Through Magic Glasses and Other Lectures

-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

d various instruments, and the rows of bright faces beyond looked u

ought 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

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 t

-ball seen f

k.) w, White of eye

o 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

ick 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 so

g.

oking at a pencil. (

eous humour. i, i, Iris. l, l, Lens. r, r, Retina. on, Optic

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, w

ee 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

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e-flame thrown o

ther 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

he thrill passes on along the optic nerve on, through the back of the eye to the brain; and our mind, followi

inging 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,

ng-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, calle

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fied by a c

agnifying-glass. A, B, En

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 fo

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's micr

ce. o, g, O

g.

cope, showing how an

s′, s′, Magnified image of same in the tube. S, S

2), 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

wards each other, so that when our eye follows them out in straight lines they are widely spread, and we see every point of ligh

nds 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

ms 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 o

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s seen under

an almost imperceptibl

way to the power of our eyes, till it seems as if there were no limit to the

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 eno

e 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

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nomical

og, Object-gl

ys 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

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 in

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 hav

d, 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 percept

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 e

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s of tel

escope with a thr

escope with a thr

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

e 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

ety-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

y 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

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.

ope, and the telescope. Besides these, however, we have two other helpers, if pos

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raphic

ing off diverging rays. c, c, S

ge 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

d like our retina, but must be flat because of printing off the pictures, and th

hat 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. Agai

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 gran

e 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 focus

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 wor

ism, 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 ea

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f's spec

receives the ray of lig

falls on the prisms. 1, 2, 3, 4, Prisms in which the rays are dispersed more and 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 close

ke 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

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 ligh

xperiments, 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 knowled

s are inches instead of feet, b

ure II.; and Short History of