The French physicist Jean Bernard Léon Foucault (1819-1868) is remembered for the Foucault pendulum, by which he demonstrated the diurnal rotation of the earth, and for the first accurate determination of the velocity of light.

Léon Foucault, son of a Paris bookseller, was born on Sept. 18, 1819. He began to study medicine but turned to physics, probably as a result of becoming assistant to Alfred Donné, who was developing a photoengraving process by etching daguerreotypes in connection with his anatomy lectures. This brought Foucault contact with the physicist Hippolyte Fizeau, who was at that time attempting to improve the daguerreotype process, and they collaborated for several years on optical topics. From 1845 Foucault was editor of the scientific section of the Journal de débats. In 1855 he was appointed physicist at the Paris Observatory; in 1864 he was elected a foreign member of the Royal...