﻿294 Mr. G. J. Stoney on the Cause of the 



In spectra of this kind the lines which fall within the limits 

 of the visible spectrum appear at first sight to be scattered at 

 irregular intervals. This may arise, and probably does in most 

 cases arise in part, from the circumstance that there may be 

 several distinct motions in each molecule of the gas, each of 

 which produces its own series of harmonics in the spectrum, 

 which by their being presented together to the eye give the 

 appearance of a confused maze of lines. But it appears also to 

 arise in part from the absence of most of the harmonics, so that 

 it is not easy to trace the relationship between the few that re- 

 main. To do so without the assistance of spectra of the First 

 Order, requires that we should have at our disposal determina- 

 tions of the wave-lengths of the lines made with extraordinary 

 accuracy ; and perhaps in a few cases, as, for example, in the 

 case of hydrogen, the marvellous determinations which have been 



o 



made by Angstrom may have the requisite precision. 



The ordinary spectrum of hydrogen consists of four lines, cor- 

 responding to C in the solar spectrum, F, a line near G-, and h. 

 To these it is possible that we ought to add a conspicuous line 

 in the solar prominences which lies near D, but which has not 

 yet been found in the artificial spectrum of hydrogen. Of these 

 lines, three, viz. C, F, and h } are to be referred to the same mo- 

 tion in the molecules of the gas. 



In fact the wave-lengths of these lines, as determined by 

 Angstrom*, are: 



h =4101*2 tenth-metres. 



F =4860-74 ,, 



= 6562-10 



These are their wave-lengths in air of standard pressure and 

 14° temperature, determined with extraordinary precision. We 

 must correct these for the dispersion of the air, so as to arrive at 

 the wave-lengths in vacuo which are proportionate to the periodic 

 times. Now, by interpolating between Ketteler's observationsf 

 on the dispersion of air, we find 



^=1-000 29952, 



^=1-000 29685, 



^c = l*000 29383 



for the refractive indices of air of standard pressure and tempe- 

 rature for the rays h, F, and C. From these we deduce that if 

 the air be at 14° of temperature, the refractive indices will become 



o 



* Angstrom's Recherches sur le Spectre Solaire, p. 31. A tenth-metre 

 means a metre divided by 10 J0 ; similarly a fourteenth-second is a second 

 of time divided by 10 u . 



t Phil. Mag. 1866, vol. xxxii. p. 345. 



