After an education received chiefly at home, he studied medicine, which, however, he speedily abandoned for physical science, the improvement of Daguerre’s photographic processes being the object to which he first directed his attention. During three years he was experimental assistant to Μ. Donné in his course of lectures on microscopic anatomy. With Μ. Fizeau he carried on a series of investigations on the intensity of the light of the sun, as compared with that of carbon heated in the voltaic arc, and of lime in the flame of the oxyhydrogen blowpipe; on the interference of heat rays, and of light rays differing greatly in lengths of path; and on the chromatic polarization of light. In 1849 he contributed to the Comptes Rendus of the Academy of Sciences, t. xxviii,, a description of an electromagnetic regulator for the electric lamp, and, in conjunction with Régnault, a paper on binocular vision. By the use of a revolving mirror similar to that used by Wheatstone for measuring the rapidity of electric currents, but having a concave mirror centred in its axis, he was enabled in 1850 to demonstrate the greater velocity of light in air than in water, and to establish the law deduced from the undulatory theory that the velocity of light in different media is inversely as the refractive indices of the media. In the same year he was created a chevalier of the Legion of Honour. For his demonstration in 1851 of the diurnal motion of the earth by the rotation of the plane of oscillation of a freely suspended, long, and heavy pendulum in an E.S.W. direction, exhibited by him at the Pantheon in Paris, and again in the following year by means of his invention the gyroscope, he in 1855 received the Copley medal of the Royal Society of London. He was also in 1855 made physical assistant in the imperial observatory at Paris. In September of that year he discovered that the force required for the rotation of a copper disc moving in its own plane becomes greater, the disc at the same time growing hotter, when the disc is made to rotate with its rim between the poles of a horse-shoe magnet. Foucault invented in 1857 the polarizer which bears his name, and in the succeeding year a method of giving to the speculum of reflecting telescopes the form of a spheroid or a paraboloid of revolution. His reflector for the great telescope in the Paris observatory was mounted in June 1859. With Wheatstone’s revolving mirror he in 1862 determined the absolute velocity of light to be 298,000 kilometres (about 185,000 miles) a second, or 10,000 kilom. less than that obtained by previous experimenters. He was created iu that year a member of the Bureau des Longitudes and an officer of the Legion of Honour, in 1864 a foreign member of the Royal Society of London, and next year a member of the Mechanical Section of the Institute. In 1865 appeared his papers on a modification of Watt’s governor, upon which he had for some time been experimenting with a view to making its period of revolution constant, and on a new apparatus for regulating the electric light; and in the following year (Compt. Rend, lxiii.) he showed how, by the deposition of a transparently thin film of silver on the outer side of the object glass of a telescope, the sun could be viewed without injuring the eye by excess of light. Foucault died of paralysis, February 11, 1868. From the year 1845 he edited the scientific portion of the Journal des Débats. His chief scientific papers are to be found in the Comptes Rendus, t. xxv., 1847—Ixix., 1869.

See Revue Cours Scient, vi., 1869, pp. 484-489; Proc. Roy. Soc. xvii., 1869, pp. lxxxiii.-lxxxiv.; Lissayous, Notice historique sur ia vie et les travaux de Léon Foucault, Paris, 1875.