[' 447 ] 



XLIX. On the Convection of Light (Fizeau Effect) in Moving 

 Gases. By C.V. RAMAN, M.A., Eon. D.Sc, Palit Pro- 

 fessor of Physics in the Calcutta University, and NlHAL 

 Karan Sethi, D.Sc."" 



[Plates VI. & VII.] 

 1. Introduction. 



IT was early in the last century that Arago tried his 

 famous experiment with the prism to detect whether 

 the sether in the interior of a material body and the light- 

 waves travelling inside it are carried along with that body 

 in its motion, and it was to explain the negative result of 

 this experiment that Fresnel propounded his well-known 

 hypothesis that the gether outside a moving body remains 

 stationary while tliat inside it drifts along with it, though 

 with a diminished velocity. He deduced a law according to 

 which this diminished velocity is given by the relation 





where u is the velocity of the body and fju its refractive index. 

 This result also explained why the aberration of the fixed 

 stars was found by Airy and Hoek to be independent of the 

 nature of the substance filling the telescope tube. And on 

 account of its fundamental importance in the theory of 

 Optics, some of the most eminent physicists have devoted 

 considerable energy to verifying the different aspects of this 

 law. More recently it has gained additional importance <«« 

 account of the fact that this law follows as a matte)- of course 

 from Einstein's remarkable principle of the relativity of 

 space and time f, and its experimental verification is now- 

 looked upon as one of the proofs of the correctness of his 

 theorem of the addition of velocities and consequently of 

 the special Principle of Relativity. 



We are, in the present note, not concerned with the first 

 part of FresneFs law, which demands a fixity of the aether 

 outside a moving body and which found support in the 

 experiments of Sir 0. Lodge t, who failed to discover a 

 drift of sether in the neighbourhood of moving matter, even 

 in the narrow space between two revolving disks or a crevice 

 in a massive sphere. The apparent disagreement of these 



* Communicated by the Authors. 



t See Cunningham's ' Theorv of Relativity,' p. 61. 



X Phil. Trans. A, 1893, p. 727. 



