﻿278 Lord Rayleigh on the 



under control, obtained by means of double refraction, and I 

 found that in this very favourable case the bands were still 

 just distinctly seen when the relative difference between I x 

 and I 2 was reduced to 4 per cent. It would seem then 

 that the estimate V = *025 can hardly be improved upoiu 

 On this basis (10) gives in terms of v 



| =i.y (%•«>) = -690;. • • • (11) 



as before. In terms of v by (6) 



f =J^-V0°g. 4 °)=- 7 * 9 7- . . (12) 



ft- 1T\/ 6 . V L 



As an example of (12), let us apply it to hydrogen 

 molecules at 0° G. Here v' = 1839 X 10 2 cm./sec.*, and 

 c = 3xl0 10 . Thus 



X/A = 1-222 x 10 5 (13) 



This is for the hydrogen molecule. For the hydrogen 

 atom (13) must be divided by \/2. Thus for absolute 

 temperature T and for radiating centres whose mass is- 

 m times that of the hydrogen atom, we have 



i- ""y'V @-«w& 



• • • (14) 

 In Buisson and Fabry's corresponding formula, which 

 appears to be derived from Schonrock, 1*427 is replaced 

 by the appreciably different number l - 22. 



The above value of X is the retardation corresponding to 

 the limit of visibility, taken to be represented by V = *025. 

 In Schonrock's calculation the retardation X 1? corresponding 

 to V = *5, is considered. In (12), y/(log e 40) would then 

 be replaced by v /(log e 2), and instead of (14) we should 

 have 



^ = 6-180 xl0 5 ^/(£) (15) 



But I do not understand how Y = *5 could be recognized in 

 practice with any precision. 



Although it is not needed in connexion with high 

 interference, we can of course calculate the width of a 



* It seems to be often forgotten that the first published calculation of 

 molecular velocities was that of Joule (Manchester Memoirs, Oct. 1848 ; 

 Phil. Mag. ser. 4, vol. xiv. p. 211.) 



