LIGHT, the natural agent which, by acting on the retina, excites in us the sensation of vision. Two leading hypo theses regarding its nature have been formed: the one, the omission or corpus cular theory, which, though supported by the great name of Sir Isaac Newton, has been abandoned, and the other, the un dulatory theory, which now obtains. The latter assumes the existence every where through the universe, or the por tion of it with which we are connected, of an extremely subtle elastic medium, called luminiferous ether, the undulations of which constitute light, and when they impinge on the retina produce vision. The particles agitated are not trans mitted but only the disturbance. The movements are held to be strictly analogous to the undulations of the at mosphere which produce and convey sound; or, as the word undulation imports, those of the ocean in producing waves. Several methods of calculating the velocity with which light is trans mitted are known. By one, the size of the minute circle through which the aberration of light makes stars appar ently revolve is carefully noted, and the relative nroportion of the earth's velocity in her orbit to that of light arriving from the stars ascertained. The result is that light is found to move about a hundred thousand times as fast as the earth, which gives the velocity about 186,000 miles per second. By another, observation is made of the time in which light actually arrives at the eye from one of Jupiter's satellites at the com mencement or the close of an occultation as compared with their calculated times.
It is found that 8' 18" are required for light to travel over half the earth's orbit, which gives, as in the former case, about 186,000 miles per second for its velocity. The velocity is also measured directly, by two instrumental methods devised respectively by Foucault and Fizeau, with the same results. The great sources of light are the sun, the fixed stars (other suns), bodies in a state of ignition, electricity, etc. The bodies sending forth rays or pencils of light are called luminous; those through which it passes easily, transparent or diaphan ous; those through which it passes less easily, translucent; and those through which it cannot pass at all, opaque. When a ray meets the surface of a body, it may be refracted and decomposed or reflected. When it encounters an opaque body it casts a shadow. Admitted into a dark chamber through a small aperture to fall on a screen, the rays make images of external objects reversed. The illuminating power on any surface is inversely as the square of the distance from the source of light. This may be measured by a photometer. Light may be defracted, it may be polarized. An abundant supply of it is essential to the healthy growth of man, the inferior animals, and plants.