Prof. A. S. Herschel on the Spectrum of the Aurora. 67 



to recognize those which appear most frequently. For this 

 purpose they should be made large, and the slit should be a 

 wide one. For ordinary miniature spectroscopes, two holes on 

 the red and five on the blue side of the slit ^ or -^ of an inch 

 apart, would suffice for recognitions and even for very useful 

 measurements. The jaws of the slit can be cut with a fine saw 

 across the middle about -^ or J of an inch deep each way ; and 

 a piece of copper foil, provided with the row of holes and a 

 sufficiently wide slit across it, can be fastened to one of them 

 inside, opposite to the crosscut and adjoining the edges of the 

 spectroscope jaws. 



Some other means may be found of piercing the jaws of a 

 pocket spectroscope at regular intervals ; but as a simple plan I 

 have found this very efficient, in finer divisions, for laboratory 

 use. The holes are pierced at T ^- of an inch apart ; and thirty of 

 them include the whole 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 diffi- 

 culty of perforated jaws could be overcome, nothing perhaps 

 could be better suited for examining auroras than a pocket spec- 

 troscope 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 yel- 

 low 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 

 respectively, not in its immediate neighbourhood, being freely 

 transmitted. 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 individualized 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 continuous 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 



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

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



F2 



