8 LIGHT SCIENCE FOR LEISURE HOURS. 



auroral action and some form of solar disturbance, would 

 at once enable us to reject some of these hypotheses. 

 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 answers 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-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 rainbow- 

 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 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 

 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- 



