THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 



NOVEMBER 1907. 



LYI. A Theory of the Displacement of Spectral Lines pro- 

 duced by Pressure. By 0. W. Richardson, M.A., JD.Sc, 

 Professor of Physics in the University of Princeton* . 



§ 1. Introduction. 



THE pressure of the surrounding gas or vapour may 

 influence the structure of the lines of emission spectra 

 — and probably also of absorption spectra — in a variety of 

 ways. Increase in the partial pressure of the vapour of the 

 emitting substance generally produces a broadening of the 

 lines, which varies greatly both in quantity and quality with 

 the particular line examined. The lines thus affected lose 

 their sharpness and become diffusely extended : the extension 

 may be more marked towards the region of either longer or 

 shorter wave-length according to the nature of the line in 

 question. This effect depends entirely in the similarity 

 between the surrounding vapour and the centres of emission. 

 Up to the present there is no experimental evidence of a 

 change of frequency of the original line due to this cause, 

 apart from the broadening which has been described. 



There is, however, another effect of pressure which is not 

 selective. Any gas other than the vapour of the emitting 

 substance affects the lines emitted, if its pressure is high 

 enough. In this case also the pressure impairs the definition 

 of the lines and broadens them out, usually about equally on 

 both sides ; but this broadening is not sufficient to mask a 



* Communicated by the Author. 

 Phil. Mag. S. 6. Vol. 14. No. 83. Nov. 1907. 2 P 



