Cornu, Marie Alfred

(b. Orléans, France, 6 March 1841; d. La Chansonnerie, near Romartin, France, 12 April 1902),

optics.

Cornu entered the École Polytechnique in 1860, graduated second in his class in 1862, and proceeded to the École des Mines. In 1864 he was appointed repetiteur at the École Polytechnique, where he became professor of physics in 1867, the year in which he was awarded his doctorate for a thesis on crystal-line reflection. He became a member of the Académie des Sciences in 1878 and was an associate of many foreign scientific bodies, including the Royal Society of London (1884) and the U.S. National Académy of Sciences (1901).

While still a probationer at the École des Mines, Cornu was attracted to experimental optics by an exhaustive study of Felix Billet’s celebrated Traité d’optique, and he repeated all the experiments in this work in his spare time. Following Jules Jamin, he showed that reflection at metallic surfaces proceeds exactly as in the case of vitreous substances, due allowance being made for ultraviolet frequencies. A series of experiments dating from 1871 led to a redetermination of the velocity of light by Fizeau’s method, for which Cornu was awarded the La Caze Prize of the Académie des Sciences and the Rumford Medal of the Royal Society of London in 1878.

Cornu made a number of important contributions to spectrum analysis, including very precise measurements of the wavelengths of certain lines in the hydrogen spectrum. By observing the edges of the solar disk and applying the Doppler-Fizeau principle, he found a means of separating the solar spectra from the terrestrial spectra; in the case of the latter he separated the influence of water vapor from that of air. Following the discovery of the Zeeman effect Cornu showed that the D line of sodium is decomposed, under normal magnetization, into four components, as opposed to three, thus forcing Lorentz to modify his theory of the Zeeman effect.

Cornu’s optical researches also included studies of conditions for achromatism in interference phenomena; his work on the measurement of the curvature of lenses; and his explanation of certain observed anomalies in the behavior of diffraction gratings in terms of minute variations in the distance between successive lines. He also engaged in acoustical researches and, with Baille, redetermined the gravitational constant by Cavendish’s method.

Apart from his many contributions to experimental physics, Cornu is remembered for the elegant method of the so-called Cornu spiral for the determination of intensities in interference phenomena.

BIBLIOGRAPHY

Cornu’s dossier at the Académie des Sciences, Paris, contains a portrait and a number of autograph letters, including six to Fizeau, as well as the orations pronounced at his funeral by Bassot, Mascart, and Poincaré.

Alfred Cornu, 1841–1902 (Rennes, 1904) contains a biographical notice by Henri Poincaré and a complete bibliography of Cornu’s published work. Also see Notice sur les titres scientifiques de M. A. Cornu par H. Fizeau (Paris, 1873); and the obituary notice in Proceedings of the Royal Society, 75 (1905), 184–188.

J. W. Herivel