318 Dr. A. Sell uster on Spectra of Lightning. 



The measurements were always made at night; and the spec- 

 troscope was left undisturbed until the following morning, 

 when the Fraunhofer lines in the neighbourhood were mea- 

 sured, so that the wave-lengths of the measurements could be 

 interpolated. 



It is of course impossible to put a pointer on a band during 

 the instantaneous flash; but a succession of flashes allows us to 

 put the pointer successively nearer and nearer until we see it 

 in coincidence with the band. In this way several readings 

 of each band were obtained. The dispersive power of the 

 spectroscopes was such that, with a higher-power eyepiece than 

 the one used in this investigation, the nickel-line could be 

 seen between the two sodium-lines. The distance between the 

 two sodium-lines was such that the two readings of the slit 

 differed by ten divisions of the micrometer, or one tenth of a 

 whole revolution. With sunlight I can measure easily to the 

 tenth part of the distance between the sodium-lines. I ob- 

 tained measurements on three different nights. Unfortunately, 

 the best nights for the work occurred before the Total Solar 

 Eclipse, which had taken me out to Colorado. A desire to 

 save my eyes prevented me from making as good use of these 

 nights as I otherwise should have done. 



July 25th, West Las Animas. — The whole horizon seemed to 

 be almost constantly illuminated with lightning, generally sheet- 

 lightning. I observed about thirty or forty different flashes. I 

 often saw the bright nitrogen-lines 5002 and 5681. I did not 

 take any measurements of these lines; but there can hardly be 

 a doubt as to their position. I saw in the part of the spectrum 

 which I was observing three bands, which, however, did not 

 always appear together. The measurements reduced to wave- 

 lengths will be given further on. Two measurements of the 

 bands ft and 7 were obtained, but one only of the band ex.. The 

 greatest difference between the two measurements amounts to 

 three times the distance between the sodium-lines. This 

 difference must be partly accounted for by the difficulty of 

 the observation, partly by the fact that the spectroscope had 

 only just been unpacked after the journey; and it was found 

 next day that it was considerably out of adjustment. The 

 micrometer-screw, also, owing to the heat and dust, had a 

 considerable backlash ; it was taken to pieces next day and 

 cleaned, which greatly improved it. 



August 3rd, Manitou. — Clouds were coming from the west 

 over Pike's Peak; and strong flashes of lightning, partly sheet 

 lightning, partly forked lightning, were observed. Only two 

 measurements were secured. One of the bands measured was 

 £. Prof. Arthur "Wright, who was present, observed that one 



