178 ASTRONOMY ON MONT BLANC. 



Another proof of the terrestrial origin of the A and B groups of the 

 solar spectrum would be furnished by their gradual decrease of in- 

 tensity during a balloon ascension, or the ascension of Mont Blanc. 

 This was the principal object of the first expedition of Janssen, and 

 he was, in fact, able to note, with the aid of the spectroscope which he 

 carried, an unmistakable diminution of the intensity of the B group 

 owing to the great elevation of the station, but the height of Mont 

 Blanc was insufficient to extinguish all the oxygen lines. M. le 

 Comte de la Baume-Pluvinel has confirmed this result by photograph- 

 ing the B group of the solar spectrum at Chamonix, and on the summit 

 of Mont Blanc in September, 1898. But the persistence of the lines 

 is readily explained, for the laboratory experiments of Janssen have 

 shown that a column of oxygen 120 meters long at atmosphere pres- 

 sure is sufficient to produce an absorption giving rise to the B lines, 

 while the atmospheric layer above Mont Blanc is equivalent to such a 

 column of oxygen 900 meters long, or seven times as long as is re- 

 quired to produce the lines. 



In order to eliminate the B group of lines it would be necessary to 

 ascend to a height of 18 or 20 kilometers. Balloons provided with 

 automatic recording spectroscopes will perhaps in time solve the 

 problem. It should not be forgotten that the study of the oxygen 

 bands is complicated by the presence among them of absorption lines 

 of water vapor. Perhaps, also, the persistent visibility of the lines 

 is partly due to the greater intensity of the light itself, owing to the 

 decrease of the general absorption of the air at high altitudes. 



Despite the difficulties which remain and which always attach to 

 new researches, the concordance of the indications furnished by the 

 several kinds of evidence which are available permits us to believe 

 that the oxygen lines found in the solar spectrum have their source 

 in our own atmosphere and that oxygen does not exist in the sun. 

 This does not prevent us from admitting the possibility that the sun 

 contains the matter going to make up oxygen, but perhaps in a state 

 of dissociation. 



II. 



For carrying on researches of considerable magnitude it is not 

 practicable to depend on simple ascents, even frequently repeated; 

 and it is indispensable to provide a permanent observatory equipped 

 with a number of instruments and suitable for a stay of considerable 

 length. M. Janssen represented this need strongly to the Academic 

 des Sciences in his report of the expedition of 1890, and his appeal 

 was not neglected. M. Bischotf'sheim, whose lamented death occurred 

 some months ago, immediately offered 150,000 francs; Prince Eoland 

 Bonaparte subscribed 100,000 francs ; Baron Alphonse de Rothschild, 



