Fig. 4.Fig. 4. Tycho and his surroundings. (From a photograph of the moon taken by Mr. De la Rue, 1863.)Tycho and his surroundings.(From a photograph of the moon taken by Mr. De la Rue, 1863.)
"This cavity measures fifty-four miles across, so that if it could be moved down to our earth it would cover by far the largest part of Devonshire, or that portion from Bideford on the north,to the sea on the south, and from the borders of Cornwall on the east, to Exeter on the west, and it is 17,000 feet or nearly three miles in depth. Even in the brilliant light of the full moon this enormous cup is dark compared to the bright rim, but it is much better seen in about the middle of the second quarter, when the rising sun begins to light up one side while the other is in black night. The drawing on the wall (Fig. 4), which is taken from an actual photograph of the moon's face, shows Tycho at this time surrounded by the numerous other craters which cover this part of the moon. You may recognise him by the gleaming peak in the centre of the cup, and by his bright rim which is so much more perfect than those of his companions. The gleaming peak is the top of a steep cone or hill rising up 6000 feet, or more than a mile from the base of the crater, so that even the summit is about two miles below the rim.
"There is one very interesting point in Tycho, however, which is seen at its very best at full moon. Look outside the bright rim and you will see that from the shadow which surrounds it there spring on all sides those strange brilliant streaks (see Fig. 3a) which I spoke of just now. There are others quite as bright, or even brighter, round other craters, Copernicus (Fig. 6), Kepler, and Aristarchus, lower down on the right-hand side of the moon; but these of Tycho are far the most widely spread, covering almost all the top of the face.
"What are these streaks? We do not know. During the second quarter of the moon, when the sunis rising slowly upon Tycho, lighting up his peak and showing the crater beautifully divided into a bright cup in the curve to the right, while a dense shadow lies in the left hollow, these streaks are only faint, and among the many craters around (see Fig. 4) you might even have some difficulty at first in finding the well-known giant. But as the sun rises higher and higher they begin to appear, and go on increasing in brightness till they shine with that wonderfully silvery light you see now in the full moon."
Fig. 5.Fig. 5. Plan of the Peak of Teneriffe, showing how it resembles a lunar crater. (A. Geikie.)Plan of the Peak of Teneriffe, showing how it resembles a lunar crater. (A. Geikie.)
"Here is a problem for you young astronomers to solve, as we learn more and more how to use the telescope with all its new appliances."
The crater itself is not so difficult to explain, forwe have many like it on our earth, only not nearly so large. In fact, we might almost say that our earthly volcanoes differ from those in the moon only by their smaller size and by formingmountainswith the crater or cup on the top; while the lunar craters lie flat on the surface of the moon, the hollow of the cup forming a depression below it. The peak of Teneriffe (Fig. 5), which is a dormant volcano, is a good copy in miniature on our earth of many craters on the moon. The large plain surrounded by a high rocky wall, broken in places by lava streams, the smaller craters nestling in the cup, and the high peak or central crater rising up far above the others, are so like what we see on the moon that we cannot doubt that the same causes have been at work in both cases, even though the space enclosed in the rocky wall of Teneriffe measures only eight miles across, while that of Tycho measures fifty-four.
"But of the streaks we have no satisfactory explanation. They pass alike over plain and valley and mountain, cutting even across other craters without swerving from their course. The astronomer Nasmyth thought they were the remains of cracks made when the volcanoes were active, and filled with molten lava from below, as water oozes up through ice-cracks on a pond. But this explanation is not quite satisfactory, for the lava, forcing its way through, would cool in ridges which ought to cast a shadow in sunlight. These streaks, however, not only cast no shadow, as you can see at the full moon but when the sun shines sideways upon themin the new or waning moon they disappear as we have seen altogether. Thus the streaks, so brilliant at full moon in Tycho, Copernicus, Kepler, and Aristarchus, remain a puzzle to astronomers still."
Fig. 6.Fig. 6. The crater Copernicus. (As given in Herschel's Astronomy, from a drawing taken in a reflecting telescope of 20 feet focal length.)The crater Copernicus.(As given in Herschel'sAstronomy, from a drawing taken in a reflecting telescope of 20 feet focal length.)
"We cannot examine these three last-named craters well to-night with the full sun upon them; but mark their positions well, for Copernicus, at least, you mustexamine on the first opportunity, when the sun is rising upon it in the moon's second quarter. It is larger even than Tycho, measuring fifty-six miles across, and has a hill in the centre with many peaks; while outside, great spurs or ridges stretch in all directions sometimes for more than a hundred miles, and between these are scattered innumerable minute craters. But the most striking feature in it is the ring, which is composed inside the crater of magnificent terraces divided by deep ravines. These terraces are in some ways very like those of the great crater of Teneriffe, and astronomers can best account for them by supposing that this immense crater was once filled with a lake of molten lava rising, cooling at the edges, and then falling again, leaving the solid ridge behind. The streaks are also beautifully shown in Copernicus (see Fig. 6), but, as in Tycho, they fade away as the sun sets on the crater, and only reappear gradually as midday approaches.
"And now, looking a little to the left of Copernicus, you will see that grand range of mountains, the Lunar Apennines (Fig. 7), which stretches 400 miles across the face of the moon. Other mountain ranges we could find, but none so like mountains on our own globe as these, with their gentle sunny slope down to a plain on the left, and steep perpendicular cliffs on the right. The highest peak in this range, called Huyghens, rises to the height of 21,000 feet, higher than Chimborazo in the Andes. Other mountains on the moon, such as those called the Caucasus, south of the Apennines,are composed of disconnected peaks, while others again stand as solitary pyramids upon the plains."
Fig. 7.Fig. 7. The Lunar Apennines. (Copied by kind permission of MM. Henri from part of a magnificent photograph taken by them, March 29, 1890, at the Paris Observatory.)The Lunar Apennines.(Copied by kind permission of MM. Henri from part of a magnificent photograph taken by them, March 29, 1890, at the Paris Observatory.)
"But we must hasten on, for I want you to observe those huge walled crater-plains which have no hill in the middle, but smooth steel-grey centres shining like mirrors in the moonlight. One of these, called Archimedes, you will find just below the Lunar Apennines (Figs. 3 and 7), and another called Plato, which is sixty miles broad, is still lower down themoon's face (Figs. 3 and 8). The centres of these broad circles are curiously smooth and shining like quicksilver, with minute dots here and there which are miniature craters, while the walls are rugged and crowned with turret-shaped peaks."
Fig. 8.Fig. 8. The crater Plato as seen soon after sunrise. (After Neison.)The crater Plato as seen soon after sunrise. (After Neison.)
"It is easy to picture to oneself how these may once have been vast seas of lava, not surging as in Copernicus, and heaving up as it cooled into one great central cone, but seething as molten lead does in a crucible, little bubbles bursting here and there into minute craters; and this is the explanation given of them by astronomers.
"And now that you have seen the curious ruggedface of the moon and its craters and mountains, you will want to know how all this has come about. We can only form theories on the point, except that everything shows that heat and volcanoes have in some way done the work, though no one has ever yet clearly proved that volcanic eruptions have taken place in our time. We must look back to ages long gone by for those mighty volcanic eruptions which hurled out stones and ashes from the great crater of Tycho, and formed the vast seas of lava in Copernicus and Plato.
"And when these were over, and the globe was cooling down rapidly, so that mountain ranges were formed by the wrinkling and rending of the surface, was there then any life on the moon? Who can tell? Our magic glasses can reveal what now is, so far as distance will allow; but what has been, except where the rugged traces remain, we shall probably never know. What we now see is a dead worn-out planet, on which we cannot certainly trace any activity except that of heat in the past. That there is no life there now, at any rate of the kind on our own earth, we are almost certain; first, because we can nowhere find traces of water, clouds, nor even mist, and without moisture no life like ours is possible; and secondly, because even if there is, as perhaps there may be, a thin ocean of gas round the moon there is certainly no atmosphere such as surrounds our globe.
"One fact which proves this is, that there are no half-shadows on the moon. If you look somenight at the mountains and craters during her first and second quarters, you will be startled to see what heavy shadows they cast, not with faint edges dying away into light, but sharp and hard (see Figs. 6-8), so that you pass, as it were by one step, from shadow to sunshine. This in itself is enough to show that there is no air to scatter the sunlight and spread it into the edges of the shade as happens on our earth; but there are other and better proofs. One of these is, that during an eclipse of the sun there is no reflection of his light round the dark moon as there would be if the moon had an atmosphere; another is that the spectroscope, that wonderful instrument which shows us invisible gases, gives no hint of air around the moon; and another is the sudden disappearance oroccultationof a star behind the moon, such as I hope to see in a few minutes.
"See here! take the small hand telescope and turn it on to the moon's face while I take my place at the large one, and I will tell you what to look for. You know that at sunset we see the sun for some time after it has dipped below the horizon, because the rays of light which come from it are bent in our atmosphere and brought to our eyes, forming in them the image of the sun which is already gone. Now in a short time the moon which we are watching will be darkened by our earth coming between it and the sun, and while it is quite dark it will pass over a little bright star. In fact to us the star will appear to set behind the dark moon as the sun sets below the horizon, and if the moon had an atmosphere like ours, the rays from the star would be bentin it and reach our eyes after the star was gone, so that it would only disappear gradually. Astronomers have always observed, however, that the star is lost to sight quite suddenly, showing that there is no ocean of air round the moon to bend the light-rays."
Fig. 9.Fig. 9. Diagram of total eclipse of the moon. S, Sun. E, Earth. M, Moon passing into the earth's shadow and passing out at M´. R, R´, Lines meeting at a point U, U´ behind the earth and enclosing a space within which all the direct rays of the sun are intercepted by the earth, causing a black darkness or umbra. R, P and R´, P´, Lines marking a space within which, behind the earth, part of the sun's rays are cut off, causing a half-shadow or penumbra, P, P´. a, a, Points where a few of the sun's rays are bent or refracted in the earth's atmosphere, so that they pass along the path marked by the dotted lines and shed a lurid light on the sun's face.Diagram of total eclipse of the moon.S, Sun. E, Earth. M, Moon passing into the earth's shadow and passing out at M´.R, R´, Lines meeting at a point U, U´ behind the earth and enclosing a space within which all the direct rays of the sun are intercepted by the earth, causing a black darkness orumbra.R, P and R´, P´, Lines marking a space within which, behind the earth, part of the sun's rays are cut off, causing a half-shadow orpenumbra, P, P´.a,a, Points where a few of the sun's rays are bent or refracted in the earth's atmosphere, so that they pass along the path marked by the dotted lines and shed a lurid light on the sun's face.
S, Sun. E, Earth. M, Moon passing into the earth's shadow and passing out at M´.
R, R´, Lines meeting at a point U, U´ behind the earth and enclosing a space within which all the direct rays of the sun are intercepted by the earth, causing a black darkness orumbra.
R, P and R´, P´, Lines marking a space within which, behind the earth, part of the sun's rays are cut off, causing a half-shadow orpenumbra, P, P´.
a,a, Points where a few of the sun's rays are bent or refracted in the earth's atmosphere, so that they pass along the path marked by the dotted lines and shed a lurid light on the sun's face.
Here the magician paused, for a slight dimness on the lower right-hand side of the moon warned him that she was entering into thepenumbraor half-shadow (see Fig. 9) caused by the earth cutting off part of the sun's rays; and soon a deep blackshadow creeping over Aristarchus and Plato showed that she was passing into that darker space orumbrawhere the body of the earth is completely between her and the sun and cuts off all his rays. All, did I say? No! not all. For now was seen a beautiful sight, which would prove to any one who saw our earth from a great distance that it has a deep ocean of air round it.
It was a clear night, with a cloudless sky, and as the deep shadow crept slowly over the moon's face, covering the Lunar Apennines and Copernicus, and stealing gradually across the brilliant streaks of Tycho till the crater itself was swallowed up in darkness, a strange lurid light began to appear. The part of the moon which was eclipsed was not wholly dark, but tinted with a very faint bluish-green light, which changed almost imperceptibly, as the eclipse went on, to rose-red, and then to a fiery copper-coloured glow as the moon crept entirely into the shadow and became all dark. The lad watching through his small telescope noted this weird light, and wondered, as he saw the outlines of the Apennines and of several craters dimly visible by it, though the moon was totally eclipsed. He noted, but was silent. He would not disturb the Principal, for the important moment was at hand, as this dark copper-coloured moon, now almost invisible, drew near to the star over which it was to pass.
This little star, really a glorious sun billions of miles away behind the moon, was perhaps the centre of another system of worlds as unknown to us as we to them, and the fact of our tiny moon crossing betweenit and our earth would matter as little as if a grain of sand was blown across the heavens. Yet to the watchers it was a great matter—would the star give any further clue to the question of an atmosphere round the moon? Would its light linger even for a moment, like the light of the setting sun? Nearer and nearer came the dark moon; the star shone brilliantly against its darkness; one second and it was gone. The long looked-for moment had passed, and the magician turned from his instrument with a sigh. "I have learnt nothing new, Alwyn," said he, "but at least it is satisfactory to have seen for ourselves the proof that there is no perceptible atmosphere round the moon. We need wait no longer, for before the star reappears on the other side the eclipse will be passing away."
"But, master," burst forth the lad, now the silence was broken, "tell me why did that strange light of many tints shine upon the dark moon?"
"Did you notice it, Alwyn?" said the Principal, with a pleased smile. "Then our evening's work is not lost, for you have made a real observation for yourself. That light was caused by the few rays of the sun which grazed the edge of our earth passing through the ocean of air round it (see Fig. 9). There they were refracted or bent, and so were thrown within the shadow cast by our earth, and fell upon the moon. If there were such a person as a 'man in the moon,' that lurid light would prove to him that our earth has an atmosphere. The cause of the tints is the same which gives us our sunset colours, because as the different coloured waves whichmake white light are absorbed one by one, passing through the denser atmosphere, the blue are cut off first, then the green, then the yellow, till only the orange and red rays reached the centre of the shadow, where the moon was darkest. But this is too difficult a subject to begin at midnight."
So saying, he lighted his lamp, and covering the object-glass of his telescope with its pasteboard cap, detached the instrument from the clockwork, and the master and his pupil went down the turret stairs and past through the room below. As they did so they heard in the distance a scuffling noise like that of rats in the wall. A smile passed over the face of the Principal, for he knew that his young pupils, who had been making their observations in the gallery above, were hurrying back to their beds.
ornate capital t
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 ama 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 magicglasses, 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.Fig. 10. Eye-ball seen from the front. (After Le Gros Clark.) w, White of eye. i, Iris. p, Pupil.Eye-ball seen from the front.(After Le Gros Clark.)w, White of eye.i, Iris.p, Pupil.
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 tothe 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 moundc, cin the centre of the white in front. In this mound we can easily distinguish two parts—first, the colouredirisor elastic curtain (i, Fig. 10); and secondly, the dark spot or pupilpin 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; forthe iris of a cat's eye contracts and expands much more than ours does."
Fig. 11.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.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.
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 coveringc. 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 fluida, called the aqueous humour, and near the back of this chamber is the dark ring or irisi, 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 lensl, which is composed of perfectly transparent fibres and has two rounded orconvex surfaces like an ordinary magnifying glass. This lens rests on a cushion of a soft jelly-like substancev, called the vitreous humour, which fills the dark chamber or cavity of the eyeball and keeps it in shape, so that the retinar, 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 pointon, 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 corneac. 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 lensl. In passing through this lens they are very much bent (orrefracted) towards each other, so much so that by the time they reach the end of the dark chamberv, 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.Fig. 12. Image of a candle-flame thrown on paper by a lens.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 linecround 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 lightcrossin the lensl, 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 thepoints of light from the pencil strike upon the retina, the thrill passes on along the optic nerveon, 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.Fig. 13. Arrow magnified by a convex lens. a, b, Real arrow. C, D, Magnifying-glass. A, B, Enlarged image of the arrow.Arrow magnified by a convex lens.a,b, Real arrow. C, D, Magnifying-glass. A, B, Enlarged image of the arrow.
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; butthere 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 arrowa, 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.Fig. 14. Student's microscope. ep, Eye-piece, o, g, Object-glass.Student's microscope.ep, Eye-piece.o,g, Object-glass.
ep, Eye-piece.o,g, Object-glass.
Fig. 15.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.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.
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 lenso, lclose down to a very minute object, say a spicule of a flint sponges, s, quite invisible to the unaided eye, the rays from this spicule are brought to a focus a long way behind it ats´, s´, making an enlarged imagebecause the lines of light have been diverging ever since they crossed in the lens. If you could put a piece of paper ats´ 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-glasse, gis 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-glasso, lis 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 beseen 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.Fig. 16. Fossil diatoms seen under the microscope. The largest of these is an almost imperceptible speck to the naked eye.Fossil diatoms seen under the microscope.The largest of these is an almost imperceptible speck to the naked eye.
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 thehidden 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 nebulæ, 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 twentymillionsofmillionsof 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 ourwonder, 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 takestwo thousand yearscoming 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.Fig. 17. An astronomical telescope. ep, Eye-piece. og, Object-glass. f, Finder.An astronomical telescope.ep, Eye-piece.og, Object-glass.f, Finder.
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 coursevery, verymuch smallerthan the house, or church, or planet it pictures. What the object-glass of the telescope does for us, is to bring a smallreal imageof 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 distancesthe 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 toreflectthe rays, and bring them to a focus instead ofrefractingthem 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 lightr,rfrom the two ends of the arrow will enter both telescopes at the same angler, x, r, cross in the lens, and pass on atexactly the same angleintothe 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 ofone footbehind it,[1]and there a small imagei, iof the arrow is formed. But B being twice the length, allows the lens to be less curved, and the image to be formedtwo feetbehind the object-glass; and as the raysr,rhave beendivergingever since they crossed atx, the real image of the arrow formed ati, iis twice the size of the same image in A. Nevertheless, if you could put a piece of paper ati, iin both telescopes, and look through theobject-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 anglei, x, iis the same in both."