16 Mr. G. G. Stokes on the Aberration of Light. 



light from the object which passes through the centr%of the one 

 shall also pass through the centre of the other. The line join- 

 ing the centres of the holes will then determine the direction 

 of the object. Now this is, in principle, just what is done in 

 the case of an astronomical instrument, only, the fixed points 

 are replaced by the optical centre of the object-glass of the 

 telescope with which the object is viewed, and by the wire to 

 which it is referred. When the image of a star is bisected by 

 the wire, we define the apparent direction of the star to be 

 that of the line joining the optical centre, of the object-glass 

 with the bisecting wire. Whether it is the wire or the star 

 which is seen out of its true place, is a question with which 

 we have no concern. The answer which we shall be disposed 

 to give to it depends on the theory of aberration which we 

 adopt. According to the theory of aberration which I ex- 

 plained in the July number of this Magazine, the answer 

 would of course be, that it is the wire which is seen in its 

 true place. 



The principal thing, however, to which I object in Prof. 

 Challis's paper, is the reasoning by which he establishes his 

 equation (5.). In the figure, a b is a 

 very small portion of a wave of light, 

 which in the small time t would be pro- 

 pagated to c d if the a'ther from a to b 

 were moving with the velocity of the 

 aether at b, whjle, in consequence of the 

 difference in the velocity of the aether at 

 a and b, the disturbance at a is propa- 

 gated to e. Now Prof. Challis takes 



cae for the angle through which the normal to the wave's 

 front is displaced as the wave passes from a b to e d. But a c 

 is only the direction in space along which the disturbance at 

 a is propagated, a direction which has no immediate relation 

 to the normal to the wave, inasmuch as it differs from it by an 

 angle which is of the order of the aberration, the very order 

 of quantities that we are considering. In fact, according to 

 the reasoning in my paper, to which Prof. Challis does not 

 appear to object, I found that the law of aberration does not 

 result from supposing the waves of light to be carried by the 

 moving aether, so long as its motion is taken arbitrary ; and 

 in order to explain aberration, I was compelled to suppose 

 udx + vdy + xvdz to be an exact differential, at least when the 

 square of the aberration is neglected. 



It is evidently immaterial whether we make the construc- 

 tion that Prof. Challis has given, or suppose ef to be the po- 

 sition into which the wave a b would come at the end of the 



