9 4 Prof. Powell on the Theory of the Aberration of Light. 



In the view which I gave in that paper of the explanation 

 of the aberration, my object was to put the principle proposed 

 by Prof. Challis in such a form as I conceived might divest it 

 of certain difficulties which had appeared to some to attach 

 to it. 



I had in fact attempted to combine Prof. Challis's principle 

 with what seemed necessary for explaining the direction of 

 vision. But that principle is in itself quite independent of this 

 last consideration; and the whole may thus be more distinctly 

 expressed by keeping them separate as follows: 



Supposing the same construction as in my former paper, 

 fig. 2, then — ^ 



(1.) The observer being at r 



the bottom of /, let the ray / 



from the star have arrived at / 



the top of/, which point may 

 represent the place of the te- 

 lescope-wire, or any other 

 terrestrial object to which the 

 star is referred, and from 

 which rays emanate in all di- 

 rections. Of these, that one 

 which proceeds in the direc- 

 tion d will pursue that direc- 

 tion along with the ray from 

 the star, and will in every 

 respect coincide with it ; and 

 they will arrive together at 

 the bottom of d at the same 

 time as that at which the ob- 

 server's eye arrives there. 

 Both objects will conse- 

 quendy be referred to the smne direction, though the wire 

 has really moved into a new position, viz, to the top of /g. 



This is all that is essential to the idea of aberration accord- 

 ing to Prof. Challis's view. It is the angular difference « 

 between these two directions which is defined to be aberration. 



(2.) What the direction is, to which the eye actually refers 

 the star and the wire, is a distinct question. By some it may 

 be regarded as altogether needless ; and so I admit it is as 

 far as the above abstract idea of aberration is concerned : yet 

 it seems to me at least desirable to take it into account ; and 

 it may, I conceive, be best put as follows : — 



"While the rays come down d the telescope has moved 

 along e parallel to itself, into the position Z^, independently of 

 the lay which had started from the wire down d : yet at every 



