Fabry's career was devoted to the design of precise optical devices based on the principles of interference of light. The Fabry-Pérot interferometer would become one of the mainstays of modern optical instrumentation. Fabry was also involved in one of the first measurements of the Doppler shifts caused by the random thermal motions of atoms and in establishing the existence of the ozone layer in the earth's atmosphere.
Fabry attended the Ecole Polytechnique and the University of Paris, receiving his doctoral degree in 1889. After holding some minor teaching posts, he joined the faculty at the University of Marseilles in 1894, moving to the Sorbonne in 1920. Fabry's doctoral thesis dealt with the interference of light waves and in 1896 he devised, with Alfred Pérot, a chamber, using partially reflecting mirrors, that allowed a light beam to interfere with itself numerous times. As a result of this interference only very precisely defined wavelengths could pass through this chamber. The Fabry-Pérot interferometer, as it came to be known, could be used in very accurate distance measurements, or to produce or select light beams of a very precisely defined frequency.
Throughout his career, Fabry was interested in the applications of interferometry, the study of optical interference, to fundamental scientific questions. The kinetic theory of gases, as developed in nineteenth century by the English mathematical physicist James Clerk Maxwell (1831-1879) and the Austrian Ludwig Boltzman (1844-1906), made it possible to predict the number of molecules of any gas moving with a given velocity at each temperature. The Austrian physicist Christian Johann Doppler (1803-1853) had explained somewhat earlier how the motion of a source, while emitting a wave, would be reflected in a slight change of the frequency of the emitted wave. By 1912 Fabry and Henri Buisson were able to confirm that the frequency shifts found for helium, neon, and krypton were exactly as predicted by the Maxwell-Boltzmann theory.
Fabry also was interested astronomy—his brother Louis had become an astronomer. He conducted a number of studies of the spectra of the stars using his interferometer. In the course of this work he was the first to recognize the absorption of ultraviolet light by ozone in the upper atmosphere. Fabry was elected a member of the French Academy of Sciences in 1927 and received medals from the National Academy of Sciences in the United States and the Royal Society in England. He also served as an advisor to the International Committee on Weights and Measures.
The precise determination of atomic spectra was to provide one of the bases for the establishment of quantum theory. The combination principle developed by Lord Rayleigh (1842-1919) and Walter Ritz (1978-1909) showed that the numerous frequencies in the visible spectrum of a given atom could each be expressed as differences of a smaller set of frequencies or "terms." The terms would eventually be identified with the energy levels of the atom concerned. The terms for hydrogen were particularly simple in form and could be fit to a simple formula, which the Danish physicist Neils Bohr (1885-1962) was able to derive in 1913 from classical physics by placing a single quantum requirement on the allowed orbits of the single electron. Explaining the spectral terms of atoms with more than one electron required the development of the full quantum theory a decade later.
DONALD R. FRANCESCHETTI