THE AURORA. II 



by the electric light are so marked that it appears easy 

 to decide definitely by prismatic analysis whether the 

 light of the aurora is or is not electric.' Singularly 

 enough, however, the first proof that the auroral light 

 is of an electric nature was derived from a very different 

 mode of inquiry. Dr. Robinson, of Armagh, discovered 

 in 1858 (a year before KirchhofFs recognition of the 

 powers of spectroscopic analysis) that the light of the 

 aurora possesses in a peculiar degree a property termed 

 fluorescence, which is a recognised and characteristic 

 property of the light produced by electrical discharges. 



* These effects,' he remarks of the appearances pre- 

 sented by the auroral light under the tests he applied, 



* were so strong in relation to the actual intensity of the 

 light, that they appear to afford an additional evidence 

 of the electric origin of the phenomenon.' 



Passing over this ingenious application of one of the 

 most singular and interesting properties of light, we find 

 that the earliest determination of the real nature of 

 the auroral light or rather of its spectrum was that 

 effected by Angstrom. This observer took advantage 

 of the occurrence of a brilliant aurora in the winter 

 of 1867-68 to analyse the spectrum of the coloured 

 streamers. A single bright line only was seen ! Otto 

 Struve, an eminent Eussian astronomer, shortly after- 

 wards made confirmatory observations. At the meeting 

 of the Eoyal Astronomical Society in June, 1868, 

 Mr. Huggins thus described Struve's results : ' In a 

 letter, M. Otto Struve has informed me that he has 

 had two good opportunities of observing the spectrum 



