AUGUSTIN JEAN FRESNEL (1788-1827), French physicist, the son of an architect, was born at Broglie (Eure) on the 10th of May 1788. His early progress in learning was slow, and when eight years old he was still unable to read. At the age of thirteen he entered the Ecole Centrale in Caen, and at sixteen and a half the Ecole Polytechnique, where he acquitted himself with distinction. Thence he went to the Ecole des Ponts et Chaussees. He served as an engineer successively in the departments of Vendee, Drome and Ille-et-Villaine; but his espousal of the cause of the Bourbons in 1814 occasioned, on Napoleon's reaccession to power, the loss of his appointment. On the second restoration he obtained a post as engineer in Paris, where much of his life from that time was spent. His researches in optics, continued until his death, appear to have been begun about the year 1814, when he prepared a paper on the aberration of light, which, however, was not published. In 1818 he read a memoir on diffraction for which in the ensuing year he received the prize of the Academie des Sciences at Paris. He was in 1823 unanimously elected a member of the academy, and in 1825 he became a member of the Royal Society of London, which in 1827, at the time of his last illness, awarded him the Rumford medal. In 1819 he was nominated a commissioner of lighthouses, for which he was the first to construct compound lenses as substitutes for mirrors. He died of consumption at Ville-d'Avray, near Paris, on the 14th of July 1827.
The undulatory theory of light, first founded upon experimental demonstration by Thomas Young, was extended to a large class of optical phenomena, and permanently established by his brilliant discoveries and mathematical deductions. By the use of two plane mirrors of metal, forming with each other an angle of nearly 180°, he avoided the diffraction caused in. the experiment of F. M. Grimaldi (1618-1663) on interference by the employment of apertures for the transmission of the light, and was thus enabled in the most conclusive manner to account for the phenomena of interference in accordance with the undulatory theory. With D. F. J. Arago he studied the laws. of the interference of polarized rays. Circularly polarized light he obtained by means of a rhomb of glass, known as "Fresnel's rhomb," having obtuse angles of 126°, and acute angles of 54°. His labours in the cause of optical science received during his lifetime only scant public recognition, and some of his papers were not printed by the Academie des Sciences till many years. after his decease. But, as he wrote to Young in 1824, in him "that sensibility, or that vanity, which people call love of glory" had been blunted. "All the compliments," he says, "that I have received from Arago, Laplace and Biot never gave me so much pleasure as the discovery of a theoretic truth, or the confirmation of a calculation by experiment." See Duleau, "Notice sur Fresnel," Revue ency. t. xxxix.; Arago, Ouvres completes, t. i.; and. Dr G. Peacock, Miscellaneous Works of Thomas Young, vol. i.
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