Through Magic Glasses and Other Lectures — Magic Glasses and how to use them. Part 1

[Illustration] The sun shone brightly into the science class-room at mid-day.

No gaunt shadows nor ghostly moonlight now threw a spell on the magic chamber above.

The instruments looked bright and business-like, and the Principal, moving amongst them, heard the subdued hum of fifty or more voices rising from below.

It was the lecture hour, and the subject for the day was, "Magic glasses, and how to use them."

As the large clock in the hall sounded twelve, the Principal gathered up a few stray lenses and prisms he had selected, and passed down the turret stair to his platform.

Behind him were arranged his diagrams, before him on the table stood various instruments, and the rows of bright faces beyond looked up with one consent as the hum quieted down and he began his lecture.

"I have often told you, boys, have I not?

that I am a Magician.

In my chamber near the sky I work spells as did the magicians of old, and by the help of my magic glasses I peer into the secrets of nature.

Thus I read the secrets of the distant stars; I catch the light of wandering comets, and make it reveal its origin; I penetrate into the whirlpools of the sun; I map out the craters of the moon.

Nor can the tiniest being on earth hide itself from me.

Where others see only a drop of muddy water, that water brought into my magic chamber teems with thousands of active bodies, darting here and whirling there amid a meadow of tiny green plants floating in the water.

Nay, my inquisitive glass sees even farther than this, for with it I can watch the eddies of water and green atoms going on in each of these tiny beings as they feed and grow.

Again, if I want to break into the secrets of the rock at my feet, I have only to put a thin slice of it under my microscope to trace every crystal and grain; or, if I wish to learn still more, I subject it to fiery heat, and through the magic prisms of my spectroscope I read the history of the very substances of which it is composed.

If I wish to study the treasures of the wide ocean, the slime from a rock-pool teems with fairy forms darting about in the live box imprisoned in a crystal home.

If some distant stars are invisible even in the giant glasses of my telescope, I set another power to work, and make them print their own image on a photographic plate and so reveal their presence.

"All these things you have seen through my magic glasses, and I promised you that one day I would explain to you how they work and do my bidding.

But I must warn you that you must give all your attention;

there is no royal road to my magician's power.

Every one can attain to it, but only by taking trouble.

You must open your eyes and ears, and use your intelligence to test carefully what your senses show you.

"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."

[Illustration: 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 glasses, for it will help us to explain how these glasses show us many things we could never see without them.

Here are two pictures of the human eyeball (Figs.

10 and 11), one as it appears from the front, and the other as we should see the parts if we cut an eyeball across from the front to the back.

From these drawings we see that the eyeball is round; it only looks oval, because it is seen through the oval slit of the eyelids.

It is really a hard, shining, white ball with a thick nerve cord (_on_, Fig. 11) passing out at the back, and a dark glassy mound _c_, _c_ in the centre of the white in front.

In this mound we can easily distinguish two parts--first, the coloured _iris_ or elastic curtain (_i_, Fig. 10); and secondly, the dark spot or pupil _p_ in the centre.

The iris is the part which gives the eye its colour; it is composed of a number of fibres, the outer ones radiating towards the centre, the inner ones forming a ring round the pupil; and behind these fibres is a coat of dark pigment or colouring matter, blue in some people, grey, brown, or black in others.

When the light is very strong, and would pain the nerves inside if too much entered the pupil or window of the eye, then the ring of the iris contracts so as partly to close the opening.

When there is very little light, and it is necessary to let in as much as possible, the ring expands and the pupil grows large.

The best way to observe this is to look at a cat's eyes in the dusk, and then bring her near to a bright light; for the iris of a cat's eye contracts and expands much more than ours does."

[Illustration: Fig. 11. Section of an eye looking at a pencil.

(Adapted from Kirke.

) _c_, _c_, Cornea.

_w_, White of eye.

_cm_, Ciliary muscle.

_a_, _a_, Aqueous humour.

_i_, _i_, Iris.

_l_, _l_, Lens.

_r_, _r_, Retina.

_on_, Optic nerve.

1, 2, Pencil.

1', 2', Image of pencil on the retina.

] "Now look at the second diagram (Fig. 11) and notice the chief points necessary in seeing.

First you will observe that the pupil is not a mere hole; it is protected by a curved covering _c_.

This is the cornea, a hard, perfectly transparent membrane, looking much like a curved watch-glass.

Behind this is a small chamber filled with a watery fluid _a_, called the aqueous humour, and near the back of this chamber is the dark ring or iris _i_, which you saw from the front through the cornea and fluid.

Close behind the iris again is the natural 'magic glass' of our eye, the crystalline lens _l_, which is composed of perfectly transparent fibres and has two rounded or convex surfaces like an ordinary magnifying glass.

This lens rests on a cushion of a soft jelly-like substance _v_, called the vitreous humour, which fills the dark chamber or cavity of the eyeball and keeps it in shape, so that the retina _r_, which lines the chamber, is kept at a proper distance from the lens.

This retina is a transparent film of very sensitive nerves; it forms a screen at the back of the chamber, and has a coating of very dark pigment or colouring matter behind it.

Lastly, the nerves of the retina all meet in a bundle, called the optic nerve, and passing out of the eyeball at a point _on_, go to the brain.

These are the chief parts we use in seeing; now how do we use them?

"Suppose that a pencil is held in front of the eye at the distance at which we see small objects comfortably.

Light is reflected from all parts of the surface of the pencil, and as the rays spread, a certain number enter the pupil of the eye.

We will follow only two cones of light coming from the points 1 and 2 on the diagram Fig. 11. These you see enter the eye, each widely spread over the cornea _c_.

They are bent in a little by this curved covering, and by the liquid behind it, while the iris cuts off the rays near the edges of the lens, which would be too much bent to form a clear image.

The rest of the rays fall upon the lens _l_.

In passing through this lens they are very much bent (or _refracted_) towards each other, so much so that by the time they reach the end of the dark chamber _v_, each cone of light has come to a point or focus 1', 2', and as rays of this kind have come from every point all over the pencil, exactly similar points are formed on the retina, and a real picture of the pencil is formed there between 1' and 2'."

[Illustration: Fig. 12. Image of a candle-flame thrown on paper by a lens.

] "We will make a very simple and pretty experiment to illustrate this.

Darkening the room I light a candle, take a square of white paper in my hand, and hold a simple magnifying glass between the two (see Fig. 12) about three inches away from the candle.

Then I shift the paper nearer and farther behind the lens, till we get a clear image of the candle-flame upon it.

This is exactly what happens in our eye.

I have drawn a dotted line _c_ round the lens and the paper on the diagram to represent the eyeball in which the image of the candle-flame would be on the retina instead of on the piece of paper.

The first point you will notice is that the candle-flame is upside down on the paper, and if you turn back to Fig. 11 you will see why, for it is plain that the cones of light _cross_ in the lens _l_, 1 going to 1' and 2 to 2'.

Every picture made on our retina is upside down.

"But it is not there that we see it.

As soon as the points of light from the pencil strike upon the retina, the thrill passes on along the optic nerve _on_, through the back of the eye to the brain; and our mind, following back the rays exactly as they have come through the lens, sees a pencil, outside the eye, right way upwards.

"This is how we see with our eyes, which adjust themselves most beautifully to our needs.

For example, not only is the iris always ready to expand or contract according as we need more or less light, but there is a special muscle, called the ciliary muscle (_cm_, Fig. 11), which alters the lens for us to see things far or near.

In all, or nearly all, perfect eyes the lens is flatter in front than behind, and this enables us to see things far off by bringing the rays from them exactly to a focus on the retina.

But when we look at nearer things the rays require to be more bent or refracted, so without any conscious effort on our part this ciliary muscle contracts and allows the lens to bulge out slightly in front.

Instantly we have a stronger magnifier, and the rays are brought to the right focus on the retina, so that a clear and full-size image of the near object is formed.

How little we think, as we turn our eyes from one thing to another, and observe, now the distant hills, now the sheep feeding close by; or, as night draws on, gaze into limitless space and see the stars millions upon millions of miles away, that at every moment the focus of our eye is altering, the iris is contracting or expanding, and myriads of images are being formed one after the other in that little dark chamber, through which pass all the scenes of the outer world!

"Yet even this wonderful eye cannot show us everything.

Some see farther than others, some see more minutely than others, according as the lens of the eye is flatter in one person and more rounded in another.

But the most long-sighted person could never have discovered the planet Neptune, more than 2700 millions of miles distant from us, nor could the keenest-sighted have known of the existence of those minute and beautiful little plants, called diatoms, which live around us wherever water is found, and form delicate flint skeletons so infinitesimally small that thousands of millions go to form one cubic inch of the stone called tripoli, found at Bilin in Bohemia."

[Illustration: Fig. 13. Arrow magnified by a convex lens.

_a_, _b_, Real arrow.

C, D, Magnifying-glass.

A, B, Enlarged image of the arrow.

] "It is here that our 'magic glasses' come to our assistance, and reveal to us what was before invisible.

We learnt just now that we see near things by the lens of our eye becoming more rounded in front; but there comes a point beyond which the lens cannot bulge any more, so that when a thing is very tiny, and would have to be held very close to the eye for us to see it, the lens can no longer collect the rays to a focus, so we see nothing but a blur.

More than 800 years ago an Arabian, named Alhazen, explained why rounded or convex glasses make things appear larger when placed before the eye.

This glass which I hold in my hand is a simple magnifying-glass, such as we used for focusing the candle-flame.

It bends the rays inwards from any small object (see the arrow _a_, _b_, Fig. 13) so that the lens of our eye can use them, and then, as we follow out the rays in straight lines to the place where we see clearly (at A, B), every point of the object is magnified, and we not only see it much larger, but every mark upon it is much more distinct.

You all know how the little shilling magnifying-glasses you carry show the most lovely and delicate structures in flowers, on the wings of butterflies, on the head of a bee or fly, and, in fact, in all minute living things."

[Illustration: Fig. 14. Student's microscope.

_ep_, Eye-piece.

_o_, _g_, Object-glass.

] [Illustration: Fig. 15. Skeleton of a microscope, showing how an object is magnified.

_o_, _l_, Object-lens.

_e_, _g_, Eye-glass.

_s_, _s_, Spicule.

_s'_, _s'_, Magnified image of same in the tube.

S, S, Image again enlarged by the lens of the eye-piece.

] "But this is only our first step.

Those diatoms we spoke of just now will only look like minute specks under even the strongest magnifying-glass.

So we pass on to use two extra lenses to assist our eyes, and come to this compound microscope (Fig. 14) through which I have before now shown you the delicate markings on shells which were themselves so minute that you could not see them with the naked eye.

Now we have to discover how the microscope performs this feat.

Going back again for a minute to our candle and magnifying-glass (Fig. 12), you will find that the nearer you put the lens to the candle the farther away you will have to put the paper to get a clear image.

When in a microscope we put a powerful lens _o_, _l_ close down to a very minute object, say a spicule of a flint sponge _s_, _s_, quite invisible to the unaided eye, the rays from this spicule are brought to a focus a long way behind it at _s'_, _s'_, making an enlarged image because the lines of light have been diverging ever since they crossed in the lens.

If you could put a piece of paper at _s'_ _s'_, as you did in the candle experiment, you would see the actual image of the magnified spicule upon it.

But as these points of light are only in an empty tube, they pass on, spreading out again from the image, as they did before from the spicule.

Then another convex lens or eye-glass _e_, _g_ is put at the top of the microscope at the proper distance to bend these rays so that they enter our eye in nearly parallel lines, exactly as we saw in the ordinary magnifying-glass (Fig. 13), and our crystalline lens can then bring them to a focus on our retina.

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

"This is simply the principle of the microscope.

When you come to look at your own instruments, though they are very ordinary ones, you will find that the object-glass _o_, _l_ is made of three lenses, flat on the side nearest the tube, and each lens is composed of two kinds of glass in order to correct the unequal refraction of the rays, and prevent fringes of colour appearing at the edge of the lens.

Then again the eye-piece will be a short tube with a lens at each end, and halfway between them a black ledge will be seen inside the tube which acts like the iris of our eye (_i_, Fig. 11) and cuts off the rays passing through the edges of the lens.

All these are devices to correct faults in the microscope which our eye corrects for itself, and they have enabled opticians to make very powerful lenses.

"Look now at the diagram (Fig. 16) showing a group of diatoms which you can see under the microscope after the lecture.

Notice the lovely patterns, the delicate tracery, and the fine lines on the diatoms shown there.

Yet each of these minute flint skeletons, if laid on a piece of glass by itself, would be quite invisible to the naked eye, while hundreds of them together only look like a faint mist on the slide on which they lie.

Nor are they even here shown as much magnified as they might be; under a stronger power we should see those delicate lines on the diatoms broken up into minute round cups."

[Illustration: Fig. 16. Fossil diatoms seen under the microscope.

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

]