Prof. Powell on the Theory of the Aberration of Light. 433 



must be had to other principles on which to find a complete 

 explanation. 



7. On this view the assumption of the rectilinear course is 

 justified; and the explanation being complete, it is wholly 

 superfluous to refer to the undulatory or any other theory of 

 light, for an explanation of the phenomenon. 



Upon this review of the principles of the explanation, it must, 

 I think, be fully admitted that its most essential and character- 

 istic feature is prominently brought out by Prof. Challis with 

 far more distinct and systematic precision than by any of his 

 predecessors in the inquiry : and from what has been remarked 

 on their explanations, it will be judged to what extent we can 

 consider them to have really implied the more precise prin- 

 ciple, though they confessedly did not explicitly or systema- 

 tically enunciate it. I may also here add, that perhaps it 

 would render the matter still plainer to many apprehensions, 

 if, instead of the telescope and its wire, we were to substitute 

 the idea of a lamp elevated so as to appear against the sky to 

 an observer below, and in coincidence with a star. Here 

 then would be distinctly two rays ; one from the star at rest, 

 another from the lamp moving with the observer, apparently 

 coinciding but really differing in direction. 



There is perhaps one point on which it may be necessary to 

 add a remark. In the above explanation it appears that the 

 course of a ray propagated directly (as from the star) coincides 

 with that given to another (propagated directly in a different 

 course, as from the wire to the eye) by the motion of the object 

 from which it originates along with that to which it comes ; and 

 it is hence inferred, that the two objects will be referred to the 

 same direction as if the light from both were naturally propa- 

 gated in the same direction. In other words, the course by 

 which the star's light actually comes to the eye, relatively to the 

 observer in motion^ can be no other than the axis of the tele- 

 scope, in which direction, the eye moving along with the light 

 from the wire, at every instant receives an impression from 

 it, along with one from the star ; the one as truly comes down 

 the tube as the other. But then, it may be asked, when we 

 consider the very different modus operandi by which they 

 each respectively take this direction, can we be sure that the 

 effect ought to be the same ? 



This question may, I conceive, be answered by the consi- 

 deration (quite independent of any theory of light), that as the 

 light from the star comes down the tube only by virtue of the 

 tube's motion, the whole effect of the ray still continues to be 

 in its own direction ; but the actual result must be estimated 

 by resolving it, only that part of it which is in the direction of 

 the tube being really effective on the eye. 



