Foucault, JEAN

BERNARD LÉON, 1819-1868. French physicist; with Fizean he carried on a series of investigations on the intensity of sunlight as compared with that of the electric arc and lime light; also as on the interference of heat rays and on chromatic polarization of light. In 1849 he described a very efficient electromagnetic regulator for an arc lamp, and with Regnault wrote a paper on binocular vision. In 1850, by the use of a revolving mirror, he demonstrated the greater velocity of light in air than in water, and thus established the law deduced from the wave theory that the speed of light in different media is inversely as their refractive indices. In 1855 he was awarded the Copley medal of the Royal Soc. of London for his demonstration of the diurnal motion of the earth, and discovered that a resistance is developed when a copper disk is made to rotate in a magnetic field. His chief papers are in the Comptes Rendus, 1847-1869.