Henry, Paul Pierre

(b. Nancy, France, 21 August 1848; d. Montrouge, near Paris, France, 4 January 1905);

HENRY, PROSPER MATHIEU (b. Nancy, 10 December 1849; d. Pralognan, Savoy, France, 25 July 1903), astronomy, optics.

The Henry brothers were united in their careers, as in their lives; and their work cannot be separated.

The death of the younger interrupted the work of the elder, who, consumed by grief, did not long survive him.

Following elementary studies in a Catholic school, they were accepted, each at the age of sixteen, into the Service Météorologique des Prévisions, recently created at the Paris observatory. But their astronomical vocation owed nothing to this institution; for it was with their own means, and in their own home, that they set up a small optical workshop and undertook, beginning in 1868, the construction of a thirtycentimeter mirror and its mounting. With this reflector and a secondhand clock they began to make a map of the stars in the ecliptic zone.

In 1871 the director of the observatory, Charles Delaunay, heard of their work and transferred the two brothers from the meteorological to the equatorial telescope section. The projected ecliptic map was then made more precise: to chart, on a band five degrees wide, the positions of all the stars up to the thirteenth magnitude. In the course of executing it, they had occasion to make a great number of observations of minor planets and discovered fourteen such bodies between 1872 and 1878. In 1884, when a fourth of the band had been explored and 36,000 stars recorded, an insurmountable difficulty arose: they reached the intersection of the ecliptic and the Milky Way, and the density of the stars was too great to permit a visual survey.

The aid of photography became essential. In their workshop at Montrouge the Henry brothers cut a sixteen-centimeter objective lens especially adapted to this technique and achromatized for the wavelengths to which photographic plates are sensitive. They coupled to the photographic telescope a visual guiding telescope, thus becoming able to control precisely the drive movement of the equatorial telescope during the course of exposures lasting as long as one hour. By 1884 they were making such remarkable photographs that the Paris observatory immediately commissioned them to build a large apparatus.

The first large photographic equatorial telescope was completed in 1885. It had an opening of thirtyfour centimeters and a focal length of 3.40 meters; the guiding telescope, with a smaller opening, was of the same length. The performances of this apparatus revealed how much photography cnuld assist astronomy. A negative of the Pleiades cluster, with its sharpness and wealth of weak stars, was frequently reproduced and contributed to the rise of photographic astronomy.

The Henry’s instrument was adopted in 1887 as the prototype for the international project of the Carte du Ciel. Seventeen identical instruments were built—more than half of them by the Henrys—and placed at various latitudes. The Carte du Ciel, which was completed only recently, has enabled scientists to collect a considerable number of documents whose value for the determination of stellar positions will increase every year.

The Henry brothers constructed many other devices. In particular they produced the lens of the great seventy-six-centimeter refractor of the Nice observatory. Their skill was considered incomparable.

Their careers remained strictly parallel. Prosper was named chief astronomer four years before Paul, in 1893; but in that year Paul was made head of the Service de la Carte du Ciel, a position which Prosper held seven years later. On three occasions they shared prizes awarded by the Academy of Sciences, and they were both admitted as associate members of the Royal Astronomical Society in 1889.

Unpretentious and modest, the Henry brothers sought relaxation from their duties at the observatory by working in their own workshop, where they conducted research at their own expense. They warmly received all who came to their laboratory to learn stellar photography, an art which they helped to create.

BIBLIOGRAPHY

I. Original Works. The Henry brothers published mainly observational results (minor planets, planets, comets), presented in the form of about fifty notes to the Comptes rendus hebdomadaires des séances de I’Académie des sciences between 1872 and 1887. The discoveries of minor planets sometimes bear just one of their names. Some results of observations were also published in Bulletin astronomique, vols. 1 (1884) and 3 (1886).

There are also the following works: “Sur la construction de cartes célestes, trés détaillés. voisines de l’écliptiquc,” in Comptes rendus hebdomadaires des séances de I’Académie des sciences,74 (1872). 246–247; “Sur un nouveau télescope catadioptrique,” ibid. 88 (1879), 556–558; “Sur la suppression des halos dans les clichés photographiques,” ibid., 110 (1890). 751; and “Méthode de mesurc de la dispersion atmosphérique.” ibid.. 112 (1891). 377–1580. published under Prosper Henry’s name.

II. Secondary Literature. See Adam. Colonel Laussedat, O. Callandreau, and C. Trepied, “Discours prononcés aux obséques de Prosper Henry,” in Bulletin astronomique, 21 (1904), 49–58; F. W. Dyson, “Obituary, Prosper Henry,” in Monthly Notices of the Royal Astronomical Society, 64 (1904).296–298; P. Puiseux, Colonel Laussedat, and B. Baillaud, “Discours prononcés aux ob séques de M. Paul Henry,” in Bulletin astronomique, 22 (1905), 97–102 ; and J, Rosch. “La mission des fréres Henry au Pic-du-Midi pour le passage de Vénus sur le soleil (6 décembre 1882),” in L’astronomie. 64 (1950), 475–490.

Jacques R. LÉvy