Imágenes de páginas
PDF
EPUB

Light-waves, journeying through ether, from sun or star or moon, dart onward at the rate of about 186,000 miles each second. If they come from lamp or candle or reflecting body on earth, their speed is the same; and they are still ether-waves, for ether not only fills space, but permeates our atmosphere, and lies between the atoms of liquid and solid substances.

The number of distinct waves or vibrations of light which enter the eye each second is enormous, varying from 450 billions to 800 billions. A good deal of likeness may be found between the scale of sound and the scale of light-that scale which we see spread out in the rainbow. Larger slower wavelets-only 450 billions in number each second!—mean Red Light, low down in the scale, answering to bass notes. Smaller quicker wavelets -800 billions each second-mean Violet Light, high up in the scale, answering to treble notes.

Between red and violet lie all the different rainbow-hues, each with its own particular number of wavelets per second. But although the waves of diverse colours differ in size, following one upon another with greater or less rapidity, light itself, passing from one place to another, does not vary in speed. A red beam, journeying through space, gets along as fast as a violet beam does; though, as red light is composed of larger waves than violet light, a smaller number of those waves must necessarily arrive in one second. The undulations of violet light being tinier, more of them have to hurry up within a given time.

A few words now about the human eye, which receives all these light-rays, and enables the brain to make use of them.

The full-grown human eyeball is usually an inch or so in diameter, and in shape almost a sphere. When we speak of 'large eyes' and 'small eyes,' we really refer to the width of the opening through which the eye is seen. The actual size of eye varies little. As a whole, the eye is an exquisitely beautiful and finished instrument, adapted in all parts to its work, guarded carefully from many dangers. Considering the extreme delicacy of the organ, and the incessant perils which surround it, one may well marvel, not that so many people are blind, but that so many retain their sight through life. Volumes have been written descriptive of the eye. We have time for only a glance

at its structure.

Roughly speaking, the eye may be said to consist of three 'membranes,' one within another, and of three different 'humours.'

The outer coat is strong and protective. Four-fifths of it, known as the 'SCLEROTIC,' is opaque, and covers the greater part of the eyeball, inclusive of 'the white'; while a small portion of it, exactly in front of the eye, known as the 'CORNEA,' is transparent.

Close within the sclerotic lies the next coat, containing a large number of blood-vessels, and known as the 'CHOROID.' The front portion of the choroid, just under the cornea, is called the 'IRIS.'

Under the choroid, again, lies the 'RETINA'; and in front, behind the iris, is the 'LENS.'

Every one has noticed the coloured portion-black or blue, brown or grey-called the 'IRIS.' It may be regarded as a kind of tinted curtain, to shut out over much light from the delicate interior; and the dark hue, common to most hot countries, is doubtless because of the need for greater shelter from greater glare. Pale blue or pale grey affords less effective protection; wherefore northern nations, dwelling in less sunny climes, have abundance of light eyes in their midst.

In the centre of this dark curtain is a small round hole or opening, through which every ray of light must pass, which is to be of use in seeing. Rays from bright objects fall upon all parts of our bodies; but elsewhere they make no impression. Only as they enter the tiny eye-pupil can they communicate with the brain, and tell whence they come. When light is strong, the sheltering retina-curtain draws closer together, lessening the opening; when it is weak, the curtain retreats, leaving a wider gap.

[ocr errors]

Between the cornea and the iris, in front of the eye, is the ⚫ aqueous humour,' or watery humour; while the 'vitreous humour,' a jelly-like substance, fills the greater part of the interior of the eyeball. The most important is the 'crystalline humour,' commonly known as the 'LENS,' situated just behind the iris. It is in appearance transparent as glass, in make firm yet elastic, in shape a double convex.' Rays of light, passing through the cornea and the pupil, fall upon the lens, and by it are caused to converge to a spot on the retina, forming a picture there of the object from which they come. The lens has a wonderful power of changing its shape, so as to suit the greater or less distance of objects under view. When it stiffens with age, the power of adjustment for near vision decreases; and if it become opaque, through the disease called 'cataract,' sight fails. In the former

case, a new outside lens, in the shape of spectacles, will to some extent remedy what is wrong. In the latter case, the lens has to be removed, and its place altogether supplied by glasses.

Behind the vitreous humour we come to the most essential part of all, the 'RETINA' of the eye. Most essential, because every other part of this complicated organ is for the express purpose of casting upon the retina a clear image of the object to be seen. The whole apparatus seems to be designed and contrived with this end in view.

The retina is exceedingly complex, being formed mainly of innumerable delicate 'nerve fibrils,' or interlacing nerve-threads of extreme fineness; all of which, after spreading in a close network over the whole back of the eye, collect together in one spot, and pass out thence to the brain, as a slender bundle, called the Optic Nerve.'

[ocr errors]

Upon the retina are depicted all objects before the eye, solid enough and bright enough to become visible. The optic nerve, which passes from the retina to the brain—and of which, indeed, the retina is made-has been said to actually feel these evanescent images or pictures, much as the nerves of touch in our fingers feel irregularities on a roughened surface. It then conveys to the brain an idea of the image which it has felt; and the brain, through long use from babyhood, has learnt the meaning of the various shapes presented, so as more or less correctly to localise the things represented in their true distant positions.

A man, blind from infancy, suddenly gaining sight, has not this experience; and such a man has been known to complain that everything seemed to press upon his eyeballs. In time he would learn to make the needful mental effort, and to project to a distance the object which has sent its picture to his brain.

Does it not look, strangely, as if we ourselves-the true EGO of each one of us-were for the time literally imprisoned in the body, with only five outlets? Close utterly those five outlets and no means whatever remain of communication for the imprisoned spirit with the outside world!

One can well understand how any serious injury to the optic nerve, or to the retina, may easily be fatal to sight.

(Questions for May on page 558.)

THE PASSÉ DÉFINI GIRL.

BY THE HON. EVA KNATCHBULL-HUGESSEN.

THE old Park gardens are not what they used to be, and they never will be again. It is not only that the authorities have done what they could to spoil them, by cutting off one corner here and another there, and, finally, by putting a great uninteresting statue of a melancholy poet just in the very middle of one of our best play places, but the children who go there are very different from those of our day, and the whole spirit of the place has changed. You may say that I don't know much about it, when I confess that I no longer have a garden key, and that I never go into the gardens now; but any one, you know, can look through those grim, grey spiked railings as he or she takes a walk into the Park, and a casual glance can tell one a great deal. I say, then, that the children whom I see there walking about with their governesses, or sometimes indulging in a game of lawn-tennis-these children conduct themselves in quite a different way from what we did. It may be a much better way, and I daresay, when I have finished my story, you will say that it is, all I maintain is that it is different. Nowadays the frequenters of the gardens go there for pleasure, or for a change after their lessons; we went there for business and for hard work. Lessons were business, too, of course, but not a bit more so than the games of 'flags,' 'wolf-and-lamb,' and 'I spy' that were to follow. From nine to twelve we studied history, geography, music, and so on, with varying degrees of attention; from twelve to two we studied the arts of running, racing, 'I spy,' and 'pulling French and English,' with the whole might of mind and body. It is very easy to be idle over one's lessons, but anybody who knows anything about it will admit that one feels much more comfortable when one has done them well, and the thing about us who used to play in the gardens was that we had just

the same kind of uncomfortable feeling about our morning's play if it had not been good and thorough, as we had about our morning's lessons if we had taken no trouble about them at all. Not quite the same, because of course there is a difference, which any one can see for himself, but very like. It did seem such a 'waste,' as we used to say, if we did not collect our forces the very instant we got to the gardens, and play hard until the moment we were called to go home. There was real excitement every day in the conjectures as to who would be there, and what 'the sides' would be, and there was elation in the prospect of the coming struggle, and chance of distinction. And when, like Meg Dacre, you had the reputation of being the fastest runner in the gardens, there was of course the additional anxiety of wondering whether you would be able to keep it up, or whether you might not find a successful rival in some new

comer.

Great was the excitement and curiosity whenever it was announced that some 'new children' had come. They would hardly have time to look round before some boy or girl would break off from the group engaged in forming sides, and come up to them with a 'Will you play with us?' We were always very sorry and rather scornful when the answer was, 'We mayn't play with any one we don't know;' but it was not often given. Generally there was a delighted consent, followed, on one side or the other, by the questions which were invariably the first stage in the process of making friends. 'What's your name?' 'How old are you?' 'How many lessons do you do?' These were often followed in the case of quite small children by the boast, I expect I can lift you,' and the attempt to make it good. But bigger ones usually went on to, 'I wonder which can run the faster, you or me?' or perhaps, 'What is your father?' or 'When is your next birthday?'

I remember once being much impressed when, at about this stage of our first conversation, a little girl asked me how far I had got in Noël et Chapsal's French grammar.

It was rather difficult for me to answer accurately, because, whenever I had got beyond the first few pages, it used to be discovered that I had forgotten the beginning, and I had to go back to it again; but what most perplexed me was that any one should take interest enough in grammar to talk about it willingly, and in the gardens. I am afraid we were an idle set at our books, and I am sure I hope that our successors are much better in that way than we

« AnteriorContinuar »