70 



Prof. A. S. Herschel on the Spectrum of the Aurora. 



Position. Middle and latter half of the third or 

 blue band in the blue gas-flame spectrum. 



Description fyc. A double band, consisting of two 

 lines ; the first rather more frequently noted 

 than the second in auroral spectra. 



Identification fyc The principal line and its com- 

 panion agree well in position with the principal 

 band in the negative-glow spectrum. 



blue'and i 5, Bri ff^ness 0-6. Wave-length 4285. 

 violet. J 6 ' » '••• » 411 °- 



Position fyc. 5 coincides nearly with G and with 

 the fourth or indigo band of the gas-flame 

 spectrum; 6 is between G and H, near the 

 hydrogen line h in the solar spectrum. 



Description fyc. 5 is a frequent but somewhat 

 difficultly visible line ; and, from its position, it 

 is possibly the limit of vision for pocket spec- 

 troscopes ; 6 was measured once by Lemstroui 

 at Helsingfors. 



Identification &;c. 5 corresponds exactly with a 

 strong band in the violet in the negative-glow 

 spectrum. 



Remarks and Suggestions. 



The general character of the subspectrum appears to be a series 

 of bright lines, bands, and shadings, more or less dimly visible 

 on a faint field of light in the blue region of the spectrum, the 

 greatest concentration occurring apparently most frequently at 

 about the positions stated above. They arise, according to 



Angstrom, from discharges of electricity from the denser to the 

 more rarefied strata of the upper air, producing there on a great 

 scale what is seen in artificial discharges of electricity in rarefied 

 air as a blue cap round the negative pole. The appearance in 

 the aurora of only one red line in the place of the many red 

 bands of the negative glow, scarcely less bright than the prin- 

 cipal one, is remarkable; and fresh observations are very desirable 

 to confirm it, or to detect other red lines if they exist. Very red 

 auroras should be examined with a wide slit, covered (if of advan- 

 tage) with light-red glass to shut off all other light as much as 

 possible. 



Mixed with the lines of the negative glow, Angstrom supposes 

 that lines of the positive disruptive spark or brush discharge 

 may appear in flashing auroras, especially near the base or arch 



