Light
the natural agent which, by acting on the retina, excites in us the sensation of vision. Two leading hypotheses regarding its nature have been formed: the one, the omission or corpuscular theory, which, though supported by the great name of Sir Isaac Newton, has been abandoned, and the other, the undulatory theory, which now obtains. The latter assumes the existence everywhere through the universe, or the portion 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 transmitted but only the disturbance. The agitate movements are held to be strictly analogous to the undulations of the atmosphere which produce and convey sound; or, as the word undulation R LIGGEIT Army Corps from 1918; and from then he 1st Army. Comat-general Oct. 16, July, 1910, he was 3d Army on the bated in the second ed in the St. Mihiel offensives. His other d command of the (1918), command of the Philippines mmand of the Westd 1919). ent which, by in us the leading hypohave been omission от согривsupported by Newton, has the other the un hich now obtains. the existen everyle elastic med Ther, the undulations ht, and when they disturban waves.
LIGHTHOUSE LIGHTHOUSE None of the early lighthouse buildings The illumination can be seen 100 miles at sea.
None of the early lighthouse buildings now exist. The Pharos of Alexandria The Romans built lighthouses (331 B. C.) gave its name to its successors.
Emperor Trajan, was re-established as a lighthouse about 1634, and in 1847 was repaired during the reign of the Emperor Trajan, was re-established as a lighthouse about 1634, and in 1847 had a dioptric apparatus placed in it.
Till the end of the 18th century the lighthouses of Great Britain and America were few in number, and of an inferior description in the great essential imports, those of the ocean in producing at Barnegat, erected in 1903, provided mitted are known. By one, the size of the velocity with which light is transmitted are known. By one, the size of the minute circle through which the relative proportion 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 one of Jupiter's satellites at the comone of Jupiter's satellites at the comas compared with their calculated times. 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 directly, by two instrumental methods devised respectively by Foucault and Fizeau, with the same results. The Fizeau, with the same 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 diaphanous; 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 abundant supply of it is essential to the healthy growth of man, the inferior animals, and plants.