14 LIGHT SCIENCE FOR LEISURE HOURS. 



tion of the number of solar spots in a period of equal 

 length. 



"We already begin to see, then, that auroras are 

 associated in some mysterious way with the action of 

 the solar rays. The phenomenon which had been 

 looked on for so many ages as a mere spectacle, caused 

 perhaps by some process in the upper regions of the 

 air, of a simply local character, has been brought into 

 the range of planetary phenomena. As surely as 

 the brilliant planets which deck the nocturnal skies 

 are illuminated by the same orb which gives us our 

 days and seasons, so are they subject to the same mys- 

 terious influence which causes the northern banners to 

 wave resplendently over the starlit depths of heaven. 

 ISTay, it is even probable that every flicker and corus- 

 cation of our auroral displays corresponds with similar 

 manifestations 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 

 apparition which from tirne'to time illuminates our 

 skies. We have learned something of the laws accord- 

 ing 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 ? 



