REPORT OF THE SECRETARY. 



67 



These lines are much crowded toward and within the ultra-violet spectrum, so 

 that perhaps this indicates a principal reason for the weakness of the sun's 

 spectrum in that region. Second, it seems probable that we are dealing with 

 a mixture of rays from sources at different temperatures. The cause and 



effect of this difference may each be twofold : For, firstly, at the center of the 

 sun's visible disk we look probably to deeper-lying and hence hotter layers 

 than at the sun's edge, where the line of sight is oblique; and, secondly, 

 since the transmission of the sun's atmosphere is probably like the earth's, 



