LIGHTHOUSE 481 LIGHTHOUSE 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 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 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. LIGHTHOUSE, a lofty tower or other structure, erected at the entrance of a harbor, or at some important or danger- ous part of a coast, and having a strong light at the top, to guide vessels, and warn them of danger. Orig:inally they were lighted up with fires, but now oil, gas, and electricity are used, the power of the light being increased by the adop- tion of glass reflectors, lenses, and prisms. In the United States these matters are under the supervision of the Lighthouse Board. The most important lighthouse in the United States is the great tower at Barnegat, erected in 1903, provided with lights of 30,000,000 candle power. The illumination can be seen 100 miles at sea. None of the early lighthouse buildings now exist. The Pharos of Alexandria (331 B. c.) gave its name to its suc- cessors. The Romans built lighthouses at Ostia, Ravenna, Puteoli, and other ports. The Phoenician Pharos at Coruna 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 Amer- ica were few in number, and of an in- ferior description in the great essential PETIT-MANON GRANITE TOWER LIGHT- HOUSE, MAINE of a lighthouse — viz., sending the great- est number of rays of light toward the horizon. As an example of a modern lighthouse tower we may take Skerry- vore, which is 139 feet in height and 42 feet in diameter at the base, containing a mass of 58,580 cubic feet of granite. (See Eddystone.) To Augustin Fresnel belongs the honor of inventing and first employing, in 1822, the dioptric system for lighthouse pur- poses in combination with a central lamp having four concentric wicks. He de- vised the lighthouse lens, which is plano- convex, 3 feet 3 inches in height by 2 feet 6 inches in breadth, composed of a central disk, surrounded by annular rings gi-adually decreasing in breadth as they recede from the center. If these lenses be assembled on a frame with eight or more sides, having a lamp in
