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CHAPTER III.

LIGHT.

NATURE AND LAWS OF LIGHT-THE SPECTRUM-REFRACTION IN THE ATMOSPHERE-THE TINTS OF SKY AND LANDSCAPE-COLOURS—THE RAINBOW-HALOS-MOCKSUNS AND MOCKMOONS-SPECTRAL ILLU

SIONS

FATA MORGANA-ENCHANTED COASTS-THE MIRAGE-SHADOWS ON MISTS AND CLOUDS—SUMMARY VIEW OF OTHER WONDERS OF LIGHT.

LIGHT is the emanation from luminous or illuminated bodies which makes them visible to the eye. Some philosophers suppose it to be a subtle and extremely attenuated fluid —a real substance, yet so fine as to have no appreciable weight; and others suppose it to be merely the undulation or vibration of an ethereal medium, which is luminous when in motion, and dark when in repose. Light may be flung from one illuminated body to another, and from a second to a third; but, in every case, it must come, in the first instance, from a self-luminous body. Its grand source throughout the solar system is the sun; a minor source of it, in this part of the universe, is the fixed stars; and the local sources of it in our world are principally the flashes of the electric fluid, the glow of phosphorescence, and the flame and red-heat of combustion. It is always of the same colour as the body from which it comes; it always moves in straight lines, and consists of rays, related to one another in some such way as

the straws of a sheaf of corn; and it streams froin every visible point of the body which emits it, and in every direction whence that point can be seen. It passes through transparent substances, whether gaseous, liquid, or solid, but is variously bent by them, or turned from the line of its previous course, according to their density and other circumstances; and it is partly absorbed and partly flung back by non-transparent bodies, in a diversity of ways, and with most gorgeous appearances, but under uniform and well-known laws. The light of the sun has been ascertained to take only seven minutes and a half to travel to the earth, or to move through a space equal to the circumference of our globe in the eighth part of a second; and all other light is presumed to travel at the same rate.

When a small sunbeam is admitted to a dark room through a little hole of a window-shutter, and projected on a white screen or on a sheet of paper, the insertion in it of a triangular piece of glass, technically called a prism, separates it into a series of splendid colours exactly similar to those of the most perfect and brilliant rainbow. This is termed the spectrum, and comprises always the same colours in the same order. The red is at the end which is least refracted, and the violet at the other end; and, if the whole be artificially divided into three hundred and sixty parts, the red will be found to occupy forty-five of these parts, the orange twenty-seven, the yellow forty-eight, the green sixty, the blue sixty, the indigo forty, and the violet eighty. When all the series, or the entire spectrum, is caught upon a lens of such form as to concentrate it to one spot, it reproduces the white sun

beam or original colourless light. The different colours possess different powers, and are technically regarded as different rays. The lightest green and deepest yellow are the most illuminating; and either the violet or a dark space immediately beyond it, is the most heating; and the spectrum as a whole affords ample scope for wondering inquiry and keen investigation as to its action on living organisms, on photographic preparations, and on select objects in the chemist's laboratory. The light of the moon, and various lights of combustion too, differ in their properties or actions from one another, and from the light of the sun. In all cases, however, the degree of brightness depends on the extent of the undulations, and the predominance of any ray or colour depends on their number..

The varied refraction of light in our atmosphere is the cause of twilight, and of all the tints and magnificence of landscape, cloud, and sky. Were the atmosphere awanting, or did it not possess the power of deflecting and diffusing light at a great variety of angles, and with a great variety of effects, the sun would rush up from night in a moment in the morning, and pass over the hemisphere in a uniform garish blaze through the day, and plunge back to night in a moment at sunset. His rising would resemble the sudden kindling of a bonfire by a great charge of gunpowder; his walk over the sky would resemble the steady flame of that fire without flicker or diversity; and his setting would resemble the instantaneous extinction of it by the fall of a waterspout. He would glare in the face of observers who looked at him, but would give them no light when they turned their backs

to him, and would never, in any circumstances, throw illumination, or colour, or the charms of perspective, on either the outspread earth or the overarching sky. Sunshine would scarcely do more for the world, or do it much differently, than a flambeau does at midnight for a mountain glen. "The number of objects in the heavens would, indeed, be augmented, for the stars would shine through a canopy as black as ebony, even when the sun was above the horizon; but all the gay colouring of the terrestrial landscape, which now de lights the eye and the imagination, would be for ever veiled from the inhabitants of the world. In such a state of things, it would be always night; and the difference between such a night and that which we now enjoy would be, that the celestial orbs, instead of being grounded on a beautiful azure sky, would appear on a black canopy, like so many white points on a dismal mourning carpet."

Sunbeams entering the atmosphere in any other direction than the perpendicular one, but especially sunbeams entering it in a very slanting manner, as all do when the sun is far down the vault and near the horizon, undergo a long series of flexure through the various densities of gas and vapour before they reach the ground. They sustain little, indeed, in the higher regions of the air, where its density is very attenuated, and where no vapours ever exist in sufficient quantity to form a cloud; but, in struggling through the increasingly thick masses of the last two or three miles, they undergo so much as to be eventually bent almost like the segment of a hoop. Hence, as the lower end of a stick inserted in clear water appears to the eye several degrees away from what is known to be its true

position, an object in the sky exactly on a line with the horizon appears to the eye to be a considerable distance above the horizon. The sun and the moon, therefore, are always visible at their rising some little time before they actually rise, and continue visible at setting some time after they actually set. And when an eclipse of the moon happens, as on rare occasions it does, exactly at the time of sunset and of moonrise, the curious spectacle is beheld of the sun in full orb on the one rim of the sky, and the moon in full orb, darkened by the earth's shadow, on the other. The earth is then in the exact line between them; yet, in consequence of the refraction of the light through her atmosphere, she appears to the eye to be considerably below both.

The sunbeams are so very diversifiedly refracted by the many different kinds of surfaces and substances on the ground, and by the many different conditions of air and vapour immediately above them, as to produce, in the very act of their falling, all the endless, sumptuous, brilliant varieties of tint and shade which adorn our landscapes. Colours, whether fixed or fleeting, are nothing more than the refractions of the precise rays or mixtures of rays which correspond to them, or, in other words, are not at all properties in objects, but simply portions of the spectrum. The very play of light is essentially a play of all the gorgeous hues of the rainbow, now flowing forth in the silver whiteness of the undivided beam,—now bursting into all the divisions, as when the beam passes through the prism,— and now dispersing and regathering and wheeling fitfully out its diversities of ray, as when the beam is thrown over a cabinet of gems, or whirled into a re

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