Through Magic Glasses and Other Lectures — Magic Glasses and how to use them, Part 2

"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 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 glasses when we think that it brings heavenly bodies, thousands of billions of miles away, so close to us that we can examine them."

[Illustration: 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.

[Illustration: 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.

] "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."

[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-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."

[Illustration: 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.

[1] 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."

[1] _Fairyland of Science_, Lecture II.

; and _Short History of Natural Science_, chapter xxxiv.

"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."

[Illustration: Fig. 20. Kirchhoff's spectroscope.

A, The telescope which receives the ray of light through the slit in O. ] [Illustration: 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.

] "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."

CHAPTER III FAIRY RINGS AND HOW THEY ARE MADE