and other Spectra with the Spectrum of the Aurora. 251 



other recognized micrometer-measured spectra when they are 

 compared. In most cases the central part of the spectrum only 

 (corresponding to the central lines of the aurora) is mapped, the 

 red line in the aurora not being found to correspond with any 

 prominent line in the gas-spectra examined, and the auroral line 

 near solar G being at preseut so indefinitely fixed as to render 

 comparison almost valueless. I have selected Dr. VogeFs spec- 

 trum as given in his Memoir ('English Mechanic/ vol. xviii. 

 No. 461, p. 446) for comparison, it being, so far as I am aware, 

 the most accurately mapped with regard to wave-length at one 

 observation of any auroral spectrum, and it seeming an unsafe 

 plan to attempt to obtain an average aurora by comparison of 

 differing observations made at various times by different observers 

 with all sorts of instruments — the difficulty, too, being increased 

 by the suspicion that the spectrum itself at times varies in num- 

 ber and position, as well as intensity of its lines. 



In illustration of the difficulty of constructing any thing like 

 a general typical aurora-spectrum I append a Table of eight 

 auroral spectra taken at hazard. 



Observers. 



Aurora Lines and Bands. 



Red. Yellow. Green. Blue. Indigo. Violet. 



Vogel, April 9, 1871 



6297 



5569 



5390 



5233 



51895004 



' 



4694] 



to \ 



4629 J 



4740 1 



to I 



4670 J 



4625 

 4640 





Barker, Nov. 9, 1871 



Barker, Oct. 14, 1872 



A. Clarke, jun., Oct. 24, 1870 



Backhouse, 1873 



Backhouse, Feb. 4, 1874 



H. R. Procter, 1870 



6230 

 6300 



6060 

 * 



5620 



5550 



5690 



5660 



5570 



* 



3 



* 



5330 1 



to 

 5200 J 

 5320 



* 

 * 



5170 



J 



5165 



5180 



5020 



5050 



to 



4990 



5015 

 4980 



4820 

 4930 

 to 

 4850 

 4850 



4830 



4310 



4350 

 4305 

 4320 



# 



* 



Lord Lindsay, 1870 



1 

 ... 1 # 



* 













* Mr. Procter's and Lord Lindsay's lines had no wave-lengths given to 

 them. No doubt some of the discrepancies are instrumental and personal; 

 but there is, I think, still enough remaining of discrepancy to lead to the 

 inference that the aurora prevails in distinct forms or types, and with the 

 fainter lines or bands differing in position. From notes and drawings 

 kindly supplied at times to me by Mr. Backhouse, there is also no doubt 

 that the relative intensity of the lines frequently differs. With a sufficient 

 number of observations these forms or types might be systematically 

 arranged and compared, with a prospect of advancing our knowledge of the 

 nature of aurorae. 



I shall feel much indebted to any observers who will furnish me, for 

 this particular purpose, with charts of spectra and wave-lengths of lines : 

 these last, however, I can, more or less perfectly, deduce from such draw- 

 ings, if the positions of a few solar or other well-known lines are marked 

 on the scale. 



S2 



