44 Trans. Acad. Sci. of St. Louis. 



believe that the number of such dark bodies is enormous; and 

 a satisfactory explanation of their condition is therefore a 

 desideratum of science. The suggestion here thrown out 

 that the smaller masses condense more quickly, and thus be- 

 come dark while the large bodies are intensely brilliant seems 

 to accord with all known phenomena of nature. 



(h.) Absorption of Light in Space. If it be conceded 

 that the nebulae are cold and that comparatively very few 

 of them are luminous, we shall be driven to the conclusion 

 that the heavenly spaces are more or less filled with dark 

 or faintly luminous matter. This matter appears telescop- 

 ically as a faint haze on the background of the sky, or as 

 diffuse nebulosit} r in photographic impressions of the vault 

 of the heavens. In any case such cosmic clouds of dark 

 or semi-opaque matter however rare act like a fog in in- 

 tercepting some portion of the light from distant regions of 

 creation, and thus ultimately limit the depths to which our 

 telescopes can penetrate. On this account it may never be 

 possible to extend our exploration of the universe beyond a 

 certain finite distance. Struve's celebrated problem of the 

 bounds of creation, in which he discussed the absorption of 

 light arising from the imperfect elasticity of the luminous ether, 

 thus appears more difficult of solution than ever. He showed 

 that if the number of stars be infinite, and they be scattered pro- 

 miscuousl} 7 throughout space, and no light be absorbed by the 

 ether, then the whole face of the sky would necessarily glow 

 like the points now occupied by the stars. As the sky is very 

 dark even in the regions most crowded by the stars, it follows 

 either that the universe is not infinite or that light is absorbed 

 or interceped by dark masses scattered throughout the 

 immensity of space. Since we now have for the first time 

 satisfactory evidence of the existence of vast clouds of cos- 

 mical dust, which intercept the light of distant stars, we know 

 that the luminiferous ether is not the only cause which extin- 

 guishes the light of distant regions of creatiou. Thus even if 

 the universe of stars be infinite we may never be able to dis- 

 cover this fact, since opaque masses limit the depth to which 

 our telescopes can penetrate. 



The significance attached to an} T line of research naturally 



