[The author is undoubtedly mistaken when he considers light “of all material bodies—the most simple,” and “an unmixed body.”
It is well known that a beam, or pencil, of light, as emitted from the sun, is not a simple body, but is capable of being divided into seven prismatic colors. The image which is formed by the refraction of the pencil, by means of a prism, is called a Spectrum, and clearly exhibits the compound nature of light. The refracted rays of the Spectrum may be collected and made to constitute a pencil of light again, which will be white, or colorless as before.
If this prismatic Spectrum be examined closely, it will be found that the different colored rays differ very much in their heating, illuminating, and chemical powers. Dr. Herschell, and other experimenters, have found that the orange rays possess a greater illuminating power than the red; and the yellow more than the orange: but the maximums of illumination lies in the brightest yellow or palest green.
There is also a very sensible difference in the heating power of these colored rays. By passing the bulb of a delicate air thermometer through the different colored rays, it indicates the greatest heat in the red rays; next in the green, and so on diminishing to the violet. But the maximum of heat has been ascertained to be immediately beyond the red rays, and of course out of the Spectrum, in an unilluminated spot: thus indicating that there are invisible rays possessing a greater heating power than any of the seven colored rays. These are called calorific rays.
By the experiments of Ritter and Wallaston it is now satisfactorily ascertained that there are also chemical rays which excite neither heat nor light, and lie on the other side of the Spectrum from the invisible calorific rays, just without the violet. It is true, the chemical effect can be distinguished even to the green rays, but this seems to be by diffusion, or a species of sympathy. The sensible chemical power is exerted just without the violet rays.
This fact is established more clearly by Berard. He concentrated, by a lens, all the portion of the Spectrum from the green to the red rays, and made them act on muriate of silver two hours without effect. He then concentrated all the portion of the Spectrum from the green to the violet rays, and made them act on muriate of silver, and they blackened it in less than six minutes. Thus, evidently, are detected very different properties in the different portions of the prismatic Spectrum.
Instead, therefore, of light being a “simple substance,” and “unmixed” it is found to be decidedly compound. It is capable of being divided into seven differently colored rays, and these rays, according to their natural properties, into three classes: the illuminating rays, calorific rays, and chemical rays.]
The rays of light always proceed in straight lines, unless diverted by some intervening body. They are subject to the laws of attraction like other small bodies. If a stream of light be admitted through a small hole into a dark room, and the edge of a knife be applied, it will be diverted from its natural course, and inflected towards it. When the rays of light are thrown back by any opposing body, they are said to be reflected. When in passing from one medium to another, they are inflected or diverted from their rectilineal course, they are said to be refracted; and this property of light is called its refrangibility. Refraction arises from this, that the rays are more attracted by a dense, than by a rare medium.
The velocity of light is prodigious, and almost incredible; it moves at the rate of near 200,000 miles in a second of time! Roemer, a Danish philosopher, was the first who found the means of determining the velocity of light, by the difference of time in the eclipses of Jupiter’s satellites, when the earth was on the same, or on the contrary side of the sun, with that planet. This point may be easily proved; for when the earth is between the sun and this planet, those eclipses will happen about 8¼ minutes sooner, than according to the tables; but when the earth is in the contrary position, the eclipses happen about 8¼ minutes later than they are predicted by the tables. Hence, therefore, light takes up about 8¼ minutes in passing from the sun to the earth, a distance of 95,513,794 miles; and it takes about 16½ minutes of time to go through a space equal to the diameter of the earth’s orbit, which is at least 190 millions of miles in length; which is near a million of miles swifter than the motion of a cannon-ball, which flies with the velocity of about a mile in eight seconds.[40] In comparing this velocity of light with that of a cannon-ball, it has been observed, that light passes through a space in about eight minutes, which a cannon-ball with its ordinary velocity, could not traverse in less than thirty-two years! The velocity of sound bears a very small proportion to that of light. Light travels, in the space of eight minutes, a distance in which sound could not be communicated in seventeen years; and even our senses may convince us, if we attend to the explosion of gunpowder, &c., of the almost infinite velocity of the one compared with that of the other.[41] Were the propagation of the rays of light less rapid, the darkness would be very slowly dissipated, and great inconveniences would result to the inhabitants of the earth.
The divisibility of the parts of matter is no where more apparent than in the minuteness of the particles of light. The unobstructed rays of light which proceed from a candle, will, almost instantaneously, fill a space of two miles; and it has been computed, says Dr. O. Gregory, that there fly out of the end of the flame of a burning candle, in a second of time, ten thousand millions of times more such particles than there are visible grains of sand in the whole earth. Dr. Nieuwentyt has computed, that an inch of candle, when converted to light, becomes divided into 269,617,040 parts, with 40 ciphers annexed; at which rate there must issue out of it, when burning, 418,660, with 39 ciphers more, particles in the second of a minute; vastly more than a thousand times a thousand million of times the number of sands the whole earth can contain; reckoning ten inches to one foot, and that 100 sands are equal to one inch.[42] As sound is propagated only at the rate of 1,142 feet in a second, a particle of light must be 786,000 times more subtile than a particle of air. If the particles of light were not extremely small, their velocity would be highly destructive. Indeed, were they equal in bulk to the two millionth part of a grain of sand, this impulse would not be less than sand shot from the mouth of a cannon. If the particles of light had more density, they would not only dazzle us by their splendor, but injure us by their heat.
There is no creature of God that diffuses itself, and whose influence reaches so far and wide, and fills so large a vacuum, as light. All that inconceivable space between this globe and the fixed stars, a distance which numbers cannot reach, is replete with light. Nay, the space in which it is diffused is not less than the universe itself; the immensity of which exceeds the conception of human understanding. It is from this almost unlimited diffusion of light that the very remotest of the heavenly bodies in the solar system become discernible, either by the naked eye or by telescopes. And had we instruments that could carry our sight as far as the light is extended, we should discover those bodies which are placed at the very extremity of the universe.[43]
Light is the medium through which objects become visible to us. It is owing to it, that we are enabled to behold and contemplate the wonderful works of the great Creator; to discover unexplored systems in the trackless regions of unbounded space, to imbibe knowledge from things created, to hold intercourse with each other, to steer the hollow bark to distant climes, and to investigate the records of all science. Without its aid, the world would have been an inhospitable wilderness, involved in sable shades of perpetual night. “Truly the light is sweet, and a pleasant thing it is for the eyes to behold it.”
Light beautifies every delightful object which comes within the reach of its rays.
“Nature’s resplendentrobe!
Without whose vesting beauty all were wrapt
In unessential gloom.”