12 LIGHT SCIENCE FOR LEISURE HOURS. 



' 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 advan- 

 tage 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 Russian astronomer, shortly after- 

 wards made confirmatory observations. At the meeting 

 of the Royal Astronomical Society in June 1868, Mr. 

 Huggins, F.R.S., 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 

 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 



monochromatic light is greenish, which surprised me ; 

 but General Sabine tells me that in his polar expedi- 

 tions 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 



