"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; 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 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 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 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 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 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. "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; 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 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 |