WOLF, CHARLES JOSEPH ÉTIENNE

(b. Vorges, near Laon, Aisne, France, 9 November 1827; d. St.-Sevan, Ille-et-Vilaine, France, 4 July 1918)

astronomy, history of science.

Wolf, whose family included a number of teachers and professors, entered the École Normale Supérieure in 1848. Agrégé in science three years later, he taught at the lycée in Nîmes and later at the one in Metz. His initial research, a study of capillarity as a function of temperature, earned him a doctorate in physical sciences in 1856. He was then named professor of physics at the Faculty of Sciences of the University of Montpellier, where in 1862 he demonstrated that the spectra of incandescent bodies, then thought to be rigorously stable, vary when the temperature of the body rises.

Also in 1862, Le Verrier had Wolf named astronomer at the Paris observatory. Assigned at first to the Service Méridien, Wolf studied the personal equation affecting meridian observations and built an apparatus for determining it. He also worked on the electric synchronization of astronomical clocks, perfecting a device that was later adopted for the clocks of Paris.

Wolf was transferred to the Service des Équatoriaux, where, in collaboration with Rayet, he photographed the penumbra of the moon during the eclipse of October 1865. Shortly afterward he observed the spectrum of a nova and noted that it contained bright lines, a new phenomenon that he subsequently sought to detect in the spectra of other stars. For this purpose, Wolf devised a direct-view spectroscope, with neither slit nor lens, that could immediately be substituted for the eyepiece of a telescope when a star was sighted. With this instrument the user could quickly carry out a spectroscopic exploration of the sky. In 1867 Wolf and Rayet discovered three stars exhibiting the phenomenon they were seeking; these were the first examples of what are called Wolf-Rayet stars, also known as stars of spectral type W.

Of Wolf’s thirty or so published notes and articles we shall mention only two of the most important. In 1869, exploiting data gathered during the transit of Mercury in the preceding year, he definitively solved the problem of the “black drop,” a phenomenon that occurs at the moment when the image of a planet comes into contact with the solar limb, making it difficult to determine the instant of contact. He showed that the phenomenon, which Lalande had attributed to irradiation, is purely instrumental and can be eliminated by the use of a sufficiently large objective that is free from aberration. From 1873 to 1875 Wolf studied the Pleiades, which serve as standards for astrometrical measurements. He established the first general catalog of the cluster (1877), containing the positions and magnitudes of 571 stars.

Appointed professeur suppléant of astronomy at the Paris Faculty of Sciences in 1875, Wolf was named titular professor in 1892. He left the observatory in the latter year and retired from teaching in 1901. Towards the end of his career Wolf became especially interested in the history of science. In the Bulletin astronomique of 1884 and 1885 he published seven articles on cosmogonic hypotheses and later collected them in a book that also contained his own complete French translation of Kant’s Universal Natural History and Theory of the Heavens. He also studied ancient standards of weights and lengths and the history of the pendulum. His most remarkable work, Histoire de l’Observatoire de Paris, is still the only full-length account of that institution. Drawing on original documents, he related “the history of the buildings and of their successive transformations, of the instruments used there, of the astronomers who lived there, and of the administration under which they lived.

Wolf was deeply religious, austere but kindly. He often spent vacations in the house where he was born. Obliged to leave it in 1914 because of the German invasion, he took refuge in St.-Servan, where he died a few months before his native city was reconquered. wolf was elected to the Académie des Sciences in 1883 and served as its president in 1898.

BIBLIOGRAPHY

I. Original Works. wolf’s physical works are “Influence de la température sur les phénoménes qui se passent dans les tubes capillaires,” in Annales de chimie et de physique, 49 (1857), 230–279; “Sur les spectres des métaux alcalins,” in Comptes rendus . . . de I’Académie des sciences, 55 (1862), 334–336, written with M. Diacon; and “Sur le pouvoir réflecteur des miroirs en verre argenté,” in Journal de physique théorique et appliquée, 1 (1872), 81–86.

His writings on instruments and their use include “Recherches sur l’équation personnellec . . .,” in Annales de l’Observatoire de Paris, Mémoires. 8 (1865), 153–208; “Description d’un nouveau spectroscope,” in Comptes rendus . . . de l’Académie des sciences, 65 (1867), 292–293; “Description du sidérostat de Foucault,” in Annales sciectifiques de l’École normale supérieure, 2nd ser., 1 (1872), 51–84; “Les applications de l’électricité à l’astronomie,” in Bulletin de la Société internationale des électriciens, 2 (1885), 105–125; and “Comparaison des divers systémes de synchronisation des horloges astronomiques,” in Comptes rendus . . . de l’Académie des sciences, 105 (1887), 1155–1159

Among his astronomical writings are “Sur le passage de Mercure du 4 novembre 1868 . . .” in Comptes rendus . . . de l’Académie des sciences, 68 (1869), 181–183, written with C. André “Description du groupe des pleïades;” in Annales de l’Observatoire de Paris, Mémoires. 14 pt. 1 (1877), A1–A81; Les hypothéses cosmogoniques (Paris, 1886); and Astronomie et géodésie, H. Le Barbier and P. Bourguignon, eds. (Paris, 1891). The discovery of the Wolf-Rayet stars is discussed in Jacques R. Lévy, “Rayet” in DSB, XI, 319–321. Various observations are reported in some twenty notes in Comptes rendus . . . de l’Académie des sciences, 62–107 (1866–1888).

The history of science is treated in “Étalons de poids et mesures de l’Observatoire . . .” in Annales de l’Observatoire de Paris, 17 (1883), C1–C78; “Rôle; de Lavoisier . . . systéme métrique,” in Comptes rendus . . . de l’Académie des sciences, 102 (1886), 1279–1284; Travaux relatifs à la théorie et aux applications du pendule, 2 vols. (Paris, 1889–1891): and Histoire de l’Observatoire de Paris de sa fondation à 1793 (Paris, 1902). Wolf also wrote seven notes, mainly on ancient standards of length, that appeared in Comptes rendus . . . de l’Académie des sciences, 95–125 (1882–1897).

II. Secondary Literature. See G. Bigourdan, “Notice sur la vie et les travaux de M. Ch. wolf,” in Comptes rendus . . . de l’Académie des sciences, 167 (1918), 46–48; and an anonymous “Notice nécrologique,” in Astronomie, 32 (1918), 255–256; and P. Painlevé, “Annonce de la mort de Ch. Wolf,” in Comptes rendus . . . de l’Académie des sciences, 167 (1918), 45–46.

Jacques R. LÉvy