294 IN STARRY REALMS. 



of landscape the distance between the objects and the 

 camera is greatly increased ; but even here the length 

 of air which the rays have to traverse is generally 

 much less than in the case where we attempt the portrait 

 of a heavenly body. The atmosphere extends above our 

 heads to an altitude which is still very uncertain. We 

 learn, however, from the phenomena of shooting stars 

 that the summit of the air is at least a couple of hundred 

 miles aloft, and perhaps much more. The upper regions 

 are so highly rarefied that they are incapable of ex- 

 ercising much deleterious influence on the rays of light ; 

 it is in the lower and the denser portions that the atmo- 

 sphere is chiefly inimical to the photographer. Now, 

 though to the portrait- taker the atmosphere signifies but 

 little, except in so far as questions of light are involved, 

 yet it is well known that the state of the atmosphere is 

 very significant in landscape- photography ; while in the 

 case of the celestial photographer the behaviour of the 

 atmosphere is of paramount importance. Even if the 

 object be immediately over his head, the rays would have 

 to make their way through two hundred miles of air 

 before they entered his apparatus ; while if the body lay 

 far away from his zenith, as of course it usually does, the 

 air-journey of the rays of light would be considerably 

 longer. Any imperfections which the atmosphere is 

 capable of producing must therefore be felt much more 

 keenly by the celestial photographer than by the brothers 

 in the craft who confine their attention to mere terrestrial 

 objects. The qualities which characterize a suitable sky 

 are steadiness, though wind is not necessarily objectionable, 

 and photographic transparency, which is a very different 



