THE MONT BLANC OBSERVATORY. 261 



One wonders how it has been possible to transport the edifice 

 to this altitnde and fix it on the snow. However, if the conditions 

 offered by the hard, permanent, and little mobile snows ot the summit 

 are carefully considered, it is soon recognized that the snows are able 

 to support very consideral)le weights,* and that they will be only slightly 

 amenable to displacements, which will render it necessary to straighten 

 again the construction which has been fixed upon them. 



On my arrival I made a rapid survey, and saw that the construction 

 had not been sunk in the snow as much as 1 had stipulated of the con- 

 tractors. I do not approve of this. My guides and myself then took 

 possession of the largest underground room. I intended at first to fix 

 the instruments for enabling observations to be commenced immedi- 

 ately, and the provisions were left on the Rocher-Rouge. This circum- 

 stance put us in a state of perplexity, for the weather suddenly became 

 very bad, and we had to remain two days separated from the stores. 

 The storm lasted from Tuesday until Thursday morning. Beautiful 

 weather then set in, and I was able to begin the observations. 



The observations have for their principal object the question of the 

 presence of oxygen in the solar atmosphere. The academy knows that 

 I worked at this important point during my ascensions to the Grands- 

 Mnlets (3,050 meters) in 1888, and at M. Vallot's observatory in 1890. 



But the novelty of the-observations of 1893 lies in the fact that they 

 have been effected on the very summit of Mount Blanc, and that the 

 instrument employed is infinitely snperior to that of the two j»receding 

 ascensions. At the first, in fact, a Duboscq sjiectroscope, incapable of 

 separating the B group into distinct lines, was employed, while the 

 instrument about to be employed at the summit of Mount Blanc is a 

 grating si)ectroscope (the dispersive piece of which I owe to the kind- 

 ness of Rowland), with telescopes having a focal length of 0-75 and 

 showing all the details of the B group. This circumstance is of consid- 

 erable imi^ortance, for it may lead to the discovery, in the constitution 

 of the group in question, of valuable elements for measuring in some 

 way the effects of the diminution of the action of our atmosphere as 

 one ascends into it, and, accordingly, to determine whether thisdiminu- 

 tion corresponds to total extinction at its limits. In fact, we shall 

 learn whether or no the double lines which make u^) the B group dimin- 

 ish in intensity as their refrangibilities diminish — that is, as their wave 

 lengths increase. 



This circumstance may perhaps be employed with profit, if not to 

 measure at least to observe the diminution of the action of the selective 

 absorption of onr atmosi)here. It has been ascertained that the most 

 feeble donbles fade away one after the other as the atmosphere is 

 ascended — that is to say, as the absorbing action is diminished. Thus, 

 under ordinary circumstances, at the surface of our seas or upon our 



* See Comples Ilendits for fiu account of expariuieuts made at Meiidoii on tlie resist- 

 ance of slightly compressed snow. {Ante, p. 259.) 



