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



pole spectrum is to represent the aurora-spectrum, it must be 

 under very different conditions from those under which it obtains 

 in air-vacuum tubes. 



4th. I feel much more in accord with Professor Angstrom's 

 memoir upon the subject of the phosphorescence or fluorescence 

 of the bright yellow-green aurora line. 



I do not notice that the Professor touches upon the external 

 features of the aurora in respect of these phenomena ; perhaps 

 the following extracts from my own note-book on this point may 

 not be uninteresting. I quote from them as made at the time. 



October 20th, 1870. — I noted the grand auroral display of 

 that evening including (( streamers of opaque white phosphorescent 

 cloud very different from the more common transparent auroral 

 diverging streams of light." (On this occasion, too, I may mention 

 I saw sharp and clear both the red and the green lines with 

 Browning's miniature spectroscope. I also noted a peculiar 

 flickering of the green line.) 



February 4th, 1872. — A fine display. The first signs were (in 

 dull daylight) " a lurid tinge upon the clouds, which suggested 

 the reflection of a distant fire, while, scattered among these, torn 

 and broken masses of white vapour having a phosphorescent ap- 

 pearance" reminded me of a similar appearance in Oc- 

 tober 1870. Then follows a description of the formation of the 

 corona. Day aurorse too, not unfrequently recorded, we should 

 suppose could hardly be seen without the presence of some phos- 

 phorescent glow. 



Having regard to the near proximity of the phosphoretted- 

 hydrogen band to the bright aurora-line, to the circumstance of 

 this band brightening by reduction of temperature (a phenomenon 

 probably connected with ozone), to the peculiar brightening of 

 one line in the " aurora" and " phosphorescent" tubes, and to 

 the observed circumstance that the electric discharge has a phos- 

 phorescent or fluorescent afterglow (I believe this was isolated 

 by Faraday), I feel there is strong evidence in favour of such an 

 origin to the principal aurora-line, if not to the red line as well. 



o 



5th. Professor Angstrom opens a wide door to discussion in 

 his proposition of the invariability of gas-spectra ; and I cannot 

 now attempt to follow this interesting part of the present sub- 

 ject. Suffice it to say that, if the Professor makes this assertion 

 in its strictest sense (I can hardly suppose he so means it), there 

 is, so far as I am aware, no one spectrum that can claim place 

 with the aurora-spectrum. Giving greater latitude to the Pro- 

 fessor's words, we may I think assume upon competent authority 

 that lines vary in number and brilliancy with temperature, and in 

 breadth with pressure ; and again, Kirchhoff, in speaking of vapour 

 films as increasing the intensity of lines, states c< it may happen 



