8 LIGHT SCIENCE FOR LEISURE HOURS. 



ently over the star-lit depths of heaven. Nay, it is 

 even probable that every flicker and coruscation of 

 our auroral displays corresponds with similar manifest- 

 ations upon every planet which travels round the sun. 

 It becomes, then, a question of exceeding interest to 

 inquire what is the nature of the mysterious appari- 

 tion which from time to time illuminates our skies, 

 We have learnt something of the laws according to 

 which the aurora appears ; but what is its true nature ? 

 What sort of light is that which illuminates the 

 heavens ? Is there some process of combustion going 

 on in the upper regions of our atmosphere ? Or are 

 the auroral streamers electric or phosphorescent ? Or, 

 lastly, is the light simply solar light reflected from 

 some substance which exists at an enormous elevation 

 above the earth ? 



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 auro- 

 ral 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 



