from Bradley to FresneL 117 



along is (pi - p) or (^ - l)/o, while a quantity of aether of 

 density p remains at rest. The velocity with which the centre 

 of gravity of the aether within the body moves forward in the 

 direction of propagation is therefore 



where w denotes the component of the velocity of the body in 

 this direction. This is to be added to the velocity of propaga- 

 tion of the light- waves within the body ; so that in the moving 

 body the absolute velocity of light is 



Many years afterwards Stokes* put the same supposition in 

 a slightly different form. Suppose the whole of the aether 

 within the body to move together, the aether entering the body 

 in front, and being immediately condensed, and issuing from it 

 behind, where it is immediately rarefied. On this assumption a 

 mass pw of aether must pass in unit time across a plane of area 

 unity, drawn anywhere within the body in a direction at right 

 angles to the body's motion; and therefore the aether within 

 the body has a drift- velocity - wp/p l relative to the body : so 

 the velocity of light relative to the body will be Ci - wplp\, and 

 the absolute velocity of light in the moving body will be 



v* 





v* k 



or ci + ^i w, as before. 



P 



This formula was experimentally confirmed in 1851 by 

 H. Fizeau,f who measured the displacement of interference- 

 fringes formed by light which had passed through a column of 

 moving water. 



* Phil. Mag. xxviii (1846) p. 76. 



t Annales de Chimie, Ivii (1859), p. 385. Also by A. A. Michelson and 

 E. W. Morley, Am. Journ. Science, xxxi (1886), p. 377. 



