"In real telescopes, where the difference between the focal length of the object-gla.s.s and that of the eye-gla.s.s can be made enormously greater, the magnifying power is quite startling, only the object-gla.s.s 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-gla.s.s 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-gla.s.s 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-gla.s.s, an eye-piece one-eighth of an inch from the image magnifies 5184 times! These giant telescopes, however, require an enormous object-gla.s.s 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-gla.s.s, 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 gla.s.s is not needed, and we lose light by using it.
"We have now three magic gla.s.ses to work for us--the magnifying-gla.s.s, 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."
[Ill.u.s.tration: 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 gla.s.s--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.[1] I need now only remind you of some of the facts. You will remember that when we pa.s.sed sunlight through a three-sided piece of gla.s.s called a prism, we broke up a ray of white light into a line of beautiful colours gradually pa.s.sing 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 gla.s.s, 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."
[1] _Fairyland of Science_, Lecture II.; and _Short History of Natural Science_, chapter x.x.xiv.
"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 pa.s.s 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 pota.s.sium vapour under the same treatment; and we shall observe these again when we study the coloured lights of the sun and stars."
[Ill.u.s.tration: Fig. 20.
Kirchhoff"s spectroscope.
A, The telescope which receives the ray of light through the slit in O.]
[Ill.u.s.tration: Fig. 21.
Pa.s.sage 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 princ.i.p.al colours are marked.]
"We see, then, that the work of our magic gla.s.s, 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 pa.s.s 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 gla.s.ses--the magnifying-gla.s.s, 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 gla.s.ses 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 a.n.a.lysis, and Darwin in the world of life."
CHAPTER III
FAIRY RINGS AND HOW THEY ARE MADE
[Ill.u.s.tration]
It was a lovely warm day in September, the golden corn had been cut and carted, and the waggons of the farmers around were free for the use of the college lads in their yearly autumn holiday. There they stood in a long row, one behind the other in the drive round the grounds, each with a pair of sleek, powerful farm-horses, and the waggoners beside them with their long whips ornamented with coloured ribbons; and as each waggon drew up before the door, it filled rapidly with its merry load and went on its way.
They had a long drive of seven miles before them, for they were going to cross the wild moor, and then descend gradually along a fairly good road to the more wooded and fertile country. Their object that day was to reach a certain fairy dell known to a few only among the party as one of the loveliest spots in Devon. It was a perfect day for a picnic. As they drove over the wide stretches of moorland, with tors to right and tors to the left, the stunted furze bushes growing here and there glistened with spiders" webs from which the dew had not yet disappeared, and mosses in great variety carpeted the ground, from the lovely thread-mosses, with their scarlet caps, to the pale sphagnum of the bogs, where a halt was made for some of the botanists of the party to search for the little Sundew (_Drosera rotundifolia_). Though this little plant had now almost ceased to flower, it was not difficult to recognise by its rosette of leaves glistening with sticky glands which it spreads out in many of the Dartmoor bogs to catch the tiny flies and suck out their life"s blood, and several specimens were uprooted and carefully packed away to plant in moist moss at home.
From this bog onwards the road ran near by one of the lovely streams which feed the rivers below, and, pa.s.sing across a bridge covered with ivy, led through a small forest of stunted trees round which the woodbine clung, hanging down its crimson berries, and the bracken fern, already putting on its brown and yellow tints, grew tall and thick on either side. Then, as they pa.s.sed out of the wood, they came upon the dell, a piece of wild moorland lying in a hollow between two granite ridges, with large blocks of granite strewn over it here and there, and furze bushes growing under their shelter, still covered with yellow blossoms together with countless seed-bearing pods, which the youngsters soon gathered for the shiny-black seeds within them.
Here the waggons were unspanned, the horses tethered out, the food unpacked, and preparations for the picnic soon in full swing. Just at this moment, however, a loud shout from one part of the dell called every one"s attention. "The fairy rings! the fairy rings! we have found the fairy rings!" and there truly on the brown sward might be seen three delicate green rings, the fresh sprouting gra.s.s growing young and tender in perfect circles measuring from six feet to nearly three yards across.
"What are they?" The question came from many voices at once, but it was the Princ.i.p.al who answered.
"Why, do you not know that they are pixie circles, where the "elves of hills, brooks, standing lakes, and groves" hold their revels, whirling in giddy round, and making the rings, "whereof the ewe not bites"? Have you forgotten how Mrs. Quickly, in the _Merry Wives of Windsor_, tells us that
""nightly, meadow-fairies, look you sing, Like to the Garter"s compa.s.s, in a ring: The expressure that it bears, green let it be, More fertile-fresh than all the field to see"?
"If we are magicians and work spells under magic gla.s.ses, why should not the pixies work spells on the gra.s.s? I brought you here to-day on purpose to see them. Which of you now can name the pixie who makes them?"
A deep silence followed. If any knew or guessed the truth of the matter, they were too shy to risk making a mistake.
"Be off with you then," said the Princ.i.p.al, "and keep well away from these rings all day, that you may not disturb the spell. But come back to me before we return at night, and perhaps I may show you the wonder-working pixie, and we may take him home to examine under the microscope."
The day pa.s.sed as such happy days do, and the glorious harvest moon had risen over the distant tors before the horses were spanned and the waggons ready. But the Princ.i.p.al was not at the starting place, and looking round they saw him at the farther end of the dell.
"Gently, gently," he cried, as there was one general rush towards him; "look where you tread, for I stand within a ring of fairies!"
And then they saw that just outside the green circle in which he stood, forming here and there a broken ring, were patches of a beautiful tiny mushroom, each of which raised its pale brown umbrella in the bright moonlight.
"Here are our fairies, boys. I am going to take a few home where they can be spared from the ring, and to-morrow we will learn their history."
The following day saw the cla.s.s-room full, and from the benches eager eyes were turned to the eight windows, in each of which stood one of the elder boys at his microscope ready for work. For under those microscopes the Princ.i.p.al always arranged some object referred to in his lecture and figured in diagrams on the walls, and it was the duty of each boy, after the lecture was over, to show and explain to the cla.s.s all the points of the specimen under his care. These boys were always specially envied, for though the others could, it is true, follow all the descriptions from the diagrams, yet these had the plant or animal always under their eye. Discussion was at this moment running high, for there was a great uncertainty of opinion as to whether a mushroom could be really called a plant when it had no leaves or flowers. All at once the hush came, as the Princ.i.p.al stepped into his desk and began:--
"Life is hard work, boys, and there is no being in this world which has not to work for its living. You all know that a plant grows by taking in gases and water, and working them up into sap and living tissue by the help of the sunshine and the green matter in their leaves; and you know, too, that the world is so full of green plants that hundreds and thousands of young seedlings can never get a living, but are stifled in their babyhood or destroyed before they can grow up.