"what soul was his, when, from the naked top
of some bold headland, he beheld the sun
rise up and bathe the world in light! he looked—
ocean and earth, the solid frame of earth
and ocean's liquid mass, beneath him lay
in gladness and deep joy."
we live in a mighty ocean whose waves are ever rushing hither and thither, always according to law, with velocity inconceivable, almost immeasurable. these waves lave the shore of that island of space which is our home, travelling to it from remotest regions, and making known to us all that we know of what lies outside our small abode. we call these waves, or rather their effects, by the name of light. we recognise in light—
"offspring of heav'n's first-born
and of th' eternal co-eternal beam"—
the antecedent of all else that exists in the universe; or, as sir john herschel said, "the superior in point of rank and conception to all other products or results of creative power in the physical world. it is light which alone can give, and does give us, the assurance of a uniform and all-pervading energy—a mechanism almost beyond conception, complex, minute, and powerful, by which that influence, or rather that movement, is propagated. our evidence of the existence of gravitation fails us beyond the region of the double stars, or leaves at best only a presumption amounting to moral conviction in its favour. but the argument for a unity of design and action afforded by light stands unweakened by distance, and is co-extensive with the universe itself."
what, then, is light? what is that mysterious movement of some essence pervading all space, whereby, from remotest depths, news is brought to us, after journeys lasting many years, though space is traversed at a rate exceeding more than ten million times the velocity of the swiftest express train?
light is in reality the result of undulations in what is called the ether of space, a perfectly transparent, almost perfectly elastic medium, occupying not only void space, but flowing as freely through the densest solids as the summer breeze flows through the forest trees. the waves of light cannot in this way pass through solid or liquid, or even aerial bodies, but either they are sooner or later brought to rest, or else they are more or less gradually deflected; just as the waves which traverse the ocean come to their end, or are deflected, when they meet the shore or shallows near the shore.
all light, however, has its real origin, not in the ethereal ocean itself, but in the movements of the minute particles of which all forms of matter known to us are composed. a tiny atom, far too small to be perceived with a microscope, even though one should be made ten thousand times more powerful than any yet constructed, when set in rapid vibration, raises minute waves in the ethereal ocean, just as a small body, vibrating on the surface of a sheet of water, would generate waves there. and as the water-waves would travel radially away from the place of their birth, so do the light-waves generated by the vibrations of one of the atoms composing a luminous body radiate forth in all directions through the ethereal ocean until, encountering some obstacle, they are sent (reduced in size) in a new direction.
in some luminous bodies there are atoms vibrating in many different periods (all very small) so as to cause light-waves of many different kinds to proceed from the body. in other cases the atoms all vibrate at one rate, or at two or three or some definite number of rates, so that only light-waves of certain kinds proceed from the body. but in all cases these light-waves only cause us to see the body when they flow in through the pupil of the eye, and falling upon the retina (or the choroid membrane, or whatever part of the eye it may be which finally receives the waves), convey to the optic nerve, and thence to the brain, the information that such and such a body, so coloured, so shaped, so moving, exists towards that direction from which the light-waves seem to come. the body so seen, as we call it, may be the original source of light, or may be a body on which light has been reflected to us.
it is in this way that we receive information from light-waves. it will be conceived how minute they must be, how perfectly they must retain their separate character, multitudinous though they are, in traversing the ether (even when that ether is clogged by the gross matter of our ordinary air), if we remember how through the tiny eye-pupil we often receive light-waves telling us of all the details, all the varieties of colour and brightness, all the movements in a rich landscape.
even more startling are the thoughts suggested by a view of the starlit heavens. from hundreds of suns at once the light-waves which have traversed varying but all enormous distances pour in upon the small circle of the eye-pupil, waves of many kinds coming in together from each sun. the waves which thus reach the eye from one bright star have been but a few years upon their journey; all that time they have been traversing an ocean swept in every part by untold millions of other waves, and yet they arrive as perfect in order and regularity as rollers which have traversed a wide sea pour in upon a level shore. from another star, as bright as the first, they have been years in travelling; from some among the fainter stars, hundreds, perhaps thousands of years. yet still they flow on, each order of waves in perfect uniformity as when they first left their parent sun.
but even this is not all. among the waves which reach the eye many, nay, most, are so small that ordinary vision cannot perceive their action. take, however, a telescope, and so gather them together as to intensify this action, and they are rendered perceptible, just as the unnoticed heaving of ocean becomes a manifest wave-motion when it reaches a regularly narrowing inlet. thus, from stars so remote that their light has required thousands, or, even in some cases, perhaps, hundreds of thousands of years in reaching us, the light-waves flow steadily in upon us. so small are these waves, that the breadth of from forty to sixty thousand of them would occupy but a single inch. through every point in space waves from all the hundred millions of stars are at all times simultaneously rushing at the rate of one hundred and eighty-five thousand miles in every second of time: yet they travel on altogether undisturbed, and each tells its story as distinctly as though the ether had conveyed no other message, and that message but for a short distance.
it would be difficult to say which thought, considered in its real significance, is more striking,—the thought of what is done for us by light regarded as a terrestrial phenomenon, or the thought of what light is doing, and has done, in presenting to us a view of the starlit heavens.
when the sun rises in splendour above the eastern horizon, tinting the sky with varied colours, lighting up the clouds which till then have been but dark patches on the heavens, bringing out the colours of hill and dale, rock and river, fields and woods, the heart gladdens at the spectacle. a pleasing melancholy falls on us as the light fades away at eventide, tint after tint vanishing, until at length the gloom of night enshrouds all. the full splendour of mid-day, the chastened splendour of a moonlit night, and the glory of the heavens when "all the stars shine, and the shepherd gladdens in his heart," stir the soul in like manner; and it might seem to many that to analyse these glories, to explain their scientific meaning, would be to rob the mind of the pleasure it had before found in such scenes. many would be disposed to think that a purer enjoyment is expressed by augustine than any student of science could find in the wonders of light, in those words in which he expresses
his sense of the loveliness of fair forms and brilliant colours. "for light, queen of the colours," he says, "bathing all i can look upon, from morning till evening, let me go where i will, will still keep gliding by me in unnumbered guises, and soothe me whilst i am busy at other things, and am thinking nothing of her." but the sensuous pleasure afforded by light is enhanced, while a purer and higher enjoyment is superadded, when the real meaning of the display is understood. as the astronomer sees in the sun a more glorious object than the sun of the poet, recognising in imagination not only the visible splendour of that orb, but the mighty energy with which it is swaying the motions of a scheme of circling worlds, the wondrous activities at work throughout its entire frame, the inconceivable tumult which must prevail in that seemingly silent globe, so the glories of light, rightly understood, are far more impressive than as they appeal simply to the senses.
fig. 1.—sunrise on the righi.
consider, for instance, the real meaning of sunrise. the orb seemingly rising above the horizon, but, in reality, at rest, is the source of all the glory which is spreading over the fair face of earth. the atoms of that remote body, vibrating with intensest activity, send forth in all directions ethereal waves, and of these relatively but a very few, about one in two thousand millions, fall upon our earth, producing the phenomena of sunlight. they have been little more than eight minutes on the road, but in that short time they have traversed more than 90,000,000 of miles. were they to fall directly upon our earth, we should see few of the splendours which attend the uprising of the sun. the deep air clothing our earth receives the onward rushing waves, and reflects them in all directions. to use biot's simile, "the air is a sort of brilliant veil, which multiplies and diversifies the sunlight by an infinity of repercussions." nor is the wonder of the scene, or its effect in filling the mind with solemn and poetic thoughts, diminished—on the contrary, it is enhanced—by the recollection that the gradually growing glory of day is brought about by the slow turning of the mighty earth,—
"that spinning sleeps
on her soft axle, as she paces even,
and bears us soft with the smooth air along."
but if this is true of a scene of terrestrial splendour, how much more fully may it be said of the glories of the heavens? no poet, if unaware of the real meaning of modern discoveries respecting the celestial bodies, can be moved by the starlit depths as the astronomer is, at least the astronomer whose study of science is not limited to mere observation and calculation. hundreds of bright points of light sparkling, and sometimes varying strangely in colour, form, no doubt, a beautiful scene; but the scene is not less beautiful, and certainly it is far more impressive, when we remember that every one of these points of light is a sun, mighty in attractive energy like ours, its whole surface glowing with fiery heat, and every particle of its substance constantly in motion, if not always in the fierce rush of cosmic hurricanes, yet with the ceaseless vibrations which generate the ethereal light-waves telling us of the star's existence.
there is one strange thought connected with the motion of light-waves through the ether of space which has not, i think, received the attention it deserves.
every one knows that when we look at the heavens we do not see the celestial bodies where they are, but where they were, and again, not where they were at any one moment of time, but some where they were a short time ago, others where they were very long ago. but it is not so generally known, or remembered by those who do know it, that if light were not so active as it is the result would be that utterly incorrect pictures of the celestial depths would continually be presented to us. as matters actually are no orb in space can appear very far from its true place. we see the sun, for instance, at any moment, not where he is, but where he was (or rather towards the direction in which he lay) about eight minutes before. but as the real velocity of the earth, and therefore the apparent velocity of the sun, amounts only to about eighteen miles per second, the sun is only thrown about 9000 miles out of his true position, which is but about the ninetieth part of his diameter: so that we see the sun very nearly in his right place. now it might seem that a star whose light takes, say, twenty years in reaching us, must be seen very far from its true place, supposing the star to be travelling along very quickly; and, in one sense, this is true. if such a star is moving at the rate of fifty miles per second, athwart the line of sight, it will be out of place by so considerable a distance as 315,000,000,000 of miles. yet the star will appear very nearly in its true position, simply because, at the star's enormous distance from us, even the great distance just named is reduced to a very small apparent amount. such a star would, in fact, be displaced by only about the thirtieth part of the sun's or moon's apparent diameters, or by about a fifteenth part of the distance separating the middle star of the great bear's tail from its small companion, sometimes called jack by the middle horse. thus the stellar heavens present very truly to us the positions of the stars; for such athwart motion as i have just imagined would be very much larger than the motion of far the greater number of the stars. but we only thus see the heavens truly pictured because of the enormous velocity with which light travels. if light swept along only at the rate of a hundred miles in a
second (a velocity still far beyond our powers of conception), there would be no believing what we should see, for every star, and our own sun, and all the planets, and even our own companion planet, the moon, would be thrown in appearance very far from their true positions. if they were all shifted in position by the same amount and in the same direction the picture would still be true, in a sense, just as we see a true picture of an object at the bottom of a clear lake, though the picture is displaced by the refractive action of the water on the rays of light. but, in the imagined case, the sun, and moon, and planets, and stars would be shifted by different amounts and in different ways, simply because they are moving at different rates and in different directions. the scene presented to us would have been utterly untrue. astronomy as a science could probably have had no existence in such a case. assuredly it could have had no existence until students of the heavenly bodies had learned to accept as the first axiom of their science the doctrine that "seeing is not believing."
fig 2.—sunset at sea.
a strange thought truly, that so active are the orbs peopling space, so swiftly do they rush onwards upon their orbits, that light, carrying its message at a rate exceeding six thousand times the velocity of the swiftest express train, would be utterly unable to give a true account of the position and movements of the celestial bodies. fortunately light gives a true record, because the qualities of the cosmic ether are such that the message of light is transmitted hundreds of times more swiftly than the swiftest bodies in the universe travel onwards upon their orbits around each other or in space.
