ASTRONOMY ON MONT BLANC. 177 



savage violence. Finally, on the 22d, the weather moderating. Jans- 

 sen continued the ascent with the 12 men still remaining under the 

 orders of their chief, Frederic Payot, for the others had demanded 

 permission to return. Thanks to the indomitable energy of this 

 faithful troop, the sledge was finally hauled to the summit, and after 

 some hours given to rapid observing in excellent weather conditions 

 they resolved to go down. At 2 o'clock they regained des Bosses, by 

 night reached Grands Mulets, and on the next day traveled to Cha- 

 monix, where they arrived at 7 o'clock in the evening. 



Professor Janssen has charmingly recounted the story of these 

 and other ascents made in following years before the academy, and 

 in a series of papers which appeared in the successive volumes of 

 TAnnuaire du Bureau des Longitudes, so that it is not needful to 

 refer to them here. These expeditions were preparatory, as we have 

 seen, to the establishment of an observatory, and they furnished at 

 the same time occasion for numerous valuable observations relating 

 principally to the solar spectrum and the structure of the sun. One 

 of the questions which interested Janssen was that of the existence 

 of oxygen in the solar atmosphere, a question whose interest is con- 

 nected Avith the important part played by this element in geological, 

 chemical, and biological phenomena. Some spectroscopists, too 

 prematurel3^ had maintained the existence of oxygen in the sun, 

 but this conclusion was strongly disputed. The experiments of Jans- 

 sen and of Egoroff had shown that the action of oxygen on light is 

 shown by the presence of certain series of fine lines in the spectrum, 

 known as A and B, and in addition to these there are faint bands 

 which are found Avhen the absorption is very strong. These fainter 

 bands disappear from the solar spectrum when the sun is at all 

 high in the heavens, and it is therefore natural to attribute them to 

 the action of the earth's atmosphere. The fine oxygen lines, however, 

 persist even when the sun is in the zenith, so that their origin is less 

 certain. 



One course of experiments would be to produce these lines arti- 

 ficially in the spectrum of the electric arc by interposing in the beam 

 an absorbing layer equal to our atmosphere, and Janssen performed 

 such an exjjeriment in 1889 by observing at Meudon the spectrum of 

 the electric light installed at the top of the Eiffel tow^er. The dis- 

 tance is about 7,700 meters and the rays traverse a layer of uniform 

 density containing about the same amount of oxygen as the layer of 

 variable density traversed by the beam of a star in the zenith. In 

 the spectrum thus obtained the fine lines were found in their normal 

 intensity, which is evidence of their terrestrial origin. A greater 

 distance w^ould be required to produce the fainter bands above men- 

 tioned. 



ISM 190(J 13 



