64 PKOCEEDINGS OF THE AMERICAN ACADEMY 



times as great as that of the true solar lines. That part of the yellow 

 of the spectrum which lies on the more refrangible side of the D line, 

 and in which during dry weather only a comparatively few lines can 

 be distinguished, was then as thickly crowded with lines as the blue or 

 the violet, but the lines were of course far less intense. 



Professor Tyndal, of London, has shown by a remarkable series of 

 experiments with the thermo-multiplier, not only that aqueous vapor 

 powerfully absorbs the obscure thermal rays, but also that the elemen- 

 tary gases of the atmosphere exert little or no action upon them. I have 

 endeavored to establish in this paper, from direct observations with the 

 spectroscope, a similar truth in regard to the luminous rays. It has 

 been estimated by Pouillet and others that about one third of the solar 

 rays intercepted by the earth are absorbed in passing through the at- 

 mosphere ; and it now appears that aqueous vapor is a most important, if 

 not the chief, agent in producing this result. It is impossible, however, 

 from any data we yet possess, to determine how great a power of absorp- 

 , tion is exerted by the oxygen and nitrogen gases which constitute the 

 great mass of our atmosphere. I have shown that a very great many, 

 and I have no doubt that almost all, the lines hitherto distinguished as 

 air lines are simply aqueous lines ; but it is very difficult to distinguish 

 atmospheric lines from the true solar lines, and our knowledge of the 

 first is as yet very incomplete. It still remains to make careful 

 comparisons throughout the whole extent of the spectrum, before we 

 can absolutely determine the relative absorbing power of the different 

 constituents of our atmosphere. 



One other inference from the facts here developed is worthy of notice 

 before closing this paper. It has been for some time suspected that the 

 blue color of the sky was in some way connected with the vapor in the 

 atmosphere ; and it is a fact of common observation, that this color is 

 more intense during the moist weather of sumnaer than during the 

 more dry weather of winter. The distribution of the aqueous lines 

 through the solar spectrum not only confirms the opinion previously 

 entertained, but also points to the cause of the color. So far as my ob- 

 servations have extended, the aqueous lines are almost wholly, if not 

 completely, confined to the less refrangible portion of the spectrum. 

 Here they are found in vast numbers, and I am not positive that they 

 exist anywhere else. If, then, the aqueous vapor absorbs most power- 

 fully the yellow and red rays of the spectrum, the blue color of the sky 

 is the necessary result. The color is therefore due to simple absorption, 



