1868.] Lieut. J. Herschel on the Lightning Spectrum. 61 



II. u On the Lightning Spectrum." By Lieut. John Herschel, R.E. 

 Communicated by Prof. Stokes, lleceived August 8, 1868. 



I have had two or three opportunities of seeing this spectrum to advantage 

 of late. The storms at the period of the setting in of the south-west monsoon 

 here are very frequent, and supply for a time almost incessant flashes, many 

 of which are of course very brilliant. The first time I examined the light in 

 the spectroscope I had no idea of measuring, but was content to realize the 

 principal facts of a continuous spectrum crossed by bright lines ; but sub- 

 sequently I made several attempts (with some success) to obtain measures. 

 That I was unable to do more in this line is due partly to the difficult}^ of 

 utilizing the short-lived appearance, partly to that fascination of waiting for 

 " one more " bright flash to verify the intersection, which can only be 

 thoroughly appreciated by the aid of a similar experience. 



The principal features of the spectrum are a more or less bright conti- 

 nuous spectrum crossed by numerous bright lines, so numerous indeed as 

 to perplex one as to their identity. This perplexity is increased by the 

 constantly changing appearance due to a variable illuminating power. This 

 variable character of the appearances is unquestionably the peculiar feature 

 of the spectrum. It is not that the whole spectrum varies in brightness in 

 the same degree, but that the relative intensities are variable, not only 

 among the various lines, but between these and the continuous spectrum. 

 The latter is sometimes very brilliant ; and when that is the case, the red 

 portion is very striking, though in general the spectrum seems to end ab- 

 ruptly at D + 0-34 (E = D+1'38, Kirchhoff's 1207 = D + 0-55). 



There is one principal line which I found equal to D + 2-20 as the result 

 of five independent measures. The probable error of this value is about 

 + •02. The general mean of all my measures of the principal nebular 

 line (obtained from twelve different nebulae) is 2-18, with a probable error 

 of about + *02. I have therefore very little doubt that these are the same, 

 viz. the nitrogen line identified in the case of nebulae by Mr. Huggins. 

 This line in the lightning spectrum is narrow and sharply defined, and is 

 conspicuously the brightest, except as noted below. 



The next in prominence is situated about D + 3-58 (F=D-f 273, Kirch- 

 hoff's 232*5 = D + 3'50). It is broader and less vivid, and not so well 

 defined at the edges. 



There are several other conspicuous lines, but none comparable to the 

 first. I noticed a sharp line in the red, but did not get a measure. 



I said that at D + 0'34 the continuous spectrum ends abruptly. A faint 

 continuation is, however, seen frequently in bright flashes, very bright ones 

 bringing out a brilliant red end crossed by a bright line. 



The whole of the ordinary spectrum seems green and blue, or rather 

 greenish blue ; but as the usual prismatic order of colours is recognizable 

 \n bright flashes, it is to be inferred that the region from E to F is so much 



