734 THE AURORA BOREALIS. 



I have endeavoured to condense the information on the Aurora 



o 



lines from Angstrom's descriptions of them in the annexed Table, 

 as a guide for further observation. 



Professor Piazzi Smyth, who has given much attention to the 

 auroral spectrum, has published, in the introduction to vol. xiii. of 

 the " Edinburgh Astronomical Observations," a set of simple com- 

 parison spectra, with notes of desiderata, which would be of great 

 service to observers well furnished with instrumental means and 

 applying them to measurements of the Aurora ; and something 

 similar would very much assist observers using direct-vision spec- 

 troscopes to map their spectra, where auroras are frequent and of 

 great brightness, in high latitudes. 



Positions and General Characters of Principal Li?ies in the 

 Auroral Spectrum, according to Angstrom. 



Red . . 1. Brightness to 4 or 8. VFave-length 6300. 



Relative position. Atmospheric absorption-line 



a, near C in the solar spectrum. 

 General descriptioii of source and frequency. Seen 

 chiefly, if not only, in red auroras ; a clearly 

 defined line, sometimes intense ; no other red 

 line visible. 

 Identification with lines of electrical-air spectra. 

 Coincides with a red band in the negative-glow 

 Yellow 1 discharge, 



or citron- > a. Brightness 25. Wave-length 5570. 

 'green. J Position. Second separable line in the first or 

 citron band of blue gas-flame spectrum. 

 Description, Sfc. The most characteristic auroral 

 line ; constant and conspicuous in all auroras 

 not divisible j sharp and bright. 

 Identification, ^c. Not identified ; possibly a 

 phosphorescent or fluorescent light emitted when 

 air is subjected to the action of electrical 

 discharge. 



rZ. Brightness 2 or ? to 6 Wave-length 5225. 



Greenish I Sa. „ 0-8 (increases 1 ^i>7n sion 



blue or { with red line) / " 6170-6iya 



blue. ISb. „ 2 or ? to 8 „ 5000. 



Uc. „ Oto4? „ 4820-4870. 



Position. 3 and 3« closely adjoin the solar line 

 b and the second or green band of the blue gas- 

 flame spectrum. Sb is at -^ {b, F) ; and the 

 line or lines 3 c are near F. 

 Description, S^c, The first three are distinct lines ; 

 the first most frequently observed ; the second 

 and third less commonly; lines in the fourth 

 place (3c) noted by Al van Clark, jun.. Barker, 

 and Angstrom. 



