THE AURORA. 9 



bow-coloured streak crossed by a multitude of dark 

 lines, and we conclude accordingly that the sun is an 

 incandescent globe shining through a complex vaporous 

 atmosphere. We feel no doubt on this point, because 

 we are absolutely certain that the sun is self-luminous. 

 Again, we observe the spectrum of the moon to be 

 exactly similar to the solar spectrum, only, of course, 

 much less brilliant. And here also we feel no doubt 

 in interpreting the result. We know, certainly, that 

 the moon is not self-luminous, and therefore we con- 

 clude with the utmost certainty that the light we 

 receive from her is simply reflected solar light. So far 

 all is clear. But now take the case of an object like 

 a comet, which may or may not be self-luminous. If 

 we find that a comet's spectrum resembles the sun's 

 and this is not altogether a hypothetical case, for a 

 portion of the light of every comet yet examined does 

 in reality give a rainbow-coloured streak resembling 

 the solar spectrum we cannot form, in that case, any 

 such positive conclusion. The comet may be a self- 

 luminous body ; but, on the other hand, its light may 

 be due merely to the reflection of the solar beams. 

 Accordingly, the spectroscopist always accompanies the 

 record of such an observation with an expression of 

 doubt as to the real nature of the object which is the 

 source of light. 



Secondly, when an electric spark flashes through any 

 vapour, its light gives a spectrum which indicates the 

 nature, not only of the vapour through which the spark 

 has passed, but of the substances between which the 



