from Bradley to Fresnel. 



119 



On the basis of this formula, Fresnel proceeded to solve 

 the problem of refraction in moving bodies. Suppose that a 

 prism A Q (7 B is carried along by the earth's motion in vacuo, its 

 face A(, C being at right angles to the direction of motion ; and 



that light from a star is incident normally on this face. The 

 rays experience no refraction at incidence ; and we have only to 

 consider the effect produced by the second surface A<>I> . Sup- 

 pose that during an interval T of time the prism travels from 

 the position A Q C Bo to the position A Ci B^ while the luminous 

 disturbance at C travels to h and the luminous disturbance at 

 A travels to D, so that B v D is the emergent wave-front. 

 Then we have 



-1 



10 



A D 



TC, 



If we write CiA\B\ = i, and denote the total deviation 

 of the wave-front by 81, we have 



AiD = AJ) - AA Q cos Si = TC - rw cos 81, 



TIC, 



