides being due to the feeble eon- 
 verging power of the crystalline lens, is also sometimes 
 caused by the eye being too short, from front to back. Far- 
 sightedness, which is a common defect in the eye-sight 
 of old people, begins to show itself by their finding it neces- 
 sary in reading a book or a newspaper to hold it much 
 farther from their eyes than usual. When not caused by 
 disease, the defect can be completely remedied by the use 
 of convex spectacle glasses ; for such glasses, as has already 
 been shown, adding to the too feeble converging power of 
 the crj-talline lens, sharply focus the image on the retina. 
 
 In the preceding chapter, I have explained the manner 
 in which the eye is able to accommodate itself for objects 
 at different distances, by producing a change in the cur- 
 vature of the crystalline lens. Now people, when old, 
 do not possess the power to accommodate the eye to dif- 
 ferent distances as readily as they did when young, so that 
 it is necessary for them to employ two sets of glasses, one 
 for far-sight, and the other for reading, or near-sight. 
 
144 THE WOFDEE BOOK OF LIGHT 
 
 In order to avoid the necessity for frequently changing 
 spectacles for distant objects and for reading, a device 
 known as bifocal spectacles is now very generally employed. 
 As shown in Eig. 63, bifocal spectacles consist of a pair 
 of glasses AA, the focal lengths of which are suitable for 
 distant objects, and a pair of smaller lenses hh, cemented 
 by Canada balsam to the lower portions of A, A. Some- 
 times, however, bifocal glasses are formed by grinding the 
 proper surfaces for the reading lenses on the lower sur- 
 face of the long-distance glasses. Sometimes the same 
 
 Fig. 62. — Bi-Focal Spectacle Glasses. 
 
 result is obtained by fusion. This possesses the advan- 
 tage, at least some people think it is an advantage, of 
 not letting others know they are obliged to wear bifocal 
 spectacles. It also possesses the real advantage of avoiding 
 the diflSculties that are frequently apt to occur when a thin 
 film of air works its way between the glass and the Canada 
 balsam that cements the two surfaces together, thus ren- 
 dering the vision indistinct. 
 
 Another defect of eyesight is what is known as astigma- 
 tism. In this defect the rays of light are refracted to a 
 different extent in different directions. In other words, an 
 eye affected by astigmatism is unable to see straight lines 
 unless they extend in certain directions. Sometimes, for 
 example, such eyes can only see straight horizontal lines, 
 and are unable to see distinctly lines extending in a verti- 
 
SPECTACLES AND EYE-GLASSES 145 
 
 cal direction. Other eyes, however, can distinctly see the 
 vertical lines but are unable to see horizontal lines. 
 
 Astigmatism is caused by differences in the length of 
 the eye. When a perfectly normal eye looks at two straight 
 lines that cross each other at right angles, both the horizon- 
 tal and vertical lines can be seen with equal distinctness. 
 If, however, the eye is astigmatic, then one of the lines only 
 is seen distinctly ; the other is blurred. In other astigmatic 
 eyes, however, the horizontal lines will be distinctly seen 
 and the vertical lines will appear blurred. 
 
 The defect of astigmatism may exist in any direction be- 
 tween the horizontal and the vertical. Various means are 
 employed for determining the amount and the direction 
 of the astigmatism. One of the commonest consists of the 
 use of what is known as an astigmatic dial. This con- 
 sists, as represented in Fig. 63, of a number of straight 
 lines radiating from the centre of the dial of an ordinary 
 watch or clock face. When such a dial is used by a person 
 with normal eyes, all these diverging lines can be seen 
 distinctly as straight lines. When, however, the dial is 
 examined by a person whose eyes possess astigmatism, 
 only some of the lines can be seen distinctly, the others ap- 
 pearing blurred or indistinct, thus showing the direction 
 in which the eyes are astigmatic. 
 
 Astigmatism is a defect that can be easily remedied by 
 the use of a cylindrical lens. Glasses for astigmatism 
 should be worn constantly. Since the direction in which 
 the eyes are affected by astigmatism is apt to change during 
 changes in the general health, as soon as people with astig- 
 matism feel that their glasses are failing to permit them 
 readily to see straight lines in all directions, they should 
 
146 THE WONDER BOOK OP LIGHT 
 
 promptly have their eyes tested for the change in its direc- 
 tion. Then the cylindrical lenses may be properly placed 
 on their glasses to overcome this defect. 
 
 There is another defect of the eye known as strabismus 
 
 Fig. 63. — Astigmatic Dial. (L. W. Fox.) 
 
 or " sqnint-eye." This is due to the extraordinary 
 strength of the muscles that control the movements of the 
 eyes in certain directions. Instead of the two eyes heing 
 directed forward, as normally, they are turned in difiEerent 
 directions, very commonly inwards towards the nose. This 
 not only gives an unpleasant appearance to the face, but, 
 what is more to the point, prevents distinct vision. 
 Since this defect is due to excessive strength of the muscles 
 
SPECTACLES AXD EYE-GLASSES 147 
 
 that are employed for turning the eyes in different direc- 
 tions, strabismus can be completely cured by cutting suf- 
 ficiently across those muscles that possess too great strength 
 until they are of equal strength with the others. When 
 this operation is skilfully performed, the eyes entirely 
 lose their squint. It should, of course, be performed only 
 by an expert surgeon. Glasses are sometimes successfully 
 employed to correct this defect. 
 
 The images produced on the retina by the crystalline 
 lens do not instantly disappear when the light causing 
 them is cut off, as, for example, by closing the eyes, or by 
 momentarily holding an opaque object between the eyes and 
 a distant object, but continues or persists for some time. 
 You have probably noticed that, if a lighted stick be rapidly 
 swung around in a circular path, as soon as the motion 
 becomes sufficiently rapid, the lighted portion is no longer 
 seen as a single luminous point but as a continuous circle 
 of fire. In a similar manner, if you carefully watch the 
 drops of falling rain, you will not see them as separate 
 spherical drops but as liquid streams. This is due to the 
 fact that you continue to see the drops in a number of 
 different positions. 
 
 Another example of the persistence of vision is seen in 
 the case of a meteorite rapidly moving across the heavens. 
 It is not seen as a single luminous body but produces a 
 luminous track across the sky. 
 
 Perhaps the best example of persistence of vision is 
 one that you can try yourself by a simple experiment. 
 If you rapidly wink your eyes, that is, open and close them 
 as often as you can in a given time, you can nevertheless 
 see the different objects at which you are looking. In this. 
 
148 THE WONDER BOOK OF LIGHT 
 
 as indeed, in all these cases, the images, by reason of the 
 persistent impressions they make on the brain, do not en- 
 tirely disappear with the light that forms them. 
 
 As we shall see in another chapter, many curious effects 
 result from the fact that images tend to remain on the 
 retina for some time after the light that has caused them 
 has disappeared. Some of these effects seem ialmost 
 magical. 
 
 In order that an image formed on the retina shall be 
 able to produce an impression on the brain, it is necessary 
 that it shall remain for a certain time. It is not enough 
 that an image sharply focused and of a deiinite size shall 
 be formed on the retina. Unless it remain there for a 
 sufficient time, it is unable to produce any effect on the 
 brain. 
 
 It is impossible to detect the motion of a cannon ball 
 when viewed at right angles to the direction of its path, 
 since, although by reason of its prodigious velocity the 
 light from it is able to form an image on the retina, yet 
 this image does not remain long enough to produce any 
 effect. If, however, the projectile is moving away from the 
 observer, it is distinctly visible, since then its image re- 
 mains on the retina for a length of time sufficient to enable 
 it to transmit its effects to the brain. 
 
 In a similar manner we are also unable to detect such 
 motion as that of the hour hand of an ordinary clock or 
 watch. On large clock dials, however, the motion of the 
 minute hand can be readily seen, and, as you well know, 
 the motion of the second hand of a watch is still more 
 easily seen. For the same reason it is impossible to see 
 the motion of the heavenly bodies in their orbits. 
 
SPECTACLES AXD EYE-GLASSES 149 
 
 There is a curious defect of some eyes that prevents 
 them from distinguishing certain colours. While such 
 eyes can see some colours, yet they are blind to others. 
 In a few rare instances, people have been found who are 
 colour-blind to aU colours. WhUe they can see coloured 
 objects, yet these images are entirely devoid of colour. To 
 such people the beautiful colours of the natural world are 
 completely invisible, natural objects appearing as differ- 
 ences of light and shade only. Generally, however, colour- 
 blind people are blind to certain colours only, and can 
 distinctly see all the others. For example, some colour- 
 blind people are unable to distinguish between greens and 
 reds. Others, though less frequently, are unable to dis- 
 tinguish between blues and yellows. 
 
 Colour-blindness is sometimes called Daltonism from 
 the fact that the celebrated philosopher, Dalton, who was 
 affected with this disease, was the first to call attention to 
 its existence, as well as to show how its presence could be 
 detected. 
 
 Dalton was unable to distinguish bright crimsons or 
 reds. To him these colours appeared as drabs or greys. 
 You can imagine the surprise and shock this good Quaker 
 occasioned, when he gravely entered his meeting-house with 
 his shapely limbs encased, not in his usual grey, as he 
 fondly thought, but in stockings of the brightest and 
 gayest crimson. 
 
 ■ When present in such persons as railroad men or sailors, 
 colour-blindness is a serious defect; for their duties oblige 
 them to be able readily to distinguish between the colours 
 of signal lights and flags. Their inability to tell readily a 
 green from a red light, or either of these from an ordinary. 
 
150 
 
 THE WONDEE BOOK OF LIGHT 
 
 white light, might easily wreck a train or a ship, and thus 
 cause great loss of life. 
 
 The ophthalmoscope, an important piece of optical ap- 
 paratus, is employed for the examination of the human eye, 
 in determining its healthy or diseased condition. The 
 ophthalmoscope was invented by Prof. Helmholtz, a well- 
 
 FiG. 64. — Hblmholtz's Ophthalmoscope. 
 
 known German physicist, in 1851. As represented in 
 Fig. 64, this apparatus consists of a concave spherical re- 
 flector M, provided at its centre with a circular aperture 
 about one-sixth of an inch in diameter, and with a con- 
 verging lens B that is held in front of the person whose 
 eye is being examined. 
 
 In employing an ophthalmoscope, the person who is 
 being examined is placed in a dark room alongside a 
 lamp provided with a screen E, placed so as to keep the 
 bead in the dark. The observer places his eye at A, im- 
 
SPECTACLES AXD EYE-GLASSES 
 
 151 
 
 mediately back of the opening in the spherical reflector that 
 he holds so as to throw a conical pencil of light into the 
 lens B, held in the other hand, before the eye which is to 
 be examined. In this manner the back of the person's 
 eye is thoroughly illumined, so that it is possible to ex- 
 amine fully its general structure and appearance. 
 
 Although the image of distant objects may be focused 
 sharply on the retina, yet when the illumination is 
 sufficiently bright, the light produces an 
 impression that is not limited to the illu- 
 mined area, but extends some little distance 
 beyond this limit. This peculiarity is 
 known as irradiation, and has the effect of 
 causing the image of an object of a light 
 colour, when seen on a dark background, 
 to appear much larger than it is in reality. 
 For example, the white square represented 
 in Fig. 65, when seen on a black back- 
 ground, appears much larger than the black square on a 
 white background shown in the same figure, although the 
 two squares are exactly of the same size. 
 
 It is irradiation that causes the stars to appear much 
 larger than they really are. In the same way, the brightly 
 illumined crescent of the two- or three-day-old moon, 
 seems to extend beyond the darkened portions and presents 
 the curious appearance of holding the moon in its grasp. 
 
 Fig. 65.— 
 Iebabiation. 
 
CHAPTEE XIII 
 HOW WE SEE SOLID BODIES 
 
 You may naturally ask why it is, since we look at an 
 object with both eyes, and an image is therefore sharply 
 focused on the retina of each eye, that we see a single 
 image only and do not see the two separately. This ques- 
 tion is not an easy one to answer. The explanation is 
 probably as follows : 
 
 The optic nerve, when it comes from both the right and 
 the left lobe of the brain, divides or branches, sending 
 its fibres into the retina of each eye. There is, therefore, 
 a correspondence of the fibres, so that an image formed in 
 either eye produces its effects through the branches from 
 the other eye. The two images thus simultaneously af- 
 fect corresponding points of the brain, and, when the two 
 eyes are properly focused on an object produce a single 
 image only. 
 
 That the retina of each eye is thus provided with 
 branches of the optic nerve which extend to the other 
 eye may be seen by the fact that, when light is permitted 
 to fall on the retina of one eye only, it produces a con- 
 traction of the pupil of the other eye. 
 
 Unless, however, both eyes are properly focused on the 
 object there will be double vision, and not single vision. 
 It is well known that persons who are either drunk or 
 have almost fallen asleep, and have, therefore, lost the 
 power of properly directing both eyes to the same object, 
 
 153 
 
HOW WE SEE SOLID BODIES 153 
 
 invariably see double. Double vision is also sometimes 
 caused by certain diseases of tlie brain or the paralysis of 
 the muscles of the eye. 
 
 But an image of an object received on the retiaa of the 
 right eye is not exactly the same as that which is received 
 from the same object on the xetina of the left eye. Al- 
 though the eye is examiaing a single object, yet if it 
 possesses solidity, or, in other words, if it has length, 
 breadth, and thickness, or extends in three dimensions, 
 then, although generally speaking the image formed ia 
 each eye is the same, yet the image of the right eye will 
 possess details that are wanting in the image of the left 
 eye, and the same thing is true of the image in the left 
 eye. This is an important fact that I wish you to under- 
 stand thoroughly. You may readily assure yourself of its 
 correctness by a simple experiment. 
 
 Hold one of your hands before your eyes, say your left 
 hand, so that when examined by the right eye with the 
 left eye shut you can see the thumb, all of the first finger 
 and part of the middle finger, which hide the other fingers, 
 the palm of the hand and some of the fingers foreshortened. 
 The back of the hand, however, is quite invisible. Xow, 
 keeping the hand in this position, close the right eye and 
 open the left eye. You will now see the thumb, forefinger 
 nearly hiding the others except, however, as you can now 
 see at the back of the hand and some of the fingers back 
 of the forefinger foreshortened. The image of the hand 
 formed by the two eyes is evidently not the same for the 
 right eye as for the left. It is because these images are 
 difEerent that when they are seen by the brain as a single 
 image they possess the appearance of solidity. 
 
154 THE WONDEE BOOK OF LIGHT 
 
 Another simple experiment will, perhaps, throw addi- 
 tional light on this matter. Bend a visiting card in the 
 middle like a triangular roof, and place it before you on a 
 table with a gable end facing the eyes. Then observe it 
 alternately with the right and the left eye, rapidly opening 
 and closing one eye after the other. You will then see 
 the card in two different positions. When seen by the 
 right eye, only one side of the flat roof will be seen strongly 
 illumined and the other side will be in the shadow. When 
 seen by the left eye only, the opposite side will be seen 
 distinctly and its other side will be in the shadow; during 
 these movements the card will seem to move from side to 
 side. We know by experience that such appearances can 
 only be produced by objects that possess solidity, or occupy 
 space in the three dimensions of length, breadth, and 
 thickness. Therefore, when we see such images, the mind 
 properly comes to the conclusion that the objects produc- 
 ing them possess solidity. Indeed, so true is this that it 
 is possible to make drawings on two separate pieces of flat 
 paper that shall represent respectively the appearance 
 presented by the front and the right and left sides of a 
 solid object. If these are placed before the two eyes so as 
 to enable them to be seen simultaneously as a single object, 
 they will appear in a marked manner to possess solidity, 
 although, as we know, the single image is due to the 
 combination of two perfectly flat pictures. 
 
 In order to obtain the correct image of a solid body, 
 it is necessary to observe it with two eyes, so that the two 
 images produced shall differ in some details from each 
 other. Indeed, without binocular vision; i. e., vision with 
 
HOW WE SEE SOLID BODIES 155 
 
 two eyes, perfect vision is, so far as the appearance of 
 solidity is concerned, an impossibility. 
 
 There is also another principle that is employed by the 
 mind in determining the correct appearance of an object. 
 This is as follows : When the images formed by the two eyes 
 are combined into a single image, the mind comes to the 
 conclusion that it is a solid object that is being viewed for 
 the following reasons: It is evident that a solid object 
 may be regarded as consisting of a number of separate 
 points situated at diflEerent distances from the eyes. While 
 being examined, the two eyes are rapidly and insensibly 
 changing their angle of convergence ; or are changing what 
 we have already defined as the optic angle. During this 
 rapid accommodation of the eyes to different distances, the 
 mind is sitting in judgment as to the difference in position 
 of these different portions of the object, and is thus enabled 
 to form an estimate of the amount and character of its 
 solidity. 
 
 If you have followed the above explanation, j'ou wiU un- 
 derstand that, in order to obtain a correct view of a solid 
 object, it is necessary that the object should be examined 
 by both eyes. This is the same as saying that binocular 
 vision, or vision with two eyes, is necessary in order to see 
 a solid object correctly, and since, except when reading 
 and during a few other occupations, all the objects we 
 look at are practically solid, binocular vision is necessary 
 for perfect vision. While, therefore, a person who has lost 
 the sight of one eye, may have fairly good vision, yet such 
 a person can never have vision as complete as if both the 
 eyes were present. 
 
156 THE WONDEK BOOK OF LIGHT 
 
 It can be shown that vision with both eyes possesses the 
 following advantages: 
 
 (1). The field of vision; i. e., the area seen, is enlarged. 
 This is almost self-evident. 
 
 (3). The image possesses a marked appearance of depth 
 because it is seen from two different positions. 
 
 (3). By the perception of the extent it is necessary to 
 converge the two eyes on the object in order to see it dis- 
 tinctly, there is secured a better idea of its distance and 
 size. 
 
 (4). When, as is frequently the case, one eye possesses 
 defects, these defects are to a certain extent compensated 
 by the image formed by the better eye. 
 
 An interesting form of an optical apparatus has been in- 
 vented by Sir Charles Wheatstone by means of which it 
 is possible readily to combine in a single picture the two 
 separate pictures that are formed by each of the eyes, and 
 thus obtain the appearance of solidity. This apparatus is 
 called the stereoscope. If you will give me your close at- 
 tention I think I can make it clear to you how this instru- 
 ment operates. 
 
 The word stereoscope is derived from two Greek words 
 meaning " to see solid." As its name indicates, the stereo- 
 scope is an apparatus by means of which two perfectly flat 
 pictures can be seen by the eye as a single picture that 
 will, therefore, appear to possess solidity. 
 
 Let us suppose that two pictures have been taken of a 
 solid object on two separate sheets of paper, or on two 
 perfectly flat surfaces just as they would appear to the 
 left and right eyes when viewed in the usual manner. One 
 of these pictures is the picture as it appears to the right 
 
HOW WE SEE SOLID BODIES 15? 
 
 eye, and the other, the picture as it appears to the left eye. 
 The two pictures are placed in the box of the stereoscope, 
 as represented in Fig. 66, before two eye-pieces. These 
 eye-pieces consist of two lenses, so constructed that the rays 
 of light coming from the pictures are refracted or bent 
 outward to the same angle. On entering the eye of an ob- 
 server, they appear to come from a single picture placed 
 in the centre of the box. They will, therefore, permit the 
 eye to observe them just as if they consisted of a single 
 picture formed by the combination of the two separate pic- 
 
 
 Fig. 66. — Stereoscope. 
 
 tures, and will, consequently, possess the appearance of 
 solidity. 
 
 Fig. 67 shows the shape of the two eye-pieces or lenses 
 employed in the stereoscope. These eye-pieces are placed as 
 represented with their thinner edges facing each other at 
 AA'. The rays of light coming from corresponding points 
 in the two pictures at PP', after their passage through the 
 lenses at LL' enter the eye of an observer as if they came 
 from a single point 0. 
 
 It is easy to take two photographs of a distant solid ob- 
 
158 
 
 THE WONDER BOOK OP LIGHT 
 
 jeet, by means of two separate cameras placed at the 
 proper distance apart so as to obtain pictures as they would 
 appear when viewed by the right and left eye respectively. 
 By afterwards placing these pictures in the stereoscope, we 
 can obtain an image of a building, or other object, that will 
 
 Fig. 67. — Action of Stebeoscope. 
 
 make it seem to stand out from the paper as if it, possessed 
 actual solidity. 
 
 In taking photographic pictures of buildings, or of land- 
 scapes, the two cameras may be placed on a stand a few 
 feet apart. Unless care is taken not to take the pictures 
 too far apart, an appearance of unnatural solidity will be 
 given to the stereoscope image. 
 
 In the stereoscope, as it was first invented by Sir Charles 
 Wheatstone, the two separate pictures were caused to pro- 
 duce the effect of a single image by means of two suitably 
 placed mirrors. 
 
HOW WE SEE SOLID BODIES 159 
 
 The appearance of solidity that can be obtained from 
 perfectly flat pictures when viewed through a stereoscope 
 is so wonderful that I would strongly urge you to take the 
 first opportunity that may present itself to look through 
 a good stereoscope, for I am sure what you will see will 
 amply repay you for the trouble you have taken. 
 
 The fact that any two pictures may be caused to produce 
 an impression on the eye as a single picture, can be made 
 to afford you and your friends no little amusement. When 
 you get hold of a stereoscope, put a friend's photograph on 
 one of the supports, and on the other a sketch of a cigar, 
 a pair of moustaches, a fool's cap, or what you will. A 
 little practice will enable you to place the drawings or the 
 photographs in such a way that your friend will appear in 
 the stereoscope to be smoking a cigar or wearing mous- 
 taches or a fool's cap. In a similar way, you can combine 
 any number of details and get a variety of amusing results. 
 
CHAPTEE XIV 
 THE NATURE OF LIGHT 
 
 We are now sufficiently acquainted with the principles 
 of light to be able to make some intelligent inquiries into 
 its nature. As you know, the ancient Greeks and Eomans 
 were not well informed concerning natural phenomena. 
 They had some very queer ideas as to the nature of light 
 and the manner in which the eye is able to see distant ob- 
 jects. At one time it was believed that, when one wished 
 to examine a distant object, it was only necessary to send 
 out rays of light from his eyes in such a direction as to fall 
 on the object to be examined. This light, after touching 
 different parts of the object, came back and, entering his 
 eyes again, gave him all the information he desired. Such 
 a ridiculous theory of the cause of light and vision is of 
 course no longer believed. We know to a certainty that the 
 rays of light coming from a distant body are not produced 
 by the eye, but by the object itself, if it be a luminous ob- 
 ject, or by some other source of light if it be an illumined 
 object; that these rays, passing through the lenses of 
 the eye, form a sharply focused image on the retina and so 
 produce an impression on the brain in the manner we have 
 already described. 
 
 I can only take time to discuss the two principal theories 
 that have been proposed to account for the phenomena of 
 light. I am the more willing thus to limit this inquiry 
 
 160 
 
NATURE OP LIGHT 161 
 
 since these theories will give you practically all that is 
 worth hearing about. One of them is now entirely re- 
 jected. The other is now practically believed in by all. 
 
 The theory that is no longer credited was known as the 
 emission or corpuscular theory of light. It was pro- 
 pounded, and ably defended, by Sir Isaac Newton, the great 
 English philosopher. This theory has so many defects that 
 it would have been overthrown much sooner than it was, 
 had it not been for the great genius of Newton, who stood 
 ready to defend it, and to endeavour to meet the many ob- 
 jections that were brought against it. I wish to say here, 
 that in discussing the nature and phenomena of light, as 
 well as in different parts of this Wonder Book, I have 
 availed myself of the excellent order of arrangement of 
 the topics on this subject found in the " Notes on Light," 
 that Prof. Tyndall prepared for the use of those who at- 
 tended his lectures on Light and Electricity at the Eoyal 
 Institution of London. 
 
 The corpuscular or emission theory asserted that light is 
 due to exceedingly small particles or corpuscles that are 
 shot ofE from all luminous or illumined bodies with an 
 almost iaconceivable rapidity. These particles or corpus- 
 cles are assumed to be so small that they are able to pass 
 through the pores of transparent media, such as glass or 
 water, almost as readily as if these media had no exist- 
 ence. The particles, too, were believed to enter the delicate 
 structure of the human eye, and, passing through its va- 
 rious parts, to strike finally against the rettaa without 
 causing the slightest injury or harm. 
 
 One of the most powerfiJ. arguments against the emis- 
 sion theory of light is the enormous velocity with which 
 
162 THE WONDER BOOK OF LIGHT 
 
 light is known to pass from place to place. If tlie cor- 
 puscles possess even the slightest weight, they must neces- 
 sarily destroy the delicate structure of the human eye, when 
 they are hurled against it with the enormous velocity 
 with which light travels. It is a simple matter of calcu- 
 lation that a body, possessing the weight of but one grain, 
 if moving with the velocity of light, would possess a mo- 
 mentum, or quantity of motion, equal to that of a cannon 
 ball weighing 150 pounds and moving with a velocity of 
 1,000 feet per second. Even supposing the corpuscles to 
 possess an almost negligibly small weight, yet if they move 
 with the velocity of light, they would, beyond question, not 
 only shatter the delicate structure of the eye, but would 
 even mow down all objects on which they fall, as would 
 the balls from a battery of guns. We could then no longer 
 speak of the merry sunshine, since even a momentary ex- 
 posure to it would result in sudden death. This objection 
 certainly makes the emission theory of light untenable. 
 
 Another objection that has been brought against the 
 corpuscular theory is connected with the reflection of light. 
 Think for a moment how this theory must explain what 
 takes place when, as in Fig. 68, a ray of light AB falls 
 perpendicularly on a plane mirror at B. As you know, the 
 light is reflected back again in the opposite direction to 
 that at which it struck the mirror, that is, from B to A. 
 
 Now, if light consists of minute corpuscles that are 
 shot off from luminous bodies with the tremendous velocity 
 of light, it would be necessary to believe that these parti- 
 cles immediately lose their tremendous velocity when they 
 strike the mirror and immediately afterwards are thrown 
 off from the surface in the opposite direction with a ve- 
 
XATUEE OF LIGHT 163 
 
 locity that is not in the slightest degree smaller than that 
 at which they struck. On this account, then, the emis- 
 sion theory of light is also untenable. 
 
 But the difficulties do not stop here. In the ease of 
 transparent substances, such as glass or water, only a por- 
 tion of the light is flung oS from the surface. Generally 
 a larger portion enters the medium, and passes through it. 
 Xow if a luminous corpuscle on striking obliquely 
 agaiast a transparent substance, such as a plate of glass 
 or a body of water, is attracted, then a 
 
 it is drawn towards the surface like 
 
 a projectile is drawn towards the * 
 
 earth, so that it should move through 
 
 i 
 
 the refracting medium with an in- g 
 
 creased velocity. But it can be Fig. 68. — ^A Sebious 
 
 shown, by actual measurements, that 2^"^£.™^ . J^„ J°^ 
 
 ' •' ' COEPUSCHLAB THEOBY 
 
 when a ray of light undergoes re- of Light. 
 fraction in a dense medium, such as glass or water, it 
 moves through the medium, not with an increased but with 
 a decreased velocity. On this count, also, we find the emis- 
 sion theory of light is untenable. 
 
 Before the corpuscular theory of light was finally over- 
 throvni, many severe battles were fought between some of 
 the world's most eminent scientific men. Newton's cor- 
 puscular theory of light was upheld by such famous men 
 as Laplace, Malus, Brewster, and Biot. Equally celebrated 
 men such as Huyghens, Euler, the mathematician, Thomas 
 Young, the celebrated English philosopher, and Eresnel, 
 a celebrated French physicist, opposed this theory and 
 asserted that light, like sound, consists of wave motions in 
 a medium. I will not take time to tell you about the ex- 
 
164 THE WONDEE BOOK OP LIGHT 
 
 ceedingly interesting and often heated discussions that 
 took place between these men during this controversy, es- 
 pecially since I have already pointed out the principal ob- 
 jections against the corpuscular theory of light, objections 
 that finally led to its complete overthrow. 
 
 Let us now try to understand the theory of light that 
 has replaced the corpuscular theory, and is now accepted 
 by scientific men all over the world. This is known as the 
 undulatory theory, and teaches that light is caused by a 
 wave motion in the universal ether. 
 
 One of the greatest difficulties that stood in the way of 
 the earlier acceptance of the undulatory theory of light 
 was the enormous velocity light is known to possess. As 
 is well known, the velocity with which light waves are 
 capable of passing through any medium depends on the 
 relations existing between the elasticity and density of that 
 medium. The greater its elasticity, the greater will be the 
 velocity with which the waves will pass through it. Then, 
 too, the smaller its density, the greater will be its velocity. 
 Any medium, therefore, to be capable of transmitting the 
 waves that produce light must not only possess an exceed- 
 ingly great elasticity, but at the same time must have an 
 exceedingly great tenuity. Now a medium possessing these 
 properties is known to exist in what is called the luminif- 
 erous ether, because of its ability to transmit waves of 
 light. The great objection to the wave theory of light, 
 therefore, has been removed. 
 
 It is now believed by practically aU scientists that this 
 ether is a highly elastic medium so extremely tenuous that 
 it can pass freely through such substances as glass, water, 
 or even the densest metals. It is imponderable or possesses 
 
NATURE OF LIGHT 165 
 
 absolutely no weight. You must not suppose, however, 
 that it in any respect resembles the atmosphere of the 
 earth, or any such traces of that atmosphere which exist 
 in any very high vacuum. 
 
 It is called the luminiferous ether because light is trans- 
 mitted through it by means of waves, or to-and-fro mo- 
 tions that take place at right angles to the direction in 
 which the ray of light is advancing. And this is very 
 curious, since, so far as we know, vibrations of this char- 
 acter; i. e., transverse vibrations, are only set up in solid 
 media. The ether is called the imiversal ether because 
 it esists everywhere, not only throughout the space be- 
 tween stars and other heavenly bodies, but also between 
 the very small particles called atoms or molecules of which 
 aU matter consists. 
 
 According to the undulatory theory of light, the small 
 particles or molecules of all luminous bodies are in a con- 
 dition of constant vibration, moving towards and from 
 one another with great rapidity. These to-and-fro motions 
 of the molecules set up in the surroxmdiag ether minute 
 waves that, spreadiag ia aU directions, produce what is 
 known as light. 
 
 It can be shown that light waves in the luminiferous 
 ether vibrate at right angles to the direction ia which the 
 rays of light are advancing. In this respect Light waves 
 difEer from the waves that produce sound, for sound waves 
 move in the same direction as that in which the sound is 
 traveUing, or, in other words, are longitudinal waves; 
 while the vibrations that produce light move at right an- 
 gles to the direction in which the light is travelling, and 
 are transverse waves. 
 
166 THE WONDEE BOOK OF LIGHT 
 
 Both the refraction and the reflection of light can be 
 thoroughly explained in accordance with the undulatory 
 theory. When you are older, and are perhaps studying 
 more deeply the undulatory theory of light, you will be 
 able to see how completely it accounts for such diflBcult 
 matters as refraction, the polarization of light, etc. 
 
 Let me now tell you how the velocity of light has been 
 measured. This velocity is so extremely great that it might 
 be supposed that no time whatever is required for light to 
 pass from place to place ; or, in other words, that this pas- 
 sage was absolutely instantaneous. Indeed, it was not 
 until some time in 1675 or 1676 that it was proved by 
 Eoemer, the celebrated Danish philosopher, that light ac- 
 tually requires time to pass through space. At this time 
 Eoemer was engaged in company with Cassini, the cele- 
 brated French astronomer, in observing the eclipse of one 
 of the moons or satellites of the planet Jupiter. You may 
 know that Jupiter, which is situated at the enormous dis- 
 tance of 475,693,000 miles from the sun, has four sepa- 
 rate moons. Eoemer was watching the moon that lies 
 nearest the planet. He could see it as long as it was mov- 
 ing in front of the planet, but as soon as it had reached 
 the edge, and turned around towards the opposite side, it 
 would disappear like a suddenly extinguished candle. 
 After a certain time, however, it would reappear on the 
 opposite edge of the planet. 
 
 By the appearance, disappearance and reappearance of 
 this moon of Jupiter, Eoemer ascertained the exact time 
 it required to make one revolution or complete passage 
 around the planet. In this way, after having made no less 
 than 100 separate observations, he knew just when the 
 
NATTJEE OF LIGHT 167 
 
 planet should reappear after it had disappeared at one edge. 
 Now these early observations were made when the earth 
 was in that part of its orbit which is nearest Jupiter. On 
 repeating the observations, six months afterwards, when 
 the planet was in that part of its orbit farthest from 
 Jupiter, to his great surprise Eoemer found that the 
 satellite was fully fifteen minutes late in reappearing, and 
 correctly inferred that it was because the light had a 
 greater distance to pass through. 
 
 Eoemer reasoned concerning this as follows: 
 
 "Had I been able to remain at the other side of the 
 earth's orbit, the moon might have appeared always at the 
 proper instant, an observer placed there would probably 
 have seen the moon fifteen minutes ago, the retardation in 
 my case being due to the fact that the light requires fifteen 
 minutes to travel from the place where my first observa- 
 tion was made to my present position." 
 
 Thus Eoemer concluded that, as the earth approached 
 Jupiter, the retardation should gradually become less, until, 
 when the earth reached the position it had when the first 
 observations were made, there should be no retardation at 
 all. He found these suppositions true by actual trials. 
 In this way Eoemer determined that light travels -nith a 
 velocity of 192,500 mUes per second. 
 
 But the velocity of light can be measured in an entirely 
 different way. This is by means of the principle known as 
 the aberration or wandering of light. If we move quickly 
 through a shower of raindrops, when the separate drops 
 are falling vertically downward, they will not appear to 
 be falling straight down, but wUl seem to be inclined to 
 an extent that depends on the velocity with which we are 
 
168 THE "WONDEE BOOK OP LIGHT 
 
 moving. It was in a similar manner that in 1723, the 
 English astronomer Bradley, showed that the motion of 
 the earth through space produced a similar deviation in 
 the direction of the rays of light. Now, if we knovr the 
 velocity with which we are moving through the falling 
 raindrops, and measure the angle at which the drops ap- 
 pear to have moved out of their vertical direction, we can 
 calculate the velocity or speed with which the drops are 
 falling. In a similar manner, knowing the velocity with 
 which the earth is moving in its orbit, we can calculate the 
 velocity with which the light is moving, and in this man- 
 ner can show that light moves with a velocity of 191,515 
 miles per second, a value quite close to that calculated by 
 Eoemer. 
 
 But the velocity of light has been actually measured by 
 means of rapidly revolving mirrors. In this manner, a 
 Prench physicist, named Pizeau, measured its velocity as 
 194,677 miles per second, while later and far more reliable 
 measurements made by another Frenchman, named Fou- 
 cault, place the velocity of light at 185,177 miles per 
 second. 
 
 Before turning from the ■ two theories of light let me 
 mention the wonderful experiments of William Crookes, 
 an English chemist and physicist, to be fully described 
 in the next chapter. While he was attempting to obtain 
 in a vacuum the molecular weight of thallium, a new 
 element he had discovered, a beam of sunlight falling on 
 the scale pan caused the balance to act as if the light 
 actually possessed weight. It looked as if the light con- 
 sisted of something like corpuscles that, striking the plate, 
 moved it in the direction in which they had been travel- 
 
NATURE OF LIGHT 169 
 
 ling. Indeed, had this thing occurred during the time 
 of the controversy respecting the corpuscular theory of 
 light it would prohably have been hailed as demonstrat- 
 ing the correctness of the theory. But the corpuscular 
 theory had been completely overthrown at this time; and, 
 moreover, Crookes, who was greatly skilled as an ex- 
 perimental philosopher, was not long in discovering its 
 cause. You will find a full description of this experi- 
 ment in the next chapter. 
 
CHAPTEE XV 
 THE LIGHT MILL 
 
 There is a curioiis piece of apparatus, somewhat re- 
 sembling a windmill, that when placed in sunlight begins 
 turning rapidly on its axis like an ordinary windmill. It 
 has, therefore, received the name of the light mill. 
 
 The appearance of the light mill, or, as it is generally 
 called by scientists, the radiometer, is represented in Pig. 
 69. It consists of a small wheel, formed of four discs 
 of some material like mica, attached to the ends of two 
 arms placed at right angles to each other, and so supported 
 on a needle point as to be capable of rotating with but 
 little friction. The opposite faces of the disc are covered 
 respectively with lamp black, that readily absorbs the 
 light and heat of the sunlight that falls on them, and with 
 some highly polished reflecting material, like silver, that 
 readily throws off both light and heat. The light wheel 
 thus formed is placed inside a glass globe, which is ex- 
 hausted to a very high vacuum and then sealed by the fu- 
 sion of the glass. When the light mill is placed in the sun- 
 light, the faces of the discs that are coated with lamp 
 black soon become much hotter than those which are coated 
 with silver, and the wheel is set into rapid rotation 
 in a direction against or away from the blackened 
 discs. 
 
 When the sunshine is bright and warm, this rotation 
 
 iro 
 
THE LIGHT MILL 
 
 171 
 
 soon becomes so rapid that the niunber of discs on the 
 
 wheel cannot be recognised by the ej'e. 
 
 The light mill was invented in 1861, by Sir William 
 Crookes. The invention was 
 brought about in so curious a 
 manner that I am sure it will 
 greatly interest you. 
 
 It appears that Crookes had 
 discovered in the residue of a 
 sulphuric acid manufactory in 
 the Hartz Mountains, a new 
 chemical element that, when 
 placed in a colourless flame, im- 
 parted to it a bright greenish 
 colour. Crookes therefore named 
 this element thallium from a 
 Greek word meaning " a twig." 
 
 Now when a chemist discovers 
 a new element, one of the first 
 things he does is to determine 
 what is called its atomic weight; 
 that is, the weight of its atoms 
 as compared with the weight of 
 the atoms of other elements. It 
 is important to know this exactly 
 
 because no two chemical elements possess the same atomic 
 
 weights. 
 Knowing that when a body is weighed in air it loses a 
 
 weight equal to the weight of the air it displaces, and that 
 
 this loss varies with the density and temperature of the air, 
 
 Crookes determined to weigh his specimen of thallium 
 
 Fig. 69. — Light Mill 
 OE Radiometeb. 
 
172 THE WONDEE BOOK OF LIGHT 
 
 in a vacuum. He, therefore, constructed a metallic box 
 with a plate of strong glass on the top, and another glass 
 plate on one of the sides, and placed the balance that was 
 to be employed for the weighing inside this box, and con- 
 necting the box with an air pump he removed as much of 
 its air as he could. Crookes had considerable experience 
 in the use of air pumps, and knew how to obtain a very 
 high vacuum, and so in this experiment he succeeded in 
 getting nearly all the air out of the box. 
 
 He then proceeded to get the exact weight of the thallium 
 by placing a small specimen on one of the scale pans of 
 the balance, and adding weights to the other pan until 
 the two sides exactly balanced, and the arm of the balance 
 came to rest in a true horizontal position. In this way 
 the exact weight of the thallium was obtained, independ- 
 ently of the buoyancy of the air. 
 
 Now it happened, at a moment when the two pans ex- 
 actly balanced each other, that a sunbeam, passing through 
 the glass side of the vessel, fell on one of the balance pans 
 only. The pan receiving the sunbeam acted as if it had 
 suddenly become heavier, and, descending, threw the other 
 pan upwards. 
 
 It was fortunate that this observation was made by a 
 man of cool intelligence. By an ordinary man it would 
 probably have been misunderstood, and this experiment 
 might, perhaps, have been given to the world as proof 
 positive that a sunbeam possesses weight. But Crookes 
 was a physicist as well as a chemist, and was therefore 
 able to deal properly with the phenomenon. 
 
 It must, however, be acknowledged that at first Crookes 
 was greatly astonished at what he saw. 
 
THE LIGHT MILL 173 
 
 " Can it be possible," he argued to himself, " that a 
 sunbeam has weight? It certainly looks so. I must study 
 this matter carefully." 
 
 At last he discovered that the movement of the illu- 
 mined pan was not due to an increase in weight caused by 
 the sunbeam falling on its surface, but to certain move- 
 ments set up by the heat of the sunbeam in the minute 
 traces of air remaining inside the vessel. 
 
 Let us endeavour to imderstand how a beam of sunlight 
 falling on a balanced scale pan in a vacuous space could 
 throw it out of equilibrium. The molecules, or the very 
 small particles of which air is composed, are never at rest, 
 but are constantly moving to-and-fro in straight lines. 
 During these motions they collide against one another, 
 or strike the walls of the vessel in which the gas is held. 
 In either case they are at once thrown ofE from one an- 
 other, or from the walls, and continue to move in straight 
 lines until they again coUide. 
 
 The number of times a molecule will collide with another 
 molecule in a given time, depends on the number of mole- 
 cules contained in the gas, as well as on the speed at which 
 they are moving. The greater the number of molecules 
 in a given volume of gas, the greater the speed with 
 which they are moving, the greater will be the number 
 of collisions per second. 
 
 But if the gas in the vessel is rarefied by removing some 
 of it by an air pump, then, the number of molecules being 
 fewer, there will be fewer collisions per second, and the 
 molecules will move through longer paths before they 
 coUide or strike one another. 
 
 If the air in the vessel is so rarefied that it only con- 
 
174 THE WONDER BOOK OF LIGHT 
 
 tains 1/1000 of its original number of molecules, the num- 
 ber of times the molecules would collide would be one 
 thousand times less than formerly. Consequently, the dis- 
 tance through which any molecule might move without 
 colliding with a neighbouring molecule, would be one thou- 
 sand times greater. In the same way, if the rarefaction 
 is such that the space contains only 1/1,000,000 of its 
 original number of molecules, the number of collisions 
 would be only 1/1,000,000 that of the original number, 
 and the space through which each molecule would be able 
 to pass without striking a neighbouring molecule would be 
 1,000,000 times greater than before. 
 
 JSTow Crookes discovered that, when a vessel containing 
 a gas is exhausted to such a degree that its remaining mole- 
 cules are able in their to-and-fro movements to pass from 
 the surfaces of objects placed in the vessel to its walls 
 without striking against one another, the exhausted space 
 will possess properties differing markedly from spaces 
 filled with ordinary air; for in such spaces the to-and-fro 
 motions of the molecules will be able to produce mechanical 
 effects, such as setting the light mill in rapid rotation. 
 
 Let us try to see how a beam of sunlight falling on 
 one of the arms of a balance placed inside a box con- 
 taining matter in the radiant or ultra-gaseous condition, 
 could cause an apparent increase in weight. Although 
 this is generally considered an extremely difficult matter, it 
 is in reality quite simple. 
 
 In the first place it must be remembered that the mole- 
 cules of a gas move to-and-fro in straight lines, and that 
 the speed of these motions increases as the gas becomes 
 hotter. If, therefore, a space filled with ultra-gaseous mat- 
 
THE LIGHT MILL 175 
 
 ter contains surfaces at different temperatures the mole- 
 cules that come in contact with the heated surfaces wiU 
 be thrown off from these surfaces with greater force than 
 from the cooler surfaces. 
 
 Let us first suppose that everything inside the box is 
 at the same temperature. Then the molecules would fly 
 off from the upper and lower surfaces of the scale pans 
 with equal force and in opposite directions; i. e., ver- 
 tically upwards and downwards with the same force. 
 
 A stream of water or gas flowing out of an oriflce in 
 an open vessel, produces a back pressure against the vessel 
 that tends to move it in a direction opposite to that in 
 which the jet is flowing. This back pressure is sometimes 
 called the reaction of the escaping jet. 
 
 Suppose, for example, the vessel D, filled with water, is 
 suspended by a cord AB, as represented in Fig. 70, at X. 
 When no liquid is escaping, the vessel will come to rest, 
 with its suspended string in a vertical position AB. As 
 soon, however, as a jet or stream of water is permitted to 
 flow out from one of the sides, the back pressure, or the re- 
 action of the escaping jet, will move the vessel from its 
 vertical position into the position AB, and the amount 
 of this deviation from its vertical position will depend 
 on the size of the opening, as well as on the velocity with 
 which the jet is escaping. 
 
 But it is evident that, if the vessel D, Fig. 70, suspended 
 as shown, at X, be provided with two jets a and &, of 
 the same size, on exactly opposite sides of the vessel, 
 when such jets are permitted to flow, their reaction will 
 tend to move the vessel with equal force in opposite direc- 
 tions, so that it will remain in a vertical position, although 
 
176 
 
 THE WONDEE BOOK OP LIGHT 
 
 back pressures are acting on it, since these pressures are 
 equal and opposite. 
 
 If, however, the volume of one of the jets as a', at Y, 
 Pig. 70, be increased, the amount of its back pressure 
 will be increased, and, since these two opposite' pressures 
 no longer neutralise each other, the vessel will move in 
 a direction towards the smaller pressure, and assume the 
 position AD'. 
 
 Now coming back to the case of the gas on the upper 
 horizontal surface of one of the scale pans of the balance 
 
 Y 
 
 Fig. 70. — Action op Equal and Unequal and Opposite 
 DiEECTED Jets. 
 
 X, Action of equal and opposite directed jets; no deflection of 
 vessel D — Y, Action of unequal and opposite directed jets; D moved 
 in the direction of the smaller jet. 
 
 in which Crookes was weighing the thallium, the air con- 
 tained within the balance being in a highly rarefied con- 
 dition, as long as the two opposite faces of the pan were at 
 the same temperature, the molecular streams that were 
 being thrown both directly upwards and downwards could 
 produce no effect on the position of the pan, since they 
 exactly balanced each other. As soon, however, as a beam 
 
THE LIGHT MILL 11^ 
 
 of sunlight fell on the upper surface of one of the pans, 
 its temperature increased and the streams of molecules 
 shot vertically upwards, being stronger than the opposite 
 stream shot vertically downwards from the lower surface 
 of the pan, the pan moved vertically downwards and thus 
 acted as if the beam of light actually possessed weight. 
 
 Now it was the discovery of the peculiar motions of 
 the molecules of gas in this highly rarefied condition, or 
 as Crookes called it in the ultra-gaseous condition, that 
 led him to the invention of the light mill or radiometer. 
 This ingenious device as shown in Fig. 69 consists of a 
 light wheel formed of thin plates of mica, supported as 
 shown at the extremities of two cross-arms. The wheel is 
 suspended on needle points so that it can move in a hori- 
 zontal plane with very little friction. 
 
 When the discs are exposed, either to the sunlight 
 or to the flame of a lighted match or candle, the light with 
 its accompanying heat is absorbed by the blackened sur- 
 faces and thrown off from the silvered surfaces. The 
 blackened surfaces therefore become heated, and the mole- 
 cules which strike them are thrown off with a greater 
 velocity than from those silvered. The wheel, therefore, 
 begins to move, in a direction away from the blackened 
 surfaces, and soon acquires an exceedingly rapid move- 
 ment. 
 
 Had it been known several hundred years ago, the radio- 
 meter would certainly have been regarded as a great won- 
 der and would probably have been claimed by the believers 
 in the emission theory of light as proving the existence of 
 corpuscles thrown off by luminous sources. 
 
 But heat is not the only agency by means of which 
 
178 THE WONDER BOOK OE LIGHT 
 
 molecular streamings can be set up in ultra-gaseous at- 
 mospheres. The same thing can be done by means of 
 electricity. 
 
 When a cannon ball strikes against a plate of steel, 
 considerable heat is produced. The same thing is true of 
 less rapid motions. A skilled blacksmith can hammer a 
 mass of soft iron, such as a horseshoe nail, so as to make it 
 suiRciently hot to use it for lighting the fire of his forge. 
 The streams of molecules that are thrown off from surfaces 
 in ultra-gaseous atmospheres move with great rapidity and 
 
 Fig. 71. — Phosphorescence by Electbical Bombabdment. 
 
 can raise bodies against which they strike to temperatures 
 sufficiently high to enable them to emit light. If a small 
 target, or plate of platinum, receives such molecular bom- 
 bardment, its temperature may be raised even to a bril- 
 liant incandescence, especially if the surface from which 
 they are thrown off is made concave in shape, so as to 
 bring all the molecules to one point or focus. 
 
 In a similar manner, substances that possess the power 
 of phosphorescence, or of becoming luminous by a short 
 exposure to sunlight, can be caused to glow or phosphor- 
 
THE LIGHT MILL 179 
 
 esce, in the dark, by exposing them to molecular bombard- 
 ment. If a phosphorescent substance, such as calcium 
 sulphide, be placed inside a glass vessel, Fig. 71, containing 
 air in an ultra-gaseous state, when a discharge from a 
 Euhmkorff coil is caused to pass between P and iSI", the 
 movements of the molecules are sufficiently rapid when 
 they strike against the phosphorescent material, to produce 
 in it a bright phosphorescent light. 
 
CHAPTEE XVI 
 THE MAGIC WAND OF PRINCE PERCINBT 
 
 You may remember my promise to show you something 
 far more wonderful in light than the manner in which 
 Prince Percinet, by a single touch of his magic wand, 
 succeeded in sorting out and arranging in separate bundles 
 the immense number of feathers from hundreds of dif- 
 ferent birds that had been thoroughly mixed together and 
 crammed into a huge barrel. There were tens of thous- 
 ands of feathers, many of which looked so much alike 
 that poor Graciosa would never have been able, without 
 help, to sort them out into separate bundles, and yet at 
 a single touch of his magic wand. Prince Percinet caused 
 the feathers to fly out of the barrel, and, in the twinkling 
 of an eye, arrange themselves in separate parcels on the 
 floor, without a single feather being out of place. 
 
 What Prince Percinet did with his magic wand was 
 certainly a wonderful thing, but, now that we are suffi- 
 ciently acquainted with the principles of light, I am ready 
 to make good my promise of proving that he could make 
 this magic wand do a far more wonderful thing. 
 
 We have seen that although a ray of sunlight, when 
 passed through a plate of glass, the opposite faces of which 
 are parallel to each other, is bent out of its course or 
 refracted both on entering and leaving the plate, it will 
 emerge parallel to its original direction. If, however, the 
 
 180 
 
MAGIC WAFD OP PEINCE PEECINET 181 
 
 opposite faces of the plate are inclined to each other; or, 
 in other words, if the plate is wedge-shaped, then the ray 
 of light on passing out of the prism will be bent out of its 
 original direction. Now this wedge-shaped piece of glass, 
 known in optics as a prism, is a magic wand far more 
 powerful than Percinet's wand. 
 
 Since a ray of light passing through a prism is bent 
 out of its original course, both while passing through 
 the glass of the prism as well as through the medium out- 
 side it, if an object is viewed through a prism, it will neces- 
 sarily be seen out of its true position. For example, if 
 a lighted candle is viewed through a prism, it will appear 
 out of its true position; for the rays of light suffer two 
 bendings. Therefore, on entering the eye of the observer, 
 they will appear to come from a candle considerably out of 
 its true position. But this ability of a prism to change 
 the apparent position of an object is by no means its most 
 curious property. 
 
 If a beam of sunlight is permitted to enter a darkened 
 room through a slot or opening in a shutter, as repre- 
 sented in Fig. 73, it will form a white or colourless image 
 of the sun on the floor of the room at K. If, however, 
 this beam is caused to pass through a prism P, placed as 
 shown in the figure, it will not only be turned out of its 
 original position, but the exceedingly great number of 
 different kinds of rays, of which a beam of sunlight consists, 
 will be turned out of their paths to different extents; so 
 that, instead of producing a white image on the floor at 
 K, it will be spread over the surface of the wall H, op- 
 posite the opening, as a wonderful band of rainbow col- 
 ours shown at v, i, I, g, y, o and r. This band of colours 
 
183 
 
 THE WONDEE BOOK OP LIGHT 
 
 is known as a spectrum, and since it has been obtained from 
 a beam of solar light, it is known as a solar spectrum. 
 
 Although it may seem to you on looking at a solar 
 spectrum that there are only a few colours, yet, in reality, 
 the band consists of an exceedingly great number of sepa- 
 rate colours that pass insensibly into one another. The 
 solar spectrum is sometimes incorrectly spoken of as con- 
 sisting of seven colours only; i. e., violet, indigo, blue, 
 
 Fig. 72. — ^A Peism Sobting Out the Diffeeent Coloueed Eats 
 OF Sunlight. 
 
 green, yellow, orange and red. In reality, however, each 
 of these consists of bands containing a very great number 
 of different shades or tints of violets, indigoes, blues, 
 greens, yellows, oranges and reds. 
 
 You will notice in the preceding figure, that the vari- 
 ously coloured rays have by their passage through a prism 
 been turned to different extents out of the original direc- 
 tion of the beam, the violet rays being diverted from 
 their course to a much greater extent than the red rays. 
 As you can see, the violet rays occupy that portion of the 
 
MAGIC WAXD OP PEIXCE PEECIXET 183 
 
 spectnim which falls on the wall nearest the ceiling, while 
 the red rays occupy the portion nearest the floor. The 
 violet rays then undergo the greatest amotait of refraction, 
 or possess what is called the greatest refrangibility while 
 the red rays are the least turned out of their course, or 
 possess the least refrangibility. The order of refrangi- 
 bility of the different colours of the spectrum, beginning 
 with the most refrangible and ending with the least re- 
 frangible, is as follows: violet, indigo, blue, green, yellow, 
 orange and red. 
 
 I wish especially to warn you not to imagine when 
 I speak of red light, or violet light, that I mean a light 
 of a single colour only. On the contrary, when I speak 
 of the red or the violet of the spectrum, I am referring to 
 bands or collections of thousands of separate reds or violets, 
 which, for the sake of convenience, we have agreed to call 
 red or violet light. And it is the same with orange, yellow, 
 green, blue, or indigo light. 
 
 It is easy to prove that each of these bands consists of 
 assemblages of different coloured tints; for, if the light is 
 successively passed through a number of prisms, rays of 
 different tints of red, violet, indigo, blue, etc., will be 
 spread out on the surface of the screen or waU that receives 
 the spectrum. 
 
 Imagine now that you are Graciosa and are in the room 
 represented in the above figure and are looking at the 
 spot of sunlight on the floor. This patch of light is your 
 barrel of mixed birds' feathers which are to be sorted 
 out and arranged in separate parcels. Even if the wicked 
 stepmother has her hand on the knob, there is plenty of 
 time for you to complete your task. By merely placing 
 
184 THE WONDEE BOOK OF LIGHT 
 
 the prism in the position shown in the figure, so that 
 the beam of sunlight, instead of falling directly on the 
 floor, passes through it, you will perform the task in 
 time to save yourself. For, lo, in an instant there appears 
 on the wall an almost infinite number of separate rays 
 or colours of the spectrum spread out in bands of sepa- 
 rate parcels. Is not this a more wonderful thing than 
 the fairy story? And fairy stories are told with the 
 intention of being as wonderful as possible. 
 
 The separation of a beam of white light into its various 
 constituents is known as dispersion. Sometimes the word 
 diffusion is carelessly and incorrectly employed for the 
 irregular scattering of light that takes place when it falls 
 on a roughened or irregular surface. As, however, I have 
 already pointed out, the irregular scattering of light is 
 properly called dispersion. 
 
 The discovery of the power of a prism thus to separate 
 sunlight into its constituent rays was made by the great 
 Sir Isaac ISTewton to whom we have already referred, about 
 the beginning of the year 1667. Kewton had built his 
 enormous reflecting telescope which, by the way, was one 
 of the first telescopes of this character, and had experi- 
 mented also with refracting telescopes, and, not being sat- 
 isfied with the images formed by their lenses, was trying 
 to improve them. 
 
 After considerable experimenting, Newton began to sus- 
 pect that the trouble with the images formed in refracting 
 telescopes was not due to the lenses, but might be caused 
 by the light itself; that, perhaps, it was impossible to 
 focus the rays coming from an object so as to prevent the 
 formation of false colouration.. He,. therefore, began ex- 
 
MAGIC WAXD OP PEIXCE PEECIXET 185 
 
 perimenting with a glass prism, placed in the opening in 
 a shutter of a darkened room, thus permitting a heam of 
 sunlight to pass through it. He found that, when per- 
 mitted to fall on the opposite wall, after its passage 
 through the prism, the light no longer had the appear- 
 ance of white light but displayed colours. He thus 
 discovered that a beam of sunlight does not consist of a 
 single colour, but of a great variety of different colours. 
 It was Xewton who gave the name solar spectrum to the 
 band of coloured rays thus produced. It was as early as 
 1667 that he discovered this magic power of a prism 
 for separating and thus sorting out the almost innumer- 
 able colours that are present in sunlight. 
 
 During his experiments, Xewton demonstrated, as you 
 also could easUy do, that when a beam of sunlight has 
 been dispersed into bands of diEEerent colours, if any one 
 of these bands of light is subsequently passed through a 
 second prism, it does not produce another group of colours 
 of the rainbow tints from the red to the violet, although it 
 does separate the band into a number of different tints of 
 the same colour. If, for example, the particular band 
 we ordinarily call violet light is caused to pass through 
 a second prism, what has seemed to the eye to be a single 
 violet coloured ray is separated into a great number of 
 differently tinted violet rays, having the general colour 
 of violet, and only called violet collectively for want of a 
 sufficient number of separate names to distinguish each 
 of them. This, however, is the point I wish you to 
 especially note. The violet light that has been separated 
 from the beam of white sunlight by its passage through the 
 first prism, although by no means a single kind of light. 
 
186 THE WONDEE BOOK OP LIGHT 
 
 yet is an entirely different thing from the white light of the 
 sun itself. Were it not different, then on its passage 
 through the second prism it would also be divided into 
 the light of the solar spectrum with all its colours between 
 the red and the violet. But as we have seen, this separation 
 does not take place. The light simply consists of a num- 
 ber of differently tinted violet rays. 
 
 The apparatus employed by Wewton in these experiments 
 is shown in Fig. 73. A beam of sunlight S, coming 
 through an opening in a shutter of a darkened room, is 
 passed through a prism A, so as to form a solar spectrum 
 
 • Fig. 73. — Newton's Experiment with Violet Light. 
 
 on the opposite wall at E. If a screen be placed so as 
 to permit only the violet rays to pass through a second 
 prism, B, there will be seen on a screen placed at H, a 
 further separation of the violet rays, but no red, orange, 
 green, blue or indigo rays will be produced. 
 
 Naturally, you will ask why the magic wand of Percinet, 
 or a prism, is capable of acting in this manner so as im- 
 mediately to sort out the different coloured rays that 
 constitute sunlight. This question can be easily answered. 
 It is because the different coloured rays pass through the 
 prism with different velocities, and, therefore, take differ- 
 ent paths, and so fall on different portions of the wall. 
 
MAGIC WAND OF PEINCB PEECIXET 187 
 
 This agrees with what I have already told you as the cause 
 of refraction; i. e., that light is refracted and takes a 
 different direction while entering a piece of glass, or a 
 surface of water, from the air, by reason of the decreased 
 speed or velocity with which it passes through the water 
 or glass, as compared with its velocity in air. 
 
 It is because each of the different coloured rays of 
 light passes through glass or water with velocities differ- 
 ent from those with which it passes through air, that a 
 prism is able to cause the rays to take different paths. 
 They can no longer keep together, or as it were, side by 
 side, as soon as they enter a medium like water or glass 
 in an oblique direction, for some of the rays move through 
 this medium .much more rapidly than through air, and, 
 moving at different speeds, must take different paths, and 
 must, therefore, be separated or dispersed. 
 
 It is almost impossible to compare the speed with which 
 this change could take place with so exceedingly slow a 
 motion as the twinkling of an eye. Since light passes 
 through air with a velocity of something in the neigh- 
 bourhood of 186,680 miles a second, the time required to 
 produce this change, and sort out the different coloured 
 bundles of rays on the opposite wall, would be the time 
 required for the light to pass through the prism and the 
 distance between the waU, a distance say of about ten feet. 
 Now since light can pass through 186,680 miles in a sec- 
 ond, the time it would take to pass through ten feet in 
 order to cause this change would of course be ridiculously 
 less in duration than the wink of the eye. 
 
 As we have seen, light is due to a wave motion in the 
 luminiferous ether, the ether vibrating very rapidly in 
 
188 THE WONDEE BOOK OF LIGHT 
 
 paths at right angles to the direction in which the ray is 
 advancing. The diflEerent coloured rays of the spectrum 
 produce their diflEerent efEects on the retina by reason of 
 the diflEerences in the rapidity of these wave motions. In 
 other words, the diflEerent colours of light diflEer from one 
 another in exactly the same way as the different notes in 
 music. As you know, the strings on a piano diflEer both 
 in length and thickness, those producing the shrill tones 
 are short, thin, and tightly stretched; those producing 
 the low or grave notes are long, thick, and less tightly 
 stretched. The short, thin, highly stretched strings move 
 to-and-fro when struck more rapidly than do the long, 
 thick, and less tightly stretched ones that produce the 
 grave tones. Now it is the same with diflEerent coloured 
 rays of light. The red rays correspond to the least rapid 
 to-and-fro motions of the ether, while the violet rays corre- 
 spond to the more rapid to-and-fro motions. 
 
 Without entering into any further explanation of this 
 matter, I would say it can be shown that a wave length of 
 light, or the distance through which light advances during 
 one complete to-and-fro motion of the ether, can be ex- 
 actly measured, and that the actual lengths of the light 
 waves required to produce the diflEerent colours are exceed- 
 ingly small. These waves are so short that, when placed 
 end to end, 40,000 of them would only extend through 
 the length of an inch, in the case of the red ray; while in 
 the case of the violet, it would require 58,000 of such rays 
 placed end to end to extend over this distance. 
 
 Ifow, since light travels at the enormous velocity of 186,- 
 680 miles per second, there will, in the case of a red ray 
 of light, be produced every second the number of separate 
 
MAGIC WAND OF PEINCE PEECINET 189 
 
 waves or vibrations that could be crowded together, not 
 in the length of an inch, but in the length of 186,680 
 miles, thus producing a number of vibrations per second 
 that is indeed too enormous to conceive. This would make 
 in the case of the red light something in the neighbour- 
 hood of (I am only giving the numbers roughly) 473,300,- 
 000,000,000 vibrations per second, and in the case of the 
 violet something in the neighbourhood of 697,300,000,000,- 
 000 vibrations per second! In other words, when a ray 
 of violet light enters the eye it is pouring waves or vibra- 
 tions through its delicate structure that foUow one an- 
 
 FiG. 74. — The Recomposition of Light. 
 
 other at such an enormous rapidity that 697,300,000,000,- 
 000 waves enter every second. If then the eye continues 
 to receive violet light for a whole minute, for an hour, or 
 for several hours, it should not be a matter of surprise 
 that its retina gets fatigued or somewhat tired. 
 
 If, by any means, the different coloured rays of the 
 spectrum are again mixed together, so that they can 
 all enter the eye of an observer at the same time, they 
 are again capable of producing the sensation of white light, 
 Suppose, for example, that all the coloured rays produced 
 by the passage of a beam of white light through a prism 
 be caused to pass through a double convex lens. Then these 
 rays, being thus brought to a single focus, will produce a 
 
190 THE WONDER BOOK OP LIGHT 
 
 light exactly the same as that of the white light of the 
 sun. 
 
 The same thing can also be done in the way shown in 
 Fig. 74, where the spectrum or collection of different 
 coloured rays produced by the passage of a beam of sun- 
 light through a prism is caused to fall on the concave 
 surface of a mirror. When all are brought to a single 
 focus as shown, they will no longer appear of different 
 
 Fig. 75. — Colotje Disc Employed fob the Eecomposition of 
 
 Light. 
 
 colours, but will possess the characteristic white of ordi- 
 nary sunlight. 
 
 There are many other ways in which the separate colours 
 can be brought together so as to produce white light. For 
 example, if the different colours of the spectrum are caused 
 to fall on a number of separate mirrors so as to be re- 
 flected to a single point on the ceiling of a room, there will 
 
MAGIC WAXD OF PRIXCE PEECIXET 191 
 
 be seen at that point, not the separate colours of the 
 different rays, but a path of white light. 
 
 There is even a simpler method of producing white 
 light by the mixing together of the different colours; or, 
 as this process is sometimes called, by the recomposition 
 of light in distinction from its decomposition or separa- 
 tion into different colours. If a blackened disc of card- 
 board has painted on its surface, in the spaces Y, I, B, G, 
 Y, 0, E, in Fig. 75, the prismatic colours, and the disc he 
 set into rapid rotation, it wiU. appear, to an eye placed in 
 front of it, not to be coloured in different segments but 
 of a uniform white. Here the mixing of the prismatic 
 colours is made in the eye of the observer; for, when the 
 rotation is sulficiently rapid and one of the coloured seg- 
 ments comes in front of the eye, say, for example, the violet 
 segment V, the eye does not cease to see the violet image 
 when the disc has moved away from it, but by the per- 
 sistence of the retinal image it continues, so that a violet 
 coloured ring is formed on the eye. In the same way the 
 other colours, the iadigo, the blue, the green, the yeUow, the 
 orange and the red segments, I, B, G, Y, and E, form 
 continuous rings on the retina and, these being superposed 
 on each other, produce on the eye the effect of white light. 
 
CHAPTEE XVII 
 COLOUR 
 
 You may be surprised to learn that a body does not 
 possess colour in itself, but that its colour is due to the 
 light that falls on it. A body has a white colour if it 
 possess the power of throwing o£E all the colours that are 
 present in sunlight. If, however, a body absorb some 
 particular colour of sunlight, then the remaining light it 
 throws off is no longer able to produce on the eye the 
 effect of white, and the body appears to be coloured. 
 
 When all the light that falls on a body is absorbed, it 
 appears black, because it throws off no light. If it par- 
 tially absorb all the different colours, thus giving off a 
 small proportion of each, it appears of a grey colour. A 
 piece of coloured fabric, such as a piece of red flannel, or 
 other red material, appears of a red colour because it ab- 
 sorbs all the colours of the light that falls on it but 
 the reds. It is only the light that has penetrated some dis- 
 tance into a coloured body, such, for example, as a red 
 fabric, that has its other colours absorbed. In many cases 
 a small quantity of light is thrown directly off its surface. 
 This light has none of its colours absorbed, so that there 
 is mingled with the red light of a red body a certain- 
 amount of white light, and the same is true of any other 
 opaque coloured body, such as a blue, an orange, or a 
 yellow. 
 
 192 
 
COLOUE 193 
 
 It is by no means an easy thing to obtain light of a 
 single colour only from an opaque coloured body, such as 
 a piece of red or crimson fabric. While such a body throws 
 off mainly the red or crimson rays which have come from 
 the light that has partially entered the mass, yet along 
 with these rays there is nearly always a certain proportion 
 of orange or yellow rays that have failed to be absorbed. 
 Eed or crimson fabrics, therefore, do not always throw off 
 pure red or crimson rays, but generally have associated 
 with them certain small proportions of orange and yellow 
 coloured rays. Of course, red coloured fabrics differ from 
 one another very greatly in this respect. Some throw off 
 nearly pure red or crimson light, while others throw off 
 red light mingled as above stated with varying proportions 
 of orange or yellow rays. And then, as we have seen, there 
 is always a certain varying proportion of white light di- 
 rectly thrown or reflected from the surface of the fabric. 
 In some coloured fabrics the amount of this white light is 
 comparatively large; in others it is quite small. 
 
 Coloured bodies, therefore, possess no colour of their 
 own. They owe their colour entirely to the light that falls 
 on them. Since they are capable of throwing off certain 
 colours onljf, if some of these colours do not exist in the 
 light by which they are illumined, they will not possess the 
 same colour as they would when illumined by daylight. 
 For this reason coloured bodies, esamined by artificial 
 light, often have an appearance entirely different from 
 that they show by daylight. For example, in many forms 
 of artificial light there is a preponderance of the red, orange 
 and yellow rays, and too small a proportion of the blue 
 and violet rays. While, therefore, such artificial light can 
 
194 THE WOFDEE BOOK OF LIGHT 
 
 give fairly true colour values to red, yellow and orange col- 
 oured fabrics, they are unable to give the true colours to 
 fabrics of green, blue, indigo or violet. 
 
 That the colour of an object lies not in itself, but in the 
 light which falls on it can be shown by the following ex- 
 tremely simple and yet wonderful experiment: An alco- 
 hol lamp, or a Bunsen burner, is practically non-luminous. 
 If such a flame is permitted to fall on a small quantity 
 of common table salt, or chloride of sodium, it becomes of 
 an intensely yellow colour, its light consisting almost en- 
 tirely of yellow light. If this light were passed through 
 a prism there would not be obtained a continuous band or 
 spectrum of different colours, as in the case of sunlight, 
 but only a single colour, or, at the most, a few bands of 
 different tints of yellow rays. 
 
 If, then, we colour the flame of a Bunsen burner, or an 
 alcohol lamp, with common table salt, and employ this 
 light in a darkened room where no other light exists, for 
 the examination of different coloured objects, we will ob- 
 tain some exceedingly curious results. For example, if a 
 piece of red sealing wax, or still better, a piece of dark red 
 flannel, is brought into the light of the yellow flame, it 
 will appear quite black. The reason is evident. The wax 
 or the flannel can only throw off red light, and since there 
 is no red light in the yellow coloured gas flame, the wax 
 or the flannel has no light to throw off, and, therefore, ap- 
 pears black. 
 
 But if a piece of yellow coloured fabric is brought into 
 the light of the yellow flame, you will be surprised at the 
 bright light it throws off; for the yellow fabric possesses 
 the power of throwing off yellow light, and since there is 
 
COLOUR 195 
 
 an abundance of this coloured light in the luminous gas 
 flame, that fabric can throw this light off readily and, 
 therefore, appears as a bright yellow. 
 
 In a similar manner if sunlight be passed through a 
 glass vessel containing a deep blue solution, obtained by 
 adding an excess of ammonia water to a solution of blue 
 vitriol, or sulphate of copper, we wiU have a light that 
 consists almost entirely of blue rays. If different coloured 
 objects are examined by this light we wUl find that, while 
 blue fabrics are readily seen, other coloured fabrics throw 
 off so little of this light that they appear either browns or 
 blacks. 
 
 A curious effect is produced on the appearance of the 
 human face when illumined by pure yeUow light. The 
 pleasing appearance of a healthy face or skin is due to its 
 ability to throw off a certaia amount of red light. The 
 face of a dead person possesses a peculiar dull grey or 
 leaden appearance, on account of the absence of the red 
 light that in life is produced by blood circulating near the 
 surface of the skin. Xow in a room where there is no light 
 except the yellow light of a sodium flame, there is neces- 
 sarily a complete absence of these reddish rays. Faces, 
 therefore, so illumined by this light present the ghastly 
 appearance of the dead. I would advise you to try this 
 experiment for yourselves, for it is exceedingly easy, and 
 is capable of producing wonderful results. A Bunsen 
 flame, or the flame of an alcohol lamp, is best for the pur- 
 pose. But if you have any difiBculty in obtaining either 
 of these things, simply pour a small quantity of alcohol 
 in a saucer and, throwing a teaspoonful of common table 
 salt in the alcohol, thoroughly saturate a piece of common 
 
196 THE WONDEE BOOK OF LIGHT 
 
 cotton lamp wick with the solution. When you set fire to 
 the lamp wick, you will have the characteristic yellow col- 
 oured flame, and if different coloured fabrics or different 
 coloured objects are examined by this flame in the dark, 
 you will be surprised at the curious appearance they pre- 
 sent. Eeds and crimsons, of almost all tints, appear either 
 as dull greys or blacks. Indeed, it is possible that in some 
 cases the people to whom you are showing these experi- 
 ments may think you have, through sleight of hand, been 
 fooling them by adroit changes in the coloured fabrics. 
 This, however, can instantly be disproved by holding in 
 front of the fabric either a lighted match or a lighted 
 taper. Either of these lights contains nearly all the col- 
 ours of the spectrum, so that the reds and the crimsons of 
 the objects at once appear and continue to be seen as long 
 as the match or taper remains lighted. When it is ex- 
 tinguished, however, and the objects are only illumined by 
 the yellow light, the peculiar browns and blacks instantly 
 reappear. 
 
 Since white fabrics owe their colour, when illumined by 
 sunlight, to their ability to throw off all the rays of the 
 spectrum, they will, when illumined by light of a particu- 
 lar colour, such for example as red, yellow, green or blue, 
 instantly appear of that colour. This fact is put to a prac- 
 tical use on the stage, at the theatre or the opera, in order 
 rapidly to change the colour of the dresses worn by the 
 ballet. If the dancers are clad in white, it is possible in- 
 stantly to change the colour of their apparel by throwing 
 different coloured beams of light on them. In this way, 
 as you have probably often witnessed, marvellous trans- 
 formations of colour are instantly brought about. 
 
COLOUR 197 
 
 But there is another kind of coloured bodies; i.e., trans- 
 parent coloured bodies. These bodies are transparent to 
 certain colours only, so that when white Ught falls on 
 them, certain of the colours are absorbed or quenched, and 
 the remaining light which passes through is coloured. An 
 example of transparent coloured bodies is found in plates 
 of coloured glass. If a beam of sunlight is permitted to 
 pass through a plate of deep red glass there can readily be 
 obtained light that consists of nearly a red colour only. If 
 a green body, such as a leaf, is illumined by this light only, 
 it may appear nearly black. Unless, however, the plate 
 of red glass employed is of a deep red colour, other rays 
 besides the reds will pass through, such, for example, as 
 the yellow or the orange rays, and even a small quantity 
 of the green, in which case the green leaf may be visible. 
 It will not, however, be of its ordinary green colour, but 
 of a sickly grey, with only a suspicion of green, that 
 wiU cause it to look very different from what it would if 
 placed in svmlight. 
 
 Some substances possess the power of showing one colour 
 by means of the light that is reflected from their surfaces, 
 and another colour by the light that passes through them. 
 Such substances are called dichroic, or two-coloured sub- 
 stances. 
 
 Some varieties of red aniline inks, prepared from certain 
 coal tar colours, are dichroic. If a plate of glass be cov- 
 ered by a thin film of such a red ink it will appear of a deep 
 red colour by the light that is passed through it, and of a 
 bright green colour by the light that is thrown off from its 
 surfaces. In a similar manner there are a number of 
 dichroic blue aniline inks. Plates of glass covered by a 
 
198 THE WONDEE BOOK OP LIGHT 
 
 thin layer of such inks appear blue by transmitted light, 
 and orange or deep yellow by reflected light. 
 
 I mnst now explain to you some peculiar properties of 
 what are called complementary colours. As we have seen, 
 when all the colours of the solar spectrum are mixed to- 
 gether in any way which will permit them to affect the eye 
 simultaneously, they will no longer appear as different 
 colours, but will produce a sensation of white light, like 
 ordinary sunlight. If, however, one group of colours, such 
 as the reds, is removed, and the remaining colours are 
 mixed together, the mixture will no longer be able to pro- 
 duce a white light, but will produce a light of a greenish 
 blue colour. In a similar manner, if the yellow rays, or 
 the band of yellows, be removed from the spectrum, the 
 remaining colours when mixed together will produce an 
 indigo blue. 
 
 Such colours are called complementary colours because 
 they are the colours that are required to complement or 
 fill out to the full measure of whiteness the colours that 
 have been removed. Eed and greenish blue are comple- 
 mentary colours. When mixed together they are capable 
 of producing white. In a similar manner yellow and 
 indigo blue are complementary colours. So also are orange 
 and cyan blue, and greenish yellow and violet. 
 
 In one sense it may be said that white and black are 
 complementary colours ; for when mixed together they are 
 capable of producing a colour that might be regarded as 
 white, though it is more properly a grey. 
 
 "We must carefully distinguish between the mixing of 
 coloured lights and coloured paints or pigments. I im- 
 agine that most of you, at one time or another in your lives, 
 
COLOUE 199 
 
 have played with boxes of paints, and have probably 
 learned, what all artists know so well, that by mixing dif- 
 ferent coloured paints together there can be produced a 
 great variety of colours. For example, yellow and blue 
 paints mixed together produce a beautiful green. If, how- 
 ever, you should mix yellow and blue lights together, you 
 would get, not a green light, but a white light; for blue 
 and yellow are complementary colours, and when comple- 
 mentary colours are mixed together, white light is the re- 
 sult. The reason for the difference in the colour produced 
 by mixing different coloured paints and different coloured 
 lights is as follows: When yellow and blue pigments are 
 mixed together, the light that penetrates them a short 
 distance has all its colours absorbed except the greens, and 
 since the greens are the only colours that are transmissible, 
 the colour of the mixture appears green. But when blue 
 and yellow lights are mixed together, being complementary 
 colours, they produce, as do all complementary colours, a 
 white light. 
 
 A number of very wonderful results can be produced by 
 reason of the fact that when the eye has looked steadily 
 for a certain time on a given colour, it gradually loses its 
 power to continue to see that colour, and becomes much 
 less sensitive to that colour. You can easily convince 
 yourself of this fact by the following simple experiment : 
 
 Stick a small red wafer at the centre of a sheet of white 
 paper, and place it where it will receive a good illumina- 
 tion by white light. Fix the eyes steadily at the centre of 
 the wafer so that they will not wander from it in. the slight- 
 est degree. If after a few moments the red wafer is re- 
 moved, and the eyes are still kept fixed at the place where 
 
200 THE WONDEE BOOK OF LIGHT 
 
 it originally was, you wUl see in its place the image of a 
 green wafer. What you are looking at is a perfectly white 
 piece of paper. What you see is a greenish circle occupy- 
 ing the place where the red wafer was. Here is what has 
 happened: the optic nerve has become so wearied by look- 
 ing at the red that it is unable to see red any longer; so, 
 although all the colours of the spectrum are thrown from 
 the surface of the paper at the place where the red wafer 
 was originally placed, the eye is unable to appreciate the 
 red colours, and, therefore, sees the colour that would be 
 produced by the removal of red from the white light. This 
 of course will be greenish blue, for greenish blue is the 
 complementary colour of red. 
 
 Similar effects can be produced by the use of coloured 
 shadows. By the use of a plate of red glass a uniform red 
 light or field can be obtained over the surface of a sheet of 
 white paper. If now an opaque body, such as a lead pencil, 
 is held in the path of the red light so as to throw a shadow 
 on the paper, and on this shadow a small quantity of dif- 
 fused light falls, it will no longer appear dark or black, 
 but will take on a greenish blue colour ; i.e., a colour com- 
 plementary to that of the red light in which the shadow 
 has been formed. In a similar manner, a faintly illumined 
 shadow formed on a yellow field will appear of an indigo 
 blue; when formed on an orange coloured field, of a cyan 
 blue; and when formed on a greenish yeUow field, of a 
 violet colour. 
 
 In the above experiment, you will observe that the green- 
 ish blue seen after the eyes have been wearied by long ob- 
 servation of red is the complementary colour to red. In 
 all such cases, the colour seen is the complementary 
 
COLOUE 201 
 
 colour either to that which has been observed, or to the 
 colour of the field in which the shadow has been formed. 
 Colours produced in this manner are called subjective 
 colours. 
 
 Subjective colours are of considerable importance from 
 the effects they produce when two colours are contrasted or 
 placed alongside each other. TMien the eye looks at two 
 colours that are placed alongside each other, each will look 
 as if it had been mixed with the complement of the other. 
 Suppose, for example, a red coloured fabric is placed along- 
 side a fabric of a greenish blue colour. Under these 
 circumstances the red will so affect the eye as to make any- 
 thing at which it is lookiag appear to possess a greenish 
 blue colour, so that the actual greenish blue will appear of 
 a deeper hue. In the same way looking at the greenish 
 blue colour wUl fiU the eye with a reddish colour and so 
 make the red appear of a deeper red. In this way com- 
 plementary colours tend to increase one another's bril- 
 liancy. 
 
 The proper contrast of colours is a matter of great im- 
 portance in the furnishing of a house, or ia the selection 
 of the different colours for articles of dress. 
 
 It is by no means an easy matter to be able to see the 
 true colour of an object when it is placed alongside some 
 other colour. I fear that an unprincipled salesman, who 
 has by experience become aware of this curious effect, may 
 sometimes take an unfair advantage of a purchaser who 
 wishes, for example, a carpet of a particular shade of 
 green. 
 
 Convinced that he has no carpet of the desired shade of 
 green, under the pretext of resting the eye of his customer, 
 
303 THE WONDEE BOOK OP LIGHT 
 
 the salesman calls attention to a beautiful shade of red or 
 crimson carpet that he skilfully unrolls on the floor. Then, 
 ■when his customer's eyes have become so affected by the 
 red that they see green in everything at which they look, 
 he pretends suddenly to remember he has a carpet of the 
 shade of green that is exactly what his customer desires. 
 With the remark : " Oh, I remember now, here is a green 
 carpet I think will just suit you," he unrolls a green carpet 
 alongside the red. Now if the customer saw this green 
 carpet alone, he would never buy it, but the green that is 
 actually in the carpet being mingled with the green in 
 his eyes, there is produced the exact shade of green he de- 
 sired, so the customer purchases the carpet and has it sent 
 home. 
 
 I need hardly say to you that such a carpet would fail 
 to suit, since it will only appear of the shade of green de- 
 sired when placed alongside a roll of red carpet. 
 
 In a similar way, as all ladies know, much difficulty is 
 often experienced in obtaining an exact match for a zephyr 
 or a coloured ribbon. You might explain to such people 
 that, if they wish to obtain an exact match, they must try 
 to come to a decision promptly before the eyes are fa- 
 tigued. They must, moreover, as far as possible, insist on 
 doing this work away from any other colours, since in 
 this way only can they obtain a satisfactory match. 
 
 The effect produced by the contrast of colours is so 
 marked that artists are unable to tell, by merely observing 
 a colour on the palette, how it will look in a painting, since 
 alongside some other colour it no longer appears as it did 
 when alone on the palette. 
 
CHAPTEE XVIII 
 
 INVISIBLE LIGHT 
 
 I AM not certain that the above chapter heading, " in- 
 visible light," is correct. Strictly speaking, if by visible 
 light we mean those vraves in the universal ether that are 
 capable of directly afEecting the eye, we cannot properly 
 call light any waves that are incapable of doing this. 
 Nor would the term invisible rays be any better, since 
 none of the ether waves are visible, unless they pass 
 through the eye and fall on the retina. 
 
 If you have followed what I have said regarding the 
 great number of different rays that exist in the solar spec- 
 trum between the red and the violet rays, you wiU agree 
 with me that ordinary sunlight is exceedingly complex, 
 consisting of a great variety of different rays. If, how- 
 ever, we examine this matter still further, it will be seen 
 that we have only commenced to understand how exceed- 
 ingly great the complexity of sunlight is. 
 
 There are three different kinds of rays present in sun- 
 light, and indeed in most luminous bodies; namely, the 
 luminous rays, or those that are capable, when enteriag the 
 eye, of producing light ; the heat rays, or those that are in- 
 capable of affecting the eye as light, but when absorbed by 
 matter, are capable of raising its temperature; and the 
 chemical or actinic rays, or those which, while incapable 
 of producing either light or marked heating effects, possess 
 the power of breaking up the combination between chem- 
 
 203 
 
304 THE WONDBE BOOK OF LIGHT 
 
 ical substances, or of causing them to unite with one 
 another. 
 
 So far as the visible part of the spectrum is concerned, 
 it would, perhaps, be more correct to say that these rays 
 are capable of producing three different kinds of effects; 
 for, in many cases, the same rays are capable of producing 
 all three of these effects; i.e., they can excite the sensation 
 of light when they fall on the retina, or may, when ab- 
 sorbed by ordinary matter, either raise its temperature or 
 exert chemical action on it. 
 
 Taking the case of the sun as a type of body that pro- 
 duces white light, let us inquire as to which of its rays 
 possess, to the greatest extent, the power of producing 
 luminous effects. Now I must ask you to remember that 
 none of the rays of the solar spectrum are capable of pro- 
 ducing light unless they actually pass through the eye and 
 fall on the retina. 
 
 While any of the rays between the red and the violet in 
 an ordinary spectrum can excite the sensation of light; 
 yet the amount of this excitation is least in the case of the 
 violet rays, and is greatest in the rays that are situated 
 in the yellow portions of the spectrum. 
 
 Practically, all portions of the solar spectrum between 
 the red and the violet, as well as the portions that lie be- 
 yond the violet and below the red, possess the power, when 
 absorbed by ordinary matter, of raising its temperature, 
 and, in so doing, of setting up heat waves in the ether. 
 Heat waves are much longer than light waves, and, there- 
 fore, produce relatively so few waves per second that they 
 are incapable of affecting the eye. It is these waves that 
 form what is sometimes called invisible light. 
 
IXVISIBLE LIGHT 205 
 
 In a similar way, all the so-called visible portions of the 
 spectrum (that is, aU the rays between the red and the vio- 
 let) when absorbed by certain kinds of matter, are also 
 capable of either producing chemical decomposition (that 
 is, of causing a separation to occur of substances that have 
 already entered into chemical combination with each other, 
 or of causing substances that have not yet combined to 
 enter immediately into chemical combination.) Like the 
 heat rays, these rays are not limited to the visible portions 
 of the spectrum, but extend both above or beyond the violet 
 and below the red. They are called the chemical or the 
 actinic rays, and, in the portions of the spectrum beyond 
 the violet and below the red, also form what is sometimes 
 called invisible light. 
 
 The effects produced by ether waves depend, therefore, 
 on the kind of matter by which they are absorbed. If, for 
 example, the rays fall on the retina and can excite in it 
 vibrations that are sufficiently rapid, they will produce 
 the sensation of light. If they fall on matter and are ab- 
 sorbed, so that they cause its molecules to move to-and- 
 fro at a less rapid rate, they will produce what are knovra 
 as heat rays, although these heat rays differ in no respect 
 from light rays save only in the rapidity with which the 
 molecules move to-and-fro, or in the number of waves they 
 produce per second. I shall explain thoroughly to you in 
 " The Wonder Book of Heat " the peculiarities of heat 
 waves. 
 
 THien, however, ether waves fall on certain kinds of mat- 
 ter and are absorbed, the atoms making up the molecules 
 are set into vibrations so great that they move beyond one 
 another's mutual attractions. The molecules are then 
 
206 
 
 THE WONDER BOOK OP LIGHT 
 
 broken up, or chemically decomposed, and the atoms may 
 enter into new combinations. In this way, the ether waves 
 produce a force known as the chemical force, or chemical 
 affinity. This, however, is a matter requiring special con- 
 sideration, and will be fully explained in " The Wonder 
 Book of Chemistry." 
 
 Various methods have been employed in order to deter- 
 mine the portions of the solar spectrum that possess the 
 greatest luminous, thermal, and actinic powers. Fig. 76 
 represents a solar spectrum produced by the passage of a 
 beam of white light through a particular kind of glass 
 known as flint glass. The spectrum is represented at the 
 
 YELLOW GREEN 
 
 Pig. 76. — Thermal Luminous and Actinic Poetions of Solae 
 Specteum. 
 
 bottom of the drawing with the rays spread out in different 
 bands from the red to the violet. In this particular spec- 
 trum the dispersing power of the prism or prisms employed 
 has been so great that, as you will see, the different portions 
 of the red, orange, yellow, green, blue, indigo, and violet 
 bands have been separated into the different regions marked 
 at the bottom of the spectrum by the letters from A to H. 
 It will' be observed that three curves, called respectively 
 the curve of luminous intensity, the curve of thermal in- 
 tensity, and the curve of chemical intensity, have been 
 drawn above the spectrum. The different heights of these 
 curves above the horizontal line represent the relative in- 
 
INVISIBLE LIGHT 207 
 
 tensities of the light, heat and chemical action at corre- 
 sponding portions of the spectrum. 
 
 The different heating portions of such spectrum are de- 
 termined by slowly moving a thermometer through the 
 spectrum not only from the red to the violet, but also in 
 the regions below the red and above the violet; or in the 
 infra-red and the ultra-violet, as these regions are respect- 
 ively called. In this way, as has been shown by Leslie, the 
 heating power of the rays is least in the violet and grad- 
 ually increases as the thermometer is moved towards the 
 red until the pale red of the spectrum is reached. 
 
 Xow whUe the results obtained in. this way are reliable, 
 yet they are reliable only for a solar spectrimi that has 
 been produced by the passage of a beam of light through a 
 prism of a certain kind of flint glass, and would not neces- 
 sarily be true in the case of a spectrum that has been 
 formed by the passage of light through a prism of some 
 other material. 
 
 It was Seebeck who was among the first to call attention 
 to the fact that the position of the greatest heating power 
 would vary according to the character of the material 
 that was employed in the construction of the prism. For 
 example, a spectrum formed by the passage of a beam of 
 sunlight through a hollow prism filled with water, has its 
 greatest heating power located not in the red but ta the 
 yellow, while, if the same prism is filled with alcohol in- 
 stead of water, its greatest heating power is found in the 
 orange-yellow. 
 
 These differences come about because the materials of 
 which the prisms are made absorb the heat waves, and the 
 measurements only give the portions of the spectrum that 
 
208 THE WONDEE BOOK OF LIGHT 
 
 have not been absorbed. This necessarily produces erro- 
 neous results. 
 
 Professor Langley, referring to the difficulty of obtain- 
 ing the position of the maximum heating or chemical power 
 of a prism, suggests that, in order to avoid the absorption 
 due to the material of the prism, a diffraction spectrum 
 should be employed. This is a spectrum formed by the 
 interference of the rays of light on their passage through, 
 or reflection from, a plate of glass ruled with a great num- 
 ber of parallel lines so close together that many thousands 
 can be crowded side by side in the space of an inch. With 
 such a spectrum the greatest heating rays, instead of being 
 found in a part of the spectrum different from that of the 
 minimum luminous rays, actually agrees with it. Both 
 of these positions are to be seen in the two curves repre- 
 sented in Fig. 77. Here, the full line represents the curve 
 of heat. Its greatest heating power is found not in the 
 infra-red but in the orange or the orange-yellow, which 
 corresponds closely to the dotted curve representing the 
 distribution of the luminosity; for, in the curves repre- 
 sented both in this and in the preceding figure, the values 
 of the heating power and the luminous power, etc., are 
 represented by the height the curve extends above the hori- 
 zontal line representing the spectrum. A curious thing 
 about the diffraction spectrum is that its curve of heat 
 extends much further below the infra-red than it does 
 above the ultra-violet. 
 
 It may be interesting to you to know that in his measure- 
 ments of the heating power of the spectrum. Prof. Lang- 
 ley employed an electrical apparatus called a bolometer. 
 This instrument depends for its operation on a well-known 
 
INVISIBLE LIGHT 
 
 309 
 
 principle in electricity, in which two wires, connected with 
 a voltaic battery or other electric source, form the two 
 arms of what is known as an electric bridge or balance. 
 When these arms or wires are at exactly the same tempera- 
 ture, the needle of a galvanometer placed in the circuit 
 
 
 
 
 
 
 
 
 
 
 
 
 ^^~ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 /- 
 
 -^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 / 
 
 ,■ 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 ,/ 
 
 / 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 / 
 
 r 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 / 
 
 / 
 
 
 
 
 
 
 •.. 
 
 
 \ 
 
 
 
 
 
 
 
 ^ 
 
 y 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 \_ 
 
 \ 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 -,^ 
 
 \ 
 
 
 
 
 
 
 
 
 
 A 
 
 
 B C 
 
 
 D 
 
 E 
 
 F 
 
 G 
 
 -A 
 
 
 
 
 
 
 
 
 
 1 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 10 
 
 
 9 
 
 
 8 
 
 
 7 
 
 
 i 
 
 
 r 
 
 
 t 
 
 
 A 
 
 Fig. 77. — Distbibution of Heat and Light in Diffbaction 
 Specteijm. 
 
 remains at rest, but if one of the wires or arms is heated 
 
 only a small fraction of a degree beyond the other, a change 
 
 at once occurs in the position of the needle. 
 
 The bolometer Langley employed was so delicate that 
 
 it could almost instantly detect a difference of temperature 
 
210 THE WONDEE BOOK OF LIGHT 
 
 of 0.00001° C. This difference of temperature was pro- 
 duced in a thin strip of metal 1-10000 of an inch in thick- 
 ness. To give you some idea of the small difference in 
 temperature the instrument could detect, I will give you 
 an illustration used by Professor Langley. While explor- 
 ing a certain portion of the ultra-violet rays of a diffrac- 
 tion spectrum, Langley was able, after an exposure of less 
 than ten seconds in a certain part of the spectrum, to detect 
 a difference of temperature so small that if the amount of 
 heat causing it had been permitted to fall for over 1000 
 years on a kilogramme of ice (2.3046 pounds avoirdupois) 
 at the temperature of melting the ice, it would have been 
 insufficient to melt it. 
 
 But let us now consider the rays of the spectrum that 
 are capable of causing chemical combination. 
 
 If a sheet of white paper, covered with a thin layer of 
 chloride of silver, is exposed to sunlight, a cheraical de- 
 composition takes place. The chloride is decomposed and 
 metallic silver, of a dark purplish black colour, is de- 
 posited on those portions of the paper that have been ex- 
 posed. 
 
 Another instance of chemical decomposition caused by 
 light occurs on a large scale in nature in the delicate 
 structures of the leaves of plants. The carbonic acid gas 
 of the atmosphere is absorbed or breathed in by the leaves. 
 When sunlight falls on them, nearly all the constituents 
 of the white light, except the greens, are absorbed. It is 
 the absorbed colours that cause the carbon and oxygen 
 atoms in the molecules of carbonic acid gas to vibrate to- 
 wards and from one another so violently that they are 
 finally carried beyond the distance at which they can mu- 
 
INVISIBLE LIGHT 211 
 
 tually hold on to each other. The carbonic acid is, there- 
 fore, decomposed, the carbon being retained for the 
 formation of the woody tissues of the plant, and the oxygen 
 thrown off into the air. 
 
 The tendency of coloured fabrics to fade or lose their 
 colours, on exposure to sunlight, is due to the chemical 
 decomposition of their colouring matter under the in- 
 fluence of sunlight. 
 
 An example of chemical combination being caused by 
 sunlight is seen in the case of a mixture of chlorine and 
 hydrogen gases. This mixture can be kept for a long 
 time without combining, provided light does not fall on 
 it, but will almost instantly enter into an explosive com- 
 bination under the influence of full sunlight. 
 
 The chemical substances employed in photographic oper- 
 ations, some of which will be referred to in a subsequent 
 chapter on photography, are influenced mainly by the light 
 waves that are situated in the neighbourhood of the blue 
 and violet rays of the spectrum. In the green leaves of 
 plants, however, it is the rays in the neighbourhood of the 
 reds, oranges and yellows, forming the absorbed portion 
 of sunlight, that cause the decomposition of the carbonic 
 acid that has been absorbed in the leaves of the plants. 
 
 It is evident, then, that there exists in the visible por- 
 tions of the solar spectrum between the violet and the 
 red, rays that are capable of causing the chemical decom- 
 position of many of the bodies on which they faU. In 
 other words, in addition to being able to produce the ef- 
 fects of light, these rays, when falling on certain kinds of 
 matter, are able to produce either heating or chemical 
 effects. 
 
312 THE WONDEE BOOK OP LIGHT 
 
 If a hollow glass prism, filled with the highly refractive 
 and transparent liquid known as bisulphide of carbon, is 
 placed in the path of a beam of sunlight, there will be 
 produced on a screen a beautiful solar spectrum containing 
 not only all the visible rays, between the reds and the 
 violets, but in addition the invisible rays of the ultra-violet 
 and the infra-red. Now mark on the screen the position 
 of the visible spectrum, replace the hollow prism filled 
 with the transparent carbon bisulphide by an exactly simi- 
 lar prism, in which the transparent" liquid has been ren- 
 dered black and opaque by dissolving in it a quantity of a 
 chemical element known as iodine. If a beam of white 
 sunlight is passed through this prism, which, while abso- 
 lutely opaque to light rays, is quite transparent to the 
 heat rays, there will be formed on the parts of the screen 
 where the coloured bands of light were seen between the 
 red and the violet, a heat spectrum absolutely invisible to 
 the eye, but capable of producing marked heating effects 
 on objects brought into its different portions. If a ther- 
 mometer or bolometer is moved along different portions of 
 this invisible spectrum, it will show the presence of heat 
 all the way from the ultra-violet to the infra-red, and, 
 indeed, would show an increase in the amount of heat in 
 the regions of the infra-red. 
 
 Kow the invisible heat rays of the spectrum so formed 
 are capable of being collected at a single point or focus 
 in the same manner as are the visible light rays. If, 
 therefore, the beam of heat rays is permitted to fall on 
 the surface of a concave mirror, or is passed through a 
 double convex lens, the rays will be brought to a point or 
 focus that I am sure you will agree possesses an exceed- 
 
INVISIBLE LIGHT 213 
 
 ingly wonderful property. Such a focus is absolutely in- 
 visible to the eye. Moreover, the air of that part of the 
 room in which it is situated, may be quite cold, as cold 
 indeed as the air in the rest of the room. So far as one 
 can see, this portion of the room possesses no property 
 different from the rest of the room. It does, however, pos- 
 sess astoxmdingly different properties; for if a piece of 
 lead, ziuc, or iron be brought into it, the metal at once 
 becomes so hot that it is melted. If a refractory material 
 (that is, a material that is capable of being raised to a high 
 temperature without melting) is brought into it, the metal 
 is raised to an intense white heat, and becomes a briLliant 
 source of light. 
 
 This curious power of obscure or invisible heat to raise 
 the temperature of a refractory body like platinum or 
 iridium to luminosity is called calorescence. Calorescence 
 is a wonderful phenomenon. Indeed, quite as wonderful as 
 any ghost or spectre you could conceive of. 
 
 I would caution you not to be so sure that seeing is 
 believing, or, what amounts practically to the same thing, 
 that because you fail to see a thing, you are, therefore, 
 justified in doubting its existence. Suppose I took you 
 into a well-illumined room and cautioned you to be careful 
 how you passed through a certain portion of the air of the 
 room, say some point near the middle of the room, situ- 
 ated about four feet above the floor. Suppose I told you 
 that that part of the room possessed the marvellous power 
 of burning jovl; that it would instantly make red hot any 
 part of your clothing that might be brought into it. You 
 possibly have so high- an opinion of your ability to see 
 things that you turn to me and declare you are surprised I, 
 
S14 THE WONDEE BOOK OF LIGHT 
 
 should think you so easy ; that you have eyes and know how 
 to use them ; that you are sure there is nothing in that part 
 of the room since you can see that there is nothing there; 
 that you will convince me I am wrong by deliberately 
 going to that particular part. If you should do this, I, 
 of course, seeing that you did not seriously injure your- 
 self by your folly, should permit you to bring say a por- 
 tion of your coat or of your pantaloons into the invisible 
 heat focus. 
 
 Instantly, you not only smell the burning cloth, and see 
 a bright glowing spot in the cloth, but you also soon feel the 
 intense heat focus, so that, running in terror from this 
 mysterious and awful place, you are ready to admit your 
 foolishness, and to acknowledge that there are things in 
 the room no one can see. You admit you are not justified 
 in believing that a thing does not exist simply because 
 you fail to see it. 
 
 But there are still other kinds of invisible rays. If the 
 ultra-violet rays of the spectrum arer brought to a focus 
 by reflection from the mirror, or by their passage through 
 a lens, there will also be formed a focus of rays beyond the 
 power of our eyes to see. This focus consists neither of 
 light rays nor of heat rays, but of the chemical rays lying 
 beyond the ultra-violet. If you permit these rays to fall 
 on a sheet of white paper that has been soaked in the 
 chemical substance known as the bisulphate of quinine, 
 the paper will become visible and will emit a peculiar 
 coloured light. 
 
 This method of causing the invisible light beyond the 
 violet, or the waves that are too rapid to affect the retina 
 of the eye, to be lowered in the number of their vibrations 
 
INVISIBLE LIGHT 215 
 
 so as to efEect the eyes is known as fluorescence, and wiU 
 be described in a subsequent chapter. 
 
 It would appear, therefore, that there are two species of 
 invisible rays; namely, those in the infra-red that are 
 unable to affect the eye because the waves they produce 
 per second are too few, and those in the ultra-violet that 
 are too rapid to afEect the eye. The first can be increased 
 as regards the number of waves or vibrations per second 
 untH they can affect the eye by calorescence. The other 
 can be decreased by fluorescence ra the number of waves 
 untU they are able to affect the eye. 
 
 But even the above does not include all the invisible 
 rays that exist in the solar spectrum. In a book entitled, 
 " "Waves and Eipples," by Dr. J. A. Pleming, of University 
 College, London, who has devoted considerable time to 
 this matter, there is an account given of the different kinds 
 of ether waves as far as they have yet been studied. In 
 order to show the extent of these waves, they are rated in 
 what are known as octaves, a word taken from music, and 
 meanrng the gamut or scale that exists between any two 
 successive notes of the same name, such as ut or C, and 
 its eighth higher note or octave; or between a re and its 
 octave, and so on. This is a convenient measure, siuce 
 the higher octave is always produced by exactly twice as 
 many vibrations as the lower note. Xow in the case 
 of light, there is, approximately, one octave between the 
 visible parts of the spectrum as contained in the red and 
 the violet ; that is, the most rapid visible rays in the violet 
 are produced by about twice as many waves per second 
 as the lowest visible rays in the red. 
 
 It has been shown that, in addition to the so-caUed lu- 
 
216 THE WONDEE BOOK OF LIGHT 
 
 minous ether waves between the red and the violet, there 
 are above the violet rays two octaves, and, in addition, a 
 number of shorter ultra-violet rays, and possibly, even be- 
 yond these, the so-called ultra-ultra-violet rays which, in 
 the opinion of some, constitute the X-rays. 
 
 Then coming down into the region of the infra-red (that 
 is, to the rays that are situated further from the red in a 
 direction away from the violet towards the red) we have 
 seven octaves of dark heat waves. 
 
 Then come five octaves of a radiation the character of 
 which is as yet unknown. Still further down in the spec- 
 trum, we come to the electro-magnetic waves, the ezist- 
 ence of which was first known by Hertz, but predicted 
 on theoretical grounds by Maxwell. Of these, there are 
 at least twelve octaves, and still beyond these are ether 
 waves, a thousand feet or more in wave length, that are 
 employed in wireless telegraphy. 
 
CHAPTEE XIX 
 SPECTRUM ANALYSIS 
 
 As you probably know, it is the province of chemistry 
 to determine the composition of a body, either by a process 
 called analysis, or a separation into its different elements ; 
 or by a process called sjmthesis which causes its constituent 
 elements to combine with one another in the proper 
 amounts so as to form the substance. The exact composi- 
 tion may be accurately determined by either of these pro- 
 cesses. To make this clearer I will give you an example. 
 Common table salt, when analysed, is found to consist of 
 two elements, a metal called sodium and a greenish yellow 
 gas called chlorine, these two substances being combiaed 
 chemically ia the proportion of one atom of sodium to 
 one atom of chlorine. 
 
 The composition of table salt can also be determined 
 by synthesis: If we cause sodium and chlorine to com- 
 bine, in the proportion of one atom of sodium to one atom 
 of chlorine, we will obtain a substance whose properties 
 in all respects resemble the properties of common table 
 salt. We will, then, again have proved that this substance 
 consists of an atom each of sodium and chloriae chemi- 
 cally combined. 
 
 The science of chemistry is now in such an advanced 
 condition that it is a comparatively easy matter to 
 determine the composition of all known substances 
 by analysis, and of many substances by synthesis.. 
 
 217 
 
318 THE WONDEE BOOK OF LIGHT 
 
 To determine the composition of a substance, it is 
 only necessary to separate a small quantity of the sub- 
 stance into its constituent elements by analysis, and then, 
 where it can be done, to cause these elements to re-com- 
 bine, so as to produce the substance by synthesis. When 
 you come to study chemistry, you will find but compara- 
 tively few cases of substances produced by animal or 
 plant life that can be formed by synthesis. 
 
 One of the most wonderful things connected with light 
 is that there is now known a variety of analysis by which 
 the chemical composition of a substance can be determined 
 without the chemist being obliged to obtain a specimen of 
 the substance that is to be analysed. All that is re- 
 quired is to examine the light given ofE from the substance 
 after it has been raised to a sufficiently high temperature 
 to change it into a vapour and to cause the vapour to glow 
 or become self luminous. 
 
 In this kind of analysis it makes no difference how far 
 off the body may be that is being analysed. If its light 
 reaches us, we can determine its chemical composition al- 
 most as well as if it were only a few feet distant. 
 
 As you know, many of the fixed stars are so far away 
 from the earth that it takes their light, although travelling 
 with the enormous velocity of 186,680 miles per second, 
 hundreds or even thousands of years to reach the earth. 
 It is possible that some of the dim or hazy bodies called 
 nebulae are even further off. But if we can only obtain a 
 specimen of their light, we can determine the chemical 
 substances that exist in them in the form of glowing 
 vapours, quite as well as if we had specimens of these sub- 
 stances in a laboratory only a few feet distant. 
 
SPECTRUM ANALYSIS 219 
 
 Before explaining how this kind of analysis is possible, 
 I wish you to think the matter over, so as to be sure you 
 appreciate how exceedingly great a wonder it is. Think 
 of bodies that are miUions of millions of millions of 
 miles distant being obliged to reveal to the inquisitive 
 physicist exactly what chemical elements are glowing 
 in their atmospheres. It is indeed a great wonder. I re- 
 member a jingle that was impressed on my mind in the 
 nursery. I then thought it very beautiful poetry. I 
 feel sure you have heard it, for it forms a portion of the 
 folk-lore that nurses are fond of reciting to their young 
 charges. It runs as follows : 
 
 "Twinkle! twinkle! little star. 
 How I wonder what you are! 
 Up above the world so high. 
 Like a diamond in the sky." 
 
 As soon as I have explained spectrum analysis to you, 
 such a star no longer possesses any mystery. It may go on 
 twinkling as it has been twinkling for many millions of 
 years. But it can no longer continue twinkling mysteri- 
 ously, so as to hide its composition. It is only necessary 
 to look at it through a certain piece of optical apparatus 
 and it can instantly be compelled to declare just what 
 kind of glowing or luminous chemical elements it has in 
 its atmosphere. 
 
 I am sure it will please you when you learn that the 
 wonderful instrument that can analyse a body many mil-, 
 lions of miles distant is no other than the slightly modified 
 magic wand of our acquaintance Prince Percinet, 
 
220 THE WONDER BOOK OP LIGHT 
 
 I think it is almost certain that Prince Percinet knew 
 all about this additional property of his fairy wand. He 
 was such a bright fellow, and did so many other wonder- 
 ful things that I am disposed to believe he not only knew 
 of this, but even of other properties that we slow mortals 
 have not yet discovered. Indeed, I can picture to myself 
 the possibility that, on a certain night he was walking with 
 Graciosa through the garden in his mother's palace, when 
 Graciosa repeated to him the nursery rhyme : " Twinkle, 
 twinkle, little star," Percinet, turning to Graciosa, re- 
 marked : 
 
 " There is nothing wonderful about that star, my dear ; 
 we fairies know exactly what it is," and then holding his 
 wand so as to look through it from one end to the other 
 he remarked: 
 
 " I can distinctly see in the atmosphere of the star 
 glowing masses of hydrogen, sodium, magnesium, calcium 
 iron, bismuth, tellurium, antimony, and mercury." 
 
 Of course this was all Greek to Graciosa, but she prob- 
 ably replied: 
 
 " Is that so, my dear ? How extremely wonderful ! " 
 
 Of course what Percinet saw when he looked through 
 his magic wand, that is, through a prism, was something 
 peculiar about the appearance of the spectrum of the star; 
 that is, the stellar spectrum. It is possible in the same 
 way to determine the composition of the sun by the ap- 
 pearance of the spectrum of sunlight. 
 
 This method of analysing a substance, or of determining 
 its chemical composition from the peculiarities of the spec- 
 trum of the light it emits, when in the form of a glowing 
 vapour, is known as spectrum analysis. Let us, therefore. 
 
SPECTRUM ANALYSIS 221 
 
 endeavour to understand the principles on which this 
 peculiar kind of analysis depends. 
 
 Spectrum analysis depends on the fact that if the rays 
 of light, coming from a vapourised Iximinous body, are ex- 
 amined through a prism, the composition of the body can 
 be determined by the peculiar character of its light. A 
 pure yellow light, as we have seen, can readily be obtained 
 by placing a small quantity of common salt in the colour- 
 less flame of a Bunsen burner. The peculiar colour of 
 this light is due to the presence of highly heated or glowing 
 particles of a volatile sodium salt. In the same way a 
 volatile salt of copper imparts a deep green tint to the light 
 of a Bunsen burner owing to the glowing particles of the 
 copper salt. Zinc imparts a purplish colour to the flame, 
 and the metal strontium a deep crimson colour. 
 
 Now when the light, given off from glowing sodium 
 salt, is passed through a prism, instead of producing a 
 continuous spectrum or band of different colours from the 
 red to the violet, it produces only a few bands of orange 
 and yellow light that are separated from each other by 
 dark spaces. In other words, the incandescent vapour of 
 the sodium salt is capable of giving off only a few different 
 rays in the neighbourhood of the orange-yellow part of 
 the spectrum. Its spectrum, therefore, consists of these 
 colours only; all the other regions are absolutely dark. 
 
 In a similar manner, the light given off by the vapour of 
 zinc consists of a few brilliant purplish bands; that is, 
 glowing zinc vapour can throw off purplish light and pur- 
 plish light only. In a similar manner the light produced 
 by the glowing copper salt gives a series of green bands 
 only. 
 
22H THE WOKDEE BOOK OF LIGHT 
 
 The possibility of determining the composition of a sub- 
 stance that exists in the form of an incandescent or glow- 
 ing vapour depends on the fact that the light the substance 
 then throws off is characteristic of that particular element 
 only. The glowing vapours of no two different elements 
 in the universe give off light of the same wave length or 
 colour. Glowing sodium vapour emits a light characterised 
 by the peculiar orange-yellow bands before referred to ; and 
 the glowing vapour of no other substance is capable of giv- 
 ing off the same kinds of light. Glowing zinc vapours give 
 off a purplish coloured light that is unlike the light 
 emitted by the glowing vapour of any other chemical sub- 
 stance, and the same is true of all the known chemical 
 substances. 
 
 The eye is unable to pick out the different coloured rays 
 present in the lights given off from different glowing sub- 
 stances. It is, however, an easy matter to examine such 
 light through a prism, and thus at once determine the name 
 of the chemical substance from the peculiarity of its light. 
 
 There are a fairly great number of different chemical 
 elements that produce more than a single coloured ray. 
 You can see, therefore, that it must have taken long and 
 patient study on the part of scientific men to determine 
 the peculiarities of the spectra produced by each of the 
 different chemical elements when in a state of incandescent 
 or glowing vapour. 
 
 I would say here that if any substance, no matter what 
 its chemical composition may be, is raised to intense 
 incandescence while in the solid or liquid state, it will 
 become white hot; that is, will emit a light that would 
 produce a continual spectrum, or a spectrum in which all 
 
SPECTRUM ANALYSIS 233 
 
 the different coloured rays are present from the red to the 
 violet. It is impossible, therefore, to analyse an elementary 
 substance, or to determine its chemical composition, by 
 examining its light when heated to incandescence in a 
 solid or a liquid condition. In order that the substance 
 may give off a characteristic light, it is necessary that its 
 molecules and atoms shall possess the freedom of motion 
 they have only when in the condition of a vapour, and that, 
 moreover, this vapour be highly heated, so that not only its 
 molecules, but also the atoms within the molecules, are 
 made to move violently to-and-fro, and thus produce char- 
 acteristic waves in the ether surrounding them. 
 
 It was Prof. Bunsen, the great German chemist, and 
 Prof. Kirchhoff, the equally great German physicist, who, 
 while examining the light given ofE from certain glowing 
 materials, detected the presence of peculiar spectra that 
 were produced by no known substances on the earth. They, 
 therefore, properly came to the conclusion that what they 
 saw was the spectra of two hitherto undiscovered chemi- 
 cal elements. After making an extended chemical investi- 
 gation, they proved that this inference was correct by iso- 
 lating the then new chemical elements, rubidium and 
 caesium. 
 
 It was in a similar manner that the English physicist 
 Crookes discovered the existence of the new element thal- 
 lium. 
 
 As in the ease of many other wonders of the physical 
 universe, the wonder of spectrum analysis, although at 
 first sight seemingly beyond the powers of the human mind 
 to comprehend, becomes ridiculously simple as soon as 
 explained. You can see how easy it should be to deter- 
 
334 THE WONDER BOOK OP LIGHT 
 
 mine the composition of a chemical substance from an 
 examination of its spectrum, if it can be shown that every 
 chemical element, when in the condition of a glowing va- 
 pour, gives off light that is characteristic of that element 
 only. As soon as this fact is appreciated, the wonder 
 completely disappears. 
 
 There are, however, some phenomena connected with 
 spectroscopic analysis that are more difiBeult thoroughly 
 to understand. 
 
 The rays of the solar spectrum lying between the red 
 and the violet possess the power of causing not only the 
 sensation of light, but also, as we have seen, when ab- 
 sorbed while passing through different kinds of matter, 
 of causing heat. 
 
 Certain substances are quite transparent to the ether 
 waves; these waves can pass through them, between their 
 molecules, without imparting any motion to the molecules, 
 that is, without heating them. There are other substances, 
 however, so opaque to ether waves that when these waves 
 endeavour to pass through them they are absorbed or 
 quenched; and, if they were originally luminous, they are 
 no longer able to produce the sensation of light, but are 
 changed into heat waves. 
 
 Now the light given off by any glowing incandescent 
 vapour is absolutely opaque to light of the same wave length 
 as its own. The glowing sodium vapour, in the flame of 
 the Bunsen burner, that emits orange-yellow light only, 
 is perfectly transparent to all the other coloured rays, per- 
 mitting them to pass without absorption. But when rays 
 of the same orange-yellow light attempt to pass through, 
 they are completely stopped. The orange-yellow light of 
 
SPECTRUM ANALYSIS 325 
 
 glowing sodium vapour is opaque to light of the same wave 
 length as its own, and this is true of the green light given 
 o£E by glowing copper vapour; of the purplish light of 
 glowing zinc vapour; of the reddish or crimson light of 
 glowing strontium vapour, etc. 
 
 An interesting discovery that at first gave scientists con- 
 siderable trouble has been made concerning the solar spec- 
 trum. In order to be able to examine this peculiarity of 
 the spectrum it is necessary that the instrument employed 
 be what is called a pure spectrum. When a beam of light 
 passing through a slot or opening in the shutter of a 
 darkened room is permitted to form a solar spectrum, un- 
 less the slot is very narrow, the spectrum will consist of a 
 series of separate bands of colour overlapping one another. 
 If we wish to examine all the peculiarities of a solar spec- 
 trum it is necessary not only that the opening in the shut- 
 ter shall be exceedingly narrow, but also that the beam of 
 light shall have its separate colours dispersed as far from 
 one another as possible, so that the separate bands shall 
 not overlap, but lie side by side. 
 
 In order to ensure a marked dispersion of the different 
 rays, it is necessary to employ more than a single prism, 
 and to permit the beam of light to pass successively 
 through the separate prisms. A narrow slice of light 
 passed successively through nine separate prisms causes a 
 great dispersion or separation of the different coloured 
 rays. 
 
 When great care has been taken to obtain a pure solar 
 spectrum, so that the successive images of the slot do not 
 overlap, and, moreover, when a spectrum has been obtained 
 in which different rays of light are widely separated by 
 
226 THE WONDER BOOK OP LIGHT 
 
 dispersion, it is found that the coloured bands lying be- 
 tween the red and the violet are not continuous. In 
 certain portions of the spectrum, some of these bands are 
 missing, and their places are occupied by numerous dark 
 lines. This peculiarity of the solar spectrum was first ob- 
 served by Dr. Wollaston, the celebrated English physicist. 
 It was afterwards studied with unusual care by Dr. Praun- 
 hofer, who carefully observed and mapped out the various 
 dark lines, marking their position by the letters. A, a, 
 B, C, D, E, b, E, 0, and H. The approximate position 
 of these lines in the solar spectrum is represented in Eig. 
 76 (page 206.) These lines are called the Eraunhofer 
 lines in honour, not of their discoverer, for that was Wol- 
 laston, but of the man who by careful and patient study 
 showed how they could be employed. 
 
 The cause of the Eraunhofer lines gave physicists con- 
 siderable trouble. The problem, however, was at last 
 solved by Eraunhofer, who gave so complete an explanation 
 of these lines that it became possible to ascertain the 
 chemical composition of a body like the sun, even though 
 its central mass gave a conscious spectrum, provided only 
 it had glowing vapours in its atmosphere. 
 
 The Eraunhofer lines are caused as follows: As we 
 have seen, a glowing vapour is opaque to light of the same 
 colour it is itself emitting. The orange-yellow light emit- 
 ted by glowing sodium vapour is opaque to light of the 
 same colour, but perfectly transparent to light of all other 
 colours. If, therefore, a beam of white light is sent through 
 a mass of glowing sodium vapour, all its coloured rays 
 except a few orange and yellow rays, will be able to pass. 
 These few orange-yellow rays, however, will be completely 
 
SPECTEUM ANALYSIS 227 
 
 stopped. If the spectnim of a beam of the white light 
 obtained from an electric arc lamp is thrown on a screen, 
 it will produce a continuous band of colour from the red 
 to the violet. If, now, before it is permitted to pass through 
 the prism, this light is caused to pass through a mass of 
 glowing sodium vapour, a few orange and yellow rays wiU 
 be absorbed, so that, instead of a continuous spectrum be- 
 ing thrown on the screen, dark bands will be seen occupying 
 the position of the few orange-yeUow rays that have been 
 absorbed. If, instead of sodium vapour, a mass of glowing 
 zinc vapour is introduced into the path of the beam, certain 
 rays of purplish light will be absorbed and dark bands 
 will be seen in the part of the spectrum formerly oc- 
 cupied by this light; and the same is true for the green- 
 ish light produced by glowing copper vapour as well 
 as by the peculiar light produced by any other glowing 
 vapour. 
 
 If, therefore, the sun, which as we know emits a continu- 
 ous spectrum from the red to the violet, should have in 
 its atmosphere glowing masses of vapours of the different 
 chemical elements, as must exist by reason of its high tem- 
 perature, these vapours, being opaque to the particular 
 coloured lights they are emitting, prevent the passage of 
 these particular rays from the sun, so that its spectrum, 
 instead of being continuous, is characterised by a series of 
 dark lines. These are the Fraunhofer lines. 
 
 If you consider the matter sufficiently you will see that 
 the Fraunhofer lines do not mark positions of absolute 
 darkness, since these lines are, in reality, illumined by the 
 light given off from the glowing vapours in the sun's atmos- 
 phere. But since their light is feeble when compared with 
 
328 
 
 THE WONDER BOOK OF LIGHT 
 
 the light given off by the sun itself, they appear dark by 
 contrast. 
 
 It will certainly interest you to know that when analysed 
 in this manner, the distant star Aldebaron is found to 
 have in its atmosphere glowing masses of hydrogen, so- 
 dium, magnesium, calcium, iron, bismuth, tellurium, anti- 
 
 FiG. 78. — ^A Specteoscope. 
 
 mony, and mercury, and that the star Alpha, in the great 
 constellation of Orion, has in its glowing atmosphere, 
 sodium, magnesium, calcium, iron, and bismuth. 
 
 When the distant nebulae are examined in the same way, 
 it is seen that they consist of masses of highly heated 
 gaseous matter in most cases the elementary gases hydro- 
 gen and nitrogen. 
 
SPECTRIJil ANALYSIS 229 
 
 Much study has been given to the solar spectrum and 
 to the mapping or locating of the Fraunhofer lines. Fraun- 
 hofer counted 600 of these dark lines in the solar spectrum ; 
 Brewster mapped out 2,000 of them, while more recently 
 over 3,000 have been accurately located. By passing the 
 light successively through a number of prisms, many of 
 the lines that were apparently single have been shown to 
 consist of two or more lines. 
 
 The apparatus by means of which spectroscopic analysis 
 is rendered possible is known as the spectroscope. This 
 apparatus consists, as shown in Fig. 78, of three telescopes 
 A, B, and C, mounted on a common stand, pointing as 
 shown towards a prism P, of iiint glass. The telescope A 
 is arranged so as to be turned around the prism, and, when 
 placed in the position desired, can be firmly clamped by 
 means of the clamp screw n. A screw m is provided for 
 the purpose of obtaining a sharp image of the spectrum by 
 the focusing of an eye-piece. 
 
 The use of the two telescopes B and C, wUl be under- 
 stood from an examination of Fig. 79. The substance 
 that is being examined is placed in the flame G. The 
 light its glowing vapour emits, passing through the lens a, 
 is converged to a focus 6, so that, when it passes through 
 the lens c, it leaves the telescope B as a beam of parallel 
 rays. These rays, after passing through the prism P, in 
 which they are dispersed, enter the lens x placed at one 
 end of the telescope A, and form an inverted image of the 
 spectrum at i, which when viewed by the observer, is seen 
 as a magnified image eight times as large as the image the 
 eye-piece is examining. 
 
 The telescope C is provided for the purpose of measur- 
 
230 
 
 THE WONDEK BOOK OF LIGHT 
 
 ing the relative distance the different lines of the spectrum 
 are apart. 
 
 It may be interesting here, before leaving the subject of 
 spectroscopic analysis, to call your attention to the fact 
 
 Fig. 79. — Passage of Light Thbough Spectboscope. 
 
 that in the above descriptions we have referred to three 
 different kinds of spectra; namely: 
 
 (1). Continuous spectra, or those produced by solids or 
 liquids heated to incandescence. 
 
 (2). Band or line spectra, or those produced by glow- 
 ing masses of incandescent vapour. These spectra are dis- 
 continuous and consist of light of a few wave lengths only. 
 
 (3). Absorption spectra due, as are the dark Fraun- 
 hofer lines, to the absorption of the light of an otherwise 
 continuous spectrum by the glowing masses of vapour that 
 are opaque to the particular coloured light they are emit- 
 ting. 
 
CHAPTER XX 
 DISEASES OR ABERRATIONS OP LENSES 
 
 Kow that we have not only studied the peculiarities of 
 the different foci of lenses, but have also learned about the 
 different coloured rays that exist in sunlight, I can keep 
 my promise to tell you something about the failure of 
 lenses properly to reproduce in their images the correct 
 outlines and colours of the objects placed before them. 
 These difficulties are known as the aberrations of lenses. 
 
 As you probably know, the word aberration means a 
 wandering or departure, and is applied to a lens to indi- 
 cate an effect different from what the lens would produce 
 if it were working properly, or were in what might be 
 called by some flight of the imagination, a healthy 
 condition. 
 
 Speaking now of lenses only, for some of these troubles 
 are found in mirrors also, I will say that a lens possesses 
 three different kinds of wanderings or aberrations, any 
 one of which will prevent the image it produces from 
 closely resembling the object. Since the word aberration 
 is difficult to remember, I have taken the liberty of re- 
 garding as unhealthy or diseased, those lenses which fail 
 to produce the images that normal healthy lenses should 
 produce. In this sense we should use the word diseases 
 as correctly expressing or meaning aberrations. 
 
 Double convex lenses are especially apt to possess dis- 
 
 231 
 
333 THE WONDEE BOOK OF LIGHT 
 
 eases in the outlines of their images. Instead of forming 
 a single image of sharp and correct outlines, they are apt, 
 under certain circumstances, to produce a number of sep- 
 arate images that are spread at different distances from 
 the lens along a line called the principal axis of the lens; 
 i. e., a straight line passing through the centres of the 
 opposite faces of the lens. This aberration makes it im- 
 possible to obtain a distinct image of the object. Another 
 kind of aberration causes the straight lines in objects to 
 appear as curved lines in the images, while curved lines 
 
 w 
 
 Fig. 80. — One of the Diseases of Lenses. 
 
 in the objects are incorrectly reproduced, and the image is 
 again indistinct. Let me try to explain to you the causes 
 of these two different kinds of lens diseases. 
 
 When a double convex lens has a width that is large in 
 comparison with the amount of the curvature of its oppo- 
 site faces, its images, as represented in Fig. 80, will possess 
 an aberration of the first character, so that a number of 
 images will be formed along the principal axis. As you 
 have already seen, a well-formed double convex lens should 
 bring parallel rays of light to a single focus on the oppo- 
 site side of the lens known as the principal focus. Now 
 
DISEASES OE ABEEEATIONS OF LEXSES 233 
 
 ■when the lens is too greatly swelled, or puffed out, at its 
 opposite faces in proportion to its width; or, in other 
 words, when its opposite faces are too greatly hulged, par- 
 allel rays of light will not be brought to a single focus F, 
 but, as shown in the above figure, to a number of separate 
 foci between F and G; for the rays of light falling at the 
 edges V and W, of the lens, are brought to a focus at GT 
 that lies nearer to the lens than the principal focus F; 
 while the rays falling near the middle of the lens are 
 brought to a focus at F, and rays intermediate between 
 these points are brought to different foci between F and 
 G. This character of aberration is known as longitudinal 
 spherical aberration. This long phrase only means that 
 the images wander out of place in the direction of a 
 straight line, because the lens has its opposite faces too 
 much curved or swelled. 
 
 The other disease of double convex lenses is that in 
 which the straight lines of its images are curved and the 
 curved lines improperly curved. Tliis disease is also 
 caused by the too great curvature of the lens; that is, by 
 the lens being too greatly swelled, curved or puffed out at 
 its opposite faces. When an object possessing straight 
 lines is placed before such a lens, if the images of its ex- 
 tremities are formed at the same distance from the centre, 
 the lines of the image will not be straight, but will be 
 curved. Since, however, the further an object is from a 
 convex lens, the nearer its foci will be to the other side of 
 the lens, the curvature is even more pronounced. 
 
 If, in order to avoid either of these false shapes the 
 image is sharply focused, say on a plate of ground glass, 
 so that the central portions shall be as correct in outlines 
 
334 THE WONDEE BOOK OP LIGHT 
 
 as possible, the distortions at the edges will be greatly 
 magnified. If, on the other hand, an effort is made to 
 focus the extremities of the object, the outlines of the cen- 
 tral part will be very irregular. This aberration is known 
 as the aberration of sphericity, and is only another way 
 of saying that the error is due to its central portions being 
 too much puffed or swelled out. 
 
 Either of these two kinds of aberrations of lenses se- 
 riously interferes with the proper operation of such optical 
 instruments as the telescope, the microscope, and especially 
 the photographic camera. 
 
 You can understand how serious a defect would be pro- 
 duced in a photograph of a person's face if a too spheri- 
 cal or bulged image-forming lens should be employed in 
 the camera. Such a lens would perhaps form an image of 
 a person's face with the line of the mouth improperly 
 curved, or an extra crook on the nose, with the eyes and 
 ears even a small fraction of an inch out of place. It 
 would produce, indeed, a dreadful effect. A photographer 
 who would offer his customers such a photograph would 
 certainly be unable to build up a trade, nor could he suc- 
 ceed in making his customers believe, as I understand is 
 sometimes done, that a photographic camera cannot lie. 
 On the contrary, if its image-forming lens is diseased, such 
 a camera is less to be believed than either Ananias or his 
 wife Sapphira. 
 
 Since both of the above errors are most marked in the 
 cases of lenses that are too greatly swelled on their oppo- 
 site faces, instead of trying to remember what these kinds 
 of aberrations are called — namely, longitudinal spherical 
 aberration, and the aberration of sphericity — I will be sat- 
 
DISEASES OE ABEEEATIOXS OF LENSES 235 
 
 isfied if you simply remember what they are and how they 
 are caused ; namely, by the opposite faces of the lens being 
 too greatly puffed out or swollen. If you should have any 
 difficulty in remembering this, I would suggest that you 
 might find a resemblance between this particular kind of 
 diseases in lenses, and a disease that I fear is not entirely 
 absent at times in young people, and which indeed, I am 
 free to confess, is sometimes present in older people. I 
 refer to a disease or defect correctly characterised by that 
 somewhat vulgar term, " a swollen head." As you well 
 know, such a diseased head leads its possessor to form too 
 high an estimate of his or her ability to observe or sit in 
 judgment on something they are contemplating. They 
 are unable to form a proper mental image of it, and, there- 
 fore, like a diseased lens, are apt to incorrectly describe its 
 nicer or finer details. 
 
 But there is another kind of aberration or disease of 
 lenses that is often far more serious than either of the 
 above. This is the kind known as chromatic aberration. 
 It results in the formation of images that fail to possess 
 the colours of the objects. Let us, therefore, carefully look 
 into this form of lens disease. 
 
 When a ray of daylight, or ordinary white sunlight, 
 passes through a lens it is not only bent out of its course 
 or refracted, but is also dispersed or separated into its 
 different coloured rays. Eor example, rays of light that 
 have passed through the extremities of a double convex 
 lens, instead of collecting at a single focus on the other 
 side of the lens, are separated into different coloured 
 rays just as they would be in passing through a prism. 
 Now, since the different coloured rays possess different 
 
236 THE WONDEE BOOK OF LIGHT 
 
 refrangibilities, the violet rays being much more refrangi- 
 ble than the red, these rays must collect at different 
 foci. If, therefore, rays of light coming from the il- 
 lumined object pass through the edges of a convex lens, 
 instead of forming a single image of the object they will 
 tend to form a number of separate coloured images that 
 occupy different positions between the violet and the red. 
 
 The presence of chromatic aberration in lenses was first 
 observed by Newton when he was studying refracting tele- 
 scopes. The telescopic images presented coloured fringes 
 at their edges. Newton endeavoured so to improve the con- 
 struction of his lenses as to avoid this difficulty and after 
 many efforts came to the conclusion that the difficulty 
 arose from the light itself and not from the lens. Under 
 the belief that the dispersive power of a lens was pro- 
 portional to its refractive power, or, in other words, 
 that if a lens formed of one kind of glass possessed a 
 greater dispersive power than another, it would, at the 
 same time possess a correspondingly greater refractive 
 power, he concluded that he could never hope to be able to 
 construct a lens free from chromatic aberration; or, as 
 such lens is now called, an achromatic lens. 
 
 In one of the other Wonder Books I had occasion to call 
 your attention to the fact that scientific men generally pos- 
 sess an advantage over men of the law, in that they care 
 very little for what is called a precedent, a thing that is so 
 highly believed in by gentlemen of the legal profession. I 
 regret to say, however, that so far as the production of an 
 achromatic lens was concerned, scientific men were very 
 much at fault, since they permitted the authority of New- 
 ton to stand in the way of an attempt to produce this 
 
DISEASES OR ABEEEATIOXS OF LEASES 23? 
 
 device. Fortunately, after the world waited some eighty 
 years, a practical optician, John DoUond, came to the 
 conclusion that he would test the correctness of Xewton's 
 conclusions. He did so and found them to be erroneous; 
 for he found that the dispersive and the refractive powers 
 were not directly proportional to each other in different 
 media. The result was that DoUond was soon able to 
 produce an achromatic lens. 
 
 Both Newton and DoUond saw that if two prisms were 
 placed with their refracting angles opposed to each other, 
 whatever one prism did in separating the rays, the other 
 prism would do in the oppo- 
 site direction, and so bring the 
 rays all together again, and 
 thus permit them to emerge 
 
 free from false colouration. 
 
 _. „ ■ Fig. 81. — Equal and Oppo- 
 
 it, lor example, as repre- site Refbaction and Dis- 
 sented in Fig. 81, two prisms pebsion. 
 A and B, be placed with their refracting angles g and g', 
 opposed, then a beam of light S, would be dispersed or 
 separated into its prismatic colors while passing through 
 the prism A. If, however, the second prism B, possess the 
 same dispersive powers, then all these colours wiU be 
 brought together again, so that the beam would pass out 
 from the prism B, as white or uncoloured light. 
 
 ISTow if, as Newton believed, the prism B must have its 
 refracting angle g' of the same value as the refracting 
 angle g, of the prism A, then the medium AB, would have 
 parallel sides, the beam would emerge parallel to its orig- 
 inal direction, and therefore would not be a prism. 
 
 When, however, it was found that some media possess 
 
238 THE WONDEE BOOK OP LIGHT 
 
 twice the dispersive powers of others, the matter assumed 
 an entirely different appearance. Eor example, flint glass 
 possesses twice the dispersive power of crown glass. If, 
 therefore, a prism of flint glass B, Pig. 82, has one of its 
 sides cemented, as shown, to a prism A of 
 crown glass, and with refracting angle b 
 twice as great as a, it would form an 
 achromatic prism; for, although a ray of 
 light passing through A would undergo 
 dispersion it would, after passing through 
 
 ciPLEop ^^'^' ^' ^^^® ^^ -""^y® dispersed equally in the 
 
 AcHEOMATisM. opposite direction, and thus be free from 
 
 false colour, yet it would also be refracted; or, in other 
 
 words, as already stated, the combination would form an 
 
 achromatic prism. 
 
 The manner in which this principle is applied to the 
 construction of an achromatic lens is shown in Pig. 83, 
 where a double convex lens B of crown glass is cemented 
 to a lens of flint glass A shaped as shown. In a i 
 
 this combination the dispersive powers of the 
 two kinds of glass completely neutralise each 
 other, but not their refractive powers, so that ' 
 
 the combination forms an achromatic lens. 
 
 But there is another advantage possessed by Fig. 83. 
 achromatic lenses. Herschel has shown that an ^f ^jp 
 achromatic lens formed of flint and crown glass. Lens. 
 shaped and arranged as shown in the preceding figure, is 
 not only capable of forming an image free from prismatic 
 colours, but also one that is nearly free from spherical 
 aberration. 
 
 Achromatic lenses, suitable for use in telescopes, are very 
 
DISEASES OR ABEEEATIONS OF LEXSES 239 
 
 unsatisfactory in microscopes. It is difficult to construct 
 achromatic lenses sufficiently small for the high magnify- 
 ing powers required in a microscope. In order to over- 
 come these difiBculties it has heen found that the hest 
 results for use in 
 microscopes are ob- 
 tained by the use of a 
 number of separate 
 achromatic lenses. 
 
 Like lenses, mirrors 
 also suffer from aberra- 
 tions. When a concave 
 mirror is formed by too 
 great a section of a 
 sphere, parallel rays of 
 light, instead of being 
 brought to a single 
 point or focus, are 
 brought to differently situated foci as represented in Fig. 
 84, in which, as will be seen, the rays falling at the edges 
 of the mirror are brought to foci nearer the mirror than 
 those reflected from near its central portions. Instead, 
 therefore, of a single focus being formed, there are a num- 
 ber of different foci by the intersection of which a brilliant 
 curve of light known as a caustic curve is produced. 
 
 Fig. 84. — Caustic Curve, a com- 
 mon Disease of Miebobs. 
 
CHAPTEE XXI 
 FLUORESCENCE AND PHOSPHORESCENCE 
 
 If the invisible violet rays of the solar spectrum are 
 brought to a focus either by a lens, or a concave mirror, 
 and this focus is caused to fall on certain substances, such 
 as a solution of bisulphate of quinine, the ether waves are 
 slowed down, so as to produce a beautiful bluish light. 
 This effect, known as fluorescence, was first carefully 
 studied by Stokes. 
 
 There are various substances that possess this power 
 of so slowing down the ultra-violet rays of light as to 
 permit them to affect the eye. When the bark of the 
 common horse-chestnut tree is boiled in water, the clear 
 solution is invisible in ordinary light, but gives out a 
 bluish light when brought into the ultra-violet rays of the 
 spectrum. The simplest way to obtain this light is to 
 permit the spectrum of sunlight, or of an electric light, to 
 fall on a sheet of white paper soaked in the horse-chestnut 
 solution. The length of the spectrum is then greatly in- 
 creased; for, instead of being visible only between the red 
 and the violet, it now extends far beyond the violet. 
 
 The light of the carbon voltaic are produces a much 
 greater quantity of the ultra-violet rays than the light of 
 the sun. Prisms formed of clear quartz crystals are almost 
 completely transparent to ultra-violet rays, and differ, in 
 this respect, from prisms made of ordinary glass that are 
 opaque to a large percentage of the ultra-violet rays. If a 
 
 240 
 
FLUORESCENCE AND PHOSPHORESCENCE 241 
 
 beam of light from an electric arc lamp is caused to 
 pass through a quartz prism, the length of the ultra-violet 
 spectrum will be from six to eight times longer than the 
 distance between the red and the violet rays. If such a 
 spectrum is received on a sheet of paper moistened with 
 the water, in which the bark of the horse-chestnut tree has 
 been boiled, the length of the visible spectrum wiU be con- 
 siderably increased. A space beyond the violet of from 
 six to eight times the distance between the red and the 
 violet rays, will now shine with fluorescent light. 
 
 You have all probably seen the greenish coloured glass, 
 known as uranium glass, the colour of which is due to 
 the presence of a small quantity of a chemical substance 
 containing uranium. This glass possesses the power of 
 fluorescence in a high degree, so that if, by means of a lens, 
 a pencil of the ultra-violet rays is thrown within its mass, 
 the path of the rays becomes visible as a bright green 
 light; for the rays as they pass through the glass become 
 luminous by fluorescence. 
 
 Some of the many other substances that also possess 
 the power of fluorescence are the green colouring matter 
 of leaves, obtained by soaking ■ bruised leaves in alcohol; 
 ordinary petroleum, or coal oil; a solution of turmeric in 
 alcohol; various kinds of fluorspar, etc. It may be inter- 
 esting to know that the word fluorescence has been given 
 to this peculiar property of light because its effects were 
 first observed in fluorspar. 
 
 If a fluorescent substance is exposed in a darkened room 
 to light from different sources, a difference will be observed 
 in the intensity of the fluorescent light produced. While 
 ordinary sunlight possesses this power in a fairly marked 
 
242 THE WONDEE BOOK OF LIGHT 
 
 degree, it is surpassed either by the light of the electric 
 arc lamp, or by the light emitted by a burning magnesium 
 wire. The reason is obvious. It is the ultra-violet rays 
 that produce the fluorescence, and the light emitted by 
 the burning magnesium wire on the electric arc, contains 
 a greater proportion of the ultra-violet rays than does 
 sunlight. 
 
 The following curious experiment can be made as re- 
 gards fluorescence. If stramonium, Jamestown or jim- 
 son weed, is soaked in water, an almost colourless solution 
 will be obtained. Writing, traced on a sheet of white pa- 
 per by means of a pen dipped in this solution, will be 
 almost invisible, but if the light from burning sulphur, or 
 brimstone, is permitted to fall on it, it instantly becomes 
 visible by fluorescence. In making this experiment care 
 must be taken not to leave the solution of stramonium 
 where it might be drunk in mistake for water; for it is 
 very poisonous. 
 
 The light emitted by the aurora borealis is especially 
 rich in the ultra-violet rays. 
 
 Some substances possess the power of fluorescence in a 
 high degree. Two of the most remarkable fluorescent sub- 
 stances are the platino-cyanide of barium, and the tungs- 
 tate of calcium. These substances are employed for cover- 
 ing the screen in an apparatus known as the fluoroseope 
 that will be described in the next chapter on the X-rays. 
 
 It has been known from early times that a substance 
 called Bologna phosphorus possesses the curious property, 
 after exposure to light, of continuing for many hours 
 to emit, in the dark, a pale light. Bologna phosphorus 
 was prepared by heating to incandescence certain minerals 
 
FLtJOEESCEXCE AND PHOSPHORESCENCE 343 
 
 and afterwards reducing them to a fine powder. In later 
 years the same kind of material is employed in the pro- 
 duction of what are called limiinous paints, or paints con- 
 sisting of various mixtures of sulphur with calcimn or 
 barium, that have been calcined and afterwards finely 
 ground. 
 
 You have probably seen luminous paints applied to the 
 surfaces of the match boxes that are permanently fixed on 
 the walls of a room. During their exposure to the light in 
 the daytime, these paints are so affected that they wUl 
 continue to shine during the greater part of the night, al- 
 though there is no other light in the room. One coming 
 into the room can, therefore, readily see where the match 
 box is. 
 
 I have sometimes thought, while searching in a darkened 
 room for a match box, with the danger, before finding it, 
 of overturning bits of china on the mantel-piece or tables, 
 how convenient it would be if such things had voices, so 
 that while you were blundering around looking for them 
 they could call out : 
 
 " Don't look there ; don't you see me. Here I am." 
 And this is practically just what the luminous paint does. 
 When you go into a room, you have only to listen carefully 
 with your eyes, to hear it calling to yon from where it has 
 been placed. 
 
 Another practical application of luminous paint is its 
 use over the surface of a clock dial. On waking at nights 
 you can tell the time by looking at the clock dial. There 
 is no need of getting up to strike a light. I would say, 
 however, that in all those cases the light produced in this 
 manner by phosphorescence is exceedingly feeble. 
 
244 THE WONDEE BOOK OP LIGHT 
 
 There are many substances that, like Bologna phos- 
 phorus and luminous paints, possess, to a greater or less 
 degree, the power of continuing to shine after exposure to 
 the light. Some of these, as in the case of the luminous 
 paints, continue to throw out light for many hours after 
 daylight ceases to fall on them. Others continue to shine 
 for a few moments only. The following substances pos- 
 sess phosphorescence in varying degrees ; i. e., the sulphides 
 of barium and calcium, diamonds, a variety of fluorspar 
 known as chlorophane, dry paper, silk, sugar, salts of the 
 alkalies and alkaline earth, compounds of uranium, 
 etc., etc. 
 
 Phosphorescence can be produced by exposure to all por- 
 tions of the spectrum; not only to the ultra-violet rays, 
 but also to all the colours between the reds and the violets, 
 and even to the invisible or heat rays in the infra-red, far 
 below the red itself. In this respect phosphorescence differs 
 markedly from fluorescence, which, as we have seen, is 
 excited only by the ultra-violet rays. If you will think a 
 moment you will understand how wonderful this is. When 
 the ultra-violet rays produce phosphorescent light their too 
 rapid wave motion has been slowed down until it can afEect 
 the eyes as light, while in the case of the invisible rays 
 below the red, the too slow vibrations or ether waves have 
 been increased or accelerated until they have become suffi- 
 ciently rapid to affect the eye. 
 
 You can easily try a wonderful experiment in phos- 
 phorescence. Heat a dull iron plate in a darkened room 
 by permitting the flame of a Bunsen burner to fall directly 
 on its lower surface. Be careful that the heating of the 
 plate is not carried sufficiently far to render it incandes- 
 
PLUOEESCEXCE AND PHOSPHOEESCENCE 245 
 
 cent. Now, fixing your eyes on the upper surface of the 
 plate, in such a position as to prevent your seeing the flame 
 of the Bunsen hurner, throw a few fragments of a variety 
 of fluorspar known as ehlorophane on the upper surface 
 of the plate. Instantly a hrUliant emerald-green phos- 
 phorescent light wiU be thrown out from the fragments. 
 This light has been produced by the increase in the rapid- 
 ity of the heat waves from the iron plate to such an extent 
 as to enable them to affect the eyes. 
 
 This increase ia the rapidity of vibrations of the fluor- 
 spar is so difficult to understand that, in the opinion of 
 some, the light so caused by heat is believed not to be due 
 to an increase in the rapidity of the heat waves, but to a 
 liberation of light energy, which, in some way that is not 
 exactly understood, has been charged on the fluorspar dur- 
 ing its exposure to light. The name ehlorophane has been 
 given to this variety of fluorspar from two Greek words 
 meaning " shining with a greenish light." 
 
 A great variety of differently coloured lights are pro- 
 duced by phosphorescence. These different colours are 
 thus described by Professor Eood in his " Text Book of 
 Colour": 
 
 "If tubes filled with the sulphides of barium, stron- 
 tium, calcium, etc., be placed in a dark room and illunii- 
 nated for an instant by a beam of sunlight, by the electric 
 light, or by burning magnesium wire, they will display a 
 charming set of tints for some minutes afterwards. Some 
 will shine with a soft violet light, and others will display 
 orange and yellow colours. Delicate blues wiU mark the ap- 
 pearance of others and will contrast well with the red hues, 
 the latter resembling in the darkness living coals of fire." 
 
246 THE WONDEE BOOK OF LIGHT 
 
 Much yet remains to be explained concerning many of 
 the phenomena of phosphorescence. Becquerel, the well- 
 known French physicist, made a series of extended experi- 
 ments and observations of these phenomena and has de- 
 scribed them in a large volume. These investigations show 
 that phosphorescence is capable of being excited in other 
 ways than by mere exposure of different substances to 
 light. As we have already stated, the exposure of chloro- 
 phane to increased temperature results in the production 
 of phosphorescent light. The same thing is true of some 
 varieties of diamonds. 
 
 Then there is a large class of substances, produced either 
 in the living bodies of animals and plants, or formed dur- 
 ing the decomposition of their tissues, that possess in a 
 marked degree the power of producing phosphorescent 
 light. The effects of such phosphorescent substances are 
 seen in the light of fireflies and glowworms, as well as in the 
 pale light given off by the water of the ocean when it is ruf- 
 fled by the wind and looks as though it were all aflame. 
 The light which makes it look this way is due to the phos- 
 phorescence caused by various zoophytes, jelly-fish, etc. 
 Here, however, the light only appears when the substance 
 undergoes a gradual burning or oxidation by contact with 
 the air, so that this variety of phosphorescent light is only 
 seen on the ocean when the air has an opportunity of 
 coming in contact with this substance where the surface 
 of the water is rufBed by the wind. This variety of 
 phosphorescence is known as chemical phosphorescence. 
 
 But the above are not the only ways in which phosphor- 
 escent light can be produced. If crystals of quartz are 
 rubbed together in a dark room, phosphorescent light will 
 
PLUOEESCEXCE AXD PHOSPHOEESCEXCE 247 
 
 appear. It is also produced when lumps of loaf sugar are 
 broken in the dark. Phosphorescent light is also mo- 
 mentarily seen when a solution of certain salts suddenly 
 crystallises in the dark. 
 
 Prom what I have said about the phenomena of phos- 
 phorescence I think you will agree with me that they can 
 be regarded as among the greatest wonders of light,. 
 
CHAPTEE XXII 
 X-RAYS AND RADIO-ACTIVITY 
 
 Although the subject of both X-rays and radio-ac- 
 tivity properly belong to electricity, and will, therefore, 
 be fully treated in that Wonder Book, yet they are so 
 closely allied with light that a short description of each 
 will be given here. 
 
 In 1895, a discovery was made in Bavaria by Pro- 
 fessor Roentgen, that, by reason of its wonderful nature, 
 caused great excitement in the scientific world. Roentgen 
 was making experiments in a darkened room with some 
 Crookes' tubes, a name given to the high-vacuum tubes 
 we have referred to in the chapter on the light mill. 
 
 In a Crookes' tube, electric spark discharges are passed 
 through the traces of gaseous atmospheres left in the tube 
 after as high a vacuum as possible has been obtained. The 
 discharges are passed through the tube by means of plati- 
 num wires fused into the glass at its opposite ends, so as to 
 make air-tight joints. As the discharges pass, the space 
 inside the tube becomes luminous, especially where the 
 moving molecules of the residual gas strike its sides. 
 
 "Wishing to prevent this light from entering the dark- 
 ened room in which he was experimenting. Prof. Roentgen 
 had covered the tubes with thick pasteboard, blackened so 
 as to prevent any leakage of the light. Believing that no 
 light could possibly pass out of the tube, you may under- 
 
An X-Ray Photograph 
 
 From Atkinson's '' Electricitf for Everybody,^' published by The Century Co. 
 
X-EAYS AND EADIO-ACTIVITY 349 
 
 stand how surprised he was to see that a sheet of paper 
 covered with a highly fluorescent substance commenced to 
 shine brilliantly. 
 
 Eoentgen was greatly puzzled. He thought he had made 
 the covering light-tight, but there was the paper shining 
 with a fluorescent light. Some light must have leaked 
 out. He therefore again examined the Crookes' tubes to 
 find where the openings were. Being unable to find any 
 he came to the conclusion that there had been produced in 
 the tube a peculiar kind of radiation that could readily 
 pass through a thick sheet of thoroughly blackened paste- 
 board. Astonished by the oddness of this discovery, he 
 made many researches, and found that there had been pro- 
 duced in the tube a new kind of radiation possessing the 
 power of readUy passing through a great variety of sub- 
 stances opaque to any of the forms of light that he knew 
 could produce fluorescence. Moreover, to his great sur- 
 prise, this radiation was not only able to produce fluores- 
 cence after it had passed through opaque substances, but 
 was also able, like ordinary light, to decompose chemical 
 substances employed in photography. In other words, the 
 new kind of radiation, besides producing fluorescence, 
 could also produce photographic pictures. 
 
 It was a great discovery. It seemed as if a new kind 
 of light had been discovered that differed from ordinary 
 light in being able readily to pass through many opaque 
 substances, but which resembled ordinary light in its abil- 
 ity to produce fluorescence and photographic effects. N'ot 
 knowing the exact nature of the new rays, Eoentgen called 
 them X-rays. As you who study algebra wfll understand, 
 the letter X is frequently used to represent an unknown 
 
250 THE WONDEE BOOK OF LIGHT 
 
 quantity. The name X-rays Eoentgen proposed for 
 these rays meant the unknown rays, and this happy name 
 is still generally retained, although they are sometimes 
 called Roentgen rays. 
 
 These rays possess the wonderful power of penetrating 
 substances opaque to ordinary light. The sheets of paper 
 on which this Wonder Book of Light is printed are not 
 very transparent, especially where they are covered with 
 the printer's ink, and yet, if you take this book with its 
 covers and hold it in the path of the X-rays, they can pass 
 readily through it and cause a paper covered with a flu- 
 orescent substance to shine brightly. Indeed, if two such 
 books be placed together in the path of the X-rays they 
 could still pass through them and be able to excite fluor- 
 escence. In the same way, the X-rays will readily pass 
 through a pack of ordinary playing cards. Indeed, even 
 after the X-rays have gone through two packs, they are 
 still capable of producing the fluorescent efEects as well 
 as the photographic effects. 
 
 But sheets of paper or pasteboard are not the only kind 
 of substances that are transparent to X-rays. Blocks of 
 wood are also quite transparent to them. Since, when 
 X-rays fall on sensitive photographic plates they produce 
 the same effects as would ordinary light, it is necessary to 
 keep such plates out of the room in which the X-rays are 
 being produced, as they would otherwise be spoiled. There 
 would be no use in placing the photographic plates in 
 drawers ; for the X-rays could easily pass through the inch 
 or so of wood at the top or bottom of the table. N'or 
 would it help to wrap the plates up in black paper since 
 they could with equal ease be ruined by the X-rays. If, 
 
X-EAYS AND EADIO-ACTIVITY 251 
 
 however, they are wrapped in sheets of thick lead which is 
 opaque to the X-rays, they would be preserved. 
 
 I will not stop now to name the many substances that 
 are opaque to ordinary light but are transparent to the 
 X-rays. The black rubber or vulcanite from which ordi- 
 nary combs are made, though almost absolutely opaque to 
 ordinary light, is quite transparent to the X-rays. So, 
 too, is the flesh, together with the blood vessels and nerves 
 that form the soft parts of the human body. As you know, 
 we cannot look through the body to see the bony skeleton 
 that lies within. We can, however, see the red colour of 
 the blood if we hold up the hand in the bright sunlight, 
 yet the bones and other portions of the interior of the hand 
 are invisible. Now I shall show you in this chapter how 
 easy it is for the X-rays to pass not only through the 
 hand, but even through the thicker parts of the body, so 
 as to form on a suitably placed sensitive photographic 
 plate a distinct photographic picture of the bones and 
 some of the tissues of the body. 
 
 The exact nature of the X-rays is unknown. When 
 they were first discovered, it was believed that they con- 
 stituted a variety of light and this belief is still held by 
 many. Since, however, the X-rays differ in so many of 
 their properties from ordinary light, not only as regards 
 their ability to pass through substances opaque to light, 
 but also because they can be neither reflected nor refracted, 
 it was necessary to explain how they difier from ordi- 
 nary light. Many attempts have been made to do this. 
 
 It is claimed by some physicists that the X-rays are 
 entirely different from light; that they consist of streams 
 of exceedingly minute fragments 'of matter, shot out with 
 
253 THE WONDEE BOOK OF LIGHT 
 
 great rapidity from that portion of the X-ray tube where 
 the X-rays are produced. In the opinion of others, and 
 probably of the majority of physicists, the X-rays are be- 
 lieved to be produced by vibrations of the ether. Some 
 claim that the vibrations, instead of taking place as in 
 ordinary light across the length of the ray, take place in 
 the direction of its length; that is, that the X-rays are 
 longitudinal ether vibrations, and not transverse vibrations. 
 
 According to still others, it is believed that the X-rays 
 differ in no respect from ordinary light, except that their 
 wave lengths are much smaller than the ultra-violet rays. 
 For this reason, the X-rays have by some been called the 
 ■ ultra-ultra-violet rays. This matter, however, is still un- 
 certain, and I will not attempt to describe it any farther 
 here. Indeed, I fear I have already gone much farther 
 than I should in a book of this scope. 
 
 When experiments were made with the X-rays, it was 
 soon discovered that these rays were not given off from the 
 cathode, or negative terminal of the tube, but from that 
 portion of the tube against which the cathode rays are 
 thrown. If an effort was made to increase the amount 
 of X-rays produced by bringing the rays to a focus against 
 some particular part of the wall of the tube, although a 
 great increase was obtained in the amount of X-rays pro- 
 duced, yet the walls of the tube were soon melted by the 
 increase of temperature and consequently the tube was 
 ruined. Now, however, X-ray tubes are made so that the 
 cathode rays strike against a heavy plate of some substance, 
 like platinum or iridium, that is extremely refractory or 
 difficult to fuse. In this way the proportion of X-rays 
 produced is greatly increased. 
 
 In order readily to examine bodies by X-rays a device 
 
X-EAYS AND EADIO-ACTIVITY 
 
 253 
 
 called a fluorescent screen may be employed. This, as 
 represented in Fig. 85, consists of a darkened box or cham- 
 ber resembling a stereoscope in shape. The smaller end 
 of the apparatus is shaped so as to fit closely over both 
 eyes of an observer. The larger end of the box is provided 
 mth a closely fitting plate, the inner surface of which 
 is covered with some highly fluorescent substance, such as 
 tungstate of calcium or barium-platino-cyanide. When the 
 X-rays fall on this plate they pass through it and produce 
 fluorescence on the side 
 facing the observer, thus 
 causing it to glow uni- 
 formly over all the parts 
 of its surface. 
 
 Kow, since after the 
 X-rays have passed through ' 
 opaque substances they are 
 capable of exciting fluor- 
 escence in fluorescent maT 
 terials, and since the light 
 so produced is capable of causing photographic images, 
 we come to this wonderful fact: that by the use of the 
 X-rays it is possible to see objects that are otherwise in- 
 visible. Think of it ! An object may be placed inside a 
 wooden box with sides so thick that it is impossible to see 
 what the box contains, and yet, if the X-rays are caused 
 to pass through the walls of the box, and are then per- 
 mitted to fall either on a fluorescent screen, or on a 
 photographic plate, we can easily see whether the box is 
 empty or not. 
 
 Suppose, for example, the box contains such objects as 
 masses of iron, steel or lead, like nailsj bullets, knives or 
 
 Fig. 85. — Fluoeescent Soeeen. 
 
S54 THE WOHDER BOOK OE LIGHT 
 
 needles. These objects prevent the X-rays from passing 
 through them, so they are seen as shadows on the flu- 
 orescent screen at the places where the light or glow is ab- 
 sent. In other words, the objects cast shadows on the 
 screen, so that the observer can see their positions and 
 shape even if the box in which they are placed were made 
 of boards six inches or a foot thick, or of 500 or 1,000 
 pages of a book. If, now, instead of a fluoroscope a sensi- 
 tive photographic plate be placed in the path of the rays, 
 its sensitive surface would have impressed on it all these 
 differences of light, so that, when properly developed and 
 fixed, it would contain an image of all it saw. 
 
 It is possible by a similar use of the X-rays to throw 
 on a fluorescent screen, or on a photographic plate, sharply 
 marked images of the bones of the hand, or even those of 
 the spinal column and ribs, by causing X-rays to pass 
 through the flesh that covers these portions of the body. 
 
 It may interest you to know the arrangement of appa- 
 ratus required for thus examining the bones of the hand. 
 If the hand be placed on a photographic plate that has 
 been carefully wrapped in several thicknesses of blackened 
 paper, a photograph of the bones as well as some of the 
 tissues will be formed on the plate. 
 
 If such objects as leaden bullets, nails, pins, needles, have 
 become lodged in any part of the human body, since, as 
 already remarked, such objects are opaque to the X-rays, 
 their position can be readily seen either on a fluorescent 
 screen, or on a photographic plate, thus enabling a surgeon 
 accurately to locate them without subjecting the patient 
 to the pain produced by the metallic probe, which, before 
 the discovery of the X-rays, was inserted in a wound and 
 wriggled about until it touched the foreign body. 
 
X-RAYS AND EADIO-ACTIVITY 255 
 
 Photographs produced as above described, by the use of 
 the X-rays, are known as radiographs or skiagraphs. Since, 
 however, they are simply photographs produced by the use 
 of the X-rays, a simpler and far better popular name is 
 X-ray photographs. 
 
 An exceedingly curious discovery was made in the year 
 1896 by the French physicist, Becquerel, to whom we have 
 before referred. This gentleman discovered that there are 
 substances which possess the power of emitting rays closely 
 resembling the X-rays in being able to pass through opaque 
 substances and afterwards produce fluorescent or photo- 
 graphic effects. These substances, however, instead of re- 
 quiring to be first placed in the light possess the exceedingly 
 strange power of being able continually to give oil this 
 peculiar form of radiation for an apparently indefinite 
 period of time. These rays have been named after their 
 discoverer, the Becquerel rays. 
 
 Some time afterwards, it was discovered that, besides 
 the salts of uranium, the name of the substance in which 
 Becquerel had detected this curious radiation, there was 
 another elementary substance, named thorium, that pos- 
 sessed a similar power of throwing off rays resembling the 
 X-rays. Then, at a still later date, it was discovered that 
 there exists in a mineral named pitchblend, the ore from 
 which uranium is obtained, a number of elements that had 
 not until that time been discovered; namely, polonium, 
 radium, and actinium, that also possess the same proper- 
 ties. In some of these, as, for example, in radium, the 
 power possessed by the rays was something in the neigh- 
 bourhood of 100,000 times greater than in the rays thrown 
 off by uranium. 
 
 The rays thus thrown off from these different substances 
 
256 THE WONDER BOOK OP LIGHT 
 
 were called respectively the polonium rays, the radium rays, 
 and the actinium rays. 
 
 The general name of radio-active substances is given to 
 these different kinds of matter that are thus capable of 
 throwing off indefinitely rays resembling the X-rays. 
 Without going any further into their peculiarities, it is 
 sufficient to say that it is believed all polonium, radium 
 and actinium rays, and indeed, the rays produced by all 
 radio-active substances, consist of very minute particles 
 of matter much smaller than the so-called chemical atoms ; 
 indeed, that they consist of small fragments of these 
 atoms. On account of their small size and prodigious ve- 
 locity they are capable of passing through matter that is 
 ordinarily opaque to light and of producing luminous ef- 
 fects when they strike the fluorescent material. The light 
 so produced causes the photographic effect. 
 
 In the opinion of some, however, it is believed that the 
 Becquerel rays, and possibly also the rays from all radio- 
 active substances, are due to vibrations in the ether like 
 those that produce ordinary light, only that they are more 
 rapid than the ultra-violet and less rapid than the X-rays. 
 This matter, however, is as yet uncertain. 
 
 It may be a matter of surprise to you to hear that the 
 above rays are thought to be produced by fragments of 
 atoms. It was not so many years ago when it was believed 
 that atoms are too hard to be cut or broken into pieces. 
 ISTow, however, it is held that particles called corpuscles, 
 whose sizes are less than .001 part of an atom, can be 
 broken off and hurled with great velocity through space 
 and are, therefore, capable of producing the peculiar ef- 
 fects of the X-rays and of the radium and other rays. 
 
CHAPTER XXIII 
 ILLUMINATION. THE SALE OF LIGHT 
 
 As illumined body differs from a luminous body in that 
 it has nothing whatever to do with the production of the 
 light it throws off. It merely throws off in all directions 
 light it has received from some luminous body. A lumi- 
 nous body not only throws off light like an illumined body, 
 but it also produces, or manufactures, the light it throws 
 off. When, however, the illumined body is coloured, the 
 light it throws off differs from the light it receives. It 
 generally receives white light, but always throws off light 
 of the particular colours that are left after certain colours 
 have been absorbed or quenched. 
 
 Sunlight is by far the best kind of light for the illumi- 
 nation of objects. In the first place it is so abundant that 
 it is capable of producing a far more uniform illumination 
 than any known artificial light. Moreover, sunlight is 
 spread out over so great an expanse of sky that it lights up 
 objects on the earth's surface far better than can any 
 artificial light. 
 
 There are a number of luminous sources from which 
 the light required for artificial illumination can be ob- 
 tained. Some of the most important of these are the arc 
 and incandescent electric lights, the acetylene lamp, the 
 gas burner, and the candle. 
 
 In order that any artificial light shall act satisfactorily 
 as an illuminant it should as nearly as possible possess 
 
 257 
 
258 THE WONDER BOOK OP LIGHT 
 
 the properties that characterise daylight. Let us see, there- 
 fore, what these properties are. 
 
 The object of all artificial illumination is not only to 
 permit objects to be seen distinctly, but also, as far as 
 possible, to give them the same appearance as when illu- 
 mined by daylight. Now, one of the most important con- 
 ditions for distinct Tision is that the images formed on 
 the retina shall not only be sufficiently illumined, but 
 shall also be steadily illumined. It is necessary, therefore, 
 that a satisfactory artificial source of light shall be steady ; 
 that is, shall continue to give ofE light of the same intensity. 
 
 There are few things more trying to the eyes than the 
 attempt to examine an object closely when it is illumined 
 by a flickering light. In such cases, the illumination of 
 the retinal image will increase and decrease in brilliancy 
 in a manner that is sure to produce a strain on the eyes. 
 To attempt under these circumstances to do any work 
 that requires close inspection, such as reading or delicate 
 mechanical tasks, is not only painful, but is sure to result 
 in bad workmanship. Now, sunlight is so wonder- 
 fully steady that no known artificial light can compete 
 with it. 
 
 In the next place, in order that objects shall be distinctly 
 seen, it is necessary that the light by which they are illu- 
 mined shall possess, as nearly as possible, the same coloured 
 rays that are present in sunlight; for, since the colour of 
 a body is due not to the body itself but to the colours in 
 the light by which it is illumined, it would, of course, 
 otherwise, be impossible for the body to possess the ap- 
 pearance it has when illumined by sunlight. 
 
 Moreover, the artificial source of light should not 
 
ILLUMINATION. THE SALE OP LIGHT 359 
 
 markedly vitiate the air by any gaseous or other emana- 
 tions. There is a great difference in this respect between 
 the different sources that are employed for artificial il- 
 lumination. Sunlight is absolutely free from obnoxious 
 vapours, and this is a property that is by no means absent 
 in most forms of artificial light. 
 
 A property to a certain extent resembling that above 
 named is the heating power of the luminous source, or 
 the extent to which it is capable of heating the surrounding 
 air. While sunlight is accompanied by a certain amount 
 of heat, in most cases the heating it produces is far from 
 being objectionable, except, perhaps, during very hot 
 weather, in close rooms. But the heating effects are by no 
 means absent in most artificial luminous sources. Nearly 
 all produce a far greater proportion of heat than light. 
 
 Another important property that should be possessed 
 by a luminous source is safety. The source should be of 
 such a character as to render it free from danger either 
 to life or to property. Sunlight is entirely safe as an 
 illuminant. Of course, the sunlight may be so intense 
 that its illumination may be too great, so great, indeed, 
 as to injure the eyes, but it is far easier to cut off a portion 
 of the light produced by a luminous source than it is to 
 increase it. 
 
 As regards its heating powers, sunlight may become at 
 times so great, especially in small and poorly ventilated 
 rooms, as to threaten life by sunstroke. With care, how- 
 ever, this danger is generally exceedingly smaU. 
 
 Sunlight is almost absolutely free from fire risks. There 
 is no other source, except, perhaps, the incandescent elec- 
 tric lamp, or the day-Hght tube, that is as free from risks 
 
360 THE WONDBE BOOK OF LIGHT 
 
 arising from the starting of fires in surrounding combus- 
 tible materials. 
 
 There is yet another property of sunlight that should be 
 mentioned; its' uses in continuing the existence of plant 
 and animal life. The presence of a certain proportion of 
 white light resembling daylight or sunlight containing 
 violet and ultra-violet rays is necessary. 
 
 As regards the amount of light they can produce the 
 arc lights and incandescents come next to sunlight. These 
 luminous sources, too, perhaps especially the arc light, 
 come nearer sunlight as regards steadiness. When properly 
 operated, the light they produce is fairly steady. Neither 
 greatly heats the space it is lighting. The arc light 
 possesses colour values that are practically the same as 
 those of sunlight, and neither of these lights markedly 
 vitiates the air or space around them. The open arc light 
 does to some extent render the air slightly impure, but the 
 enclosed are light and the incandescent electric light are 
 practically free from any injurious effects on the air of the 
 space surrounding the illuminant. 
 
 One of the most serious objections to the use of the arc 
 light as a source of illumination is the difficulty of sepa- 
 rating a single light into a number of less brilliant lights. 
 The difficulty exists in the case of most artificial lights, 
 but not to so great an extent as in the arc light. 
 
 When only a single luminous source is employed for 
 lighting a room ; or, indeed, when but a limited number of 
 sources are employed for this purpose, the objects fail to 
 be uniformly illumined. Disagreeable shadows are apt 
 to be produced that make it difficult for the objects to be 
 distinctly seen. Now, incandescent lamps possess, in this 
 
ILLUMINATION. THE SALE OF LIGHT 261 
 
 respect, a great advantage over arc lamps, since the cur- 
 rent employed for incandescent electric lamps can. be 
 readily split or divided among a great number of separate 
 lamps. The objects illumined, therefore, are almost en- 
 tirely free from shadows. 
 
 The above advantage, however, is attended by many dis- 
 advantages. In the first place, the light produced by in- 
 candescent electric lamps contains a large proportion of 
 the reds, yellows, and orange coloured rays, with a small 
 proportion of the greens, blues, and violet rays. Conse- 
 quently, coloured objects so illumined do not possess the 
 same appearance as when illumined by the daylight, or by 
 the arc light. Fabrics that are blue by daylight necessarily 
 appear dull when illumined by the incandescent light, 
 which, as we have seen, is deficient in the blue and violet 
 rays. The marked improvements, however, that have re- 
 cently been made in incandescent lamps have to a great 
 extent removed this objection. In that they do not vitiate 
 the air of the room, incandescent lamps are almost as 
 satisfactory as daylight or any other source of illumi- 
 nation. 
 
 Oil and gas lamps possess many disadvantages when 
 compared with sunlight. Their fire risks are greater than 
 either are or incandescent lamps. They vitiate the air by 
 pouring into it the gases formed during the burning of the 
 oil or gas. They also render the air less fit to breathe by 
 removing the oxygen that is so necessary for the continu- 
 ance of animal life. 
 
 Another serious objection to all oil and gas lamps, is 
 the high temperature of the gases produced in them during 
 combustion. Moreover, these gases are produced in great 
 
362 THE WONDEE BOOK OF LIGHT 
 
 volume. During hot weather the use of gas lights in the 
 air of confined rooms, such as in weaving and spinning 
 mills, results in so high a temperature, and the char- 
 acter of the work is often so poor, that it has been 
 found necessary to shut down the mills during the night. 
 This objection is now, however, largely a matter of the 
 past, since practically all modern mills are illumined by 
 incandescent electric lamps. 
 
 Another serious objection to oil and gas lamps is found 
 in their deficient colour values. Their light possesses a 
 preponderance of reds, yellows, and orange coloured rays, 
 and a decreased proportion of the blues and violet rays, 
 that is even more marked than in the case of the incandes- 
 cent electric lamps. It is true that this objection has been 
 greatly overcome by the use of acetylene gas, a variety of 
 hydro-carbon gas that is readily obtained by the action of 
 water on calcium carbide, a chemical substance produced in 
 the electric furnace. 
 
 An effort has recently been made to improve common 
 illuminating gas by the use of incandescent mantles, such 
 as those employed in the "Welsbach and similar lamps. 
 While a marked advantage is obtained as to the amount 
 of light yielded, such lamps producing from five to six 
 times more light than would the same amount of gas 
 burned with an ordinary gas burner, yet they possess the 
 disadvantage of producing a light containing so large a 
 proportion of greenish rays, and so small a proportion of 
 red rays, that coloured objects, when illumined by their 
 light, do not appear of the same colours as they do by 
 daylight. 
 
 Perhaps one of the most satisfactory sources of arti- 
 
ILLUMI^-ATION. THE SALE OF LIGHT 263 
 
 ficial light is a variety of light called the Moore Electric 
 Daylight Tube. This light is obtained by the passage of 
 electric discharges through long vacuum tubes. The light, 
 which closely resembles in its properties that produced by 
 the well-known Geissler tubes, is enclosed. This luminous 
 source cannot, therefore, render the air unfit to breathe 
 by fumes given off, is perfectly free from fire risks, and 
 does not heat the air of the room. Its daylight values are 
 fairly satisfactory. The principal objection is that, as 
 generally employed, the intensity of the light it produces 
 is comparatively small. 
 
 You will probably be surprised when I tell you that 
 practically all artificial sources of light with, perhaps, 
 the exception of the iloore Electric Daylight Tube light, 
 are very deficient in one exceedingly important respect. 
 They are all far better sources of heat than of light. 
 Even in the case of a good incandescent lamp there is 
 produced only four parts of light in every one hundred 
 parts of radiation. In other words, the ordinary incan- 
 descent lamp gives only 4% of light while the remaining 
 96% consists of heat. When you remember that light is 
 what is desired from a luminous source, the heat being 
 highly objectionable, an incandescent lamp is exceedingly 
 unsatisfactory as an artificial iUuminant, since it gives 24 
 times more heat than light. 
 
 The proportion of light to heat in the case of an ordinary 
 candle is still more unsatisfactory, the candle emitting only 
 1|% of light and 98^% of heat. The Welsbach incan- 
 descent mantle gives 3|% of light, and 971% of heat. 
 And even in the case of the electric arc light there are 
 only 13% of light and 87% of heat produced. 
 
364 THE WONDEE BOOK OF LIGHT 
 
 It seems unfortunate that all the artificial luminous 
 sources produce so small a quantity of light in proportion 
 to the heat. If we knew the secret of the glowworm or 
 the firefly, we would be able to produce an artificial light 
 far more satisfactory in its relative proportions of light 
 and heat. And yet, even if we could do this, while we 
 would have a very cheap light, yet it would be far from 
 a satisfactory light source, since it would be characterised 
 by so marked a deficiency of the reds, yellows and orange 
 rays, and so great a preponderance of the greenish rays 
 that would make its colour values exceedingly unsatis- 
 factory. 
 
 In order to get the best results in the illumination of 
 objects by artificial light, care must be taken that the 
 light falls on the objects directly, and does not enter the 
 eyes of an observer until after it has been thrown off from 
 them. Under no circumstances should the light of the 
 luminous source be permitted directly to enter the eye of 
 the workman. When this is permitted, the image of the 
 source will be so much brighter than the image of the 
 illumined objects that it will be difficult, if not impossible, 
 distinctly to see them. 
 
 Examples of poor artificial illumination are frequently 
 seen in store windows. Take a walk some night through 
 the business portions of your city or town, until you come 
 to a window that is illumined by an arc light hung di- 
 rectly in the middle of the window. The arc lamp certainly 
 gives an abundance of light to the articles in the window, 
 but it doesn't throw its light on the objects so as to en- 
 able them to be seen; for, unless that side of the lamp 
 which faces the street is covered by an opaque shield or 
 
ILLUMINATION. THE SALE OF LIGHT 265 
 
 reflector, its light directly enters the eyes of any one 
 looking into the wiadow to examine the goods and pro- 
 duces such a brilliant image of the lamp that it is diffi- 
 cult, and, indeed, almost impossible to see anything else, 
 much less closely to examine the objects in the window. 
 Nor wiU. there be much improvement if the lamp is hung 
 up outside the window, since, in this case, there wiU still 
 be seen so bright an image of the lamp apparently in the 
 window that the lamp is stiU the only thing you can 
 distinctly see. 
 
 The proper lighting of store windows is now much better 
 understood, so that it will be strange if you do not find 
 somewhere in your town or city a store window that has 
 been illumined on a different principle. In this case the 
 luminous sources are entirely out of sight. The only 
 light that enters the eye is from the illumined objects 
 and under these circumstances you can see them distinctly. 
 The luminous sources are either placed in the upper part 
 of the window back of an opaque curtain, or they are 
 placed back of metal shades at the sides or at the top or 
 bottom of the window, where they are completely out of 
 sight. 
 
 In the commercial sale of light, means must be provided 
 for the correct measurement of the quantity of light for 
 which a purchaser is paying. In the case of the electric 
 light this is' done by the use of the electric meter, which 
 shows the amount of electricity that is passing through 
 the lamps. In a similar way a gas meter is employed for 
 measuring the quantity of gas that flows from the gas 
 main in the street into the house. But it is not so much 
 the quantity of the electricity or gas delivered to the pur- 
 
366 
 
 THE WONDEE BOOK OF LIGHT 
 
 chaser that is of especial interest to him. What he wants 
 to know is the amount of light a given quantity of elec- 
 tricity or gas is producing. This can be done by means 
 of an instrument called a photometer that gives the in- 
 tensity of an electric lamp or gas burner, measured in what 
 is called candle-power, or an amount of light equal to so 
 many candles of a definite size and composition burning 
 so as to consume a given weight of the wax, stearine or 
 tallow per minute. 
 
 When a luminous source consisting of a single point 
 
 ■Thb Peinciple of the Inveesb Squakes. 
 
 of light throws its light on an object, the amount of light 
 received will be inversely as the square of its distance from 
 the source. Suppose, for example, the light of a candle A, 
 Pig. 86, falls on a screen B, placed say one inch from the 
 candle. The screen will receive a certain quantity of light 
 from the candle. If now the screen be moved twice as 
 far from the "candle, say to C, since the same quantity of 
 light is now spread over a surface four times greater than 
 that at B the intensity of the illumination, or the amount 
 of light per square inch, will be only one-quarter as great 
 as it was at B, In the same way, if the screen be moved 
 
ILLUMINATIOX. THE SALE OP LIGHT 267 
 
 to D, three times further from the candle than B, the 
 same amount of light will now be spread over an area 
 nine times as great, so that the intensity of the illumina- 
 tion at D, must necessarily be only one-ninth as great as 
 it was at B. In other words, the intensity of the illumina- 
 tion will be inversely as the square of the distance; for at 
 distances twice or three times as great as one, the in- 
 tensity will be inversely as these numbers squared; for, 
 two multiplied by two equals four, and " inversely as 
 four" is represented by 1 divided by 4; and similarly 
 
 Fig. 8T. — The Bunsen Photometee. 
 
 three multiplied by three equals nine, and 1 divided by 
 9 or one-ninth will be "inversely as nine." 
 
 There are many ways in which the intensity of luminous 
 sources can be measured. One of the simplest consists in 
 a form of instrument invented by Bunsen, the German 
 chemist. A screen of paper S, with a grease spot at its 
 centre, is placed in the vertical position shown in Pig. 87. 
 If one side of this disc is examined by reflected light, the 
 grease spot will seem darker than the rest of its surface, 
 since some of the light passes through this part of the 
 screen. If, however, the disc is examined on the opposite 
 side by transmitted light the grease spot will appear 
 brighter than the surrounding parts since more light passes 
 
368 THE WONDEE BOOK OF LIGHT 
 
 through it. In both these cases it is supposed that the 
 instrument is placed in a dark room where no other light 
 than the lights that are being examined are present. 
 
 Suppose now it is desired to know how much greater is 
 the amount of light given off from the gas burner B, 
 placed on one side of the screen S, than that from the 
 candle A, placed on the opposite side of the screen. To do 
 this it is only necessary to move the gas light towards 
 and from the screen until the grease spot disappears when 
 viewed from either side. Of course, both sides of the 
 screen must necessarily be seen simultaneously. This is 
 readily accomplished by means of suitably inclined mirrors 
 placed on each side of the screen. When this is done the 
 intensity of the light emitted by the two sources will be 
 inversely as the square of their distance from the screen. 
 If, for example^ the gas burner B, is three times further 
 from the screen than the candle A, then since the squares 
 of three and one, are nine and one respectively, it is 
 evident, that the intensity of the candle is one-ninth that 
 of the gas burner, or that the intensity of the gas burner 
 is nine times that of the candle. 
 
 It might seem, since the amount of light the eye re- 
 ceives from a distant object is inversely as the square 
 of its distance from the eye, that the brightness of the 
 image as seen in the eye should rapidly decrease as the 
 distance increases, Now, in point of fact as you may have 
 observed, objects at different distances from the eye appear 
 to be of equal brightness. The explanation is obvious. As 
 an object recedes from the eye, the size of its image 
 formed on the retina decreases in exactly the same propor- 
 tion as the intensity of its illumination decreases, If it 
 
ILLUMINATION. THE SALE OP LIGHT 269 
 
 is twice as far from the eye, although the intensity of the 
 light it receives is only one-fourth as great as it was 
 previously, yet since this light is concentrated on an 
 image one-fourth the size it will, of course, appear to be 
 as brightly illumined when twice as far off as it was in 
 the first instance. 
 
CHAPTEE XXIV 
 
 LIGHTHOUSE ILLUMINATION AND SEARCHLIGHTS. 
 ILLUMINATION OF THE STAGE 
 
 The intensity of the light received by an illumined ob- 
 ject decreases with the square of its distance from the 
 luminous source. That is, an object illumined by an 
 artificial source twenty yards away receives, in accordance 
 with the law of inverse squares, a much smaller illumina- 
 tion than it would at a distance of ten yards. It is for 
 this reason that the signal lights employed in lighthouses 
 in order that distant vessels may know the general location 
 of a coast, or of dangerous shoals, or the lights employed 
 on locomotives that are drawing railroad trains, would 
 soon become invisible if their rays were permitted to con- 
 tinue to diverge. 
 
 I remember not so very long ago, when the public 
 buildings were erected in the city of Philadelphia, that the 
 question arose whether it would not be easy so thoroughly 
 to illumine the marble tower and the statue of "William 
 Penn placed on the top of the tower, at a distance of 
 some 535 feet above the level of the street, that it could 
 be seen distinctly not only all over the city but even at a 
 distance of many miles from the city. The great amount 
 of light given off by an are light was well known, so it was 
 commonly believed that, if a circle of twelve or more arc 
 lights should be placed around the base of the pedestal 
 supporting the statue, they woxild not only light up the 
 
 270 
 
LIGHTHOUSE, SEAECHLIGHT, STAGE 271 
 
 white marble shaft of the column, with a dazzling white 
 light, but would also so illumine the bronze statue that 
 it oould be seen from a great distance. This was the 
 more confidently believed because the upper portions of 
 the tower, including the clock dials, were covered with 
 plates of metal that were to be covered with a coating of 
 metallic aluminium, a metal which reflects light as well 
 as burnished silver. But the aluminium coating proved 
 a total failure; and, even if it had not, the fact that the 
 illumined tower and statue, like all illumined bodies, 
 threw off their light as diverging pencils, the decrease 
 in intensity, as the square of the distance, would have 
 resulted in the failure of this plan. It is not surprising, 
 therefore, that both the marble tower and the bronze statue 
 of Penn are absolutely invisible at a distance of a few 
 miles only. 
 
 The rapid decrease in the intensity of the light thus 
 emitted can perhaps be better understood from a few 
 figures. Suppose, for example, an object is situated at a 
 distance of one foot from a luminous source. In accord- 
 ance with the law of inverse squares, the same object, if 
 situated at a distance of 100 feet from the source, or 100 
 times greater, would have an intensity of illumination of 
 ! or _i_ of what it would have at a distance 
 
 lUO X 100 10,000 
 
 of one foot. If at a distance of five miles or 36,400 feet, 
 it would have an intensity of . ^ or 1 It is 
 
 26,400 X 86,400 698,960,000 
 
 not difficult, therefore, to realise why an object should 
 be practically invisible at a distance of five miles if the 
 rays of light from the luminous source be permitted to 
 diverge. 
 
 Of course this diflBculty can be easily avoided by causing 
 
272 THE WONDER BOOK OF LIGHT 
 
 the rays that come from the luminous source to be so re- 
 flected from a concave mirror, or to pass through a lens 
 in such a way as to leave the source in directions parallel 
 to one another. In such a case the law of inverse squares 
 would no longer be true. The only loss in intensity of 
 illumination would be that which would arise from the 
 want of transparency of the air. 
 
 An extremely important ease in which it is necessary 
 to be able to see the light from a luminous source at con- 
 siderable distance from it, is that of the light em- 
 ployed in a lighthouse. 'No matter how strong such 
 a light may be, if its rays are permitted to diverge 
 in all directions from the lighthouse, the light will 
 so rapidly decrease in intensity that it will prove of 
 little or no value at comparatively short distances. 
 The means employed for causing the light in 
 lighthouses to leave the house in the form of nearly 
 parallel rays is to cause it to pass through various 
 forms of lenses. Mirrors are unsuitable for this 
 Ljggj. purpose, since, of course, they would shut off the 
 HOUSE light from the side opposite the metallic 
 reflectors. 
 If an attempt be made to obtain parallel rays of light 
 by the use of a single lens, this lens must necessarily be 
 comparatively wide. It would, therefore, be apt to possess 
 a marked spherical aberration. Moreover, it would be so 
 thick that too great an amount of light would be lost dur- 
 ing its passage through the glass. For this reason light- 
 house lenses are constructed with a plano-convex lens 
 surrounded by several rings or hoop-like segments with 
 their flat faces turned towards the luminous source. This 
 
LIGHTHOUSE, SEAECHLIGHT, STAGE 273 
 
 construction can be well seen from a cross-section of the 
 lens shown in section in Fig. 88, where a single plano- 
 convex lens C is provided with two concentric annular 
 segments A and B, all of which have their plane surfaces 
 on the same side as the luminous source. 
 
 The luminous sources of lighthouses consist either of 
 powerful oil lamps, in which the supply of oil to the wicks 
 is obtained by means of pumps, or of powerful electric 
 arc lamps placed so that their diverging pencils of light 
 after passing through the lens emerge parallel to one 
 another. 
 
 In a well-constructed lighthouse lens, which produces a 
 nearly parallel beam of light, the light, if situated at the 
 top of a high tower, can readily be seen by vessels at dis- 
 tances of forty miles away. 
 
 The lighthouse lens above described was devised by 
 Fresnel, a French physicist ; so lenses of this character are 
 generally known as Presnel lenses. Where a broad beam 
 of parallel rays is desired, a form of apparatus known as 
 the Fresnel fixed light is employed. Here, as represented 
 in Fig. 89, a series of triangular hoop-shaped prisms are 
 placed above and below the central portion as shown. 
 This figure shows a cross section. The actual lantern 
 is barrel-shaped. From this lantern there would issue 
 a broad sheet of parallel rays thrown out generally in a 
 direction nearly parallel to the horizon. 
 
 Since there are often a great number of lighthouses 
 situated near one another on different portions of a coast, 
 it is necessary that some method be employed for readily 
 distinguishing them from one another. To a certain 
 extent this can be done by having the lights of different 
 
2U 
 
 THE WONDEE BOOK OF LIGHT 
 
 colours. But since very few of the different coloured 
 rays can pass through great thicknesses of air, but little 
 
 difference in this di- 
 rection is possible, 
 for red is practically 
 the only colour be- 
 sides that of the 
 nearly white light 
 given off from the 
 lamp that can be 
 used. Some other 
 method, therefore, 
 must be emploj^ed. 
 This is generally 
 done by means of 
 what is called the 
 flashing light. 
 
 A form of flashing 
 light is represented, 
 partly in elevation 
 and partly in cross- 
 section, in Fig. 90. 
 Here, as will be seen, instead of having a lantern with the 
 cylindrical outline of a barrel, its sides consist of eight 
 separate panels formed as shown of separate Presnel lenses, 
 while above and below are the prismatic hoops described in 
 connection with the fixed light. As before, the luminous 
 source is placed at the centre of the cage or lantern. As 
 an examination of the preceding figure will show, parallel 
 beams of light are thrown out of the sides as indicated. 
 If, now, by any suitable means, such as by clockwork. 
 
 Fig. 89. — Feesnel Fixed Light. 
 
LIGHTHOUSE, SEARCHLIGHT, STAGE 275 
 
 the cage is rotated or turned at a given rate, a distant 
 ship will be able to see the light only when any of the 
 
 
 -. \ 
 
 
 \ N ■> 
 
 \ 
 
 "'. \ 
 
 
 
 \ 
 
 •- N " 
 
 \ \ 
 
 N '>- 
 
 \^^,'^-- 
 
 \ ^ 
 
 \- 
 
 -'k^ 
 
 '• ' ! 
 : • ! 
 
 1 : : I ; ; ; 
 Fig. 90. — Flashing Light. 
 
 panels are turned directly towards it, the light entirely 
 disappearing when the angular space between two neigh- 
 bouring panels is turned towards the ship. Now, since, 
 
376 THE WONDER BOOK OF LIGHT 
 
 by this means, the number of times the lights appear and 
 disappear per minute can be easily varied, it is not diflB- 
 eult to distinguish with certainty one light from another, 
 especially if one or more of the panels are provided vpith 
 red glasses; or, v?hat is equally satisfactory, if one or 
 
 Fig. 91. — Aec Headlight fob LocoMOirvE. 
 
 more of the panels are entirely omitted. Both of these 
 methods are in use. 
 
 It is necessarily an exceedingly expensive matter prop- 
 erly to place such a system of lighthouses on the navigable 
 waters of any great country so as to prevent loss to ship- 
 ping. Take, for example, a country like the United States. 
 Here it would not only 'be necessary to locate long lines 
 of lighthouses at different portions of the Atlantic, the 
 Gulf of Mexico, and the Pacific coasts, but also at different 
 
Fig. 92 — Tiiiktt-i^'Ch Projectok ox Mount 
 Wasiiixgtox 
 
LIGHTHOUSE, SEAECHLIGHT, STAGE 277 
 
 points on the Great Lakes and along all of its navigable 
 rivers. 
 
 While, of course, if the rays of light coming from light- 
 house lenses are absolutely parallel, they could be seen by 
 vessels much farther ofP, yet under these conditions, the 
 width of the space where such rays would fall on the 
 ocean might be too small for use, since the light would 
 be invisible as soon as the vessel crossed this lighted 
 area. For this reason, a certain amount of divergence 
 is purposely given to the rays that have passed through 
 the lenses. 
 
 The illumination of the tracks of railroads is generally 
 obtained by what is known as a locomotive headlight. This 
 consists of a luminous source so placed in front of a re- 
 flector or mirror supported on the smokestack of the 
 locomotive that it throws a slightly diverging beam of 
 light on the track ahead. A locomotive headlight provided 
 with an arc lamp is represented in Fig. 91. 
 
 An entirely different form of light is employed at sea, 
 as well as on land, for the purpose of throwing a parallel 
 beam of light in any direction, so as to discover the position 
 of an enemy, or thoroughly to illumine some distant object. 
 For this purpose, various forms of devices known as search- 
 light apparatus are employed. These generally consist 
 of a powerful electric arc lamp so placed in front of a 
 good reflecting mirror that the rays, after being thrown 
 off from the mirror, shall be only slightly diverging or 
 nearly parallel. 
 
 A form of searchlight apparatus is represented in Fig. 
 92. Here a nearly parallel beam of light is being thrown 
 in the direction shown. 
 
 Speaking of searchlights, I might mention here that a 
 
378 
 
 THE WONDEE BOOK OF LIGHT 
 
 mueh better means for lighting the statue of William Penn 
 on the tower of the public buildings in Philadelphia, as 
 well as making the tower itself 
 visible at a fairly considerable dis- 
 tance, would be by means of search- 
 lights that throw strong beams of 
 nearly parallel rays on the tower 
 and statue. This has been done with 
 such marked success, that the light 
 thus concentrated on the building 
 was sufficient to enable it to be seen 
 at a considerable distance, although 
 the rays of light it throws ofE from 
 illumined surfaces were, of course, 
 diverging. 
 
 Various devices are employed at 
 the theatre or opera, for throwing 
 either white or coloured light on 
 objects on the stage., A common 
 form of lamp known as a stage lamp 
 is represented in Pig. 93. This, as 
 can be seen, consists of an arc lamp 
 placed at the focus of a metallic re- 
 flector, so as to throw either parallel 
 or slightly diverging light on the 
 object on the stage. 
 When it is desired to obtain on the stage effects by 
 illumining objects, such as the dresses of ballet dancers or 
 actresses, with different coloured lights, a lantern is em- 
 ployed consisting of a light placed back of a mirror and 
 provided in front with means for rapidly changing a num- 
 ber of coloured glasses so as to obtain the particular colour 
 that is desired. 
 
 Fig. 93. — Stage Lamp. 
 
CHAPTEE XXV 
 SEEING IS NOT ALWAYS BELIEVING 
 
 I WAS always careful, when teaching natural philosophy, 
 to impress on the minds of my students, especially those 
 who were beginning work in the physical laboratory, that 
 they could by no means always rely on what they thought 
 they saw; that unless they had learned by extended prac- 
 tice, how to see, they might often be mistaken in what they 
 believed they saw. In order to convince them of this, I 
 advised them to take the first favourable opportunity for 
 attending an exhibition of good legerdemain, to see how 
 easily their eyes could mislead them. 
 
 That seeing is not always believing can be proved by a 
 great variety of cases of what may properly be called 
 optical illusions; that is to say, images of external objects 
 projected on the brain through the eyes, hut so modified 
 by their surroundings that they are incorrectly interpreted. 
 
 It is only after considerable training that we are able 
 properly to judge of the distance, size and shape of external 
 objects. Indeed, it is often only by their surroundings 
 that we are able to see them in their proper proportions. 
 Under certain conditions the interpretation of images 
 formed on the retina is so incorrect, that what we think 
 we see is quite different from what we actually see; so 
 optical illusions result. There are a great variety of such 
 illusions. Of these I wiU describe a few, selecting those 
 I think cannot fail to interest you. 
 
880 THE WONDEE BOOK OF LIGHT 
 
 One of the most curious optical illusions results from 
 the fact that, under certain circumstances, we are unable 
 to form correct ideas of relative lengths. I think that 
 most of you, if asked to look at Fig. 94, and say which of 
 the two lines seems to be the longer, would unhesitatingly 
 declare that the vertical line is much longer than the 
 horizontal line. Now, if you take the trouble to measure 
 them carefully you will find that they are of exactly the 
 same length. This difference in apparent length may, per- 
 haps, be increased by making one of the two lines thicker 
 or heavier than the other. The ex- 
 act cause of this and many similar 
 illusions is unknown. 
 
 Of course you will ask the explana- 
 tion for this illusion. Now I might, 
 in reply, enter into a lengthy explana- 
 tion, such as is given in many books 
 
 Fig. 94.— An Op- that treat of the subject of optical 
 
 TICAL IlXUSION. .,, . -r , r. .i . i, 
 
 illusions. I must confes that such 
 explanations are so laboured that it is difficult to under- 
 stand what they mean, if, indeed, they have any meaning 
 whatever, which, in some cases, I greatly doubt. Such 
 explanations remind me somewhat of the answer Shake- 
 speare makes Hamlet give to the following question: 
 
 " What are you reading, my lord ? " 
 
 " Words, words, words." 
 
 So I think that until you are old enough to be able to 
 read these explanations for yourself we had better let 
 the matter pass with the statement that the exact causes 
 of such illusions are not thoroughly understoood. My 
 object now is to prove to you that seeing is not always 
 
SEEIXG IS NOT ALWAYS BELIEVING 281 
 
 believing. There is no reason for our carrying this mat- 
 ter further. 
 
 This inability properly to determiQe the relative lengths 
 of two lines is not limited to liaes drawn at right angles 
 
 Fig. 95^ — ^Anothee Optical Illusion. 
 
 to each other. For example, look at Fig. 95, where the 
 lower of two horizontal lines has exactly twice the length 
 of the upper line. I think that most of you would say that 
 the lower line appears less than twice the length of the 
 npper. 
 
 The apparent difference in length may be made more 
 deceptive if two lines of exactly the same length have 
 iaclined lines drawn at their extremities. If, moreover. 
 
 ^^^ 
 
 J^ 
 
 <- 
 
 > 
 
 ^ 
 
 < 
 
 Fig. 96. — Otheb Optical Illusions. 
 
 the inclined lines are doubled, the illusion as to difference 
 in length is still more marked. 
 
 Have you ever tried to catch your friends by asking 
 them to look at a silk hat worn by a taU man, and then 
 have them mark off on the wall of a room a point to which 
 they thought the top of the hat would come if it were 
 
383 
 
 THE WONDEE BOOK OF LIGHT 
 
 placed on the floor? In nearly every case where the 
 persons have not made such measurements before, the 
 mark will be placed much higher than the point to which 
 the hat actually comes. 
 
 In Pig. 96, the two squares formed by drawing a number 
 of equi-distant parallel lines that are horizontal at C, and 
 vertical at D, are of exactly the same size, and yet you 
 cannot help acknowledging that the square at A seems 
 to be higher than it is broad, while that at B appears to be 
 
 Fig. 97. — ^A Strange Optical Illusion. 
 
 broader than it is high; still both are true squares, as you 
 can convince yourself by actual measurement. 
 
 Sometimes in mechanical drawings, where shadings 
 have been placed on the outside of parallel lines for the 
 purpose of representing cross sections, the appearance of 
 the drawings has been greatly injured by this peculiar op- 
 tical illusion. If, again, as in Eig. 97, the oblique lines 
 alternate, the straight lines appear to be inclined differently 
 to each other alternately towards and from one another 
 from the bottom to the top. You should notice, however, 
 
SEEING IS NOT ALWAYS BELIEVING 283 
 
 that in order to produce this illusion, the oblique lines 
 should not extend across the separate parallel lines. 
 
 It is well known to printers that in some capital letters 
 and Egures, such, for example, as in the capital letters 
 and the numerals that follow, the upper and lower lobes 
 of the two are not exactly equal : 
 
 ssssssssssss 
 
 Now, if these were actually made equal the letter S, or 
 the numeral 8, would lose its apparent symmetry. The 
 top of the letter is therefore made larger than the bottom 
 as you can easily see by turning the page upside down, or 
 by looking at the following lines in which the same char- 
 acters are inverted. 
 
 SSSSSSSSSSSS 
 
 8888888888888 
 
 A number of small black circles placed closely together 
 as in Fig. 98, wiU appear, when viewed by the eye, not to 
 be circles but six-sided figures, or hexagons. This effect 
 is produced on the retina by the black and white portions 
 of the images. It will interest you to know how this 
 optical illusion was discovered. It appears that a French 
 physicist who was examining under the microscope minute 
 forms of vegetable life known as the algae was desirous 
 of producing a drawing that should correctly represent 
 on paper the appearance they thus presented. I will, 
 however, let him tell you the story as to how he succeeded 
 in obtaining this result; 
 
384 THE WONDEE BOOK OF LIGHT 
 
 " For a long time I occupied myself with the examina- 
 tion of the hexagonal appearance of the points constituting 
 the streaks. Why should these hexagons show themselves, 
 and how could they be other than the visible bases of small 
 pyramids piled very closely one on the other; and if this 
 were the case, why were not the points of the little pyra- 
 mids visible? Or, was the structure before me analogous 
 to that of the eyes of insects? Then the carapace would 
 be but a surface of perforated polygonal openings. This 
 latter hypothesis was attractive enough, and would have 
 
 
 Fig. 98. — Optical Illusion Causing Circles to Appeab as Six- 
 SiDED Figures. 
 
 explained many things, but some careful observations with 
 very powerful object-glasses, quite free from blemishes, 
 had shown me that these hexagons had round points, con- 
 trary to the descriptions of micrographs. These observa- 
 tions, corroborated by the micrographic photographs of 
 Lackerbauer, the much regretted designer, and by Colonel 
 "Woodward, of Washington, left not the slightest doubt 
 that it was necessary to discover why the eye persistently 
 saw hexagons where there were circles. To elucidate this 
 point, it was necessary to find some means of reproducing 
 
SEEIXG. IS ICOT ALWAYS BELIEVING 285 
 
 artificially what nature had accomplished with so much 
 precision on the surfaces of algse. After many fruitless 
 attempts, I decided on making a 
 trial of a stereotype plate covered 
 with dots arranged in quincunxes 
 (i. e., in square groups of five) 
 very close together. The result 
 was more successful than I had 
 hoped; the effect produced is ex- 
 actly that of the arrangement of 
 the so-called hexagons of the pj^, 99.— Sytvanus p. 
 most beautiful of the algse, the Thompson's Optical 
 Pleurosigma angulata. If these I'^^usion. 
 stereotypes are examined with one eye only, we shall be 
 immediately convinced that we have to do with hexagonal 
 polygons." 
 
 Professor Sylvanus P. Thompson, the English electri- 
 cian and physicist, gives the following curious instances 
 
 Oof optical illusions: In Pig. 
 99 a series of concentric 
 black circles, alternating with 
 white circles, are drawn as 
 shown. If now a card on 
 which these circles are drawn 
 be . given a circular motion, 
 the circles will seem to ro- 
 tate, this rotation being in 
 Fig. 100. Opticai, Illusion, the same direction as the 
 Apparent Rotation or Cog rotation of the card. If, 
 
 however a cogged wheel in 
 white is drawn with the black enclosing line as represented 
 
286 THE WONDEE BOOK OF LIGHT 
 
 in Eig. 100, then, on the rotation of the sheet, the cogged 
 wheel will appear to rotate, but this time in the opposite 
 direction to that of the rotation. 
 
 The illusion is more satisfactory if a number of concen- 
 tric circles are drawn around the cogged wheel. 
 
 There are many other curious optical illusions, but I 
 think the preceding will suffice to prove the statement 
 made that seeing is not always believing. 
 
CHAPTEE XXVI 
 EFFECTS CAUSED BY PERSISTENCE OF VISION 
 
 There are a number of curious effects caused by the 
 persistence of the retinal image ; i. e., by the fact that the 
 images of objects, viewed by the eye, do not immediately 
 disappear when the light producing them is cut off. So 
 many different effects are produced in this manner that I 
 can only call your attention to some of the more striking. 
 
 It is easy to see both sides of a silver half doUar at 
 the same time. To do this it is only necessary to spia the 
 
 Fig. 101. — The Thaumateope ob Wootjee Tuenee. 
 
 coin on its edge by giving it a sudden twisting motion. 
 As the opposite sides are turned alternately towards and 
 from the eyes the retinal images continue, so that both 
 sides can be distinctly seen at the same time. 
 
 An optical toy known as the thaumatrope — or, as the 
 word means, " the wonder turner " — is based on the same 
 principle. This toy, which, by the way, has been known for 
 many years, consists, as represented in Fig, 101, of a stiff 
 card, on the opposite faces of which two different pictures 
 
 28T 
 
S88 THE WONDER BOOK OE LIGHT 
 
 have been drawn. When the card is rapidly twirled, by 
 twisting the ends of strings attached to two of its op- 
 posite edges, the pictures placed on the opposite faces 
 appear as a single picture. You can show your ingenuity 
 by devising a number of laughable effects by drawing dif- 
 ferent pictures on the opposite faces of a card ; for example, 
 a pair of spectacles on one side, and on the other a man's 
 face. If on one side of the card you make a drawing of a 
 cigar or pipe, you can readily add these objects to any 
 photograph of a man or woman that is placed on the op- 
 posite face. 
 
 A similar form of optical apparatus is known as the 
 zootrope. In this apparatus successive pictures of men 
 or animals are placed at suitable intervals on a long strip 
 of paper, so as to leave between them blank spaces of a 
 certain length. The figures represent animals or men at 
 different times during a certain movement, such as jump- 
 ing over a fence, or running after and catching something. 
 The paper strip with its figures is placed inside a cylindri- 
 cal box, so as to be viewed by a person on the outside 
 through a number of vertical slots. A rapid rotation is 
 given to the cylinder and, since the figures inside can only 
 be seen while the slots are passing the eyes of the observer, 
 the pictures are only seen momentarily. They do not, how- 
 ever, entirely disappear, owing to the fact that their 
 images persist or remain on the retina. If the pictures 
 have been properly drawn, so as to represent correctly the 
 successive movements of the animals, or the person, they 
 will appear as a single continuous picture in which the 
 moving objects appear correctly to go through their proper 
 motions. 
 
PEESISTENCE OF VISION 289 
 
 Of course you wiU understand that, in any device in 
 ■which motions are produced by viewing successively a 
 number of pictures, it is necessary that these pictures cor- 
 rectly represent the positions occupied by the animals or 
 persons at different parts of their motions. The best 
 method of obtaining such pictures is to take successive 
 photographs at different times. 
 
 Edison has devised the familiar moving picture ap- 
 paratus known as the biograph and by a number of other 
 names. The successive pictures for this apparatus are 
 taken on a long photographic film that is wrapped around 
 the surface of a cylinder, from which it is rapidly un- 
 wrapped by any suitable mechanical means. Each picture 
 is brought successively before the image-forming lens of a 
 projecting lantern, and, being powerfully illumined by an 
 electric light, casts a succession of images on a distant 
 screen. These images, being successively seen by the audi- 
 ence do not appear as separate pictures but as if they con- 
 sisted of a single picture, representing people, animals, etc., 
 as going through lifelike movements. In this way repre- 
 sentations of people running, dancing, jumping, walking, 
 etc., are given in a very comical manner, and, since the 
 photographs correctly represent the expression of their faces 
 at different times, the effects produced are often exceed- 
 ingly ludicrous. This is especially the case if, as is some- 
 times done, the figures are apparently made to talk to one 
 another by the use of a phonograph. 
 
 I remember seeing a very remarkable picture thrown on 
 a screen by a moving picture apparatus. It represented a 
 locomotive approaching the audience from a distance. In 
 order to add to the effect, the picture of the locomotive. 
 
290 THE WONDEE BOOK OP LIGHT 
 
 when first seen on the screen, is not only small but is 
 purposely poorly illumined. It gradually grows more and 
 more distinct and somewhat larger, and appears to be 
 coming rapidly nearer and nearer the stage. In order to 
 increase the illusion, rumbling and rattling sounds are 
 sometimes added, and, just as the locomotive seems ready 
 to hurl itself off the stage onto the audience, a man is 
 represented as crossing the track safely just before the 
 locomotive rushes by. 
 
 A variety of moving discs or spinning tops, that depend 
 for their effects on the persistence of vision, are employed 
 for the mixing of different coloured lights in the eyes of 
 an observer. One of these has already been referred to. 
 You will remember that this apparatus consists of a cir- 
 cular disc capable of being rapidly rotated. The disc is 
 divided into segments of different sizes and painted with 
 the colours of the solar spectrum. The colour in each seg- 
 ment is seen as a continuous band or ring just as would 
 be a bit of glowing coal or wood if whirled rapidly in a 
 circle. The different coloured bands or circles being mixed 
 together in the eyes produce the appearance of white light. 
 
 As is well known, a mixture of black and white paints 
 produces grey. There are various ways of ensuring this 
 mixture of the white and black. Eor example, a disc, 
 painted with alternate segments of black and white, when 
 rapidly rotated, completely loses the marked contrast it 
 presents when at rest, but seems to be covered with 
 a uniform coating of grey. This effect, however, will 
 not be seen until a certain rapidity of movement has been 
 reached. 
 
PEESISTENCE OP VISIOX 291 
 
 If, on the contrary, the disc is painted as represented 
 in Fig. 103 with a six-pointed star as shown, when a suiJi- 
 cient rapidity of rotation has been reached, its surface, 
 instead of being laniformly coloured, will show a series of 
 concentric circles, the centre of 
 which will be pure black and the 
 remaining circles of different 
 shades of grey that rapidly be- 
 come lighter towards the edge of 
 the disc. A little thought will 
 explain the cause of this. Around 
 the centre of the disc there is a 
 continuous dark circle so that Fig. 102.— ilixixo Black 
 here black only will be seen, 
 
 while, as we proceed towards the circumference of the disc, 
 the amount of white that will be mixed with the black will 
 become greater and greater, thus producing lighter shades 
 of grey. 
 
 There are other ways in which these rings can be 
 produced; for example, by the rapid rotation of a disc 
 with jagged spokes. The result is a series of concentric 
 rings of varying shades of grey produced by the admix- 
 ture of the blacks and whites. 
 
 If, instead of employing whites and blacks, two dif- 
 ferent colours are used, then instead of havtag greys pro- 
 duced by the mixing of the colours on the retina, various 
 colours will result. 
 
 It makes no difference what means are employed for the 
 rapid rotation of the disc. A simple form of top devised 
 for this purpose by Helmholtz consists of a heavy brass 
 
393 THE WONDER BOOK OF LIGHT 
 
 disc, the weight of which causes the top to continue spin- 
 ning for a long time after it has been set in motion by the 
 rapid pulling of a string wrapped around its vertical axis. 
 Discs of coloured paper can be placed on the upper part 
 of the top. By cutting a slot from the centre to the edge, 
 as shown in Fig. 103, two differently coloured discs, say 
 a red and a blue, can be slipped over each other through 
 the slot so that a half, a third, or a fourth, etc., of the 
 red disc shall cover the blue disc. Then, when the top is 
 rotated, a colour will be produced such as would result 
 from the mixing of two, three, or four times as much red 
 as blue. In the same way different coloured discs can be 
 employed. When blue and yellow 
 discs are employed, a great variety 
 of different greens are obtained, 
 or when red and yellow discs are 
 employed, different shades of 
 'orange result, etc. 
 
 Perhaps, one of the most in- 
 teresting devices in the way of 
 tops for producing curious effects 
 
 Fig. 103.-PAPEB Discjjaused by' the persistence of 
 FOB Colour Top. . . . „ , . ^ . ,, , 
 
 vision, IS found m a device that 
 
 is sometimes called a dazzling top. 
 
 The dazzling top consists of a modified form of the top 
 
 devised by Helmholtz. Its axis is provided with a hollow 
 
 stem, so that when the top has been set in motion in the 
 
 usual manner, by pulling a string, different shaped wires 
 
 or pieces of metal can be slipped into the hollow axis. 
 
 These wires, or pieces of metal, being highly polished, as 
 
PEESISTENCE OF VISIOK 293 
 
 soon as they acquire a sufficiently rapid motion, produce, 
 like the point of a rapidly rotated lighted stick, a series 
 of luminous circles. In this way the gauzy outlines of 
 vases, bowls, saucers, plates, and discs are produced in an 
 exceedingly wonderful manner. 
 
CHAPTEE XXVII 
 GHOSTS 
 
 There are two kinds of ghosts; ghosts that have no 
 existence whatever save in a disordered brain, or in 
 a brain over which the mind has no control, and ghosts 
 that are caused by actual pictures formed on the retina 
 by invisible objects, and therefore believed to be of super- 
 natural origin, that is, belonging to beings from another 
 world. 
 
 We have, so to speak, two kinds of eyes, the eyes of the 
 mind, and the eyes of the body. The eyes of the mind are 
 able to conjure up images that at times seem quite as 
 distinct as those seen by the eyes of the body. It is 
 these two kinds of eyes that give rise to the two kinds of 
 ghosts above referred to. 
 
 And first as to the ghosts that have no existence, save in 
 a mind that is beyond the control of the will power, 
 and, therefore, is not to be relied on as to the accuracy of 
 its images. When we are asleep, and dream, our eyes 
 are shut; there is no possibility of the existence of any 
 true retinal images. Our will power is lost; our thoughts 
 flow on automatically, sometimes in a sensible coherent 
 order, but generally in an extremely curious way, so that 
 the things we dream of consist of strange mixtures of 
 the possible and the impossible. 
 
 Now this peculiarity characterises all our dreams. No 
 
 294 
 
GHOSTS 293 
 
 matter how eurious and wonderful they may be, thev never 
 surprise ns. It makes no difEerence whether the times, 
 places, or eircnmstances are contrary to what we conld 
 have expected them to be. "We see and talk with people 
 we know have long ago been dead, but they appear as we 
 knew them during life. They talk with the same voices 
 they had during their lifetime. It seems natural that they 
 should be talking with us. The same is true, if these 
 imaginary people happen to have died himdreds and thou- 
 sands of years before our time. It is all one and the same 
 thing, and seems perfectly natural. We may dream we are 
 talking with Christopher Columbus, the discoverer of 
 America, with George Washington, the saviour of his 
 country, with the great Lincoln, or with any other of the 
 people who have long ago been dead and buried, and 
 nevertheless this fact fails to surprise us. 
 
 Xo one can teU just how different ideas get into our 
 brains during dreams. It seems, however, that our dreams 
 are based on actual impressions received during our wak- 
 ing hours. Dr. William B. Carpenter, in his Principles 
 of Mental Physiology speaks as follows concerning this 
 point: 
 
 " There can be no doubt that the materials of our dreams 
 are often furnished by the traces left upon the brain by 
 an occurrence that has long since passed and which has 
 completely faded out of the conscious memory." 
 
 As an illustration of this fact Carpenter says: 
 
 "The following circumstance recently mentioned to the 
 writer by an eminent judge, one of whose mental ex- 
 periences has already been cited, affords a characteristic 
 illustration of this kind of cerebral action. Having 
 
1896 THE WONDEE BOOK OP LIGHT 
 
 been retained, before his elevation to the bench, in a ease 
 which was to be tried in the north of England, he slept 
 at the house of one of the parties in it; and dreamed 
 through the night that lizards were crawling over him. 
 He could not imagine what had suggested such an idea 
 to his mind, until, on going into the apartment in which 
 he had passed the evening he noticed a mantel-piece clock 
 on the base .of which were figures of crawling lizards. 
 This he must have seen without noticing it; and the 
 sight must have left a " trace " in his brain, though it 
 left no record in his . conscious memory." 
 
 Ideas of time appear to be strangely lost sight of in our 
 dreams. Instances are on record in which people have 
 dreamed of occurrences that ,have continued for long 
 periods of time, when it was certain the person had been 
 asleep only a few minutes, and, possibly, only a few sec- 
 onds. Indeed, it is believed by some, who have given this 
 matter careful thought, that dreams covering a long 
 succession of events have been started by a sound that 
 awakened the sleeper; that it is only during the short 
 interval between a previous condition of deep sleep and 
 being fully awake, that all the dream impressions have 
 been produced on the mind, or in other words, that all 
 our dreams occur during that short time which exists 
 between sleeping and waking. 
 
 It is not an uncommon experience that when there 
 seems to be an imminent danger of death, as, for example, 
 by drowning, that the entire previous life of the person 
 appears to pass almost instantaneously before his mind lika 
 a rapidly changing panorama. 
 
 There are^ however, what may be called day dreams 
 
GHOSTS 297 
 
 that occur wlien our eyes are wide open, and when we 
 believe we see some object to which, however, we attach 
 entirely inconsistent ideas. If, now, we can, in our 
 dreams distinctly see pictures either of the living or 
 dead, it is not surprising that we should also see in day 
 dreams, appearances of people who might, perhaps, prop- 
 erly be called ghosts. IN' or is it only to weak, disordered 
 brains that such day dreams come. I am sure no one 
 would accuse Sir Walter Scott, the great writer, of possess- 
 ing a weak intellect, and yet day dreams were common 
 to him. In Scotfs book on "Demonology and Witch- 
 craft," he gives an interesting account of one of these day 
 dreams : 
 
 While attentively reading, shortly after the death of 
 his friend. Lord Byron, an account of his habits and 
 opinions, Scott tells the following of a friend: 
 
 " Passing from his sitting-room into the entrance-hall, 
 fitted up with the skins of wild beasts, armour, &c., Scott 
 saw right before him, and in a standing posture, the exact 
 representation of his departed friend, whose recollection 
 had been so stronglv brought to his imagination. He 
 stopped for a single moment, so as to notice the wonder- 
 ful accuracy with which fancy had impressed upon the 
 bodily eye the peculiarities of dress and posture of the 
 illustrious poet. Sensible, however, of the delusion, he 
 felt no sentiment save that of wonder at the extraordinary 
 accuracy of the resemblance, and stepped onwards towards 
 the figure, which resolved itself, as he approached, into 
 various materials of which it was composed. These were 
 merely a screen occupied by great-coats, shawls, plaids, 
 and such other articles as are usually found in a country 
 
298 THE WOKDEE BOOK OF LIGHT 
 
 entrance-hall. Sir Walter returned to the spot from which 
 he had seen this product of what may be called imagina- 
 tion proper, and tried with all his might to recall it by the 
 force of his will, but in vain — a good illustration of 
 the slight influence of volition over sensation, compared 
 with that of a vivid mental image or idea acting upon 
 the sensorial centres and distorting or moulding into 
 other forms the impressions leceived from objects of 
 sense." 
 
 When one has formed a vivid mental picture of a certain 
 thought he may, while looking at some object, see not the 
 object itself but the mental picture so distinctly that it 
 seems to possess as much reality as if it actually existed. 
 Thus Dr. Carpenter tells the following story that illus- 
 trates this matter : 
 
 " During the conflagration at the Crystal Palace in 
 the Winter of 1866-67, .when the animals were destroyed 
 by the fire, it was supposed that the chimpanzee had suc- 
 ceeded in escaping from his cage. Attracted to the roof, 
 with this expectation in full force, men saw the unhappy 
 animal holding on to it, and writhing in agony to get 
 astride of one of the iron ribs. It need not be said that 
 its struggles were watched by those below with breath- 
 less suspense, and, as the newspapers informed us, 'with 
 sickening dread.' But there was no animal whatever there ; 
 and all this feeling was thrown away upon a tattered piece 
 of blind, so torn as to resemble, to the eye of fancy, the 
 body, arms, and legs of an ape ! " 
 
 Here, a tattered piece of window blind only was seen, 
 and yet it seemed to consist of the body of a chimpanzee, 
 that was dying in great agony in the, flames. 
 
GHOSTS 299 
 
 Carpenter also tells the following somewhat similar 
 story : 
 
 " A whole ship's company was thrown into the utmost 
 consternation by the apparition of the cook who had died 
 a few days before. He was distinctly seen walking ahead 
 of the ship, with a peculiar gait by which he was dis- 
 tinguished when alive, through having one of his legs 
 shorter than the other. On steering the ship towards the 
 object, it was found to be a piece of floating wreck." 
 
 It is not surprising, therefore, that there are apparently 
 well authenticated stories of ghosts who have been seen 
 under different circumstances by people, and, moreover, 
 by people of unquestioned mental ability, people of strong 
 mentality concerning whose integrity and reliability there 
 can be no question. But we can no more believe in the 
 reality of these illusions than we can in the bodily exist- 
 ence of the people we see in our dreams. These appear- 
 ances consist of those unsubstantial things of which dreams 
 are made. They are mere creations of the imagination. 
 
 We come now to ghosts of the second kind ; i. e., appear- 
 ances of objects whose images actually exist on the retina. 
 These objects being invisible, their images are assumed 
 to be of supernatural origin, or to constitute beings from 
 another world. 
 
 To a certain extent, our estimate of distance depends 
 not only on the size of the object, hut also on the amount 
 of its illumination. Suppose now, while in a room that is 
 too dark to prevent any one from seeing the position of 
 a distant screen, that, by means of a magic lantern, the 
 picture of a small object suddenly appears on the screen; 
 for example, a skeleton. The illumination given to 
 
300 THE WONDEE BOOK OF LIGHT 
 
 this image being small, and the image itself being of small 
 size, it seems to those in the room to be situated at an 
 enormous distance. If now by a slight movement of. the 
 lantern, the size of the image is caused to increase rapidly 
 and the amount of its illumination is also increased there 
 will be presented to the audience the optical illusion of a 
 skeleton rushing with terrible speed towards them. 
 
 The magic lantern employed for the above purpose is 
 called a phantasmagoria. It differs from an ordinary 
 lantern only in its optical details, and in being mounted 
 on rollers so that it can be moved towards or from the 
 stage. This form of lantern was produced by an Eng- 
 lish optician named Robertson, and was first brought out 
 in the city of Paris. By its use Eobertson succeeded in 
 giving a number of exhibitions in the city of Paris which 
 created an unusual excitement even in that readily excit- 
 able city. 
 
 It appears that Eobertson from a very early age was 
 greatly interested in the so-called black art. He thus 
 referred to this fact : 
 
 " Who has not in his younger days believed in witches, 
 hobgoblins, and compacts with the devil? I know I did, 
 and worse ; for I imagined and fully believed that an inno- 
 cent old woman who was a neighbour of ours really had 
 dealings with Lucifer, as every one asserted. I even went 
 so far as to envy her the power of conferring with the 
 Evil One, and once shut myself up in my room with an 
 unhappy live cock, whose head I cut off in the most bar- 
 barous manner, having heard that that was the most 
 approved manner of summoning into one's presence the 
 great head of all the demons, I waited for him for several 
 
GHOSTS 301 
 
 hours, calling on him to appear, threatening to deny his 
 existence for the future if he did not appear, but all to no 
 purpose. The books on magic and the black art that I 
 had read had completely turned my head. I believed every- 
 thing that was in them, and I desired ardently to perform 
 the wonders they described, even with the aid of the devil. 
 ' The 2Iagia Xaturalis of Porta ' and the ' Recreations of 
 Midorge,' which treated simply of natural phenomena, 
 had no effect upon me, but I was at last obliged to fall 
 back on the principles involved in them, in order to create 
 the diabolical appearances I had sought after in what I 
 considered a truly supernatural manner, until at last my 
 dweUing became a true pandemonium. 
 
 " It is only our grandmothers, it has been said for a long 
 time, who believe in magic, witches, and supernatural 
 appearances; but the statement is hardly true, seeing how 
 easily the country people fall a prey to the first cheat who 
 chooses to invest himself with supernatural powers. We 
 have sufficiently ridiculed the superstitions of the ancients, 
 and numberless instances may be adduced which are a 
 shame to their intelligence, and which give, so to speak, 
 a denial to the stories we have heard of their high state 
 of civilisation. But, I believe, if we were to make a col- 
 lection of all the stories of ghosts, of mysterious appear- 
 ances, of communications between the living and the 
 departed, of the discoveries of hidden treasures, &c., &c., 
 which have taken place, even since the Eevolution, before 
 whose power so many dark things have been brought to 
 light, the collection would hardly be less bulky than that 
 of the ancient superstitions now happily passed away." 
 
 Perhaps one of the most wonderful public productions 
 
302 THE WONDER BOOK OP LIGHT 
 
 of ghosts before large audiences was that of M. Eobin, 
 a French prestidigitatenr, one of the most celebrated suc- 
 cessors of the great Robert Houdin. Robin produced the 
 ghost I am about to describe in Paris during the year 
 1847. Since that time it has been modified and greatly 
 improved. 
 
 Of course you wish to know how such a ghost can be 
 raised on strictly scientific principles. If you examine 
 Fig. 104 I think you will have no difficulty in understand- 
 ing how this is accomplished. The figure shows a vertical 
 cross-section of a theatre with the audience on the left, 
 and the stage on the right-hand side. Beneath the stage is 
 a person dressed as a ghost. A powerful light is thrown 
 on him from a lantern, and the diverging pencils of light 
 from his illumined body, being reflected from the surface 
 of an inclined sheet of clear glass, are seen by the audi- 
 ence as an image on the other side of the glass immediately 
 in front of an actor placed as shown. , 
 
 In order to keep up this illusion, it is necessary that this 
 actor shall never come into such a position on the stage 
 as to appear directly before the plate of glass. If he did, 
 the glass plate, hitherto invisible to the audience, would 
 at once be seen, thus of course destroying the illusion. 
 
 In this exhibition every care is taken to raise the expecta- 
 tion of the audience. The lights are lowered in the theatre, 
 and to the accompaniment of faint music by the orchestra, 
 the villain, in this case the actor, becomes reminiscent 
 and tells of the many murders he has committed. At last 
 the ghost comes, as it were from the grave, to wreak ven- 
 geance on the man who had brought his existence on the 
 earth to a premature end. The murderer rises, sends an 
 
GHOSTS 
 
 303 
 
304 
 
 THE WONDEE BOOK OP LIGHT 
 
 imaginary bullet through the ghost, and even pierces its 
 heart with a rapier. But, to the intense delight of the 
 audience, who has paid to be fooled, and insists on having 
 full value for its money, the ghost, aided by a hollow 
 laugh from a person in the wings, shows its scorn for pistol 
 balls or rapier stabs, and the audience goes home to tell 
 in language, that is more enthusiastic than accurate, of 
 
 Fig. 105. — The Phantom Bouqijet. 
 
 the wonderful things it has seen, though, of course, you 
 or I could tell them that seeing is not always believing. 
 I might add to this chapter a number of other illusions 
 produced either by reflection from mirrors, or refraction 
 from lenses. For example, in a well known illusion called 
 the " Phantom Bouquet," a bouquet is seen by the aid of 
 
GHOSTS 
 
 305 
 
 a concave mirror M placed as represented in Fig. 105, un- 
 der circumstances where the object producing it is invisible. 
 The bouquet B, is supported in an inverted position on the 
 open side of a box placed as shovm. By giving the mirror 
 an inclination the image of the bouquet will appear to 
 the observer in an upright position B' resting on the top of 
 the box, just as if it were a real bouquet, when, of course, 
 a stick can readily be passed 
 through the image without in- 
 juring it. 
 
 Sometimes images produced 
 by lenses or mirrors are formed 
 on clouds of smoke or incense 
 acting as a screen. This is 
 probably the reason ancient 
 necromancers, who appear to 
 have had some knowledge of 
 optics, always burned incense 
 when they desired to raise peo- 
 ple from the dead, or called on 
 genii to appear. 
 
 All the cases I have given of 
 ghosts, especially the ghost of 
 Eobin, required complicated 
 apparatus. Now I will show you how easily you can com- 
 mand a spectre to appear on the ceiling of a room by no 
 more complicated mechanism than the figure of a devil or 
 any other grotesque shape, cut from a piece of white paper, 
 with a little hole at the centre, as shown in Fig. 106, and 
 pl'aced on a black cloth or sheet of black paper, where it 
 will receive a flood of light. After keeping your eyes fixed 
 
 Fig. 
 
 106.— An Eash-t 
 Raised Devil. 
 
306 THE WONDER BOOK OP LIGHT 
 
 on the hole at the centre of the paper, for at least half a 
 minute or longer, suddenly direct them to the ceiling of the 
 room, preferably to a portion of the ceiling that is not 
 brightly illumined. You will then see, as plain as plain 
 can be, a demon face grinning at you from above, only 
 now all the dark parts appear illumined, and the white 
 parts appear dark. If you wish to vary this appearance, 
 it is only necessary to use coloured instead of white paper ; 
 for if you cut the devil out of green paper he wUl appear 
 on the ceiling in his everyday suit of fiery red. 
 
CHAPTER XXVIII 
 
 SUNSHINE AND GLADNESS, MOONSHINE AND 
 MADNESS 
 
 I AM sure you will have no hesitation in believing that 
 sunlight produces gladness, but I hope you will not 
 believe that moonlight tends in any degree to produce 
 madness; for I feel sure this is not true. Indeed, there 
 is no reason why it should be true, because, after aU, moon- 
 light is only sunlight thrown to the earth from the surface 
 of the illumined moon. 
 
 The merry, merry simshine has long been recognised as 
 ensuring that buoyancy of feeling, or exuberance of bodily 
 health and mental vigor, that is sure to result in happiness 
 and gladness. One feels happy in the sunlight and gloomy 
 and sad in the darkness. Animals, like plants, do not 
 flourish in the dark and, if deprived for too long a time 
 of the stimulating power of light, will gradually droop 
 and die. 
 
 It is but fair, however, to say that there can be so great 
 a stimulation produced by prolonged exposure to sunlight 
 that it may eventually lead to decreased vitality. In the 
 temperate zone, where there is practically never a marked 
 excess of light, we can safely expose ourselves to the light 
 and heat of the sun for long periods of time, although even 
 here we may overdo the matter and receive so great an 
 amount of light as to be seriously injured. But in the 
 tropics, where there is so great an abundance of sunlight, 
 
 30T 
 
308 THE WONDER BOOK OP LIGHT 
 
 it is necessary that people like you and me, -whose skins 
 are white, beware of exposing ourselves too freely to the 
 full rays of the sun, since, otherwise, we may be greatly 
 injured. While this injury is produced by general exposure 
 of the body to sunshine, it is especially the eyes that should 
 be guarded under these circumstances. It often happens 
 that people from the temperate zones, on going to live in 
 the tropics, unless they take the precaution to protect their 
 eyes by coloured glasses, find their eyesight seriously af- 
 fected by the great flood of light. 
 
 On the higher mountain ranges, where the slopes are 
 covered with snow, unless the eyes are protected from the 
 intense light thrown ofE from the snowfields, a form of 
 blindness frequently occurs. It will interest you to know 
 that there is a disease of the eyes similar to snow-blindness 
 that sometimes occurs in the tropics, and is due to the ex- 
 cessive light of these parts of the world. 
 
 While an excess of light is apt to be attended by bad 
 results; yet, when taken in moderation, light unquestion- 
 ably acts as a stimulant to our bodies as well as to the 
 bodies of animals. As is well known, exposure to light 
 increases the ability of the blood to carry with it the life- 
 giving oxygen that is taken from the atmosphere during 
 breathing. Moreover, in a similar manner, an exposure to 
 the light aids the blood in giving off, or getting rid of, the 
 carbonic acid gas that is produced in different parts of the 
 body, by the oxygen burning the impurities out of the 
 blood. 
 
 Under certain conditions, what may be called light hun- 
 ger exists in animals. This hunger is apt to occur in 
 the case of dark skinned races like the negroes. When 
 
SUNSHINE AND GLADNESS 309 
 
 negroes are brought from a tropical climate to this 
 country, they will often manifest light hunger in an un- 
 mistakable manner. You have probably noticed how in 
 some portions of the temperate zones of the United States, 
 negroes appear to take delight in lying, or even sleeping, 
 in the full sunlight. If you or I should do this, we 
 would soon get too much of a good thing. The negro 
 is able to do it for two reasons; first, because his body 
 has become accustomed to it during many generations; 
 and, second, because his sMn is dark coloured, and, there- 
 fore more opaque to light than our white skin. You know 
 that the life in the open air and the exposure to the full 
 sunshine you have when you go to the country, and espe- 
 cially to the seashore, produce a bronzing, tanning or 
 browning of the skin. Now beyond doubt this darkening 
 of the skin is an effort on the part of nature to protect 
 the body from too free exposure to the sun. 
 
 Exposure to sunlight produces stimulation just as a 
 drink of wine or alcohol produces stimulation. But it 
 does not foUow that because a moderate exposure to the 
 merry sunshine stimulates the body and mind, thus mak- 
 ing you feel glad and happy, that an over-exposure is 
 desirable. 
 
 Exposure to sunlight causes gladness by a stimulation of 
 the mental powers. Some of our best known literary men 
 actually basked in the sunlight. Charles Lamb greatly 
 enjoyed the bright sunny skies of Italy. Goethe talks 
 about sun-thirst. Southey was fond of writing while 
 lying in the hot sunshine on the white beach at the sea- 
 shore. 
 
 The lower animals appear to recognise this stimulating 
 
310 THE WONDEE BOOK OF LIGHT 
 
 power of light. Dr. WoodrufE tells about a certain species 
 of ants that are in the habit of bringing the eggs out of 
 their dark underground nests for a short exposure to the 
 bright sunlight. As is well known, if left too long in this 
 light the eggs would be killed. A short exposure, how- 
 ever, appears greatly to aid in their germination and 
 subsequent growth. 
 
 Perhaps one of the best instances of the good effects of 
 sunlight is found in a statement recently made in the 
 New Yorh Medical Journal (January 34th, 1903) that 
 houses in New York City, situated on the north side of the 
 streets that run east and west, show a higher death rate 
 in such diseases as pneumonia, consumption and kidney 
 troubles, than houses on the south side. He concludes 
 that the reason for this is that the people living on the 
 south side of the street get more light and air since, dur- 
 ing the winter, they spend so much of their time in the 
 rear rooms, which are sunnier, lighter and warmer. 
 
 It is because of the well-known effects produced on the 
 human body by sunlight that various kinds of treatment 
 have arisen for the curing of disease by exposure either 
 to the white light of the sun, or to certain of its coloured 
 rays. Such methods for the treatment of diseases are 
 known by the scientific name of actino-therapy, or, what 
 you may call, light-cure. 
 
 The lower or infra-red rays of the spectrum, as well as 
 the ultra-violet rays, have been especially employed in the 
 cure of diseased conditions of the body. In this treat- 
 ment light, passed through coloured glasses, is permitted 
 to fall directly on the body of the patient, or is concen- 
 trated on particular parts by means of lenses. 
 
SUNSHINE AND GLADNESS 311 
 
 The violet or the ultra-violet rays are, perhaps, employed 
 most frequently in light-cures. While, in certain cases, 
 unquestioned good has resulted from the exposure of the 
 body to concentrated violet light, yet it appears that serious 
 effects may follow a too prolonged exposure. 
 
 I was once told by a well-known English Army officer 
 that, during the prevalence of cholera ia India, some thirty 
 odd years ago, it was his experience that the best method 
 of sterilising water, or rendering it fit to drink by killing 
 any cholera germs it might contain, was to permit the 
 water to run in a thin stream where it was exposed to 
 direct sunshine. 
 
 There are various methods employed for the treatment 
 of diseased bodies by their exposure to the light. Some- 
 times the patients are merely bathed in light of different 
 colours by placing them in a room in which all the light 
 that enters has passed through variously coloured glasses 
 in the windows. Sometimes, however, powerful pencils 
 of light are concentrated on different parts of the body 
 by means of lenses. Now while many diseases are greatly 
 improved by such an exposure to violet light (for this 
 unquestionably results in the death of the germs causing 
 those diseases) yet, unless care has been taken, there may 
 be so great a stimiilation of the surrounding tissues that 
 there is a danger of gradually destroying them. There- 
 fore, while the patient may be cured of some particu- 
 lar disease, yet he may die of another and even worse 
 disease produced by the death of the surrounding tissues. 
 
 Various forms of what are called light baths are em- 
 ployed. These for the greater part consist of small cabi- 
 nets containing a great number of incandescent electric 
 
313 THE WONDEE BOOK OF LIGHT 
 
 lamps and in some cases arc lamps. Such a quantity 
 of light in a small space, especially in the case of incan- 
 descent electric lamps, may cause the temperature of the 
 air in the bath to rapidly rise, soon reaching as high as 
 122° F., or even 140° F., so that the patient soon begins 
 to perspire freely. 
 
 When intelligently administered, and not used in ex- 
 cess, light has been known to produce beneficial results. 
 
 It is believed by some that exposure of the body to differ- 
 ent coloured rays of light produce effects that \avj with 
 the colour. For example, it is said by some that exposure 
 to the red rays produces a stimulation or excitement of the 
 nervous system. This is probably the reason red excites a 
 bull. It is also claimed, on the contrary, that exposure to 
 the ultra-violet rays produces a depression of the nervous 
 system. 
 
 But I think I have now said enough to prove that sun- 
 light tends to gladness. And now as to the dismal state- 
 ment that moonlight tends to madness. I am glad to say 
 that as far as I have been able to discover from my read- 
 ing, no evidence whatever has been adduced to show the 
 correctness of this statement. It was, however, at one 
 time generally believed. Indeed, the word lunatic, in its 
 derivation from the Latin word luna, the moon, is based 
 on the belief that lunatics owed their madness to exposure 
 to moonlight. 
 
 A belief in the ability of the moon to work spells or 
 produce ill effects on people has existed from the earliest 
 times. These beliefs are so ridiculous that I only men- 
 tion them here to show you how far people will let them- 
 selves be fooled. For example, in some parts of the world 
 
SIJXSHIXE AND GLADXESS 313 
 
 it is firmly believed that, if an unbaptised cbUd is exposed 
 to the rays of the moon, it wiLL be uafortunate all the rest 
 of its life. If yon have ever had a run-around on your 
 finger, or a felon or swelling under the nail, you will not 
 need to be told that they are painful; for you wUl know 
 that by experience. It may interest you, however, to know 
 that there are countries where it is claimed that you 
 brought these swellings on yourself by pointing this finger 
 at the moon. Another equally foolish belief is that if the 
 hair of the head is cut while the moon is increasing, or 
 apparently growing bigger every day, headache wiU re- 
 sult. 
 
 The astrologers firmly believed that the moon governed 
 the brain, and thus caused madness. Every now and then 
 the newspapers, possibly with a desire to " poke fun " 
 at people who are weak enough to believe that the moon's 
 light can affect the brain, publish a new moon hoax. This 
 willingness to ridicule believers in the moon seems to have 
 existed as long ago' as the time of the Grecian author, 
 Lucian. Lucian, who was a great wag, asserted that one 
 night, while asleep on the earth, he was wafted to the 
 moon where he met Endymion, the shepherd. You may 
 possibly know that the great ambition of Endymion was 
 that he should always remain young, and should be per- 
 mitted to sleep as long as he wished. Lucian declares 
 that when he reached the moon he found Endymion asleep, 
 and since he was himself wafted there while asleep, the 
 account he gives of the moon may be regarded as moon- 
 shine, the stufE that dreams are made of. In describing 
 the people on the moon, Lucian says they lived on frogs 
 that they cooked over a fire, but that, instead of eating 
 
314 THE WONDEE BOOK OP LIGHT 
 
 the flesh, they simply breathed in the smell that came 
 from the cooking. Another peculiarity of these people 
 was the odd habit they had of taking their eyes out of 
 their heads so as to keep them from seeing any thing that 
 displeased them. Now this was certainly an exceedingly 
 foolish habit; for, when they mislaid their eyes, they were 
 obliged to borrow those of their friends. But enough of 
 this nonsense, which I only mention to show you that it is 
 possible for one to be madder than a March hare by having 
 one's head filled with this kind of moonshine. 
 
CHAPTER XXIX 
 
 PHOTOGRAPHY 
 
 "When you get to thinking about it there are very few 
 things that are really more wonderfiil than the taking of a 
 photograph. I have sometimes heard people praised be- 
 cause they are able to see so much at a single glance. 
 Such people, looking but for a few moments at a collection 
 of objects in a large store window, are able to remember, 
 and record in writing, a greater number of the objects 
 than you would believe possible. These people are prop- 
 erly praised both for their powers of observation and for 
 their great memories. But the best of them cannot begin 
 to compare with a photographic plate, either in its quick- 
 ness of observation, or in its wonderful memory. When 
 suitably placed in a camera, focused on a crowded audi- 
 ence, and permitted to remain exposed but for a small 
 fraction of a second, a photographic plate is able both to 
 record and to remember the smallest details of all objects 
 in view. Xot a single person in the audience who is placed 
 so as to be in the view of the camera is missed. And not 
 only this, but even all the peculiarities of size, height, 
 clothing and sex are clearly marked. In the same way, if 
 the camera is pointed at a landscape, there is not a single 
 blade of grass, a flower, a tree, or even a leaf on the trees, 
 in sight, that the photographic plate does not recognise and 
 record. It is, indeed, a wonderful thing, and I have often 
 
 315 
 
316 THE WONDEE BOOK OF LIGHT 
 
 thought how much better the memory of a photographic 
 plate apparently is than our memory. I use the word 
 " apparently " here purposely, because I am by no means 
 sure that, if we had more carefully trained memories, or, 
 as Shakespeare calls them, " the tablets of our brains," they 
 would not only equal, but would even surpass the most 
 highly sensitive photographic plate that has ever been 
 produced. 
 
 When describing the camera obscura, I called your at- 
 tention to the fact that it would be an easy matter, by 
 placing in the darkened room of the instrument, a sheet 
 of paper covered with any substance sensitive to light, per- 
 manently to record on it the minutest details of the image. 
 Now in point of fact, the wonderful invention of photog- 
 raphy was the result of actual trials of this character. 
 
 At first thought, photography certainly might not seem 
 to be an invention of a very high order. It was known 
 that there are substances that change colour on exposure to 
 light. It would only require, it seemed, the placing of a 
 sheet of paper on the glass directly in the light. But the 
 camera obscura was invented by Baptista Porta, of Na- 
 ples, as early as 1538. It was known as early as 1777 that 
 light possesses the power of affecting certain compounds of 
 silver. And yet it was not until long afterwards, in 1814, 
 that the great invention of photography was actually 
 made. 
 
 Like many other great inventions, this was not made by 
 a single individual. The first discovery in this art may, 
 perhaps, be regarded as beginning in the year 1803, when 
 Josiah Wedgwood, an English potter, and Sir Humphrey 
 Davy, an English philosopher, described a method they had 
 
PHOTOGEAPHY 317 
 
 employed for copying shadow pictures, or the profiles of 
 opaque objects, by light acting on paper or leather covered 
 with a small quantity of silver chloride. On exposure to 
 sunlight this material changes in colour from a nearly 
 pure white to a purplish black. The shadow pictures so 
 obtained were, in reality, photographs. They possessed but 
 little value, however, since no means had then been discov- 
 ered for making them permanent by fixing them; i.e., by 
 dissolving off the silver chloride that had remaiaed unaf- 
 fected by the light. Unless this was done the photographs 
 were necessarily spoiled by a general blackening of their 
 surfaces when taken into the light. 
 
 At a much later date, in 1839, an Englishman, named 
 Fox Talbot, in a paper read before the Eoyal Society, 
 described a method by means of which pictures thus 
 formed could be fiLxed by the use of a solution of common 
 table salt. This was a great improvement, but unfortu- 
 nately the fixing agent employed was imperfect in its ac- 
 tions, so that even these pictures gradually browned or 
 darkened on prolonged exposure to the light. 
 
 But before this time a French experimenter, Joseph 
 Xiepce of Chalons, after working for some fifteen years 
 in an endeavour permanently to fix the images produced in 
 the camera obseura, described in 1829, a process by which 
 he was able to do this, iloreover, about 1824, another 
 Frenchman, Daguerre, an artist, who had been trying to 
 do the same thing, had opened in the city of Paris an ex- 
 hibit called the diorama. Daguerre employed the camera 
 obseura to aid him in painting his pictures. He tried for 
 many years to fibs permanently the pictures formed in the 
 camera obseura by the action of the light on plates cov- 
 
318 THE WONDEE BOOK OP LIGHT 
 
 ered with substances that were acted on by light. Niepee 
 and Daguerre finally entered into a partnership and worked 
 together with the idea of more thoroughly solving this 
 important problem. 
 
 Before these men had formed their partnership, how- 
 ever, Niepce had invented the following process : Dissolv- 
 ing bitumen or pitch in a solvent liquid, he flowed the so- 
 lution uniformly over the surface of a metallic plate, and, 
 when dried in the dark, placed the plate in the camera 
 obscura, so that the image of the object was focused on its 
 surface. The action of the light rendered insoluble the 
 portions of the pitch surface on which it fell, so that when 
 the plate was developed by flowing a suitable solvent liquid 
 over its surface, the portions not acted on by the light were 
 removed. In this way a picture remained on the surface of 
 the bitumen in relief, or standing up higher than the por- 
 tions that had been dissolved. This process, though 
 successful, was objectionable in that the action of light 
 was so feeble that an exposure for several hours was 
 necessary. 
 
 Mepce and Daguerre worked together but without any 
 marked success. Niepce died in 1833, but Daguerre, con- 
 tinuing his experiments, at last made a great discovery. 
 By exposing a silver plate to the vapour of iodine, he formed 
 on its surface a uniform and thin layer of iodide of silver, 
 a substance very sensitive to the action of light. This 
 action, however, had long remained unknown because it 
 left no visible traces on the plate. 
 
 You can convince yourself of this by asking some one 
 to let you come into the dark room when they are about to 
 develop a photograph. When the plate which has been 
 
PHOTOGEAPHY 319 
 
 exposed in the camera is taken out of the plate-holder for 
 development, no traces of an image can be seen on its sur- 
 face. Some peculiar action has taken place in its sensi- 
 tised surface, the nature of which is not exactly under- 
 stood. The picture is iuvisible, but when the plate is sub- 
 jected to a process called " development," you will see the 
 picture flash out in a very wonderful manner. 
 
 Daguerre, failing to see any image on his silver iodide 
 plate, thought that the light had not afEected it. One of 
 these silver plates, that he put in a closet until he could 
 find the time to clean it, recover it with iodide of silver, 
 and use it again, was unintentionally exposed to the fumes 
 of mercury, and these fumes instantly developed the latent 
 picture and thus made it visible to the eyes. 
 
 Some people seem to get no little pleasure in the belief 
 that many great scientific discoveries were the results of 
 mere chance. They are apt to cite the discovery of this 
 great principle in photography as an example. This is 
 not at all fair. It is true that Daguerre's discovery was, 
 to a certain extent, accidental, but you must remember that 
 Daguerre had been working for years in this line in an en- 
 deavour to find some way of obtaining a visual picture by 
 the action of light. He, therefore, was ready to take the 
 slightest hint that Dame Xature would give him. She 
 gave him the hint and he took it. 
 
 Daguerre announced his great discover}' in 1839. His 
 first photographs were taken on plates of silver, but in 
 order to decrease the cost, he afterwards employed plates 
 of copper, electro-plated with silver, and afterwards ex- 
 posed the silvered surface to the action of iodine. Pic- 
 tures taken by Daguerre's process were called Daguerreo- 
 
320 THE WONDER BOOK OP LIGHT 
 
 types. They are positive pictures, and, therefore, possess 
 the disadvantage of permittting only a single picture to 
 be obtained. 
 
 Shortly after the announcement of Daguerre's discov- 
 ery. Pox Talbot, already referred to, made a great im- 
 provement in the photographic process by the employment 
 of iodide of silver, fixed in a uniformly thin coating on 
 the surface of a transparent glass plate. The iodide was 
 caused to cling to the plate by covering one of its surfaces 
 with a material that would adhere with considerable force. 
 But since it is not our iatention in this Wonder Book to 
 give any more than a general description of the more im- 
 portant facts of photography, and since to a certain ex- 
 tent, most of these processes are based on the original 
 process of Talbot, I will make this description of the art 
 of photography general rather than specific. 
 
 There is a great difference between the taking of photo- 
 graphs now and forty or fifty years ago. Then, the pho- 
 tographer was obliged to make his own sensitive plates, 
 and, in fact, to perform all the various processes necessary 
 to produce a picture. 
 
 There are two kinds of sensitive plates employed to-day 
 in photography; namely, wet plates and dry plates. Dry 
 plates have come into almost universal use, owing to the 
 fact that after they have been made they can be readily 
 carried from place to place; for, with proper care, they 
 are as good a year or more after they have been made, as 
 they were on the day they were prepared. 
 
 Wet plates, on the contrary, are difficult to prepare; re- 
 quire the use of a dark photographic room in the immedi- 
 ate neighbourhood; and must be used shortly after they 
 
PHOTOGEAPHY 321 
 
 are made. I think you will imderstand this better when 
 I describe to you the process required for the production 
 of a wet photographic plate. 
 
 The plate of glass that is to be employed for a wet pho- 
 tographic plate must first be thoroughly cleansed. An ex- 
 ceedingly small speck of dirt may ruin the photograph. 
 The plate is then covered with a uniform layer of a liquid 
 substance called collodion, produced by dissolving a variety 
 of gun-cotton in a mixture of alcohol and ether. Thus 
 prepared, collodion is a colourless, transparent liquid. 
 When flowed over the surface of the glass plate, the alco- 
 hol and ether rapidly evaporate, leaving a uniform and 
 firmly adhering film of gun-cotton. 
 
 Before flowing the plate with the collodion, a certain 
 quantity of iodide or bromide of silver, or sometimes mix- 
 tures of both of these substances, are dissolved in it. Be- 
 fore thoroughly dry, the plate is immersed in a bath of 
 nitrate of silver to which a small quantity of common salt 
 has been added. In this way a coating of iodide of silver, 
 or bromide of silver, or a mixture of both of the substances, 
 is formed on the plate. The plate is then set aside to drain, 
 and is afterwards exposed in the camera while in a moist 
 state to the action of light. 
 
 It is not difiBcult to see how inconvenient the wet-plate 
 photographic process must be, when pictures are to be 
 taken at any distance from a dark room, since one must 
 take along a large amount of apparatus necessary to pro- 
 duce the plates in the dark. 
 
 'After the exposure of the plate it is necessary to bring 
 out the latent image. This is done by the process known 
 as developing, where certain chemical substances are em- 
 
322 THE WONDEE BOOK OF LIGHT 
 
 ployed, such as salts of iron and pyrogallic acid. The de- 
 veloper acts hy reducing to the metallic state those por- 
 tions of the iodide or bromide of silver that have been 
 acted on by the light. Since the amount of the change 
 that takes place is proportional to the amount of light that 
 has fallen on different portions of the plate, there will be 
 seen on the developed plate a negative picture, or one in 
 which the light portions of the image are reproduced in 
 black, while the dark portions, or those on which little or 
 no light, has fallen, remain either but slightly affected, or 
 are even entirely unaffected, so that the plate will remain 
 of the same colour it had before it was exposed to the light. 
 As soon as the development of the plate has gone on to a 
 sufficient degree, the plate is washed in water so as to re- 
 move the developer thoroughly and thus check its further 
 action. 
 
 Were the plate now exposed to the light, the photo- 
 graphic picture on its surface would be ruined, since it 
 still contains a considerable quantity of the sensitive silver 
 salts that have not been acted on by the light. A process 
 known as fixing must therefore be employed. This is done 
 by washing the plate with a chemical solution known as 
 hypo-sulphite of soda which dissolves and thus removes the 
 unaffected silver salts. 
 
 The next step is to wash the plate thoroughly so as to 
 remove the hypo-sulphite. Unless this is done the picture 
 is apt to fade. 
 
 The photographic plate thus obtained forms what is 
 called a negative, since in it the lights and shades of the 
 natural object are reversed. A negative, however, pos- 
 sesses the great advantage of readily producing a great 
 
PHOTOGEAPHY 323 
 
 number of pictures in which the lights and shades are as 
 seen La nature ; or, in other words, are positive. 
 
 There are various ways in. which positives can be ob- 
 tained from negatives. One of the simplest consists in 
 coating a sheet of paper with a thin iilm of silver chloride, 
 a chemical substance whose power of turning black on 
 exposure to light is well known. The dried, photographic 
 negative is placed over a sheet of this paper and exposed 
 to light. The portions of the paper immediately under 
 the clear parts of the glass are blackened by the light, 
 while those immediately under the blackened portions of 
 the negative remain white, so that in this way, a positive 
 picture is produced. Of course, as soon as this is done, aU 
 the silver chloride that has been unacted on must be 
 removed ; that is, the picture must be fixed. This is done 
 by washing it in a solution of hypo-sulphite of soda. 
 
 Pictures produced as above are called silver prints. 
 They consist in reality of coatings of metallic sUver, spread 
 in different thicknesses over the surface of the paper, thus 
 producing the necessary shading. Since metallic sUver 
 readily tarnishes on exposure to the air, it is necessary to 
 subject it to a process called toning. The method gener- 
 ally adopted for this purpose consists in depositing a thin 
 layer of metallic gold over the silver. Since gold does not 
 tarnish on exposure to the air, a toned picture will last 
 for practically an indefinite length of time. 
 
 Most of the photography that is carried on out of doors 
 to-day is done by what are called photographic dry plates. 
 These consist of glass plates, one surface of which is cov- 
 ered with a coating of gelatine in a condition known as 
 emulsion ; that is, the gelatine has been acted on by a sol- 
 
334 THE WONDEE BOOK OF LIGHT 
 
 vent so as to assume the shape of minute globules sus- 
 pended in the liquid. This gelatine has been rendererd 
 highly sensitive by being mixed with sensitive bromide of 
 silver, and is then spread uniformly in a thin layer over 
 the surface of the glass plate. 
 
 There are many processes employed for the preparation 
 of this emulsion by which the degree of its sensitiveness 
 may be varied. I will not attempt, however, to describe 
 them in this book. It is enough to say that dry photo- 
 graphic plates thus prepared can readily be made ex- 
 tremely sensitive. 
 
 Sometimes, instead of employing glass for the prepara- 
 tion of dry photographic plates, a long, thin sheet of cel- 
 luloid, a variety of gun-cotton is used, with a sensitised 
 coating of gelatine spread over its surface. The sheet is 
 tightly rolled and placed in the camera so that a portion 
 may receive the image. When an exposure is made, this 
 portion is replaced by another portion of the roll that has 
 not yet been exposed. Those of you who have worked with 
 cameras of the kodak type, remember how, after you have 
 focused the camera as well as you can on a distant object, 
 and have exposed a portion of the film, you must turn it so 
 as to bring a new portion ready for exposure. In some 
 forms of cameras, such as those employed for taking the 
 successive pictures that are used in moving-picture ap- 
 paratus, the film is rapidly moved by clockwork. 
 
 It is possible to make dry photographic plates or films 
 so exceedingly sensitive that their action is practically in- 
 stantaneous. With such films, excellent pictures can be 
 taken of waves at the seashore when in the act of break- 
 ing; of a yacht that is rapidly moving past the camera; 
 
PHOTOGEAPHY 325 
 
 of a bird while in flight; of a racehorse on the track; or 
 of a rapidly moving express train. Since the films are 
 thin, many dozen separate pictures can be placed on 
 a single roll. This is a matter of great convenience, since 
 a camera, once loaded, can be employed for a considerable 
 time. Moreover, it is not necessary to at once develop 
 such pictures on the film. They can be kept for a long 
 time after they have been taken without injury. 
 
 The ferrotype is a form of photograph with which many 
 of you are familiar, although the ferrotype is not as much 
 employed now as it was formerly. In the ferrotype the 
 photograph is taken on the surface of a thin plate of iron 
 covered with black varnish so as to protect the iron plate 
 from the action of the chemical solutions in which it is 
 placed. The sensitised surface of the ferrotype is coated 
 in various ways with sensitive salts, sometimes a wet 
 process being employed, and sometimes a gelatine emul- 
 sion being spread over the surface of the plate. 
 
 There are a great number of processes called photo- 
 graphic printing processes, whereby it is possible to em- 
 ploy a photographic positive in connection with priuter's 
 ink, and so produce an indefinite number of copies. It 
 is possible, also, to make a photographic picture on a litho- 
 graphic stone and thus produce an indefinite number of 
 pictures by photo-lithography. 
 
CHAPTEE XXX 
 SOAP-BUBBLE COLOURS 
 
 Most of yoy have blown soap-bubbles. If you have not, 
 I would earnestly advise you to begin, no matter how old 
 you may be ; for it will give you much information as well 
 as pleasure. 
 
 When you first begin to blow a soap-bubble it possesses 
 a good reflecting surface, and, therefore, acts as an excel- 
 lent convex spherical mirror, which, like all mirrors of this 
 kind, produces amusing and curious contortions in the 
 shapes of its images. As the bubble increases in size, and, 
 consequently, as the thickness of the film decreases, in 
 addition to the curious images, prismatic colours begin to 
 be seen on its surface. At first these are so weak that it 
 is difficult to distinguish them, but as the walls of the soap- 
 bubble film grow thinner the colours become more and more 
 distinct, until, when the bubble becomes large, they rival 
 in brilliancy the colours of either the rainbow, or of the 
 solar spectrum as caused by the passage of sunlight through 
 a prism. Now, it can be shown, as indeed I hope you wiU 
 be able to understand after reading this chapter, that a 
 soap-bubble film is by no means of uniform thickness at 
 all places. If it were, the same colour would be seen over 
 all parts of its surface. As you know, this is never the 
 case. 
 
 A soap-bubble soon breaks, for its life is short. If, how- 
 336 
 
SOAP-BUBBLE COLOTJES 327 
 
 ever, it is guarded against the causes that tend to break it, 
 its life may be greatly prolonged. Now there are mainly 
 two causes for the breaking or bursting of a soap-bubble. 
 One is its exposure to jarring or sudden blasts of air. 
 The other is the thinning of its film by rapid evaporation. 
 The latter is the chief cause of a high mortality among 
 soap-bubbles; for since its extended surface offers an ad- 
 mirable chance for evaporation, the bubble dries up, or 
 evaporates at its surface, and thus soon becomes so thin 
 as to break. 
 
 There is a simple way by means of which it is possible 
 so greatly to increase the life of a soap-bubble that it be- 
 comes so to speak a veritable Methuselah among soap- 
 bubbles. 
 
 If a soap-bubble, which ordinarily lives a fraction of 
 a minute, can be kept from breaking for four, five, six, 
 or even eight hours, its lifetime will be many times that 
 of an ordinary bubble. Such a bubble would certainly 
 out-Methuselah Methuselah; for he lived only 969 years. 
 Taking the life of a man at present as three score years 
 and ten, or seventy years, we see that Methuselah lived 
 not quite fourteen times as long as the average man. A 
 soap-bubble that was kept from breaking for six hours 
 would live 360 minutes, or 31,600 seconds, as compared 
 with one-quarter of a minute, or fifteen seconds, for the 
 life of an ordinary bubble. So the bubble would have 
 existed 1440 times as long as an ordinary bubble, whereas 
 Methuselah lived only fourteen times as long as the 
 ordinary man. 
 
 I am sure you will be interested to know how you can pro- 
 long the life of a soap-bubble. It is not at aU a difficult 
 
328 
 
 THE WONDEE BOOK OF LIGHT 
 
 matter. You must simply protect the bubble from blasts 
 of air and prevent it from evaporating. In addition to 
 this, you must keep the bubble in as quiet a place as pos- 
 sible, so as to protect it from jarrings. Since my six-hour 
 bubble spent its life in a school-house, where there were 
 about seven hundred boys, I think you may expect to keep a 
 bubble alive at least as long as I did. Of course, to do 
 
 Fig. 107. — An Exceedingly Venebable Soap-Bubble. 
 
 this, you must get a good variety of soap-bubble liquid, 
 preferably a mixture of good castile soap and glycerine. 
 
 And now for the details of the experiment. You will 
 need a large bell-jar A, Eig. 107, although a large candy 
 jar will answer. Place on a table, in as quiet a room as 
 you can, a tumbler B, with the mouth upwards. Select for 
 this purpose a tumbler whose edge is smooth. Blow a 
 good-sized bubble 0, and, before it has reached its full 
 size, permit the bottom to come in contact with the mois- 
 tened top of the tumbler. When the bubble has become 
 fairly large, gently disengage it from the pipe. This re- 
 
SOAP-BUEBLE COLOUES 329 
 
 quires some skill, but I think you wiU soon be able to do 
 it. Having previously placed some water on the top of 
 the table, and moistened the inside of the bell-jar A, place 
 it over the bubble as shown. The bell-jar wiU now 
 protect the bubble from having its life knocked' out 
 by draughts of air, and since the air inside the jar will 
 soon become filled with moisture, by the evaporation of 
 water purposely placed inside it, the bubble wiU be pro- 
 tected from the principal thing that would tend to extin- 
 guish it; that is, evaporation. You have now caged your 
 bubble and can watch it grow old. 
 
 What wUl most probably eventually cause the death 
 of the bubble is that the soapy water which forms its film 
 will gradually drain downwards from the top of the bub- 
 ble to the place where it rests on the edge of the glass 
 tumbler. In this way the top of the bubble will gradually 
 become thinner and thinner and at last will necessarily 
 break. 
 
 The colours produced by very thin soap-bubble films are 
 wonderful both in their variations of hue, as well as in their 
 intensity. As the caged bubble grows older, its film be- 
 comes thinner, especially at the top, where the most won- 
 derful display of colours is apt to occur. At last, how- 
 ever, when the bubble is nearing its end, it begins to show 
 some of the signs of old age. In the case of a soap-bubble, 
 these signs are especially seen by the colour of its top or 
 head approaching a black. In this way a very aged soap- 
 bubble differs from men and women in whom as you know 
 the approach of old age is indicated by the whitening of 
 the hair. In the case of the soap-bubble, it is by the black- 
 ening of the top of the bubble. This blackening is caused 
 
330 THE WONDEE BOOK OF LIGHT 
 
 by the soap-bubble film becoming so thin at the top that its 
 thickness is somewhere in the neighbourhood of the diam- 
 eter of a molecule of water. 
 
 The colours seen in soap-bubbles are produced in an en- 
 tirely different manner from those of ordinary coloured 
 fabrics. The material of the soap-bubble film is trans- 
 parent or colourless, and it is white light that falls on 
 the bubble. What is it then that causes the brilliant dis- 
 play of colours? It is certainly neither the absorption of 
 certain of the rays, nor is it their dispersion as in the case 
 of a prism. It is due to an entirely new cause that has not 
 yet been described; i.e., to the fact that light is produced 
 by wave motion in the imiversal ether. 
 
 If you were asked what would happen if two candles 
 could be lighted so absolutely at the same time, that the 
 ether waves from each candle, moving outward in all di- 
 rections, would reach the ether situated somewhere between 
 the two candles, so that both sets of waves would simul- 
 taneously cause it to move to-and-fro, what would you say? 
 Would the motions and consequently the light at these 
 points be greater or less in intensity than the motions and 
 light at other points? I imagine that most of you would 
 say : " Why, of course, the light would be twice as intense 
 at all points, since if one candle can give out light of a 
 given intensity, then two candles must give out light of 
 twice the intensity." This answer would be correct, pro- 
 vided the distance in which the waves from each of the 
 candles tended to move the ether was the same. 
 
 But suppose, owing to the light from one candle moving 
 through a distance sufiBciently greater than the light from 
 the other candle, that it would reach the ether between the 
 
SOAP-BUBBLE COLOUES 331 
 
 two candles at such a time that it would try to move the 
 ether in a direction opposite to that in which the other 
 candle was trying to move it, and that these two forces were 
 exactly equal in strength and continued to act for the same 
 length of time. I am sure you can see that then there 
 would be no motion whatever. The waves from one candle 
 would exactly neutralise the waves from the other candle, 
 and since in light the absence of ether motion means dark- 
 ness, there would be places between the two candles where 
 the light would be entirely absent. At first sight it seems 
 here as if this is a case of one being added to one and pro- 
 ducing nothing. It is not, however, but is rather the case 
 of one minus one being equal to zero; that is, one taken 
 from one leaves nothing, which must of course be the 
 case. 
 
 Perhaps you will understand this better if I put it in 
 another way. You all know the game of tug-of-war. Now 
 the case of the waves from the two candles trying to move 
 the ether in opposite directions, for equal times, with equal 
 forces, is similar to the tug-of-war. The ether forms the 
 rope; the equal forces oppositely acting for equal lengths 
 of time, are the teams pulling on the halves of the rope. 
 If the teams are exactly matched, the rope does not move. 
 
 "Without going any further into this subject, I may say 
 if a ray of light R, Pig. 108, of a single colour (say a ray 
 of light produced by burning sodium in an alcohol flame) 
 be permitted to fall on the thin surface of a soap-bubble 
 film A, represented as of varjong thickness, a portion of 
 the light 1 wiU be reflected or thrown off from the upper 
 surface of the bubble, and that a part of the remainder, 
 passing through the film, wiU be reflected from its lower 
 
333 THE WOFDEE BOOK OP LIGHT 
 
 surface 2, and, after passing through the film, will again 
 emerge at its upper surface at 3 so near 1 that the rays 1 
 and 3 will endeavour simultaneously to affect the same 
 ether. Now calling the ray reflected from the upper sur- 
 face the first ray, and the ray which has emerged after 
 passing twice through the film, the second ray, it will be 
 seen that the second ray has passed through a length of 
 path greater than that of the first ray, by a distance, ap- 
 proximately, equal to twice the thickness of the film, and 
 has, therefore, been retarded back of the first ray by this 
 distance. If now this retardation is equal to the half of a 
 
 wave length of light, or three- 
 halves of a wave length, or any 
 odd number of half wave 
 lengths, then the two rays will 
 neutralise each other, since they 
 
 ^ „ will attempt to move the ether 
 
 Fig. 108.— a Soap- . ,i -^ ,. ,. 
 
 Bubble Film. m exactly opposite directions tor 
 
 equal periods of time. Conse- 
 quently, no light will be seen at these points. At such 
 portions of the film, however, where the retardation is equal ' 
 to a whole wave length of yellow light, or two whole wave 
 lengths, the first and second ray will simultaneously at- 
 tempt to move the ether in the same direction, and a band 
 of bright yellow light will result. Now what is true of 
 yellow light is true of any of the coloured rays of the 
 spectrum, only the position where the rays can extinguish 
 or augment each other will vary with the different coloured 
 light. 
 
 It was Sir Isaac Newton who first studied the cause of 
 soap-bubble colours. He did this by means of a piano- 
 
SOAP-BUBBLE COLOURS 333 
 
 convex lens whose curved surface was rested against a flat 
 sheet of glass. The film of air between the two plates 
 gradually increased in thickness from the centre outwards. 
 AVhen a beam of sunlight was permitted to fall on such a 
 plate, a series of rings, of the colours of the solar spectrum, 
 were seen arranged around the point where the curved 
 surface touches the flat glass. Here, as you may be able to 
 understand, the successive circles of colour were due to the 
 fact that each of the coloured rays produced an increase 
 in intensity of light at places where the retardation of 
 one ray was equal to a whole wave length or any number 
 of whole wave lengths, and that these colours, falling in 
 regular order according to the increased thickness of the 
 film, produced the coloured prismatic rings. 
 
 Colours produced in this manner are said to be produced 
 by the interference of light. Such colours are also produced 
 in many other ways. For example, the colours seen when 
 films of oil or tar float on water are soap-bubble colours. So, 
 too, are the colours produced by films of oxide of iron 
 on the surfaces of articles of tempered steel. So, too, 
 curiously enough, are the brilliant colours seen in the 
 wings of certain insects; the iridescent colours of rainbow 
 coal and some other minerals, and on a great variety of 
 other substances, such as mother-of-pearl. 
 
CHAPTBE XXXI 
 
 RAINBOWS AND OPALESCENCE 
 
 Although the subject of rainbows was discussed in 
 " The Wonder Book of the Atmosphere," yet, since we have 
 now studied the general principles of light,, it may be well 
 again to discuss it here at somewhat greater length. 
 
 The rainbow may properly be regarded as among the 
 most beautiful and wonderful of all natural phenomena. 
 It is produced by a variety of causes and is in reality quite 
 complex. While I shall make no attempt to give a full 
 description of its causes, yet I can tell you now much about 
 it that I am sure you can understand, and so form some 
 idea of its complexity as well as its beauty. 
 
 Of course you all know that rainbows are only seen 
 when raindrops are falling, and are due to the rays of light 
 entering your eyes after having passed through the drops. 
 In order to see a rainbow, you must stand with your back 
 to the sun directly facing the falling drops. Under these 
 circumstances you may see an are or bow spanning the 
 heavens containing the colours of the solar spectrum. The 
 centre of this bow is situated where the shadow of your 
 eye would fall on the sky, provided it were possible for 
 such a shadow to be formed. The size, therefore, of the 
 rainbow increases as the sun sinks lower and lower in 
 the sky. 
 
 Generally speaking, when the rays of sunlight fall 
 
 334 
 
EAINBOWS AND OPALESCEXCE 335 
 
 ojjIiqTielY ^n raindr op s, they e nter th e drops a nd, whi le 
 pagain g th iongh^^theiiu are refracted. Then, on reaching 
 thp harV f)f +hp djopfi^-fchg y are reflected j nd_agaiii re- 
 fracted. In other words, the rays that enter the drops are 
 twice refracted and once reflected. Iliiring.jthis igfrac- 
 tion t hey also undergo di spersion or separation into their 
 prisiaatic_cfilffiurs. They emerge, therefore, from the drop, 
 separated into their different coloured rays just as they 
 do after having passed through a prism. 
 
 When these coloured rays emerge from the drops they 
 are divergent, and since the raindrops through which they 
 have passed are situated at fairly great distances from the 
 observer's eyes, they reach the eyes so decreased in inten- 
 sity, owing to the space over which they have been spread, 
 that they are too feeble to produce any marked effects of 
 colour. 
 
 It is not aU the rays, however, that are divergent and, 
 therefore, unable to affect the eyes. A certain proportion; 
 i.e., those produced by the rays of white light that strike 
 the drop at a certain angle, emerge as parallel coloured 
 rays, and these rays, like those that are passed through a 
 Fresnel light-house lens, have not decreased in intensity 
 and, therefore, are capable of impressLag their colours on 
 the eyes. 
 
 Owing to the fact that the coloured rays are divergent, 
 it will be impossible for one observer to see any more than 
 a single collection of rays from a given set of drops. For 
 example, red rays can only be seen that have been formed 
 by the light which passes through raindrops occupying a 
 certain angular position as regards the sun and the eye 
 of the observer. Of course any drops that occupy this 
 
336 THE WONDEE BOOK OP LIGHT 
 
 angular position will also have their red rays enter the eye 
 of the observer. It can be shown that these drops would lie 
 along portions of a circle at the centre of which the eye 
 of the observer is situated. The observer would, there- 
 fore, see an arc or bow of red light only, although un- 
 doubtedly other coloured rays have entered his eye, which, 
 
 however, have been so 
 weakened by divergence 
 that they failed to produce 
 any effect of colour. 
 
 But other raindrops 
 also . produce parallel 
 emergent rays, and since 
 different coloured rays of 
 light are differently dis- 
 persed, the drops produc- 
 ing the other coloured 
 Fig 109^ACTION OF Eain Dbop j^ ^ j^uated on 
 
 ON Beam of Sunlight. ■' 
 
 different circles all of 
 
 which, however, will have the eye of the observer in their 
 centre. 
 
 An arc or bow will, therefore, be seen projected on the 
 sky, with the eye of the observer as a centre, that will con- 
 tain all the colours of the prismatic spectrum, and vrill, 
 moreover be arranged in the order of their refran- 
 gibility. 
 
 In Pig 109 is shown the position of a beam of light S I, 
 coming from the sun and falling on a raindrop at I. This 
 beam passes through the drop and is reflected as shown at 
 A and emerging takes the path I' M. It is, therefore, 
 twice refracted and once reflected. Since while it is under- 
 
EAINBOWS AND OPALESCENCE 337 
 
 going refraction it is separated or dispersed into its pris- 
 matic colours, it emerges in the form of the colours of the 
 spectrum. Now I, is that part of the drop where several 
 parallel rays passing into the drop while undergoing dis- 
 persion as well as refraction emerge parallel to their origi- 
 nal direction, and, therefore, are able to affect the eye of 
 the observer. 
 
 Tyndall thus describes in simple language the cause of 
 the rainbow : 
 
 " The Kiijibow is in fact a. spe ctrum, in which the rai n- 
 drops.play_^e^artoi4)rif5ffls. — Tfee-^idth-oliheJjowjrom 
 ifidto^jviolet is about two de gTe^;__J!]ig^ize of the arc 
 visible -at_any time manifpafly fjeperfls npnn the-pAisition 
 oj the^un. The jbow is grandes t, when it is form ed by-4he 
 ^rising^-er^th e setting su n. An entire semicircle is then seen 
 by an observer on a plain, while from a mountain top a still 
 greater arc is visible." 
 
 In the rainbow the order of the colours is such that the 
 red is at the top of the bow and the violet below. This 
 bow is called the primary rainbow, in order to distinguish 
 it from a fainter bow called the secondary rainbow, that 
 is formed above the primary bow as shown in Pig. 110. 
 The secondary bow is produced by rays of light that enter- 
 ing at the bottom of the raindrop undergo two reflections 
 from the back of the drop and two refractions at the sur- 
 faces. Since the beam is reversed by this double reflec- 
 tion, the order of the colours of the secondary bow will 
 be reversed; i.e., the violet rays are at the top of the bow, 
 and the red are below. This double reflection of the light 
 also greatly weakens its intensity, so that the secondary 
 bow is fainter than the primary. On some rare occasions 
 
338 THE WONDEE BOOK OP LIGHT 
 
 a third bow is seen below the primary bow. Its colours, 
 however, are generally very faint. 
 
 Eainbows can of course be caused by particles of water 
 that are distributed through the air in any way, whether 
 as falling raindrops, or as the spray from waterfalls, by 
 
 Fig. 110. — Cause of the Primabt and 
 Secondaey Rainbows. 
 
 waves, or breakers during stormy weather, or by being 
 thrown into the air from a jet or nozzle. 
 
 As we have seen in some of the preceding chapters, the 
 white light of the sun may produce colour in the bodies on 
 which it falls in several ways. In opaque coloured bodies 
 some of the rays are absorbed so that those which are 
 thrown off from the surface are necessarily coloured. In 
 
EAINBOWS AXD OPALESCENCE 339 
 
 transparent coloured bodies some of the rays are absorbed 
 so that the remaining rays that are transmitted must nec- 
 essarily be coloured. On the passage of light through a 
 prism the separate coloured rays are so differently re- 
 fracted by dispersion that when they emerge from the 
 prism they are too far apart to simultaneously affect the 
 e3'e and therefore appear coloured. And then, again, we 
 have soap-bubble colours that are produced by the inter- 
 ference of the rays of light. 
 
 There yet remains to be explained another method of 
 producing colour that is called opalescence, which takes 
 its name from the peculiar colours produced in a variety 
 of mineral known as the opal, a variety of quartz. 
 
 The colours of opalescence are produced in a manner en- 
 tirely different from any of the before mentioned ways. 
 The opal owes its display of colours to the presence of a 
 great number of very small reflecting particles that are 
 distributed through the solid transparent material of the 
 mineral. When sunlight falls on an opalescent substance, 
 owing to the small size of the reflecting particles, the 
 blues and violets are reflected from them, so that the sub- 
 stance throws back the reflected light. The rest of the 
 light either passes through or is partially absorbed. 
 
 I am sure it will interest you to know that the blue 
 colour of the sky is due to opalescence. In this case, how- 
 ever, the minute reflecting particles are distributed, not 
 through a solid mass of a transparent solid, as in the opal, 
 but through a mass of transparent air. These sky parti- 
 cles are so small that they are capable of reflecting the 
 blue rays of light only, thus giving to the sky its character- 
 istic colour. The other coloured rays are but little ab- 
 
340 THE WONDEE BOOK OF LIGHT 
 
 sorbed and pass through the air as can be seen from the 
 fact that the transmitted light, coming from the sun, gives 
 to the sun a yellow colour, when the number of sky par- 
 ticles is small, and a deep red colour when the number is 
 great. The latter, as you know, occurs when the air is 
 filled with smoke particles as it is apt to be during the 
 time of year known in our latitude as Indian summer. It 
 also occurs when finely divided volcanic ashes are sus- 
 pended in the air, as was the case during the explosive 
 eruptions of Kr'akatoa and other volcanoes. 
 
 The blue colour of the sky is, therefore, a phenomenon 
 of opalescence, and is due to the presence of a great num- 
 ber of what might be called sky particles consisting of 
 numerous exceedingly minute particles of floating dust or 
 other matter that are so small as to be capable of reflect- 
 ing the very short wave lengths of light only; i.e., the 
 blues and the violets. The depth of the colour increases 
 with the decrease in the size of the particles as well as with 
 a decrease in their number. After a heavy rain, when the 
 face of nature has been as it were washed, the blue colour 
 deepens, and, indeed, in very high mountain regions, often 
 takes on a colour almost as deep as an indigo blue. On 
 the other hand, when the number and size of the particles 
 increase, especially when the particles consist of minute 
 drops of water, the air assumes a hazy or whitish appear- 
 ance, since then nearly all colours of light are reflected. 
 
 Any transparent medium will appear of a bluish colour 
 if small reflecting particles are distributed throughout it. 
 As you well know, very clear water, when deep, takes on 
 a marked bluish tint. When the light enters the eye after 
 it has been reflected from the great number of very ex- 
 
EAINBOWS AND OPALESCENCE 341 
 
 ceedingly small particles that are spread through the water, 
 it takes the characteristic blue colour because generally 
 blue light only is reflected from these small particles. Gla- 
 cial ice show a similar phenomenon. I have seen, in the 
 glaciers on the Alps, while standing at an edge of a deep 
 fissure, or crevasse, and looking down its vertical walls, the 
 wonderful blue colour that is assumed by the ice. 
 
 You have all probably heard of sky-blue milk. This is a 
 name given in pleasantry to milk that has been too greatly 
 diluted with water; for, if a large quantity of water is 
 mixed with milk, a bluish opalescence results, the particles 
 of milk being scattered in exceedingly minute drops 
 throughout the water. 
 
 There is a simple experiment you can try in opalescence 
 to illustrate the blue colour of the sky. Eau-de-Cologne, 
 that is, ordinary cologne water, consists of pure alcohol in 
 which has been dissolved various odorous resins, gums, and 
 essential oils. If a drop or two of cologne be permitted to 
 fall in a large glass jar filled with clear water, the resins, 
 gums and oils being deprived of their solvent alcohol, are 
 thrown down in the shape of a fine cloud of exceedingly 
 small particles. This cloud, when seen by reflected light, 
 assumes the blue colour of the sky. In trying this experi- 
 ment, it is best to permit a single drop of the cologne to 
 fall into the water, and, if this does not produce the de- 
 sired results, to follow it by a second drop. If too many 
 of the resinous or oily substances are placed in the water, 
 it will take on a milky appearance, owing to the particles 
 being too great in number and of too large size. 
 
 There is another instance of a blue colour produced by 
 opalescence, when the smoke of a fire is seen against a dark 
 
343 THE WONDEE BOOK OP LIGHT 
 
 background, such, for example, as the foliage of a tree. 
 Here, the smoke appears of a distinct blue color. You may 
 have noticed this while out on a picnic. In a similar man- 
 ner, the smoke of a cigarette or cigar, when in small quan- 
 tities, takes on a blue colour, but when thrown out by the 
 smoker in too large volumes, it appears of a whitish tint. 
 
CHAPTEE XXXII 
 POLAHISED LIGHT 
 
 Before closing The Wonder Book of Light, I wish to tell 
 you a little about some complex matters in optics, which, 
 though far too difiScult to discuss fully in a book of this 
 scope, yet may be made tolerably plain. 
 
 There is a variety of transparent mineral known as 
 calcspar, or carbonate of lime, that possesses the curious 
 property of causing a single ray of light, passing through 
 it in a certrain direction, to be broken up into two separate 
 rays that take different paths. Both of these rays undergo 
 refraction. In order to distinguish this peculiar kind of 
 refraction from ordinary refraction, it is called double 
 refraction. This variety of calcspar is quite transparent. 
 If objects are looked at through the mineral, they will, in 
 most positions, appear double, since each of the rays of 
 light into which the single ray is split up produces a sepa- 
 rate image. 
 
 Now the two rays, into which the single ray of light is 
 divided by double refraction, differ markedly in many 
 respects from ordinary light, and this is due to the fact 
 that they are polarised. In other words, these rays possess 
 different properties in different directions, just as a mag- 
 net possesses different properties at its opposite ends or 
 poles. 
 
 Ordinary light is produced by ether waves in which the 
 343 
 
344 THE WONDEE BOOK OF LIGHT 
 
 ether moves in directions at right angles to the direction 
 in which the ray is passing. Ifow, while in ordinary light, 
 all these vibrations take place at right angles to the path 
 of the ray, yet the successive to-and-fro motions occur 
 in paths that are inclined to one another. I know this 
 is hard to understand, but the successive paths, in which 
 the ether moves, are something like the steps of a circular 
 staircase. All the steps are horizontal; that is, all are at 
 right angles to the direction of the vertical support about 
 which they wind, but the successive steps are inclined to 
 one another. 
 
 In a ray of ordinary light, the vertical support of the 
 staircase may be taken as the direction in which the ray 
 is advancing, while the successive steps represent the succes- 
 sive ether paths. One path, say that near the bottom of the 
 flight, is in the direction of the bottom step. The next, 
 is in the direction of the next step, and so on throughout 
 the entire flight from the bottom to the top. 
 
 In polarised light, however, while the ether paths are 
 also all at right angles to the direction in which the ray is 
 moving, yet all the successive paths are parallel to one 
 another. 
 
 As already mentioned, the two rays into which a single 
 ray of light is broken by double refraction, consist of 
 polarised light, and, like all polarised light possess many 
 peculiarities that are quite difEerent from those of ordinary 
 light. 
 
 There are a number of ways in which ordinary light 
 can be polarised. One of these was discovered by Malus, 
 in 1808, while looking at one of the windows in the Palace 
 at Luxembourg, through a prism of a double refracting 
 
POLAEISED LIGHT 345 
 
 substance. He noticed, when the prism was rotated or 
 turned in front of his eye, that in certain positions one 
 of two images produced by the double refracting substance 
 completely disappeared, but that if he continued the rota- 
 tion, it reappeared, while the other image disappeared. 
 Without going any further into this difiBcult matter, I 
 would say that the alternate disappearance and reappear- 
 ance of the images^of the window was due to the polarisa- 
 tion of the light reflected from the wiadow, and, that inves- 
 tigations made by several scientific men, resulted in the 
 discovery that under certain circumstances the light re- 
 flected from any surface is polarised. 
 
 Light may also be polarised by its passage through 
 certain minerals, such as plates of tourmaline. Light is 
 also polarised by single refraction, provided the light falls 
 on a reflecting surface at such an angle of incidence that 
 the reflected ray is thrown off at right angles to the path 
 taken through the material by the refracted ray. In this 
 case the reflected ray is also polarised. 
 
 Any device for polarising light is called a polariser. 
 Any device for detecting the presence of polarised Hght 
 is called an analyser. 
 
 One of the simplest polarisers consists of slices, cut from 
 a six sided prism of a mineral called tourmaline at right 
 angles to its greatest length. Different specimens of tour- 
 maline vary in transparency from those that are completely 
 opaque, to those that are partially transparent. When 
 thin slices of such crystals are obtained, the light that 
 passes through them is completely polarised, so that such a 
 section forms a polariser. 
 
 Now any polariser may also act as an analyser; for, 
 
346 
 
 THE WONDER BOOK OF LIGHT 
 
 if two such tourmaline polarisers, such as h a, are placed as 
 shown in Pig. Ill the light is polarised by its passage 
 through ttj and this no matter how far a may be separated 
 from b. If, however, b is slowly rotated it will be found 
 that the light from a no longer passes freely through it; 
 for, when a slight inclination is reached, as shown at b' a', 
 some of the light of a is cut off, and when the two plates 
 occupy the positions shown at B A, the light polarised by 
 its passage through a is completely opaque to the tourma- 
 line analyser B. 
 
 It is always true that light, polarised by its passage 
 through any polariser, can only pass through another 
 
 Fig. 111. — TouEMALiNE Polabisees and Analysees. 
 
 polariser when that polariser occupies a certain position, 
 so that when an analyser is held between the eye and a 
 beam of polarised light, there will be formed two posi- 
 tions in which the light freely passes through it, and 
 two positions, at right angles to the former, where no 
 light whatever is able to pass. Although polarised light 
 cannot be distinguished from ordinary light by the un- 
 assisted eye, yet it can, therefore, be detected readily by 
 the use of an analyser. Examined in this way, the light 
 from certain parts of the blue sky can be shown to be par- 
 tially polarised. 
 A much better form of polariser is the Kicol's prism. 
 
POLAEISED LIGHT 
 
 347 
 
 <r 
 
 This consists of a prism. Fig. 112, formed of a piece of 
 doubly refracting calcspar, about an inch in height and a 
 third of an inch in breadth, that is sawn or cut in two 
 along the diagonal plane Idac, as shown. The two halves 
 are then cemented together ia their former position by 
 means of a transparent cement known as Canada balsam. 
 In this way the prism is in no 
 wise changed from its original 
 condition except that a thin slice 
 of Canada balsam has been in- 
 troduced into its mass at the 
 diagonal cut. Now a result of 
 this construction is that the 
 Xicol's prism possesses the prop- 
 erty of transmitting only one of 
 the two rays into which a ray 
 of ordinary light on passing 
 through it is divided; for, on °'' 
 meeting the first surface of the 
 Canada balsam, one of these 
 rays is totally reflected, and 
 passes out at the side of the^jg ^-^r, 
 crystal, while the other ray passes 
 
 through the Canada balsam and emerges at the end of the 
 prism completely polarised. 
 
 This form of polariser is more frequently employed than 
 any other since all the light passing through it is com- 
 pletely polarised; and, owing to the transparency of the 
 calcspar it permits more light to pass through it than does 
 a plate of tourmaline. 
 
 There are a variety of curious effects produced by the 
 
 •Nicol's Pbism. 
 
348 THE WONDER BOOK OP LIGHT 
 
 passage of polarised light through different transparent 
 substances. Among these are the exceedingly beautiful 
 colours that are produced by the interference of the rays 
 of polarised light on their passage through various trans- 
 parent double-refracting media. These interference col- 
 ours are produced in a manner not unlike that explained 
 in the production of soap-bubble colours. 
 
 Other sets of phenomena are caused by the passage of 
 polarised light through various transparent substances 
 such as certain prisms of quartz or rock crystal. Some 
 varieties of quartz possess the curious property of causing 
 a beam of polarised light, while passing through them, to 
 rotate or turn the plane in which the light has been 
 polarised. This property is known as rotary polarisation. 
 While a beam of polarised light is still polarised after its 
 passage through a crystal of quartz, yet when it emerges 
 from the crystal, the plane in which it has been polarised is 
 rotated or changed. 
 
 Some substances possess the power of rotating the plane 
 of polarisation to the right hand, or are said to possess 
 right-handed rotary polarisation. Others turn it to the 
 left, or have left-handed rotary polarisation. Strange to 
 say, some substances, like quartz, are able, in certain speci- 
 mens, to rotate the plane of polarisation to the right, 
 and, in other specimens, to rotate the plane of polarisation 
 to the left. 
 
 Polarised light is employed to a considerable extent in 
 sugar refineries for the purpose of making a rapid analysis 
 of the quantity of sugar present in a solution. There are 
 two kinds of sugar, one of which, like loaf-sugar, is crys- 
 tallisable, and the other, like molasses, is uncrystallisable. 
 
POLAEISED LIGHT 349 
 
 Now it happens that these two kinds of sugar rotate thq 
 plane of polarised light in different directions, so that by 
 the use of an apparatus known as the saceharimeter it is 
 possible, by simply causing a beam of polarised light to 
 pass through a clear solution containing both kinds of 
 sugar, to tell, by the appearance of certain colours, not only 
 the kind but also the amount of sugar present. 
 
 With this I must close The Wonder Book of Light. Many 
 of the subjects I have discussed are very difBcult, and 
 some of you may at times have found it hard to f oUow me. 
 I feel sure, however, that most of you cannot have failed 
 to obtain considerable valuable information from what 
 you have read, and even though, now and then, the mat- 
 ters have been too difficult, yet it will not hurt you to 
 put the book aside for a year or so, when you can again 
 read it to see if you cannot then understand what now 
 bothers you. 
 
 THE END