12 LIGHT SCIENCE FOR LEISURE HOURS. 



of the aurora borealis. The spectrum consists of one 

 line, and the light is therefore monochromatic. The 

 line falls near the margin of the yellow and green por- 

 tions of the spectrum. . . . This shows that the mono- 

 chromatic light is greenish, which surprised me ; but 

 General Sabine tells me that in his polar expeditions he 

 has frequently seen the aurora tinged with green, and 

 this appearance corresponds with the position of the line 

 seen by M. Struve.' 



The general import of this observation there is no 

 mistaking. It teaches us that the light of the aurora 

 is due to luminous vapour, and we may conclude, with 

 every appearance of probability, that the luminosity 

 of the vapour is due to the passage of electric dis- 

 charges through it. It is, however, possible that the 

 position of the bright line may be due to the character 

 of the particles between which the discharges take 

 place. 



But the view we are to take must depend upon the 

 position of the line. Here a difficulty presents itself. 

 There is no known terrestrial element whose spectrum 

 has a bright line precisely in the position of the line 

 in the auroral spectrum. And mere proximity has no 

 significance whatever in spectroscopic analysis. Two 

 elements differing as much from each other in character 

 as iron and hydrogen may have lines so closely ap- 

 proximating in position that only the most powerful 

 spectroscope can indicate the difference. So that when 

 Angstrom remarks that the bright line he has seen lies 

 slightly to the left of a well-known group of lines be- 



