230 THE CHEMISTRY OF THE SUN. [CHAP 



1. Variations between Solar and Terrestrial Spectra. 



Attention has been called to Kirchhoff's statement that the 

 existence of the terrestrial elements in the sun is established by 

 the fact of the coincidence of wave-length and intensity between 

 the lines visible in our laboratories and the lines recorded as 

 existing in the solar spectrum. 



We have already seen what enormous differences there are 

 in the spectrum of calcium under different conditions. In the 

 diagram of the calcium spectrum (Fig. 75) we saw that H and 

 K, the most important lines in the spectrum of the sun, are 

 really not the thickest lines in the spectrum of that substance 

 at the temperature of the electric arc. But when we pass from 

 calcium, which occupied the attention of observers several years 

 ago, to other elements, as the photographs enable us easily to do, 

 and when we go still more into the minute anatomy of the thing, 

 we find that the further we go the less final is the statement 

 that the matching in intensity of the lines is perfect. 



Nor is this all. Not only is the matching less perfect in inten- 

 sity, but many lines in various spectra are left out, which omission 

 cannot be accounted for on the long and short principle. It has 

 been before pointed out that of the 26 lines of aluminium, it is 

 easy to explain how 2 only are left in the solar spectrum, be- 

 cause the 24 dropped were short lines. But when we come to 

 other elements, we find of adjacent lines lines of equal length, 

 which, so far as we should expect, ought to be equally repre- 

 sented in the sun one is absent, and one is present, probably 

 with more intensity than it would seem to deserve from its 

 behaviour among other lines of the spectrum. A table will 

 best exhibit the sort of variation that crops up and insists on 

 being recorded when the solar spectrum is photographed in 

 anything like the detail which it absolutely demands. The 

 method of recording will be at once understood. 



