(b. Paris, France, 24 July 1853; d. Paris, 15 January 1948)
Deslandres was born into a family typical of the mid-nineteenth-century French bourgeoisie. He graduated from the École Polytechnique in 1874 and entered the army, in which he served until 1881, when a strong interest in physical sciences led him to resign his military position. He worked first in the physical laboratories of the École Polytechnique and the Sorbonne, devoting himself to ultraviolet spectroscopy under the guidance of M. A. Cornu.
In 1889 Deslandres joined the staff of the Paris observatory, then headed by Admiral Ernest Mouchez, who sought to develop astrophysics in the institution long dedicated to celestial mechanics under Le Verrier. In 1897 he joined Jules Janssen at his astrophysical observatory in Meudon and became its director in 1908, following Janssen’s death. In 1926 the Paris and Meudon observatories were united under his management. Deslandres retired in 1929 but pursued an active scientific life almost until his death at the age of ninety-four.
During his long and successful career Deslandres was elected to essentially all the scientific societies of significance, including the Académie des Sciences (1902; president, 1920), the Royal Society, the Royal Astronomical Society, the Accademia dei Lincei, and the National Academy of Sciences of the United States.
In his bearing, his character, and his style of life Deslandres always remained more akin to the soldier (and the officer) than to the scholar. These consequences of his education also appeared in his scientific work: he was more successful in the experimental and technical aspects of physics and astrophysics than in creating new theories (even though he worked at it with tenacity and ultimately with intuition).
In his thesis Deslandres studied the spectra of molecules such as nitrogen, cyanogen, CH, and water and recognized two simple laws in the disconcerting complexity of the numerous bands, each of which is made up of many tens of lines. These laws bear his name.
First, the frequencies (or wave numbers) v of lines inside a given band can be represented by the parabolic formula
ν=A + 2Bm +Cm2
where m is an integer (m ≠ 0). Then, the constants A associated with various bands of a given system can all be fitted to another parabolic formula involving the integers v′ and v″ These laws were at first useful in the empirical study and classification of molecular spectra; later, with the elaboration of quantum mechanics, it became easy to explain them in terms of the structure of molecules. The integers m, v′, and v″ were simply the quantum numbers identifying the possible rotational and vibrational energies of the molecule in the initial and final states of a radiative transition; and the various empirical constants in Deslandres’s formulas could be related to physical constants that were characteristics of each molecule.
After this splendid success in laboratory spectroscopy, Deslandres turned in 1889 to astrophysics without losing interest in molecules, as is shown by his fine observations of the Zeeman effect on molecular lines. In numerous publications of his late years he sought a unified theoretical interpretation of molecular spectra; he paid little attention to such modernist developments as quantum mechanics.
At the Paris observatory Deslandres attached a spectrograph to the recently built 120-cm. mirror telescope and began observing the spectra of stars and planets, devoting himself to the measurement of line-of-sight velocities through the Doppler-Fizeau effect, the most essential tool of the astronomer in studying the motions and dynamics of celestial bodies. He continued the same type of observations in Meudon with a spectrograph attached to a large refractor with an 83-cm. aperture. Among his most valuable results were the law of rotation of Saturn’s ring (1895), which was shown to rotate as a system of independent particles and not as a solid body, and the proof that Uranus (like its known satellites) rotates in the direction opposite to all other planets, a fact of significance to cosmogony.
As an astronomer Deslandres is still better known for his important contributions to the physical study of the sun, particularly for the invention of the spectroheliograph, an instrument that he completed at the Paris observatory in the spring of 1894. It allows one to make photographs of the sun in nearly monochromatic light, the narrow spectral band being selected at will with the help of a dispersing spectrograph. By choosing radiations to which the solar atmosphere is very opaque, one “sees” only its outermost layers, the chromosphere; and a wealth of important phenomena are thus revealed, such as solar plages, prominences, and flares. The spectroheliograph was invented independently by the great American astronomer George Ellery Hale, who actually completed his first instrument more than a year before Deslandres, at Kenwood Observatory in Chicago. However, Deslandres explored with particular tenacity the possibilities of the new method and found useful variants; and for many years he contributed to the description of the structure of the chromosphere and of the complex phenomena of solar activity.
Like Hale, Deslandres believed (and correctly so) that solar activity is dominated by electromagnetic causes; and he attempted imaginative explanations, now of limited interest. However, he showed remarkable intuition in maintaining that the sun produced radio waves, although the crude experiments of Charles Nordmann in 1902 failed to detect them. Solar Hertzian radiation was actually observed only in 1942 by J. S. Hey, and this finding was published shortly after World War II. It is not known whether “the brave soldier” (as Stratton described Deslandres), who then led a rather secluded life, was ever informed that he had been right forty years before.
I. Original Works. Deslandres’s most important publications are “Spectre du pôle négatif de l’azote. Loi générale de répartition des raies dans les spectres de bandes,” in Comptes rendus hebdomadaires des séances de l’Académiedes sciences, 103 (1886), 375–379; “Loi de répartition des raies et des bandes, communes à plusieurs spectres des bandes...,” ibid., 104 (1887), 972–976; “Spectres de bandes ultra-violets des métalloïdes avec une faible dispersion,” his thesis (Faculté des Sciences de Paris, 1888), no. 619; “Étude des gaz et vapeurs du soleil,” in L’astronomie(Dec. 1894); and “Recherches sur l’atmosphère solaire. photographie des couches gazeuses supérieures...,” in Annales de l’Observatoire d’astronomie physique de Paris, 4 (1910).
II. Secondary Literature. Short biographies of Deslandres are L. d’Azambuja, in Bulletin de la Société astronomique de France, 62 (1948), 179–184; and F. J. M. Stratton, in Monthly Notices of the Royal Astronomical Society, 109 (1949), 141–144.