THE AURORA. 9 



is a luminous solid or liquid, the instrument converts 

 its light into a rainbow-coloured streak. If the object 

 is a luminous vapour, its light is converted into a few 

 bright lines. And, lastly, if the object is a luminous 

 solid or liquid shining through any vapours, the rain- 

 bow-coloured streak again makes its appearance, but 

 it is now crossed by dark lines, corresponding to the 

 vapours which surround the object and absorb a por- 

 tion of its light. 



But I must not omit to notice two circumstances 

 which render the interpretation of a spectrum some- 

 what less simple than it would otherwise be. 



In the first place, if an object is shining by reflected 

 light its spectrum is precisely similar to that of the 

 object whose light illuminates it. Thus we cannot 

 pronounce positively as to the nature of an object 

 merely from the appearance of its spectrum, unless we 

 are quite certain that the object is self-luminous. For 

 example, we observe the solar spectrum to be a rain- 

 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 



