2 LIGHT SCIENCE FOR LEISURE HOURS. 



canopy. Startling coruscations add splendour to the 

 scene, while the noble span of the auroral arch, from 

 which the waving curtain seems to depend, gives a 

 grandeur to the spectacle which no words can ade- 

 quately describe. Gradually, however, the celestial 

 fires which have illuminated the gorgeous arch seem 

 to die out. The luminous zone breaks up. The scene 

 of the display becomes covered with scattered streaks 

 and patches of ashen grey light, which hang like 

 clouds over the northern heavens. Then these in 

 turn disappear, and nothing remains of the brilliant 

 spectacle but a dark smoke-like segment on the 

 horizon. 



Such is the aurora as seen in arctic or antarctic 

 regions, where the phenomenon appears in its fullest 

 beauty. Even in our own latitudes, however, strik- 

 ingly beautiful auroral displays may sometimes be 

 witnessed. Yet those who have seen the spectacle 

 presented near the true home of the aurora, recognise 

 in other auroras a want of the fulness and splendour of 

 colour which form the most striking features of the 

 arctic and antarctic auroral curtains. 



Physicists long since recognised in the aurora a 

 phenomenon of more than local, of more even than 

 terrestrial, significance. They learned to associate it 

 with relations which affect the whole planetary scheme. 

 Let us inquire how this had come about. 



So long as men merely studied the appearances pre- 

 sented by the aurora, so long, in fact, as they merely 

 regarded the phenomenon as a local display, they could 



