Gilbert 
 Light Experiments 
 
 FOR BOYS 
 
 BY 
 
 CARLETON JOHN LYNDE, Ph.D. 
 
 Professor of Physics 
 MacDONALD COLLEGE 
 QUEBEC PROVINCE, CANADA 
 
 Under the Direction of 
 
 ALFRED C. GILBERT 
 
 Yale University • 1909 
 PUBLISHED BY 
 
 THE A. C. GILBERT COMPANY 
 
 NEW HAVEN, CONN. 
 
 New York Chicago San Francisco Toronto London 
 
Copyrighted, 1920, by A. C. Gilbert 
 New Haven, Conn. 
 
 THE GETTY CENTER 
 LIBRARY 
 
Foreword 
 
 The things that are the most common are often the ones that 
 people know the least about. This is true of light. Few boys 
 have even taken the trouble to get the essential facts about this 
 subject. You can realize how important it is when you are 
 told that without the sun — our main source of light — there 
 would be no life at all. There would be no growth of plants, 
 that's sure. I know as a boy my curiosity was always prompt- 
 ing me to ask questions. I wanted to know the facts and reasons 
 for everything. I believe most boys are the same in that respect. 
 
 This book has been written so that you can get information 
 first hand on a mighty interesting subject. It is in plain language, 
 which you can readily understand, and a study of it will soon 
 make you familiar with great scientists, who have made laws 
 of great importance. Their discoveries make possible the use 
 of a number of instruments that you know so well. The telescope 
 - — the opera glass — the moving-picture machine — are just a few 
 which can be mentioned, and you know how necessary they are in 
 the world to-day. 
 
 In learning facts about light and some of the inventions made 
 by great men you will get a knowledge that very few boys 
 have. You will be able to talk very interestingly about light 
 and it will be easy for you to explain a great many questions 
 that come up in connection with it. Best of all, you will have 
 a whole pile of fun and at the same time get a good understand- 
 ing of the fundamentals of the science of light. 
 
 Sincerely yours, 
 
Digitized by the Internet Archive 
 in 2013 
 
 http://archive.org/details/gilbertlightexpeOOIynd 
 
GILBERT LIGHT EXPERIMENTS 
 
 FUN WITH BRIGHT SUNLIGHT 
 
 Experiment No. 1. To obtain more 
 than one million miles of sunshine. Go 
 outside with your watch in your hand 
 and stand in the sunlight (Fig. 1) for just 
 six seconds. In that short six seconds 
 you will have received more than 1,000,- 
 000 miles of sunlight. Light travels at 
 the enormous velocity of 186,000 miles 
 per second, and therefore in six seconds 
 you receive on your body 186,000 X 6 = 
 1,116,000 miles of sunlight. 
 
 Experiment No. 2. To receive over 
 two thousand million million light waves. 
 Hold your hand in a beam of sunlight 
 (Fig. 2) for six seconds and then with- 
 draw it. 
 
 Fig. 1. You receive 
 1,000,000 miles of sunshine 
 
 Fig. 2. You receive 2,000,- 
 000,000,000 light waves 
 
 You will show soon that white light 
 is a mixture of lights of all colors. Now 
 the red light waves in the sunlight fall 
 on your hand at the rate of 390 million 
 million per second and in six seconds 
 390 X 6 = 2340 million million fell on 
 your hand. Your hand received many 
 more than this because waves of all colors 
 fell on it, and of violet light alone it 
 received twice the above number. 
 
 Repeat this experiment with a candle 
 flame, or oil lamp, or electric light. In 
 each case your hand receives more than 
 two thousand million million light waves 
 in the six seconds. 
 
6 
 
 GILBERT BOY ENGINEERING 
 
 TO MAKE YOUR DARK ROOM 
 
 Boys, you are going to make 
 many experiments with a beam of 
 sunlight let into a darkened room, 
 so prepare for them now thor- 
 oughly, as follows : 
 
 Select in your home a room 
 with only one window, facing the 
 south or east or west. Now cover 
 this window so that no light can 
 enter the room except through a 
 small slit. The best way to do 
 
 Fig. 3. Shutter to darken room 
 
 this is to make a solid wooden shut- 
 ter, with tar paper on one side, to 
 cover the whole window and make 
 a slit in it three inches wide and 
 two inches high. If the window is 
 too large for this, make a wooden 
 shutter (Fig. 3) for the lower part 
 and cover the upper part with black 
 cloth, black paper, a quilt, a heavy 
 blanket (Fig. 4), or anything that 
 will shut out all light. Make the 
 slit in the wooden part because you 
 will want to change the size and 
 
 Fig. 4. Shutter in window with 
 other side showing and with card- 
 board tacked over slit 
 
GILBERT LIGHT EXPERIMENTS 
 
 7 
 
 shape of the slit. To clo this you will 
 make slits of the right kind in black 
 tar paper or cardboard and then tack 
 these over the slit in the wooden shut- 
 ter. Make the shutter so that you can 
 take it down and put it up quickly, be- 
 cause you will want to experiment 
 many times and your mother will, 
 probably, not want to leave the win- 
 dow permanently darkened. 
 
 TO MAKE A DARK BOX 
 
 Fig. 5. Your dark box 
 
 If you cannot make a dark room, 
 you can make a dark box (Fig. 5), as follows : Take a packing box 
 V/2 X V/ 2 ' X 2' or larger, bore a hole 2 T / 2 inches in diameter in 
 the center of one end, cover the open top with a piece of dark 
 light-proof cloth, 4' X 3', tacked to the ends and one side. Plait 
 this along the side to leave room for your head and shoulders. 
 Make the box light-proof, turn it on its side with sunlight enter- 
 ing the slit, and you are ready to make your experiments. 
 
 It improves the box to paint it black on the inside. When 
 you need a slit, cut it in cardboard and tack the cardboard over 
 the 2^-inch hole. This hole will just take your large ring lens 
 holder when you experiment with lenses. 
 
 A dark room is more fun than a dark box, and the directions 
 in this book assume that you have made one. You will find, how- 
 ever, that you can make most of the dark-room experiments in 
 the dark box. It is an excellent idea to have both. 
 
 Experiment No. 3. To show that you cannot see a beam of 
 light unless it falls on some object or directly on your eyes. 
 Darken the room on a day when the sun is shining in the win- 
 dow. Leave the room for five minutes to let the dust settle and 
 then return. Do you find that you cannot see the beam between 
 
8 
 
 GILBERT BOY ENGINEERING 
 
 the slit and a 
 screen or the wall 
 or floor? 
 
 Make a dust 
 near the beam by 
 shaking your coat 
 or a carpet. Do 
 you now appear to 
 see the beam (Fig. 
 6)? You do, be- 
 cause the dust par- 
 
 rig. 6. Dust makes the beam visible tides reflect light 
 
 to your eyes. 
 
 On a clear night you cannot see the beam from the headlight 
 of a locomotive ; but when there is mist, rain, or snow, you appear 
 to see it because particles of these reflect light to your eyes. In 
 either case you can see any object the beam falls on because it re- 
 
 Fig. 7. A beautiful inverted picture 
 From Appleton's School Physics, published by the American Booh Oo. 
 
GILBERT LIGHT EXPERIMENTS 
 
 9 
 
 fleets light to your eyes. You 
 can also see the light if it falls 
 directly on your eyes. 
 
 Experiment No. 4. To show 
 that light travels in a straight 
 line. Make a dust near the 
 beam. Does the light travel in 
 a straight line from the slit to 
 the wall or floor or paper 
 screen? 
 
 Experiment No. 5. To get Fig> 8> You see a beau tifui picture in 
 
 a picture of all OUt-of-doorS. natural colors 
 
 Punch a nail hole in a piece of black paper or cardboard, tack the 
 paper or cardboard over the slit in your darkened room, and hold 
 a sheet of tissue paper about one foot from the hole. Do you find 
 on the paper a picture (Figs. 7 and 8) of the whole view out-of- 
 doors opposite the hole? Is the picture inverted and in natural 
 colors? Can you see men, horses, and automobiles moving? 
 
 This is a fascinating experiment, and it shows best when the 
 sun is shining on the landscape and not on the window. 
 
 The picture is inverted because light travels in straight lines. 
 The sunlight which falls on any part of a cloud, for example, is 
 reflected in all directions in straight lines, and a very small part of 
 this light passes through the hole to the bottom of the tissue paper. 
 Also the sunlight which falls on any object on the ground is re- 
 flected in all directions in straight lines, and a very small part of 
 this passes through the hole to the top of the tissue paper, and so 
 on. That is, the picture is inverted because light travels in a 
 straight line from each object through the hole to the paper. 
 
 The picture is in natural colors because each object reflects 
 light of its own color and absorbs the remainder. That is, the 
 blue sky, green grass, and red bricks reflect blue, green, and red 
 light respectively, and so on. 
 
10 GILBERT BOY ENGINEERING 
 
 Move the paper farther from the hole. Is the picture larger 
 but dimmer? It is larger because the rays from different parts of 
 the view cross at the hole and diverge afterward. It is dimmer 
 because only a certain amount of light passes through the hole, 
 and it covers a larger area the farther the paper is from the hole. 
 Punch a second nail hole two inches from the first. Do you nowget 
 two pictures? Do they blur where they overlap? Punch many holes. 
 Do you get as many pictures, but do they blur more and more? 
 
 Open the slit to its full size. Do you find that there is no 
 picture at all, but just white light? There is no picture because 
 light from all parts of the view falls on all parts of the picture 
 and the combination of all colors produces white light. 
 
 Make a hole the size of a lead pencil in a new piece of black 
 paper and tack the paper over the slit. Do you get a brighter pic- 
 ture, but is it more indistinct than with the nail hole ? Remove the 
 tissue paper. Is there a picture on the opposite wall ? This will show 
 only if the sun is shining brightly on the view, and if your room is 
 completely dark except for the light which passes through the hole. 
 
 Experiment No. 6. To get a picture of the sun. Allow a beam 
 of sunlight to pass through a nail hole into your darkened room, 
 
 catch it on a piece of 
 paper and move the 
 paper back and forth. 
 Is the image round 
 (Fig. 9) and is it larger 
 the farther the paper is 
 from the hole? 
 
 The image is round 
 because the sun is 
 round. It increases in 
 size because the light 
 rays from the sun trav- 
 el in straight lines and 
 
GILBERT LIGHT EXPERIMENTS 11 
 
 cross at the hole. The 
 upper side of the sun 
 sends out light in all 
 directions in straight 
 lines ; a very small part 
 of this passes through 
 the hole (H, Fig. 11) 
 and makes an image of 
 itself at the bottom of 
 the picture ; similarly 
 light from the lower 
 side of the sun makes Fig 10 you gee two images of the sun 
 
 an image of itself at the 
 
 top of the picture, and so on. Reflect the light to the farthest part 
 of the room by means of a mirror. Is the image larger the farther 
 it is from the hole? Punch two holes. Do you get two images 
 (Fig. 10) ? Punch many holes. Do you get many images, but 
 do they overlap? 
 
 Open the slit entirely. Do you get only a bright spot in the 
 shape of the slit? This spot is made up of many, many round 
 images, and you will notice that the edges and corners are some- 
 what blurred and not sharp. 
 
 Take a new piece of black paper, make a triangular hole one- 
 quarter inch on a side, tack it over the slit and get an image of the 
 sun at one inch from the hole, then at greater and greater dis- 
 tances. Is the image at first triangular and does it become more 
 
 and more blurred at the 
 sides and corners until 
 finally it is round? The 
 image is made up of 
 many small, round im- 
 ages of the sun, and 
 
 Fig. 11. The rays cross at the nail hole when these are large 
 
12 
 
 GILBERT BOY ENGINEERING 
 
 compared to the size of the hole they 
 overlap and produce a round image. 
 
 Repeat with a square hole one- 
 quarter inch on each side. Are the re- 
 sults similar? 
 
 You have probably noticed that sun- 
 light produces round images of the sun 
 when it passes through any small open- 
 ing; for example, in a shutter or blind, 
 between the leaves of trees, and so on. 
 The explanation is that given above. 
 
 Experiment No. 7. To make a 
 pinhole camera. Make a nail hole 
 in the middle of the bottom of a 
 cardboard box, cover the open top with 
 a piece of 
 
 Fig. 12. The open side of the 
 box is covered with tissue paper 
 
 tissue paper 
 (Fig. 12), 
 hold the hole 
 
 toward a brightly lighted landscape, 
 cover your head and the tissue paper 
 with a black cloth or blanket to shut out 
 all the light (Fig. 13), and look at the 
 tissue paper. Do you see a beautiful 
 image of the landscape inverted and in 
 natural colors? 
 
 This is a beautiful experiment and 
 it is explained as above. 
 
 You can actually make pictures 
 through a pinhole, as follows : Remove 
 the lens from a camera, cover the open- 
 ing with heavy tin foil and pierce the 
 foil with a pin. Now to take the pic- 
 
 see a beautiful 
 picture on the paper 
 
GILBERT LIGHT EXPERIMENTS 13 
 
 Fig. 14. Spherical waves and 
 straight rays 
 
 ture, cover the pinhole, arrange the 
 plate or film in position, uncover the 
 pinhole for a short time, cover it, and 
 develop your negative as usual. 
 
 SOMETHING ABOUT LIGHT 
 
 Now, boys, before we go any further 
 let us get some clear ideas about light. 
 
 Light is that which produces on the 
 eyes the sensation of sight. 
 
 Medium. A medium is anything 
 through which light travels ; for example, air, water, glass, and 
 the ether. 
 
 Ether. The ether is supposed to be a very thin and elastic 
 medium which fills all space, not only the space between the 
 planets, but also the space between the smallest particles (mole- 
 cules) of solids, liquids, and gases. 
 
 How Light is Produced. Light is produced by the vibration 
 of very hot particles of matter. 
 
 For many reasons, scientists believe that the smallest particles 
 of all substances are vibrating, that is, moving back and forth in 
 all directions, all the time, and that the hotter they are the faster 
 they vibrate. Now in the flame of a candle, oil lamp, or gas jet 
 there are particles of unburned carbon which are very hot and are, 
 therefore, vibrating rapidly. These vibrating particles set the 
 ether in the flame in vibration, and these vibrations spread out 
 
 wwwwww wwww 
 
 Fig. 15. A beam of light. W = parallel waves. R = parallel rays 
 
14 GILBERT BOY ENGINEERING 
 
 in all directions in the form of spherical waves in the ether. These 
 ether waves are light waves or heat waves. 
 
 Similarly the light of an incandescent electric arc light or of 
 the sun is produced by rapidly vibrating hot particles of matter. 
 
 Note. Heat waves are longer than light waves and do not 
 produce the sensation of sight, but they are similar to light waves 
 in all other respects. 
 
 Waves and Rays. If the dot in the center of Fig. 14 is a rap- 
 idly vibrating particle, the circles about it will give the position of 
 
 Fig. 16. The lens produces a converging pencil and a diverging pencil 
 W = waves. R = rays 
 
 its light waves after equal intervals of time, but the light waves 
 are spherical instead of circular. The straight arrows drawn from 
 the center represent light rays. They give the path along which 
 the light is traveling in all directions from the center. The light 
 waves are real and produce the sensation of sight ; the rays are not 
 real, they are imaginary, straight lines which give the direction 
 of the light and they are always at right angles to the waves. 
 
 Parallel Waves and Rays. The waves from the dot are larger 
 the farther they are from the center, and when they are one hun- 
 dred yards or a mile from the center they are very large indeed. 
 If your eye receives light from any such distant point the small 
 part of the waves which enter it are nearly parallel straight lines, 
 
GILBERT LIGHT EXPERIMENTS 15 
 
 and since the rays are always at right 
 angles to the waves they are also nearly 
 parallel. This is particularly true if the 
 distant point is the sun, at a distance of 
 ninety million miles. Parallel waves and 
 rays then are those from a distant source. 
 Beam. Pencil. A beam (Fig. 15) 
 
 is a group of parallel waves and rays. A Fig. 17. The candle sends rays 
 
 •1 /t-« - n\ • r 1 in all directions 
 
 pencil (Fig. 16) IS a group Of waves and From Appleton's School Phys- 
 , . , ' . , ics. published by the American 
 
 rays which converge at a point or diverge Book Co. 
 
 from it. The eyes (Fig. 17) are receiving diverging pencils of 
 
 light from the candle which is sending out light in all directions. 
 
 Luminous and Non-luminous Bodies. Luminous bodies are 
 those which give out light, such as the sun, electric light, gas jet, 
 oil lamp, candle, and match. Non-luminous bodies are those 
 which do not give out light, and which can be seen only by 
 means of light from luminous bodies. 
 
 Transparent, Translucent, and Opaque Bodies. Bodies which 
 you can see through are called transparent; such as air, water, 
 and glass. Bodies which let light through, but which you cannot 
 see through, are called translucent; as paper, ground glass, and 
 cotton cloth. Bodies which do not transmit light are called 
 
 opaque; for example, wood, 
 brick, and metal. 
 
 No substance is entirely 
 opaque; for example, even 
 metals let through some light 
 when they are in very thin 
 sheets. 
 
 Straight Lines. Light al- 
 ways travels in straight lines 
 from its source until it falls 
 Fig. 18. Unburned carbon in a flame on some object or Until it 
 
16 
 
 GILBERT BOY ENGINEERING 
 
 passes into another me 
 dium. It then changes I 
 direction but again | 
 travels in straight lines I 
 from the object or in j 
 the new medium. 
 
 Experiment No. 8. 
 Carbon particles in a 
 flame. Hold a saucer 
 in a candle flame (Fig. 
 18). Is it blackened? 
 The blackening is caused by the unburned carbon particles. 
 
 Experiment No. 9. Water waves. Look along the surface of 
 water in a pan and dip a pencil in and out. Do you observe 
 circular waves (Fig. 19) ? Throw a stone into a still pond or 
 lake. Do you see circular waves? These illustrate light waves, 
 but the light waves are spherical. 
 
 Fig. 19. Circular waves on water 
 
 FUN AT NIGHT 
 
 Experiment No. 10. How you see things. Boys, when you 
 have attended movie shows where they have animated cartoons 
 you have perhaps seen a dotted line move from the eye of the 
 hero (or villain) to the object he is looking at. You might think 
 from this that you see 
 an object by means of 
 light which goes from 
 your eye to it. This 
 is not the case, how- 
 ever, as you will now 
 prove. 
 
 Take an unlighted 
 candle into an abso- 
 lutely dark room and Fig. 20. You see the book by reflected light 
 
 K — 1 
 
GILBERT LIGHT EXPERIMENTS 17 
 
 look around. Can you 
 see anything? You can- 
 not because all the ob- 
 jects in the room are 
 non - luminous, includ- 
 ing yourself and your 
 eyes. This proves that 
 you do not see things 
 by means of light which 
 goes from your eye to 
 
 the thing you are look- Fig - 21 - Li S nt travels in a straight line to your eye 
 
 . ing at. Now light the candle. The flame is a luminous body and 
 you see it by means of light which goes from it to your eye. 
 
 Can you now see the non-luminous bodies ? You can because 
 light travels from the candle flame to these objects and from them 
 to your eye (Fig. 20). 
 
 You have proved here that you see any object by means of 
 light which travels from it to your eye and not the reverse. 
 
 Experiment No. 11. To show that light, when not reflected or 
 refracted, travels in a straight line from the object to your eye. 
 
 Note. The thing you are 
 looking at directly or indi- 
 rectly is called the object. 
 
 Close one eye, look at 
 the flame of a candle, and 
 then move a book slowly be- 
 tween the flame and your 
 eye (Fig. 21). Do you find 
 that you cannot see the 
 flame when the book has 
 crossed the straight line be- 
 tween the flame and your 
 Fig. 22. Light travels in a straight line eye ? This proves that the 
 K — 2 
 
18 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 
 
 23. You see an inverted image of the 
 candle 
 
 light from the flame 
 travels in a straight line 
 to your eye. 
 
 Close one eye and look 
 at any part of some other 
 object, then again move 
 the book across the 
 straight line from the 
 part to your eye. Do 
 you find again that you 
 cannot see the part when 
 the book has crossed 
 the straight line between the part and your eye? 
 
 Cut three pieces of cardboard about 5"X3", punch a small hole 
 in each at the same height, stand them upright on the table, place 
 a candle flame in front of an end hole, and look at the flame 
 through the three holes (Fig. 22). Shift the cardboards one at 
 a time. Do you find that you can see the flame only when the 
 holes are in a straight line? 
 
 Note. You will show later that 
 light is bent out of the straight line 
 when it is reflected from a mirror and 
 when it is refracted in passing from 
 air to water, air to glass, and so on. 
 
 You have shown here that light, 
 which is not reflected or refracted, 
 travels in a straight line from the 
 object to your eye. 
 
 Experiment No. 12. Picture of a 
 pandle flame. Punch a nail hole in 
 card C and arrange as shown in Fig. 
 23. Do you see an inverted image of 
 the flame and is it larger the farther Fig. 24. Flash-light telegraphing 
 
GILBERT LIGHT EXPERIMENTS 19 
 
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 GILBERT BOY ENGINEERING 
 
 D is from C? Make the hole larger. 
 Is the image brighter but more 
 blurred? Can you explain these 
 facts as in Experiments 5 and 6 and 
 as illustrated in Figs. 11 and 23? 
 
 Experiment No. 13. Flash-light 
 telegraphing. You can telegraph 
 to your friends at night by means of 
 flash-light signals (Fig. 24) and the 
 Morse code. Use a short flash for 
 a dot and a long flash for a dash. 
 For the Morse code see page 19. 
 Experiment No. 14. Telegraph- 
 Fig. 25. Light telegraphing ing with a candle or lamp. Arrange 
 
 the apparatus as shown in Fig. 25 
 and have a friend make a similar arrangement in a window facing 
 you. You can then telegraph by means of the Morse code. Un- 
 cover the light for a short time to produce a dot and for a longer 
 time for a dash. 
 
 INTENSITY OF LIGHT 
 
 If you hold a book 1 foot from a lighted candle, it receives a 
 certain amount of light ; if you hold it 2 feet from the candle, it 
 receives only one-fourth as much light ; if you hold it 3 feet from 
 the candle, it receives only one-ninth as much light, and so on. 
 That is, the intensity 
 of the light on any ob- 
 ject varies inversely as 
 the square of the dis- 
 tance between the ob- 
 ject and the source of 
 light. 
 
 In Fig. 26 the light 
 
 Fig. 26. The light which covers one square at one 
 foot covers four and nine at two and three feet, 
 and is then only one-fourth and one-ninth as 
 intense 
 
GILBERT LIGHT EXPERIMENTS 21 
 
 which would cover 1 square 
 at 1 foot would cover 4 
 and 9 equal squares at 2 
 and 3 feet and would there- 
 fore be one-fourth and one- 
 ninth as intense. 
 
 Experiment No. 15. To 
 prove the law of intensity. 
 Screen A, Fig. 27, has a 
 hole just 1 inch square and 
 screen B a square 3 inches 
 on each side, divided into 9 
 
 Fig. 
 
 28. The spot is dark by re- 
 fleeted light 
 
 Fig. 27. The light is one-ninth as strong on 
 B as on A when B is 3 feet from the candle 
 
 square inches. Place A 1 foot from 
 the candle and B 2 feet. Does the 
 light which passes through 1 
 square inch in A cover 4 square 
 inches on B? Is it, therefore, only 
 one-fourth as intense on B as it is 
 on A? Place B 3 feet from the 
 candle. Does the light now cover 
 9 square inches? Is it, therefore, 
 only one-ninth as intense? This 
 proves that the intensity of light 
 varies inversely as the square of 
 the distance. 
 
 Experiment No. 16. A greased 
 spot. Rub a small piece of butter 
 on the center of a piece of paper 
 and melt it. 
 
 Look at the spot by reflected 
 light (Fig. 28). Is the spot dark? 
 Look at it by transmitted light 
 
22 
 
 GILBERT BOY ENGINEERING 
 
 (Fig.29). Is it bright? It is darker 
 than the paper in the first case and 
 brighter in the second, because more 
 light goes through the greased spot 
 than through the paper. 
 
 Experiment No. 17. Four can- 
 dles against one. Put four candles 
 2 feet from the greased-spot screen 
 and one candle 1 foot on the other 
 side (Fig. 30). Trim the wicks 
 until the flames are of equal size. 
 
 Is the greased spot as bright 
 as the paper? It is, because the 
 light which goes through from one 
 side is exactly equal to that which 
 goes through from the other, that 
 is, the one candle throws as much 
 light on the screen as do the four 
 candles. This is explained by the 
 
 Fig. 29. The spot is bright by trans- 
 mitted light 
 
 law of intensity men- 
 tioned above. Each of 
 the four candles is 2 feet 
 from the screen and 
 therefore throws just 
 one-fourth as much light 
 on the screen as one 
 candle does at 1 foot. 
 
 Experiment No. 18. 
 Candle power of a 
 lamp. The candle 
 power of a lamp is the 
 
 Fig. 30. 
 
 One candle at one foot is equal to four at 
 two feet 
 
GILBERT LIGHT EXPERIMENTS 23 
 
 number of times greater 
 or less its light is than that 
 given by a standard candle. 
 
 Put the greased - spot 
 screen justlfoot fromacan- 
 dle (Fig. 31) and move the 
 lamp until the greased spot 
 is as bright as the paper. 
 Measure the distance from 
 the lamp to the screen. If 
 this is 2 feet, the lamp is Fig - 81 - mn ^ s the candle power of the lamp 
 4-candle power; if 3 feet, 9-candle power; 4 feet, 16-candle power; 
 and so on. This follows from the law of intensity of light. 
 
 The instruments used to measure the candle power of lamps 
 are called photometers, and the one you have here illustrated is 
 named after its inventor, Bunsen's photometer (Fig. 32). A is the 
 greased-spot screen, B the standard candle, and C the light tested. 
 
 Experiment No. 19. Four against one. Place a candle 1 foot 
 from a screen and four equal candles just 2 feet from the screen, 
 and place an object in front of the screen as in Fig. 33. Are the 
 shadows of equal darkness? 
 
 If the candle flames are all of the same size, the shadows are 
 equally dark because each light illuminates the shadow produced 
 by the other and because the candle at 1 foot sends as much light 
 
 to the screen as 
 the four candles 
 do at 2 feet from 
 the screen. This 
 is explained by the 
 law of intensity. 
 
 E x p e r i ment 
 No. 20. Shadow 
 
 Fig. 32, Bunsen's photometer photometer. Put a 
 
24 
 
 GILBERT BOY ENGINEERING 
 
 candle 1 foot from a screen 
 (see Fig. 34, illustrating shad- 
 ow photometer), stand a pen- 
 cil in front of the screen, and 
 move the lamp back and forth 
 until the two shadows are 
 equally intense. Measure the 
 distance from the screen to 
 the lamp. If this is 2, 3, 4, or 
 A}/2 feet, the candle power of 
 the lamp is 4, 9, 16, or 20%, 
 and so on. 
 
 Each light illuminates the 
 shadow cast by the other and 
 therefore when the shadows are equal the lights are equal, and the 
 candle power is calculated as above from the law of intensity of 
 light. This illustrates the shadow photometer. 
 
 Fig. 33. One candle equal to four 
 From Millihan and Gale's First Course in 
 Physics, Ginn & Co. 
 
 SHADOWS 
 
 Experiment No. 21. 
 
 Enlarging shadow. 
 Move a pencil from the 
 screen toward the 
 candle (Fig. 35). Does 
 the shadow increase in 
 size? It does, because 
 light goes in all direc- 
 tions in straight lines 
 from the flame, and the 
 pencil intercepts more 
 light the nearer it is to 
 the flame. 
 
 Fig. 34. Illustrating the shadow photometer 
 
GILBERT LIGHT EXPERIMENTS 25 
 
 SHADOW ENTER- 
 TAINMENTS 
 
 Boys, you can have 
 the greatest kind of fun 
 bygiving shadow shows 
 to your friends, and the 
 
 Fig. 35. Shadows 
 
 preparation you need is very slight. 
 Hang a sheet over a folding doorway 
 as shown in Fig. 36. 
 
 Now opposite the door put a strong 
 lamp on a stool, chair, or table, accord- 
 ing to the show (Fig, 37). The audi- 
 ence is in darkness on the other side 
 of the screen. 
 
 Show 1. The Dentist. Dentist 
 
 Fig. 36. A sheet over a 
 doorway- 
 
 Seated, bell rings, boy comes in 
 with bandaged head, dentist 
 seats him and examines tooth, 
 boy howls, dentist takes very, 
 very large pliers, and pulls out 
 a very large cardboard tooth 
 (Fig. 38). The tooth, of course, 
 was stuck in boy's coat collar 
 
 Fig. 37. Showing lamp on a chair 
 
26 GILBERT BOY ENGINEERING 
 
 Fig. 38. The dentist 
 
 heart, all with much panto- 
 mime, takes large coal tongs, 
 shoves them down boy's throat 
 (apparently, of course), and 
 
 at one side. Use much panto- 
 mime all through the show. 
 
 Show 2. The Doctor. Doctor 
 seated with very large plug hat 
 and very long beard, bell rings, 
 boy enters rubbing stomach and 
 groaning. Doctor seats him, takes 
 pulse, examines tongue, listens to 
 
 Fig. 40. How it is done 
 
 Fig. 3U. The doctor 
 
 pulls up a long snake (Fig. 39). 
 More pantomime. Boy not yet 
 well, doctor again shoves tongs 
 down his throat and pulls up an 
 alligator, and so on. Much panto- 
 mime of boy feeling fine. 
 
 The snake and alligator are cut 
 out of stiff paper or cardboard and 
 are handed up by a third boy as 
 shown in Fig. 40, 
 
GILBERT LIGHT EXPERIMENTS 27 
 
 Show 3. A Surgical Case. 
 
 Scene 1. A boy is seated at 
 a table with large plate of pota- 
 toes. He swallows them whole, 
 then swallows knife, fork, spoon, 
 saltcellar, and so on. Much pan- 
 tomimeof enjoyingagood meal. 
 
 Scene 2. Doctor seated at 
 table, boy rushes in rubbing 
 stomach, doctor lays him on 
 table, takes large knife, jabs it 
 
 Fig. 42. Quieting the patient 
 
 ing fine, shakes hands with 
 doctor and thanks him. 
 
 The knife and axe are cut 
 out of cardboard, the plug hat 
 is a tube of stiff paper on an or- 
 dinary hat, the whiskers are an- 
 other tube of paper. The boy 
 swallowing potatoes really 
 hands them to another boy hid- 
 den beside the chair. 
 
 Show 4. A Boxing Match. 
 
 Fig. 41. A surgical case 
 
 into boy's stomach (Fig. 41), 
 boy raises head to object, doc- 
 tor hits him on head with hatch- 
 et (Fig. 42) and proceeds to cut 
 him open, throws back coat 
 to imitate opening stomach, 
 and takes out all the potatoes, 
 knife, fork, spoon, and so on. 
 Doctor sews boy up, hits him 
 on head with hatchet, boy 
 comes to, pantomime of feel- 
 
 Fig. 43. A boxing match 
 
28 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 44. Living shadows 
 
 Put one boy near the 
 screen and another 
 nearer the light. The 
 first is natural size, 
 the second is enor- 
 mous (Fig. 43). If they 
 now pretend to fight 
 it is very, very funny 
 from the audience. In 
 one of the fights, have 
 the lamp on the stool, 
 let the little fellow 
 beat the big fellow, 
 and if the big fellow finally runs away and Steps over the lamp 
 to the chair it looks as though he had jumped into the ceiling. 
 Little fellow then struts around as winner. 
 
 Show 5. Living Shadows. Cover a mirror with two pieces 
 of paper, out of each of which you have cut identical eyes, 
 nose, and mouth with teeth, as shown in Fig. 44. Paste 
 the under paper against the mirror, but paste the outer paper 
 only at the top. Arrange the light and boy as shown and sway 
 the outer paper back and forth. Do you see goggling eyes and 
 snapping mouth? 
 
 Now have the boy, whose shadow is shown, make a speech 
 with proper gestures, while you sway the paper. The effect 
 will be extremely amusing to the spectators. 
 
 Show 6. Living Shadow Dialogue. Arrange two mirrors 
 as above and place one on each side of the screen. Have the 
 two shadows carry on a dialogue while two other boys sway 
 the papers. 
 
 You will have plenty of fun inventing shows of your own, 
 and with a few beards, mustaches, and false noses made of paper 
 or of other material you can have very, very funny times. 
 
GILBERT LIGHT EXPERIMENTS 29 
 
 Fig. 45. The angle 
 of reflection R ia 
 equal to the angle 
 of incidence I 
 
 REFLECTION OF LIGHT 
 The Law of Reflection: Angle of reflection equals angle of 
 incidence. If a beam of sunlight is allowed to fall on a mirror 
 and the beam before and after reflection is made visible by dust 
 in the air, it is found that the beams make equal angles with a 
 ruler held perpendicular to the mirror, and 
 that they are in the same plane. The beam 
 I, Fig. 45, which strikes the mirror is called 
 the incident beam, and the beam R which is 
 reflected is called the reflected beam. The 
 angle i which the incident beam makes with 
 the perpendicular PN is called the angle of 
 
 incidence, and the angle r which the reflected beam makes with 
 the perpendicular is called the angle of reflection. This experi- 
 ment illustrates the Law of Reflection, which is : The angle of 
 reflection equals the angle of incidence and the reflected and 
 incident beams are in the same plane. 
 
 FUN WITH SUNLIGHT 
 Experiment No. 22. To prove the law of reflection. Allow 
 a small beam of sunlight to pass through the slit of your shutter 
 
 and fall on a mirror 
 placed on the floor 
 or table of your 
 darkened room. 
 Make sufficient dust 
 to show the sunlight. 
 Is the sunlight re- 
 flected and does it 
 make a bright spot 
 on the ceiling or op- 
 posite wall? 
 
 Fig. 46. The beams make equal angles with , T , , , , 
 
 your ruler JNow hold a ruler 
 
30 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 47. The beams are in the same plane 
 
 perpendicular to the 
 mirror opposite the 
 spot where the light 
 strikes the mirror (Fig. 
 46). Do you find that 
 the angle between the 
 reflected beam and the 
 ruler is equal to the 
 angle between the in- 
 cident beam and the 
 ruler? 
 
 Hold a sheet of 
 cardboard edge-wise to the incident and reflected beams in such a 
 position that the incident beam is split in two (Fig. 47). Is the 
 reflected beam also split in two? That is, are the reflected and 
 incident beams in 
 the same plane? 
 
 You have here 
 proved the law of 
 reflection. 
 
 Experiment No. 23. 
 Irregular reflection. 
 Let the beam fall on 
 a piece of white un- 
 glazed paper (Fig. 
 48). Is there no re- 
 flected beam ; is the 
 
 light reflected in all directions and does it make 
 everything around it brighter? The light is 
 reflected in all directions because the surface 
 
 riS ^?flection gUlar is rOU g h ( See Fi §-' 49 )' 
 
 fngh 8chooi n phy8- You see a11 non ^ m ' m ous objects by means 
 %? & VSuS of % ht which the y reflect irregularly. 
 
 Fig. 48. There is no reflected beam from rough paper 
 
GILBERT LIGHT EXPERIMENTS 31 
 
 Experiment No. 24. Twice the 
 angle. Hold the mirror perpen- 
 dicular to the beam. Is the beam 
 reflected back to the slit? Now 
 turn the mirror to an angle of 45° 
 to the beam. Is the reflected beam 
 turned through an angle of 90°? 
 That is, is the reflected beam 
 turned through twice the angle 
 the mirror turns? Try other an- 
 gles. The reflected beam turns 
 through twice the angle because 
 the angles of incidence and reflec- 
 tion are equal and each is equal 
 to the angle through which the 
 mirror turns, therefore, together 
 they are equal to twice this angle. 
 
 Fig. 50. HeliograpMng 
 
 FUN BY DAY WITH ONE MIRROR 
 Experiment No. 25. The heliograph. Reflect sunlight from 
 your window (Fig. 50) to a distant building, and have your friend 
 reflect sunlight from near this building to your window. 
 
 Now send a message to your friend by the Morse code. Un- 
 cover your mirror for a short time for a dot and for a longer 
 time for a dash. He reads the message on the wall of the build- 
 ing. He replies and you read the message on the inside wall of 
 your room. This is the principle of the heliograph used for 
 military signaling. 
 
 Experiment No. 26. Height of any point on a building. Drive 
 one end of a straight stick into the ground and make the stick 
 exactly vertical. Place the mirror B, Fig. 51, beside it flat on the 
 ground and adjust until the stick and its image are in a straight 
 line ; the mirror is then exactly horizontal. 
 
32 
 
 GILBERT BOY ENGINEERING 
 
 J 
 
 Fig. 51. You find the height of the 
 window by reflection 
 
 Now if you want to find the 
 height of the topmost window, for 
 example, stand back until you can 
 just see the top of the window, 
 then measure : your distance BC 
 from the mirror ; the distance BE 
 from the mirror to the building; 
 and your height CA from sole to 
 eye. 
 
 The triangle ABC which you 
 make with the mirror is similar to 
 the triangle DBE which the top of 
 the window makes (Fig. 51), that 
 is, they have equal angles, there- 
 fore, 
 
 DE AC 
 
 EB 
 
 CB 
 
 Examples. If you are 12 feet 
 from the mirror, your height to 
 your eye is 5 feet, and the mirror 
 
 is 120 feet from the building at the ground level, then 
 Height of window 5 
 
 12 
 
 Height of window 
 
 120 
 5 X 120 
 
 12 
 
 50 feet 
 
 FUN BY DAY OR NIGHT WITH ONE MIRROR 
 
 Experiment No. 27. An object and its image. Look at yourself 
 in a vertical mirror and move back and forth. Does your image 
 always appear to be as far behind the mirror as you are in front ? 
 
GILBERT LIGHT EXPERIMENTS 33 
 
 Fig. 52. 
 
 The candles are at equal distances from your 
 mirror 
 
 Arrange the 
 window glass ver- 
 tically, place a can- 
 dle in front and an- 
 other behind (Fig. 
 
 52) , and make the 
 rear candle coin- 
 cide with the image 
 of the front candle. 
 Measure the dis- 
 tance from each 
 candle to the mir- 
 ror. Are they ex- 
 actly equal? 
 
 Draw a line on a piece of paper and call it the mirror line (Fig. 
 
 53) . Draw three lines across it, and perpendicular to it, at 2-inch 
 intervals. Place the window glass vertically on the mirror line, place 
 
 the front candle on each perpendicular 
 in turn. Is its image candle always at 
 the same distance from the mirror? 
 
 You have proved here that an 
 image is always the same distance 
 behind the mirror that the object 
 is in front; and that it is on 
 pendicular drawn from the 
 across the mirror line. 
 Experiment No. 28. Slanting object. 
 Place a pencil in front of the vertical 
 window glass (Fig. 54), and slanting 
 toward the glass. Make a second pencil 
 of equal length coincide with the image. 
 Does the image also slant toward the 
 mirror? It does, because each part of 
 
 Fig. 53. The candles are on 
 the same perpendicular 
 
 a per- 
 object 
 
 Fig. 
 
 54. Both pencils slant 
 toward your mirror 
 
 K — 3 
 
34 GILBERT BOY ENGINEERING 
 
 the image is as far be- 
 hind as the corre- 
 sponding part of the 
 object is in front, and 
 both are on a line per- 
 pendicular to the mir- 
 ror line. 
 
 Experiment No. 29. 
 To copy a drawing. 
 Arrange drawing, ver- 
 tical window glass, 
 
 Fig. 55. You copy a drawing easily book, paper, and light, 
 
 as shown in Fig. 55. Do you find it easy to copy the drawing? 
 Why is the drawing reversed? 
 
 WHY THE IMAGE IS AS FAR BEHIND THE MIRROR 
 AS THE OBJECT IS IN FRONT 
 
 We will explain this first by means of rays and then by means 
 of waves, but you must remember that what you actually receive 
 in your eyes is light waves and not rays. The rays are only im- 
 aginary lines which show the direction the waves are moving. 
 
 In Fig. 56 the eye sees image B, but the light, of course, goes 
 from A to the mirror and is reflected to the eye. The angles r 
 and i are equal by the law of reflection ; also, since CD and AB 
 are parallel, angle A is equal 
 to i and angle B is equal to r. 
 Therefore angles A and B are 
 equal ; also the angles at F are 
 equal, since they are right an- 
 gles. The triangles CFA and 
 CFB then have two angles 
 equal and a side CF in com- 
 
 2* 
 
 mon, therefore they are equal Fig . 5 e. The ray appears to come from b 
 
GILBERT LIGHT EXPERIMENTS 35 
 
 Fig. 57. The waves appear to come from B 
 
 and FB is equal to AF. This 
 shows that the image is as far 
 behind the mirror as the object 
 is in front because by the law 
 of reflection r is equal to i. 
 
 We will now explain by 
 means of light waves why the 
 image and objects are at equal 
 distances from the mirror, as 
 follows : 
 
 The waves of light from the 
 candle A strike the mirror as in 
 1, Fig. 57, and are reflected as 
 
 in 2. The curvature, of the waves is exactly reversed by reflec- 
 tion. The eye estimates the distance of an object partly by the 
 curvature of the waves which enter it, and the image appears at 
 B as far behind the mirror as the object at A is in front, because 
 the reflected waves which enter the eye have exactly the curva- 
 ture they would have if the mirror were absent and the object 
 were at B. 
 
 We see, then, that the image of A is at an equal distance B 
 because the waves from A are reversed by the mirror but un- 
 altered in any other 
 way. 
 
 Experiment No. 30. 
 
 To illustrate reflected 
 waves. Fill with wa- 
 ter a cake tin with flat 
 sides (Fig. 58), place 
 it near a good light, 
 dip a pencil in, two 
 inches from one side. 
 Fig. 58. You see circular waves reflected Is the reflected wave 
 
 . . 
 
 4k 
 
 ■f : 
 
 4 
 
 ■ppp 
 
 ':. : ;.-..":-V,/ 
 
 
 
 
 
 % , ~~ 'J 
 
36 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 59. 
 
 Your right hand is an image of your left 
 and vice versa 
 
 curved in the opposite 
 direction to the orig- 
 inal wave? Is it 
 larger and is it the 
 size it would be if it 
 came from a point at 
 an equal distance out- 
 side the pan? This is 
 what happens when a 
 light wave strikes a 
 mirror. 
 
 Experiment No. 31. 
 
 Why your image is reversed. Hold your two hands in front of 
 you (Fig. 59). Is one a reversed image of the other? 
 
 Hold your left hand in front of a mirror (Fig. 60). Does it 
 appear to be your right hand? It does, because each point of 
 the image is the same distance behind the mirror that the corre- 
 sponding part of the left hand is in front. 
 
 Experiment No. 32. Reversed words. Print such words as 
 STAR, STUN, and TOP on paper and look at their images 
 (Fig. 61). Are they reversed? Why? 
 
 Experiment No. 33. 
 Reading a blotting pa- 
 per. Write a sentence 
 in ink, blot it on fresh 
 blotting paper. Try 
 to read it. It is hard. 
 Read it in the mirror 
 (Fig. 62). Is it easy? 
 Why? 
 
 Experiment No. 34. 
 
 To see behind you. 
 TT ^ . . Fig. 60. Your left hand appears to be the image 
 
 Hold the mirror in of the right 
 
GILBERT LIGHT EXPERIMENTS 37 
 
 front of you as in Fig. 63. 
 Can you see behind you 
 with ease? Why? 
 
 Experiment No. 35. 
 To see around a corner. 
 Hold a mirror as in 
 Fig. 64. Can you see 
 around a corner with 
 ease? Why? 
 
 EXPERIMENTAL 
 
 Fig. 61. You see a different word 
 
 MAGIC 
 
 Experiment No. 36. A candle burning in a glass of water. 
 Place candle in front of window glass and glass of water behind it, 
 as in Fig. 65. Does the candle appear to burn in the water? 
 
 Experiment No. 37. Phantom candle in boy's head. Arrange 
 apparatus as in Fig. 66. Does the candle appear to burn in the 
 boy's head? 
 
 Experiment No. 38. Phantom flame. Arrange the apparatus 
 as in Fig. 67 and hold your hand behind in the image of the flame. 
 Can you do this quite safely? 
 
 Experiment No. 39. To make magical transformations. 
 
 Arrange the appa- 
 ratus as in Figs. 68 
 and 69. Place the 
 lighted candle in po- 
 sition 1 and adjust 
 block A until it coin- 
 cides with the image 
 of block B. 
 
 Now to prepare a 
 transformation, fold 
 
 Fig. 62. You read a blotting paper easily a sheet of paper once 
 
38 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 63. You see behind your back 
 
 over at the middle, 
 and on the top half 
 make a drawing of a 
 man. Make it with a 
 lead pencil and bear 
 down so as to crease 
 the lower half. Now 
 draw on the lower 
 half inside the creases 
 the skeleton of a man. 
 Tear the halves apart, 
 attach the man to B 
 and the skeleton to A, 
 and adjust until the 
 man seems to turn into a skeleton when you move the candle 
 from position 1 to position 2. 
 
 Now darken the room and ask a friend to look into the glass, 
 as in Fig. 69, while you move the candle from 1 to 2. He will be 
 much mystified. 
 
 Use photos of the same size, one of a boy and the other of a 
 girl, and make the 
 transformation. Make 
 many other trans- 
 formations. 
 
 You may have seen 
 similar magical trans- 
 formations at 
 the theater or in side 
 shows. 
 
 Experiment No. 40. 
 A magic box. Make 
 out of stiff cardboard 
 
 a square box 24 inches Fig. 64. You see around a corner 
 
GILBERT LIGHT EXPERIMENTS 39 
 
 Fig. 65. You see the candle burning in water 
 
 long and 3^4 inches 
 wide and high. Cut 
 the box into two equal 
 parts at an angle of 
 45° (see 1, Fig. 70), 
 turn the halves as in 
 2, cut a hole in one 
 half, its center being 
 2 inches from the end. 
 Cut two trapdoors 
 and attach to each a 
 loop of cord for open- 
 ing it. Now insert the 
 window glass at the division, see 3. 
 
 Now to have fun with your friends, hide the box behind a 
 large sheet of paper or cardboard and have your friends look 
 through a hole in the paper or cardboard into the hole in 
 the box. 
 
 Have the box in a good light and have different objects be- 
 neath the trapdoors. If you now open one trapdoor, the object 
 beneath is seen by your friends. If now you close the first door 
 and open the second, the object will appear to be transformed 
 
 into the other ob- 
 ject. 
 
 Use an empty 
 glass and a glass half 
 full of milk and you 
 can make the tumbler 
 empty and fill at 
 will. 
 
 You can make 
 many other very fun- 
 ny transformations. 
 
 Fig. 66. You see the candle burning in the boy's head 
 
40 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 
 
 to 
 
 FUN AT NIGHT 
 
 Hypnotism 
 
 Boys, you can put on imitation 
 hypnotism shows which will mystify 
 and amuse your friends, as follows : 
 Get a piece of window glass 
 2 feet by 3 feet, or larger. Put it in 
 a box (Figs. 71 and 72) at an angle 
 of 45° to the length of the box and 
 exactly vertical. Paint the inside of 
 the box black. The box is open to- 
 ward the audience, also at the op- 
 posite end, Bl, B2, B3, and at the 
 side, Al, A2, A3. A strong light 1 
 is placed at the side and throws light 
 on a person sitting opposite Al, A2, 
 or A3, but not on the glass. A 
 weaker light 2 is placed so that 
 it will throw light on a person 
 seated opposite Bl, B2, or B3, t 
 but not on the glass. When 
 light 1 is up, a boy seated oppo- 
 site Al, A2, or A3 will be seen 
 by the audience by reflected 
 light as seated at Bl, B2, or B3. 
 When light 2 is up. a boy seated 
 at Bl, B2, or B3 will be seen at 
 Bl, B2, or B3 by direct light. 
 The, direct image is brighter 
 than the reflected, and to cut it 
 down tack two or three layers 
 of green or black mosquito 
 
 67. The candle appears 
 burn in your hand 
 
 SKELETON 
 
 WINDOW GLASS 
 
 Fig. 68. Transformations 
 
GILBERT LIGHT EXPERIMENTS 41 
 
 netting over the B 
 opening*. Use black 
 backgrounds behind A 
 and B; this is impor- 
 tant. 
 
 You should always 
 try out your appa- 
 ratus just before you 
 use it. Look at the 
 window glass from Fig 69 How you arrange glass> book> pictures, 
 the audience side, have and candles 
 
 both lamps lighted, and the boy A will appear to be seated beside 
 boy B. If A appears too low, tilt the glass toward A ; if he ap- 
 pears too high, tilt the glass toward B. If the glass is exactly 
 vertical, A and B will be at exactly the same height. 
 
 Fig. 70. How to make a magic box 
 From Magical Experiments, published by David McKay Co., Philadelphia 
 
42 
 
 GILBERT BOY ENGINEERING 
 
 Now have an assistant turn down lamp 1 and A will dis- 
 appear, turn up lamp 1 and A appears again. Also turn light 
 2 down and up and B disappears and reappears. 
 
 These shows need five boys. One is the hypnotist, who 
 stands out in front and gives the patter talk to the audience. 
 He can wear a dress suit, mustache, tall hat, and so on, if he 
 desires. Two boys are needed as the hypnotism subjects, who 
 occupy chairs A and B, as required. The remaining two boys 
 are needed to operate the lamps. 
 
 You will invent all kinds of shows for yourselves and make 
 up your own patter talk, but we suggest a few that will make 
 plenty of fun. 
 
 Show 1. To hypnotize a boy at a distance of ten feet and to 
 make him disappear and reappear. 
 
 THE BOX IS BLACK ON THE INSIDE - 
 
 WINDOW I 
 
 STRONG UCHT 
 
 Fig. 71. Your hypnotism box 
 
GILBERT LIGHT EXPERIMENTS 43 
 
 TABLE 
 
 Fig. 72. Plan of hypnotism apparatus 
 
 Patter. "Ladies and Gentlemen, I have much pleasure in 
 announcing that I will to-night give you an exhibition of hyp- 
 notic power more wonderful than anything you have yet seen. 
 It may surprise those of you who know me to learn that I have 
 been studying hypnotism for years, and it will surprise you still 
 more to learn that I have discovered a secret source of hypnotic 
 power of immensely greater strength than any such power dis- 
 covered up to this time. Former hypnotists have hypnotized 
 their subjects at a distance of one or two feet ; but I dare not do 
 this because my power is so great that it might injure the subject. 
 I will do all my work at a distance of ten feet, which I have found 
 to be a perfectly safe distance. I do not care to explain my power 
 any more than to say that I have in the cabinet powerful bodies 
 
GILBERT BOY ENGINEERING 
 
 which bathe my subjects in electro-magnetic forces and help 
 me in my work." (Note. This is perfectly true because lamps 
 give out light and light is electro-magnetic in nature ; of course, 
 do not tell the audience this.) "Now without further talk 'I will 
 give an exhibition of my hypnotic power by hypnotizing Charles 
 at a distance of ten feet." (Raises curtain.) 
 
 Note. Charles is seated at center position, B2 (see Fig. 72). 
 Light 2 is turned up and 1 is turned down. Chair at side is in 
 position A2. 
 
 Hypnotist : "Charles, attention !" (Makes passes with hands 
 slowly and says, "Sleep ! Sleep ! Sleep !" slowly. Charles shivers, 
 gets rigid, and slowly closes eyes.) 
 
 Hypnotist: "Now, Ladies and Gentlemen, former hypnotists 
 have made their subjects do such simple things as rise in the air, 
 remain floating in the air, and so on. On another occasion I will 
 show you some of these simple things, but I will now give you a 
 much more wonderful example of hypnotic power. I will make 
 Charles dissolve into thin air and disappear entirely." 
 
 Hypnotist (to Charles) : "Charles, avaunt! Avaunt! Avaunt!" 
 (slowly with passes). (Lamp 2 is slowly turned down and at the 
 same time 1 is slowly turned up. Charles gradually fades into 
 nothing, but the chair is apparently left.) 
 
 Hypnotist : "Ladies and Gentlemen, Charles has now joined 
 the spirit world ; but for the sake of his family and friends, I will 
 call him back. I must do this quickly or he may get beyond my 
 power, great as this is." (Looking at ceiling) "Charles, appear!" 
 (Looking at glass) "Appear! Appear!" (slowly). (Light 1 is 
 slowly turned down and 2 slowly turned up and Charles slowly 
 appears.) 
 
 Hypnotist snaps his fingers. Charles wakes up and smiles. 
 Hypnotist drops curtain, bows to the audience, and goes behind 
 the curtain to help arrange for the next show. 
 
 Show 2. To hypnotize a boy, turn him into another boy, and 
 
GILBERT LIGHT EXPERIMENTS 45 
 
 then turn him back again. (Charles is seated at position B2, 
 lamp 2 is up ; Henry is seated at position A2, lamp 1 is 
 turned down.) 
 
 Hypnotist: "Ladies and Gentlemen, I will now give you an 
 even greater example of my hypnotic power. Other hypnotists 
 make their subjects believe that they are some one else, but I will 
 actually turn my subject into another being and right before 
 your eyes. Now watch carefully and please do not talk, because 
 I have to concentrate my will to make this transformation, and 
 if my attention is diverted my subject might be left half changed, 
 which would be very serious indeed." 
 
 (Raises curtain, speaks to Charles) "Charles, attention !" 
 (Makes short passes and says) "Sleep! Sleep! Sleep!" (slowly). 
 
 (To audience) "Now, Ladies and Gentlemen, if you will keep 
 perfectly quiet I will change Charles into another boy." 
 
 (To Charles) "Charles, change! Change! Change!" (slowly). 
 
 (Light 2 is slowly turned down and 1 slowly turned up. 
 Charles turns into Henry slowly. Henry is also asleep. Hyp- 
 notist snaps fingers — Henry wakes up and smiles.) 
 
 Hypnotist : "Now, Ladies and Gentlemen, it would never do 
 to leave these boys mixed up in this way because their mothers 
 would never know which is which, not to mention their best 
 girls, so I will turn them back again. Now quiet, please. Henry, 
 attention !" (He mesmerizes Henry with passes and saying, 
 "Sleep ! Sleep ! Sleep !" Henry stiffens and closes eyes. He 
 then says, "Change ! Change ! Change !" and Henry slowly 
 changes to Charles as light 2 is turned up and 1 down.) 
 
 Hypnotist wakes up Charles, drops curtain, bows, and retires 
 behind curtain again. 
 
 Show 3. Transmigration of souls. 
 
 Hypnotist: "Ladies and Gentlemen, my next exhibition of 
 my hypnotic power will deal with the transmigration of souls. 
 You have all heard of this strange Hindu theory, namely, that 
 
46 
 
 GILBERT BOY ENGINEERING 
 
 our souls have passed down the ages and have migrated from 
 one animal or man to another. Now I have traced Charles's 
 soul history, and it is very interesting. I have not the time to 
 show you all the forms it has taken, but I will show you the 
 animals his soul inhabited one thousand, two thousand, and three 
 thousand years ago." 
 
 (Raises curtain, mesmerizes Charles as before, then says) 
 "O ancient soul form ! Come ! Come ! Come !" (Makes passes 
 and bows three times toward Charles.) 
 
 (Charles slowly changes to a dog. Charles is at B2, dog 
 is on a box at A2. At first light 2 is up and light 1 is down. 
 The dog appears as 2 is lowered and 1 is turned up.) 
 
 Hypnotist: "Charles is a very nice boy, and you see that 
 one thousand years ago his soul inhabited the body of a very 
 nice dog. I will now show you the body his soul inhabited two 
 thousand years ago." 
 
 (Turns toward dog, makes passes, bows three times, and says) 
 "O more ancient soul form! Come! Come! Come!" 
 
 (While hypnotist is talking Charles has left his seat and a 
 cat on a box is put in his place. The dog now changes to a cat 
 as light 2 is turned up and light 1 down.) 
 
 Hypnotist : "You see that Charles's soul two thousand years 
 ago occupied the body of a very beautiful cat. I will now show 
 you the body his soul occupied three thousand years ago." 
 
 (Makes passes toward cat, bows three times, and says) "O 
 most ancient soul form ! Come ! Come ! Come !" 
 
 (The dog has been replaced by a bird in a cage and the cat 
 changes to a bird as light 1 is turned up and light 2 down.) 
 
 Hypnotist: "Ladies and Gentlemen, Charles's soul occupied 
 the body of a beautiful bird three thousand years ago. I will 
 now turn the bird back to Charles. Otherwise the cat might eat 
 Charles up, and I am afraid Charles's mother would not forgive 
 the cat or me." 
 
GILBERT LIGHT EXPERIMENTS 47 
 
 (Makes passes at bird and says) "Charles, appear! Appear! 
 Appear V s 
 
 (Charles has taken his place again at B2 and appears as 
 light 2 is turned up and 1 down.) 
 
 Hypnotist snaps fingers and awakens Charles, drops curtain, 
 bows, and retires. 
 
 Show 4. The transmutation of metals. 
 
 Hypnotist : "Ladies and Gentlemen, those of you who know 
 the history of the sciences know that all through the ages and 
 down to the present time, scientists have been trying to change 
 the base metals into the noble metals, — lead into silver, iron 
 into gold, and so on. All such attempts have previously failed, 
 but I wish to announce modestly to-night that I have succeeded, 
 with the help of my marvelous hypnotic power. I will now prove 
 this to you by changing iron into other metals. ,, 
 
 (Raises curtain showing a flatiron — or any iron object — 
 on a box. He makes passes at the iron and says) "O spirit of 
 iron, depart! O spirit of silver, come! Come! Come !" (The 
 iron slowly changes to a silver cake basket, or any object of 
 silver.) (The iron object is on a box at B2, the silver object is 
 on an exactly similar box at A2. At first light 2 is up, the iron 
 disappears and the silver appears as 2 is turned down and 1 up.) 
 
 Put a gold object in place of the iron, and change silver to 
 gold, and so on. 
 
 Show 5. To hatch an egg. Have an egg on one box and a 
 chicken on the other, and slowly change the egg to a chicken. 
 It is even funnier if you have a full-grown hen. Pretend that 
 your power is not strong enough, great as it is, to change the 
 hen back to the egg. 
 
 Show 6. Astral bodies. 
 
 Hypnotist : "Ladies and Gentlemen, I will next give you an 
 exhibition of occultism, and I will show you the results of a 
 marvelous discovery I have made. I have discovered a liquid 
 
48 
 
 GILBERT BOY ENGINEERING 
 
 with remarkable powers. If a person drinks this liquid he is 
 immediately changed to his astral body. This body appears to 
 the eye to be the same as ever, but it is composed of bound ether 
 only and has no substance. I may say that this has nothing 
 whatever to do with hypnotism ; the effects are produced entirely 
 by the liquid. " 
 
 (Raises curtain, disclosing Charles and Henry apparently 
 seated side by side with a glass of liquid — water or milk — in 
 front of each. Charles is at Al and Henry is at B3. Both 
 lights are up.) 
 
 Hypnotist : "You now see Charles and Henry. Boys ! Drink 
 some of the powerful liquid. " (The boys do so.) "Now, Ladies 
 and Gentlemen, the boys appear to you the same as before, but 
 they are not. 
 
 "Charles ! Put your hand gently through Henry." (Charles 
 does so.) "Henry! Do you feel anything?" (Henry shakes his 
 head to indicate, no.) 
 
 "Henry ! Put your hand gently through Charles." (Henry does 
 so.) "Charles! Do you feel anything?" (Charles moves lips.) 
 "You say 3^011 don't feel anything, but you wish he would wash 
 his hands." 
 
 Hypnotist : "You see, Ladies and Gentlemen, their bodies have 
 no substance. They are simply astral bodies made up of bound 
 ether. I will prove this further. 
 
 "Charles! Slice Henrygently with the butcher's knife." (Charles 
 does so.) "Henry! Does it hurt?" (Henry moves lips.) "What! 
 You like it?" (Henry nods yes and moves lips.) "You like it be- 
 cause it makes you feel like a sliced orange?" (Henry nods, yes.) 
 
 "Henry! Chop Charles gently with a hatchet." (Henry does 
 so.) "Charles! Does it hurt you?" (Charles shakes head and 
 moves lips.) "It doesn't hurt you and you like it because it makes 
 you feel like minced meat?" (Charles nods, yes.) 
 
 Hypnotist; "Now, Ladies and Gentlemen, I will show you 
 
GILBERT LIGHT EXPERIMENTS 49 
 
 another evidence of the marvelous power of this liquid. I will 
 have Charles pour some of the liquid on an apple and thereby 
 turn it into an astral apple. He will then give it to Henry to eat. 
 
 "Charles ! Change the apple and give it to Henry." (Charles 
 changes apple by pouring a little liquid on it out of the glass, but 
 instead of giving it to Henry he starts eating it himself. Henry 
 objects and apparently knocks the apple out of Charles's hand. 
 They sit in their chairs and each punches many times to the right. 
 Their fists go right through each other.) 
 
 Hypnotist drops curtain and apologizes solemnly to the audi- 
 ence, saying that he is sorry the astral bodies got beyond his 
 control. He bows and retires. 
 
 Show 7. Power of the will over supernatural beings. 
 
 Hypnotist: "Ladies and Gentlemen, I will now conclude the 
 entertainment of the evening by giving an exhibition of the power 
 of the human will over supernatural beings. Charles has just 
 had a terrifying experience and I am going to help him out." 
 
 (Raises curtain and shows Charles seated at Bl. Charles 
 is frightened and keeps looking over first one shoulder then 
 the other.) 
 
 Hypnotist : "Now, Charles, calm yourself and tell us exactly 
 what happened." (Charles moves lips.) "You say you just saw 
 a ghost up the dark road near Fred's house." (Charles nods and 
 moves lips.) "Did you run?" (Charles nods, yes.) "Were you 
 afraid?" (Charles shakes head, no.) "Why did you run then?" 
 (Charles moves lips.) "Oh, you just wanted to see whether you 
 could beat him running?" (Charles nods, yes.) "Did you beat 
 him?" (Charles nods, yes.) "Where did you leave the ghost?" 
 (Charles moves lips and waves hand toward door.) "You left it 
 at the front door ?" (Charles nods, yes.) "Can it get in ?" (Charles 
 shakes head and moves lips.) "Oh, you locked the door. Well, 
 it doesn't matter because you aren't afraid of ghosts, anyway." 
 (Ghost gradually appears beside Charles. Henry, covered with 
 
 K — 4 
 
50 
 
 GILBERT BOY ENGINEERING 
 
 sheet, is seated at A3 and appears at B3 as light 1 is turned up.) 
 
 (Charles is much startled, strikes at ghost with fists, then with 
 knife, drops knife with a clatter, takes up hatchet and strikes at 
 ghost ; Charles is much agitated. Ghost is calm all through this ; 
 it just looks at Charles, but now it moves over into Charles. 
 (Henry moves from chair A3 to Al.) Charles claws at his own 
 neck, trying to tear out ghost.) 
 
 Hypnotist now calls out, "Charles, calm yourself! I will 
 help you. You cannot get rid of the ghost because it is your own 
 ghost, but now just sit steady and I will pin the ghost to the 
 chair by my will power and when I say, 'Come !' get up quietly 
 and come out here in front." 
 
 (Hypnotist looks intently at ghost, makes passes, and says 
 quietly) "Come !" (Charles comes out in front.) 
 
 Hypnotist : "Now, Charles, look at your own ghost. Do you 
 want to get rid of him entirely?" 
 
 Charles: "Yes." 
 
 Hypnotist: "All right. Now watch quietly and I will send 
 him away." (Looking at ghost and pointing finger at him) "O 
 ghost of Charles, disappear and never come back! Disappear! 
 Disappear!" (slowly). (Ghost disappears as light 1 is turned 
 down.) 
 
 Hypnotist: "Ladies and Gentlemen, this concludes our enter- 
 tainment for this evening. Thanking you all for your kind atten- 
 tion, I bid you good-night." (Bows and retires.) 
 
 ELEVATIONS 
 
 The illusion show (Fig. 73) has a sheet of plate glass GG 
 at an angle of 45°. You can put on a similar show by turning 
 your box on its side. You can make a boy appear to rise in 
 air and stay there. The boy is lying on his back on a rug in 
 place of T and has his legs folded as though he were sitting 
 with crossed legs. The hypnotist then makes proper passes and 
 
GILBERT LIGHT EXPERIMENTS 51 
 
 Fig. 73. Illusion show 
 Permission of Hurst and Company, publishers of children's books and toys 
 
 commands him to rise. Two concealed boys at T pull the rug and 
 the boy appears to rise. He can then be turned upside down and 
 back again. You can repeat with the boy lying down. 
 
 FUN BY DAY OR NIGHT WITH TWO MIRRORS 
 Experiment No. 41. Magic money. Stand the two mirrors 
 
 vertically on the table sidewise to a good light and place a coin 
 
 between them. Look 
 
 over each mirror in 
 
 turn into the other 
 
 (Fig. 74). Have you 
 
 multiplied your mon- 
 ey wonderfully? 
 
 Experiment No. 42. 
 
 Magic lights. Repeat 
 
 the above in the dark 
 
 with a lighted candle 
 
 between the mirrors Fig , 74 . You see ma ny coins 
 
52 
 
 GILBERT BOY ENGINEERING 
 
 (Fig. 75). Do 
 you find many, 
 many lights? 
 
 E x p e r i ment 
 No. 43. Magic 
 army. Put a 
 number of lead 
 soldiers on a nar- 
 row strip of pa- 
 per and draw 
 them between the 
 
 Fig. 75. You see many lights vertical mirrors 
 
 (Fig. 76). Do you 
 see an immense army marching in perfect order? 
 
 Experiment No. 44. Magic dancers. Cut out of paper or 
 cardboard a small figure of a man dancing. Attach him to a 
 string and make him dance between the mirrors in a good light 
 (Fig. 77). Do you find a multitude of dancers who keep time 
 perfectly ? 
 
 Experiment No. 45. Magic silver or copper mine. Separate 
 the mirrors by two blocks, place them one above the other and 
 face to face (Fig. 78) ; 
 
 place a silver or cop- 
 per coin on the lower 
 mirror. Do you find 
 yourself looking 
 down into a very 
 deep hole with many 
 silver or copper coins 
 in it? 
 
 Why you see 
 Many Images in Par- 
 
 allel Mirrors. YOU Fig. 76. You see an army 
 
GILBERT LIGHT EXPERIMENTS 53 
 
 Fig, 77. You see many dancers 
 
 see many images 
 between two par- 
 allel mirrors be- 
 cause the image 
 formed in one 
 mirror is an ob- 
 ject in the other, 
 and so on. 
 
 In Fig. 79, two 
 mirrors, A and B, 
 4 inches apart are 
 facing each other 
 and a candle be- 
 tween them is 1 inch from B and 3 inches from A. 
 
 In B the image Bl is formed 1 inch behind B and in A the 
 image Al is formed 3 inches behind A. 
 
 Now image Al is 7 inches in front of B and it forms 
 an image B2 7 inches behind B; similarly image Bl is 5 inches 
 in front of A and forms an image A2 5 inches behind A. 
 Again, A2 is 9 inches in front of B and forms an image B3 
 9 inches behind B, and so on. 
 
 You see many im- 
 ages because the light 
 which enters your 
 eyes has been reflect- 
 ed one or more times. 
 
 If you are looking 
 at Bl, the light which 
 enters your eye ap- 
 pears to come from 
 Bl, but it comes from 
 
 the candle and is re- 
 Fig. 78. You look into a very deep hole containing n . - 
 
 much money fleeted from B. 
 
54 GILBERT BOY ENGINEERING 
 
 ! ! . ! ! ! ! ! ! • 1 
 
 A 2 Ai Bi B2 Ba 
 
 If you are looking at B2, the light appears to come from B2, 
 but B2 is an image of Al, and the light goes from the candle 
 and is reflected twice before it enters your eye. 
 
 Image B3 is an image of A2, which in turn is an image of Bl, 
 and you see B3 by means of light which has been three times 
 reflected. Similarly you would see B4, BIO, and B50 by means 
 
 of light reflected 4, 10, 
 and 50 times. 
 
 It is good practice 
 to locate the images in 
 parallel mirrors and to 
 trace the paths of the 
 light. 
 
 Why the Images 
 become dim. The im- 
 ages become dimmer 
 Fig. 80. You see over the book the farther they are 
 
GILBERT LIGHT EXPERIMENTS 55 
 
 Fig. 81. You see over your head 
 
 away : first, because 
 some light is absorbed 
 by the mirrors at each 
 reflection ; and, second, 
 because the light has 
 traveled a long dis- 
 tance in being reflect- 
 ed back and forth be- 
 tween the mirrors. 
 
 Experiment No. 46. 
 The trench periscope. 
 To illustrate how the periscope works, look over the top of a tall 
 book as shown in Fig. 80. Place one mirror against the book at 
 an angle of 45° and hold the second mirror above the book at 
 the same angle. Can you see over the top easily without being 
 seen yourself? Turn the upper mirror until it looks backward 
 (Fig. 81). Can you see back over your head, but is everything up- 
 side down? Turn the upper mirror until it looks sidewise(Fig. 82). 
 Can you see things, but are they turned on their sides? 
 
 THE "WHY" OF THE PERISCOPE 
 
 Now let us see 
 why the image is 
 right side up in 
 some cases and 
 not in others. 
 
 The mirrors in 
 the regular peri- 
 scope are parallel 
 to each other, and 
 you can locate the 
 image in each mir- 
 
 Fig. 82. You see things at the side ror in turn as you 
 
56 
 
 GILBERT BOY ENGINEERING 
 
 Let the arrow, Fig. 
 83, represent the ob- 
 ject; its image in A is 
 Al and the top and 
 bottom of Al are as 
 
 did in the case of 
 parallel mirrors. 
 
 far behind the mirror 
 extended as the top 
 and bottom of the ar- 
 row are in front. 
 
 Now let us suppose 
 mirror B to be extend- 
 
 ing. 83. The "why" of the periscope 
 
 ed as shown by the dotted line, then Bl is the image of Al in 
 this extended mirror and the top and bottom of Bl are as far 
 behind B as the top and bottom of Al are in front of B, and 
 therefore Bl is right side up. 
 
 In the second case, the mirrors are at right angles (Fig. 84). 
 Al is the image of the arrow in A extended and Bl is the image 
 of Al in B extended; Al is on its side and Bl is inverted for the 
 reasons given 
 
 above. Ai 
 
 In the third case, 
 the image is on its 
 side in the upper 
 mirror, and since 
 the lower mirror is 
 parallel to this im- 
 age, the image in 
 the lower mirror is 
 
 \ ARROW 
 
 V 
 
 / 
 
 No. 47. To make a 
 
 Fig. 84. Why things are upside down 
 
\ 
 
 GILBERT LIGHT EXPERIMENTS 57 
 
 trench periscope. Get a block of 
 wood 4" X 4" X 6", measure down 
 1 inch from each end and draw 
 a line across diagonally. This line 
 will be at 45° to the length of 
 the block. Cut the block through 
 on this diagonal line, see right side 
 Fig. 85. 
 
 Now attach a mirror to each diag- 
 onal face by means of tacks. Cut 
 a piece of stiff cardboard 17 inches 
 wide and as long as you wish to 
 make the periscope. Tack this to the 
 block, overlapping 1 inch on one 
 side. Paste the overlapping parts 
 together. Cut a hole 3" X 3" oppo- 
 site the upper mirror and a hole 
 2" X 2" opposite the lower mirror, and your periscope is finished. 
 
 You can use this periscope in your trench battles ; also you 
 can use it on a train to see forward without putting your head 
 out of the window. In this case, however, you should fasten the 
 window glass over one of the holes to keep cinders out of your eyes. 
 
 Fig. 85. The completed periscope 
 and the block used in it 
 
 Fig. 86. You see four candles 
 
 FUN WITH MIR- 
 RORS AT DIFFER- 
 ENT ANGLES 
 Experiment No. 48. 
 
 Mirrors at different 
 angles. Stand the mir- 
 rors vertically and at 
 right angles on the 
 table (Fig. 86) and 
 place a lighted candle 
 
58 
 
 GILBERT BOY ENGINEERING 
 
 between them. Do you see four candles, the real candle and 
 three images ? 
 
 Make the angle 60°. Do you see six candles, the original 
 candle and five images? 
 
 Make the angle 45°. Do you see eight candles? 
 
 Make the angle 30°. Do you see twelve candles? 
 
 There are 360° in a complete circle, and the number of candles 
 you see in each case is 360 divided by the angle between the 
 
 360 
 
 mirrors. For example, when the angle is 90°, you see or 4 
 
 360 90 
 
 candles ; and when the angle is 60°, )^ou see ■ or 6 candles ; 
 
 A 60 
 
 and so on. 
 
 Experiment No. 49. A one-boy crowd. Stand the mirrors 
 at 90° and put your face close to the mirrors. Are there four 
 of you, yourself and three images? 
 
 Repeat with the mirrors at the angles mentioned above. Do 
 you find yourself a crowd all in a circle? 
 
 Experiment No. 50. Arrows. Stand the mirrors at 90° on a 
 piece of white paper and draw an arrow pointing at one of the 
 mirrors. Do some of the arrows point in one direction and 
 some in the opposite direction? Keep one mirror in such a posi- 
 tion that the arrow points directly at it and move the other 
 mirror until the angle is 60°. Do the six arrows point toward 
 each other in pairs? 
 
 Repeat with the mirror at the other angles mentioned above. 
 
 Experiment No. 51. An infinite number of candles. Light 
 a candle and stand the mirrors close to it and gradually make 
 them parallel. Do you see very, very many candles? 
 
 When the mirrors are parallel the angle between them is 
 360 
 
 0°, and is infinity, so you should see an. infinite number of 
 
 
 
GILBERT LIGHT EXPERIMENTS 59 
 
 I 
 
 I 
 
 --h 
 
 I 
 i 
 
 A2 B£ 
 
 
 - 2" — 
 
 
 II" 
 
 
 4- 
 I 
 
 l 
 
 Fig. 87. Tho "why" of mirrors at right angles 
 
 images. You cannot, 
 because some light is 
 lost at each reflection 
 and finally all is lost. 
 
 Experiment 
 No. 52. To locate the 
 images in mirrors at 
 an angle. Draw two 
 lines 4 inches long at 
 right angles to rep- 
 resent two mirrors at 
 right angles (Fig. 87) 
 and extend them 
 backward by dotted 
 
 lines to represent the extended mirrors. Place a dot 1 inch from 
 A and 2 inches from B, then image Al will be 1 inch behind A 
 and image Bl 2 inches behind B. The third image A2B2 is an 
 image of both Al and Bl ; it is 1 inch behind A extended and 
 2 inches behind B extended. 
 
 It is harder to locate the images 
 when the angle is 60° or smaller, but 
 it will help you to know that the 
 images are always all on the circum- 
 ference of a circle of which the angle 
 of the mirrors is the center. 
 
 Practice locating the images in 
 mirrors at 60°. 
 
 Experiment No. 53. The kaleido- 
 scope. The kaleidoscope (Fig. 88) 
 consists of two mirrors at an angle 
 of 30° in a tube which has an eye 
 opening at one end and at the other 
 Fig. 88. The kaleidoscope a chamber containing pieces of col- 
 
60 GILBERT BOY ENGINEERING 
 
 ored glass. When you 
 look through the tube 
 and revolve it, the 
 colored pieces of glass 
 make beautiful twelve- 
 sided figures by mul- 
 tiple reflection. 
 
 Illustrate the work- 
 ing of the kaleidoscope 
 as follows : Draw two 
 
 Fig. 89. You see many twelve-sided figures lines at an angle of 
 
 30° on a piece of white paper. Stand the mirrors on a block above 
 these lines with the angle toward a good light (Fig. 89). Now 
 put pieces of colored paper and other small objects on a strip 
 of paper and draw the paper under the angle, while you look 
 down between the mirrors with your eye near the angle. Do 
 you see a series of twelve-sided figures? 
 
 ILLUSIONS 
 
 The Sphinx. This 
 illusion shows an 
 Egyptian head with- 
 out a body (Fig. 90). 
 The hypnotist shows 
 the audience an empty 
 box with a glass front. 
 He closes and locks 
 the door over the 
 front, places the box 
 exactly on the center 
 of the table, unlocks 
 it, opens the door, and, 
 
 behold, there is an Fig. 90. The Egyptian head 
 
GILBERT LIGHT EXPERIMENTS 61 
 
 CURTAINS 
 
 PLAN 
 
 Egyptian head in the 
 box. The hypnotist 
 stands near the audi- 
 ence and addresses 
 the head. "O an- 
 cient Sphinx, awake ! 
 Awake! Awake I" 
 The sphinx slowly 
 opens its eyes and 
 stares straight ahead. 
 The hypnotist then 
 addresses questions 
 to it and it answers 
 in very deep and very 
 dead tones, and so on. 
 Finally the hypnotist 
 locks the box, brings 
 it forward to the au- 
 dience, opens it, and 
 there is nothing in it 
 butahandfulof ashes. 
 
 The mechanism of this illusion is illustrated in Fig. 91. The 
 table is on three legs, A, B, C, with mirrors at 60° between A, C, 
 and A, B. The curtains at the back and sides are exactly alike, 
 and to the audience the images of the side curtains appear to be 
 the back curtain, and the space under the table appears quite 
 empty. 
 
 Cabinet of Proteus. The performer puts his assistant into 
 the cabinet (Fig. 92), closes the doors a moment, makes passes, 
 open doors (Fig. 93), and the assistant is gone. Closes doors 
 again, makes passes, opens doors, and out comes an entirely 
 different man. Closes doors again, makes passes, opens doors, 
 and out comes a lady. Closes doors again, makes passes, 
 
 Fig. 91. Plan of the table 
 
62 
 
 GILBERT BOY ENGINEERING 
 
 opens doors, and out comes assistant. 
 
 To the audience, the cabinet appears 
 entirely empty except for a post, C, with 
 a strong light at the top. There are, 
 however, two hinged mirrors, ab and 
 ab, Fig. 94, at an angle of 60° and the 
 post covers the angle. The sides and 
 back are exactly alike and the images 
 of the sides in the mirrors appear to the 
 audience to be the back. The man, 
 lady, and as- 
 
 Fig. 92. Cabinet closed 
 From Hoffman's Modern Magic 
 
 sistant, of 
 course, hide 
 behind the mirrors. Members of the 
 audience stand behind and beside the 
 cabinet all through the performance. 
 The assistant swings the mirrors 
 against the sides before he comes out 
 the last time, and then members of the 
 audience are asked to examine the cab- 
 inet, when, of course, they find nothing. 
 
 ^^^^^^ 
 
 Fig. 94. Plan of cabinet 
 From Hoffman's Modern Magic 
 
 Cabinet open 
 From Hoffman's Modern Magic 
 
 Illusion Show. Pharaoh's 
 thumb. Make a table out of 
 cardboard (Fig. 95) and stand it 
 on three legs, each of which is 
 exactly 5% inches from the other 
 two, and place your two mir- 
 rors between A and B and A and 
 C. Surround it by screens on 
 three sides, making the sides and 
 
GILBERT LIGHT EXPERIMENTS 63 
 
 back exactly alike 
 and exactly the same 
 distance from the 
 table. 
 
 Now have an as- 
 sistant put his arm 
 through a hole in the 
 back curtain and put 
 his blackened thumb 
 up through a hole in 
 the table top, and you 
 are ready to begin 
 the act. 
 
 _ , . . Fig. 95. Pharaoh's thumb 
 
 Explain to the au- 
 dience that you have succeeded in bringing to life the thumb 
 of an ancient pharaoh by your hypnotic power. Explain that 
 the thumb was lost in battle, fell on the sands of the desert and 
 dried but did not decompose. This pharaoh was a great hyp- 
 notist, which makes it easier for you to bring his thumb back 
 to life. Explain also that the thumb will answer any question 
 about the future. If the thumb moves forward it is, yes ; if it 
 doesn't move at all it is, no. 
 
 Now open the curtains, address the thumb, "O Thumb of an 
 ancient Pharaoh, awake! Awake! Awake!" (slowly and with 
 passes). The thumb does not move. You now ask, "O ancient 
 and sacred Thumb, will Charles get his wish?" (Thumb slowly 
 nods, yes.) "O ancient and sacred Thumb, will Henry get 
 through his examinations?" (Thumb does not move. No.) And 
 so on. 
 
 Vaudeville Act. The acrobat. You can put on a short but 
 very funny act with a mirror (Fig. 96) placed at an angle to the 
 audience. 
 
64 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 96. You become a marvelous acrobat 
 From Magical Experiments, published by David McKay Co., Philadelphia 
 
 FUN WITH THE CURVED MIRROR 
 
 Experiment No. 54. Converging sunlight. Open the slit in 
 your darkened room to its full size and allow the sunlight to 
 fall on the concave (curved-in) side of your curved mirror. 
 Make a dust. Is the sunlight converged to a point and does it 
 diverge beyond this point (Fig. 97) ? This point is the focus of 
 the mirror. 
 
 Experiment No. 55. Diverging sunlight. Turn the convex 
 (curved-out) side of the mirror to the sunlight (Fig. 98). Is 
 the sunlight reflected and diverged or spread? 
 
GILBERT LIGHT EXPERIMENTS 65 
 
 Experiment No. 56. 
 
 Picture of the sun. 
 Remove the shutter, 
 stand the mirror on 
 the table in the sun- 
 light, and focus the 
 sunlight on a strip of 
 paper inch wide 
 (Fig. 99). Is the pic- 
 ture of the sun round 
 and very bright? 
 
 Experiment No. 57. Fig * 97, You see a brilliant focus in dusty air 
 The focus is very hot. Focus the sunlight on your hand with 
 the concave mirror (Fig. 100). Is it hot? It is, because all the 
 heat of the sunlight is concentrated at the focus. 
 
 Experiment No. 58. To light a match with sunlight. Place 
 a match in front of a narrow strip of paper (Fig. 101) and focus 
 the sunlight on the head. Does the match light? 
 
 Experiment No. 59. A magic cannon. Stick a needle into 
 the under side of a cork and stick a match on the other end of 
 
 the needle (Fig. 102), 
 with a small piece of 
 paper at one side of the 
 head. Insert the stopper 
 in an empty bottle, focus 
 the sunlight on the 
 match head through the 
 glass sides (Fig. 103). 
 Does the match light 
 and are the cork, nee- 
 dle, and match driven 
 out with a satisfactory 
 
 You see the light spread P°P ? 
 
66 
 
 GILBERT BOY ENGINEERING 
 
 
 
 ■ 
 
 
 
 The lighted match 
 heats the air and the 
 expanding air drives 
 out the cork. 
 
 Experiment No. 60. 
 Focal length of con- 
 cave mirror. Focus 
 the sunlight on a nar- 
 row piece of paper 
 and measure the dis- 
 tance between the 
 back of the mirror and the paper. This is the focal length of 
 the mirror. Do you find it to be about two inches ? 
 
 Experiment No. 61. Focal length of convex mirror. Make 
 two pencil dots just 2 inches apart on a piece of cardboard and 
 between these punch two holes just 1 inch apart. Hold the 
 cardboard between the convex mirror and the sun and move it 
 until the light which passes through the holes 1 inch apart is 
 reflected to the dots 2 inches apart, and measure the distance 
 from the back of the mirror to the card. This is the focal 
 
 Fig. 99. You see a brilliant picture of the sun 
 
 Fig. 100. You find the focus hot 
 
GILBERT LIGHT EXPERIMENTS 67 
 
 length of the convex 
 mirror. Do you find 
 it to be 2 inches? 
 
 There is no real 
 focus for a convex 
 mirror because it 
 spreads the light, but 
 the reflected rays ap- 
 pear to come from a 
 point 2 inches behind 
 
 
 
 
 
 
 i 
 
 
 
 
 jj 
 
 
 
 
 
 
 
 
 ■ 
 
 
 
 Fig. 102. How to arrange cork, needle, and match 
 
 the focus (Fig. 104). 
 Do you find a small 
 inverted picture in 
 natural colors of the 
 window and of the 
 things outside the 
 window? 
 
 Have a friend 
 move about near the 
 window. Do you get 
 his picture? 
 
 Fig. 101. You light a match with sunlight 
 
 the mirror. An un- 
 real focus of this kind 
 is called avirtualfocus. 
 
 Experiment No. 62. 
 Pictures. Go to the 
 back of the room, 
 turn the concave mir- 
 ror toward the win- 
 dow, and hold a piece 
 of paper three-quar- 
 ters inch wide near 
 
 Fig. 103. You light the match in the bottle 
 
68 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 104. You get a picture 
 
 Experiment 
 
 No. 63. Your own 
 image. Look at 
 yourself in the con- 
 cave mirror. Are 
 you upside down and 
 small? Bring your 
 eye closer to the 
 mirror than the focus 
 (2 inches) (Fig. 105). 
 Is your eye large 
 and right side up? Look at yourself in the convex side. Are 
 you small and right side up in all cases? 
 
 THE "WHY" OF THE CURVED MIRRORS 
 Waves. When parallel waves (1), Fig. 106, strike the con- 
 cave side of the mirror, they are reflected and so curved in that 
 they converge at the focus and then diverge. 
 
 When parallel waves strike the convex side (2), they are 
 reflected and so curved out that they diverge and never meet. 
 
 Rays. The curved 
 mirror is part of a 
 sphere and the cen- 
 ter of the sphere is 
 at. C, Fig. 107(1). The 
 lines CA are radii of 
 the sphere and they 
 are perpendicular to 
 the mirror. When par- 
 allel rays strike the 
 concave mirror they 
 make equal angles 
 
 with these perpendic- Fig. 105, Your eye is enlarged 
 
GILBERT LIGHT EXPERIMENTS 69 
 
 Fig. 106. (1) Parallel waves are curved in. (2) Parallel waves are curved out 
 
 ular radii and cross at the focus F. The line through the center 
 O of the mirror and through the center C of the sphere is called 
 the principal axis of the mirror. You will notice that the parallel 
 
 Fig. 107 (1). How parallel rays are converged by a 
 concave lens and diverged by a convex lens 
 
 Fig. 107 (2) 
 
Fig. 108. A searchlight 
 Courtesy of the Scientific American 
 
GILBERT LIGHT EXPERIMENTS 
 
 71 
 
 Fig. 109. 
 
 A parabolic 
 reflector 
 From Black and Davis' 
 Practical Physics, published 
 by The Macmillan Co. 
 
 rays which are above the principal axis 
 before they strike the concave mirror are 
 below it afterward and vice versa. This 
 explains why the images you see in the 
 concave mirror are reversed. 
 
 When your eye is nearer than the 
 focus, it intercepts the rays before they 
 can cross, and your image appears to be 
 behind the mirror, right side up and 
 large. 
 
 When parallel rays strike the convex 
 side of the mirror, Fig. 107(2), they make equal angles with the 
 radii (CA extended) ; they diverge but appear to come from the 
 focus F. This is the unreal or virtual focus. 
 
 The rays above the principal axis before reflection are above 
 it afterward, and, therefore, the images in the convex mirror are 
 right side up. 
 
 Searchlight Reflectors. The reflectors on battleship search- 
 lights (Fig. 108) are made in the shape of a parabola (Fig. 109). 
 Parallel rays which strike parabolic reflectors converge exactly 
 at the focus, and conversely if a light is placed exactly at the 
 focus the reflected light consists of parallel rays which go 
 straight forward. The reflectors on automobile and locomotive 
 headlights are also parabolic, and the lamp is placed at the focus. 
 Spherical Aberration. Spherical mirrors do not converge all 
 parallel rays at the focus because those 
 which strike near the edge are reflected be- 
 hind the focus (Fig. 110). This is called the 
 spherical error or spherical aberration of 
 the mirror. Conversely if a light is placed 
 at its focus a spherical mirror does not re- 
 F %he 1 r icai : aberi^tioTi ° f fleet it in parallel rays. This explains why 
 ^S<cnTp^sic*"pM6M«2ed *t is not used as a first-class reflector, 
 
 by The Macmillan Go, 
 
 
 
 
 
 
 
 
 
 
 — * ~ — -jr. 
 
 — » 
 
 
 c' *tftf\ 
 
 
72 
 
 GILBERT BOY ENGINEERING 
 
 REFRACTION OF 
 LIGHT 
 
 When light passes in a 
 slanting direction from one 
 medium to another, — for ex- 
 ample, from air to water 
 or the reverse, or from air 
 to glass or the reverse, — 
 part of it is reflected 
 at the surface between the 
 two media and part of it 
 enters the second medium but 
 is bent out of its path, from ABC to ABD, Fig. 111. This bending 
 is called refraction. When light passes from air to any denser 
 medium as water or glass, it is bent toward a line NN drawn 
 perpendicularly through the surface at the point it enters. See 
 
 Fig. 112 (1). When light 
 passes from water or glass to 
 air, it is bent away from 
 the perpendicular NN. See 
 Fig. 112 (2). 
 
 Fig. 111. The light is bent or refracted 
 From Lynde's Physics of the Household, 
 published by The Macmillan Co. 
 
 Fig. 112 (1). Part of the 
 light is reflected and part 
 refracted 
 
 Fig. 112 (2) 
 
GILBERT LIGHT EXPERIMENTS 73 
 
 Fig. 113. You see light bent toward the perpendicular NN 
 
 FUN WITH SUNLIGHT 
 Experiment No. 64. Air to water. Allow a beam of sunlight 
 to pass through the slit in your darkened room. Cut a slit 1 inch 
 long and % inch wide in a piece of cardboard, put this over your 
 mirror, and reflect sunlight into a glass pitcher full of water 
 into which you have put 2 or 3 drops of milk (1), Fig. 113. Vary 
 the slant of the beam of sunlight which strikes the water and 
 view the beam in the water through the sides of the pitcher. Is 
 some of the light reflected at the surface of the water? Does 
 some of it enter the water and is it bent or refracted? Make 
 the beam split on the side of the pitcher so that half is inside 
 and half outside. Is the beam in the water bent toward an im- 
 aginary perpendicular at the point it enters? Repeat this with a 
 glass of milky water (2). Repeat with a bottle of milky water (3). 
 Use a bottle with flat sides. 
 
74 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 114. You see part of the light reflected and 
 part refracted 
 
 E xper iment 
 No. 65. Air to glass. 
 Make the beam split 
 on the thick glass 
 plate standing on its 
 edge, on two match- 
 es, on paper (Fig. 
 114). You cannot 
 see the light in the 
 glass but you can 
 see it on the paper 
 below after it has 
 passed through the 
 
 glass. Is the light which passes through the glass bent toward an 
 imaginary perpendicular NN drawn at the point it enters? 
 
 Let the sunlight enter through a slit 1 inch long and 
 % inch wide. Split the beam of light on the edge of the 
 glass plate and hold a piece of paper behind the plate. Tilt 
 the plate to different angles. Is the light which passes through 
 the glass plate always bent toward the perpendicular NN? 
 
 Experime n t 
 No. 66. A glass of 
 water. Remove your 
 shutter and stand a 
 glass of water in 
 sunlight near the 
 window ; fill the 
 glass to the top and 
 put paper around 
 the sides to keep 
 out the sunlight. Is 
 the sunlight which 
 
 Strikes the water Fig. 115. The light is bent down 
 
GILBERT LIGHT EXP ERIMENTS 75 
 
 surface bent down, as shown in ^ \n Smooth 
 
 Fig. 115? yS\S/\ b Growi& 
 Explanation of Refraction. A 
 
 beam of sunlight is bent or re- 
 fracted when it passes from air to 
 water because light travels more 
 slowly in water than it does in air. ^Lond 
 Its velocity in water is only three- 
 fourths of its velocity in air. Fig 11G The ^ hy v of refrac ti n 
 
 "Mrkw +r* cpp +hp> rnnnprtinn hp- From the Ontario High School 
 JNOW tO See tne Connection De- PhysicSj by permission of the 
 
 tween change in direction and change publishers 
 in velocity, let us consider what would happen if a regiment of soldiers 
 marched in a slanting direction BD from smooth ground to rough 
 ground, as shown in Fig. 116. The men would march less rapidly on 
 the rough ground and the direction of the marching lines would be 
 changed. The line AB is still on smooth ground and is straight. Part 
 of the line ab is on rough ground and this part is somewhat behind. 
 The line cd has a larger part on rough ground and this part is behind. 
 The line CD is wholly on rough ground and it is marching in a direc- 
 tion DE different from BD, and it would continue in this new direc- 
 tion. This is exactly what happens to parallel light waves. They are 
 bent toward the perpendicular when they pass at a slant from air to 
 water or glass because they travel more slowly in water or glass than 
 they do in air. They are bent away from the perpendicular when 
 they pass at a slant from water or glass to air because they 
 travel faster in air than they do in water or glass. 
 
 REFRACTION OF SPHERICAL WAVES 
 Experiment No. 67. A coin under water. Put a coin in a glass 
 
 of water and look down at it through the water (Fig. 117). Does 
 
 it appear to be nearer than it really is? 
 
 You see the coin because light passes from it to your eyes. 
 
 This light is in the form of spherical waves in the water, but 
 
76 
 
 GILBERT BOY ENGINEERING 
 
 when these waves 
 enter the air they 
 become more 
 curved (see BC, 
 Fig. 118) because 
 the parts which en- 
 ter the air first 
 travel faster and get 
 ahead of the parts 
 still in water. 
 
 Now your eye 
 
 Fig. 117. The coin appears closer to you than it is estimates the dis- 
 tance of an object 
 
 partly by the curvature of the waves 
 which enter it from the object. The 
 curvature of the waves which enter 
 your eye from the coin is the same as 
 though the coin were at a point A 
 only three-fourths the depth, and this 
 is the reason the coin appears to be 
 at A. 
 
 If you look at the coin in a slant- 
 ing direction, it appears to be nearer 
 the surface still, because the light is 
 bent more and more the greater the 
 slant of the rays from the coin to 
 the surface. 
 
 RELATION BETWEEN ANGLES 
 OF REFRACTION AND 
 INCIDENCE 
 
 If the light ray in Fig. 119 is pass- 
 
 r • .< .1 «> Fig. 118. Why the coin appears 
 
 ing from air to water, then the line closer to you 
 
GILBERT LIGHT EXPERIMENTS 77 
 
 RM is always exactly three- 
 fourths the length of IM no 
 matter how large or small i 
 may be. If the light passes in 
 the opposite direction, the same 
 relation holds until the critical 
 angle is reached. (See page 87 
 for definition of critical angle.) 
 
 If the ray is passing from 
 air to glass, RM will always be 
 two-thirds of IM, and this rela- 
 tion holds if the light passes 
 from glass to air, until the criti- 
 cal angle is reached. 
 
 This gives you the relation between the angle of incidence 
 and the angle of refraction in all cases. 
 
 Fig. 119. RM is throe-fourths of IM in 
 water and two-thirds of IM in glass 
 From Lynde's Physics of the House- 
 hold, published by The Macrnillan Co. 
 
 FUN BY DAY OR NIGHT 
 Experiment No. 68. Magic lead pencil. Put a pencil in 
 a glass of water in a slanting direction and sight along it 
 
 (Fig. 120). Does 
 
 it appear to be 
 bent up? It does, 
 because the light 
 from it is bent 
 as shown in 
 Fig. 121. 
 
 E x p e r i ment 
 No. 69. Magic 
 ruler. Put a ruler 
 vertically in a 
 pitcher of water 
 
 Fig. 120. You see a bent pencil to a depth of 4 
 
78 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 121. The light is bent 
 From Black and Davis' Practical Phys- 
 ics, published by The Macmillan Co. 
 
 inches (Fig. 122). Does the 
 part under water appear to be 
 only 3 inches long when viewed 
 vertically? It does, because 
 light travels in water only 
 three-fourths as fast as it does 
 in air. 
 
 Does it appear much shal- 
 lower when viewed at a slant? 
 It does, because light is bent more the greater the angle at which 
 it leaves the water. 
 
 Experiment No. 70. An elastic ruler. Shove the ruler to 
 the bottom of a pail of water and lift it out. Does it appear to 
 stretch? 
 
 Experiment No. 71. Magic glass. Stand a ruler at one end 
 of the glass prism held on one edge, Fig. 123 (1). Does the 
 bottom appear only two-thirds its real depth when viewed ver- 
 tically? It does, because light travels only two-thirds as fast 
 in glass as it does in air. 
 
 Does it appear even shallower when viewed at a slant? It 
 does, because light is bent more the greater the angle at which 
 it leaves the glass. 
 
 Repeat this with 
 the prism on end. 
 
 Repeat with the 
 glass plate on its 
 edge, Fig. 123 (2). 
 
 Experiment No. 72. 
 Phantom coin. Fill a 
 long, deep pan with 
 water. Put a coin on 
 the bottom and view 
 it vertically and then Fig. 122. You see the ruler shortened in water 
 
GILBERT LIGHT EXPERIMENTS 79 
 
 Fig. 123 (1). The prism and the glass plate 
 appear shallower than they are 
 
 at greater and greater 
 slants. Does the coin 
 seem to rise? Why? 
 
 Experiment No. 73. 
 A disappearing coin. 
 Stand a coin on edge 
 in a tin funnel full of 
 water, ask a friend to 
 stand so that he can 
 just see the top over 
 the edge of the funnel, 
 and then let the water 
 run out. Does he find 
 
 that he can no longer see the coin from where he stands? Why? 
 
 Experiment No. 74. A broken looking-glass. Play this trick on 
 your family. Take a piece of soap and mark a star with radiating 
 lines near one edge of a looking-glass (Fig. 124). The family will 
 think the glass is broken. A real break shows up because the 
 light is refracted at the break and this gives a fair imitation. 
 Where is the Fish? The three boys 1, 2, and 3 in Fig. 
 
 125 are looking at 
 the same fish and 
 they see it at the 
 three different posi- 
 tions 1, 2, and 3, 
 because the light 
 from the fish is bent 
 more the greater the 
 slant it has when it 
 reaches the water 
 surface. None of 
 them see the fish 
 Fig. 123 (2) where it is. 
 
80 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 124. A broken mirror 
 
 How Deep is the Water? If you have gone swimming in 
 very clear water you know that it always looks shallower than 
 it is. If the water is at the same depth everywhere it will look 
 to you shallower in the distance, for the reasons given 
 above. 
 
 How Tall are You to a Fish? If you are 6 feet tall a fish 
 (Fig. 126) sees you as 8 feet tall, because the curved waves from 
 you are made less curved in water and, therefore, appear to 
 come from a more distant point. 
 
 Experiment No. 75. Breaking a pencil without touching it. 
 Look at a pencil in a slanting direction through a bottle of 
 water with flat sides or through the edges of the glass plate. 
 Does it appear to be broken into three parts? Why? 
 
GILBERT LIGHT EXPERIMENTS 81 
 
 Fig. 125. They see the fish at different depths 
 
 Fig. 126. You appear to a fish to be four-thirds bigger than you are 
 K — 6 
 
82 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 127. The light bends at A 
 
 Experiment No. 76. Shifting pin. 
 Put the glass plate flat on a piece of 
 paper on the table. Stick the pins 
 A, B on each side (Fig. 127) and sight 
 from pin A to B through the glass. 
 Does B appear to be shifted? Draw 
 lines around the edges of the plate, 
 aim a ruler at the two pins through 
 the glass, and draw a line along the 
 ruler; then draw a line from B to A 
 and draw a line perpendicular to the edge of the plate at A. This 
 shows that the light which passes from B to A in glass is bent 
 away from the perpendicular when it enters air. 
 
 Experiment No. 77. Shifting line. Put the plate on a piece 
 of paper (Fig. 128) and draw lines around the edge. Now draw 
 a slanting line AB, sight along a ruler through the glass at this 
 line, and draw the line CD along the ruler. Is the line parallel 
 to AB but shifted? Draw per- 
 pendiculars at B and C and 
 draw a line from B to C. The 
 light from A passes into the 
 glass at B and is bent toward 
 the perpendicular at B; it pass- 
 es from glass to air at C and 
 is bent away from the perpen- 
 dicular at C. 
 
 Experiment No. 78. Things 
 are not where they seem. Look 
 at a lighted candle through 
 your glass prism (Fig. 129). 
 Does the candle appear to be 
 in a different place? 
 
 Does it also appear to be Fig. 128. The light bends at b and c 
 
GILBERT LIGHT EXPERIMENTS 83 
 
 beautifully colored ? 
 You will experiment 
 with colors soon. 
 
 Experiment No. 79. 
 Bending light around 
 a corner. Stand the 
 prism on end on paper 
 and draw a triangle 
 around the end (Fig. 
 130). Now draw a 
 line AB slanting to- 
 
 ward one side. Sight Fig 129 You gee a gMfted candle 
 
 along a ruler through 
 
 the prism at this line and draw a line CD along the ruler. Now 
 remove the prism, draw short perpendiculars at B and C, and 
 join BC. 
 
 The light from A enters glass at B and is bent toward the 
 perpendicular ; it enters air again at C and is bent away from the 
 second perpendicular. This is why the light is bent around a 
 corner by your prism. 
 
 Experiment No. 80. To see under water from a boat. You 
 cannot see things under water from air usually, because the 
 light reflected from the surface blinds you to the light coming 
 from beneath the surface. You can easily see through the sur- 
 
 Fig. 130. The light bends around a corner 
 
Fig. 131. You can see under water through a tube 
 
GILBERT LIGHT EXPERIMENTS 85 
 
 Fig. 132. Spotting submarines 
 
86 
 
 GILBERT BOY ENGINEERING 
 
 face, however, through a pipe 
 of any kind, as shown in Fig. 
 131, because the sides of the 
 pipe keep the reflected light 
 out of your eyes. Try this with 
 a pipe 2 or 3 feet long. 
 
 Experiment No. 81. To see 
 the fish you are trying to catch. 
 If you can fish under a boat- 
 house or under a wharf, you 
 can see the fish deep down under water because the house or 
 wharf prevents surface reflection. You can do this also as fol- 
 lows : Stretch a blanket or tarpaulin between two boats and put 
 your head under it. The surface reflection is removed and you 
 will be able to see to great depths. 
 
 Spotting Submarines. Submarines are easily spotted at great 
 depths from a dirigible or airplane at a great height above the 
 surface (Fig. 132) because at these great heights the light re- 
 
 Fig. 133. How light passes from water 
 to air 
 
 From Black and Davis* Practical Phys- 
 ics, published by The Macmillan Co. 
 
 Fig. 134. You see a phantom pin 
 Permission of Hurst and Company, publishers of children's books and toys 
 
GILBERT LIGHT EXPERIMENTS 
 
 87 
 
 fleeted vertically upward is not so great as the vertical light 
 received from objects beneath the surface. 
 
 Total Reflection. When light passes from water to air in 
 a slanting direction (Fig. 133), part of it is reflected and the part 
 which passes through is bent away from the perpendicular. As 
 the slant becomes greater the bending is greater, and finally 
 the light which passes into air is at right angles to the per- 
 pendicular. If the light in water is still more slanting when 
 it reaches the surface, it does not pass into air at all, but is all 
 reflected back into the water. This is called total reflection. 
 The angle at which this takes place in water is any angle greater 
 than 48.5°, and in crown glass and hard flint glass, any angle 
 greater than 41° and 37° respectively. These angles are called 
 the critical angles for these substances. 
 
 Experiment No. 82. A phantom pin. Cut a slice of cork, 
 attach a pin to the under side, float the cork on the surface of 
 water in a full glass, stand the glass on the table, and look at 
 the cork from the level of the table. Can you see a phantom 
 pin above the cork (Fig. 134) ? You see it by means of light 
 reflected from the under side of the water surface. 
 
 Experiment No. 83. A broken spoon. Put a spoon in a glass 
 half filled with water and look at the under side of the water 
 surface through the side of the tumbler. Do you find a brilliant 
 image of the part of the spoon in water? You see this by light 
 reflected at the under surface. 
 
 Prism Glass. These prism glasses, Figs. 135 (1) and (2), 
 are used to throw light to the rear of a store, or from the sidewalk 
 into the basement. They J JL t f 
 
 are made of glass and 
 have prisms on one side. 
 The light which enters 
 them is totally reflected 
 from the inside surface 
 
 Fig. 135 (1) Fig. 135 (2) 
 
 Light is reflected from inside surface of prism 
 From the Ontario High School Physics, by per- 
 mission of the publishers 
 
88 
 
 GILBERT BOY ENGINEERING 
 
 of the prisms and is directed to the back of 
 the building or basement. 
 
 Right-angled Prisms. These are made of 
 glass and act as mirrors, in some opera glasses 
 and other optical instruments. Light which 
 enters one right-angled face, AC, Fig. 136, is 
 totally reflected at the slanting face and passes 
 out through the other right-angled face BC. 
 
 ATMOSPHERIC REFRACTION 
 
 Mirages. A ship at sea sometimes appears upside down 
 (Fig. 137) because the air near the cold water is colder and 
 denser than the air above and the light from the ship is refracted 
 as it passes from each layer of cold air to the warmer layer 
 above and is finally totally reflected. The light which enters the 
 sailor's eye appears to come from the image above. 
 
 Mirages on the hot deserts are caused by light from the 
 
 Fig:. 137. A mirage 
 From Aldons' Elementary Course of Physics, published by The Macmillan Co. 
 
 Fig. 136. A right- 
 angled prism 
 
GILBERT LIGHT EXPERIMENTS 89 
 
 clouds which passes from the upper cold air through warmer 
 and warmer lower layers. It is refracted and finally totally re- 
 flected and the clouds look like a lake of water on the ground. 
 Sunset and Sunrise. You see the sun before it is up and 
 
 Fig. 138. Why you see the sun before it rises and after it has set 
 From Appleton's School Physics, published by the American Book Co. 
 
 after it has set because light from it is refracted by successive 
 layers of air which are denser the nearer they are to the 
 earth. 
 
 The direct ray SD, Fig. 138, could not be seen at A because 
 the earth is in the way, but the light SB is seen because it is 
 refracted to A. 
 
 COLOR 
 
 Spectrum. When a beam 
 of sunlight passes through a 
 glass prism as shown in Fig. 
 139, it is spread out into a 
 colored band called the spec- 
 trum. This spectrum contains 
 all the primary colors, of 
 which those most easily rec- 
 ognized are in order: red, 
 orange, yellow, green, blue, 
 indigo, and violet. 
 
 Fig. 139. A prism produces a spectrum 
 
 from white light 
 From Lynde'8 Physics of the Household, 
 published by The Macmillan Co, 
 
90 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 140. A spectrum recombined to produce white light 
 
 White Light made up of all Colors. The experiment above 
 shows that white light is made up of lights of all primary colors. 
 
 This is proved again 
 by passing the spec- 
 trum through a prism 
 turned in the opposite 
 direction (Fig. 140) ; 
 the colors are recom- 
 bined to produce white 
 light. 
 
 It can be proved 
 also by turning the 
 prism back and forth 
 quickly (Fig. 141). 
 The colors overlap at 
 the center and pro- 
 duce white light. 
 
 Dispersion. You 
 know that light is re- 
 fracted or bent when 
 
 !Fig. 141. White light is made up of many 
 primary colors 
 Courtesy of the Scientific American 
 
GILBERT LIGHT EXPERIMENTS 91 
 
 Fig. 142. A beautiful spectrum by reflection under water 
 
 it passes from air to water or glass or the reverse, because it 
 travels more slowly in water and glass than it does in air. Now 
 the waves of red light are longer than those of orange, the waves 
 of orange are longer than those of yellow, and so on, the waves 
 of each light beginning at the red end of the spectrum are longer 
 than those next to it until we get to the very shortest, namely, 
 the waves of violet light. It has been found by experiment that 
 the shorter the waves, the more slowly they travel in water or 
 glass and, therefore, the more they are refracted or bent when 
 they pass from air to water or glass, or the reverse. When white 
 light passes through a prism then, the shorter waves are bent 
 
92 
 
 GILBERT BOY ENGINEERING 
 
 •* ONE WAVE LENGTH 
 
 \ 
 
 or refracted more than the 
 longer waves and as a result 
 the white light is spread out 
 into the spectrum. This 
 spreading of light is called 
 dispersion. 
 
 Pig. 143. The waves destroy each other 
 or interfere 
 
 INTERFERENCE 
 
 Spectrum by Reflection. 
 
 Another beautiful method 
 of producing a spectrum is 
 illustrated in Fig. 142. A 
 mirror is placed in a slant- 
 ing position under water in 
 a pan and a beam of sunlight 
 is allowed to fall on the mir- 
 ror. The sunlight, in going 
 through the water to the 
 
 mirror and back, really passes through a prism of water and it is 
 spread out or dispersed into a beautiful spectrum. 
 
 If, after the spectrum is formed, the surface of the water is 
 stirred, the colors of the spectrum are mixed and the reflected 
 beam is white. This proves again that white light is made up 
 of all the colors of the spectrum. 
 
 Interference. In a water wave the particles of water simply 
 move up and down, but the wave moves forward. A wave length 
 is a hill and a hollow. 
 
 If, now, two waves of exactly the same length come together 
 in such a way that one is one-half wave behind the other (Fig. 
 143), the hill of one coincides with the hollow of the other, the 
 particles of water do not move at all, and one wave destroys the 
 other. This is called interference. 
 
 The same thing occurs in light waves ; two streams of waves 
 may come together and destroy each other, that is, produce 
 darkness. 
 
GILBERT LIGHT EXPERIMENTS 93 
 
 Colors by Interfer- 
 ence. If a beam of 
 sunlight is allowed to 
 fall on a soap film held 
 in a vertical position 
 on the end of a lamp 
 chimney (Fig. 144), it 
 is found that the soap 
 film when viewed by 
 reflected light is 
 
 Crossed by horizontal Fig. 144. Colors in a soap film 
 
 colored bands. These 
 
 colors are formed by interference as follows : The soap film has 
 two surfaces with water between, and when it stands on edge 
 the water runs toward the bottom and the film becomes a narrow 
 prism. Now the light is reflected partly from the front film and 
 partly from the back film, and where the films are 1-4, 3-4, 5-4 
 waves of red light apart, the red waves from the rear are 1-2, 
 1 1-2, 2 1-2 waves behind the red waves from the front when they 
 enter your eye. These two sets of waves, then, interfere and de- 
 stroy each other, and all that your eye sees is blue. Similarly a 
 
 little above and below 
 these points the blue 
 waves destroy each 
 other, and you see red 
 light. 
 
 FUN WITH SUN- 
 LIGHT 
 
 Fig. 145. You produce a beautiful spectrum by 
 means of your prism 
 
 Experiment No. 84. 
 
 The prism spectrum. 
 Allow a beam of sun- 
 light to pass through 
 
94 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 146. 
 
 You produce a beautiful spectrum by 
 reflection under water 
 
 the slit in your dark- 
 ened room and fall on 
 the prism supported be- 
 tween blocks as shown 
 in Fig. 145. Cut a piece 
 of cardboard of the ex- 
 act size of one face of 
 the prism and put it on 
 the upper face. Do you 
 find a beautiful spec- 
 trum on the wall or ceil- 
 ing? Do you find that 
 the violet end is nearest 
 the base of the prism and the red end nearest the angle, that is, 
 is the violet end the most bent? Turn the prism over. Do you 
 get a spectrum on the floor? 
 
 Get the spectrum on the wall or ceiling again and rock the 
 prism quickly. Is the center of the spectrum white? This proves 
 that white light is made up of all spectrum colors because they 
 mix at the center. 
 
 Experiment No. 85. Spectrum by reflection. Place a mirror 
 in a slanting position under water and arrange it so that the 
 beam of sunlight falls on the mirror (Fig. 146). Do you find a 
 beautiful spectrum on the wall above the slit? Stir the water. 
 Do the colors mix and produce white light? 
 
 Experiment No. 86. Colors by interference. Make soap suds 
 as you would for blowing soap bubbles. Put the suds in a saucer. 
 Dip the end of a lamp chimney in the suds and support the chim- 
 ney on its side in sunlight (Fig. 144). Look at the film by reflected 
 light. Do you find that the film at the top is crossed by beautiful 
 horizontal colored bands? These colors are produced by inter- 
 ference. The colors in a soap bubble and in a film of oil on water 
 are produced by interference. 
 
GILBERT LIGHT EXPERIMENTS 95 
 
 WHY OBJECTS ARE COLORED 
 
 An object has a certain color because something in the object 
 absorbs all other colors. For example, a blue dress is blue be- 
 cause the dye in the dress absorbs all the other colors of the spec- 
 trum. Also a red dress is red because the dye absorbs the colors 
 in the blue end of the spectrum, and so on. 
 
 An object is white when all of the colors of the spectrum are 
 partly absorbed and all are reflected. 
 
 An object is black when all the colors are completely ab- 
 sorbed and none reflected. 
 
 Experiment No. 87. Changing colors. Darken your room and 
 allow sunlight to enter through a slit smaller than your colored- 
 glass plates. Hold the red glass over the slit and hold colored 
 objects in the red light. Are red objects red, but all other colored 
 objects dark or black? They are dark or black because the dye 
 in them absorbs the red light. Repeat with the blue glass. 
 Are the results similar? 
 
 Note. The blue glass lets through a little red, yellow, and 
 green, as you will now show. 
 
 Experiment No. 88. Changed spectrum. Get the spectrum 
 with the prism and then put the red glass against the prism. 
 Does the red glass absorb all colors except red ? Repeat with the 
 blue glass. Does it absorb nearly all colors except blue, but 
 does it let through a small amount of the other colors? 
 
 Experiment No. 89. Changing colors in spectrum. Get the 
 spectrum with the prism and hold colored objects in the differ- 
 ent colors. Do they change colors according to the part of 
 the spectrum they are in? They are black in the part of the 
 spectrum which they absorb completely. 
 
 FUN BY DAY OR NIGHT 
 Experiment No. 90. A colored strip. Cut a strip of white 
 paper about 1-16 inch wide and 2 inches long and pin it to a 
 
96 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 147. The white paper is colored 
 
 black object. Put 
 it in a good light 
 and look at it 
 through the prism 
 (Fig. 147). Do you 
 find a spectrum in- 
 stead of the white 
 paper? 
 
 Do you find also 
 that the spectrum is 
 reversed, that is, 
 that the red is nearest the base of the prism and the violet 
 nearest the angle? This is so because your eye sees an object 
 in the direction the light enters it from the object. The red 
 is least bent but appears to be most bent, and the violet the 
 reverse. 
 
 Experiment No. 91. Combining spectra. Cut a strip of 
 white paper 1 inch wide and 2 inches long and look at it through 
 the prism (Fig. 148). Do the edges appear colored, but is the 
 center white? The center is white because the spectra formed 
 by the edges overlap at the center and this combination of all the 
 colors of the spectrum produces white light. 
 
 Experiment 
 No. 92. Colored can- 
 dle, flame. Look at 
 the flame of a candle 
 through a prism. 
 Is it beautifully col- 
 ored, but does 
 the center tend to 
 be white and are the 
 colors reversed as 
 
 above ? j?ig. 148. The paper is colored only at the edge 
 
GILBERT LIGHT EXPERIMENTS 97 
 
 COMPLEMENTARY COLORS 
 
 Complementary colors are those which, when combined, pro- 
 duce white light. If any colors are taken out of the spectrum, 
 the remaining colors are complementary to those taken out, 
 because together they produce white light. 
 
 MIXING PAINTS 
 
 A paint which absorbs the colors in the blue end of the spec- 
 trum is red in color and a paint which absorbs the colors in 
 the red end of the spectrum is blue in color. If, now, these 
 paints are mixed, they do not produce white paint but black 
 paint because together they absorb all the colors. 
 
 FUN WITH SUNLIGHT 
 Experiment No. 93. Colored glasses. Stand the red and 
 blue glasses side by side on a piece of white paper in sunlight. 
 The red absorbs the blue end of the spectrum and lets through 
 red light. The blue absorbs the red end of the spectrum and 
 lets through blue light. Now place one behind the other. Do 
 they absorb all the light and is the shadow black? 
 
 THE RAINBOW 
 
 The rainbow (Fig. 
 149) is formed by the 
 internal reflection and 
 dispersion of sunlight 
 by falling drops of 
 water. You see itwhen 
 the sun is . behind you 
 and not over 42° above 
 the horizon. The first 
 or primary rainbow is 
 formed by two refrac- 
 K — 7 
 
 Fig. 149 (1). The "why" of the rainbow 
 From the Ontario High School Physics, by permis- 
 sion of the publishers 
 
98 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 149 (2) 
 From the Ontario High School Physics, by permis- 
 sion of the publishers 
 
 tions at A and C and 
 one internal reflection 
 at B (2) ; it is violet 
 below and red above 
 and the angle at which 
 the light enters your 
 eye is about 41° to the 
 direction of the sun- 
 light. The secondary rainbow is formed by two refractions A 
 and D and two internal reflections B and C (3) ; it is red below 
 and violet above and the angle of the light is about 52°. 
 
 Experiment No. 94. An artificial rainbow. Place a glass full 
 of water (Fig. 150) on a table in sunlight and projecting beyond 
 the edge. Do you get two or more beautiful rainbows on the floor? 
 Stand the glass on a mirror. Do you get two beautiful rainbows 
 on the ceiling? These bows, however, are not reversed, This 
 
 Fig. 150. You make an artificial rainbow 
 
GILBERT LIGHT EXPERIMENTS 99 
 
 Fig. 151. You see a devil 
 
 experiment will show 
 best in your darkened 
 room. 
 
 FUN AT NIGHT 
 Experiment No. 95. 
 
 A changing devil. Cut 
 a little devil out of 
 cardboard and arrange 
 as shown in Fig. 151. 
 Hold the red glass in 
 
 front of the candle at the right. Is the devil at the right red and 
 is the devil at the left very dim but of the complementary color, 
 green? Use blue glass. Is one devil blue and the other very 
 dim but of the complementary color, orange? 
 
 Experiment No. 96. A tri- 
 colored star. Fold a piece of 
 cardboard. Cut a four-pointed 
 star in one half. Fold the 
 points back, make a tracing on 
 the other half, and cut out a 
 star very carefully with the 
 points exactly between those 
 of the first. Arrange as shown 
 in Fig. 152 and hold the red 
 glass in front of one can- 
 dle. Do you get an eight- 
 pointed star with the points 
 alternately red and green and 
 with a white or pink eight- 
 pointed star inside? Repeat 
 with the blue glass. Are the 
 
 Fig. 152. You see an eight-pointed star points blue and Orange? 
 
100 GILBERT BOY ENGINEERING 
 
 Fig. 153. Trench faces 
 
 Experiment No. 97. A ghost party. Mix a half teaspoonfnl 
 of salt in three or four teaspoonfuls of alcohol in a saucer, stand 
 the saucer on a cup on the table (to prevent burning the table), 
 
GILBERT LIGHT EXPERIMENTS 101 
 
 seat the party around the table in the dark, light the alcohol, and 
 look at your neighbors' faces and at your own in a mirror. Do 
 you all look like ghosts ? You do, because the salt in the flame 
 gives only yellow light, and since your rosy cheeks and rosy lips 
 absorb this color they appear black. 
 
 TRENCH FACES 
 
 Our boys at the front painted their faces black (Fig. 153) 
 before they started out on night raids, because the black paint 
 absorbed the light and prevented their faces from being seen. 
 
 Fig. 154. Spectroscope 
 Courtesy of the Scientific American 
 
 THE SPECTROSCOPE 
 
 When substances are vaporized in a flame and the flame is 
 viewed through a spectroscope (Fig. 154) the spectrum seen is 
 crossed by bright lines. Each substance has its own particular 
 lines, and when we know these lines we can tell what substances 
 are in the flame. This is the basis of spectrum analysis. In the 
 spectroscope shown here the light passes through a narrow slit, 
 through tube A, through four prisms, and into the telescope B 
 in which the enlarged spectrum is seen. 
 
102 GILBERT BOY ENGINEERING 
 
 WHAT IS IN THE SUN AND STARS? 
 
 When the light from the stars is viewed in the spectroscope, 
 the spectrum is crossed by dark lines exactly corresponding to 
 the bright lines mentioned above. These are called the Fraun- 
 hofer lines, after their discoverer. If, in the spectrum of light 
 from the sun, for example, we see dark lines exactly corre- 
 sponding to the bright lines produced by iron in the spectrum 
 on the earth, we know that there is iron in the sun, and so on. 
 
 3 
 
 Fig. 155. A lighthouse lens 
 From Aldons* Elementary Course of Physics, published by The Macmillan Go. 
 
 LIGHTHOUSE LENSES 
 
 Lighthouse lenses have at the center a comparatively thin 
 lens and around this prismatic sections with greater and greater 
 angle toward the edge, (1) Fig. 155. Panels (2) made up in 
 this way are placed completely around the light F (3). This 
 gives a large, short focus lens which does not absorb as much 
 light as a solid thick lens would absorb. 
 
 LENSES 
 
 Lenses are of two kinds, converging and diverging. 
 Converging lenses are thicker at the middle than at the edges, 
 and we may think of them as made up of sections of prisms, 
 
GILBERT LIGHT EXPERIMENTS 103 
 
 Fig. 156 (1). A converging Fig. 156 (2). A diverging 
 
 lens as sections of prisms lens as sections of prisms 
 
 From Lynde's Physics of the Household, published by The Macmillan Co. 
 
 Fig. 156 (1), the angles of the prisms being greater the nearer 
 they approach the edges. These lenses converge parallel rays 
 to a point F, called the focus. 
 
 (Diverging lenses are thinner at the middle than at the edges, 
 and we may think of them as made up of sections of prisms, 
 Fig. 156 (.2), with their thin edges toward the center. These 
 lenses diverge parallel rays and make them appear to come 
 from a point F, called an unreal or virtual focus. 
 
 FUN WITH SUNLIGHT 
 
 Experiment No. 98. 
 
 pass through the slit in 
 ing lens in the beam 
 (Fig. 157) and make a 
 dust. Do you see that 
 the light comes to a 
 point and diverges af- 
 terward ? 
 
 Repeat with the 
 other converging lens. 
 Is the light again 
 brought to a point but 
 at a different distance 
 from the lens? 
 
 Converging lenses. Allow sunlight to 
 your darkened room, hold a converg- 
 
 Fig. 157. You see a brilliant focus in dusty ail 
 
104 
 
 GILBERT BOY ENGINEERING 
 
 Experiment No. 99. 
 
 Diverging lens. Re- 
 peat this experiment 
 with your diverging 
 lens. Is the light di- 
 verged or spread? 
 
 Experiment 
 No. 100. Focal lengths. 
 Remove your shutter, 
 
 Fig. 158. You measure the focal length focus the light with a 
 
 converging lens, hold 
 a piece of paper at the point where you get the smallest and 
 brightest image of the sun (Fig. 158) and measure the distance 
 from the lens to the paper. The point is the focus and the dis- 
 tance is the focal length of the lens. 
 
 Repeat with the other converging lens. 
 
 £)o you find the focal lengths of the lenses to be 4 inches 
 and 8 inches respectively? 
 
 Experiment No. 101. Focal length of diverging lens. Punch 
 two nail holes exactly 1 inch apart in a piece of paper, put this 
 in front of the diverging lens, and measure the distance at which 
 the spots of sunlight appear 2 inches apart on a paper behind 
 the lens. This is the 
 virtual focal length. 
 Is it 4 inches? 
 
 Experiment 
 No. 102. Is it hot? 
 Put your hand at the 
 focus of each converg- 
 ing lens in turn (Fig. 
 159). Is the sunlight 
 hot? It is, because all 
 
 the light and heat Fig. 159. The focus is hot 
 
GILBERT LIGHT EXPERIMENTS 105 
 
 which falls on the lens 
 is concentrated at the 
 focus. 
 
 Repeat with the di- 
 verging lens. Is there 
 no heat? 
 
 Experiment 
 No. 103. To light a 
 match with sunlight. 
 When the sun is hot 
 at mid-day put a Fig * 160, ^ ou ^S^t a match with sunlight 
 
 match on a piece of 
 
 paper and focus sunlight on it with the short focus lens (Fig. 160). 
 Does it light? Why? 
 
 Experiment No. 104. Magic cannon. Repeat Experiment 
 No. 59, but light the match by means of the short focus lens 
 (Fig. 161). 
 
 THE "WHY" OF IT 
 
 When the parallel waves from the sun fall on a converging 
 lens, which is thicker at the middle than at the edges (Fig. 162), 
 
 the portions of the 
 waves that go through 
 the thick part are 
 slowed up more than 
 the portions which go 
 through the thinner 
 parts, and as a result 
 the waves are so 
 curved in that they 
 converge at the focus 
 and diverge afterward. 
 Fig. 161. You light the match in the bottle The waves are shown 
 
106 GILBERT BOY ENGINEERING 
 
 i 2 
 
 Fig. 162. Parallel wave3 and rays are converged 
 
 in 1 and the rays in 2. This explains why these lenses con- 
 verge the light. 
 
 When parallel waves fall on a diverging lens, which is thinner 
 at the center than at the edges, the portions which go through 
 the center are less delayed than the portions which go through 
 the edges and the waves are so curved out that they diverge 
 after passing through the lens. The waves are shown in 1, 
 Fig. 163, and the rays in 2. This explains why these lenses 
 diverge the light. 
 
 If the light comes from an object near a converging lens 
 the waves are curved when they reach it, and one of three things 
 may happen. 
 
 If the object is at a distance from the lens greater than the 
 focal length (1, Fig. 164), the curvature of the waves is reversed 
 and the light is brought to a point on the other side of the lens 
 
 Fig. 163, Parallel waves and rays are diverged 
 
GILBERT LIGHT EXPERIMENTS 107 
 
 at a distance greater 
 than the focal length. 
 
 If the light is at 
 the focus (2, Fig. 164), 
 the curvature of the 
 waves is so altered 
 that they are parallel 
 after they pass 
 through the lens. 
 
 If the light is 
 nearer to the lens than 
 the focus (3, Fig. 164), 
 the curvature of the 
 waves is altered by 
 the lens, but they still 
 diverge and will never 
 converge. 
 
 Fig. 164. Light and a converging lens 
 
 FUN BY DAY OR NIGHT 
 Experiment No. 105. Images. Arrange a candle, 4-inch con- 
 verging lens, and screen as in Fig. 165. Place the lighted candle 
 
 3 feet from the lens 
 and move the screen 
 until you get an image. 
 Is it inverted and 
 small ? Repeat with 
 candle at 2 feet and 1 
 foot. Is the image 
 larger each time? 
 
 Place candle at 
 twice the focal length, 
 that is, 8 inches. Are 
 Fig. 165. You see a picture of the candle image and candle the 
 
108 
 
 GILBERT BOY ENGINEERING 
 
 same size? Place can- 
 dle at 6 inches. Is the 
 image larger? Place 
 candle at 5 inches. Is 
 the image larger still? 
 Place candle at the fo- 
 cus. Is the image very- 
 large? Place candle at 
 3 inches and 2 inches, 
 that is, closer than focus. 
 
 Pig. 166. You see a picture of your hand A ™ no images formed ? 
 
 Repeat with the 
 
 converging lens of 8-inch focus. Place candle at distance of 
 4 feet, 3 feet, 2 feet, 16 inches or twice the focal length, 15 inches, 
 12 inches, 8 inches, and 6 inches. Are the results similar? 
 
 Is the image smaller than the candle when the candle is at a 
 greater distance from the lens than twice the focal length? Is it 
 larger when the candle is at a distance less than twice the focal 
 length and greater than the focal length? 
 
 Experiment No. 106. Picture shows. With the candle, con- 
 verging lens, and screen, as in Fig. 166, get the image of the 
 candle on the screen, then hold your hand behind the candle 
 and close to it. Do you get an inverted picture of your hand in 
 natural colors? 
 
 Hold a black and white drawing upside down and close to 
 the candle. Do you get a picture right side up? 
 
 Repeat with colored drawings, colored flowers, and so on. 
 Do you get colored pictures? 
 
 Repeat with all kinds of things and use four or five candles 
 to get more light. 
 
 Experiment No. 107. A picture of out-of-doors. In the day- 
 time, go to the side of the room away from the window and get 
 a picture of distant objects on the screen (Fig. 167). Do you 
 
GILBERT LIGHT EXPERIMENTS 109 
 
 find a beautiful invert- 
 ed picture in natural 
 colors of everything 
 out-of-doors ? 
 
 Measure the dis- 
 tance from lens to 
 screen. This is again 
 the focal length of the 
 lens. At night get a 
 picture of a distant 
 light and measure the Fig 167 You see a picture of thin g S out-of-doors 
 focal length. 
 
 Experiment No. 108. The lenses and your eyes. Hold the 
 converging lenses in turn at arm's length and look at distant 
 objects. Is the image small and inverted? 
 
 Hold them about one foot from your eye and look at your 
 finger held closer to the lens than its focal length. Is the image 
 large and right side up? 
 
 Repeat with the diverging lens. Is the image always right 
 side up and small? 
 
 HOW THE IMAGES ARE FORMED 
 
 In Fig. 168 (1) the object OB is at a greater distance than the 
 focal length. All the rays which fall on the lens from any point 
 B meet at the point M and, therefore, the image of B is at M. 
 We cannot trace all the rays, but it is necessary to trace only two. 
 The two most easily traced are the parallel ray BR and the ray 
 BP which goes through the center of the lens. Ray BR goes 
 through the focus F after it goes through the lens ; ray BP goes 
 straight ahead, or nearly so, because the two sides of the lens 
 are nearly parallel at the center. 
 
 The rays from all other points between B and O meet at points 
 between M and I and, therefore, MI is the inverted image of BO. 
 
110 
 
 GILBERT BOY ENGINEERING 
 
 (2) (3) 
 Fig. 163. How images are formed 
 From Lynde's Physics of the Household, published by The Macmillan Co. 
 
 In (2), BO is inside the focus; therefore BR and BP diverge 
 after they pass through the lens and do not form an image. Your 
 eye, however, makes an image because it sees the rays as though 
 they came from MI. This explains why you see anything inside 
 the focal length as enlarged and right side up. 
 
 In (3), BO is outside the virtual focus of the diverging lens. 
 BR and BP diverge after they pass through the lens and your 
 eye sees the image MI. This explains why diverging lenses 
 always give images small and right side up. 
 
 POWER OF A LENS 
 
 Spectacles are lenses, and opticians measure the power of the 
 spectacle lenses as follows : If the lens has a focal length of 
 1 meter it is said to have a power of 1 diopter ; if it has a focal 
 length of 1-2, 1-3, or 1-10 meter it is said to have a power of 2, 3, 
 or 10 diopters ; and so on. That is, the shorter the focal length 
 the greater the power. 
 
GILBERT LIGHT EXPERIMENTS 
 
 111 
 
 A meter is 100 centimeters long". You will find on most or- 
 dinary rulers 30 divisions on the side opposite the inch divisions ; 
 
 Fig. 169. Conjugate foci 
 
 each of these divisions is 1 centimeter, and 100 of these make 
 a meter. 
 
 Experiment No. 109. Power of your lenses. Measure in 
 centimeters the focal length of the 8-inch lens. Do you find 
 
 100 
 
 it to be 20 cms. ? Is the power of the lens then = 5 diopters ? 
 
 20 
 
 Repeat with the 4-inch lens. Is its focal length 10 cms. and 
 100 
 
 its power == 10 diopters ? 
 
 10 
 
 Experiment No. 110. Power of spectacles. Measure in centi- 
 meters the focal length of your father's or mother's spectacles 
 and calculate their power in diopters. 
 
 Experiment No. 111. Conjugate foci. Get the image of a 
 candle as in Fig. 169, mark the position of the screen and the 
 candle, and then exchange them. Do you again find an image, 
 but of different size? 
 
 Repeat at different distances. 
 
 Two points so situated with respect to a converging lens that 
 an object at either forms an image at the other are called conjugate 
 foci. There are an infinite number of pairs of such points for 
 each converging lens. 
 
112 
 
 GILBERT BOY ENGINEERING 
 
 RELATION BETWEEN OBJECT AND IMAGE 
 
 If D Q is the distance of an object from a lens and D i is the 
 
 111 
 
 distance of its image from the lens, then — + — — — , where F is 
 
 D D, F 
 
 the focal length of the lens. This is one relation between the 
 object and its image. 
 
 The magnification of an image is the number of times it is 
 larger or smaller than the object, and you can always find it by 
 dividing Dj by D Q ; that is, the magnification = Dj-f- D Q . 
 
 Experiment No. 112. Where is the image? Arrange the 4- 
 inch lens with the candle 6 inches from it. Calculate where the 
 image will be as follows : 
 
 1 11 1 1 _ 1 1 _ 1 1_3 2 _ 1 
 
 D~ Q F OI 6 D" i ~4 0r D" i ~4 — 6~12"~12 ~12 
 
 .*. is 12. The image will be 12 inches from the lens. Try it. 
 Now calculate and try where the image will be if the object 
 is 5 inches, 7 inches, 8 inches, 12 inches, 20 inches from the lens, 
 and so on. 
 
 Repeat with the 8-inch lens, using D Q greater than 8 inches. 
 
 Experiment No. 113. How big will the image be? Arrange 
 the candle 6 inches from the 4-inch lens and the image will be 
 at 12 inches, as you found above. 
 
 Now, since magnification = Dj D Q , it is 12 -r- 6 = 2, and the 
 image will be 2 times as large as the object. Measure the height 
 of the flame and of its image. Is the image 2 times as high as 
 the flame? Try other distances and then the other lens. 
 
 MAGIC 
 
 Experiment No. 114. Cylindrical lens. Look at your finger 
 through a tumbler of water. Does the tumbler of water act as 
 a cylindrical lens and is your finger broad? 
 
GILBERT LIGHT EXPERIMENTS 1 1 3 
 
 Experiment No. 115. Treble your money. Put a quarter in a 
 tumbler half full of water, put a saucer over the tumbler, and in- 
 vert both. Do you see a half dollar on the saucer and a quarter 
 higher up? Why? 
 
 Experiment No. 116. Heat through ice. Place the concave 
 mirror upside down on a sheet of clear ice inch thick and let 
 it melt into the ice. Do you get an ice lens ? At noon, when the 
 sun is hot, hold your hand at the focus of this lens. Is it hot? 
 
 Experiment No. 117. A spectrum from ice. Take a clear piece 
 of ice, shave it to the shape of a prism, and hold it in sunlight. 
 Do you get a beautiful spectrum? 
 
 OPTICAL INSTRUMENTS 
 
 FUN BY DAY OR NIGHT 
 
 A Magnifying Glass is simply a converging lens (Fig. 170) 
 with the object PQ closer than the focus. The eye receives rays 
 which are still diverging and 
 sees the image pq enlarged. 
 You have illustrated this 
 above. 
 
 The Astronomical Tele- 
 scope (Fig. 171) consists of 
 two converging lenses, or 
 systems of lenses, connected 
 by a long tube. The lens 
 nearest the object is called 
 the objective, and the lens nearest the eye, the eyepiece. 
 
 The objective (Fig. 172) forms a real inverted image im of 
 the object BO inside the focus of the eyepiece. The eyepiece 
 magnifies this, just as a magnifying glass does, and the eye sees 
 the enlarged image IM. 
 
 When the telescope is focused on a distant object: the dis- 
 
 K — 8 
 
 Fig. 170. A magnifying glass 
 From Lynde's Physics of the Household, 
 published by The Macmillan Co. 
 
114 GILBERT BOY ENGINEERING 
 
 Fig. 171. Astronomical telescope at Lick Observatory 
 Courtesy of the Scientific American 
 
GILBERT LIGHT EXPERIMENTS 
 
 115 
 
 Fig. 172. How you see the image 
 From Lynde's Physics of the Household, published by The MacmilJan Go. 
 
 tance between the lenses is equal to the sum of their focal 
 lengths ; and the magnification is equal to the focal length of the 
 objective divided by the focal length of the eyepiece. 
 
 Terrestrial telescopes have, between the objective and eye- 
 piece, other lenses which turn the image right side up. 
 
 Experiment No. 118. An astronomical telescope. Arrange 
 the converging lenses on a piece of board (Fig. 173) and focus 
 on a distant object. 
 
 Measure the distance between the lenses. Is it equal to the 
 sum of their focal lengths, that is, 8 -f- 4 = 12 inches ? 
 
 Look at a distant object through the telescope with one eye 
 and outside the telescope with the other eye. Is the magnifi- 
 cation equal to focal 
 length of objective -f- 
 focal length of eye- 
 piece, that is, 8 -T- 4 
 = 2 times ? 
 
 Hold a piece of 
 paper at the focus of 
 the objective. Do you 
 get an image? 
 
 Experiment 
 No. 119. To make a 
 telescope. Place 8- 
 Fig. 173. You illustrate the telescope inch lens in ring hold- 
 
116 
 
 GILBERT BOY ENGINEERING 
 
 Fig. 174. You make a telescope 
 
 into the first and your telescope is 
 made (Fig. 174). Focus it on a 
 distant object. 
 
 The Compound Microscope 
 (Fig. 175) is the same in principle 
 as the astronomical telescope, but 
 the objective has very great power, 
 that is, it has a very short focal 
 length. The objective forms a 
 real image, im, Fig. 176, of BQ, 
 and the eyepiece forms the en- 
 larged image IM of im. 
 
 The Opera Glass (Fig. 177) 
 has a converging lens C for objec- 
 tive and a diverging lens c for 
 eyepiece. The objective would 
 form an inverted image ab of AB, 
 but the eyepiece diverges the light 
 and the eye sees the erect image 
 A'B'. The ordinary opera glass 
 consists of two such instruments; 
 
 er and wind dark 
 wrapping paper 
 around the holder to 
 make a tube 10 inches 
 long. Place 4-inch 
 lens in the other 
 ring holder and wind 
 wrappingpaper around 
 the holder to make 
 a tube 6 inches long. 
 Slip the second tube 
 
 Fig. 175. A compound microscope 
 
GILBERT LIGHT EXPERIMENTS 1 1 7 
 
 Fig. 176. Illustrating how images are formed in the microscope 
 From Lynde's Physics of the Household, published by The Macmillan Go. 
 
 they are shorter than the ordinary telescope and, therefore, more 
 convenient. 
 
 Fig. 177. How you see things in an opera glass 
 From Lynde's Physics of the Household, published by The Macmillan Co. 
 
 Experiment No. 120. An opera glass. Arrange the lenses 
 on a piece of board as in Fig. 178. Focus on an object. Is 
 the image erect and 
 are the lenses closer 
 together than in the 
 telescope ? 
 
 Experiment 
 No. 121. To make an 
 opera glass. Place 8- 
 inch lens in ring holder 
 and wind around it a 
 tube of wrappingpaper 
 
 3 inches long. Place Fig. 178. You illustrate tne opera glasa 
 
118 GILBERT BOY ENGINEERING 
 
 I 
 
 Fig. 179. Binoculars 
 
 Courtesy of the Scientific American 
 
 the diverging lens in the other ring holder and wind a tube 
 2 inches long. Insert the second tube in the first and your opera 
 glass is made. Focus it on a distant object. 
 
 The Prism Binoculars (Fig. 179) are made with lenses sim- 
 ilar to those in an astronomical telescope, but the light is re- 
 flected four times by means of glass prisms. This reflection 
 makes the image erect and shortens the length of the tube. 
 
 The Projecting 
 Lantern (Fig. 180) 
 consists of a light- 
 proof box, a source 
 of bright light, a con- 
 densing lens, a lantern 
 slide, and a projecting 
 lens. The bright light, 
 
 Fig. 180. A projecting lantern produced by eleCtriC- 
 
 From Lynde's Physics of the Household, published m t 
 
 by The Macmiiian Co. lty, acetylene, or, as 
 
GILBERT LIGHT EXPERIMENTS 
 
 119 
 
 Fig. 181. You make a postcard lantern 
 
 here, by a limelight, 
 is converged on the 
 lantern slide by the 
 condensing lens and 
 an image of the in- 
 verted slide is thrown 
 on the screen by the 
 projecting lens. 
 
 The Postcard Lan- 
 tern consists of a 
 light-proof box, two 
 electric lights which 
 throw light on the postcard but not directly on the lens, a 
 postcard slide, and a converging lens which throws an image 
 of the postcard on the screen. 
 
 Experiment No. 122. Magic-lantern shows. Place 4-inch 
 lens in ring holder in a hole in a large piece of cardboard, place 
 a black book 6 inches from lens and a white screen 12 inches 
 from lens on the other side, light the candles, and hold small 
 objects against the book. Are their images thrown on the 
 screen in natural colors and magnified twice? 
 
 Experiment 
 No. 123. To make a 
 postcard lantern. You 
 can have lots of fun 
 with a lantern made as 
 follows : 
 
 Get a cardboard or 
 wooden box (Fig. 181) 
 about 8" X 6" X 6", 
 put the 8-inch lens in 
 ring holder and in a 
 
 Fig. 182, You hold a magic-lantern show wrapping paper tube 
 
120 
 
 GILBERT BOY ENGINEERING 
 
 A, 
 
 a 
 
 4 inches long ; put the 
 tube into a hole in 
 one side of the box 
 and paint the opposite 
 side of the box black. 
 Place an electric light 
 or oil lamp on each 
 
 Fig. 183. A camera 
 From Lynde's Physics of the Household, published 
 by The Macmillan Co. 
 
 side of the postcard and close to it, and arrange two shades 
 to prevent the direct light from falling on the lens. Hold a 
 postcard, or other object, against the black end, focus the lens 
 on a white screen about 2' X 2', and your lantern is finished. The 
 illustration shows the lantern with the top and one side re- 
 moved. The top should have a trapdoor at the rear end through 
 which you can insert and remove the postcards. The audience, 
 is seated on the side of the screen away from the lantern. 
 
 Experiment No. 124. Fun at night. You can put on a magic- 
 lantern show with oil lamps or electric lights as shown in Fig. 
 182. The doorway between two rooms is covered by two heavy 
 curtains and the 8-inch lens in a ring holder is inserted in a 
 hole in a piece of cardboard and pinned between the two cur- 
 tains. A black book stands 10 inches from the lens, and is illu- 
 minated by two strong 
 
 lamps ; two screens 
 prevent the direct 
 light of the lamps 
 from striking the lens. 
 A white tissue paper 
 or cloth screen, 2' X 2', 
 is on the opposite side 
 of the door 40 inches 
 from the lens, the au- 
 dience is beyond the 
 
 Screen, and if now you Fig. 184. You illustrate the camera 
 
GILBERT LIGHT EXPERIMENTS 
 
 121 
 
 Fig. 185. The camera obscura 
 
 hold postcards, drawings, 
 and other small objects 
 upside down against the 
 book, the lens will throw 
 erect and enlarged images 
 on the screen, and your 
 show is on. 
 
 The Photographic 
 Camera is simply a light- 
 proof box with a converg- 
 ing lens in one side and 
 a plate holder in the 
 other. The lens L (Fig. 
 183) throws an inverted 
 image ba of the object 
 AB on the plate S. 
 
 Experiment No. 125. 
 To illustrate the camera. Put your converging lenses in turn in 
 a ring holder, and put the holder in a hole in one end of a card- 
 board box (Fig. 184). Cover the box and your head with a dark 
 cloth and move the screen back and forth until you get a picture. 
 
 The Camera 
 Obscura(Fig.l85) 
 has a combined 
 lens and reflecting 
 prism at the top 
 which throws a 
 picture down on 
 the table in front 
 of the artist. 
 
 E x p e r i ment 
 No. 126. To make 
 
 Fig. 186. You make a camera obscura a Camera ob- 
 
122 
 
 GILBERT BOY ENGINEERING 
 
 scura. Arrange the 8-inch lens, mir- 
 ror, and box as in Fig. 186. Cover the 
 front of the box and your head with 
 a black cloth. Do you get a beau- 
 tiful picture on the white paper at 
 the bottom of the box? 
 
 Experiment No. 127. A moving- 
 picture show. Use the camera ob- 
 scura on a table outdoors or near a 
 window and let two of you get under 
 the black cloth and look at the pic- 
 ture, while two others go through 
 funny antics outdoors about 30 feet 
 from the camera. Do those under 
 the cloth see a very funny moving- 
 picture show? Change places and 
 repeat. 
 
 Experiment No. 128. A sub- 
 marine periscope. Arrange the apparatus as in Fig. 187 with 
 the mirror at 45° at the top of a long cardboard tube and observe 
 the paper under the black cloth. Do you get a fine picture on 
 the paper? 
 
 This illustrates the construction of one type of submarine 
 periscope. 
 
 The Stereoscope (Fig. 
 188) turns two pictures into 
 one that stands out. The 
 glasses are prismatic lenses 
 placed edge to edge; they 
 take light from the two pic- 
 tures AA, A 2 B 2 , Fig. 189, 
 and diverge it so that it ap- 
 pears tO COme from One piC- Fig. 188, The stereoscope 
 
 Fig. 187. You make a submarine 
 periscope 
 
GILBERT LIGHT EXPERIMENTS 123 
 
 tare AB. The pictures are taken in a stereo- 
 scopic camera, which is simply two cameras 
 side by side and a short distance apart. 
 
 Your Eye (Fig. 190) has an outer horny 
 membrane called the cornea and behind this 
 a watery liquid called the aqueous humor, 
 behind this a muscular lens called the 
 crystalline lens and inside this another fluid 
 called the vitreous humor. At the back is 
 the nerve layer, the retina, which receives the 
 sight impression, and behind the retina is a 
 black coating which shuts out all light ex- 
 
 189. How the 
 stereoscope works 
 From Lynde's Phys- 
 ics of the Household, 
 published by The 
 Macmillan Co. 
 
 tmpl 
 
 optic aervK 
 
 Fig. 190. Your eye 
 From Black and Davis' Practical Phys- 
 ics, published by The Macmillan Co. 
 
 focused by moving the lens 
 back and forth ; but the eye is 
 focused by changing the shape 
 of the lens and, therefore, its 
 focal length. The muscles of 
 the eye make the crystalline 
 lens more convex when we view 
 an object near at hand and less 
 convex when we view one at a 
 distance. 
 
 cept that which comes through 
 the lens. The colored part of 
 the eye is the iris and the open- 
 ing in the iris is the pupil. The 
 iris contracts the size of the 
 pupil in a strong light and en- 
 larges it in a dim light. 
 
 The eye is very much like a 
 camera, but there is one strik- 
 ing difference: the camera is 
 
 Fig. 191. The reason for the use of 
 
 spectacles 
 
 From Lynde's Physics of the Household, 
 published by The Macmillan Co. 
 
124 
 
 GILBERT BOY ENGINEERING 
 
 Spectacles. The eyes of short-sighted people focus the light 
 in front of the retina F, Fig. 191 A, and this difficulty is overcome 
 by spectacles with diverging lenses, L. 
 
 The eyes of long-sighted people focus behind the retina F, 
 Fig. 191 B, and this difficulty is corrected by spectacles with 
 converging lenses, L. 
 
 Experiment No. 129. To look through your hand. Your two 
 eyes look along converging lines when you look at any object, 
 
 Fig. 192. To make the bird enter the cage 
 
 and this leads to the following apparent magic. Roll a piece of 
 paper into a tube, hold it beside your hand, look at your hand 
 with one eye and through' the tube with the other. Do you 
 appear to see through your hand? Look through other things 
 in this way. 
 
 Experiment No. 130. To put the bird into the cage. Draw 
 a cage and a bird with centers about 2 inches apart on paper, 
 stand a card on the line AB between them (Fig. 192), then look at 
 
GILBERT LIGHT EXPERIMENTS 125 
 
 the cage with one eye and at the bird with the other. Does the 
 bird enter the cage ? 
 
 The Moving-Picture Machine (Fig. 193) throws 12 to 16 
 pictures on the screen each second and shuts off the light while 
 one picture is changing to the next. The pictures are taken at 
 the same intervals and differ very slightly one from the next 
 (Fig. 194). 
 
 Fig. 193, A moving-picture machine 
 
 The "Why" of the Movies. The reason you see the pictures 
 continuously and are not aware that the light has been shut 
 off is that your eyes retain each picture for a short time after 
 it has left the screen. You will now illustrate this. 
 
 Experiment No. 131. Circles of fire. Go into a dark room, 
 light a match, blow it out but keep the live coal, and then wave 
 
126 
 
 GILBERT BOY ENGINEERING 
 
 it in the air. Do you see circles of fire? You do, because your 
 eye retains the impressions for some time. 
 
 Experiment No. 132. To put the bird into the cage. Draw 
 a bird on one side of a piece of cardboard and a cage exactly 
 opposite on the other side. Attach cords above and below and 
 spin the cardboard. Does the bird appear to enter the cage? 
 It does, because your eyes retain the pictures of the cage 
 and bird for a short time. 
 
 Fig. 194. The "why" of the monies 
 
In the Dark ! 
 
 A knock on the head with 
 a hatchet or a stab with a 
 knife doesn't sound pleasant, 
 but you'll enjoy apparent 
 treatment of this kind and 
 so will your friends who 
 watch your shadow show. 
 Make your boy friend rise 
 in the air— change him into 
 a bird or a cat — create 
 freakish images. It's easy ! 
 And laugh —your audience 
 sure will enjoy it because it's 
 new— nothing like it. An 
 entertainment made for 
 boys who want real fun. 
 But that's only a few of the 
 many things you can do with 
 
 GILBERT 
 LIGHT EXPERIMENTS 
 
 One of these outfits will help you to understand a great many facts about light. 
 You can perform a number of experiments which explain the laws of light. 
 Learn about the movie machine, the telescope and other optical instruments. 
 There's a big book on Light with each set, it's a handy size, just right to put in 
 your pocket 
 
 From this book and your set you'll get a knowledge of light that will be helpful 
 to you always. It's great fun too, the kind you like. The outfit is complete with 
 prisms, mirrors and all the apparatus you'll need to perform the experiments. 
 
 Ask your dealer to show you this new Gilbert toy. 
 If he hasn't it write 
 
 THE A. C. GILBERT COMPANY 
 
 507 Blatchley Ave., New Haven, Conn. 
 
 In Canada — The A. C. Gilbert-Menzies Co., Limited, Toronto, Ont. 
 
 In England — The A. C. Gilbert Co., 125 High Holborn, London, W. C. 1 
 
The Greatest 
 Book for Boys 
 in Years 
 
 BOY ENGINEERING 
 
 The Most Helpful 
 Book for Boys Ever 
 Published 
 
 Think of it! "Football Strategy," by Walter 
 Camp— "How to Pole Vault," by Former 
 World's Champion, A. C. Gilbert— "Flying," 
 by Eddie Rickenbacker, and " Athletic 
 Training," by the famous Yale trainer, 
 Johnny Mack. Chapters about signalling, 
 wireless, wonderful heat, sound and 
 light experiments, how to build a real 
 weather bureau station of your own, 
 chemistry for boys, electrical, hydraulic 
 and pneumatic engineering and surveying, 
 practical carpentry — all in one finely illus- 
 trated book. It's yours for a quarter and 
 worth dollars to you. 
 
 Buy it from your dealer, or 
 send us 25c to-day. You'll 
 never be sorry 
 
 The A.C. Gilbert 
 Company 
 
 507 Blatchley Avenue 
 New Haven : Conn. 
 
GETTY CENTER LIBRARY 
 
 3 3125 00019 0385