﻿142 Royal Society : — 



In clearness of diction, in comprehensiveness., in beauty of illus- 

 tration this little volume is all that can be desired. In Lesson 

 XLIV. we have an extremely useful summary of the methods by 

 which the true positions of the heavenly bodies are obtained ; and 

 at the end of the volume we have a very complete and withal ex- 

 tremely intelligible account of the law of gravitation. 



XX, Proceedings of Learned Societies, 



ROYAL SOCIETY. 



[Continued from p. 75.] 



November 19, 1868. — Lieut, -General Sabine, President, in the 



Chair, 



f J^HE following communications were read : — 

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



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 con- 

 tinuous spectrum crossed by bright lines ; but subsequently 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 difficulty of utili- 

 zing 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 

 continuous 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 some- 

 times very brilliant ; and when that is the case, the red portion is 

 very striking, though in general the spectrum seems to end abruptly 

 at D + 0'34 (E = D+1*38, KirchhofT's 1207 = D-f 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. 



