JANSSEN, PIERRE JULES CESAR French astronomer, was born in Paris on Feb. 22, 1824, and studied mathematics and physics at the faculty of sciences. Vari ous scientific missions were entrusted to him. In 1857 he went to Peru in order to determine the magnetic equator; in 1861-62 and 1864, he studied telluric absorption in the solar spectrum in Italy and Switzerland; in 1867 he carried out optical and magnetic experiments at the Azores; he successfully observed both tran sits of Venus, that of 1874 in Japan, that of 1882 at Oran in Algeria; and he took part in a long series of solar eclipse-expedi tions. At the great Indian eclipse of 1868 he demonstrated the gaseous nature of the red prominences, and devised a method of observing them spectroscopically under ordinary daylight condi tions. One main purpose of his spectroscopic enquiries was to answer the question whether the sun contains oxygen or not. For
this, he perceived the advantage of reducing the thickness of air through which observations have to be made, and established an observatory on Mont Blanc (1893). In 1875 he was appointed director of the new astrophysical observatory established by the French Government at Meudon, and set on foot there in 1876 the remarkable series of solar photographs collected in his great Atlas de photographies solaires (1904). The first volume of the Annales de l'observatoire de Meudon was published by him in 1896. He died at Paris on Dec. 23, 1907.
See A. M. Clerke, Hist. of Astronomy during the 19th Century (1903) ; H. Macpherson, Astronomers of To-Day (1905).