6 MOUNTAIN OBSERVATORIES IN AJUERICA AND EUROPE. 



is more absorbed, proportionally, than tlie red. A familiar example 

 of tliis is sliown in the redness of the setting sun. If we measure the 

 heat which comes to us from the sun we shall find that it is refracted, 

 altered in quantity and also in quality by our own atmosphere. 



One of the chief problems of astrophysics is to evaluate the amount 

 of these alterations, so as to obtain the true and not merely the 

 apparent effect of celestial radiations. 



In order to measure the effect of the earth's atmosphere in these re- 

 gards there are two obvious experimental methods. The observer 

 may, first, remain in the same place, and make his measurements whei 

 the heavenly body is near the zenith (when its rays traverse the leasli 

 depth of air) and again, when it is near the horizon (when its rays 

 traverse the maximum depth). By a comparison of such observations 

 the effect of the atmosphere can be concluded. Or, again, the observei 

 may occupy two stations, one near the sea-level (and thus under the 

 whole of the atmosphere), the other on a high mountain (and thus free^ 

 from the effect of the air beneath). A comparison of such measures 

 will, again, determine the influence of the earth's envelope. In many 

 of the delicate problems of astronomy and physics, recourse must be 

 had to both these devices. High-level observing stations are called 

 for in many special researches. 



Stars seen from the summit of a high mountain of about 10,000 

 feet in altitude appear considerably brighter than from sea-level, and 

 the effect to an observer seems to be a brightening of the whole heavens. 



This brightening is, however, not uniform over the entire sky. 

 Stars at and near the zenith are but slightly more brilliant, while those 

 near the horizon are about two and a half times brighter than at sea- 

 level. The very vivid impression made upon an observer who first 

 sees a clear night-sky from a high peak is chiefly due to the marked 

 increase in the brilliancy of the stars, and of the Milky Way, close 

 down to the horizon.* 



If while the stars are more brilliant, because the air is more trans- 

 parent, they are at the same time more steady (twinkle less), because 

 the air is more tranquil, the advantages of a mountain station for 

 astronomical purposes become very great. If these advantages are 

 noteworthy for observations made with the eye and telescope, they are 

 still more so when the eye is replaced by the photographic plate. The 

 blue rays pass through the higher air relatively more freely than 

 through the lower and denser. 



* At an elevation of 14,000 feet in the Sierras and Rocky Mountains the sky, on 

 a cloudless and smokeless day, is violet, not blue. The skies of the paintings of 

 Bierstadt, Moran, and others seem false to those who have never lived at these 

 high altitudes, but they are not so. 



