ASTRONOMY OX MONT BLANC." 



By II. Kauau. 



Quo non asceiidam ^ This is the intrepid device whicli the dean of 

 French astronomers appears to have adopted, when, undeterred by 

 dangers and fatigue, he persevered in founding- an observatory, in the 

 midst of the eternal snow, upon the highest summit of tlie European 

 continent. Professor Janssen was well fitted for this enterprise by 

 his previous mountain experiences — in the studies which he made in 

 18()4: at the summit of the P^aulhorn to determine the terrestrial 

 absorption lines in the solar spectrum; in the researches carried out 

 on Mount Etna in 18^7 to test for the existence of water vapor in the 

 atmosphere of Mars; in the expedition of 1868, when he passed a 

 winter in the Himalayas, occupied with the spectroscopic study of 

 the sun and stars, and in that of 1871 to the Nilgherries, Avhen he con- 

 vinced himself of the existence of the coronal atmosphere of the sun. 

 Besides these expeditions, there was his visit in 1867 to the observa- 

 tory at the Pic du Midi and, finally, his successive ascensions of 

 Mont Blanc to resolve the question of the presence of oxygen in the 

 sun. It was indeed these latter ascensions which led to the establish- 

 ment of the astronomical observatory on the summit of Mont Blanc. 



I. 



It is worth while to state briefly why the astronomers of our day 

 have chosen to establish their instruments at high altitudes. When 

 one has passed some time under an overcast sky, which obscures the 

 light and renders all observations impossible, he recognizes the 

 advantage of a station above the clouds. But even Avlien the sky is 

 perfectly cloudless the atmosphere is still a serious obstacle, for it is 

 in fact a translucent and changeable veil between the observer and 

 the heavens. This veil is very thick at sea level, and both distorts 

 and changes the nature of the images of celestial objects. First of 

 all, by its refraction it falsifies the' position, of the stars more and 

 more as they approach the horizon, because the denser layers of the 



a Translated, by permission, from Revue des Deux Mondes, Paris, February 15, 

 1907, pp. 876-892. 



