FRESNEL, AUGUSTIN JEAN, a French physicist, was b. at Broglie, in the department of Eure, 10th May, 1788; educated at Caen at the Ecole Polytechnique, and finally at the Eeole des Fonts et Chaussees. On the €ompletion of his studies, he was sent as government engineer to La Vendee, and afterwards to the department of Drisne; where he remained till Mar., 1815. On the return of Napoleon from Elba, F. offered his services to the Bourbons, but ill health prevented him from actively engaging in mili tary life. At the restoration, he resumed his duties as government engineer; but in the interval he had been devoting his enforced leisure to physico-mathematical researches, particularly the polarisation of light, with so much success, that although in a letter, dated 28th Dec., 1814, we find him writing to a friend to get him some books on the subject, as he did not know what the phrase " polarization of light meant " (" Je ne sais ce qu'on entend par la polarisation de hi lumiere"), yet of the he ranked among the first authorities on the question. In ignorance,
it is said, of the labors of Young, F. ,demonstrated to his countrymen the error of the Newtonian theory of the propagation of light by the'emission of material particles, and ably advocated the undulatory hypothesis. The result dr his researches was exibited iu amemoir, crowned by the French academie des sciences in 1819. Along with Arago, he investigated the action exercised by polarized rays of light on each other, and their discoveries, publiShed in a joint memoir, confirmed his previous theory on the mode of the propagation of light. His practical application of the new theory to the improve ment of the light-house system, was of incalculable value, and has quite abolished the old metnod of illuminating light-houses. • See LIGHT-DOUSES. In 1823, F. was elected a member of the academie des sciences; in 1825 a member of the royal society of London; and in 18'27, received from the same society the Rumford medal for his discoveries con cerning light and'heat. He died July 14, 1827.