THE AURORA BOREALIS. 729 



object, but partly also, it may be, on account of the variability of 

 the aurora. The red does not ahvays appear, and when it does is 

 often so weak that it cannot be observed in the spectroscope. If 

 now it be assumed that the aurora has its final cause in electrical 

 discharges in the upper strata of the atmosphere, and that these 

 discharges, whether disruptional or continuous, take place some- 

 times on the outer boundary of tlie atmosphere, and sometimes 

 near to the surface of the earth, this variability will easily show 

 in the appearance of the spectrum what the observations appear to 

 confirm. 



"If we consider the conditions under which the electric light 

 appears on the boundary of the atmosphere, moisture in that 

 region must be set. down as nil, and consequently the oxygen and 

 hydrogen there must alone act as conductors of electricity. Ang- 

 strom has tried to reproduce these conditions on a small scale. 

 Into a flask, the bottom of which is covered with a layer of phos- 

 phate, the platinum wires are introduced, and the air is pumped 

 out to the extent of several millimetres. If the inductive current 

 of a Ruhmkorff" coil be sent through the flask, the whole flask 

 will be filled, as it were, with that violet light which otherwise 

 only proceeds from the negative pole, and from both electrodes a 

 spectrum is obtained consisting chiefly of shaded violet bands. 



" If this spectrum be compared with that of the aurora, Ang- 

 strom thinks that the agreement between the former and some of 

 the best established bands of the latter is satisfactory. 



Lines. 



Of the aurora 

 spectrum - 



'According to Barker - 

 „ „ Vogel - 

 „ „ Angstrom 

 „ „ Lemstrom 



Mean 

 a of the violet light 



Wave-lengths. 

 431 470-5 



— 469-4 523-3 



— 472 521 

 426-2 469-4 523-5 



Of the spectruE 



428-6 470-3 522-6 

 427-2 470-7 522-7 



"In the neighbourhood of the line 469*4 Herr Vogel has more- 

 over observed two weak light bands, 466 * 3 and 462 * 9. The spec- 

 trum of the violet light has also two corresponding shaded bands, 

 465-4 and 460-1. 



" Should the aurora be flamy and shoot out like rays, there is 

 good reason for assuming a disruptive discharge of electricity, and 

 then there ought to appear the strongest line in the line-spectrum 

 of the air, the green, whose wave-length is 500-3. Precisely this 

 has been actually observed by Vogel, and has moreover been seen 

 by Angstrom and others. 



" Finally, should the aurora be observed as it appears at a 

 less height in the atmosphere, then are recognised both the 

 hydrogen lines and also the strongest of the bands of the dark- 

 banded air-spectrum, as e.g.^ 497-3. There are found also again 

 nearly all the lines and light bands of the weak aurora spectrum, 

 whose position has with any certainty been observed. 



