220 Mr. J. N. Lockyer. On the Classification [Jan. 10, 



rays, it may still be said that the light of the luminous arc is sensibly 

 monochromatic. 



" Here is a circumstance which gives this observation on the 

 spectrum of the aurora borealis a greater and even cosmic importance. 

 During a week of the month of March, 1867, 1 succeeded in observing 

 the same spectral line in the zodiacal light which had then an extra- 

 ordinary intensity for the latitude of Upsala. At last, during a 

 starlight night, the whole heavens being in a manner phosphorescent, 

 I found traces of it even in the faint light emitted from all parts of 

 the firmament. A very remarkable fact is that the line in question 

 coincides with none of the known lines in the spectra of simple or 

 compound gases, at least so far as I have studied them at present. 

 It follows from what I have said that an intense aurora borealis, such 

 as may be observed above the polar circle, will probably give a more 

 complicated spectrum than that which I saw. Supposing that to be 

 the fact, it may be hoped that in the future ifc will be possible to 

 explain more easily the origin of the lines found and the nature of 

 the phenomenon itself. Not being able to give this explanation at 

 present, I propose to return to it another time." 



Zollner 1 s View. 



In the * Report to the Royal Saxon Academy of Sciences,' October, 

 1871, Zollner expressed the opinion that the temperature of the 

 incandescent gas of the aurora must be very low. He affirms that 

 the spectrum does not correspond with that of any known substance, 

 and suggests, therefore, that it may be one given by air under some 

 peculiar condition which cannot be experimentally reproduced. (A 

 translation of Zollner 's paper is given in the ' Philosophical Maga- 

 zine,' vol. 41, 1871, p. 122.) 



VogeVs Views. 



Vogel also makes the same affirmation, and comes to the same 

 conclusion as Zollner, namely, that the spectrum of the aurora is one 

 which cannot be artificially produced. He suggests that it may be 

 the integrated spectrum of several layers which exist under different 

 conditions (' Reports of the Royal Saxon Academy of Sciences/ 

 1871).* He points out that the characteristic line in the aurora 

 spectrum observed by Angstrom is coincident with a very faint line 

 of nitrogen. That this line should appear in the aurora spectrum 

 with enhanced intensity he regards as quite consistent with the 

 known variability of gas spectra under various conditions of tem- 

 perature and pressure. He also points out the possible coincidence 

 of one of the lines with a line in the negative-pole spectrum of 



* A translation of Vogel's paper is given by Capron (' Aurorse,' p. 194). 



