736 



THE AURORA BOREALIS. 



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Mixed with the lines of 

 the negative glow, Ang- 

 strom supposes that lines 

 of the positive disruptive 

 spark or brush discharge 

 may appear in flashing 

 auroras, especially near 

 the base or arch as 

 distinguished from the 

 tops of the streamers, 

 giving to the subspec- 

 trum a different appear- 

 ance according to the 

 strength and agitation 

 of the streamers : this 

 may perhaps be trace- 

 able in the appearance 

 and disappearance of the 

 lines 3 a, 3 c, and per- 

 haps of other faint 

 lines, whose positions 

 should be noted. Such 

 lines should also be 

 searched for in quies- 

 cent parts, such as sta- 

 tionary auroral bands 

 and the tops of very 

 bright streamers. 



A correct recogni- 

 tion of some one or 

 more of the lines de- 

 scribed above, other 

 than the citron line, is 

 however, of chief im- 

 portance in observing 

 with small spectro- 

 scopes, as the leading 

 lines themselves must 

 supply the only stand- 

 ard intervals of com- 

 parison for eye-estima- 

 tions of such faint 

 spectra. 



