CHAPTER XIV.

LIGHT.

334. =Nature of Light.=--We do not know what light is. There are two suppositions in regard to it. One is that of Sir Isaac Newton, called the theory of _emission_. According to this light is a substance, but so ethereal that it has no weight, and is capable of passing through various substances of even great density. The other supposition is what is called the _undulatory_ theory. The advocates of this, which is now quite generally received, believe light to consist of undulations, waves, or vibrations in an ether which is supposed to exist every where, pervading all space and every substance. You perceive an analogy here to sound, the vibrating medium in the case of sound, however, being always some palpable substance--solid, fluid, or aeriform. Heat is supposed, as stated in § 271, to be a vibration of the ethereal substance, as light is, though the two vibrations must of course be somewhat different in character. Any body that is capable of communicating the light-vibration to this ether is said to be _luminous_.

335. =Sources of Light.=--The chief source of light to our earth is the sun, which is a permanently luminous body. Then we have the light of combustion in its various forms. Electricity is another source of light. Light is sometimes emitted during decay or putrefaction of some substances. Some animals--as fire-flies, glow-worms, and phosphorescent animals in the sea--have the power of emitting light.

336. =Light Moves in Straight Lines.=--Light, like heat and sound, radiates in straight lines in all directions from its source. We can see this to be true by admitting rays of light into a darkened room through small openings in the shutters, the rays making straight lines across the darkness, as may be seen by the motes which are flying in the air. The fact is recognized by the marksman in taking aim, and by the engineer in making his levels. The carpenter acts upon it when he tests the smoothness of any surface by letting the light pass along over it to his eye.

337. =Diffusion of Light.=--As light passes in all directions from any body or point, the farther we go from its source the less will the light be. If we take any two rays of light, the farther we trace them from their source the farther are they separated from each other, and what is true of any two rays is true of all the rays. It follows that the farther removed any surface is from a source of light the less light will there be upon it. This decrease of light in proportion to distance is a perfectly regular decrease, and it is as the square of the distance; or, in other words, the intensity of light is inversely as the square of the distance. Take a screen, Fig. 223, and a candle, placing a square piece of pasteboard between them at one foot from each. The shadow on the screen, you see, covers a space four times as large as the pasteboard. That is, the light that shines on the pasteboard, if allowed to pass on to the screen, would be diffused over four times the space, and therefore would have only one-quarter of the intensity. So if as shown in Fig. 224, the screen be placed at twice the distance from the pasteboard that the light is, the shadow will cover a space nine times as large as the pasteboard, and therefore the light there would have one-ninth of the intensity which it has where the pasteboard is. Again, it is seen by Fig. 225 that if the screen be placed at the distance of three feet the intensity of the light is one-sixteenth of that which it is at the pasteboard. While the distances, therefore, are as 1, 2, 3, 4, etc., the intensity of the light is _inversely_ as the numbers 1, 4, 9, 16, etc., that is _inversely as the squares of the distance_.

338. =Velocity of Light.=--The velocity of light is so great that within any ordinary distances it may be considered as instantaneous. Thus when we measure the distance of a cannon by the difference between the time of its flash and the report, we do not reckon the light to consume any time in its passage to the eye. But when we come to look at objects as distant as the sun and other heavenly bodies, we reckon in our calculations the time of the passage of light. It takes light eight minutes to travel from the sun to us, a distance of ninety-five millions of miles. With the telescope stars have been seen which have been ascertained to be at such a distance that it requires over ten years for their light to come to the earth. Others have been seen which are much farther off, but their distances have not been absolutely ascertained. Some have been seen supposed to be at such a distance that the light coming from them to the eye of the astronomer was a hundred thousand years in its passage.

339. =Roemer's Observations.=--The velocity of light was first determined by Roemer, a Danish astronomer, in 1676. It was done in his calculations and observations of the eclipse of one of Jupiter's moons. After making the calculation of the time it would take for the satellite to pass through the shadow of the planet, he observed its passage, and found that it did not come out from the shadow as soon as his calculation required by fifteen seconds. What was the difficulty? If the earth had remained in one spot from the beginning to the end of the passage of the satellite, the observation would have come out exactly according to the calculation. But the earth had moved during this time (about forty-two hours and a half) the immense distance of 2,880,000 miles. The light of the emerging satellite therefore had to travel over this additional distance to overtake the earth, and it took fifteen seconds to do it. If we divide, then, this distance by 15 we get the distance which light travels in a second, which is 192,000 miles. All this can be made clear by the diagram, Fig. 226. Let S be the sun, J Jupiter, and C one of its moons emerging from its shadow. Let A be the earth as it is when the eclipse of Jupiter's moon begins. When it emerges the earth has passed to B, and the light from the satellite has to travel as much farther to reach it now as B C is longer than A C. Roemer made other observations with the earth at some other parts of her orbit with the same result.

340. =Reflection of Light.=--Light, like sound and heat, is reflected in straight lines when it strikes upon any resisting substance. We can see this to be the case when it strikes upon any smooth and plane surface. And it is true of light, as it is of heat, that the angles of incidence and reflection are equal. Thus if _c_, Fig. 227, be a reflecting surface, and _b c_ a line perpendicular to it, then a ray of light, _d c_, will be reflected in the line _c a_, and the angle of incidence, _d c b_, will be equal to the angle of reflection, _b c a_.

341. =How we See.=--We see the various objects around us by the light which is reflected from them. Every point of every surface that we see reflects rays or vibrations of light to our eyes. Thus if we see a person there are rays of light reflected into our eyes from every part of him. These rays form an image of him in the back part of each eye, and it is by this image that we see him, as will be explained in full in another part of this chapter. Reflected light is painting the images of objects in the eye every moment in great abundance and variety. If a speaker have an audience of a thousand persons all looking at him, his image is at the same time in two thousand eyes, and in each of these two thousand images every motion and every changing expression are faithfully depicted.

342. =Mirrors.=--That reflected light does thus form images of objects you see in the common mirror. The image formed in it of any object comes from the light reflected from that object into the glass. Then in seeing the image light is reflected from it into the eye, there to form a similar image, though of much less size. By using two or more mirrors the reflections of the image can be multiplied, and by some arrangements of them to a very great extent. That the image appears to be at the same distance beyond the surface that the object is before it, is owing to the fact that the reflected rays come from the glass at the same angle that the incident rays strike upon it. This may be shown from Fig. 228 (p. 263). Suppose _m m'_ is a looking-glass, and an arrow, A B, is before it. Rays of light come from it at all points to the glass. We will take only two of these rays at each end of the arrow. The ray A _g_ will be reflected to the eye at the same angle in the ray _g o_, and the ray A _f_ will be reflected in the ray _f_ E. And the reflected rays will have the same rate of divergence as the incident rays. The same can be shown in regard to rays from B or any other point on the arrow. Now if the lines _o g_ and E _f_ be extended, they will meet at the point _a_, which is at the same distance behind the mirror as A is before it. The same thing can be shown of the rays from B or any other point. Therefore the image of the arrow will appear to the eye to have the same relative position behind the glass that the arrow itself has before it.

343. =The Kaleidoscope.=--I have already noticed the multiplication of the images of objects by using two or more mirrors. In the kaleidoscope, by a particular arrangement of mirrors, the images are multiplied, and by changes in the position of the objects the relative positions of the images are infinitely varied. Fig. 229 will serve to explain the operation of the instrument. Let A B and B C be two plane mirrors placed at right angles to each other, and _a_ an object before them. Let I be the position of the eye looking at the mirrors. The rays _a f_ and _a g_ will be reflected to I as represented, and the eye will see two images, which appear to be at _b_ and E. But the ray _a_ K will be reflected to _c_, and then to I, so that a third image will be seen at _d_. Here is but a single second reflection, or reflection of an image; but by placing the mirrors at an angle of 60°, 45°, and 30° the images may be increased to six, eight, and ten, having a circular arrangement. In the kaleidoscope two mirrors are placed in a tube at an angle of 30°, and variously-colored pieces of glass in the farther end of the instrument, changing their relative position with every movement of it, give an endless variety of images symmetrically arranged.

344. =Curved Mirrors.=--These may be concave or convex. The action of a concave mirror upon light may be illustrated by Fig. 230. If parallel rays, as represented, strike upon the mirror they will, in their reflection, be made to _converge_, or come together, at the focus, _a_. But suppose the light comes from this focus, the rays of course _diverging_, or going away from each other; then the rays, as reflected, will be parallel. If the light or object be nearer to the mirror than the focus, and the rays of course be more diverging, then the effect of the mirror will be to lessen the divergence when the rays are reflected. You see that the tendency is to make the rays converge. And hence concave reflectors are much used when it is desired to throw a great amount of light in one direction. The effect of the concave mirror upon the apparent size and position of objects placed before it varies with the relation of their position to the focus. The action of a convex mirror upon light is the opposite of that of the concave. Its tendency is to make the rays diverge. Thus (Fig, 231), if parallel rays strike upon a convex mirror they diverge, as if they came from a focus behind the mirror, as _b_, as indicated by the dotted lines.

345. =Refraction of Light.=--When light passes from one medium into another it is bent from its course. This may be illustrated by Fig. 232, in which A B C D is a box, into which a candle, E, is shining. The candle is so placed that the shadow of the side A C falls at D. But let the box be filled with water, and now the shadow is removed to _d_, as if the candle were at _e_. This is because the rays of light from the candle, in passing from the air into the water, are bent or refracted so as to take a different direction. Here we have light passing from a rarer into a denser medium. Let us see now how it is when light passes from a denser medium into a rarer. This can be illustrated on Fig. 233. Let the vessel, A B C D, be empty, and let a coin be placed at O. Let the eye, E, be in such a position that a straight line, O G E, from the coin to the eye would strike the side of the vessel a little below the edge, or, in other words, that the edge of the vessel would prevent the eye from seeing it. If now, keeping the eye in this position, water be poured in up to a certain level, say F G, the coin comes into view. This is because light coming from the coin to L is bent into another direction, L E, and the coin therefore appears to the eye to be at K. In this case the refraction is _from_ the perpendicular, P Q, let down through the point L, where the light emerges from the denser into the rarer medium. But When light passes from a rarer into a denser medium the refraction is reversed--it is _toward_ the perpendicular. It is from this refraction of light that a stick partly immersed in water appears to the eye to be broken just at the surface of the water.

346. =Dawn and Twilight.=--The light of the sun, in passing from space into our atmosphere, is refracted. If it were not we should have no daylight preceding the rise of the sun, or twilight after its setting; but light would burst upon the darkness of night at once when the sun appeared above the horizon, and darkness would suddenly succeed to the light of day at sunset. As it is, in the morning the light bends toward us as it strikes across the atmosphere long before we see the sun, and after the sun has disappeared from view at evening its light bends toward us in the same manner. And farther, we really see the sun in the morning before it gets above the horizon, and in the evening after it has gone below it. This may be made clear by Fig. 234. Let the central ball represent the earth. Now as the atmosphere is most dense near the earth, and is rarer as you go outward from the earth, it is represented in the figure as having different layers in order that the operation of the refraction may be more clear to you. The outermost layer is exceedingly rare, and each layer is more dense than the previous one as you go in toward the earth. The light coming from the sun, S, below the horizon into the first layer of air, instead of passing on straight to _a_, as indicated by the dotted line, bends toward the earth. Then in entering the second layer, instead of passing on to _b_, it will be bent or refracted still more, as this layer is denser; and so on through all the layers, being refracted in each more than in the previous one. The result is, that as every object is seen in the direction in which the rays from it at length reach the eye, the sun, though really below the horizon, appears to be above it, as represented. The path of light from the sun, as it passes through the air, is a curved line. This is because the air, instead of being of uniform density, lessens in density as we go from the earth. If it were of uniform density the light would be refracted in straight lines, as in the experiments in § 345.

347. =Mirages.=--Sometimes inequalities occur in the density of the lower portions of the atmosphere, causing, of course, unequal refraction, and producing some strange appearances, termed _mirages_. For example, at Ramsgate, on the coast of England, there was seen, at one time, as represented in Fig. 235 (p. 268), a ship at such a distance that only her topsails were visible; and above in the air there were two complete images of the ship, the uppermost being erect and the under one inverted. Captain Scoresby, in a voyage to Greenland, saw an inverted image of a ship so well defined that he decided that it was the image of his father's ship, the _Fame_, which was afterward verified. The ship itself was at that time at a distance of 30 miles. An incident in the early history of the author's place of residence may be cited as an example of mirage. A ship left for England freighted with a valuable cargo, and having on board a large number of the best citizens of the colony. Some time after there was immense excitement in New Haven, because the inhabitants saw, with great distinctness, what they supposed to be this vessel, at only a little distance, apparently sailing against the wind. But it soon disappeared from view, part after part, until the whole was gone. The ship itself was never heard from, and it was supposed at the time that this appearance was a manifestation of Providence for the purpose of informing the colonists what had become of their friends. But what was seen was undoubtedly the reflected image of this or some other ship. It is such appearances as these that have given rise to the stories which have been sometimes told of phantom ships. Mirages are very common in the extensive deserts in hot climates, exhibiting to the eye of the traveler various deceptive appearances, as islands, lakes, etc. In Bonaparte's campaign in Egypt such an appearance caused whole battalions of thirsty soldiers to rush forward, supposing at the moment that a plentiful supply of water was at hand.

The most astonishing instance of mirage of which I have ever heard is thus narrated: "The cliffs on the French coast are 50 miles distant from Hastings, on the coast of Sussex, and they are actually hidden from the eye by the convexity of the earth; that is to say, a straight line drawn from Hastings to Calais or Boulogne would pass through the sea. A year or two ago, however, a Fellow of the Royal Society, who was residing at Hastings, was surprised to see a crowd of people running to the sea-side. Upon inquiry as to the cause of this he was informed that the coast of France could be seen by the naked eye. He immediately went down to the shore to witness so singular a sight, and there discovered distinctly the French cliffs extending for some leagues along the horizon, and so vividly that they appeared to be only a few miles off. The sailors and fishermen, with whom Mr. Latham walked along the water's edge, could hardly at first be persuaded of the reality of the appearance; but as the cliffs gradually became more elevated they were so convinced that they pointed out to Mr. Latham the different places they were accustomed to visit--such as the bay and the wind-mill at Boulogne, St. Vallery, and other places on the coast of Picardy, even as far as Dieppe, all the French shores appearing to the English sailors as if they were sailing at a short distance from them toward the harbors. With the aid of a telescope the French fishing-boats were plainly seen at anchor; and the different colors of the land upon the heights, together with the buildings, were perfectly discernible. The day when this occurred is said to have been extremely hot, without a breath of wind stirring, and the phenomenon continued visible in the highest splendor until past eight o'clock in the evening, having been seen for three hours continuously."

348. =Visual Angle.=--In order that you may understand the operation of lenses in relation to vision I must first explain to you what is meant by the visual angle. In Fig. 236 (p. 270) are represented arrows of the same size at different distances from the eye. From the ends of each of the arrows are drawn lines to the eye. The angle which these lines make in each case as they meet at the eye is termed the visual angle. Now the apparent size of an object depends upon the size of this angle. The degrees of the angles are marked upon the figure. Thus the visual angle of the nearest arrow is 120 degrees, and that of the second is 60, only half as large. The first arrow therefore appears twice as large as the second. For the same reason it appears four times as large as the third, eight times as large as the fourth, and twelve times as large as the fifth. The same thing is illustrated in another way in Fig. 237. Here the arrows _e f_, _g h_, and _i k_ appear to the eye as large as A B, because they have the same visual angle, and for this reason make an image of the same size in the eye, as you see is indicated in the figure. It is hardly necessary to say that what is true of objects as a whole is true also of any part of them. Each part, however small, has its visual angle, and this governs its apparent size.

349. =Lenses.=--Transparent bodies having curved surfaces are called lenses. There are six kinds, represented in Fig. 238. The lenses in most common use are the double convex and double concave. The explanation of the mode in which these act upon light will sufficiently illustrate the operation of the others. They act by refraction, the convex collecting the rays, or bringing them nearer together, and the concave putting them farther apart. You can at once see, then, that a convex lens by causing the rays coming from an object to converge more, increases the visual angle, and therefore makes the object to appear larger than it otherwise would. This effect is illustrated by Fig. 239. The rays of light coming from the arrow are made by the lens so to converge as to meet at _a_, instead of _b_, where they would meet if they did not pass through the lens. That is, by passing through the lens they have a larger visual angle, and therefore the object is magnified. The distance between, _c_ and _d_ shows the size which the arrow would appear to have to the eye placed at _a_.

350. =Microscopes and Telescopes.=--What has been said of the action of the convex lens upon the visual angle will serve to explain the operation of the microscope. This instrument may be single or compound. The compound microscope has more than one lens, and is used to magnify very minute objects. Its operation may be seen by the diagram, Fig. 240. Rays from the object, E F, passing through the first lens, or object-glass, as it is called, form a magnified inverted image, G H, which is still more magnified by the eye-glass, C D. In the telescope we have also convex lenses, but they are arranged differently from those of the microscope, as the objects to be magnified are distant.

351. =Magic Lantern.=--This is an instrument by which pictures made upon slips of glass with coloring substances which allow the light to pass readily are thrown upon a screen magnified. It is a metallic lantern, A A, Fig. 241, with a concave reflector, _p q_, and two convex lenses, _m_ and _n_. At _c d_ is a space between the lenses into which the pictures are introduced. L is a strong light, which is in the focus both of the mirror and the lens _m_. The picture is therefore illuminated strongly by the rays reflected from the mirror and passed through the lens. The lens _n_ which is movable, is so adjusted as to throw a highly magnified image of the picture upon the screen. As the image is an inverted one the pictures must be inserted upside down, that the images on the screen may be upright. The _solar microscope_ is, in its essential parts, like the magic lantern, the sun being used as the illuminator.

352. =Camera Obscura.=--This instrument differs from the magic lantern in giving us diminished images of objects. An instrument of this kind can be arranged extemporaneously any where. Thus, if into a darkened chamber light be admitted through a small opening, inverted images of any objects in front of the opening will be formed upon a white screen in the opposite part of the chamber. Such an arrangement is represented in Fig. 242 (p. 273), C D being the chamber, L the opening, and _a b_ the image of the object A B. The images in such a case, however, are faint, because the opening must necessarily be small, and therefore but few rays, comparatively, come from the objects. By making the opening larger, and gathering the rays that enter it with a double convex lens, we can have well-defined and bright images of objects. Though the camera obscura may have various forms, I have described what is essentially the arrangement of the instrument. One form of it, for sketching either single objects or groups of them in landscapes, is represented in Fig. 243. Here the rays of light coming from objects strike upon a mirror, A B, and are reflected through a convex lens, C D, upon white paper on the bottom, E F, of the box, where the outlines of the images are traced by the sketcher. The light can enter only at the opening above, for on the side of the box which is open there hangs down a curtain on the back of the artist as he sketches.

353. =The Eye.=--The eye is essentially a camera obscura. It is a dark chamber in which images are formed upon a screen in its back part, and the light which comes from objects is admitted through an opening in front, where there is a double convex lens. That you may understand the manner in which the images are formed, I give you, in Fig. 244, a map of the eye. At _a_ is the thick, strong white coat called the _sclerotic_ coat, from a Greek word meaning hard. This, which is commonly the white of the eye, gives to the eyeball its firmness. Into this is fastened in front, like a crystal in a watch-case, _e_, the _cornea_. The sclerotic and cornea, you see then, make together one coat of the eye, the outer one. The cornea is the clear, transparent window of the eye through which the light enters. Next to the sclerotic coat comes the _choroid_ coat, which is dark, to prevent too much reflection back and forth in the eye. Then you have a very thin membrane, _c_, the _retina_, the screen on which the images are formed. This is composed chiefly of the fine fibres of the nerve of sight, _d_. To return to the front of the eye where the light enters--behind the cornea is the iris, _g g_, which is immersed in a watery fluid, _f_, called the _aqueous humor_. The light passing through the cornea and the aqueous humor comes to the crystalline lens, _h_, which, you see, is a double convex lens. Passing through this and through a jelly-like substance, called the vitreous humor, which fills all that large space _i_, it strikes upon the retina, _c_, where it forms the images of the objects from which it came.

You see now how the eye is like a camera obscura. You have in it the dark chamber with its screen, the opening through the iris, the pupil, for the admission of the light, and just behind this opening the lens for gathering or concentrating the light before it falls upon the retina. The refraction of the light is not, however, done wholly by this lens. The projecting cornea, with its contained aqueous humor, refracts it considerably, for it forms a convex lens.

354. =Distinct Vision.=--In order that vision may be perfectly distinct, it is necessary that the rays coming from each point of the object which is seen should, on converging, meet together, or be brought to a focus on the screen of the eye, the retina. Thus, in Fig. 245, the rays which come from _a_, the end of the arrow, meet on the retina at _b_, and those from _c_, the other end, are brought to a focus at _d_. Now the muscles of the eye have considerable power in adjusting the eye to objects at different distances, so as to bring the rays in most cases together exactly at the retina. They fail to do it with objects that are very near. You can see that this is so if you bring any object, as your finger, nearer and nearer to the eye. You will at length find that you can not see it distinctly. The reason is, that the rays from it diverge so much that the cornea and lens can not make them converge enough to meet at the retina. This divergence of rays at different distances is illustrated in Fig. 246. Suppose that you are looking at some very minute object. The nearer you bring it to the eye the better you can see it, till you come to a certain point. There the rays are so divergent, as you can readily see by the figure, that the lenses of the eye can not make them converge sufficiently for distinct vision. Now just here the microscope comes in to help the eye by causing these divergent rays to come nearer together before they enter the window of the eye, the cornea.

355. =Near-Sighted and Far-Sighted.=--Some persons have their eyes so shaped that they can not fully adjust them to objects at different distances. Thus the near-sighted can see with distinctness only objects that are near. The reason is that the rays converge too much, and are brought to a focus before they arrive at the retina, as represented in Fig. 247. The images therefore of distant objects are indistinct. If the retina could in any way be brought forward a little the difficulty would be obviated. But as this can not be done, concave glasses are resorted to, which counteract the effect of the too highly refractive power of the eye. In the far-sighted the difficulty is of an opposite character. The refractive power is so feeble that when near objects are viewed the rays are not brought to a focus soon enough, as seen in Fig. 248. Convex glasses are used in this case, making the divergent rays of near objects less divergent before they enter the cornea.

356. =Images in the Eye Inverted.=--The images formed on the retina are inverted. This can be proved by taking the eye of an ox and carefully paring off the back of it, leaving little else than the retina itself. Holding now a candle before the eye, its image may be seen inverted upon its rear part. The question arises why it is that we see objects erect when their images on the retina are inverted. On this point I will quote from my _Human Physiology_: "It has been supposed by some that we really see every thing reversed, and that our experience with the sense of touch, in connection with that of vision, sets us right in this particular. And this it is supposed is the more readily done from the fact that our own limbs and bodies are reversed as pictured on the retina, as well as objects that are around us, so that every thing is _relatively_ right in position. But if this be the true explanation, those who have their sight restored after having been blind from birth should at first see every thing wrong side up, and should be conscious of rectifying the error by looking at their own limbs and bodies. But this is not so. The above explanation of erect vision, and other explanations of a similar character, are based upon a wrong idea of the office which the nerve performs in the process of vision. It is not the image formed upon the retina which is transmitted to the brain, but an impression produced by that image. The mind does not look in upon the eye and see the image, but it receives an impression from it through the nerve; and this impression is so managed that the mind gets the right idea of the relative position of objects. Of the way in which this is done we know as little as we know of the nature of the impression itself."

357. =Single Vision.=--Whenever we see any object with both eyes there is an image formed in each eye, and impressions go from both eyes by the optic nerves to the brain. And yet with these two impressions there is no double vision so long as the two eyes correspond with each other in situation. This is because the image in one eye occupies the same place on the retina that the image in the other eye does. The correspondence is ordinarily perfect, the two eyes turning always together in the same way, upward, downward, or laterally, without the least variation. You can observe the effect of a want of this correspondence by pressing one of the eyes in some direction with the finger while the other is left free to move in obedience to the muscles. When this is done every object appears double, because its image occupies in one eye a different part of the retina from what it does in the other, and so two different impressions are carried to the brain. The same thing occurs in squinting, in which the action of the muscles of the two eyes does not agree. Ordinarily in squinting there is not double vision, because the mind has the habit of disregarding the impressions that come from the defective eye. But when squinting occurs suddenly from disease there is double vision, for it takes a little time to form the habit referred to.

358. =Stereoscope.=--The images of objects in the two eyes, though always similar, are not generally perfectly alike. They are so only when the object presents a simple surface, as in the case of pictures. When the object presents two or more surfaces to the sight the images are more or less unlike. This can be illustrated in a very simple way. Hold a book up straight before your eyes with its back toward you. You see the back and both sides. Now if you shut your right eye you will see with the left the back of the book and the left side. That is, these two parts of the book are imaged on the retina of the left eye. By shutting the left eye it will appear that the image in the right is different, for you see now with the back the right side of the book. Here you have the explanation of the stereoscope. In the right side of this instrument you have the picture of the object as the object itself would appear to the right eye, and in the left side you have the picture of it as it would appear to the left eye. Thus, if a book in the position alluded to above were the object, in the right picture there should be represented the back together with the right side of the cover, and in the left the back with the left side of the cover. The two impressions, carried to the brain by the optic nerves, give together the impression of a solid book. The same principles apply to the representation of all solids in the stereoscope.

359. =Thaumatrope.=--Each impression made upon the optic nerve by light lasts about the eighth part of a second. No distinct impressions can be made, therefore, upon the retina unless they succeed each other with less rapidity than this. If, for example, in the revolution of a wheel, eight or more spokes pass by one point in a second, they can not be seen as distinct spokes, but will be mingled together, producing one continuous impression. So, too, if a light revolve so as to describe a circle in an eighth part of a second it will appear to the eye as one unbroken circle of light. It is this continuous impression on the retina that makes small objects, as the cars pass swiftly along, appear to run in long lines along with us. The fact thus developed is made use of in the contrivance of a toy called the thaumatrope. A picture is made on each side of a circular card, and whirling the card around very rapidly by means of two strings fastened to it, the two pictures are made to mingle together as one. Thus in Fig. 249 are represented the two sides of such a card, on the one side there being the picture of a dog, and on the other that of a monkey. When made to revolve rapidly the monkey will be seen sitting on the back of the dog.

360. =Light Compound.=--I have thus far spoken of light as if it were a simple thing. But it is compound. Every ray of white light has in it seven different colors. That this is so we can prove by taking a beam of light by itself and dissecting it, as we may say, or separating it into its seven parts. I will show you how this can be done. Let D E, Fig. 250, a beam of the sun's light, pass through a small opening in a shutter into a dark room. The rays will pursue a straight course, and if a screen be placed at F they will make a spot of white light. But if a glass prism, A B C, be held in the position represented the rays will be refracted, and when received upon the screen M N the light will be separated into seven colors in the order which is given. The figure thus produced is called the solar spectrum. Observe why it is that the colors are separated. It is because they are refracted unequally. If they were equally refracted the light upon the screen would be white, as before it was refracted. The violet rays are most refracted, the indigo next, the blue next, etc., and the red are the least refracted of all.

361. =Proportion of the Colors in Light.=--The colors in light are not equal in amount. If we divide the spectrum into 360 equal parts the proportion in the colors will be as follows: red, 45; orange, 27; yellow, 40; green, 60; blue, 60; indigo, 48; violet, 80.

Some suppose that there are really but three simple colors, red, yellow, and blue, the other colors being produced by a combination of these. Thus red and yellow will together form orange, and yellow and blue will form green.

362. =Recomposition of Light.=--After decomposing light by passing it through a prism we can bring the separated colors together again and form from them white light. The manner in which this is done is represented in Fig. 251. The beam of light, after passing through the prism S A A', instead of proceeding in the direction indicated by the dotted lines to form the spectrum, is made to pass through the prism S' B B', placed in a reversed position, and its rays are refracted so as to assume their original relation, making a white beam, M. Here the second prism counteracts the effect of the first, because its position is exactly the reverse.

Newton very justly considered the decomposition and the recomposition of light as affording the most positive proof that white light contains all the seven colors. He tried various experiments to prove the same thing. Thus he mingled together intimately seven powders having the seven prismatic colors, and found that the mixture had a grayish-white aspect. He also painted a circular board with these colors, and found that on whirling it so rapidly that the colors could not be distinguished the whole board appeared to be white. In order to have this succeed perfectly the proportion between the colors must be observed, as in Fig. 252. A very pretty way of illustrating the composition of light is to have a top painted in this way. When the top is whirling rapidly it is white, but as it slackens its motion the seven colors appear.

363. =Colors of Objects.=--The color of any object depends upon the manner in which it reflects light. Thus, if it be red, it reflects the red rays of the spectrum, absorbing the other rays; and if it be green, it reflects the green rays, etc. If it reflect all the colors together, it is white; and if it reflect none, or almost none, of the light, it is black.

You can readily see why the color of an object varies with the kind of light that shines upon it. If an object which is red in sunlight be exposed to a yellow light, as a yellow flame, or sunlight that has passed through a yellow-colored glass or curtain, it loses its red color, for there are no red rays in the light to be reflected by it into our eyes. A person exposed to such a light has a deathlike paleness, the lips and skin losing entirely their red color. This effect can be witnessed at any time by mixing alcohol with a little salt on a plate and setting fire to it. You see in what has been said the reason that, in examining goods in the evening, especially by candle-light, we find the colors often differ somewhat from those which they have in the day.

In some substances the colors are changeable with varying positions, though the light be the same. We see this often in shells and minerals. We see it also in some fabrics, as changeable silk. This is owing to the arrangement of the particles, it being such as to occasion variety in reflection with changes of position.

364. =Colors of the Clouds.=--There is no more gorgeous display of colors than we sometimes see in the clouds at morning or evening, especially the latter. These colors are occasioned simply by refractions and reflections in the minute vesicles (§ 288) of which the clouds are composed. How simple are the materials, light, water, and air, and yet how grand and diversified are the results!

365. =The Rainbow.=--In producing the colors of the rainbow the materials are less even than in producing those of the clouds. They are only light and water. The colors come from the reflection and refraction of light in the drops of the falling rain. I will illustrate the manner in which these reflections and refractions take place. Take a single drop, represented in Fig. 253. Let S be a beam from the sun. This entering the drop at A, is refracted, and passes to B, at the farther side of the drop. Here a portion of it is lost by its proceeding on in the line B C. The remainder is reflected to D, and passes to E, being refracted as it thus passes out into a rarer medium, the air. Here you have a single reflection and two refractions. But in the second bow, which is sometimes formed, there are two reflections as well as two refractions, as represented in Fig. 254. The beam of light, S, from the sun enters the drop at A, is refracted, and passes to B. Here a portion proceeds on in the direction B C. The other portion is reflected to D. Then this is lessened by a part of it proceeding on in the line D E. What remains is reflected to E. You see here the reason that the second bow is not so bright as the primary one. In the latter there is but one reflection in each drop, and therefore there is but one point where there is loss of light by its passing on out of the drop; while in the former there are two reflections, and therefore loss at two points.

366. =Circumstances under which Rainbows are Seen.=--A rainbow is seen when the spectator stands between the sun and falling rain. This commonly can not be the case, except in the latter part of the day. It sometimes, though very rarely, happens that a shower passes from the east to the west in the morning, and then a rainbow can be seen in the west. Fig. 255 is intended to show under what circumstances a rainbow is seen. Let a horizontal line be drawn from O, the observer, to P, a point directly under the middle point of the arch. If this line were extended backward from the observer it would be precisely in the direction of the sun from him. That is, the sun is directly opposite the middle of the bow. Now if the drop at A reflect a red ray to the eye of the spectator all other drops similarly situated in the arch will reflect red rays. So if B reflect a green ray all other drops similarly situated will do the same. And so of C, reflecting the violet ray. For the sake of clearness there are only three reflections represented, but the same is true of all the seven colors. In the secondary bow the arrangement of the colors is reversed, the red being at the inner part of the bow and the violet at the outer part. The double reflections are manifest in the drops D, E, and F. What I have described as taking place in a few drops takes place in countless multitudes of them in forming the bow. As the exact place of the rainbow depends not only upon the direction of the rays of the sun but also the position of the spectator, it is clear that no two spectators see precisely the same bow, for the drops that form it for the one are not the same drops that form it for the other. This is very obvious if the two be quite distant from each other; but it is equally true if they are very near together, although in this case the bow for the one would be very nearly coincident with the bow for the other. It is also true that the rainbow of one moment is not the rainbow of the next, for as the drops that reflect it are falling drops there must be a constant succession of them in any part of the bow.

367. =Colors in Dew-Drops and Ice-Crystals.=--We often see something very analogous to the rainbow in the dew. As the sun rises, if, with our backs to it, we look at the dew-drops, we see all the colors of the rainbow glistening every where before us, as if the grass were filled with gems of every hue. Here we have the same refraction and reflection in drops of water, and the resemblance fails only in the regularity of arrangement which the rainbow presents. We see the same thing also if the ground is strewed with bits of ice which have fallen from the branches of the trees, and the sun shines aslant upon them.

368. =Heat and Light.=--We have not yet finished our dissection of the beam of light, begun in § 360. In the beam of light which is separated into the seven colors there is heat also; and in the separation it is found, as represented in Fig. 256, that the rays of heat are most abundant just beyond the red rays, while they are very sparing indeed at the other end of the spectrum. The greatest degree of light is at the boundary between the orange and the yellow rays.

369. =Chemistry of Light and the Daguerreotype.=--There is a chemical power also in light, producing every where, quietly but thoroughly, important effects. The chemical rays are most abundant at the end of the spectrum opposite to that where the heat-rays abound. It is these which do the work in Daguerreotyping. In this art light has been said to be the painter; but this is not strictly true. Light makes the image of the object, just as it does in the camera obscura and in the eye, but it has no power to fasten that image upon the metallic plate. This is done by the chemical rays, which, like the rays of heat, go along with the light. Without going into particulars, which will be given in Part Second, the process of Daguerreotyping is simply this: A metallic plate is so prepared that the chemical rays of light shall act upon it sensibly. Then the object to be taken--a person or any thing else--being before the instrument, a slip of ground glass is inserted, and when the operator gets the lens so adjusted that a good image of the object is seen on the glass he takes this out and puts in its place the metallic plate. Rays of light coming from the object make the image, and the chemical rays bound up with the light act upon the plate so as to fix the image there.