FOUCAULT, JEAN BERNARD LE'ox, 1819-68; a French physicist. He studied medi cine, but devoted himself to science, acting as Donne's experimental assistant in the latter's lectures on microscopic anatomy; and investigating, with Figeau, the intensity of sunlight as compared with that of carbon heated in the voltaic arc, and that of lime in the oxyhydrogen blowpipe, and also the duomatic polarization of light. He pub lished in the Comptes Rendus of the academy of sciences, 1849, an account of an electro magnetic regulator for the electric lamp. The following year he proved the velocity of light in air than in water, measuring it by the use of a revolving mirror with a concave mirror centered in its axis, and established the law that the velocity of light in different media is inversely as the refractive indices of the media. In 1831, he demonstrated the rotation of the earth on its axis by the diurnal rotation in an e.s.w. direction of the plane of oscillation of a loon. heavy pendulum freely suspended. The following year, he invented the gyroscope. long became physical assistant in the Paris
imperial observatory in 1855. In 1857, he invented the polarizes called by his name, and in 1858, succeeded in giving to the speculum of reflectiog telescopes the form of a spheroid or a paraboloid of revolution. He set the great reflector in the telescope of the Paris observatory in 1859. In 1865, he a series of papers on a modifica tion of Watt's governor, showing how its period of revolution could be made constant, and on an apparatus for regulating the electric light. He also showed how the sun can be observed without injury to the eve from the excess of light. He was editor of the scientific portion of the Journal des D, from 1845. In conjunction with Regnault, he published an important paper on binocular vision. He received the decoration of the legion of honor in 1850, and was made an officer in 1864. He also received many honors from scientific associations in France and England.