452 Messrs. Michelson and Morley on the Relative Motion 



the angle bab l being equal to the aberration =«, is returned 

 along ba n (aba^Sa), and goes to the focus of the telescope, 

 whose direction is unaltered. The transmitted ray goes 

 along ac, is returned along ca n and is reflected at a„ making 

 ca t e equal 90— a, and therefore still coinciding with the first 



Fig. 2. 



ray. It may be remarked that the rays ba t and ca y do not 

 now meet exactly in the same point a n though the difference 

 is of the second order ; this does not affect the validity of 

 the reasoning. Let it now be required to find the difference 

 in the two paths aba t and aca r 

 Let V = velocity of light. 



v = velocity of the earth in its orbit. 



D = distance ab or ac, fig. 1. 



T=time light occupies to pass from a to c. 



T y = time light occupies to return from c to a t (fig. 2). 



Then 



T = 





D 



