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VII. On the Spectrum of the Aurora. (Extract from a Letter 

 from Professor A. S. Herschel to R. H. Scott, Esq., F.R.S.*) 



o 



THE paper by Angstrom, noticed in ' Nature ' (vol. x. 

 No. 246, July 16, 1874), was a rare legacy of his 

 last days to spectroscopists of auroras. Nothing more conclu- 

 sive and satisfactory, that I know of, has been written on the 

 subject; and little more will be done now, I expect, by future 

 observers than to verify his conclusions and to extend the 

 research in the direction that he points out, in its more nume- 

 rous details. 



The spectrum of the aurora is no doubt in the main the same 

 as that of the pale blue light round the negative pole in an air 

 or nitrogen vacuum-tube, with the induction-spark passing 

 through it. There are so many well marked lines in this spec- 

 trum that, looking at Angstrom's representation of them^ it is 

 probably owing to the insignificant appearance of that part of 

 the vacuum-spark that its. proper spectrum has O not been more 

 frequently studied with reference to the aurora, as Angstrom seems 

 to have done by an experiment specially adapted for the purpose. 



There are several forms or modes (apparently four or five) of 

 electrical discharge through rarefied gases f. When very much 

 rarefied, air transmits the electricity so as to discharge the 

 Ruhmkorff poles without a spark. In that state there is still a 

 glow of heated air round both poles, which increases in size and 

 length along the tube as the air-pressure is increased, faster 

 round the positive than round the negative pole. This has been 

 accounted for by showing that the air offers far greater resistance 

 to the passage of electricity when it surrounds a cathode or 

 negative, than when it surrounds an anode or positively electrified 

 pole. The difference becomes more obvious as the pressure and 

 density of the gas are increased. The negative glow shrinks into 

 a very small space, while the positive brush extends through 

 nearly the whole length of the tube, abolishing at last the dark 

 space that at very low tensions separates the two lights from 

 each other. At pressures not exceeding one or two millimetres 

 the positive glow is stratified ; but if the pressure is increased it 

 becomes continuous ; and if the air-pressure amounts to that of 

 \ or \ an inch of mercury, or upwards, it again gathers into 



* Communicated by R. H. Scott, Esq. 



t An examination of these with revolving mirrors, by A. Wullner, at 

 Aix la Chapelle, appeared in the " Jubelband " of Poggendorff's Annalen 

 this year at the same time as Angstrom's paper in that volume, which also 

 contains some other tracts (by A. de la Rive and others) tracing the effects 

 of magnets and of metallic vapours in augmenting the discharge through 

 air. 



Phil. May. S. 4. Vol. 49. No. 322. Jan. 1875. F 



