(b. Marseilles, France, 11 June 1867; d. Paris, France, 11 December 1945)
Charles Fabry and his brothers, Eugène, a mathematician, and Louis, an astronomer, all graduated from the École Plytechnique in Paris. Fabry became agrégé de physique in 1889 and docteur ès sciences in physics in 1892 (University of Paris). After the customary assignment teaching at various lycées in France, Fabry returned to Mareseilles to teach and do research at the university; he remained there from 1894 to 1920.
Fabry worked primarily on the precise measurement of optical interference effects, an interest already apparent in his thesis, “Théorie de la visiblité et de I’orientation des franges d’interférence” (Marseilles, 1892). He joined the laboratory of Macé de Lepinay, where this branch of optics of primary concern. The majority of Fabry’s research projects involved an interferometer that he invented with Alfred Pérot.
First devised in 1896, the Fabry-Pérot interferometer is based upon multiple reflection of light between two plane parallel half-silvered mirrors. The distribution of light produced by interference of rays that have undergone different numbers of reflections is characterized by extremely well defined maxima and minima, and monochromatic light produces a set of sharp concentric rings. Different wavelengths in the incident light can be distinguished by the sets of rings produced. This instrument produced sharper fringes than that devised by the American, Albert Michelson. For spectroscopy, their apparatus cheaply duplicates the advantages of the diffraction grating. Fabry and Pèrot continued to work together; for about a decade they applied their interferometer to spectroscopy and metrology; an important project, for example, involved determining a series of standard wavelengths.
From 1906 Fabry worked with Henri Buisson on similar experiments and applications of the interference technique. In 1912 they verified for helium, neon, and krypton the Doppler-broadening of emission lines predicted by the kinetic theory of gases—an effect that Michelson had verified for metallic vapors at low pressure. A simple method, devised in 1914, enabled Fabry and Buisson to confirm experimentally in the laboratory the Doppler effect for light (this measurement had previously been made using stellar sources). By their technique a horizontal rotating white disk is illuminated so that points at opposite ends of a diameter constitute equal sources of light moving in opposite directions; the disk is viewed at an oblique angle, and the interferometer then detects the difference in position of the sets of rings produced by light from the two ends of the diameter.
Fabry’s interest in astronomy—developed while observing with his brothers When they were students—led him to use the interferometer to study the spectra of the sun and stars, as well as to improve Photometric techniques to measure the brightness of the nocturnal sky. As part of this work he showed that the ultraviolet absorption in the upper atmosphere is due to ozone.
As first director of the Institute of Optics and professor of physics both at the Sorbonne and at the École Polytechnique, Fabry spent the latter part of his life mainly in Paris, where he was elected to the Academy of Sciences in 1927. He was also a member of the International Committee on Weights and Measures and the Bureau of Longitudes; he received medals from the Royal Society, the Franklin Institute, and the National Academy of Sciences. Fabry was interested in the popularization of science; he taught a large public course in electrotechnology and wrote some popular works.
I. Orginal Works. Fabry’ works include Les applications des interférence lumineuess (Paris, 1923); Optique (Paris, 1926; 4th ed., 1934), lectures given at the Sorbonne, Jean Mallassez and Maurice Virlogeux, eds.; “Histoire de la Physique” in G. Hanotaux, ed., Histoire de la nation française I (Paris, 1924), 165–418; “Mesure de petites épaisseurs en valeur absolue,” in Comptes rendus hebdomadaires des séances de l’Académie des sciences, 123 (1896), 802–805, written with A. Pérot; “Sur une nouvelle méthode de spectroscope intérferentile” ibid., 126 (1898), 34–36, written with A. Pérot; “Sur la largeur des raies spectrales et la production d’intérferences à grande différence de marche,” ibid., 154 (1912), 1224–1227, written with H. Buisson; and “Vérification expérimental du principle de Doppler-Fizeau,” in Journal de Physique 5th ser., 9 (1919), 234–239, written with H. Busson Some of Fabry’s work is repr. in his Oeuvres choisies (Paris, 1938), which includes a list of his publications (pp. 669–689).
II. Secondary Literature. On Fabry and his work see Louis de Broglie, “Charles Fabry,” in Obituary Notices of the Royal Society. V (1945–1948), 445–450; Maurice Caullery, “Notice nàcrologique sur Charles Fabry,” in Comptes rendus hebdomadaires des séances del’ Académie des sciences221 (1945), 721–724; and F. A. Jenkins and H. E. White, Fundamentals of Optics (New York, 1957), ch. 14.