﻿274 Lord Rayleigh on the 



specimen o£ greater capacity. These results agree with the 

 facts as indicated by the curves of fig. 4. 



It seems probable, therefore, that a satisfactory theory of 

 f rictional electricity can be formed on the lines indicated, 

 though in the form given above the theory is doubtless far 

 from perfect. 



In conclusion, I desire to express my best thanks to 

 Professor E, Taylor Jones for much valuable help and advice 

 during the course of the work. 



Physics Laboratory, 

 University College of N. Wales, Bangor, 

 October 1914. 



XXXI. On the Widening of Spectrum Lines. 

 By Lord Rayleigh, O.J/., F.R.S* 



MODERN improvements in optical methods lend ad- 

 ditional interest to an examination of the causes 

 which interfere with the absolute homogeneity of spectrum 

 lines. So far as we know these may be considered under 

 live heads, and it appears probable that the list is ex- 

 haustive : — 



(i.) The translatory motion of the radiating particles in 

 the line of sight, operating in accordance with Doppler's 

 principle. 



(ii.) A possible effect of the rotation of the particles. 



(iii.) Disturbance depending on collision with other par- 

 ticles either of the same or of another kind. 



(iv.) Gradual dying down of the luminous vibrations as 

 energy is radiated away. 



(v.) Complications arising from the multiplicity of sources 

 in the line of sight. Thus if the light from a flame be 

 observed through a similar one, the increase of illumination 

 near the centre of the spectrum line is not so great as towards 

 the edges, in accordance with the principles laid down by 

 Stewart and Kirchhoff ; and the line is effectively widened. 

 It will be seen that this cause of widening cannot act alone, 

 but merely aggravates the effect of other causes. 



There is reason to think that in many cases, especially 

 when vapours in a highly rarefied condition are excited 

 electrically, the first cause is the more important. It was 

 first considered by Lippich f and somewhat later inde- 

 pendently by myself J. Subsequently, in reply to Ebert, 



* Communicated by tlie Author. 



t Pogg. Ann. t. cxxxix. p. 465 (1870). 



% 'Nature,' vol. viii. p. 474 (1873) ; Scientific Papers, vol. i. p. 183. 



