OXYGEN IN THE SUN. 15 



existence." I had still earlier given a similar explanation 

 of the absence of all spectroscopic evidence of hydrogen in 

 the case of the bright star Betelgeux.* 



Let us more closely consider the significance of what we 

 learn from the spectral evidence respecting the gas hydro- 

 gen. We know that when the total light of the sun is dealt 

 with, the presence of hydrogen is constantly indicated by 

 dark lines. In other words, regarding the sun as a whole, 

 hydrogen constantly reduces the emission of rays of those 

 special tints which correspond to the light of this element 

 When we examine the light of other suns than ours, we find 

 tliat in many cases, probably in by far the greater number 

 of cases, hydrogen acts a similar part But not in every 

 case. In the spectra of some stars, notably in those of 

 Betelgeux and Alpha Herculis, the lines of hydrogen are 

 not visible at all ; while in yet others, as Gamma Cassio- 

 peiae, the middle star of the five which form the straggling 

 W of this constellation, the lines of hydrogen show bright 

 upon the relatively dark background of the spectrum. When 

 we examine closely the sun himself, we find that although 

 his light as a whole gives a spectrum in which the lines of 

 hydrogen appear dark, the light of particular parts of his 

 surface, if separately examined, occasionally shows the 

 hydrogen lines bright as in the spectrum of Gamma Cas- 

 siopeiae, while sometimes the light of particular parts gives, 



* In "Other Worlds than Ours," I wrote as follows : " The lines 

 of hydrogen, which are so well marked in the solar spectrum, are not 

 seen in the spectrum of Betelgeux. We are not to conclude from this 

 that hydrogen does not exist in the composition of the star. We know 

 that certain parts of the solar disc, when examined with the spectroscope, 

 do not at all times exhibit the hydrogen lines, or may even present them 

 as bright instead of dark lines. It may well be that in Betelgeux 

 hydrogen exists under such conditions that the amount of light it sends 

 forth is nearly equivalent to the amount it absorbs, hi which case its 

 characteristic lines would not be easily discernible. In fact, it is impor- 

 tant to notice generally, that while there can be no mistaking the 

 positive evidence afforded by the spectroscope as to the existence of 

 any element in sun'or star, the negative evidence supplied by the absence 

 of particular lines is not to be certainly relied upon. " 



