184 ASTRONOMY ON MONT BLANC. 



the solar constant, if indeed it be constant, for to be sure no one has 

 proved that the solar radiation is not variable. It would be of inter- 

 est also to study in an analogous manner the chemical effectiveness 

 of the solar radiation and also of its photometric brightness, all of 

 which comprises a gi-eat field of observation. 



We have already discussed at some length the observations of 

 Janssen relative to the question of the existence of oxygen on the 

 sun and the confirmation of his results by the experiments of Comte 

 de la Baume-Pluvinel. In 1899 new experiments on the photogra- 

 phy of the spectrum were undertaken by Tikhoff, an astronomical 

 assistant at Meudon, and these included observations at Meudon, 

 Chamonix, and the summit of Mont Blanc. In 1902 Aubert carried 

 on a piece of work in which he employed a spectroscope having 

 lenses and prisms of quartz, for the purpose of studying the modifi- 

 cation of the intensity of the ultra violet part of the spectrum caused 

 by change of altitude, a piece of work which was begun by Cornu. 

 Finally, in 1905 and 1906, M. Stefanik, of the University of Prague, 

 made spectral observations in the same conditions. 



In 1895 and 1896 one of the most experienced of the astronomers 

 of the Paris Observatory, M. Bigourdan, attempted with the Def- 

 forges apparatus to measure the force of gravity at the summit of 

 Mont Blanc and at different points in the region; but his experi- 

 ments were hindered by bad weather. They were repeated in 1898 

 by M. Hansky with the apparatus of Sterneck, which is well adopted 

 for relative determinations. The measurements were conducted first 

 at Meudon, then on the summit of Mont Blanc and at Grands Mulcts, 

 Brevent, and Chamonix. 



In 1900 M. Hansky came once more from Eussia to repeat his actino- 

 metric observations, and he ascended the mountain for that purpose 

 on July 23 and September 1, spending six days on the summit at 

 each visit. On September 4 he had occasion to observe the sun at 

 rising, and saw the famous green ray. He writes : " The atmosphere 

 was transparent, and the horizon of an extraordinary clearness, so 

 that mountains more than 100 kilometers distant could be seen dis- 

 tinctly. At the instant of sun rising I was struck by an intense pure 

 green light, Avhich lasted about a half second. The sun appeared 

 directly after, brilliant and all yellow without a tinge of red. Hygro- 

 metric observations showed a very Ioav percentage of water vapor in 

 the atmosphere and there were few solid particles." It is well known 

 that this phenomenon, which is sometimes observed on the open ocean 

 in calm clear weather, is explained by the dispersion of the sun rays, 

 which graze the horizon under conditions such that the green j)art of 

 the spectrum is not unduly Aveakened by absorption. On the preAaous 

 night M. Hansky had observed the occultatioii of Saturn by the moon. 



