256 



THE CHEMISTRY OF THE SUN. 



[CHAP. 



It will be seen that the ratio between the affected and un- 

 affected lines is very variable. What strikes one, indeed, is the 

 wonderful irregularity in the behaviour of the various lines ; 

 there is no relation, for instance, between the widening of the 

 lines in the spots and their appearances in the prominences. 



It may here be asked, " But what has this to do with the 

 lines common to two or more spectra ? " I answer, it would 

 have nothing to do with such lines if Thalen had not observed 

 them ; but in his observations, which are the ne plus ultra of 

 spectroscopic accuracy, he came across them abundantly. 



Among the 345 lines given ly ThaUn are 18 with identical 

 readings in two spectra. They are, therefore, the exact equiva- 

 lents of those lines found to be coincident in work on another 

 part of the spectrum. 



Now, for the reasons above given, if the explanation of their 

 basic character suggested by the consideration of the hypothetical 

 furnaces be the correct one, then we should expect a con- 

 siderable development of these lines in the spectrum of the 



