262 Mr. J. R. Capron on the Comparison of some Tube 

 Of course no memoir on the aurora from so high an autho- 



o 



rity as the late Professor Angstrom can be aught than highly in- 

 teresting and instructive ; and I quite appreciate that the Pro- 

 fessor has opened up new and apt discussion (especially upon the 

 subject of the presumed phosphorescent or fluorescent character 

 of the bright aurora-line) ; but, on the other hand, I find reason 

 to differ from him in his absolute conclusions as to the violet- 

 pole spectrum ; and on some other points in his memoir, I 

 further feel strongly that whatever advance the learned Pro- 

 fessor's memoir may have made in that direction, there is still 

 plenty of auroral ground left for spectroscopists to work upon, 

 and that the legacy Professor Herschel speaks of, though unde- 

 niably rich, may 5 like some other legacies, give rise to a good 

 deal of discussion among the legatees before its benefits are 

 reached. 



Professor Angstrom's principal propositions seem to be 

 these : — 



1st. Two auroral spectra. I agree in this, but question 

 whether the fainter lines may not comprise more than one 

 spectrum. 



2nd. I agree also that the bright yellow-green line falls, as 

 Professor Angstrom describes, just behind the second line in the 

 hydrocarbon yellow group. And 1 find, in common with the Pro- 

 fessor, no well-marked or prominent line in the air-spectrum 

 with which it accords. 



3rd. This may be conveniently divided into two parts, viz. : — 



A. The proposition that " moisture in the region of the aurora 

 must be regarded as nil" 



Here I see reason to differ, since (to quote a letter of Mr. 

 Procter's) " the vapour-density of OH 2 is only 9 against 14 for 

 N and 16 for i" and again, " electrical or heat-repulsion (vide 

 Crookes and Faye) may carry water-dust up to enormous 

 heights." There are, too, I think, circumstances connected with 

 the aurora itself which make the fact of moisture being nil in 

 the auroral regions quite unlikely. The first of these is the fact 

 that the white arc, streamers, and floating patches of light 

 found in some aurorse have frequently the peculiar dense and 

 solid look of vapour clouds — a circumstance with which I have 

 been frequently struck. Mr. Procter also suspects that the 

 aurora is generally formed in a sort of "mist or imperfect vapour." 

 The second, that aurorse, or portions of them, are, as I believe, 

 frequently close to the earth's surface (for instances see e English 

 Mechanic,' vol. xviii., No. 460, p. 419). The arc of a fine white 

 aurora which I saw at Kyle Akin (Skye) last autumn had quite 

 a solid and misty look, while one of its extremities had the ap- 

 pearance of being behind a near range of mountains, but in 



