174 ASTRONOMY ON MONT BLANC. 



air form there a largei proportion of the path of the beam of light. 

 Another inconvenience, more serious from the point of view of phys- 

 ical astronomy, is the ditfuse reflection of the atmosphere, which 

 causes a beam of light to illuminate in all directions the air it 

 traverses. The light of the sky is a great obstacle to investigations 

 of objects near the sun, and for a long time it prevented the observa- 

 tion of the solar prominences excepting at the rare occasions of total 

 solar eclipses. It is now^ possible to view them in full daylight, 

 thanks to Janssen's discovery of a method of enfeebling the diffuse 

 sky light by the aid of the spectroscope, while leaving nearly 

 unchanged the monochromatic light of the prominences. 



Among the effects due to the presence of the atmosphere should be 

 mentioned the absorption which it exercises upon the radiations of 

 the sun and stars, for by retaining a portion of such radiation it 

 profoundly alters the quality of the rays which reach the earth's 

 surface. Not only is the intensity of the rays reduced by the passage 

 through the atmosphere, but their composition is essentially changed. 

 This is the reason why the sun appears red near the horizon, and it is 

 also the cause of some of the dark lines in the solar spectrum, lines 

 whose terrestrial origin is attested by their variation at high and low 

 sun. The effects we have been considering are much diminished 

 when the observer stations himself at an altitude of several thou- 

 sand meters. 



At the summit of Mont Blanc (about 4,810 meters above the level 

 of the sea) the barometer stands at 425 millimeters, from which it 

 follows that the weight of the atmosphere above is still a little more 

 than half that of the entire atmosphere. Upon the highest peaks 

 of the Himalayas (8,800 meters) the barometric pressure is only 

 about a third of its value at sea level, where each square meter of the 

 earth's surface supports a weight of 10,000 kilograms of air. By 

 choosing a station at a great elevation the difficulties which embarrass 

 the observations of physicists and astronomers clue to the presence 

 of the atmosphere ma}^ be partly overcome. The measure of success 

 which has already crowned tentative efforts in this direction has 

 encouraged many wdio were hesitating to make the sacrifices accom- 

 panying perilous and costly ascents, and now we see mountain obser- 

 vatories r"_ultiplying and equipped with the most powerful instru- 

 ments. 



The most celebrated of these establishments is the magnificent Lick 

 Observatory, on the summit of Mount Hamilton in California, in an 

 admirable site at 1,300 meters above sea level. According to the 

 report of its first director, E. S. Holden, the nights are continually 

 fine during six or seven months of the year, and about half the time 

 remaining may be employed for observing. This observatory was 



