Leon Foucault, a French natural philosopher, born in Paris, Sept. 18, 1819, died Feb. 11, 1868. While studying medicine he was impressed by the discoveries of Daguerre, and turned his attention exclusively to optics. He rapidly acquired proficiency in this branch of natural philosophy, and in 1844 invented an electric lamp, which has been adopted by natural philosophers for physical experiments, and used as a means of lighting large factories or yards. With Hippolyte Fizeau he made a series of delicate experiments upon the phenomena of light. He solved a problem which had attracted the attention of Wheatstone, Arago, and many others, demonstrating that the velocity of light differs materially while passing through a vacuum or through transparent bodies. He was no less successful in mechanics than he had been in optics. By means of the pendulum he gave a new and striking demonstration of the rotatory motion of the earth. The gyroscope, another instrument with which he experimented, not only affords new indication of the earth's rotation, and serves to measure it, but furnishes a means of determining astronomical positions without observation of the heavens. (See Gyroscope.) Foucault was rewarded for his labors by an appointment to an important post in the observatory at Paris, and received in 1855 the Copley medal of the royal society.