On the Origin of the Light of the Sun and Stars. 449 



the universe, in the same proportion are we perplexed to assign 

 the producing cause of such a wonderful outflow of luminosity. 



All speculations on this subject naturally divide themselves 

 into two groups. We have, in the first place, those which 

 assume that the sun and stars are fed from within ; and, in the 

 second, those which assert that they are fed from without. A 

 little explanation will make this distinction clear. If we sup- 

 pose the sun to be a huge mass at a very high temperature 

 which is gradually cooling, and therefore giving out light and 

 heat, or if we suppose his brightness to be due to chemical 

 combination of the substances which form his mass, in either 

 case we assert that he is fed from within. But, on the 

 other hand, if we suppose that he is fed by comets, or by 

 meteors impinging against his atmosphere, and having their 

 motion converted into heat (just as they have when they 

 impinge against the atmosphere of the earth), or by an ex- 

 ternal ether, or in any way by planets, then we assert that 

 he is fed from without. In presuming to add another to the 

 list of these speculations, let us begin by laying down certain 

 rules to guide us in our discussion. 



Now, first of all, our hypothesis must not be inconsistent, 

 or only barely consistent with appearances on the sun's disc ; 

 and, in the second place, it must be susceptible of application 

 to other systems, and capable, by a legitimate extension, of 

 explaining the very strange and even startling phenomena 

 which reach us from those distant regions. The sun, in fine, 

 must not be regarded as an individual apart by himself, but 

 rather as that member of a large family with whom we are best 

 acquainted, and who, if questioned aright, may perhaps inform 

 us of the habits of his race. What, then, are the phenomena 

 which he presents ? It is well known that his surface, although 

 generally appearing uniformly luminous to the naked eye, is not 

 so in reality. Setting aside spots for the moment, the centre 

 of his disc is decidedly brighter than the circumference ; leading 

 us to infer that the sun, like our own earth, is surrounded by 

 an atmosphere which absorbs much of the light which passes 

 through it in an oblique direction. It is likewise worthy of 

 remark, that this atmosphere must have a lower temperature 

 than the region which gives rise to the luminosity, since other- 

 wise it would not exercise an absorptive influence upon the light 

 emitted, but would add as much as it took away, or even more, 

 if its temperature were higher. The dark lines in the solar 

 spectrum are likewise a proof of the presence of an absorbing 

 atmosphere of low temperature. 



But the spots which appear from time to time on the sun's 

 surface are at once the most interesting and instructive of all 

 solar phenomena. Their existence has been known for a long 



