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 
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Wie. 2.—Hnergy spectrum of sun compared with black body. 
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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, 
