V.] STELLAR ATMOSPHERES. 51 



Assuming that the most valid absorbing vapours in any particular 

 stafr are all near one temperature, we can proceed to investigate the 

 origins of the spectrum lines by first getting a clue as to the probable 

 temperature from the extent of continuous spectrum, and then inquir- 

 ing into the presence or absence of the lines which are longest in the 

 spectra of various substances at that temperature. If, however, the 

 absorptions take place at different levels in the atmosphere of a star, 

 the proper spectrum of each substance to be thus investigated can only 

 be determined by a comparison of the stellar with the terrestrial lines 

 of the substance under varying temperature conditions. 



TMs method of looking for the longest lines will fail in the case of 

 stars which are hotter than our hottest spark. In such case, therefore, 

 we must necessarily rely on a comparison with lines which, from our 

 study of the spectra at different temperatures, would most probably be 

 longest in the spectrum at a temperature higher than any at which 

 experiments can be carried on. 



It is in connection with such an inquiry as this that the study of 

 the conditions of the sun's atmosphere is of supreme importance, that 

 is why I have devoted the previous chapter to it. It is obvious that a 

 knowledge of the solar conditions must be of the utmost value in 

 enabling us to apply a well-established series of facts, gathered in the 

 case of the star nearest to us, to the phenomena presented by the more 

 distant bodies. 



By doing this we have obtained facts which suggest in what parts 

 of the atmosphere the absorption takes place which produces the 

 various phenomena on which the chemical classification can be based ; 

 these facts we are bound to accept in a discussion of the origin of 

 stellar absorption in the absence of evidence to the contrary. And we 

 are justified in extending these general conclusions to all the stars that 

 shine in the heavens. I go further than this, and say that in the 

 presence of such definite results, it is not philosophical to assume that 

 the absorption may take place at the bottom of the atmosphere of one 

 star, or at the top of the atmosphere of another. The onus probandi 

 rests upon those who hold such views. 



So much then, in brief, for solar teachings in relation to the record 

 of the absorption of the lower parts of stellar atmospheres. 



If we are justified in arguing from a star with a photosphere as 

 well developed as that of the sun to one in which it is in all probability 

 much less marked in consequence of a much higher temperature, then 

 we must consider that the absorptions which mark our the various star 

 groups are more conditioned by the temperatures of the absorbing 

 regions merely than by the thickness of the absorbing atmospheres, or 

 by the densities of the various vapours. Another consideration to be 



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