Rev, J. Challis o« the Aberration of Light. 



323 



It will be seen that in the above explanation it is assumed 

 that light travels from the object to the eye in a straight line. 

 According to the emission theory of light such is the case, and 

 if this view of the nature of light be adopted, nothing further 

 need be said. The same would be true on the undulatory 

 hypothesis, if we might suppose the aether to be absolutely at 

 rest. But it is impossible to conceive that the earth can move 

 through space without communicating some motion to the 

 aether which surrounds and pervades it. It is therefore ne- 

 cessary, if we adopt the undulatory theory, to inquire how 

 far the preceding explanation will be modified by taking ac- 

 count of the motion of the aether. And here I may remark, 

 that the theory I am explaining is widely different from that 

 which Mr. Stokes has proposed in the July number of this 

 Journal. According to Mr. Stokes's views, the phaenomenon 

 of aberration is entirely owing to the motion which the earth 

 impresses on the aether, and which at the earth's surface he 

 supposes to be equal to the earth's motion. On the contrary, 

 I have to show that the amount of aberration will be the same 

 whatever be the motion of the aether, and if this cannot be 

 shown, the undulatory theory, and not the foregoing expla- 

 nation, is at fault. 1 am, however, so fully persuaded of the 

 truth of the undulatory theory, that I have no doubt this pro- 

 position admits of proof, and the following I consider to be 

 satisfactory. 



Let, as before, e and iso (fig. 2) be simultaneous positions of 

 the eye of the spectator and a terrestrial object, as the wire of 

 a telescope, and let luminous waves from a distant fixed object, 



Fig. 2. 



as a star, proceed in the direction 5 w e', accompanied, after 

 reaching w, by waves fromw, so that the eye at ef receives the 



Y 2 



