450 Prof. C. V. Raman and Dr. X. K. Sethi on the 



This, combined with the primary wave cos^— lot — x), 

 will give cos— Ibt — x — B I, Avhere 



,==nld ' v ^jy{ 1 --b} 



If /a' be the effective refractive index of the medium as 

 modified by the moving molecules, that of the medium 

 without the particles being taken as unity, 



S' = ( fjL '-})d.r. 

 And therefore 





But it has been shown by Lord Rayleigh that if /a be the 

 refractive index with the molecules when they do not have 

 the velocity v, 



D'-D 



/-l = (^~l)(l~^) 



i.e., fi—fi' = 2(/j,— l)^, 



which, in terms of the corresponding velocities of light V 

 and V, reduces to 



<v- 1) 



V'-V=». 



s 



In the case when fi does not much differ from 1, as in air, 

 this becomes 



V-Y ^ + ^r^ [since ,, + 1 = 2] 



(-*> 



2. Experimental Methods. 



It was proposed to employ the same optical arrangements 

 as had been used by Fizeau and which constitute in effect 

 a compensated Rayleigh Interferometer, and which are 

 eminently suited for this work. Michelson's arrangement 

 might have been better, but owing to the great length of 





