20 LIGHT SCIENCE FOR LEISURE HOURS. 



of the bright line may be due to the character of the 

 particles between which the discharge takes 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 charac- 

 ter as iron and hydrogen may have lines so closely 

 approximating 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 belonging to the metal calcium (the principal 

 ingredient of common chalk), we are by no means to 

 infer that he supposes the substance which causes the 

 presence of the bright line has any resemblance to that 

 element. Until we can find an element which has a 

 bright line in its spectrum absolutely coincident with 

 the bright line detected by Angstrom in the spectrum 

 of the aurora,* all speculation as to the real nature of 

 the vapor in which the auroral electric discharge takes 

 place, or of the substance between which the spark 

 travels, is altogether precluded. 



But interesting as the discovery undoubtedly is, 



* Other green lines have since been discovered in the auroral spec- 

 trum ; and occasionally a red line is seen. 



