Pierre Janssen

Thus in 1857 he went to Peru in order to determine the magnetic equator; in 1861–1862 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 transits 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-expeditions, e.g. to Trani, Italy (1867), Guntur, India (1868), Algiers (1870), Siam (1875), the Caroline Islands (1883), and to Alcossebre in Spain (1905).

[7][8] At the great Indian eclipse of 1868 that occurred in Guntur, Janssen also demonstrated the gaseous nature of the red prominences, and devised a method of observing them under ordinary daylight conditions.

An indispensable preliminary was the virtual elimination of oxygen-absorption in the Earth's atmosphere, and his bold project of establishing an observatory on the top of Mont Blanc was prompted by a perception of the advantages to be gained by reducing the thickness of air through which observations have to be made.

This observatory, the foundations of which were fixed in the hard ice that appeared to cover the summit to a depth of over ten metres, was built in September 1893, and Janssen, in spite of his sixty-nine years, made the ascent and spent four days making observations.

[2][10] In 1875, Janssen 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).

[2] (see also Meudon Great Refractor) Janssen was the President of the Société Astronomique de France (SAF), the French astronomical society, from 1895 to 1897.