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with the sum total that is lavishly poured away into space. Poured away recklessly, poured away uselessly, so we might feel disposed to say ; but what do we know about the matter?
That which to our limited understanding seems like waste, need not be waste. That which to us seems purposeless, may have some mighty cause underlying. If nothing else can be learnt from this abounding outflow of heat and light, so much at least is apparent, the Royal richness and fulness of DIVINE giving. Man may dole out his little gifts, just enough at a time for the moment's need. God gives grandly to His Universe, enough and to spare !
Still, it would be the height of rashness to assert that for all this wealth of sunlight and sun-heat, no use does, or can exist. We simply know nothing whatever about the matter!
An eye is that organ, by means of which a man perceives, or becomes conscious of, objects near or far, of which he would not know by means of his other senses.
True, those other senses can often be exercised in addition. When he sees a thing, he may also feel it, taste it, smell it, hear it,—any or all. But by sight alone he can learn that an object exists, which he is unable to hear or smell, to taste or feel. He can discover something of its size and colour, its shape and position, through sight alone.
Remember, however, that the eye is only an organ. The eye itself does not see. It is merely the camera or optical instrument,' by means of which a man's brain is made aware of things outside himself; things often too distant for him to be conscious of them in any other mode. Moreover, that which your brain sees is not the object itself, but only a reflection of that object, thrown upon the retina of your eye,-more strictly still, only a message or idea of that image, telegraphed from the eye to the brain.
You say that you see the moon. Yet the moon is not touched by your eye. When you feel a table, the sensitive nerves of your finger-tips are in actual contact with the table. When you taste sugar, the sugar is within you, in actual contact with your nerves of taste. When you smell a rose, particles of the sweet essence of the rose have floated into your nose, coming into actual contact with the olfactory nerve.
In all these cases, though the brain receives a message, it is a inessage direct from
the source, caused by positive touch between your nerves of sensation and the thing which is felt, tasted, or smelt.
But with hearing and with seeing actual contact ceases. You may hear an explosion miles away. You may see a sun millions or billions of miles away.
Yet something analogous to the sense of touch still comes in. When you hear a gun fired, half a mile off, the gun is not touched by your ear; nevertheless, something is sent from the exploding powder to the nerves within your ears, by means of which a message of noise reaches the brain. When you see a moon, tens of thousands of miles away, that moon is not touched by your eye; nevertheless, something is despatched from the moon to the nerves of your eyes, by means of which a message of light reaches the brain.
What is this something which travels to ear and to eye, like a telegraphic message? Take the ear first. Sound journeys to the ear in waves of air.
. Any manner of concussion, however slight, produces such waves. One may even say that any manner of touch, however delicate, produces waves of sound. Our ears are so formed as to hear only certain degrees of air-vibrations ; but, doubtless, innumerable vibrations take place which we are unable to hear, because they are too high, or too low, or too gentle, to affect our organs of hearing. A fly cannot walk across the window-pane without noise, though to us his footsteps are inaudible. A spider cannot crunch his victim without noise, though the crunching is unheard by us. A grain of sand cannot drop to the ground without noise, though finer ears than ours are required to detect the tiny jar. When a rock falls, or a lion munches, or a horse walks, our coarser sense of hearing is at once aware of the air-waves set in motion.
It must not be supposed that a wave of air is at all the same as wind. Wind is air in actual motion, travelling from one place to another; whereas, in a wave, the air-particles move very slightly, and do not travel at all.
A wave of water, out at sea, does not mean an onward motion of the water. Close to shore waves break and rush a little way up the beach ; but away from land, where the real nature of a wave is apparent, it simply rises and falls. The wave itself travels onward; the water does not. The wave passes through the water, not carrying the water along. It is a vibration or impulse, communicated from particle to particle of water.
the wave passes, each water-particle makes a little motion, and returns to its former position. A piece of floating seaweed is not borne onward by the wave. It has a passing lift, and is left where it was.
Just so with waves: of sound. Each wave is a vibration or impulse, which passes from particle to particle of air, giving it a stir, but not sweeping it onward. When rock is blasted at a distance, and the crash of explosion reaches your ears, you must not suppose for a moment that the air which was close to the blasted rock has come rushing to you. That would, indeed, be a hurricane ! All that the explosion has done is to set in motion waves of air,-undulations passing swiftly, far more swiftly than the most rapid hurricane, from particle to particle of air, and at length reaching the particles situated within your ears. By means of those undulations, news of the distant explosion is conveyed through your ear-drums to your brain ; and after long practice your brain has learnt to ascribe the airwaves in question to their true source.
We believe that light also is conveyed by waves, or by something analogous to waves; not waves of air, for our atmosphere is far too coarse a medium, but waves of luminiferous ether.
The Ether of Space has neither been seen nor felt, nor can its existence be absolutely proved ; but that some such medium does exist no reasonable doubts can be felt. Light appears to be conveyed by exceedingly small and rapid waves of ether ; waves set in motion by the clash of minute particles in any blazing or heated body: much as sound-waves are set in motion by the clash or friction of larger substances.
Sound-waves travel at the rate of 1140 feet each second ; and many distinct waves or vibrations enter the ear during one second of time, from the source which causes the noise. Musical waves, adapted to our power of hearing, vary from about sixteen per second, to forty thousand per second. That is to say—the lowest bass-note, which we can detect, means 16 waves striking upon the ear-drum in the course of each second ; while the highest treble-note means 40,000 waves striking upon the ear-drum in the course of each second. Waves of sound, coming to our ears in the shape of ten to twelve waves per second, or in the shape of fifty or sixty thousand waves per second, are absolutely unheard by us. Yet any amount of such waves may be travelling through the air at all times.
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.
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