THOLLON, LOUIS

(b. Ambronary, Ain, France, 2 May 1829; d. Nice, France, 8 April 1887)

solarphysics.

Thollon’s scientific career began in 1878, when Raphaël Bischoffsheim, the founder of the Nice observatory, requested his assistance in setting up the spectroscopic equipment for the observatory. Thollon designed a high dispersion light spectroscope containing four dispersive prisms, two of which were made with carbon disulfide. Each prism was traversed twice by the ray of light, and the measurement of the position of the line was effected with the aid of a micrometer. In order to eliminate thermal effects, the apparatus was frequently tested on several solar lines selected as reference points. Work on the spectroscope was completed in 1883.

At the start of his own research, in 1879, Thollon carried out an experiment that has become classic. He demonstrated the Doppler-Fizeau effect in a concrete manner. If one alternately projects on the slit of a spectroscope the eastern and western sides of the sun, the spectral lines exhibit a displacement representing the difference of 4 km per second between the radial velocities of the two sources. The telluric lines that originate in the terrestrial atmosphere remain fixed. This method of using the telluric lines for reference was employed by N. DunÉr in 1890 to measure the rotational velocity of the sun.

Thollon established a great chart of the solar spectrum, which was published posthumously, in 1890. The spectrum extends from the visible red to the middle green (from 7,600 Å to 5,100 Å); it contains 3,448 lines, of which 2,336 are solar and 1,112 are telluric, with 246 lines being common. In addition to the position of the lines, the chart gives their intensities at a solar height of 10° and 30° above the horizon, for dry air and saturated air, and also, by extrapolation, for the case of observations made outside the earth’s atmosphere. This chart is the last and most important of the documents for which broad data were gathered about the solar spectrum by means of a spectroscope. Such information was subsequently obtained by spectrography.

Thollon was awarded a prize by the AcadÉmie des Sciences in 1885.

BIBLIOGRAPHY

I. Original Works. Thollon published eight notes on spectroscopic technique to the Comtes rendus hebdomadaires des sÉances de l’AcadÉmie des sciences, 86–96 (1878–1883); and nine notes on spectroscopic observations of comets, eclipses, and novae, ibid., 92–102 (1881–1886).

Thollon’s most important works on solar spectroscopy are “DÉplacement de raies spectrales, dû au mouvement de rotation du soleil,” ibid., 88 (1879), 169–171; “Constitution el origine du groupe B du spectre solaire,” in Bulletin astronomique, 1 (1884), 223–230; “Nouveau dessin du spectre solaire,” ibid., 3 (1886), 330–343; “Spectroscopie solaire,” in Annales de l’Observatoire de Nice, 2 (1886), D1–D28; and “Nouveau dessin du spectre solaire,” ibid., 3 (1890), A1–A112, with 17 plates in the atlas.

II. Secondary Literature. See J. Janssen, “Allocution . . . á l’occasion de la mort de M. Thollon,” in Comptes rendu hebdomadaires des sÉances de l’AcadÉmie des sciences, 104 (1887), 1047–1048; “Dr. L. Thollon,” in Observatory, 10 (1887), 207; and E. W. Maunder, “M. Thollon’s Atlas of the Solar Spectrum,” in Monthly Notices of the Royal Astronomical Society, 51 (1891), 260–261. See also A. Cornu, “Sur la mÉthode Döppler-Fizeau.” in Annuaire publiÉ par le Bureau des longitudes (1891), D25–D26.

Jacques R. LÉvy