EXPERIMENTAL SCIENCE SERIES FOR BEGINNERS. 
 I. 
 
 L I G-H T: 
 
 A SERIES OF 
 
 SIMPLE, ENTERTAINING, AND INEXPENSIVE EX- 
 PERIMENTS IN THE PHENOMENA OF LIGHT, 
 FOR THE USE OF STUDENTS 
 OF EVERY AGE. 
 
 BY 
 
 ALFRED M. MAYER AND CHARLES BARNARD. 
 
 NEW YORK: 
 D. APPLETON AND COMPANY, 
 
 549 AND 551 BROADWAY. 
 1879. 
 
COPYRIGHT BY 
 
 D. APPLETON AND COMPANY, 
 
 187T. 
 
PEEFATOET NOTE. 
 
 IT is the design of tliis book to furnish a number 
 of simple and easy experiments in the phenomena of 
 light, that any one can perform with materials that 
 may be found in any dwelling-house, or that may 
 be bought for a small sum in any town or city. By 
 the aid of this book the reader becomes an experi- 
 menter. The student of Nature may read in books, 
 and soon forget. The experimenter w T ho questions 
 Nature himself, who constructs his own apparatus, 
 and who performs his own experiments, learns past 
 forgetting. He knows because he has observed. 
 
 It is believed that this book will occupy a place 
 hitherto unfilled in scientific literature. It is specially 
 prepared for the boy or girl student, and for the 
 teacher who has no apparatus and who wishes his 
 pupils to become experimenters, strict reasoners, and 
 exact observers. Nearly all the experiments described 
 are new, and all have been thoroughly tested. The 
 
4 PREFATORY NOTE. 
 
 materials employed are of the cheapest and most 
 common description, and all the experiments may be 
 performed at an expense of less than fifteen dollars. 
 The apparatus is, at the same time, suitable for regu- 
 lar daily use in both the home and school, and with 
 care should last for years. 
 
 The origin of this series of books, and the manner 
 of their production, may be briefly stated. For sev- 
 eral years Professor Mayer has been studying how to 
 give to every teacher and scholar the knowledge of the 
 art of experimenting. To accomplish this very desir- 
 able object, he had invented the simplest and cheapest 
 apparatus, and he and his scientific friends had been 
 satisfied with their performance. It remained to 
 describe these instruments and the ways of using 
 them. He found, however, that his leisure from pro- 
 fessional duties was not sufficient for this work, and, 
 not to delay further the publication of his labors, 
 Professor Mayer called in Mr. Charles Barnard to 
 assist him in preparing the books for the press. The 
 construction and arrangement of the instruments were 
 explained to Mr. Barnard, and the experiments were 
 made before him. Mr. Barnard then wrote out the 
 descriptions, which were revised by Professor Mayer. 
 The engravings have been made under Professor 
 Mayer's special direction, and care has been taken to 
 
PREFATORY NOTE. 5 
 
 render them accurate representations of the appara- 
 tus and experiments. 
 
 The nature of light is not touched upon in this 
 volume. The authors propose to explain, in another 
 book, the phenomena of interference and polarization 
 of light, and to explain fully the structure of the eye 
 and the nature of vision. 
 
 THE AUTHOKS. 
 
CONTENTS 
 
 PAGK 
 
 PREFATORY NOTE .... 3 
 
 CHAPTER I. 
 Introduction . 
 
 CHAPTER H. 
 
 The Sources of Light . . . . . .12 
 
 The Heliostat . . . . . . .16 
 
 First Experiment with Heliostat . . . . 24 
 
 Experiment with Cards and a Lamp . . . .26 
 
 Experiment with Shadows . . . .30 
 
 Experiment in measuring Light . . . . .35 
 
 CHAPTER III. 
 
 Reflection of Light ...... 38 
 
 Experiment in Multiple Reflection . . . . .44 
 
 Second Experiment in Multiple Reflection . . . 47 
 
 Experiment with Mirror on Pulse . . . . .48 
 
 Experiment with Glass Tube ..... 50 
 
 Experiments in Dispersed Reflection . . . .51 
 
 Experiment with Jar of Smoke .... 55 
 
 The Milk-and-Water Lamp 58 
 
CONTENTS. 
 
 CHAPTER IY. 
 
 Refraction of Light 59 
 
 The Water-Lens 71 
 
 Experiments in Projection ..... 76 
 
 The Fountain of Fire . . . . . .80 
 
 The Water-Lantern . . . , . 84 
 
 The Solar Microscope ...... 90 
 
 CHAPTER V. 
 
 Decomposition of Light ..... 92 
 
 Experiments with Solar Spectrum . . . .95 
 
 The Color-Top . . . . . . . 99 
 
 Direct Recomposition of the Colors of the Spectrum . . 103 
 Experiments in Reflected Colors . . . .103 
 
 Experiments in Contrasted Colors . . . .106 
 
 CONCLUSION ....... Ill 
 
 LIST OF APPARATUS AND THEIR COST . . . .113 
 
LIGHT. 
 
 CHAPTER I. 
 
 INTROD UGTION. 
 
 ALL about us are men busy with their various 
 trades and professions : sailing ships, digging in mines, 
 making all manner of useful tools and machinery, 
 planting seeds and reaping harvests, and doing many 
 other works and labors according to certain fixed rules 
 that they found printed in books, or that they learned 
 of others, or that they discovered for themselves. Each 
 one has to do with the physical phenomena around 
 him. The more he knows about these phenomena 
 the more he knows about things, their relation to 
 each other, and their action one upon another the 
 better he can work. ,A knowledge of the phenomena 
 of Nature is the most important knowledge one can 
 have who wishes to succeed in life. More than this, 
 
10 LIGHT. 
 
 the observation of facts in Nature gives readiness of 
 perception, and study and reading upon the causes of 
 these facts stimulate the mind to healthful and pleas- 
 urable action. 
 
 The laws that govern the physical phenomena 
 about us were not told to us by ancient gods, or 
 divinely-instructed men. They were discovered by 
 experiment or observation. Men asked questions of 
 Nature ; they watched her phenomena, till they felt 
 sure they saw a reason for their action. Sometimes 
 they did not understand all that happened, and 
 made strange guesses at the laws that governed the 
 happenings. Other men repeated the experiments 
 and got new answers ; and thus, in time, the truth 
 about things became known. Many of these facts in 
 Nature, and the laws that govern their action, are 
 now known of all men. Others are still obscure, or 
 dimly known, and are being investigated every day in 
 the hope that they may be better understood. 
 
 The farmer, the sailor, the mechanic, and artisan, 
 most familiar with these facts and laws of Nature, is, 
 other things being equal, the most likely to be suc- 
 cessful in his work. You hope to have a share in the 
 world's work, and you wish to study Nature and her 
 phenomena. You can read about these things in 
 books. A better way is to make experiments to ask 
 
INTRODUCTION. 11 
 
 Nature yourself to examine the phenomena of light, 
 heat, sound, electricity, etc., to study these phenom- 
 ena, and find their causes for yourself. 
 
 To try an experiment means to put certain things 
 in certain relations with other things, for the purpose 
 of finding out how- they affect each other. Experi- 
 menting is thus a finding out. 
 
 It is the design of this book to tell you something 
 about experiments in the phenomena of light to 
 show how these experiments illustrate the action of 
 light, and to explain briefly some of the elementary 
 laws that govern this action. All of these experi- 
 ments may be performed with the cheapest and most 
 common materials that can be found. They are all 
 easy and simple, and they are, at the same time, in- 
 teresting and entertaining. Some of the things here 
 described are capable of affording amusement for a 
 large number of people, and many of the exhibitions 
 and displays that may be made with them are won- 
 derfully attractive and beautiful. 
 
12 LIGHT. 
 
 CHAPTEE II. 
 
 THE SOURCES OF LIGHT. 
 
 WHEN the sun rises in the morning, the darkness 
 of the night seems to fade away, and, wherever we 
 look, without or within, all the air and space about us 
 appears to be full of light. "When evening comes 
 again, the daylight disappears, and the moon and the 
 stars give us another light. In the house we start 
 the lamps, and they give us another light. Out-of- 
 doors, in the dusky meadows, we see the fire-flies dart- 
 ing about, and giving out pale sparkles of yellow light 
 as they fly. We look to the north in the night and 
 see the aurora, or we watch the lightnings flash from 
 cloud to cloud, and again we see more light. 
 
 This light from sun and moon, the stars, the fire, 
 the clouds, and sky, is well worth studying. It will 
 give us a number of the most beautiful and interest- 
 ing experiments, and, by the aid of a lamp, or the 
 light of the sun, we can learn much that is both 
 strange and curious, and perhaps exhibit to our friends 
 a number of charming pictures, groups of colors, 
 magical reflections, spectres, and shadows. All light 
 comes from bodies on the earth or in the air. or from 
 
THE SOURCES OF LIGHT. 13 
 
 bodies outside of the atmosphere ; and these bodies 
 we call the sources of light. Light from sources out- 
 side of the atmosphere we call celestial light, and the 
 sources of this light are stars, comets, and nebulae. 
 The nebulae appear like flakes and clouds of light in 
 the sky, and the comets appear only at rare intervals, 
 as wandering stars that shine for a little while in the 
 sky and then disappear. The stars are scattered 
 widely apart through the vast spaces of the universe, 
 and they give out their light both day and night. 
 The brightest of these stars is the sun. "When it 
 shines upon us, the other stars appear to be lost 
 in the brighter light of this greater star, and we 
 cannot see them. At night, when the sun is hid, 
 these other stars appear. "We look up into the sky 
 and see thousands of them, fixed points of light, each 
 a sun, but so far away that they seem mere spots 
 and points of light. Besides these stars are others, 
 called the planets, that move round the sun. These 
 give no light of their own, and we can only see 
 them by the reflected light of the great star in the 
 centre of our solar system. Among these stars are 
 the Moon, Yenus, Mars, Jupiter, and many others. 
 "We might call celestial light starlight ; but the light 
 from the great star, the sun, is so much brighter 
 than the light of the others, that we call the light it 
 
14 LIGHT. 
 
 gives us sunlight, and the light from the other suns 
 we call starlight. For convenience, we also call the 
 reflected light from the planets starlight, and the 
 light from our nearest planet we call moonlight. 
 
 Terrestrial light includes all the light given out 
 by things on the earth, or in the air that surrounds 
 the earth. The most common light we call firelight, 
 or the light that comes from combustion. When we 
 light a lamp or candle, we start a curious chemical 
 action that gives out light and heat. The result of 
 this action is fire, and the light that comes from the 
 flame is firelight. When a thunder-storm rises, we 
 see the lightning leap from the clouds, and give out 
 flashes of intensely bright light. Sometimes, at night, 
 the northern sky is full of red or yellow light, dart- 
 ing up in dancing streamers, or resting in pale clouds 
 in the dark sky. You have seen the tiny sparkles of 
 light that spring from the cat's back when you stroke 
 her fur in the dark, or have seen the sparks that leap 
 from an electrical-machine. All these the aurora, 
 the lightning, and the electric sparks are the same, 
 and we call such light electric light. 
 
 Sometimes, in the night, we see shooting-stars 
 flash across the sky. These are not stars, but masses 
 of matter that, flying through space about the earth, 
 strike our atmosphere and suddenly blaze with light. 
 
THE SOUKCES OF LIGHT. 15 
 
 The friction with the air as they dart through it is so 
 great that these masses glow with white heat, and 
 give out brilliant light. Two smooth white-flint peb- 
 bles, or two lumps of white sugar, if rubbed quickly 
 together, will give out light, and this light we call 
 the light from mechanical action. 
 
 Sailors upon the ocean sometimes see, at night, 
 pale-yellow gleams of light in the water. A fire-fly 
 or glow-worm imprisoned under a glass will show, in 
 the dark, bright spots of light on his body. A piece 
 of salted fish or chip of decayed wood will sometimes 
 give a pale, cold light in the night; and certain chemi- 
 cals, like Bologna phosphorus and compounds of sul- 
 phur, lime, strontium, and barium, if placed in the 
 sunlight in glass vessels and then taken into the dark, 
 will give out dull-colored lights. All these the 
 drops of fire in the sea, the glow-worm, the bit of de- 
 cayed wood, and these chemicals are sources of the 
 light called phosphorescence. 
 
 These are the sources of light the stars, the fire, 
 electricity, friction, and phosphorescent substances. 
 We can study the light from all of them, but the 
 light from the sun or a lamp will be the most con- 
 venient. The light of the sun is the brightest and the 
 cheapest light we can find, and is the best for our ex- 
 periments. A good lamp is the next best thing, and 
 
16 LIGHT. 
 
 in experimenting we will use either the sun or a 
 lamp, as happens to be most easy and convenient. 
 
 THE HELIOSTAT. 
 
 In looking out-of-doors in the daytime we find 
 that the sunlight fills all the air, and extends as 
 far as we can see. It shines in at the window and 
 fills the room. Even on a cloudy day, and in rooms 
 where the sunshine cannot enter, the light fills every- 
 thing, and is all about us on every side. Now, in 
 studying light we do not wish a great quantity. "We 
 want only a slender beam, and we must bring it into 
 a dark room, where we can see it and walk about it 
 and examine it on every side, bend it, split it up into 
 several beams, make it pass through glass or water, 
 and do anything else that will illustrate the laws that 
 govern it. 
 
 Choose a bright, sunny day, and go into a room 
 having windows through which the sun shines. Close 
 the shutters, curtains, and blinds, at all the win- 
 dows save one. At this window draw the curtain 
 down till it nearly closes the window, and then cover 
 this open space with a strip of thick wrapping-paper. 
 Cut a hole in this paper about the size of a five-cent 
 piece, and at once you will have a slender beam of 
 sunlight entering the hole in the paper and falling on 
 
THE SOURCES OF LIGHT. 17 
 
 the floor. Close the upper part of the window with 
 a thick shawl or blanket, and, when the room is per- 
 fectly dark, our slender beam of light will stand out 
 clear, sharp, and bright. 
 
 As soon as we begin to study this beam of light, 
 we find two little matters that may give us trouble. 
 The sun does not stand still in the sky, and our beam 
 of light keeps moving. Besides this, the beam is not 
 level, and it is not in a convenient place. "We want 
 a horizontal beam of light, and some means of keep- 
 ing it in one place all day. An instrument that will 
 enable us to do this, and that can be adjusted to the 
 position of the sun in the sky at all seasons of the 
 year and every hour of the day, may be readily made, 
 and will cost only a small sum of money. 
 
 On the next page are several drawings, giving 
 different views of such an instrument and some of 
 its separate parts. It is called a heliostat, and we 
 shall find it of the utmost value in our experiment- 
 ing in light, heat, sound, electricity, and other branches 
 of physical science. 
 
 The first drawing represents a front-view of the 
 heliostat. The second drawing gives an end-view, 
 and we can now make one by simply following these 
 few directions : The part marked A in the two draw- 
 ings is a piece of pine board, 23 inches (58.4 centi- 
 
2 3 4. 5 6 
 
 SCALE Ys 
 
 FIG. 1. 
 
THE SOURCES OF LIGHT. 19 
 
 metres) wide and two or more feet long, or as long as 
 the window where it is to be used is wide. Any boy 
 who can use plane and saw can make this piece of 
 work out of common inch-board, and, if you have no 
 pieces so wide as that, it can be made of two or more 
 pieces fastened together with cleats ; but, in this case, 
 all the cracks must be close and tight. In the middle 
 of this board, cut a round hole 5 inches (12.7 centi- 
 metres) in diameter, with its centre 8 inches from 
 the bottom of the board. In the first drawing this 
 hole can be seen at B, and in the second drawing 
 it is shown by dotted lines at B. On one side of 
 the board screw two iron brackets, using brackets 
 measuring 14 inches (35.5 centimetres) by 12 inches 
 (30.5 centimetres). These brackets are placed one 
 on each side of the hole in the board, and are 
 placed 14 inches (35.5 centimetres) apart, and with 
 the short arm of the bracket against the board. In 
 the first drawing the two brackets are shown, and in 
 the second drawing one is shown in profile, and they 
 are marked C in both drawings. On the end of the 
 brackets is placed a flat piece of board, 6J inches 
 (16.5 centimetres) wide and 14 inches (35.5 centi- 
 metres) long, or long enough to reach from one 
 bracket to the other. This board may be screwed 
 up to the brackets, and thus make a shelf. Oare 
 
20 LIGHT. 
 
 must be taken in fastening this shelf to the brackets 
 to place it so that the outside edge of the shelf will 
 be 16 inches (40.6 centimetres) from the large board. 
 On the outside edge of this shelf another board, 7 
 inches (1T.8 centimetres) wide, is placed upright, and 
 secured with screws and small strips of wood at the 
 ends, as in the drawing. This shelf, with the wooden 
 back, is marked D in the drawings. 
 
 These things make the fixed parts of the heliostat, 
 and we have next to make the movable parts, or the 
 machinery whereby it can be adjusted to the move- 
 ment of the sun in the heavens. First, get out a flat 
 piece of board 10^ inches (26.7 centimetres) long, 6J 
 inches (16 centimetres) wide, and J inch (12 milli- 
 metres) thick. Then make a flat, half-round piece, 
 shaped like the figure marked G. This piece must 
 be J inch (T millimetres) thick, 5 inches (14 centi- 
 metres) along the straight side, and with the circular 
 part with a radius of 3 inches (7.6 centimetres). A 
 hole, inch (12 millimetres) in diameter, is made in 
 this, as represented in the drawing, and then the half- 
 round piece must be screwed to the flat piece of wood 
 we just cut out. In the figure marked JVyou will see 
 these two pieces fastened together. Fig. / is the 
 most difficult piece of all. It should be made of ash 
 or some hard wood. One end is square, and has a 
 
THE SOURCES OF LIGHT. 21 
 
 deep slot cut in it ; the rest is round, and may be 1 
 inch (32 millimetres) in diameter. The square part 
 must be large enough to slip over the half-circular 
 piece, G, as is shown at If. A hole, J inch (12 milli- 
 metres) in diameter, is cut in the two ends, as marked 
 by dotted lines at J, and through these holes an iron 
 bolt and nut are fitted, so as to hold the circular piece, 
 G, and yet allow it to turn freely in every direction. 
 A hole, 1J inch (32 millimetres) in diameter, is cut 
 through the triangular piece of wood IT, as shown 
 by the dotted lines, and then this block is securely 
 fastened to the back of the large board, as shown in 
 the second drawing. An opening of the same di- 
 ameter, and having the same direction, is also cut 
 through the board, and the movable piece, marked 
 /, is put through this hole, as in the drawing. Final- 
 ly, we want a wooden washer, 3 inches (8.7 centi- 
 metres) wide, as represented at M. This we slip over 
 the long wooden handle, as shown in the second draw- 
 ing, and this washer rests on the block j^ the top 
 of which is 3J inches square. This makes all the 
 movable parts of the heliostat, and, when we have 
 put in the mirrors, the instrument is finished and 
 ready for use. We must have two mirrors, one 6 
 inches (15.2 centimetres) square and one 10 inches 
 (25.4 centimetres) long and 6 inches (15.2 centimetres) 
 
22 LIGHT. 
 
 wide. These may be made of common looking-glass ; 
 but plate-glass with silvered back is far better, and 
 costs only a little more. 
 
 Any carpenter can make this instrument, and the 
 cost will be about as follows : Wood, 50 cents ; labor, 
 $1.75 ; glass, $1 ; iron nut, 5 cents ; brackets, 50 cents 
 total, $3. 80. When finished, the instrument should 
 have a coat of shellac-varnish, and, when this is done, 
 the mirrors may be put in place, and fastened on with 
 very heavy bands of rubber. This will enable us to 
 take the glasses off when the instrument is not in use, 
 and, if the elastic bands or rings are very strong, they 
 will answer perfectly. The long mirror is to go on 
 the movable piece at N", and the small mirror stands 
 on the shelf, facing the opening in the board, at 0. 
 This mirror stands at the angle shown in the next 
 drawing (Fig. 2), and the other mirror is adjusted to 
 the sun at its various positions in the sky at different 
 seasons of the year. 
 
 Here is a diagram showing the position of the 
 handle of the heliostat, and the mirror for different 
 seasons and in different parts of the country. The 
 handle must be placed on a line parallel with the 
 axis of the earth, and the four dotted lines give its 
 position when the heliostat is to be used in Boston, 
 New York, Washington, and New Orleans. This 
 
THE SOURCES OF LIGHT. 
 
 also causes the block of wood marked K to have a 
 slightly different shape, so that the hole through it will 
 be in the middle. The dotted line marked "At Equi- 
 nox " shows the path of the light from the sun, and 
 
 FIG 2. 
 
 the three dotted lines show the paths of the reflected 
 light as it passes from one mirror to the other. The 
 position of the movable mirror is also shown in the 
 positions it has at summer and winter solstices. 
 
24= LIGHT. 
 
 FIRST EXPERIMENT WITH THE HELIOSTAT. 
 
 Choose a bright sunny day, and take the heliostat 
 into a room having a window facing the south. Raise 
 the sash and place the instrument in the window, and 
 fasten it there so that it will be firm and steady. 
 Before closing the window down upon it, move the 
 larger mirror on its axis till it reflects a beam of light 
 into the small mirror. Then turn the handle to the 
 right or left, and a round, horizontal beam of light 
 will enter the room. When this is done, close all the 
 windows, so as to make the room as dark as possible. 
 To do this, shawls or blankets or enameled cloth 
 will be found useful inside the curtains and shutters. 
 Then get a piece of cardboard, about 6 inches (15.2 
 centimetres) square, and lay a five-cent piece in the 
 centre, and, with a knife, cut a hole in the card just 
 the size of the coin. Then fasten this, with pins or 
 tacks, over the opening in the heliostat. 
 
 We have now a slender beam of light in a dark 
 room. Walk about and study it from different sides. 
 See how straight this slender bar of light is ; it bends to 
 neither the left nor right, but extends across the room 
 in an absolutely straight line. As the sun moves, turn 
 the handle of the heliostat to keep the light in place. 
 
 Here is a picture of a dark room, in the window 
 
THE SOURCES OF LIGHT. 
 
 25 
 
 of which is the heliostat. In the centre of the piece 
 of cardboard is the small hole where the light enters 
 the room. A boy is holding one end of a long piece 
 of linen thread just at the bottom of the hole in the 
 
 FIG. 3. 
 
 card, and another boy has drawn the thread out 
 straight and tight, so that it just touches the beam of 
 light throughout its length. 
 
 Were you to try this experiment, you would see 
 
26 LIGHT. 
 
 that the thread would suddenly be lighted up through- 
 out its whole length, and would shine in the dark 
 room like silver. Then if the boy allows the thread 
 tol>ecome slack and loose, or if he lowers it even a 
 very little, it will disappear in the darkness. If he 
 raises and lowers it quickly, it will seem to appear 
 and disappear as if by magic. 
 
 This is a very pretty experiment \ but we must 
 not stop to look at its merely curious effects. Try it 
 over several times, and see if it does not show you 
 something about the beam of sunlight. Plainly, if 
 the thread is lighted up its whole length when it is 
 straight, then the beam of light must be straight also. 
 Here we discover something about light; we learn 
 that it has a certain property. Our experiment 
 shows that light moves in straight lines. 
 
 EXPERIMENT WITH CARDS AND A LAMP. 
 
 Here is a picture representing three little wooden 
 blocks placed in a row upon a flat, smooth table, and 
 fastened to them are three postal-cards, so that they 
 will stand upright. At the end of the table is a 
 small lamp. This is all we need to perform an- 
 other experiment, that will show us the the same 
 thing we observed with the beam of light from the 
 heliostat. To make these things, get a piece of 
 
THE SOURCES OF LIGHT. 27 
 
 wood 10 inches (25.4 centimetres) long, 3 inches 
 (76 millimetres) wide, and 1J inch (37 millimetres) 
 thick, and saw it into five pieces, each 2J inches 
 (64 millimetres) long. Next make three slips of 
 pine, 4 inches (10 centimetres) long, 3 inches (76 
 
 FIG. 4. 
 
 millimetres) wide, and inch (4 millimetres) thick. 
 Having made these, get three postal-cards, and lay 
 them flat on a board, one over the other. Just here 
 we need a tool for making small holes and doing 
 other work in these experiments ; and we push, with 
 a pair of pliers, a cambric needle into the end of a 
 wooden penholder, or other slender stick, putting the 
 eye-end into the wood, and thus making a needle- 
 pointed awl. Measure off one-half inch from one end 
 
28 LIGHT. 
 
 of the top postal-card, and with the awl punch a 
 hole through them all, just half-way from each side. 
 Lift the cards up, and with a sharp penknife pare 
 off the rough edges of the holes, and then run the 
 needle through each, so as to make the holes clean 
 and even. 
 
 Take one of these cards and one of the wooden 
 slips, and put the card squarely on one of the wooden 
 blocks and place the slip over it, and tack them both 
 down to the block. This will give us the cards and 
 blocks as shown in the picture. When each card is 
 thus fastened to a block, we shall have two blocks 
 left. These we can lay aside, as we shall need them 
 in another experiment. 
 
 Now light the lamp, and place one block on the 
 table, quite near the lamp. Look at the lamp care- 
 fully, and see that the flame is just on a level with the 
 hole in the card. If it is too high or too low, place 
 some books under it, or put the lamp on a pile of 
 books on a chair near the table. Take a chair and sit 
 at the opposite end of the table, and place another 
 card before you. Now look, through the hole in this 
 card, at the first card before the lamp. If the table 
 is level, you will see a tiny star or point of light 
 shining through the holes in the two cards. With- 
 out moving the eye, draw the third card into line 
 
THE SOURCES OF LIGHT. 29 
 
 between the others, and in a moment you will see 
 the yellow star shining through all three cards. 
 
 Next take a piece of thread and stretch it against 
 the sides of the three cards, just as they stand, and 
 immediately you see that they are exactly in line. 
 The holes in the cards we know are at the same dis- 
 tance from the edges of the cards, and our experiment 
 proves that the beam of light that passed through all 
 the holes must be straight, or we could not have seen it. 
 The cards are in a straight line, and the beam of light 
 must also be straight. This experiment, like the 
 first, shows us that there is a law or rule governing 
 the movement of light, and that law is, that light 
 moves in straight lines. 
 
 Move the lamp as near to the edge of the table as 
 possible, and then bring one of the cards close to the 
 lamp-chimney. Then change your seat, and repeat 
 this experiment several times in different directions. 
 Each time you will see exactly the same thing, no 
 matter in what direction the light moves from the 
 lamp. The lamp may be moved from one side of 
 the table to the other, and in every direction we shall 
 find the light moving in exactly straight lines from 
 the source of light. This is true whether the source 
 be the sun, a lamp, or a star. One can walk all about 
 the lamp and see it from every side, and we can place 
 
30 LIGHT. 
 
 our three cards in any direction, north or south, up or 
 down, east or west, or in any and every direction, and 
 every time it will give the same result. 
 
 Thus we have found out the law by which light 
 moves, viz., it moves in straight lines in all directions 
 from the source of light. 
 
 Knowing this, you can readily think of a number 
 of things in which these laws are made useful. A 
 farmer planting an orchard, an astronomer fixing the 
 positions of stars, a sailor steering his ship by night, 
 employs this law: the first, to arrange his trees in 
 straight lines ; the second, to measure out vast angles 
 in the sky ; and the third, to lay the courses of his 
 ship in safety. Each employs these laws with cer- 
 tainty and safety, because they are fixed and never 
 change. 
 
 EXPERIMENT WITH SHADOWS. 
 
 This picture represents a sheet of white note-paper, 
 standing upright, like a small screen, upon a table. 
 Near it is a bit of square paper, fastened to the end 
 of our needle-pointed awl, and beside this is a lamp, 
 and next to the lamp is a postal-card, having a slit 
 cut in it near the top. On the screen you will no- 
 tice that there is a shadow of the bit of paper held 
 on the needle. The paper screen may be made of 
 
THE SOURCES OF LIGHT. 31 
 
 any sheet of white paper, and it may be held up- 
 right by placing some books behind it. The bit of 
 paper on the needle is just 1 inch (25 millimetres) 
 square; and to hold the awl in place, the handle 
 
 FIG. 5. 
 
 may be stuck in a mass of wax on the table. The 
 slit in the postal-card should be 1 inch (25 mil- 
 limetres) long and J inch (7 millimetres) wide, and 
 should be horizontal. The card may rest against the 
 lamp, and, if it is not high enough, put something 
 under it, so that the slit will be opposite the flame. 
 These things are easily procured, and, when you have 
 them, light the lamp, place the postal-card before it, 
 and then make the room quite dark, or, if it is night, 
 put out all the other lights. Set up the needle-awl 
 with the bit of paper on the end about 12 inches (30.5 
 centimetres) from the lamp, and make it firm and 
 
32 LIGHT. 
 
 steady with a bit of wax softened in the fingers. 
 Then bring the screen in a line with the paper square 
 and the lamp, and about 24 inches (61 centimetres) 
 from the lamp. If everything is right, there will be 
 a square shadow of the bit of paper on the screen. 
 Look carefully at everything, and have the paper just 
 on a level with the slit in the postal-card, and have 
 the lamp, paper, and screen, just in line, and then the 
 square shadow will appear sharp and clear on the 
 white screen. With a lead-pencil trace an outline of 
 this shadow on the screen ; then move the screen 
 just 12 inches (30.5 centimetres) farther from the 
 lamp. Look at the shadow. See how it has increased 
 in size. With the pencil trace this shadow on the 
 screen, and then, laying the screen on the table, meas- 
 ure the two shadows, and see how they compare in 
 size, and see how they both compare with the size 
 of the paper square that cast the shadows on the 
 screen. 
 
 Fig. 6 shows how light spreads out, and how 
 shadows expand as the distance increases. A is the 
 lamp, and B is the postal-card, having a slit for the 
 light to pass. C is the paper screen, and D is the 
 first shadow made on the screen when it was 24 
 inches from the lamp. E\& the second shadow made 
 on the screen when it was 36 inches from the lamp. 
 
THE SOURCES OF LIGHT. 
 
 33 
 
 If you lay the paper C on the tracing of the small 
 shadow D, you will observe that it only covers one- 
 fourth of the surface, and that the shadow is four 
 times as large. Place it on the larger shadow E, and 
 you will see that it covers only one-ninth of its sur- 
 face. In the diagram the first shadow is marked off 
 into quarters, and the second into ninths, by dotted 
 lines. The diagram also shows how the rays of light 
 spread out wider and wider the farther they travel 
 from the source of light. 
 
 Now, make two squares of paper, one the size 
 of D and the other the size of E. Then place D 
 
 24 inches (61 centimetres) from the lamp, and E 
 36 inches (91.4 centimetres), and both in a line. If 
 7, D, and E> have the positions shown in the dia- 
 gram, it will be found that D and E are both in 
 shadow, while the square C is illuminated. Eemove 
 the square (7, and D will be lighted, which shows 
 
34 LIGHT. 
 
 that all the light that falls on D previously fell on 
 C. E yet remains in darkness. Next, remove D 
 and replace (7, and E still remains in shade ; but, on 
 removing (7, E\& fully illuminated. This shows that 
 the quantity of light that fell on C spreads over four 
 times the surface at the distance Z>, and nine times 
 the surface at the distance E. Hence each one of the 
 squares on D is one-fourth as bright as the square 
 <7, and any one of the squares on E is one-ninth as 
 bright as C. 
 
 Here we are coming upon another fact about 
 light ; we find another law governing its action. 
 At one foot from the lamp the light had a certain 
 power ; at two feet it had only one-quarter as much 
 power ; at three feet it only had one-ninth as much 
 power or intensity. So, if we approach the lamp, 
 at a certain distance the light has a certain bright- 
 ness ; at half that distance it has four times the 
 brightness ; at one-third the distance it has nine times 
 as much brightness. The above relation, existing 
 between the intensity of the light on a surface at 
 different distances from the source of light, is often 
 stated as follows : The illumination of a given sur- 
 face varies in brightness inversely as the square of 
 its distance from the source of light. 
 
THE SOURCES OF LIGHT. 
 
 35 
 
 EXPERIMENT IN MEASURING LIGHT. 
 
 This picture represents a sheet of white paper, 
 standing upright upon a table. A few inches from 
 this screen is our needle-pointed awl, stuck upright 
 in the table. (If you do not care to do this, the awl 
 can be stuck into a block of wood or bit of wax.) A 
 lighted candle is placed on the table, about 22 inches 
 (55.8 centimetres) from the screen. Beyond this is a 
 lamp, placed upon a pile of books, so as to bring the 
 
 FIG. 7. 
 
 flame of the lamp on a level with the flame of the 
 candle. The lamp should stand, say, at 44 inches (112 
 centimetres) from the screen ; and, if it has a flat 
 wick, it must be so placed that the wick stands di- 
 agonally to the screen. 
 
 These are all the things we need to make a most 
 
36 LIGHT. 
 
 interesting experiment in measuring light, and we 
 have only to make the room dark, or put out all the 
 other lights, if it is evening, and we can go on with 
 the work. Let the candle burn a moment or two, 
 and then bend the wick down, so as to give a large 
 flame. If you have no lamp, a gaslight will answer. 
 Upon the paper screen are two shadows of the awl 
 side by side. Move the lamp to the right or left till 
 the two shadows just touch, and make one broad 
 band. Study this double shadow carefully. Per- 
 haps one half is darker than the other. Move the 
 lamp backward or forward, and you will see that its 
 shadow changes becomes darker or lighter. Pres- 
 ently you will find a place for the lamp where the 
 double shadow appears of a uniform depth. 
 
 Now both lamp and candle cast just as deep a 
 shadow, and yet one is much farther from the screen 
 than the other. Measure off the distance. Perhaps 
 the candle is 22 inches (55.8 centimetres) from the 
 the screen, and the lamp is 44 inches (112 centi- 
 metres). 
 
 In our last experiment we found that the illu- 
 mination of a given surface varies inversely as the 
 square of its distance from the source of light. The 
 square of 22 is 484, and the square of 44 is 1,938. 
 Now, if we divide 1,938 by 484, we get 4, and thus 
 
THE SOURCES OF LIGHT. 37 
 
 we find that our lamp is four times as bright as the 
 candle. It casts just the same depth of shadow on 
 the screen as the candle, and it is four times as 
 bright, because the square of the distance of the can- 
 dle will divide the square of the distance of the lamp 
 four times. If we measure it another way we find 
 the candle is half the distance from the lamp to the 
 screen, and gives only one-quarter as much light. 
 
 Such a measurement as this is both easy and 
 simple, and by means of such an experiment we can 
 find out how much light any lamp gives. In this 
 case, we find one lamp gives just four times as much 
 light as the candle, or as much light as four candles 
 would give at once. This is called a photometric 
 experiment, from two Greek words meaning light- 
 measurement. You may sometimes hear people say 
 that a certain gas-lamp gives a sixteen or eighteen 
 candle light, and our experiment shows us what they 
 mean by this expression. They mean that the lamp 
 has a photometric value of so many candles, or gives 
 a light equal to the light of sixteen or eighteen can- 
 dles burning at the same time. 
 
38 LIGHT. 
 
 CHAPTEE HI. 
 
 REFLECTION OF LIGHT. 
 
 PLACE the heliostat in position, and bring a slen- 
 der beam of light into the darkened room. Then 
 get a small looking-glass, or hand-mirror, and a car- 
 penter's steel square, or a sheet of stiff paper, having 
 perfectly square corners. Hold the mirror in the 
 beam of light. At once you see there are two beams 
 of sunlight, one from the heliostat and another from 
 the mirror. Hold the glass toward the heliostat, and 
 you will see this second beam going back toward the 
 window. 
 
 This is certainly a curious matter. Our beam of 
 light enters the room, strikes the mirror, and then we 
 appear to have another. It is the same beam, thrown 
 back from the glass. This turning back of a beam of 
 light we call the reflection of light. 
 
 Place a table opposite the heliostat, and place the 
 mirror upon it, against some books. Turn the mirror 
 to the right, and the second or reflected beam of light 
 moves round to the right. Turn the glass still more, 
 and the beam of light will turn off at a right angle, 
 and there will be a spot of light on the wall at that 
 
REFLECTION OF LIGHT. 
 
 39 
 
 side of the room. Now bring the carpenter's square 
 or the piece of square paper close to the mirror, so 
 that the point or corner will touch the glass just 
 where the sunlight falls upon it. Now one edge of 
 
 FIG. 8. 
 
 the square is brightly lighted by the sunbeam, and if 
 the mirror is placed at an angle of 45 degrees with 
 the sunbeam, the other edge of the square is lighted 
 up by the second beam. 
 
 In this diagram, A is the beam of light from the 
 
40 LIGHT. 
 
 heliostat, and B is the beam reflected from the mirror, 
 that is marked M. To make this more simple, we 
 call the first beam the beam of incidence, and we say 
 that it travels in the direction of incidence, as shown 
 by the arrow. The second beam, marked B, we call 
 the beam of reflection, and the course it takes we call 
 the direction of reflection. The point marked 0, 
 where the light strikes the mirror, is called the point 
 of incidence. 
 
 In the diagram is a dotted line representing a 
 quarter of a circle reaching from the beam of inci- 
 dence to the beam of reflection. A quarter of a 
 circle, as you know, is divided into 90 degrees. 
 Another dotted line extends from at the mirror to 
 X on the quarter-circle, and divides it into two parts. 
 Half of 90 is 45, and hence the mirror stands at an 
 angle of 45 degrees with both beams of light. Now 
 the line A and the dotted line reaching from to X 
 make the angle of incidence, and the angle between 
 B and the line from to JTis the angle of reflection ; 
 and the curious part of this matter is, that these two 
 angles are always equal. Here they are both angles 
 of 45 degrees. 
 
 Move the mirror about in any direction, and meas- 
 ure the angles of incidence and the angles of reflec- 
 tion, and these angles will always be exactly equal. 
 
REFLECTION OF LIGHT. 
 
 41 
 
 If you look at the diagram you will see tliat the 
 mirror is at an angle of 45 degrees with the beam of 
 incidence, and that the beam of reflection is at an 
 angle of 90 degrees with the incident beam. Hence, 
 if the mirror is tilted through a certain angle, the 
 reflected beam is tilted through twice this angle. 
 For instance, if the mirror is moved 1 degree the 
 
 FIG. 9. 
 
 beam of reflection moves 2. degrees. Place the mir- 
 ror at an angle of 22 with the beam of incidence, 
 and the beam of reflection is at an angle of 45. 
 Move the mirror to an angle of 67, and the beam 
 of reflection will move round to an -angle of 135 
 degrees. 
 
 This drawing represents the two postal-cards fitted 
 
42 LIGHT. 
 
 on blocks of wood that we used in a former experi- 
 ment, and the three blocks of wood we cut out at 
 that time. The five blocks are placed close together 
 in a line, and with the postal-cards at the ends. A 
 lighted lamp is placed near one of the cards, and on 
 the middle block is a small piece of window-glass 
 that has been painted with black varnish. A single 
 coat of black varnish on one side of the glass is all 
 that is required to give us the black mirror needed in 
 this experiment. Place the lamp close to the card in 
 such a position that the flame will be just on a level 
 with the hole in the card. If the lamp is not con- 
 venient, the blocks and cards may be placed upon a 
 table facing a north window in full daylight. 
 
 When everything is ready, look through the hole 
 in the postal-card marked B, down upon the black 
 mirror, and on it you will see a single spot of light, 
 the reflection from the lamplight or the light from 
 the window shining through the hole marked A in 
 the drawing. Get the needle-pointed awl and place 
 it so that the point will just touch the spot of light 
 in the black mirror, and then fasten the awl in this 
 position with a piece of wax, as represented in the 
 picture. 
 
 You will readily see that this experiment is the 
 same as the last. Again we have a beam of light 
 
REFLECTION OF LIGB^T. 43 
 
 reflected from a mirror. The beam of incidence 
 passes through the postal-card at A and finds its point 
 of incidence on the mirror, and the beam of reflection 
 extends from the point of incidence to the second 
 card at J?. 
 
 Take a sheet of stiff paper 10 inches (25.4: centi- 
 metres) long, and about 4 inches (10 centimetres) 
 wide, and hold it upright between the two cards, with 
 the bottom resting on the mirror. .With a pencil 
 make a mark on the edge of this at the point of in- 
 cidence, marked by the awl, and at the hole in the 
 card where the beam of incidence enters, and marked 
 A in the drawing. Draw a line between these two 
 points and you have an angle formed by this line and 
 the base of the paper. This angle marks the angle 
 of incidence. Put the paper on the blocks, with the 
 ruled line toward the card -Z?, and you will find that 
 the line fits here equally well. It now extends from 
 the point of incidence to B, and proves that this 
 angle is the same as the other, that both sides are 
 alike, and that the angle of incidence and the angle 
 of reflection are equal. 
 
 Take out the block in the middle, and move the 
 others nearer together till they touch. Repeat the 
 experiment: make a measurement with a piece of 
 paper as before, and draw a line on it from the point 
 
44 , LIGHT. 
 
 of incidence to either of the holes on the cards, and 
 then compare the angles thus found, and in each case 
 they will be exactly alike. Take out another block 
 and try it again, and you will reach the same result. 
 
 These experiments show us that there is a fixed 
 law in this matter, and the more we study it the more 
 we are convinced that it has no exceptions. 
 
 EXPERIMENT IN MULTIPLE REFLECTION. 
 
 Choose a south room on a sunny day, and close 
 the blinds and shutters at all the windows save one, 
 and at this window draw down the curtain until only 
 a narrow space is left at the bottom. Close this space 
 with a strip of thick wrapping-paper, and then cover 
 the rest of the window with a blanket or shawl, so 
 as to make the room perfectly dark. Then cut a 
 round hole, the size of a five-cent piece, in this paper, 
 and through this hole a slender beam of sunlight will 
 fall into the darkened room. 
 
 Bring a hand-mirror into this beam of light, and 
 the beam of reflection will make a round spot of sun- 
 light on the wall above the window. This spot of 
 light is a picture of the sun thrown by the mirror 
 upon the wall. Hold the mirror at an oblique angle 
 in the sunbeam, and direct the beam of reflection 
 upon the opposite wall. Now there are several re- 
 
REFLECTION OF LIGHT. 45 
 
 flections, brilliant spots of light. If the spots of light 
 do not stand out sharp and clear, turn the mirror 
 slowly round and you will soon find a position for 
 the glass that will give six or more reflections. 
 
 How does it happen that a common looking-glass 
 can thus split a single sunbeam into several beams ? 
 If you touch a pencil to a mirror you will notice that 
 while the point of the pencil touches the glass the 
 point of the reflected pencil sepn in the mirror does 
 not meet the point of the real pencil, and that there 
 is a little space between them. The reflection we see 
 in the glass is from the smooth surface of the quick- 
 silver at the back of the glass, and the space between 
 the reflection and the pencil is filled by the glass. 
 
 Hold a sheet of common window-glass before a 
 lighted lamp or candle, and you will see a faint re- 
 flection of the flame in the glass, and at the same 
 time you can readily see through the glass. This 
 shows us that the outside of any piece of smooth glass 
 will reflect light, and our experiment is designed to 
 show a still more curious matter. 
 
 Fig. 10 represents the single beam reaching the 
 point of incidence on the outside of the mirror at 0, 
 and reflected to the wall at 1. Part of the light goes 
 through the glass to B, and here is another point of 
 incidence, and a new beam of reflection is .thrown 
 
46 
 
 LIGHT. 
 
 through the glass to the wall at 2. If you look at the 
 reflections on the wall, you will see that the second 
 spot of light is the brightest. This comes from the 
 quicksilver, for, as this is a better reflector than the 
 glass, it sends out a brighter beam of reflection. When 
 
 FIG. 10. 
 
 this second beam of reflection passes through the glass, 
 a part of its light is reflected from the under side of 
 the surface, and is turned back against the quicksilver 
 again. Once more it is reflected, and a new beam of 
 reflection makes number 3. The drawing shows the 
 path these beams of light take in the glass, and the 
 quivering spots of light on the wall show how one 
 beam of light may be reflected again and again in dif- 
 ferent directions. If the reflector was perfect and 
 
REFLECTION OF LIGHT. 47 
 
 returned all the light, these multiple reflections might 
 be repeated many times over ; but every time light is 
 reflected from any bright surface, a part of the light 
 is lost, and thus each reflection grows fainter and 
 fainter till the light is spent. Look at the multiplied 
 reflections on the wall, and you will see that the first 
 reflection from the glass is bright, and that the sec- 
 ond, from the quicksilver at the back of the glass, is 
 brighter still ; and that the others grow fainter and 
 fainter till all the light is spent, and the reflections 
 disappear. 
 
 SECOND EXPERIMENT IN MULTIPLE REFLECTION. 
 
 Light a lamp and place it on a table, and get the 
 two postal-cards and the blocks that we used in the 
 experiment in reflection. With a sharp knife cut a 
 slit in one card, just at the pin-hole, about f inch (J9 
 millimetres) long and -fa i nc ^ (1 millimetre) wide. 
 Then place this card close to the lamp, as in the other 
 experiment, and set up the other card about fifteen 
 inches away from it. Then lay a looking-glass on the 
 table between the two. Look at the picture (on page 
 41) and arrange the cards as there represented, and 
 put the mirror in place of the blackened glass on the 
 blocks. On looking through the small hole in the 
 postal-card (marked E in the drawing), you will see 
 
48 LIGHT. 
 
 in the mirror several bars of yellow light, placed one 
 over the other. Again we have an instance of multi- 
 plied reflection. Instead of seeing the reflections 
 thrown upon the wall, we can look down upon them 
 and see them, just as they stand, each at its point of 
 incidence on the glass and the quicksilver. Study 
 these brilliant bars of light, examine the diagram 
 carefully, and you will readily see that this experi- 
 ment simply exhibits in a different manner the same 
 thing that we saw in the last experiment. 
 
 EXPERIMENT WITH MIRROR ON PULSE. 
 
 Get a small bit of looking-glass, about an inch (25 
 millimetres) square, and some wax. "Warm the wax 
 in the hand till it is soft, and then make three small 
 pellets about the size of a pea. Put one of these on 
 the back of the little mirror, near the edge and half- 
 way between two corners. Place one at each of the 
 opposite corners, so that the mirror will have three 
 legs or supports placed in a triangle. Put the helio- 
 stat in place, and bring a small beam of sunlight into 
 the dark room. If this is not convenient, any beam 
 of sunlight in a dark room (as in former experi- 
 ments) will answer. 
 
 Turn back your coat-sleeve, and, while standing 
 near the beam of light, place the little mirror on the 
 
REFLECTION OF LIGHT. 49 
 
 wrist, with one of the wax legs resting on the pulse. 
 Then bring the arm into the beam, so that the light 
 will fall on the mirror. Hold the arm steady, and 
 watch the spot of reflected light thrown upon the 
 wall. See ! It moves backward and forward with a 
 curious, jerking motion. It is like the ticking of a 
 clock, or like the beating of one's pulse. It is the mo- 
 tion of your pulse. The mirror moves with the pulse, 
 and the beam of reflection thrown on the wall moves 
 with it, and, though this movement is very slight, 
 the reflection on the wall moves over a space of sev- 
 eral inches, and we can see it plainly. In our first ex- 
 periment in reflection we learned that when a mirror 
 was moved to the right or left, the beam of light re- 
 flected from it moved also to the right or left, and each 
 time through twice as great an angle as the mirror. 
 
 This experiment is a wonderfully interesting one, 
 and may be tried with a number of boys or girls, and 
 each may see the peculiar beating of his or her pulse 
 pictured on the wall in the most singular and startling 
 manner. If any of the persons whose pulse-beats are 
 thus exhibited get excited, laugh at the exhibition, 
 or are in any way disturbed, the change in the move- 
 ment of their pulse will be quickly repeated on the 
 wall, where a hundred people can see it. 
 3 
 
50 LIGHT. 
 
 EXPERIMENT WITH GLASS TUBE. 
 
 Procure a glass tube, about inch (19 millime- 
 tres) in diameter and 12 inches (30.5 centimetres) 
 long, and paint the outside with black varnish. If 
 this is not convenient, cover the tube with thick 
 black cloth, and fasten it down with mucilage, taking 
 care to have the cloth square at the ends. Punch 
 
 ""T\ 
 
 A 
 
 FIG. 11. 
 
 a hole in a postal-card with the sharp point of a pair 
 of scissors, and with a knife make the ragged edges 
 of the hole smooth. Hold the card at one end of 
 the tube so that the hole will come just at the centre 
 of the opening, and then, while facing a window or a 
 bright lamp, look through the tube with one eye, and 
 you will see a spot of light surrounded by a number 
 of beautiful rings. 
 
REFLECTION OF LIGHT. 51 
 
 Here we have another example of multiplied re- 
 flection. The light entering the tube, through the 
 hole in the card, falls on the smooth surface of the 
 interior of the tube, and appears to the eye in the 
 form of rings. 
 
 Fig. 11 represents a section of the tube, and shows 
 the paths the different rays of light take, and shows 
 how each is reflected from side to side till they all 
 meet in the eye. The dotted lines and the rings 
 projected beyond the tube show how they appear to 
 the eye. By studying this drawing carefully, and 
 trying cross cuts and slits in the card in place of the 
 single hole, you will get a very correct idea of re- 
 peated reflection, and find the tube a source of con- 
 siderable amusement. 
 
 EXPERIMENTS IN DISPERSED REFLECTION. 
 
 Get a small piece of black velvet or cloth and 
 take it to a dark room, where the heliostat will give 
 us a slender beam of sunlight. If this is not con- 
 venient, use a common beam of sunlight in a dark 
 room, as in some of our former experiments. Hold 
 the velvet in the hand between the fingers, and so as 
 to leave the palm of the hand clear. Turn back the 
 coat-sleeve so as to expose part of the white cuff, and 
 then bring the velvet into the beam of sunlight. You 
 
52 LIGHT. 
 
 will observe nothing in particular, for the black, rough 
 cloth does not reflect the light at all. Now move the 
 hand, so that the spot of light will fall on the palm. 
 See what a pretty, rosy glow of light falls on the wall ! 
 This is the reflected light from the hand. The skin 
 is rough, and the light is diffused and scattered about, 
 and, instead of a bright spot of reflected light, as with 
 a mirror, we have this glow spread all about on the 
 wall and furniture. Now move your hand, so that 
 the sunlight falls on your cuff. Immediately there is 
 a bright light shining on the wall, and lighting the 
 room with a pale, bluish-white glare. Move the hand 
 quickly, so that the black cloth, the hand, and the 
 white cuff, will pass in succession through the beam 
 of light. Observe how the different things reflect 
 the light in different degrees. The cuff is the smooth- 
 est and whitest, and gives the brightest reflection ; the 
 hand gives less light, because it is less smooth ; and 
 the cloth, that has a very dark and rough surface, 
 gives no reflection at all, and the spot of sunlight 
 falling upon it seems dull and faint. 
 
 This experiment shows us something more in the 
 reflection of light. A piece of glass, the surface of 
 water, polished metals, ice, and all substances having 
 very smooth surfaces, reflect light in one direction. 
 The linen cuff also reflected light, but apparently in a 
 
REFLECTION OF LIGHT. 53 
 
 very different manner from the mirrors we have been 
 using. 
 
 Place a lighted lamp upon a table and lay a mirror 
 before it, and you can see a clear and distinct reflec- 
 tion of the lamp and the flame pictured on the glass. 
 Put a sheet of white paper before the lamp, and you 
 can see only a confused spot of reflected light on the 
 brightly-lighted paper. Lay a freshly-ironed napkin 
 or handkerchief before the lamp, and even the indis- 
 tinct spot of light has disappeared, and the white 
 cloth reflects light equally from every part. 
 
 These drawings are intended to show how light is 
 reflected from different surfaces. The first represents 
 a smooth surface, like glass, that sends all the beams 
 in one direction, because the points of reflection for 
 the beam are in the same plane. (See 1, 2, 3, Fig. 12.) 
 
 The second drawing represents a slightly-rough- 
 ened surface, like paper. Some of the points of re- 
 flection turn the light one way, some another, and the 
 beam of reflection is no longer formed of parallel 
 rays. They are scattered about, and the image they 
 form is confused and indistinct. In the third draw- 
 ing we have a rough surface, like cloth, and here 
 the rays of the beam of reflection are scattered in 
 every direction, and we can see no image. 
 
 It is in this manner that we are enabled to see the 
 
LIGHT. 
 
 FIG. 12. 
 
 people and things about us. The light of the sun or 
 a lamp falls upon them, and is reflected into our eyes, 
 and we say we see the objects. Very few things re- 
 
REFLECTION OF LIGHT. 55 
 
 fleet light so brightly that we obtain from them a 
 reflected image of the source of the light, and we 
 generally see only dispersed and scattered light, that 
 does not blind or dazzle the eye, and enables us to 
 look upon these objects with ease, and to readily see 
 all their parts. 
 
 The clouds, the water, the grass, rocks, the ground, 
 buildings, the walls inside, clothing and furniture, 
 and everything we can see, reflect light in every 
 direction again and again, and thus it is that all 
 spaces, without and within, are filled with light so 
 long as the sun shines. At night the sun sinks out of 
 sight, and still it is light for some time after, for the 
 sunlight is reflected from the sunset-clouds and the 
 sky. 
 
 Sometimes, upon a summer's day, when broken 
 clouds partly hide the sun, you will see long bars of 
 dusky light streaming from openings in the clouds. 
 These long bars are beams of sunlight shining upon 
 dust and fine mist floating in the air, and we see 
 them because each speck and particle reflects light in 
 every direction. 
 
 EXPERIMENT WITH JAR OF SMOKE. 
 
 Fig. 13 represents a large, clean glass jar, such 
 as one sees at the confectioner's. It is standing 
 
56 
 
 LIGHT. 
 
 upon a black cloth laid upon a table in a dark room, 
 and on top of the mouth is laid a postal-card, hav- 
 ing a slit, 1 inch (25 millimetres) long and -$ inch 
 (1 millimetre) wide, cut in it. Above the jar is a 
 
 Fio. 13. 
 
 hand-mirror, so placed that the beam of sunlight from 
 the heliostat (or from a hole in the curtain) will be 
 reflected downward upon the postal-card on top of 
 the jar. 
 
 This simple apparatus is designed to show how 
 
REFLECTION OF LIGHT. 57 
 
 light is reflected from small particles floating in the 
 air. Set fire to a small bit of paper and drop it into 
 the jar. Place your hand over the mouth of the jar, 
 and in a moment it will be filled with smoke. When 
 the paper has burned out, put the postal-card in place, 
 so that the slit will be in the centre of the mouth of 
 the jar. Let the beam of reflected light from the 
 mirror fall on this slit. 
 
 Look in the jar and you will see a slender ribbon 
 of light extending downward through the jar. Else- 
 where it is quite dark and black. Here we see the 
 light streaming through the opening in the card, and 
 lighting up the particles of smoke in its path. 
 
 Take off the card, and let the reflected beam fall 
 freely into the jar. The smoke is now wholly illu- 
 minated, and the jar appears to be full of light, and 
 every part of the bottle shines with a pale-white 
 glow. 
 
 Put the postal-card on again and let the light -fall 
 through the slit. The smoke has nearly all disap- 
 peared, and the ribbon of light in the jar is quite dim. 
 Curious streaks and patches of inky blackness run 
 through it. What is this ? Nothing simply noth- 
 ing. The smoke is melting away, and the beam of 
 light disappears because there is nothing to reflect it 
 and make it visible. 
 
58 LIGHT. 
 
 This part of the experiment appears quite magi- 
 cal in its effects, and is exceedingly interesting. 
 
 THE MILK-AND-WATER LAMP. 
 
 Take away the jar and put a clear glass tumbler 
 in its place. Fill this with water and throw the 
 beam of reflected light down upon it, and the water 
 will be lighted up so that we can easily see the 
 tumbler in the dark. "Now add a teaspoonful of milk 
 to the water and stir them together. Throw the 
 beam of light down once more. This is indeed re- 
 markable. The tumbler of milk-and-water shines like 
 a lamp, and lights up the room so that we can easily 
 see to read by its strange white light. Move the 
 mirror and turn aside the beam of light, and instantly 
 the room becomes dark. Turn the light back again, 
 and once more the glass is full of light. 
 
 Here the minute particles of milk floating in the 
 water catch and reflect the light in every direction, so 
 that the entire goblet seems filled with it, and the 
 room is lighted up by the strange reflections that 
 shine through the glass. 
 
REFRACTION OF LIGHT. 59 
 
 CHAPTEE IY. 
 
 REFRACTION OF LIGHT. 
 
 CERTAIN things, like glass, water, mica, and ice, 
 allow light to pass directly through their substance. 
 We hold them before the eyes, and see the light very 
 nearly as well as through the air. Such substances, 
 we say, are transparent. Other objects, like porce- 
 lain or oiled paper, do not permit all the light to 
 pass, and such things, we say, are semitransparent or 
 translucent. Many other things do not permit light 
 to pass through them, and cast shadows behind them 
 when brought into a beam of light. These things 
 cut off all the light, and we call them opaque. 
 
 Here is a common glass bottle with straight sides 
 and about three inches (76 millimetres) broad, or as 
 wide as a postal-card (Fig. 14). On one side is pasted 
 a piece of white paper having a perfectly round hole 
 cut in it. On the glass, in the clear space made by 
 the circular opening in the paper, are two lines drawn 
 at right angles, in ink. These two lines divide the 
 circle into four equal parts, and are to serve as guides 
 in some new experiments. 
 
 Fill the bottle with clear water up to the hori- 
 
60 
 
 LIGHT. 
 
 Fio. 14. 
 
 zontal line in the circle, and then, holding the bot- 
 tle in a small horizontal beam of sunlight from the 
 heliostat, you will see that the light passes directly 
 through the water in the bottle or through the air 
 above the water. To make this more distinct, cut a 
 
REFRACTION OF LIGHT. 61 
 
 slit, 1^ inch (38 millimetres) long and -$ inch (1 
 millimetre) wide, in a postal-card, and place this 
 against the side of the bottle, so that the light will 
 pass through the slit. This gives a sharp, clear beam 
 of light, and by studying it carefully, we see that the 
 beam in the air and its continuation in the water 
 
 FIG. 15. 
 
 preserve the same direction. If we place the bottle 
 on the floor or table, and with the mirror send a per- 
 
62 LIGHT. 
 
 pendicular beam down into the water, we shall see 
 exactly the same thing. 
 
 Fig. 15 represents the bottle of water standing 
 upon a table, under a window, where the beam of 
 sunlight enters from the heliostat. The opening 
 where the light comes in, the mirror, and the re- 
 flected beam of light thrown down upon the bot- 
 tle, are plainly shown in the picture. The postal- 
 card is held in such a position that the beam falls 
 upon the slit and then enters the bottle. Look into 
 the bottle through the opening in the paper, and see 
 where the beam falls, and then move the mirror and 
 the card till the beam enters the bottle above the 
 water and strikes the water just where the two lines 
 meet in the centre of the circle. Draw the postal- 
 card forward so that some of the light will cross the 
 outside of the bottle, and appears to make a white 
 mark across the paper circle. Study the two beams 
 outside and inside the bottle, and see if you can dis- 
 cern anything peculiar about them. The part of the 
 beam inside the bottle and above the water follows 
 the same direction as the beam outside till it touches 
 the water-line, and then it turns down and takes a 
 new direction. This bending, that takes place when 
 a beam of light passes from air into water, is called 
 refraction. It takes place very generally when light 
 
REFRACTION OF LIGHT. 
 
 63 
 
 passes from one transparent medium to another, and 
 gives rise to a number of curious matters in regard 
 to light. 
 
 Here is a drawing of the beam of light cross- 
 ing the opening in the paper, and showing 'how it 
 is bent. It passes through the air above the water, 
 
 in the upper half of the circle, and then takes a new 
 direction through the water in the lower half. You 
 will observe in the drawing dotted horizontal lines 
 extending from B to J., and from C to Z>. Look at 
 the beam of light carefully, and with a pen mark 
 these places A and C on the edge of the paper circle. 
 Take the bottle to the light and measure off the dis- 
 tances from A to the perpendicular line, or along the 
 line A B in the drawing, and from C to the perpen- 
 dicular line, or the line C D in the drawing. Make 
 a record of these measurements, and then take the 
 
64 LIGHT. 
 
 bottle to the dark room. Place it nearer to the mir- 
 ror, and let the reflected beam of light fall upon it 
 at a different angle, being careful that the beam 
 strikes the water at the centre of the circle. Examine 
 the beam of light in the bottle, and you will observe 
 that it is bent, but at a different angle. Mark the 
 two points where the beam crosses the circle above 
 and below the water, and measure their distances 
 from the perpendicular line, and then compare these 
 distances with those we obtained the first time. Di- 
 vide the distance between A and B by the distance 
 between C and D, and you will obtain a certain quo- 
 tient. Divide the two sets of figures obtained the 
 second time (that is, the distance from the edge of 
 the circle to the perpendicular line above by the 
 same below the water), and the quotient or ratio of 
 the one to the other will be exactly the same as be- 
 fore. For instance, if the distance from A to IB is 
 four units, the distance from C to D will be three 
 units, and in every experiment this proportion will 
 be the same. In this case, where the light passes 
 from the air to the water, we get a quotient that is 
 one and a third, and this quotient we call the index 
 of refraction. These experiments show us that there 
 is a fixed law of refraction. When the light met 
 the surface of the water at a right angle, it passed 
 
EEFRACTION OF LIGHT. 65 
 
 through the water without bending. In such instances 
 the light is said to meet the water in a normal direc- 
 tion. If it meets the water on either side of this 
 normal, it is refracted. Glass, diamonds, mica, and 
 every transparent substance, have their own peculiar 
 refraction. Glass has an index of refraction of 1.5. 
 A diamond has quite another index of refraction, 
 
 FIG. 17. 
 
 and it is by comparing these that we are able- to 
 prove whether a stone is a real diamond or only 
 an imitation made of glass. 
 
 Above is a picture representing the bottle in a new 
 position. The beam of sunlight enters the darkened 
 window, and falls upon a mirror lying flat on the 
 
66 
 
 LIGHT. 
 
 table. It is then reflected upward toward the bottle 
 that stands upon a pile of books. The postal-card 
 is put up as before, and the beam of light passes 
 through the slit and enters the bottle below the sur- 
 face of the water. Look at the beam of light in the 
 bottle through the circular opening. Instead of pass- 
 ing through the water into the air above the surface, 
 
 it is bent and turns downward into the water again. 
 If, at first, you do not see this curious effect, raise the 
 mirror slightly, tip it up toward the bottle, and take 
 out some of the books under the bottle till the beam 
 of light enters the bottle, in the direction C 0, as in 
 the above drawing. Here the line A B represents 
 
REFRACTION OF LIGHT. 67 
 
 the surface of the water in the bottle, and the line 
 Y X is the perpendicular line in the circle. In 
 this experiment the light must enter the bottle at G 
 and pass to at the surface of the water, and then 
 you will see a most curious phenomenon, the reflec- 
 tion of the light from the surface of the water at O 
 downward to D. To understand this singular matter 
 we must study the diagram. 
 
 In the diagram a beam of light is represented as 
 entering the circle at 6r, and is then refracted to H. 
 Another beam goes from / to J. Dotted lines are 
 drawn from each of these beams to the perpendic- 
 ular, both above and below the water. You can 
 easily compare the relations of these dotted lines 
 above and below, and you will see that they still pre- 
 serve the same relation to each other that we dis- 
 covered in former experiments. First, we must ob- 
 serve that light may pass from air to water, as from 
 G to and H^ or from water to air, as from H to 
 and G, and the amount of bending will be the 
 same in both cases. In other words, the light takes 
 the same path in going from air to water as when 
 moving from water into the air. A beam of light 
 passing to 0, just above the surface of the water, 
 will be refracted as already described. To study 
 this matter further, we must reverse the direction of 
 
68 LIGHT. 
 
 the light and cause it to pass from the water to the 
 air. The beam of light entering at (7, below the 
 water, passes to O. Now, if we measure a line from 
 G to the perpendicular X, we shall find it is too 
 long to be three units, if we call the length of the 
 longest line (0 B) that can be drawn from the cir- 
 cumference of the circle to its centre four units. The 
 beam of light entering the water passes to the sur- 
 face at 0, and finds itself a prisoner, and it turns 
 back and dives down into the water again. None of 
 our experiments have shown a more singular result 
 than this. The lines which we have so often drawn 
 perpendicular to the diameter, Y X, of the circle, 
 are called sines, and the law of refraction is always 
 thus stated: The sines of the angles of incidence 
 have a constant relation to the sines of the angles 
 of refraction. In the case of light passing from air 
 into water, the ratio of the sines is as 4 to 3 ; in the 
 case of light passing from air into glass, the ratio of 
 the sines is as 3 to 2. 
 
 The beam of light entering the water at G is said 
 to have reached the critical angle A O C, and hence 
 is totally reflected. By this is meant that it has gone 
 beyond the critical point, where the law of the sines 
 comes to an end, and reflection takes the place of r& 
 fraction. 
 
REFRACTION OF LIGHT. 69 
 
 Sometimes, when walking along a road on a warm 
 day, you may observe a curious quivering in the air 
 just where the road seems to meet the sky, as it goes 
 over a hill. The objects near this point appear to be 
 distorted and to tremble, or they assume fantastic 
 shapes. Here we have an instance of refraction 
 caused by the heated air just above the surface of 
 the road. The light passing through these layers of 
 unequally-heated air is refracted unequally, and the 
 objects that reflect the light appear distorted. In 
 some instances the refraction may pass the critical 
 angle, and we may see the objects apparently doubled 
 by reflection. Warm, calm days by the sea show the 
 same thing, when .distant vessels appear repeated in 
 the sky, or when distant land that is really below the 
 horizon " looms " up and glimmers upon the horizon 
 in trembling headlands. This illusion is called the 
 mirage, and takes place when refraction exceeds the 
 critical angle and becomes reflection. 
 
 Fill a clear glass tumbler with water, and put a 
 spoon in it, or dip one finger in the water, and hold it 
 above your head so that you can look into the water 
 from below. You will find that you cannot see through 
 the water up into the air above. The under surface 
 of the water will appear to shine like burnished sil- 
 ver, and the spoon or your finger will be reflected in 
 
70 LIGHT. 
 
 it, as in a beautiful mirror. This illustrates total 
 reflection, and shows that in this case all light thrown 
 upward through the water is reflected from its sur- 
 face. Look into the tumbler from above, and it ap- 
 pears full of clear water. Look into it from below, 
 and it seems as if an opaque sheet of silver rested on 
 the water, and shut out the view of everything above. 
 Take a small glass tube, and roll up a piece of 
 colored paper or printed paper and slip it inside the 
 tube, and then place the tube in the goblet jof water. 
 Hold the goblet in the hand near the eyes, and you 
 can see the paper in the tube through the water. 
 Lower the goblet till you can look down into the 
 water from above, and the tube- will appear as if 
 made of silver, and the paper will totally disappear. 
 To vary the experiment, lift the tube up and down 
 in the water, and the paper will appear and disappear 
 in the most surprising manner. This also illustrates 
 total reflection. The light reflected from the paper 
 passes through the glass tube into the water, and is 
 refracted. In certain positions the light passes the 
 critical angle, and is reflected from the outer surface 
 of the glass tube, and fails to reach the eye. Look 
 into the goblet from below, and there is the colored 
 paper pictured by total reflection on the under side of 
 the water. 
 
REFRACTION OF LIGHT. 
 
 71 
 
 THE WATER-LENS. 
 
 Fig. 19 shows an oblong box of pine, 14 inches 
 (35.7 centimetres) high, 6| inches (16.5 centimetres) 
 square at the outside at each end, and made of thin 
 
 FIG. 19. 
 
 boards, nailed or screwed together. One side is en- 
 tirely open, and at the top is a round hole, 5 inches 
 (12.T centimetres) in diameter. On this opening 
 rests a hemispherical glass dish, made by cutting 
 
72 LIGHT. 
 
 off the round top of a glass shade. This makes a 
 thin glass bowl, 5 inches (14 centimetres) in diam- 
 eter, and it rests in the hole, partly above and partly 
 below the top of the box. 
 
 Inside the box two strips of wood are fastened, 
 one on each side, at an angle of forty-five degrees. 
 On these strips rests a sheet of silvered glass, 5f 
 inches (13.7 centimetres) wide and 8^ inches (21.4 
 centimetres) long, or just large enough to slip into the 
 box, as shown by the dotted lines in the picture. To 
 keep the glass from sliding out, a tack or brad may be 
 driven in the box at the end of the mirror. 
 
 Put the heliostat in the window, and bring a full 
 beam of sunlight into the darkened room. Then 
 place this box on the window-seat, or on a table next 
 to the window, with the open side toward the window, 
 and in such a position that the beam from the helio- 
 stat will fall on the mirror. By this arrangement the 
 light will be reflected upward through the glass bowl. 
 Then fill the bowl with clear water, choosing the 
 purest and cleanest that can be found. Adjust the 
 box carefully, and see that the beam from the helio- 
 stat strikes the mirror fully, and that the reflected 
 beam meets the bowl on every side, so that there are 
 no shadows inside the box. 
 
 Here we have a broad beam of light passing from 
 
REFRACTION OF LIGHT. 73 
 
 the air into water, and our experiments have shown 
 us that in such an event the light may be refracted. 
 
 Hold a sheet of paper in a horizontal position 
 just above the bowl, and you will see that it is fully 
 lighted up by the light thrown up by the mirror 
 through the water. Raise the paper slowly, and the 
 circle of light on the paper will grow smaller and 
 brighter, till it is reduced to a small dot of intense 
 white light. 
 
 Put a match just at this bright spot of light, on 
 the under side of the paper, and instantly it begins to 
 burn. Touch the lighted match to the paper, and 
 hold the burning paper beside the bowl of water, and 
 gently blow the smoke over the water. See what a 
 strange cone of light appears in the smoke ! It is 
 pale below, next the water, and grows brighter and 
 brighter till the top of the cone is reached, and here 
 it is intensely bright. Above this cone appears an- 
 other, upside down, with its point touching the point 
 of the cone beneath it. Above, on the ceiling, is a 
 large circle of light, perhaps several feet in diameter. 
 
 Fig. 20 represents a number of rays of light en- 
 tering at the left, and reflected upward from the 
 mirror. From our experiments we learned that light 
 passing from the air into water, and reaching the sur- 
 face in a normal direction, goes straight on through 
 
74 
 
 LIGHT. 
 
 the water in the same path. If it enters the water 
 on either side of this normal, it is refracted or 
 
 turned aside, and takes a new path. The greater the 
 angle at which it enters the water, the greater the re- 
 
REFRACTION OF LIGHT. 75 
 
 fraction. In the diagram the line in the middle repre- 
 sents the ray of light in the centre that meets the 
 water at a normal, and passes straight through it 
 and on into the air above. On either side the rays 
 are represented as refracted, or bent out of their 
 track, and obliged to take new paths. The greater 
 the distance of the ray from the normal, the greater 
 its refraction. Now, as all the rays at the same dis- 
 tance from the normal are refracted to the same de- 
 gree, it follows that there must be a place where all 
 these rays of refraction will meet. 
 
 Look at the cone of light over the bowl of water, 
 and you can see the spot where all the rays of light 
 are concentrated. Here they meet in what is called a 
 focus. You can readily remember this word, because 
 it means a hearth, or turning-place^ and we saw our 
 match take fire just at that point. The rays of sun- 
 light contain heat, as well as light, and, if we gather 
 them altogether in a bundle, of course we shall con- 
 centrate both the heat and light. A bit of paper held 
 in the focus glows with intensely white light, and 
 presently begins to smoke and burn in the concen- 
 trated heat. 
 
 This bowl of water is called a lens, and, by means 
 of refraction, we may use it to concentrate light and 
 heat. Beyond the focus you observe the light spreads 
 
76 LIGHT. 
 
 out again till it meets the ceiling, where it appears 
 as a broad disk of light. In the diagram each ray is 
 represented as meeting at the focus, and then all 
 pass each other and go on in their previous direc- 
 tions; and you can readily see that a new cone of 
 light will be formed, upside down, above the focus ; 
 and beyond it all the rays will spread out wider and 
 wider the farther they go. Hold a piece of paper 
 just above the focus, and you will see a small circle of 
 light upon it. liaise it higher, and the circle grows 
 larger and larger, and on the ceiling it is several feet 
 wide. 
 
 By means of lenses of glass or water we can spread 
 out a beam of light, gather a whole bundle of rays 
 into a single focus we can make distant objects ap- 
 pear near, make small things appear large, and large 
 things appear small. Telescopes, microscopes, specta- 
 cles, and all kinds of optical instruments, are founded 
 on this simple law of refraction, as shown by this 
 bowl of water. 
 
 EXPERIMENTS IN PROJECTION. 
 
 At the optician's you can purchase a small glass 
 plano-convex lens, 3 inches (76 millimetres) in diam- 
 eter, and of a focal length of about 8 inches, for per- 
 haps less than fifty cents. Such a lens is flat on one 
 
REFRACTION OF LIGHT. 
 
 77 
 
 side and convex on the other, and from this it takes 
 its name. Take this lens into a room, and close the 
 curtains at all the windows save one. Soften a piece 
 of wax, and stick the lens into it, so that it will serve 
 as a handle, and then hold the lens a few inches from 
 the wall, or, if the wall is dark-colored, before a sheet 
 of paper pinned upon the wall, and just opposite the 
 
 
 FIG. 21. 
 
 window. On the wall will then appear a picture 
 of the window, and the trees, houses, and other 
 objects that may be seen through it. Move the 
 lens backward or- forward, and you will find a place 
 where the image on the wall becomes distinct, and 
 gives a miniature view of the window in its natural 
 colors, and upside down. 
 
78 
 
 LIGHT. 
 
 Here the light from the window falls upon the 
 lens, and is refracted to a focus. This focus consists 
 of points, each of which is formed by the conver- 
 gence of rays which come from a similar point in 
 the window. This we call a projection^ because the 
 light is projected or thrown upon the wall by the lens. 
 To understand this we must notice that every part of 
 the window sends light into the room in every direc- 
 tion. Every part sends light into the entire lens, and 
 each beam is refracted and takes a new direction be- 
 
 FIG. 22. 
 
 yond it, so that, ultimately, all the rays meet at the 
 focus. In examining this projection, you will notice 
 that the glass is quite near the wall when the focus is 
 clear and sharp. If we measure the distance from 
 the lens to the projection, we shall get a certain meas- 
 urement. This measure we call the focal distance of 
 the lens. 
 
 Fig. 22 represents two rays from the top of the 
 window and two from the bottom, and shows the 
 
REFKACTION OF LIGHT. 79 
 
 path they take. To draw every ray would confuse 
 the picture, and by examining these four we can 
 form an idea how they all travel together in a crowd 
 and meet beyond the lens in new positions, and all 
 closely drawn together in a focus. Those from the 
 top of the window are refracted in one direction, 
 those from the bottom in another, and thus they cross 
 each other, and the projected image of the window 
 appears to be upside down. 
 
 The picture on page YV in this section represents 
 the heliostat in position in a dark room. On a table 
 in front of the instrument is the plano-convex lens, 
 mounted on a lump of wax, fastened to a block of 
 wood, and placed with the convex side toward the 
 sun. The opening of the heliostat is covered by a 
 piece of smoked glass, having a figure of an arrow 
 drawn upon it. The light passes through the glass 
 where the smoke was brushed away in drawing the 
 arrow, and falls upon the lens. By refraction the 
 beams of light form an image of the arrow upon a 
 white screen. This screen is made of white cotton 
 cloth, and is hung about 15 feet (4.57 metres) from 
 the lens. The result is a large projection of the 
 arrow, upside down, and in white on a black ground. 
 Move the lens backward or forward slightly, and you 
 will find a place where the projection is sharp and 
 
80 LIGHT. 
 
 clear, and then the lens may be fixed there while we 
 project other images on the screen. 
 
 This simple and inexpensive apparatus thus makes 
 an excellent magic lantern. Common painted or 
 photographic lantern - slides may be placed upside 
 down at the opening of the heliostat, and will be pro- 
 jected on the screen clearly and distinctly, as with the 
 best magic lanterns. Concerning the use of this lan- 
 tern, and the slides that may be used in it, more may 
 be found under the section on the water-lantern. 
 
 If it is not convenient to use a heliostat, this lan- 
 tern may be used by taking a beam of sunlight, as it 
 enters through a hole 4 inches (10 centimetres) in 
 diameter in the shutter, and reflecting it in a hori- 
 zontal direction through the lens by means of a hand- 
 mirror. 
 
 THE FOUNTAIN OF FIRE. 
 
 Fig. 23 represents a flat-bottomed flask, used by 
 chemists. It has a narrow neck at the top, a flat 
 base, and a hole at the side. This hole may be cut 
 in the flask by means of a tube of brass one-quar- 
 ter inch in diameter. This tube has a square end 
 which is scored by two or more cross-cuts with a V- 
 shaped file. A block of wood, having a hole of one- 
 quarter inch in diameter, is placed against the flask ; 
 and then the tube, armed with emery and water, is 
 
REFRACTION OF LIGHT. 
 
 81 
 
 inserted in this hole, and by twirling the tube in the 
 fingers the hole in the glass is made. The tube may 
 also be put in a lathe. In the picture the flask 
 stands upon a shelf in front of the heliostat, and 
 
 FIG. 23. 
 
 just beneath it on the floor is placed a tub or a 
 water-pail. These few things and some pieces of 
 colored glass will enable us to perform a most inter- 
 
82 LIGHT. 
 
 esting and beautiful experiment both in refraction 
 and reflection. Place the finger over the hole in the 
 side of the flask and fill it with water. Place the 
 flask on the shelf so that the beam of light from the 
 heliostat will strike the glass opposite the hole in 
 the side. 
 
 Look at the beautiful cone of light in the water. 
 The beam of light is refracted and brought to a focus 
 as in our other experiments, except that here the 
 cone is entirely under water. Study this singular cone 
 carefully, and adjust the flask so that the point of 
 the cone shines on the finger at the hole in the side. 
 When this is done, make the room as dark as possi- 
 ble, and then remove the finger and let the water fall 
 in a stream into the tub on the floor. 
 
 How magical ! The curving stream of water is 
 fall of light, and appears like a stream of molten iron. 
 The spot where it falls seems touched with fire. 
 Put your finger in the stream of water, and it is 
 brightly illuminated. Of course, the water soon runs 
 down, and the display stops. To prevent this, bring 
 water in a rubber tube from the water-pipes in the 
 house, and then regulate the supply so that the re- 
 ceiver may be kept full as fast as the water runs out. 
 
 Place a piece of red glass behind the flask in 
 the beam of sunlight, and the stream of water will 
 
I 
 
 REFRACTION OF LIGHT. 83 
 
 look like blood. Touch it, and the hand will be crim- 
 son, and the scattered drops that fall in a shower into 
 the tub will shine like drops of red fire. Place a 
 green or blue glass behind the flask, and the stream 
 of water will turn green or blue, and present a most 
 singular appearance. Hold a goblet in the stream, 
 and it will overflow with liquid light. Flashes and 
 sparkles of fire will appear in it, and foam over the 
 sides, shining with brilliant light. 
 
 This beautiful experiment is as interesting as it is 
 strange and magical, and it illustrates both refraction 
 and total reflection. The flask makes a lens, and 
 the falling stream of water is lighted up by the cone 
 of light that enters it at the hole in the flask. Both 
 the water and the light pass out of the hole to- 
 gether, the light inside of the water. That this is so, 
 may be proved by permitting the water to escape, 
 when the light will be seen shining out of the hole 
 horizontally into the room. Why, then, does it not 
 shine out into the room while the water is escaping ? 
 When the stream of water is flowing out, it falls in a 
 curve into the tub on the floor. The beam of light, 
 passing out with the water, meets its curved surface 
 at such an angle that it is totally reflected. This 
 beam of reflection again meets the surface of the 
 water, and is again totally reflected. In this manner 
 
84 LIGHT. 
 
 it is reflected from side to side, again and again, till 
 it reaches the tub, and there we see it shining brightly. 
 It is a prisoner in the water, and follows it down into 
 the tub. When you put your hand in the falling 
 water, you see that it is lighted brightly, and yet the 
 stream by comparison is rather dark. If it were 
 pure, distilled water, it would hardly be visible. As 
 it is full of floating specks and motes, each of these 
 reflects light, and these cause the water to appear full 
 of light. 
 
 This fountain of fire is a charming experiment 
 for a school, and its double lesson makes it as inter- 
 esting as it is beautiful. 
 
 THE WATER-LANTERN. 
 
 Fig. 24: represents the water-lens used in the 
 last experiment but two. The water -lens stands 
 in the wooden box containing the mirror, and at 
 the back of the box is a wooden slide holding a 
 horizontal shelf at the top. This slide has a long 
 slot cut in it, and, by means of a bolt and nut fast- 
 ened at the back of the box, it can be made fast to 
 the box in any desired position. This slide is 16 
 inches (40.6 centimetres) long, 5 inches (12.7 cen- 
 timetres) wide, and f inch (19 millimetres) thick ; and 
 the slot cut in it extends nearly the whole length. 
 
REFRACTION OF LIGHT. 
 
 85 
 
 FIG. 24. 
 
 shelf on the top is 7 inches (17.8 centimetres) 
 
86 LIGHT. 
 
 long, and 5 inches (12.7 centimetres) wide, and has a 
 hole, 3J inches (8.3 centimetres) in diameter, cut in 
 the centre. The iron bolt and nut must go through 
 the back of the box, and must have a washer wide 
 enough to cover the slot in the slide. A few inches 
 below the bolt a block of wood is fastened to the 
 back of the box in the slot of the slide, to serve as a 
 guide in raising and lowering the slide carrying the 
 lens. In the hole in the shelf rests a large watch- 
 glass, or shallow dish, about 4 inches (10.1 centime- 
 tres) in diameter. The plano-convex lens used in our 
 experiments in projection may be here used in place 
 of the watch-glass. On each side of the shelf are 
 two upright wooden arms, and between them is placed 
 a looking-glass 7 inches (17.8 centimetres) long, and 
 4 inches (10.1 centimetres) wide. To hold this mir- 
 ror in place, screws may be put through the top of 
 uprights into the frame, so that it will hang sus- 
 pended, and turn freely up or down. 
 
 This apparatus can be made for about $3.20, the 
 woodwork costing $1, the two mirrors $1.50, the two 
 lenses costing 70 cents ; and when it is ready for work 
 it will be a fine lantern suitable for projecting large 
 pictures upon a screen. Place the lantern before the 
 heliostat, so that the full beam of light will be re- 
 flected from the mirror upward through the glass 
 
REFRACTION OF LIGHT. 87 
 
 bowl and the watch-glass. Fill each of these with 
 clear water, and then place the swinging mirror at 
 the top at an angle of 45. Hang up a large screen 
 of white cotton cloth or sheet in front of the lantern, 
 and from 15 to 40 feet (4.57 to 12.20 metres) from the 
 lantern. On this screen will appear a circle of light 
 projected from the lantern. The sunlight from the 
 mirror is refracted in the large water-lens and brought 
 to a focus. It is again refracted in the small glass of 
 water, and is reflected by the mirror on the screen. 
 Get a piece of smoked glass, and trace upon it some 
 letters, and then lay it on the water-lens with the top 
 (upper side of the writing) toward the screen: im- 
 mediately the letters will appear on the screen, in 
 white on a black ground. If the projection is not 
 distinct, loosen the nut at the back of the box, and 
 move the wooden slide up or down till the right focus 
 is obtained. 
 
 This water-lantern may now be used for all the 
 work performed with ordinary magic lanterns. Place 
 a sheet of clear glass over the large lens to keep 
 the dust out of the water, and then lay common lan- 
 tern-slides on this as in a magic lantern. 
 
 The most simple slides for such a lantern can be 
 made by laying thin paper over engravings or draw- 
 ings, and tracing the picture with lines of holes 
 
88 LIGHT. 
 
 pricked with a pin. In the lantern such a paper slide 
 will show the lines of the picture in dots of light on 
 a dark ground. Another way is to write or draw on 
 sheets of smoked glass. A curious effect may be 
 made by placing the smoked glass in the lantern and 
 writing upon it, upside down and backward, when the 
 letters will appear to grow out in big white characters 
 on the dark screen, and afford much amusement to 
 all who see it. Of course, the film of smoke will 
 easily rub off, and each scratch and finger-mark will 
 be shown on the screen, and the work is often dirty 
 and troublesome ; but it has the advantage of being 
 quickly done, and, if the picture is not right, it can be 
 rubbed out and another put in its place. 
 
 A better kind of slide may be made by draw- 
 ing with a needle on sheets of gelatine. Sheets of 
 gelatine, 18 inches (45.7 centimetres) square, can be 
 bought for 35 cents, either pure and transparent or in a 
 variety of colors. Lay a piece of this on an engraving, 
 and trace the picture, drawing, map, or outline, with the 
 point of a large needle do not press very hard on the 
 gelatine ; a mere scratch is enough and in the lan- 
 tern every line and dot will be visible, in black upon 
 a white or colored ground. To preserve these sheets 
 of gelatine, put them between sheets of glass, and 
 bind them together with paper pasted over the edges. 
 
REFRACTION OF LIGHT. 89 
 
 Another kind of slide may be made by flowing 
 skimmed milk over sheets of glass. When the white 
 film of milk is dry, drawings may be traced in it with 
 a sharp pencil or pointed stick. Another plan is to 
 rub Castile soap over glass, and to draw on this in the 
 same way. By this plan you can destroy the picture 
 by rubbing on more soap, and you may then make a 
 new picture in it. 
 
 This lantern is quite as good as the best magic 
 lanterns. For schools, where one boy or girl wishes 
 to show a sum in arithmetic, an example in alge- 
 bra, a map, or sample of penmanship, to the whole 
 school, the sun-lantern and piece of smoked glass, or 
 a sheet of gelatine, will enable him to project it on 
 a screen, so that a hundred boys and girls can see it 
 at once. 
 
 Another interesting experiment may be made with 
 this lantern by taking the glass cover off of the large 
 lens, and dropping a very small chip of wood in the 
 water. It will be pictured in gigantic size upon the 
 screen, and curious fringes of shade will gather round 
 it, showing where the water clings to the wood. A 
 drop of camphor or of oil of coriander or oil of cin- 
 namon, let fall into the water, will exhibit geometri- 
 cal figures and strange motions on the screen ; and a 
 few drops of indigo or carmine ink will color the 
 
90 LIGHT. 
 
 screen blue or red, and make an excellent background 
 for some of the pictures. 
 
 To describe all that could be done with this water- 
 lantern and heliostat would fill a book. Having 
 made them, you can consult other books on making 
 projections, and find the lantern a source of amuse- 
 ment and instruction for hundreds of people for a 
 very long time. 
 
 THE SOLAR MICROSCOPE. 
 
 Fig. 25 represents a common round glass flask, 
 about 6 inches (15.3 centimetres) in diameter; a com- 
 mon pocket-microscope lens of 1 inch focus (costing 
 25 cents), and a glass slide, carrying a microscopic 
 object. The flask is filled with water, and is placed 
 on a table just at the opening of the heliostat, so that 
 the light will be refracted in it and brought to a 
 focus. It is thus a water-lens, and may be used to 
 bring a focus of light upon any object placed near it. 
 Just behind this focus we place a glass slide, contain- 
 ing some object to be examined in a microscope. To 
 hold this slide upright, we stick it in a mass of wax. 
 The magnifying-glass is fastened to a bit of wax rest- 
 ing on a block of wood, so that it may be moved 
 backward or forward along a strip nailed down on 
 another block. About 15 feet (4.57 metres) from the 
 
REFRACTION OF LIGHT. 
 
 91 
 
 table is placed a screen, and on this is projected a 
 large image of the minute object on the slide. A 
 cloth is hung over the upper part of the water-lens, 
 to shut out the light, and all other light is excluded 
 from the room. This apparatus makes a solar micro- 
 scope, that may be used to project all kinds of micro- 
 
 FHJ. 25. 
 
 scopic objects, so that they can be exhibited before a 
 large number of people. Tanks for holding animal- 
 cules, and all objects used in microscopes, may be 
 placed in this solar lantern and exhibited upon a large 
 scale, with very little trouble, and at only the ex- 
 pense of the flask, the pocket-lens, the screen, and 
 the heliostat. 
 
92 LIGHT. 
 
 CHAPTER Y. 
 
 TEE DECOMPOSITION OF LIGHT. 
 
 CUT a vertical slit, an inch (25 millimetres) long 
 and j of an inch (1 millimetre) wide, in a piece of 
 cardboard. Make the slit with sharp, clean edges, 
 and then fasten the cardboard over the opening in 
 the heliostat, and a slender ribbon of light will 
 enter the dark room. In front of this slit place a 
 small block of wood, and on this put a lump of wax. 
 At the optician's you can purchase for 50 cents a 
 good glass prism. Stand this upright in the wax, as 
 in Fig. 26. Behind this, at a distance of about 15 
 feet (4.57 metres), hang up the screen we used in the 
 lantern projections. 
 
 Here A is the opening in the heliostat, but some- 
 what exaggerated in size. The prism is at P, and /S 
 shows how the screen is placed, but gives its position 
 much too near the prism. 
 
 On the screen will be projected a band of brilliant- 
 colored light, resembling the rainbow. "We have seen 
 that light may be reflected, and that it may be re- 
 fracted ; here we discover that, by refraction, it may 
 be decomposed that a single beam of white sun- 
 
THE DECOMPOSITION OF LIGHT. 
 
 93 
 
 light may be split into a vast number of rays, each 
 having a color of its own. This beautiful band of 
 color is called the solar spectrum. Study it carefully. 
 
 FIG. 26. 
 
 It is quite impossible to count the colors, for they 
 mingle together and merge into each other by invisi- 
 ble gradations, so that we cannot say where one color 
 
94 LIGHT. 
 
 begins and another ends. Yet with a little care you 
 can make out a number of colors, that seem quite dis- 
 tinct. Seven colors can very easily be counted by 
 beginning at the red, or left end of the spectrum. 
 These colors are red, orange, yellow, green, blue, in- 
 digo, and violet. Some people count ten, and call 
 them red, orange, yellow, yellowish-green, green, 
 bluish -green, ultramarine -blue, indigo, violet, and 
 lavender. All these colors, and the countless shades 
 of colors that lie between them, are in the beam of 
 sunlight. Decompose the light by means of a prism, 
 and they stand side by side on the screen in a beauti- 
 ful band or belt. Each color has a different degree 
 of refraction, the refraction increasing from the red 
 to the violet ; and thus they meet the screen at differ- 
 ent places, and we see them spread out side by side 
 like a band or ribbon upon the screen. 
 
 To prove that this solar spectrum is the solar 
 light decomposed, and to show that all these colors 
 may be found in a beam of white light, place a hand- 
 mirror in the beam of refracted light just beyond the 
 prism. The spectrum may thus be reflected to a dis- 
 tant part of the room, on the wall or ceiling. Then, 
 holding the mirror in the fingers, make it vibrate to 
 and fro, so that the reflected spectrum will move, in 
 the direction of its length, from side to side very 
 
THE DECOMPOSITION OF LIGHT. 95 
 
 quickly. At once the spectrum on the wall changes 
 into a streak of white light, with colored spots at 
 each end. To understand this, you must remember 
 the common experiment of whirling a lighted stick 
 or bit of live coal. The spot of fire changes into a 
 ring of light. When light falls upon the eye its effect 
 lingers for a short time, even after the source of the 
 light has moved away, or has ceased to give light. 
 The vision is said to persist or stay after the light has 
 really gone. So in this case the colors of the moving 
 spectrum on the wall persist or stay in the eye while 
 they are moving to and fro, and thus one color over- 
 laps another, and we seem to see them all at once in 
 one place. This mingling of every color in the eye 
 gives us a band or streak that appears white, and 
 thus, indirectly, proves that all the colors of the 
 spectrum make white, and that white light contains 
 all these colored lights. At the ends of this band of 
 white are bright spots of color. As the mirror moves 
 backward and forward, it stops at each end of its lit- 
 tle journey to change its direction, and here the spec- 
 trum becomes visible. 
 
 EXPERIMENTS WITH THE SOLAR SPECTRUM. 
 
 Send to the dealer in artists' materials and get a 
 cake of red vermilion, emerald-green, and aniline vio- 
 
96 LIGHT. 
 
 let (Hoffman's violet B. B.). If this color cannot be 
 found, buy " Nuremberg violet." Get these shades, 
 and no others, and then cut out three narrow strips of 
 cardboard, and give one a coat of the red, one a coat 
 of violet, and paint the other green. Take pains to 
 give them a good thick coat, so as to hide the white 
 paper. Study the solar spectrum on the screen care- 
 fully, and you will see that these shades of red, green, 
 and violet, are in it. When the painted strips are 
 dry, take the red vermilion strip and hold it in the 
 spectrum at the left or red end, and you will see that 
 it matches the red exactly. Tip the paper backward a 
 trifle so that the surface of the paper will not shine or 
 glisten in the light, and then move it slowly to the 
 right, keeping it before the spectrum. As it passes the 
 orange it grows dark ; in the yellow it is darker still ; 
 opposite the green it is perfectly black. Move it to the 
 very end, and everywhere the red strip is quite black. 
 Place it before the red again, and its color comes out 
 clear and bright. Try the violet strip in the same 
 way. In the same manner, the green strip is green 
 when it is in the green part of the spectrum, and black 
 everywhere else. 
 
 This experiment shows that green, red, arid violet, 
 are visible in green, red, and violet light, and that 
 in light of any other color they are invisible, and the 
 
THE DECOMPOSITION OF LIGHT. 97 
 
 strip of card appears to be black. Hence, an object 
 appears of its proper color because it absorbs all 
 colors of the white light except its own color which 
 it reflects. 
 
 Look at the spectrum closely, and you will notice 
 that the red is at one end, the green near the mid- 
 dle, and violet is at the other end. Between the 
 red and the green you will notice many shades 
 of yellow, from deep-orange to yellowish-green, 
 and between the green and the violet are many 
 shades of blue, from greenish-blue to deep indi- 
 go. 
 
 It is thought that, when we see a red light, cer- 
 tain nerves in the eye are affected, and convey a 
 peculiar sensation to the brain, that we call Ted. 
 These nerves are sensitive to red light, but are not 
 sensitive to any other light, except in a moderate 
 degree. Another set of nerves in the eye are pecul- 
 iarly sensitive to green light, and still another set are 
 affected by violet light. Hence the sensations caused 
 by these three colors are called the three elementary 
 color sensations, and from the combinations of these 
 sensations come all countless shades of color. "When 
 one of these colors falls on the eye, we see it dis- 
 tinctly. When two say the red and green 
 
 meet the eye, both sets of nerves are affected at 
 5 
 
98 LIGHT. 
 
 once, and we get a sensation that is neither red nor 
 green, but yellow. In the same manner, when green 
 and violet meet in the eye, the two sets of nerves are 
 excited, and we see not green and violet, but blue. 
 In the same manner, if red, green, and violet light 
 enters the eye, all the nerves are excited at once, and 
 we see not three colors, but one, which is white. 
 
 Purple. 
 
 Bed. 
 
 Violet. 
 
 This diagram will assist us to remember the rela- 
 tion the color sensations bear to each other. The 
 red and the green combine to make yellow ; the green 
 and the violet unite to make blue ; all three mingled 
 
THE DECOMPOSITION OF LIGHT. 99 
 
 together give us white. We may also combine red 
 and violet light, and get purple light. 
 
 THE COLOR-TOP. 
 
 This picture represents a common iron top, that 
 may be found at the toy-shops. If you cannot find 
 
 Fro. 28. 
 
 one exactly like it, there are others having a straight 
 handle at the top, instead of the curved handle. 
 
100 LIGHT. 
 
 Just under the flat part of the disk are two or 
 three round pieces of drawing-paper, and under these 
 is a thick disk of pasteboard. Each of these has a 
 hole in the centre, so that it can be slipped over the 
 leg of the top. In some tops, however, it may be 
 easier to put these disks above on the handle. Such 
 a top as this may be made to spin in a dinner-plate 
 on the table by winding a string round the leg, and 
 then pulling it away with the right hand, while the 
 top is held upright by the left hand. 
 
 Get some thick drawing-paper, and cut out three 
 disks, each 4 inches (10 centimetres) in diameter, and 
 make a hole in the centre of each, so that it will slip 
 over the leg of the top. Cut each disk open from the 
 circumference to the centre with a pair of scissors. 
 Paint one with the red vermilion, one with the 
 emerald-green, and the other with the violet that we 
 used in the last experiment, and then, while these are 
 drying, make a disk of thick pasteboard, and cut a 
 hole in the middle, so that it will slip tightly over the 
 leg of the top. 
 
 "When these are ready, take the red and green 
 disks and hold them side by side with the cut places 
 opposite, and slip one into the other, and then turn 
 them round, so that the green covers the red. Then 
 put them both on the leg of the top, and put the 
 
THE DECOMPOSITION OF LIGHT. 101 
 
 pasteboard disk under them, to hold them in place. 
 Now, if you hold the top upright in a plate and make 
 it spin, you will see a beautiful ring of green color 
 round the spinning top. 
 
 When it stops, take off the pasteboard, and revolve 
 the colored disks one on the other, so that half of 
 the red and half of the green can be seen. Now spin 
 them on the top, and instantly you have a ring of 
 yellow. Move the disks again, so as to display one- 
 quarter of the green and three-quarters of the red, 
 and when the top spins you get a deep-orange ring. 
 Move them again, and let the green hide nearly all the 
 red, and the top shows a greenish-yellow ring. In this 
 manner you may mix the red and green in greater 
 or less proportions, and the ring of color on the top 
 will exhibit new shades of yellow with every change. 
 
 In the same way, combine the green disk and the 
 violet, and the spinning top will show a new shade of 
 blue with every proportion in which the green and 
 violet are mixed. Put on the red and violet disks, 
 and let each show more or less, and shades of purple 
 will be shown. Put on all three disks the red, 
 green, and violet and arrange them so that one-third 
 of each is shown, and the ring will be gray. Change 
 the proportions, and you will see each time new 
 shades of gray or white. 
 
102 LIGHT. 
 
 This is a very simple toy, but it serves to show 
 how these three colors may be combined to produce 
 every color in the solar spectrum. The color will 
 vary very greatly, and new and beautiful shades of 
 yellow, blue, purple, and gray, will be found at every 
 trial. Red, green, and violet, may be tinted with 
 other colors in the most charming manner, and hours 
 can be filled with amusement and instruction by ex- 
 perimenting with this color-top and its ever-chang- 
 ing colored rings. 
 
 To exhibit the colored rings on this top before a 
 number of people, make a disk of stiff cardboard about 
 5 inches (12.7 centimetres) in diameter, and cut out three 
 holes at equal distances from each other near the edge. 
 Over these holes place pieces of red, green, and violet, 
 or ultramarine-blue glass, one color on each hole, and 
 fasten them down with little bands of paper at the 
 edges, and secured with mucilage. Place this disk on 
 the color-top, and hold it upside down just above the 
 large lens in the water-lantern. Have the lantern 
 prepared to give projections on the screen (see sec- 
 tion on water-lens), and then you will see three 
 spots of colored light on the screen, and, by making 
 the top spin round on the handle by means of the 
 string, the three spots of color will whirl round in 
 a ring, and, if the top moves fast enough, we shall see 
 
THE DECOMPOSITION OF LIGHT. 103 
 
 a ring of white or gray. Cover the violet glass so as 
 to shut out all the light, and then make the top spin, 
 and the two spots of red and green will appear on the 
 screen in the form of a yellow ring. In this manner 
 all the effects exhibited by the color-top may be pro- 
 jected on a large scale on the screen, and make a most 
 interesting and beautiful exhibition that will be sure 
 to please all who see it. 
 
 DIRECT RECOMPOSITION OF THE COLORS OF THE 
 SPECTRUM. 
 
 Let the spectrum fall on a mirror, and throw its 
 reflection upon a distant part of the room. Procure 
 a slip of looking-glass half an inch wide and about 
 three inches long, and place this on the mirror in any 
 color of the spectrum. By tilting the slip of looking- 
 glass, any color can be thrown on to any other color 
 of the spectrum, and thus an endless variety of colors 
 can be formed by compounding their elementary com- 
 ponent colors. 
 
 EXPERIMENTS IN REFLECTED COLORS. 
 
 Fig. 29 represents a flat block of wood having a 
 short stick set up at one corner. On this stick is 
 fastened, by means of a lump of wax, a strip of clear 
 window -glass about 1 inch (25 millimetres) wide, 
 
104 
 
 LIGHT. 
 
 and 3 inches (7.5 centimetres) long. Just behind the 
 stick is another piece of glass of the same size fast 
 ened to the block of wood by a mass of -wax. Place 
 the instrument on a table near a window, and then sit 
 before it with your back to the light. Cut out small 
 
 Fm. 29. 
 
 bits of paper and paint one red-vermilion, anoth- 
 er emerald-green, and the third violet, as in our 
 experiments with the color-top. Then place one 
 of these in front of the instrument, at the spot 
 marked C in the drawing, and, sitting close to the 
 instrument, look down into the glass on the stick 
 and you will see the bit of colored paper reflected in 
 the glass. Suppose this is the red piece. Then place 
 the green piece at the ring marked A. On looking 
 
THE DECOMPOSITION OF LIGHT, 1Q5 
 
 into the glass you can now see both the green piece 
 reflected in the glass and the red behind it. While 
 thus looking at both, move the one or the other till 
 they appear to come in line, or one over the other, 
 and then, in place of seeing a red and a green piece, 
 you will see a single yellow piece. 
 
 Again we have a combination of colors, and we 
 can place the red and violet or the violet and green 
 pieces before and behind the glass, and see the colors 
 combine precisely as in the color-top. If it is not 
 convenient to make this instrument, these effects can 
 be shown with a piece of clear window-glass, by 
 simply holding it in the hand so that one color can 
 be seen reflected in the glass, and the other directly 
 through it. 
 
 With the instrument we can combine all three 
 colors by placing them in the positions marked A, B, 
 and Oy and then looking through both glasses at once. 
 The color at A will be seen through both glasses, the 
 color at C will be seen in the upper glass and in a line 
 with the first, and the color at B will be seen reflected 
 on the surface of the lower glass; and, if all three 
 are in the right places, we shall see only one piece, 
 and that will be white or gray. 
 
106 LIGHT. 
 
 EXPERIMENTS IN CONTRASTED COLORS. 
 
 Cut out three pieces of drawing-paper about 2 
 inches (5.1 centimetres) square, and paint one red, 
 another green, and another violet, using the paints 
 we bought for the color-top. If these are not at 
 hand, cut out squares of red, green, and violet paper, 
 and squares of yellow, pink, blue, or any other colors 
 you can obtain. Lay a piece of black cloth on a table, 
 near the window, and then sit before it with your 
 back to the light, and place the red square of paper on 
 the cloth. Take a sheet of white or, still better, light- 
 gray blotting-paper in the right hand, and hold it just 
 above the red square in such a position that it can be 
 quickly slipped over it, so as to hide it from sight. 
 ISTow look steadily at the red square for a minute or 
 two, and then slip the gray paper over it. In a few 
 seconds there will appear on the gray paper a curious 
 image just the size and shape of the red square, but 
 of a bluish-green color. It will grow brighter quickly, 
 and then fade away, leaving nothing but the gray 
 paper. Put the green square on the black ground, 
 and, after looking at it for a moment, cover it with 
 the gray paper, and a pink image of the square will 
 seem to shine out of the gray paper for a moment, 
 and then fade away. Look at the violet square, and 
 
THE DECOMPOSITION OF LIGHT. 1Q7 
 
 then suddenly hide it, and a pale, greenish-yellow 
 image will be seen. Get a square of yellow paper, 
 and repeat the experiment, and you will see a violet- 
 blue image. In the same way try an orange square 
 and get a violet image ; try greenish-yellow, and get 
 a pink image. 
 
 These strange, ghostly after-images that linger 
 after any color has been suddenly removed, result 
 from an action in the eye. Having looked at red till 
 the eye becomes weary, and then having suddenly 
 taken the red away and replaced it by a white surface, 
 the nerves of the eye send us another sensation that 
 we call bluish-green. The nerves sensitive to red 
 having become fatigued, the nerves sensitive to green 
 and violet are fresh and fully sensitive to the green and 
 violet rays in the white light reflected from the paper. 
 
 Every color gives a particular after-image, and this 
 image is always of a color that is said to be comple- 
 mentary to it. Eed is complementary to bluish-green, 
 orange to sky-blue, yellow to violet-blue, green to 
 pink, and so on through all the colors. 
 
 To vary this experiment, you may take a small bit 
 of green paper, say about the size of a wafer, and lay 
 it on the middle of a square of orange-colored paper, 
 and then, after looking at the two colors for a mo- 
 ment, hide them both with the gray paper, and the 
 
108 LIGHT. 
 
 after-image will be blue, with an orange spot in the 
 centre. Take off the green wafer and put on a blue 
 one, and make the experiment. Put a large spot of 
 ink in the centre of this orange square, and the blue 
 after-image will have a gray spot in the centre. Black, 
 we know, is the absence of color, and on looking at 
 the black spot the eye is not excited at all, and the 
 blue image appears to have a hole in the middle 
 through which we see the gray paper. If we put 
 a white wafer on the orange square, we shall see a 
 blue after-image with an orange spot. 
 
 These strange and curious after-images, appearing 
 like colored ghosts on the gray paper, may afford both 
 amusement and instruction, and will give us the com- 
 plementary color of any color we use in the experi- 
 ments. These complementary colors, when placed side 
 by side, always give the eye a pleasing sensation, and 
 we say that the colors look well together. 
 
 Take a piece of red cloth or paper and hang it up 
 before a white screen, or upon a white wall, in a dark 
 room. Stand near the window and look at the red 
 cloth, and you will not be able to see it. Then 
 slowly open the window-shutter, and permit a little 
 light to enter the room. Now the red cloth looks 
 like a black patch on a gray wall. Let in a little 
 more light, and it turns to a deep, dark red. Gradu- 
 
THE DECOMPOSITION OF LIGHT. 109 
 
 ally let in more and more light, and the deep-red 
 cloth will change to a lighter and lighter shade, 
 till the room is fully lighted, when it will appear 
 in its real color. Bring the red cloth into the full 
 sunlight, or throw a beam from the heliostat upon it, 
 and it assumes a still lighter shade of red. Close 
 the shutters again, and it will change back from 
 red to dark-red, and through every shade to per- 
 fect black. Try any other color in the same man- 
 ner, and you will produce precisely the same effects. 
 This experiment may be tried in the night by turning 
 the gas (or other lamp) slowly down till the light dis- 
 appears, and turning it up again, while you watch the 
 changing shades of color as the light decreases and 
 increases. 
 
 Here we have quite a different matter, showing 
 that the shade or brightness of a color depends upon 
 the amount of light it receives. If it has plenty of 
 light, it appears of its normal shade ; if it has less 
 light, it takes a darker shade; if more, it has a 
 brighter shade. If, in using our color-top, we put a 
 bit of black cloth or paper on the colors, the ring of 
 color, when the top spins, will take a darker shade. 
 If we put on a piece of white paper, we shall get 
 rings of a lighter shade.. 
 
 When the sunlight falls on any object, the object 
 
110 LIGHT. 
 
 absorbs part of the light and reflects the rest. If it 
 absorbs all the light and reflects nothing, the eye sees 
 no reflected rays, and we say the object is black, or is 
 invisible. If it reflects all the light, all colors enter 
 the eye, and we say it is white. If it absorbs all the 
 rays of the spectrum except red, our eyes receive 
 these red-reflected rays, and we call the object red. 
 If it absorbs all the rays except red and green, the 
 eye receives these two rays, two sets of nerves are 
 excited at once, and we say that the object is yellow. 
 It is in this manner that we see the things about us, 
 and are enabled to recognize the colors in which they 
 appear to be clothed. 
 
CONCLUSION. HI 
 
 CONCLUSION. 
 
 have now seen how light moves through air, 
 water, and other transparent substances; we have 
 learned something of the manner in which it may be 
 reflected and refracted ; and we have examined a few 
 of the more simple facts about colors. Yet we have 
 not by any means learned all that is known about 
 light, nor have we exhausted half the capabilities 
 of our apparatus. We have studied reflection from 
 plane surfaces : all the wonderful effects produced by 
 reflection from curved mirrors remain for further 
 study. We have examined only one or two of the 
 different kinds of lenses ; and in the beautiful science 
 of colors we have, as it were, only opened the gate 
 into a strange and marvelous country. 
 
 You might go on for a year and still make experi- 
 ments every day, and even then not reach the end. 
 You have seen that it is not difficult to make experi- 
 ments ; and, should you take up other books on light 
 and make new experiments, you would find much 
 that would be of the greatest value and interest. 
 
 Should. you learn nothing else, you will see for 
 yourself with what skill, wisdom, and goodness, all 
 
112 LIGHT. 
 
 these beneficent laws Lave been arranged. These 
 things came not by chance, or of themselves. They 
 all point to a great and wise Creator, who has given 
 the light a pathway, and filled it with bewildering 
 and perpetual beauty. It is the light that paints the 
 flowers, tints the clouds, and decks the sky in blue. 
 Everything selects its own particular color out of the 
 solar spectrum, and shines with all the beauty and 
 glory of the light. No man hath counted all the 
 glories of light, nor hath any man yet traced all its 
 paths. It brings us strange messages from distant 
 suns ; it makes all Nature beautiful. 
 
 Having made a fair start in the art of experi- 
 menting, let us go on to new experiments in sound, 
 heat, magnetism, electricity, and mechanics. 
 
 THE END. 
 
List of Apparatus used in the Experiments on Light, 
 with the prices, as supplied by Samuel Hawk- 
 ridjje, successor to George Wale Co., Hobolcen, 
 New Jersey. 
 
 Heliostat $6 00 
 
 Water-lantern 6 00 
 
 Glass tube 05 
 
 Pulse-mirror 05 
 
 Square bottle for refraction 15 
 
 Plano-convex lens 75 
 
 Small double-convex lens 50 
 
 Flask for condenser of solar microscope 75 
 
 Glass cylinder for experiment of the illuminated jet to be used 
 with plano-convex lens in place of the large flask shown in 
 
 Fig. 23 1 50 
 
 Glass prism 50 
 
 Top 10 
 
 Cake of vermilion paint 15 
 
 Cake of emerald green paint 15 
 
 Nuremberg violet, in powder, to be used with gum-water 15 
 
 Two small slips of clear glass 05 
 
 $14 85 
 
LIGHT: 
 
 A SERIES OF SIMPLE, ENTERTAINING, AND INEXPENSIVE EX- 
 
 PERIMENTS IN THE PHENOMENA OF LIGHT, FOR THE 
 
 USE OF STUDENTS OF EVERY AGE. 
 
 By ALPKED M, MAYEB and CHARLES BABNABD. 
 
 Price, $1.00. 
 
 From the New York Evening Post. 
 
 " A singularly excellent little hand-book for the use of teachers, parents, and chil- 
 dren. The book is admirable both in design and execution. The experiments for 
 which it provides are so simple that an intelligent boy or girl can easily make them, and 
 so beautiful and interesting that even the youngest children must enjoy the exhibition, 
 while the whole cost of all the apparatus needed is only $15.00. The experiments here 
 described are abundantly worth all that they cost in money and time in any family 
 where there are boys and girls to be entertained." 
 
 Front the New York Scientific A merican. 
 
 " The experiments are capitally selected, and equally as well described. The book 
 is conspicuously free from the multiplicity of confusing directions with which works of 
 the kind too often abound. There is an abundance of excellent illustrations." 
 
 Front the A nterican Journal of Science and A rts. 
 
 "The experiments are for the most part new, and have the merit of combining pre- 
 cision in the methods with extreme simplicity and elegance of design. The aim of the 
 authors has been to make their readers ' experimenters, strict reasoners, and exact ob- 
 servers,' and for the attainment of this end the book is admirably adapted. Its value 
 is further enhanced by the numerous carefully-drawn cuts, which add greatly to its 
 beauty." 
 
 From the Boston Globe. 
 
 " The volume seems well adapted to the needs of students who like to have their 
 knowledge vitalized by experiment. The fact that nearly all the experiments described 
 are new, and have been tested, is an additional recommendation of this handy volume. 
 The illustrations add to its interest and value, and its simplicity, both of design and 
 execution, will commend it to beginners and others seeking information on the subject." 
 
 From the 'Philadelphia Press. 
 
 "It supplies a large number of simple and entertaining experiments on the phe- 
 nomena of light, that any one can perform with materials that maybe found in any 
 dwelling-house, or that may be bought for a small sum in any town or city. This 
 actually is philosophy in sport, which thoughtful or ready minds can easily convert 
 into science in earnest." 
 
 D. APPLETON & CO., 549 & 551 BROADWAY, NEW YORK. 
 
THE EXPERIMENTAL SCIENCE SERIES. 
 
 SOUND: 
 
 A Series of Simple, Entertaining, and Inexpensive Experiments in the 
 Phenomena of Sound, for the Use of Students of Every Age. 
 
 By ALFRED MARSHALL MAYER, 
 
 Professor of Physics in the Stevens Institute of Technology; Member of the 
 National Academy of Sciences, etc. 
 
 UNIFORM WITH "LIGHT," FIRST VOLUME OB 1 THE SERIES. 
 
 Neat 12mo volume, bound in cloth, fully illustrated. Price, $1.00. 
 
 " It would really be difficult to exaggerate the merit, in the sense of consum- 
 mate adaptation to its modest end, of the little treatise on ' Sound ' which forms 
 the second number of Appletons' 'Experimental Science Series. 1 The purpose 
 of these hand-books is to teach the youthful student how to make experiments 
 for himself, without the help of a trained operator, and at very little expense. 
 How successful the authors were in attaining that end is attested by the remark- 
 able and constantly-increasing demand for the initial volume. These hand-books 
 of Professor Mayer should be in the hands of every teacher of the young." New 
 York Sun. 
 
 " The present work is an admirably clear and interesting collection of experi- 
 ments, described with just the right amount of abstract information and no more, 
 and placed in progressive order. The recent inventions of the phonograph and 
 microphone lend an extraordinary interest to this whole field of experiment, 
 which makes Professor Mayer's manual especially opportune." Boston Courier. 
 
 "Dr. Mayer has written a second beautiful book of experimental science, the 
 subject being * Sound.' It is a little volume, is surprisingly comprehensive, and, 
 although intended for beginners, contains many pages that will be read with 
 pleasure by those most familiar with the subject." N. Y. Independent. 
 
 " l Sound ' is the second volume of the ' Experimental Science Series.' Like 
 its predecessor, it is deserving of hearty commendation for the number of inge- 
 nious and novel experiments by which the scientific principles are illustrated. 
 These little volumes are the best manuals ever written for the use of non-scien- 
 tific students, and their study will more than repay the labor devoted to them." 
 Boston Gazette. 
 
 " An interesting little treatise on ' Sound.' A carefully-prepared price-list of 
 articles needed for tests and experiments adds to the value of the volume." 
 Boston Evening Transcript. 
 
 D. APPLETON & CO., PUBLISHERS, 649 & 551 BROADWAY, K. Y. 
 
INTERNATIONAL SCIENTIFIC SERIES. 
 
 READY. 
 
 I. FORMS OF WATER, in Clouds, Rain, Rivers, Ice, and Glaciers. By Prof. 
 
 JOHN TYNDALL. 1 vol. Cloth. Price, $1.50. 
 
 II. PHYSICS AND POLITICS ; or, Thoughts on the Application of the Prin- 
 ciples of "Natural Selection" and " Inheritance " to Political Society. By 
 WALTER BAOEHOT. 1 vol. Cloth. Price, $1.50. 
 III. FOODS. By EDWARD SMITH, M. D., LL. B., F. R. S. 1vol. Cloth. Price, 
 
 $1.75. 
 [INI 
 
 IV. MIND AND BODY. The Theories of their Relations. By ALEXANDEB 
 
 BAIN, LL. D. 1 vol., 12mo. Cloth. Price, $1.50. 
 
 V. THE STUDY OF SOCIOLOGY. By HEEBEET SPENCEE. Price, $1.50. 
 VI. THE NEW CHEMISTRY. By Prof. JOSIAH P. COOKE, Jr., of Harvard 
 
 University. 1 vol., 12mo. Cloth. Price, $2.00. 
 VII. THE CONSERVATION OF ENERGY. By Prof. BALFOUR STEWART, 
 
 LL. D. F. R. S. 1 voL, 12mo. Cloth. Price, $1.50. 
 
 VIII. ANIMAL LOCOMOTION; or, Walking, Swimming, and Flying, with a Dis- 
 sertation on Aeronautics. By J. BELL PETTIGREW, M. D. 1 vol., 12mo. 
 Illustrated. Price, $1.75. 
 IX. RESPONSIBILITY IN MENTAL DISEASE. By HENRY MAUDSLEY, 
 
 M.D. 1vol., 12mo. Cloth. Price, $1.50. 
 X. THE SCIENCE OF LAW. By Prof. SHELDON AMOS. 1 vol., 12mo. Cloth. 
 
 Price, $1.75. 
 
 XI. ANIMAL MECHANISM. A Treatise on Terrestrial and Aerial Locomo- 
 tion. By E. J. MAEEY. With 117 Illustrations. Price. $1.75. 
 XII. THE HISTORY OF THE CONFLICT BETWEEN RELIGION AND 
 SCIENCE. By JOHN WM. DRAPER, M.D., LL. D., author of "The In- 
 tellectual Development of Europe." Price, $1.75. 
 
 XIII, THE DOCTRINE OF DESCENT, AND DARWINISM. By Prof. OSCAR 
 
 SCHMIDT, Strasburg University. Price, $1.50. 
 
 XIV. THE CHEMISTRY OF LIGHT AND PHOTOGRABHY: In its Applica- 
 
 tion to Art, Science, and Industry. By Dr. HERMANN VOGEL. 100 Illus- 
 trations. Price, $2.00. 
 
 XV. FUNGI ; their Nature, Influence, and Uses. By M. C. COOKE, M. A., LL. D. 
 Edited by Rev. M. J. BERKELEY, M.A., F.L.S. With 109 Illustrations. 
 Price, $1.50. 
 
 XVI. THE LIFE AND GROWTH OF LANGUAGE. By Prof. W. D. WHIT- 
 NEY, of Yale College. Price, $1.50. 
 
 XVII. MONEY AND THE MECHANISM OF EXCHANGE. By W. STANLEY 
 
 JEVONS, M. A., F. R. 8. Price, $1.75. 
 
 XVIII. THE NATURE OF LIGHT, with a General Account of Physical Optics. 
 
 By Dr. EUGENE LOMMEL, Professor of Physics in the University of Erlangen. 
 With 88 Illustrations and a Plate of Spectra in Chromo -lithography. Price, 
 
 XIX. ANIMAL PARASITES AND MESSMATES. By Monsieur VAN BENEDEN, 
 
 Professor of the University of Lou vain. With 83 Illustrations. Price, $1.50. 
 
 XX. ON FERMENTATIONS. By P. SCHUTZENBERGEB, Director at the Chemical 
 
 Laboratory at the Sorbonne. With 28 Illustrations. Price, $1.50. 
 
 XXI. THE FIVE SENSES OF MAN. Bv JULIUS BEENSTEIN, O. O. Professor of 
 
 Physiology in the University of Halle. With 91 Illustrations. Price, $1.75. 
 XXII. THE THEORY OF SOUND IN ITS RELATION TO MUSIC. By Prof. 
 
 PIETEO BLASEBNA, of the Royal University of Rome. With numerous 
 
 Woodcuts. 1 vol., 12mo. Cloth. Price, $1.50. 
 XXIIL STUDIES IN SPECTRUM ANALYSIS. By J. NORMAN LOCKYEE. With 
 
 Illustrations. lvol.,12mo. Cloth. Price, $2.50. 
 XXIV. A HISTORY OF THE GROWTH OF THE STEAM-ENGINE. By 
 
 ROBEET H. THURSTON, A. M., C. E., Professor of Mechanical Engineering 
 
 in the Stevens Institute of Technology, Hoboken, New Jersey, etc., etc. 
 
 With 163 Illustrations. 1 vol., 12mo. Cloth. Price, $2.50. 
 XXV. EDUCATION AS A SCIENCE. By ALEXANDER BAIN, LL. D., Professor 
 
 of Logic in the University of Aberdeen. Price, $1.75. 
 
 D. APPLETON & CO., Publishers, 549 & 551 Broadway, New York. 
 
INTERNATIONAL SCIENTIFIC SERIES. 
 
 VOLUMES IN PREPARATION. 
 
 THE SUIT. By Prof. YOUNG, of Princeton College. With numerous Illustrations 
 
 CHROMATICS. From the Modern Point of View. By Prof. O. N. ROOD, of 
 Columbia College. 
 
 ILLUSTRATIONS OF THE LOGIC OP SCIENCE. By CHARLES S. 
 PEIRCE. 
 
 BREAD : The Theory and Science of its Production. By Prof. E. N. HORSFORD. 
 
 THE FIRST PRINCIPLES OF THE EXACT SCIENCES EX- 
 PLAINED TO THE NON-MATHEMATICAL. By Prof. W. KINGDON 
 CLIFFORD. (In press.) 
 
 THE PHYSICAL GEOGRAPHY OF THE SEA. By W. B. CAR. 
 PENTER, LL. D., F. R. S. 
 
 THE BRAIN AND ITS FUNCTIONS. By J. LUYS, Physician to the 
 Hospice de la SalpStriere, With Illustrations. (In press.) 
 
 THE BRAIN AS AN ORGAN OF MIND. By H. CHARLTON BASTIAN, 
 M. D., F. R. S. (In press.) 
 
 THE STARS. By the Rev. A. SECCHI, late Director of the Observatory at Rome. 
 (/ press.) 
 
 GENERAL PHYSIOLOGY OF MUSCLES AND NERVES. By 
 
 Prof. J. ROSENTHAL, of the University of Erlangen. 
 
 THE HUMAN RACE. By Prof. A. DE QUATREFAGES, Membre de 1'Institut 
 
 PS YCHOMETRY. By FRANCIS GALTON, F. R. S. 
 
 THE LAWS OF VOLCANIC ACTION. By J. W. JUDD ; F. R. S. 
 
 THE EMBRYONIC PHASES OF ANIMAL LIFE. By Prof. F. N. 
 BALFOUR. 
 
 THE CRAYFISH : an Introduction to the Study of Zoology. By T. H. HUX- 
 LEY, F. R. S. 
 
 ANIMALS AND THEIR CONDITIONS OF EXISTENCE. By 
 
 Dr. CARL SEMPER. 
 
 ATOMS AND THE ATOMIC THEORY. By Prof. WURTZ. 
 
 ANIMAL INTELLIGENCE. By GEORGE J. ROMANNES, F. L. S. 
 
 A MANUAL OF CRYPTOGAMIC BOTANY. By ALFRED W. BEN- 
 
 NETT, F. L. S. 
 
 ON ANTS AND BEES. By Sir JOHN LUBBOCK, Bart., F. R. S. 
 
 FORM AND HABIT IN FLOWERING PLANTS. By Prof. W. T. 
 THISELTON DYER, B. A., B. Sc. 
 
 PROTOPLASM AND THE CELL THEORY. By Prof. MICHAEL 
 FOSTER, M. D. 
 
 EARTH-SCULPTURE: Hills, Valleys, Mountains, Plains, Rivers, Lakes; 
 how they were Produced, and how they have been Destroyed. By Prof. A. C 
 RAMSAY, LL. D., F. R. S. 
 
 FORMS OF LIFE AND OTHER COSMICAL CONDITIONS. By 
 
 P. BERT, Professor of Physiology, Paris. 
 
 D. APPLETON & CO., PUBLISHERS, 549 & 551 BROADWAY, NEW YORK. 
 
D. APPLETON & CO.'S PUBLICATIONS. 
 
 I. 
 
 HEREDITARY GENIUS. An Inquiry into its Laws and Con- 
 sequences. By FRANCIS GALTON, F. R. S. 1 vol., 12mo. Cloth, 
 
 $2.00. 
 
 II. 
 
 ENGLISH MEN OF SCIENCE : their Nature and Nurture. By 
 FRANCIS GALTON, F. R. S. 1 vol., 12mo. Cloth, $1.00. 
 
 III. 
 
 A HISTORY OF THE GROWTH OF THE STEAM- 
 ENGINE. By ROBERT H. THURSTON, A. M., C.E., Professor of 
 Mechanical Engineering in the Stevens Institute of Technology, 
 Hoboken, N. J. With 163 Illustrations, including 15 Portraits. 
 1 vol., 12mo. Price, $2.50. 
 
 IV. 
 
 SOUND. A Series of Simple, Entertaining, and Inexpensive Experi- 
 ments in the Phenomena of Sound, for the Use of Students of 
 Every Age. By ALFRED MARSHALL MAYER, Professor of Physics 
 in the Stevens Institute of Technology. Uniform with " LIGHT," 
 first volume of the series. 12mo. Cloth. Fully illustrated. 
 Price, $1.00. 
 
 V. 
 
 LESSONS IN COOKERY. Hand-book of the National Training- 
 School for Cookery, South Kensington, London. To which is 
 added an Essay on " The Principles of Diet in Health and Dis- 
 ease," by THOMAS K. CHAMBERS, M. D. Edited by ELIZA A. You- 
 MANS. In 1 vol., 12mo, 382 pages. Cloth. Price, $1.50. 
 
 VI. 
 
 ALL AROUND THE HOUSE ; or, How to make Homes Happy. 
 By Mrs. HENRY WARD BEECHER, author of " Motherly Talks," etc. 
 1 vol., 12mo. Cloth. Price, $1.50. 
 
 This volume, as its title implies, consists of papers upon topics concerning 
 the ordering and well-heing of the household. It contains, in addition to a large 
 number of receipts for cooking, and rules for marketing, numerous hints for the 
 management of servants and children, directions as to furnishing, repairing, 
 cleansing, etc., and information on all the innumerable things on which house- 
 keepers need information, while, in addition to its usefulness as a guide to prac- 
 tical knowledge and economical methods, it is eminently interesting and sug- 
 gestive, in its various essays on home topics, to every one concerned in the 
 welfare and happiness of the household. 
 
 D. APPLETON & CO., PUBLISHERS, 549 & 551 BROADWAY, NEW YORK. 
 
ELEMENTARY WORKS ON MECHANICAL AND 
 PHYSICAL SCIENCE. 
 
 FORMING A SERIES OF 
 
 TEXT-BOOKS OF SCIENCE 
 
 ADAPTED FOR THE USE OF 
 
 ARTISANS AM STUDENTS ffl PUBLIC AM SCIENCE SCHOOLS, 
 
 Fully illustrated ; size, 16mo. 
 
 VOLUMES ALREADY PUBLISHED: 
 
 The Elements of Mechanism. By Profes- 
 sor T. M. GOODEVE, M. A. Cloth, $1.50. 
 
 Metals: Their Properties and Treatment. 
 
 By Professor C. L. BLOXAM. Cloth, 
 
 $1.60. 
 Introduction to the Study of Inorganic 
 
 Chemistry. By W. A. MILLER, M. D., 
 
 D.C.L^LL.D. Cloth, $1.50. 
 Theory of Heat. By Professor J. C. 
 
 MAXWELL, M. A., LL.D. Cloth, $1.50. 
 
 The Strength of Materials and Struc- 
 tures. By J. ANDERSON, C. E., LL. D., 
 F.B.S.E. Cloth, $1.50. 
 
 Electricity and Magnetism. By Profes- 
 sor F. JENKIN, F. E. SS. L. & E., M. 
 I. C. E. Cloth, $1.50. 
 
 Workshop Appliances, including Machine- 
 Tools used by Engineers. By C. P. B. 
 SHBLLEY, C. E. Cloth, $1.50. 
 
 Principles of Mechanics. By Professor T. 
 M. GOODEVE, M. A. Cloth, $1.50. 
 
 Introduction to the Study of Organic 
 Chemistry. By Professor H. E. ARM- 
 STRONG, Ph. D., F. C. 8. Cloth, $1 50. 
 
 Qualitative Chemical Analysis and 
 
 Laboratory Practice. By Professor 
 
 T. E. THORPE, Ph.D., F.B.S.E.,and 
 
 M. M. P. MUIR, F. E. 8. E. Cloth, $1.50. 
 Telegraphy. By W. H. PREECE, C. E., 
 
 and J. SIVEWHIGHT, M. A. Cloth, $ 1 .50. 
 Railway Appliances. By J. W. BARRY, 
 
 C.E. Cloth, $1.50. 
 The Art of Electro- Metallurgy. By G. 
 
 GORE, LL.D., F.E. 8. Cloth, $2.50. 
 Introduction to the Study of Chemical 
 
 Philosophy. By W. A. TILDEN, D. Sc. 
 
 Lond., F. C. 8. Cloth, $1 .50. 
 The Elements of Machine Design. By 
 
 Professor W. C. UNWIN, C. E. Cloth, 
 
 $1.50. 
 Treatise on Photography. By Captain 
 
 W. De WIVE LESLIE ABNEY, F. li. 8. 
 
 Cloth, $1.50. 
 
 (Other- volumes in preparation.) 
 
 " This admirable series of text-books is invaluable for the use for which it was origi- 
 nally planned. Several of the authors are preeminent in their own specialty, and their 
 works must have been, of immense service to the numerous class of students for whom 
 they are chiefly intended. Taking the series as a whole, it would be a difficult task to 
 single out another list of text-books on the same or collateral subjects in our language 
 which could be compared with them, either in regard to quality and price, or that 
 are so well fitted for the instruction of engineering students, or for students generally 
 in our public and science schools." London Examiner. 
 
 D. APPLETON & CO., Publishers, 549 and 551 Broadway, New York. 
 
UNIVERSITY OF CALIFORNIA LIBRARY 
 
 THIS BOOK IS DUE ON THE LAST DATE 
 STAMPED BELOW 
 
 JVJl 
 
 MAR 4 1919 
 9 1919 
 
 30m-l,'15 
 
YB 46443 
 
 w