The following experiment has likewise been suggested as a proof of the impression which light makes upon the eye. If a card, on both sides of which a figure is drawn, for example, a bird and a cage, be made to revolve rapidly on the straight line which divides it symmetrically, the eye will perceive both figures at the same time, provided they return successively to the same place. M. D’Arcy found by various experiments, that, in general, the impression which light produces on the eye, lasts about the eighth of a second. M. Plateau, of Brussels, found that the impression of different colours lasted the following periods; the numbers here stated being the decimal parts of a second. Flame, 0.242. or nearly one fourth of a second; Burning coal, 0.229; White, 0.182, or, a little more than one sixth of a second; Blue, 0.186; Yellow, 0.173; Red, 0.184.
5. Light, though extremely minute, is supposed to have a certain degree of force or momentum. In order to prove this, the late ingenious Mr. Mitchell contrived the following experiment. He constructed a small vane in the form of a common weather-cock, of a very thin plate of copper, about an inch square, and attached to one of the finest harpsicord wires, about ten inches long, and nicely balanced at the other end of the wire, by a grain of very small shot. The instrument had also fixed to it in the middle, at right angles to the length of the wire, and in an horizontal direction, a small bit of a very slender sewing needle, about half an inch long, which was made magnetical. In this state the whole instrument might weigh about ten grains. The vane was supported in the manner of the needle in the mariner’s compass, so that it could turn with the greatest ease; and to prevent its being affected by the vibrations of the air, it was enclosed in a glass case or box. The rays of the sun were then thrown upon the broad part of the vane or copper plate, from a concave mirror of about two feet diameter, which, passing through the front glass of the box, were collected into the focus of the mirror upon the copper plate. In consequence of this the plate began to move with a slow motion of about an inch in a second of time, till it had moved through a space of about two inches and a half, when it struck against the back of the box. The mirror being removed, the instrument returned to its former situation, and the rays of the sun being again thrown upon it, it again began to move, and struck against the back of the box as before. This was repeated three or four times with the same success.
On the above experiment, the following calculation has been founded: If we impute the motion produced in this experiment to the impulse of the rays of light, and suppose that the instrument weighed ten grains, and acquired a velocity of one inch in a second, we shall find that the quantity of matter contained in the rays falling upon the instrument in that time amounted to no more than one twelve hundred-millionth part of a grain, the velocity of light exceeding the velocity of one inch in a second in the proportion of about 12,000,000,000 to 1. The light in this experiment was collected from a surface of about three square feet, which reflecting only about half what falls upon it, the quantity of matter contained in the rays of the sun incident upon a foot and a half of surface in one second of time, ought to be no more than the twelve hundred-millionth part of a grain. But the density of the rays of light at the surface of the sun is greater than that at the earth in the proportion of 45,000 to 1; there ought therefore to issue from one square foot of the sun’s surface in one second of time, in order to supply the waste by light 1/45,000th part of a grain of matter, that is, a little more than two grains a day, or about 4,752,000 grains, or 670 pounds avoirdupoise, nearly, in 6,000 years, a quantity which would have shortened the sun’s diameter no more than about ten feet, if it were formed of the density of water only.
If the above experiment be considered as having been accurately performed, and if the calculations founded upon it be correct, it appears that there can be no grounds for apprehension that the sun can ever be sensibly diminished by the immense and incessant radiations proceeding from his body on the supposition that light is a material emanation. For the diameter of the sun is no less than 880,000 miles; and, before this diameter could be shortened, by the emission of light, one English mile, it would require three millions, one hundred and sixty-eight thousand years, at the rate now stated; and, before it could be shortened ten miles, it would require a period of above thirty-one millions of years. And although the sun were thus actually diminished, it would produce no sensible effect or derangement throughout the planetary system. We have no reason to believe that the system, in its present state and arrangements, was intended to endure for ever, and before that luminary could be so far reduced, during the revolutions of eternity, as to produce any irregularities in the system, new arrangements and modifications might be introduced by the hand of the All Wise and Omnipotent Creator. Besides, it is not improbable that a system of means is established by which the sun and all the luminaries in the universe receive back again a portion of the light which they are continually emitting, either from the planets from whose surfaces it is reflected, or from the millions of stars whose rays are continually traversing the immense spaces of creation, or from some other sources to us unknown.
6. The intensity of light is diminished in proportion to the square of the distance from the luminous body. Thus, a person at two feet distance from a candle, has only the fourth part of the light he would have at one foot, at three feet distance the ninth part, at four feet the sixteenth part, at five feet the twenty fifth part, and so on for other distances. Hence the light received by the planets of the Solar system decreases in proportion to the squares of the distances of these bodies from the sun. This may be illustrated by the following figure,
Figure 1.
Suppose the light which flows from a point A, and passes through a square hole B, is received upon a plane C, parallel to the plane of the hole—or, let the figure C be considered as the shadow of the plane B. When the distance of C is double of B, the length and breadth of the shadow C will be each double of the length and breadth of the plane B, and treble when AD is treble of AB, and so on, which may be easily examined by the light of a candle placed at A. Therefore the surface of the shadow C, at the distance AC—double of AB, is divisible into four squares, and at a treble distance, into nine squares, severally equal to the square B, as represented in the figure. The light, then, which falls upon the plane B being suffered to pass to double that distance, will be uniformly spread over four times the space, and consequently will be four times thinner in every part of that space. And at a treble distance it will be nine times thinner, and at a quadruple distance sixteen times thinner than it was at first. Consequently the quantities of this rarified light received upon a surface of any given size and shape when removed successively to these several distances, will be but one-fourth, one-ninth, one-sixteenth, of the whole quantity received by it at the first distance AB.
In conformity with this law, the relative quantities of light on the surfaces of the planets may be easily determined, when their distances from the sun are known. Thus, the distance of Uranus from the sun is 1,800,000,000 miles, which is about nineteen times greater than the distance of the earth from the same luminary. The square of 19 is 361; consequently the earth enjoys 361 times the intensity of light when compared with that of Uranus; in other words, this distant planet enjoys only the 1/361 part of the quantity of light which falls upon the earth. This quantity, however, is equivalent to the light we should enjoy from the combined effulgence of 348 full moons; and if the pupils of the eyes of the inhabitants of this planet be much larger than ours, and the retina of the eye be endued with a much greater degree of nervous sensibility, they may perceive objects with as great a degree of splendour as we perceive on the objects which surround us in this world. Following out the same principle, we find that the quantity of light enjoyed by the planet Mercury is nearly seven times greater than that of the Earth, and that of Venus nearly double of what we enjoy—that Mars has less than the one half—Jupiter the one twenty-seventh part—and Saturn only the one ninetieth part of the light which falls upon the Earth. That the light of these distant planets, however, is not so weak as we might at first imagine appears from the brilliancy they exhibit, when viewed in our nocturnal sky, either with the telescope or with the unassisted eye—and likewise from the circumstance that a very small portion of the Sun—such as the one fortieth or one fiftieth part diffuses a quantity of light sufficient for most of the purposes of life, as is found in the case of total eclipses of the Sun, when his western limb begins to be visible, only like a fine luminous thread, for his light is then sufficient to render distinctly visible all the parts of the surrounding landscape.
7. It is by light reflected from opake bodies that most of the objects around us are rendered visible. When a lighted candle is brought into a dark room, not only the candle but all other bodies in the room become visible. Rays of the sun passing into a dark room render luminous a sheet of paper on which they fall, and this sheet in its turn enlightens, to a certain extent, the whole apartment, and renders objects in it visible, so long as it receives the rays of the sun. In like manner, the moon and the planets are opake bodies, but the light of the sun falling upon them, and being reflected from their surfaces, renders them visible. Were no light to fall on them from the sun, or were they not endued with a power of reflecting it, they would be altogether invisible to our sight. When the moon comes between us and the sun, as in a total eclipse of that luminary, as no solar light is reflected from the surface next the earth, she is invisible—only the curve or outline of her figure being distinguished by her shadow. In this case, however, there is a certain portion of reflected light on the lunar hemisphere next the earth, though not distinguishable during a solar eclipse. The earth is enlightened by the sun, and a portion of the rays which fall upon it is reflected upon the dark hemisphere of the moon which is then towards the earth. This reflected light from the earth is distinctly perceptible, when the moon appears as a slender crescent, two or three days after new moon—when the earth reflects its light back on the moon, in the same manner as the full moon reflects her light on the earth. Hence, even at this period of the moon, her whole face becomes visible to us, but its light is not uniform or of equal intensity. The thin crescent on which the full blaze of the solar light falls, is very brilliant and distinctly seen, while the other part, on which falls only a comparatively feeble light from the earth, appears very faint, and is little more than visible to the naked eye, but with a telescope of moderate power,—if the atmosphere be very clear—it appears beautifully distinct, so that the relative positions of many of the lunar spots may be distinguished.