﻿Widening of Spectrum Lines. 275 



who claimed to have discovered that the high interference 

 actually observed was inconsistent with Doppler's principle 

 and the theory of gases, I gave a more complete calculation *, 

 taking into account the variable velocity of the molecules 

 as defined by Maxwell's law, from which it appeared 

 that there was really no disagreement with observation. 

 Michel son compared these theoretical results with those of 

 his important observations upon light from vacuum-tubes 

 and found an agreement which was thought sufficient, 

 although there remained some points of uncertainty. 



The same ground was traversed by Schonrock f , who made 

 the notable remark that while the agreement was good for 

 the monatomic gases it failed for diatomic hydrogen, oxygen, 

 and nitrogen ; and he put forward the suggestion that in 

 these cases the chemical atom, rather than the usual molecule, 

 was to be regarded as the carrier of the emission-centres. 

 By this substitution, entailing an increase of velocity in the 

 ratio ^/2 : 1, the agreement was much improved. 



While I do not doubt that Schonrock's comparison is sub- 

 stantially correct, I think that his presentation of the theory 

 is confused and unnecessarily complicated by the introduction 

 (in two senses) of the " width of the spectrum line/' a quantity 

 not usually susceptible of direct observation. Unless I mis- 

 understand, what he calls the observed width is a quantity 

 not itself observed at all but deduced from the visibility of 

 interference bands by arguments which already assume 

 Doppler's principle and the theory of gases. I do not see 

 what is gained by introducing this quantity. Given the 

 nature of the radiating gas and its temperature, we can 

 calculate from known data the distribution of light in the 

 bands corresponding to any given retardation, and from 

 photometric experience we can form a pretty good judgment 

 as to the maximum retardation at which they should still be 

 visible. This theoretical result can then be compared with a 

 purely experimental one, and an agreement will confirm 

 the principles on which the calculation was founded. I 

 think it desirable to include here a sketch of this treatment 

 of the question on the lines followed in 1889, but with a few 

 slight changes of notation. 



The phenomenon of interference in its simplest form occurs 

 when two equal trains of waves are superposed, both trains 

 having the same frequency and one being retarded relatively 



* " On the limits to interference when light is radiated from moving 

 molecules," Phil. Mag-, vol. xxvii. p. 298 (1889) ; Scientific Papers, 

 vol. iii. p. 258. 



t Ann. der Physik, xx. p. 995 (1906). 



T2 



