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."
[Ill.u.s.tration: Fig. 10.
Eye-ball seen from the front. (After Le Gros Clark.)
_w_, White of eye. _i_, Iris. _p_, Pupil.]
"Let us try to understand this before we go on to artificial gla.s.ses, for it will help us to explain how these gla.s.ses 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) pa.s.sing out at the back, and a dark gla.s.sy 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."
[Ill.u.s.tration: 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-gla.s.s. 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 gla.s.s" 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 gla.s.s. 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 pa.s.sing 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 pa.s.sing 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"."
[Ill.u.s.tration: Fig. 12.
Image of a candle-flame thrown on paper by a lens.]
"We will make a very simple and pretty experiment to ill.u.s.trate this.
Darkening the room I light a candle, take a square of white paper in my hand, and hold a simple magnifying gla.s.s 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 pa.s.ses 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 s.p.a.ce 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 pa.s.s 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."
[Ill.u.s.tration: Fig. 13.
Arrow magnified by a convex lens.
_a_, _b_, Real arrow. C, D, Magnifying-gla.s.s. A, B, Enlarged image of the arrow.]
"It is here that our "magic gla.s.ses" come to our a.s.sistance, 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 gla.s.ses make things appear larger when placed before the eye. This gla.s.s which I hold in my hand is a simple magnifying-gla.s.s, 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-gla.s.ses you carry show the most lovely and delicate structures in flowers, on the wings of b.u.t.terflies, on the head of a bee or fly, and, in fact, in all minute living things."
[Ill.u.s.tration: Fig. 14.
Student"s microscope. _ep_, Eye-piece. _o_, _g_, Object-gla.s.s.]
[Ill.u.s.tration: Fig. 15.
Skeleton of a microscope, showing how an object is magnified.
_o_, _l_, Object-lens. _e_, _g_, Eye-gla.s.s. _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-gla.s.s. So we pa.s.s on to use two extra lenses to a.s.sist our eyes, and come to this compound microscope (Fig. 14) through which I have before now shown you the delicate markings on sh.e.l.ls 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-gla.s.s (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 pa.s.s on, spreading out again from the image, as they did before from the spicule. Then another convex lens or eye-gla.s.s _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-gla.s.s (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-gla.s.s _o_, _l_ is made of three lenses, flat on the side nearest the tube, and each lens is composed of two kinds of gla.s.s 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 pa.s.sing 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 gla.s.s 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."
[Ill.u.s.tration: 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 s.p.a.ce around us and discover as much as we can about the myriads of suns and planets, comets and meteorites, star-mists and nebulae, 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 gla.s.ses when we think that it brings heavenly bodies, thousands of billions of miles away, so close to us that we can examine them."
[Ill.u.s.tration: Fig. 17.
An astronomical telescope.
_ep_, Eye-piece. _og_, Object-gla.s.s. _f_, Finder.]
"A Telescope (Fig. 17) can, like the microscope, be made of only two gla.s.ses: an object-gla.s.s 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-gla.s.s 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-gla.s.s 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 s.p.a.ce 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-gla.s.s in the direction where that star lies in infinite s.p.a.ce.
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-gla.s.s; 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-gla.s.ses; so that while a small good hand telescope, such as you use, may have an object-gla.s.s 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-gla.s.s, then we can magnify them; and the longer the telescope, and the stronger the eye-piece, the more the object is magnified.
[Ill.u.s.tration: 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-gla.s.s. _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.]
"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 weatherc.o.c.k 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 pa.s.s 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-gla.s.s 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-gla.s.s; 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-gla.s.s_ (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."
[1] In our Fig. 18 the distances are inches instead of feet, but the proportions are the same.
"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-gla.s.s, 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_.