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Through Magic Glasses and Other Lectures By Arabella B. Buckley Characters: 38316

Updated: 2017-12-06 00:03

he sun shone brightly into the science class-room at mid-day. No gaunt shadows nor ghostly moonlight now threw a spell on the magic chamber above. The instruments looked bright and business-like, and the Principal, moving amongst them, heard the subdued hum of fifty or more voices rising from below. It was the lecture hour, and the subject for the day was, "Magic glasses, and how to use them." As the large clock in the hall sounded twelve, the Principal gathered up a few stray lenses and prisms he had selected, and passed down the turret stair to his platform. Behind him were arranged his diagrams, before him on the table stood various instruments, and the rows of bright faces beyond looked up with one consent as the hum quieted down and he began his lecture.

"I have often told you, boys, have I not? that I am a Magician. In my chamber near the sky I work spells as did the magicians of old, and by the help of my magic glasses I peer into the secrets of nature. Thus I read the secrets of the distant stars; I catch the light of wandering comets, and make it reveal its origin; I penetrate into the whirlpools of the sun; I map out the craters of the moon. Nor can the tiniest being on earth hide itself from me. Where others see only a drop of muddy water, that water brought into my magic chamber teems with thousands of active bodies, darting here and whirling there amid a meadow of tiny green plants floating in the water. Nay, my inquisitive glass sees even farther than this, for with it I can watch the eddies of water and green atoms going on in each of these tiny beings as they feed and grow. Again, if I want to break into the secrets of the rock at my feet, I have only to put a thin slice of it under my microscope to trace every crystal and grain; or, if I wish to learn still more, I subject it to fiery heat, and through the magic prisms of my spectroscope I read the history of the very substances of which it is composed. If I wish to study the treasures of the wide ocean, the slime from a rock-pool teems with fairy forms darting about in the live box imprisoned in a crystal home. If some distant stars are invisible even in the giant glasses of my telescope, I set another power to work, and make them print their own image on a photographic plate and so reveal their presence.

"All these things you have seen through my magic glasses, and I promised you that one day I would explain to you how they work and do my bidding. But I must warn you that you must give all your attention; there is no royal road to my magician's power. Every one can attain to it, but only by taking trouble. You must open your eyes and ears, and use your intelligence to test carefully what your senses show you.

Fig. 10. Eye-ball seen from the front.

(After Le Gros Clark.) w, White of eye. i, Iris. p, Pupil.

"We have only to consider a little to see that we depend entirely upon our senses for our knowledge of the outside world. All kinds of things are going on around us, about which we know nothing, because our eyes are not keen enough to see, and our ears not sharp enough to hear them. Most of all we enjoy and study nature through our eyes, those windows which let in to us the light of heaven, and with it the lovely sights and scenes of earth; and which are no ordinary windows, but most wonderful structures adapted for conveying images to the brain. They are of very different power in different people, so that a long-sighted person sees a lovely landscape where a short-sighted one sees only a confused mist; while a short-sighted person can see minute things close to the eye better than a long-sighted one."

"Let us try to understand this before we go on to artificial glasses, for it will help us to explain how these glasses show us many things we could never see without them. Here are two pictures of the human eyeball (Figs. 10 and 11), one as it appears from the front, and the other as we should see the parts if we cut an eyeball across from the front to the back. From these drawings we see that the eyeball is round; it only looks oval, because it is seen through the oval slit of the eyelids. It is really a hard, shining, white ball with a thick nerve cord (on, Fig. 11) passing out at the back, and a dark glassy mound c, c in the centre of the white in front. In this mound we can easily distinguish two parts-first, the coloured iris or elastic curtain (i, Fig. 10); and secondly, the dark spot or pupil p in the centre. The iris is the part which gives the eye its colour; it is composed of a number of fibres, the outer ones radiating towards the centre, the inner ones forming a ring round the pupil; and behind these fibres is a coat of dark pigment or colouring matter, blue in some people, grey, brown, or black in others. When the light is very strong, and would pain the nerves inside if too much entered the pupil or window of the eye, then the ring of the iris contracts so as partly to close the opening. When there is very little light, and it is necessary to let in as much as possible, the ring expands and the pupil grows large. The best way to observe this is to look at a cat's eyes in the dusk, and then bring her near to a bright light; for the iris of a cat's eye contracts and expands much more than ours does."

Fig. 11.

Section of an eye looking at a pencil. (Adapted from Kirke.)

c, c, Cornea. w, White of eye. cm, Ciliary muscle. a, a, Aqueous humour. i, i, Iris. l, l, Lens. r, r, Retina. on, Optic nerve. 1, 2, Pencil. 1′, 2′, Image of pencil on the retina.

"Now look at the second diagram (Fig. 11) and notice the chief points necessary in seeing. First you will observe that the pupil is not a mere hole; it is protected by a curved covering c. This is the cornea, a hard, perfectly transparent membrane, looking much like a curved watch-glass. Behind this is a small chamber filled with a watery fluid a, called the aqueous humour, and near the back of this chamber is the dark ring or iris i, which you saw from the front through the cornea and fluid. Close behind the iris again is the natural 'magic glass' of our eye, the crystalline lens l, which is composed of perfectly transparent fibres and has two rounded or convex surfaces like an ordinary magnifying glass. This lens rests on a cushion of a soft jelly-like substance v, called the vitreous humour, which fills the dark chamber or cavity of the eyeball and keeps it in shape, so that the retina r, which lines the chamber, is kept at a proper distance from the lens. This retina is a transparent film of very sensitive nerves; it forms a screen at the back of the chamber, and has a coating of very dark pigment or colouring matter behind it. Lastly, the nerves of the retina all meet in a bundle, called the optic nerve, and passing out of the eyeball at a point on, go to the brain. These are the chief parts we use in seeing; now how do we use them?

"Suppose that a pencil is held in front of the eye at the distance at which we see small objects comfortably. Light is reflected from all parts of the surface of the pencil, and as the rays spread, a certain number enter the pupil of the eye. We will follow only two cones of light coming from the points 1 and 2 on the diagram Fig. 11. These you see enter the eye, each widely spread over the cornea c. They are bent in a little by this curved covering, and by the liquid behind it, while the iris cuts off the rays near the edges of the lens, which would be too much bent to form a clear image. The rest of the rays fall upon the lens l. In passing through this lens they are very much bent (or refracted) towards each other, so much so that by the time they reach the end of the dark chamber v, each cone of light has come to a point or focus 1′, 2′, and as rays of this kind have come from every point all over the pencil, exactly similar points are formed on the retina, and a real picture of the pencil is formed there between 1′ and 2′."

Fig. 12.

Image of a candle-flame thrown on paper by a lens.

"We will make a very simple and pretty experiment to illustrate this. Darkening the room I light a candle, take a square of white paper in my hand, and hold a simple magnifying glass between the two (see Fig. 12) about three inches away from the candle. Then I shift the paper nearer and farther behind the lens, till we get a clear image of the candle-flame upon it. This is exactly what happens in our eye. I have drawn a dotted line c round the lens and the paper on the diagram to represent the eyeball in which the image of the candle-flame would be on the retina instead of on the piece of paper. The first point you will notice is that the candle-flame is upside down on the paper, and if you turn back to Fig. 11 you will see why, for it is plain that the cones of light cross in the lens l, 1 going to 1′ and 2 to 2′. Every picture made on our retina is upside down.

"But it is not there that we see it. As soon as the points of light from the pencil strike upon the retina, the thrill passes on along the optic nerve on, through the back of the eye to the brain; and our mind, following back the rays exactly as they have come through the lens, sees a pencil, outside the eye, right way upwards.

"This is how we see with our eyes, which adjust themselves most beautifully to our needs. For example, not only is the iris always ready to expand or contract according as we need more or less light, but there is a special muscle, called the ciliary muscle (cm, Fig. 11), which alters the lens for us to see things far or near. In all, or nearly all, perfect eyes the lens is flatter in front than behind, and this enables us to see things far off by bringing the rays from them exactly to a focus on the retina. But when we look at nearer things the rays require to be more bent or refracted, so without any conscious effort on our part this ciliary muscle contracts and allows the lens to bulge out slightly in front. Instantly we have a stronger magnifier, and the rays are brought to the right focus on the retina, so that a clear and full-size image of the near object is formed. How little we think, as we turn our eyes from one thing to another, and observe, now the distant hills, now the sheep feeding close by; or, as night draws on, gaze into limitless space and see the stars millions upon millions of miles away, that at every moment the focus of our eye is altering, the iris is contracting or expanding, and myriads of images are being formed one after the other in that little dark chamber, through which pass all the scenes of the outer world!

"Yet even this wonderful eye cannot show us everything. Some see farther than others, some see more minutely than others, according as the lens of the eye is flatter in one person and more rounded in another. But the most long-sighted person could never have discovered the planet Neptune, more than 2700 millions of miles distant from us, nor could the keenest-sighted have known of the existence of those minute and beautiful little plants, called diatoms, which live around us wherever water is found, and form delicate flint skeletons so infinitesimally small that thousands of millions go to form one cubic inch of the stone called tripoli, found at Bilin in Bohemia."

Fig. 13.

Arrow magnified by a convex lens.

a, b, Real arrow. C, D, Magnifying-glass. A, B, Enlarged image of the arrow.

"It is here that our 'magic glasses' come to our assistance, and reveal to us what was before invisible. We learnt just now that we see near things by the lens of our eye becoming more rounded in front; but there comes a point beyond which the lens cannot bulge any more, so that when a thing is very tiny, and would have to be held very close to the eye for us to see it, the lens can no longer collect the rays to a focus, so we see nothing but a blur. More than 800 years ago an Arabian, named Alhazen, explained why rounded or convex glasses make things appear larger when placed before the eye. This glass which I hold in my hand is a simple magnifying-glass, such as we used for focusing the candle-flame. It bends the rays inwards from any small object (see the arrow a, b, Fig. 13) so that the lens of our eye can use them, and then, as we follow out the rays in straight lines to the place where we see clearly (at A, B), every point of the object is magnified, and we not only see it much larger, but every mark upon it is much more distinct. You all know how the little shilling magnifying-glasses you carry show the most lovely and delicate structures in flowers, on the wings of butterflies, on the head of a bee or fly, and, in fact, in all minute living things."

Fig. 14.

Student's microscope.

ep, Eye-piece. o, g, Object-glass.

Fig. 15.

Skeleton of a microscope, showing how an object is magnified.

o, l, Object-lens. e, g, Eye-glass. s, s, Spicule. s′, s′, Magnified image of same in the tube. S, S, Image again enlarged by the lens of the eye-piece.

"But this is only our first step. Those diatoms we spoke of just now will only look like minute specks under even the strongest magnifying-glass. So we pass on to use two extra lenses to assist our eyes, and come to this compound microscope (Fig. 14) through which I have before now shown you the delicate markings on shells which were themselves so minute that you could not see them with the naked eye. Now we have to discover how the microscope performs this feat. Going back again for a minute to our candle and magnifying-glass (Fig. 12), you will find that the nearer you put the lens to the candle the farther away you will have to put the paper to get a clear image. When in a microscope we put a powerful lens o, l close down to a very minute object, say a spicule of a flint sponge s, s, quite invisible to the unaided eye, the rays from this spicule are brought to a focus a long way behind it at s′, s′, making an enlarged image because the lines of light have been diverging ever since they crossed in the lens. If you could put a piece of paper at s′ s′, as you did in the candle experiment, you would see the actual image of the magnified spicule upon it. But as these points of light are only in an empty tube, they pass on, spreading out again from the image, as they did before from the spicule. Then another convex lens or eye-glass e, g is put at the top of the microscope at the proper distance to bend these rays so that they enter our eye in nearly parallel lines, exactly as we saw in the ordinary magnifying-glass (Fig. 13), and our crystalline lens can then bring them to a focus on our retina.

"By this time the spicule has been twice magnified; or, in other words, the rays of light coming from it have been twice bent towards each other, so that when our eye follows them out in straight lines they are widely spread, and we see every point of light so clearly that all the spots and markings on this minute spicule are as clear as if it were really as large as it looks to us.

"This is simply the principle of the microscope. When you come to look at your own instruments, though they are very ordinary ones, you will find that the object-glass o, l is made of three lenses, flat on the side nearest the tube, and each lens is composed of two kinds of glass in order to correct the unequal refraction of the rays, and prevent fringes of colour appearing at the edge of the lens. Then again the eye-piece will be a short tube with a lens at each end, and halfway between them a black ledge will be seen inside the tube which acts like the iris of our eye (i, Fig. 11) and cuts off the rays passing through the edges of the lens. All these are devices to correct faults in the microscope which our eye corrects for itself, and they have enabled opticians to make very powerful lenses.

"Look now at the diagram (Fig. 16) showing a group of diatoms which you can see under the microscope after the lecture. Notice the lovely patterns, the delicate tracery, and the fine lines on the diatoms shown there. Yet each of these minute flint skeletons, if laid on a piece of glass by itself, would be quite invisible to the naked eye, while hundreds of them together only look like a faint mist on the slide on which they lie. Nor are they even here shown as much magnified as they might be; under a stronger power we should see those delicate lines on the diatoms broken up into minute round cups."

Fig. 16.

Fossil diatoms seen under the microscope.

The largest of these is an almost imperceptible speck to the naked eye.

"Is it not wonderful and delightful to think that we are able to add in this way to the power of our eyes, till it seems as if there were no limit to the hidden beauties of the minute forms of our earth, if only we can discover them?

"But our globe does not stand alone in the universe, and we want not only to learn all about everything we find upon it, but also to look out into the vast space around us and discover as much as we can about the myriads of suns and planets, comets and meteorites, star-mists and nebul?, which are to be found there. Even with the naked eye we can admire the grand planet Saturn, which is more than 800 millions of miles away, and this in itself is very marvellous. Who would have thought that our tiny crystalline lens would be able to catch and focus rays, sent all this enormous distance, so as actually to make a picture on our retina of a planet, which, like the moon, is only sending back to us the light of the sun? For, remember, the rays which come to us from Saturn must have travelled twice 800 millions of miles-884 millions from the sun to the planet, and less or more from the planet back to us, according to our position at the time. But this is as nothing when compared to the enormous distances over which light travels from the stars to us. Even the nearest star we know of, is at least twenty millions of millions of miles away, and the light from it, though travelling at the rate of 186,300 miles in a second, takes four years and four months to reach us, while the light from others, which we can see without a telescope, is between twenty and thirty years on its road. Does not the thought fill us with awe, that our little eye should be able to span such vast distances?

"But we are not yet nearly at the end of our wonder, for the same power which devised our eye gave us also the mind capable of inventing an instrument which increases the strength of that eye till we can actually see stars so far off that their light takes two thousand years coming to our globe. If the microscope delights us in helping us to see things invisible without it, because they are so small, surely the telescope is fascinating beyond all other magic glasses when we think that it brings heavenly bodies, thousands of billions of miles away, so close to us that we can examine them."

Fig. 17.

An astronomical telescope.

ep, Eye-piece. og, Object-glass. f, Finder.

"A Telescope (Fig. 17) can, like the microscope, be made of only two glasses: an object-glass to form an image in the tube and a magnifying eye-piece to enlarge it

. But there is this difference, that the object lens of a microscope is put close down to a minute object, so that the rays fall upon it at a wide angle, and the image formed in the tube is very much larger than the object outside. In the telescope, on the contrary, the thing we look at is far off, so that the rays fall on the object-glass at such a very narrow angle as to be practically parallel, and the image in the tube is of course very, very much smaller than the house, or church, or planet it pictures. What the object-glass of the telescope does for us, is to bring a small real image of an object very far off close to us in the tube of the telescope so that we can examine it.

"Think for a moment what this means. Imagine that star we spoke of (p. 41), whose light, travelling 186,300 miles in one second, still takes 2000 years to reach us. Picture the tiny waves of light crossing the countless billions of miles of space during those two thousand years, and reaching us so widely spread out that the few faint rays which strike our eye are quite useless, and for us that star has no existence; we cannot see it. Then go and ask the giant telescope, by turning the object-glass in the direction where that star lies in infinite space. The widespread rays are collected and come to a minute bright image in the dark tube. You put the eye-piece to this image, and there, under your eye, is a shining point: this is the image of the star, which otherwise would be lost to you in the mighty distance.

"Can any magic tale be more marvellous, or any thought grander, or more sublime than this? From my little chamber, by making use of the laws of light, which are the same wherever we turn, we can penetrate into depths so vast that we are not able even to measure them, and bring back unseen stars to tell us the secrets of the mighty universe. As far as the stars are concerned, whether we see them or not depends entirely upon the number of rays collected by the object-glass; for at such enormous distances the rays have no angle that we can measure, and magnify as you will, the brightest star only remains a point of light. It is in order to collect enough rays that astronomers have tried to have larger and larger object-glasses; so that while a small good hand telescope, such as you use, may have an object-glass measuring only an inch and a quarter across, some of the giant telescopes have lenses of two and a half feet, or thirty inches, diameter. These enormous lenses are very difficult to make and manage, and have many faults, therefore astronomical telescopes are often made with curved mirrors to reflect the rays, and bring them to a focus instead of refracting them as curved lenses do.

"We see, then, that one very important use of the telescope is to bring objects into view which otherwise we would never see; for, as I have already said, though we bring the stars into sight, we cannot magnify them. But whenever an object is near enough for the rays to fall even at a very small perceptible angle on the object-glass, then we can magnify them; and the longer the telescope, and the stronger the eye-piece, the more the object is magnified.

"I want you to understand the meaning of this, for it is really very simple, only it requires a little thought. Here are skeleton drawings of two telescopes (Fig. 18), one double the length of the other. Let us suppose that two people are using them to look at an arrow on a weathercock a long distance off. The rays of light r, r from the two ends of the arrow will enter both telescopes at the same angle r, x, r, cross in the lens, and pass on at exactly the same angle into the tubes. So far all is alike, but now comes the difference. In the short telescope A the object-glass must be of such a curve as to bring the cones of light in each ray to a focus at a distance of one foot behind it, [1] and there a small image i, i of the arrow is formed. But B being twice the length, allows the lens to be less curved, and the image to be formed two feet behind the object-glass; and as the rays r, r have been diverging ever since they crossed at x, the real image of the arrow formed at i, i is twice the size of the same image in A. Nevertheless, if you could put a piece of paper at i, i in both telescopes, and look through the object-glass (which you cannot actually do, because your head would block out the rays), the arrow would appear the same size in both telescopes, because one would be twice as far off from you as the other, and the angle i, x, i is the same in both."

Fig. 18.

Skeletons of telescopes.

A, A one-foot telescope with a three-inch eye-piece.

B, A two-foot telescope with a three-inch eye-piece.

e, p, Eye-piece. o, g, Object-glass. r, r, Rays which enter the telescopes and crossing at x form an image at i, i, which is magnified by the lens e, p. The angles r, x, r and i, x, i are the same. In A the angle i, o, i is four times greater than that of i, x, i. In B it is eight times greater.

"But by going to the proper end of the telescope you can get quite near the image, and can see and magnify it, if you put a strong lens to collect the rays from it to a focus. This is the use of the eye-piece, which in our diagram is placed at a quarter of a foot or three inches from the image in both telescopes. Now that we are close to the images, the divergence of the points i, i makes a great difference. In the small telescope, in which the image is only one foot behind the object-glass, the eye-piece being a quarter of a foot from it, is four times nearer, so the angle i, o, i is four times the angle i, x, i, and the man looking through it sees the image magnified four times. But in the longer telescope the image is two feet behind the lens, while the eye-piece is, as before, a quarter of a foot from it. Thus the eyepiece is now eight times nearer, so the angle i, o, i is eight times the angle i, x, i, and the observer sees the image magnified eight times.

"In real telescopes, where the difference between the focal length of the object-glass and that of the eye-glass can be made enormously greater, the magnifying power is quite startling, only the object-glass must be large, so as to collect enough rays to bear spreading widely. Even in your small telescopes, with a focus of eighteen inches, and an object-glass measuring one and a quarter inch across, we can put on a quarter of an inch eye-piece, and so magnify seventy-two times; while in my observatory telescope, eight feet or ninety-six inches long, an eye-piece of half an inch magnifies 192 times, and I can put on a 1/8-inch eye-piece and magnify 768 times! And so we can go on lengthening the focus of the object-glass and shortening the focus of the eye-piece, till in Lord Rosse's gigantic fifty-six-foot telescope, in which the image is fifty-four feet (648 inches) behind the object-glass, an eye-piece one-eighth of an inch from the image magnifies 5184 times! These giant telescopes, however, require an enormous object-glass or mirror, for the points of light are so spread out in making the large image that it is very faint unless an enormous number of rays are collected. Lord Rosse's telescope has a reflecting mirror measuring six feet across, and a man can walk upright in the telescope tube. The most powerful telescope yet made is that at the Lick Observatory, on Mount Hamilton, in California. It is fifty-six and a half feet long, the object-lens measures thirty-six inches across. A star seen through this telescope appears 2000 times as bright as when seen with the naked eye.

"You need not, however, wait for an opportunity to look through giant telescopes, for my small student's telescope, only four feet long, which we carry out on to the lawn, will show you endless unseen wonders; while your hand telescopes, and even a common opera-glass, will show many features on the face of the moon, and enable you to see the crescent of Venus, Jupiter's moons, and Saturn's rings, besides hundreds of stars unseen by the naked eye.

"Of course you will understand that Fig. 18 only shows the principle of the telescope. In all good instruments the lenses and other parts are more complicated; and in a terrestrial telescope, for looking at objects on the earth, another lens has to be put in to turn them right way up again. In looking at the sky it does not matter which way up we see a planet or a star, so the second glass is not needed, and we lose light by using it.

"We have now three magic glasses to work for us-the magnifying-glass, the microscope, and the telescope. Besides these, however, we have two other helpers, if possible even more wonderful. These are the Photographic camera and the Spectroscope."

Fig. 19.

Photographic camera.

l, l, Lenses. s, s, Screen cutting off diverging rays. c, c, Sliding box. p, p Picture formed.

"Now that we thoroughly understand the use of lenses, I need scarcely explain this photographic camera (Fig. 19), for it is clearly an artificial eye. In place of the crystalline lens (compare with Fig. 11) the photographer uses one, or generally two lenses l, l, with a black ledge or stop s between them, which acts like the iris in cutting off the rays too near the edge of the lens. The dark camera c answers to the dark chamber of the eyeball, and the plate p, p at the back of the chamber, which is made sensitive by chemicals, answers our retina. The box is formed of two parts, sliding one within the other at c, so as to place the plate at a proper distance from the lens, and then a screw adjusts the focus more exactly by bringing the front lens back or forward, instead of altering the curve as the ciliary muscle does in our eye. The difference between the two instruments is that in our eye the message goes to the brain, and the image disappears when we turn our eyes away from the object; but in the camera the waves of light work upon the chemicals, and the image can be fixed and remain for ever.

"But the camera has at least one weak point. The screen at the back is not curved like our retina, but must be flat because of printing off the pictures, and therefore the parts of the photograph near the edge are a little out of proportion.

"In many ways, however, this photographic eye is a more faithful observer than our own, and helps us to make more accurate pictures. For instance, instantaneous photographs have been taken of a galloping horse, and we find that the movements are very different from what we thought we saw with our eye, because our retina does not throw off one impression after another quickly enough to be quite certain we see each curve truly in succession. Again, the photograph of a face gives minute curves and lines, lights and shadows, far more perfectly than even the best artist can see them, and when the picture is magnified we see more and more details which escaped us before.

"But it is especially when attached to the microscope or the telescope that the photographic apparatus tells us such marvellous secrets; giving us, for instance, an accurate picture of the most minute water-animal quite invisible to the naked eye, so that when we enlarge the photograph any one can see the beautiful markings, the finest fibre, or the tiniest granule; or affording us accurate pictures, such as the one at p. 19 of the face of the moon, and bringing stars into view which we cannot otherwise see even with the strongest telescope.

"Our own eye has many weaknesses. For example, when we look through the telescope at the sky we can only fix our attention on one part at once, and afterwards on another; and the picture which we see in this way, bit by bit, we must draw as best we can. But if we put a sensitive photographic plate into the telescope just at the point (i, i, Fig. 18), where the image of the sky is focused, this plate gives attention, so to speak, to the whole picture at once, and registers every point exactly as it is; and this picture can be kept and enlarged so that every detail can be seen.

"Then, again, if we look at faint stars, they do not grow any brighter as we look. Each ray sends its message to the brain, and that is all; we cannot heap them up in our eye, and, indeed, after a time we see less, because our nerves grow tired. But on a photographic plate in a telescope, each ray in its turn does a little work upon the chemicals, and the longer the plate remains, the stronger the picture becomes. When wet plates were used they could not be left long, but since dry plates have been invented, with a film of chemically prepared gelatine, they can be left for hours in the telescope, which is kept by clockwork accurately opposite to the same objects. In this way thousands of faint stars, which we cannot see with the strongest telescope, creep into view as their feeble rays work over and over again on the same spot; and, as the brighter stars as well as the faint ones are all the time making their impression stronger, when the plate comes out each one appears in its proper strength. On the other hand, very bright objects often become blurred by a long exposure, so that we have sometimes to sacrifice the clearness of a bright object in order to print faint objects clearly.

"We now come to our last magic glass-the Spectroscope; and the hour has slipped by so fast that I have very little time left to speak of it. But this matters less as we have studied it before.[2] I need now only remind you of some of the facts. You will remember that when we passed sunlight through a three-sided piece of glass called a prism, we broke up a ray of white light into a line of beautiful colours gradually passing from red, through orange, yellow, green, blue, and indigo, to violet, and that these follow in the same order as we see them in the rainbow or in the thin film of a soap-bubble. By various experiments we proved that these colours are separated from each other because the many waves which make up white light are of different sizes, so that because the waves, of red light are slow and heavy, they lag behind when bent in the three-sided glass, while the rapid violet waves are bent more out of their road and run to the farther end of the line, the other colours ranging themselves between."

Fig. 20.

Kirchhoff's spectroscope.

A, The telescope which receives the ray of light through the slit in O.

Fig. 21. Passage of rays through the spectroscope. S, S′, Slit through which the light falls on the prisms. 1, 2, 3, 4, Prisms in which the rays are dispersed more and more. a, b, Screen receiving the spectrum, of which the seven principal colours are marked.

"Now when the light falls through the open window, or through a round hole or large slit, the images of the hole made by each coloured wave overlap each other very much, and the colours in the spectrum or coloured band are crowded together. But when in the spectroscope we pass the ray of light through a very narrow slit, each coloured image of the upright slit overlaps the next upright image only very little. By using several prisms one after the other (see Fig. 21), these upright coloured lines are separated more and more till we get a very long band or spectrum. Yet, as you know from our experiments with the light of a glowing wire or of molten iron, however much you spread out the light given by a solid or liquid, you can never separate these coloured lines from each other. It is only when you throw the light of a glowing gas or vapour into the slit that you get a few bright lines standing out alone. This is because all the rays of white light are present in glowing solids and liquids, and they follow each other too closely to be separated. But a gas, such as glowing hydrogen for example, gives out only a few separate rays, which, pouring through the slit, throw red, greenish-blue, and dark blue lines on the screen. Thus you have seen the double, orange-yellow sodium line (3, Plate I.) which starts out at once when salt is held in a flame and its light thrown into the spectroscope, and the red line of potassium vapour under the same treatment; and we shall observe these again when we study the coloured lights of the sun and stars."

"We see, then, that the work of our magic glass, the spectroscope, is simply to sift the waves of light, and that these waves, from their colour and their position in the long spectrum, actually tell us what glowing gases have started them on their road. Is not this like magic? I take a substance made of I know not what; I break it up, and, melting it in the intense heat of an electric spark, throw its light into the spectroscope. Then, as I examine this light after it has been spread out by the prisms, I can actually read by unmistakable lines what metals or non-metals it contains. Nay, more; when I catch the light of a star, or even of a faint nebula, in my telescope, and pass it through these prisms, there, written up on the magic-coloured band, I read off the gases which are glowing in that star-sun or star-dust billions of miles away.

"Now, boys, I have let you into the secrets of my five magic glasses-the magnifying-glass, the microscope, the telescope, the photographic camera, and the spectroscope. With these and the help of chemistry you can learn to work all my spells. You can peep into the mysteries of the life of the tiniest being which moves unseen under your feet; you can peer into that vast universe, which we can never visit so long as our bodies hold us down to our little earth; you can make the unseen stars print their spots of light on the paper you hold in your hand, by means of light-waves, which left them hundreds of years ago; or you can sift this light in your spectroscope, and make it tell you what substances were glowing in that star when they were started on their road. All this you can do on one condition, namely, that you seek patiently to know the truth.

"Stories of days long gone by tell us of true magicians and false magicians, and the good or evil they wrought. Of these I know nothing, but I do know this, that the value of the spells you can work with my magic glasses depends entirely upon whether you work patiently, accurately, and honestly. If you make careless, inaccurate experiments, and draw hasty conclusions, you will only do bad work, which it may take others years to undo; but if you question your instruments honestly and carefully, they will answer truly and faithfully. You may make many mistakes, but one experiment will correct the other; and while you are storing up in your own mind knowledge which lifts you far above this little world, or enables you to look deep below the outward surface of life, you may add your little group of facts to the general store, and help to pave the way to such grand discoveries as those of Newton in astronomy, Bunsen and Kirchhoff in spectrum analysis, and Darwin in the world of life."

[1] In our Fig. 18 the distances are inches instead of feet, but the proportions are the same.

[2] Fairyland of Science, Lecture II.; and Short History of Natural Science, chapter xxxiv.

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