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



spectrum of the substances in the iron dust which came out under the 

 lowest conditions of electrical excitation. I have previously shown that 

 under these circumstances what we do get is invariably the spectrum 

 of manganese with its first fluting at 558, and that long before the 

 spectrum of iron itself is seen. 



Should this line of argument be accepted, we have in it an addi- 

 tional proof of the suggestion that the luminosity of nebulas is really 

 due in great part to collisions, not to electrical excitation of any kind 

 in the first instance. 



I think it will be granted after what has preceded, that there is 

 strong evidence of an intimate relation between the spectrum of the 

 aurora and the spectra of meteorites and meteor- swarms. Certainly 

 the coincidence is such as to justify us in regarding meteoric dust 

 as the origin of the spectrum until a better and more probable origin 

 is demonstrated. 



How this view will meet the periodicity and geographical distribu- 

 tion of auroraa remains to be investigated ; the question may be asked 

 whether the earth sometimes meets greater quantities of aurora- 

 producing matter revolving round the sun than at other times, and 

 whether in this way the periodicity may be explained. 



IX. The Aurora and the Zodiacal Light. 



Since the shooting star ignition level lies between 75 and 50 miles 

 in height, and auroraa have been seen at heights of over 100 miles, it 

 seems probable that the matter which reaches the earth from space 

 is in the main of three degrees of fineness, and gives evidences 

 of its existence at three different heights, the finest furnishing 

 materials for auroral displays at heights reaching to 130 miles,* the 

 mean finenesses igniting at a height of 75 miles, and giving rise to 

 the appearance of falling stars, till a height of 50 miles is reached, 

 when it is all consumed ; and the coarsest of all, which at times reach 

 the surface itself as meteoric irons or stones. 



An additional argument in favour of the meteoric theory of the 



* Capron and Herschel, " On the Auroral Beam of November 17, 1882 " (' Phil. 

 Mag.,' May, 1883). Professor Herschel, from measurements made 1863-67, deter- 

 mined the height of long white stationary auroral arches to be close upon 100 

 miles. 



Herr Sophus Tromholt (' Nature,' vol. 27, p. 394) gives 90 miles, and Baron 

 Nordenskjbld (' Scientific Work of the " Vega" Expedition,' Part I, p. 401-450), 

 gives 115 uiiles. 



Professor Herschel has also referred to measurements of auroral arches by Dr. 

 Dalton (' Phil. Trans.,' 1828, p. 291), who found 100 miles. Professor Potter's 

 determinations (' Cambridge Phil. Trans.,' 1845) of the heights of auroral arches 

 observed in September and October, 1833, ranged, on the other hand, between 55 

 and 85 miles. 



