Foucault, JEAN BERNARD LÉON

Foucault, JEAN BERNARD LÉON, French physicist, was born in Paris, 18th September 1819. His first work in the physical sciences was the improvement of Daguerre's photographic processes, and his next a series of investigations, conducted in co-operation with Fizeau, on the properties of light. Continuing his optical inquiries, he was enabled to demonstrate in 1850 that the velocity of light is greater in air than in water, and that in different media the velocity varies inversely as the refractive indices of the respective media. For his remarkable proof in 1851 of the rotation of the earth by means of a freely-suspended pendulum, see PENDULUM. Two years after his appointment (1855) as physical assistant in the observatory at Paris Foucault invented his polariser; and two years later (1859) his reflector for the great telescope at Paris was completed. Foucault also invented apparatus for the better application of the electric light, and showed that the sun may be viewed without injury to the eyesight if the object glass of a large telescope be covered with a thin film of silver. He died at Paris on 11th February 1868. Foucault edited the scientific part of the Journal des Débats from 1845, and was elected a foreign member of the Royal Society of London in 1864. His papers were printed in Comptes Rendus (1847-69). Compare also Recueil des Travaux Scientifiques de Foucault (ed. by Gariel and Bertrand, 1878).