THE AUROKA BOIIEALIS. 733 



include the -svhole visible spectrum. Sodium-light, which is 

 common in laboratory flames, exhibits the punctures with ad- 

 mirable distinctness ; and each fifth hole being punched double, 

 the scale is very easily read off. There are ten holes on the red, 

 and twenty on the blue side of the slit. If the mechanical difficulty 

 of perforated jaws could be overcome, nothing perhaps could be 

 better suited for examining auroras than a pocket spectroscope so 

 prepared with a few close but clear and tolerably open holes on 

 each side of the slit. 



The secondary auroral lines can only be seen (in small spec- 

 troscopes) with a pretty broad slit ; and the strength of the yellow 

 line might then prove embarrassing. I would abolish it, if so, by 

 a blue* glass nearly covering one half, and a red glass the other 

 half of the slit — the blue and red parts of the spectrum re- 

 spectively, not in its immediate neighbourhood, being freely trans- 

 mitted. The slit might also be made longer than usual for 

 auroral study. 



I have been here supposing that special spectroscopes would be 

 provided for Arctic observers. But it is quite certain that much 

 may be done with common pocket spectroscopes without any such 

 provision. They should have adjustable slits and good dispersion, 

 as the secondary lines are faint ; and though abundant enough in 

 the blue to make the spectrum there pretty luminous, they can 

 only be individualised by varying the slit-aperture. On the only 

 occasion when I have seen this spectrum (in February 1872), they 

 seemed to run into each other, and presented a light so nearly con- 

 tinuous in the blue part that, although the slit of the Browning's 

 pocket spectroscope which I was using was extremely fine, and 

 was focused on the yellow line, no interruption or appearance of 

 lines could be made out. It was probably also through not opening 

 the slit that I missed seeing a red line which another observer, 

 using a similar instrument and looking with me at the Aurora, saw 

 very plainly. Although its red colour was intensely brilliant, I 

 failed to see the slightest tr?ce of light on the red side of the yellow 

 line. Had I opened the slit, or perhaps opened and closed it 

 alternately (as the yellow line, though fine, was still very bright), 

 the result would probably have been different. 



I can confirm the appearance of the negative or " cathode " 

 spectrum which Angstrom gives, from the results of some exami- 

 nations of it which 1 have lately made. On projecting the recorded 

 lines in wave lengths, there is a very exact agreement with the 

 chief lines and shadings as figured in the plate. Some fainter 

 lines, however, are visible, which Angstrom has, perhaps, omitted 

 purposely to avoid encumbering the drawing. During the years 

 1871 and 1872 there were several resumes of the Aurora spec- 

 trum, accompanied with new measurements, in Poggendorff's 

 " Annalen " and the " American Journal of Science " (by Vogel, 

 Barker, and others). 



* Some care would be necessary in selecting the blue glass, as these gene- 

 rally transmit a yellow ray closely corresponding with the auroral line. 



