THE AURORA. 15 



All these views have from time to time found sup- 

 porters among scientific men. It need hardly be said 

 that what we now know of the association between 

 auroral action and some form of solar disturbance, 

 would at once enable us to reject some of these hy- 

 potheses. But we need not discuss the subject from 

 this point of view ; because a mode of research has 

 recently been rendered available which at once an- 

 swers our inquiries as to the general character of any 

 kind of light. I proceed to consider the application 

 of this method to the light from the auroral streamers. 



The spectroscope, or, as we may term the instru- 

 ment, the "light-sifter," tells us of what nature an 

 object which is a source of light may be. If the object 

 is a luminous solid or liquid, the instrument converts 

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

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

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

 solid or liquid shining through any vapors, the rain- 

 bow-colored streak again makes its appearance, but it 

 is now crossed by dark lines, corresponding to the 

 vapors which surround the object and absorb a portion 

 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 



