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



neglecting the changes in the earth's motion during the pas- 

 sage of light, the apparent place of the sun, or of any star 

 or planet, is its true geocentric place for the instant at which 

 the light was emitted by the sun or star, or reflected by the 

 planet." 



This explanation appears to me, I must confess, itself in- 

 volved in no inconsiderable obscurity. It may however indi- 

 cate the kind of notions which were commonly prevalent, and 

 which called for such an answer; and at any rate, the precise 

 point of view in which the distinguished author, thus placed 

 the subject, will be at once seen to be quite distinct from that 

 taken in the recent investigations. 



Other writers seem to have felt in like manner the defective 

 notions which prevailed, and sought to aid the more correct 

 conception by various analogies. Lalande* illustrated the 

 case by supposing a carriage with a front window open while 

 rain falls vertically ; when at rest no rain will enter, but when 

 in motion it will. Maupertuis referred to the instance of hit- 

 ting a bird flying, by aiming at a point in advance of its true 

 place ; and others have imagined various like cases. 



But these popular illustrations fail in conveying precisely 

 the idea of which we are in quest. The light from the star 

 after all comes directly to us in the position where the earth is 

 for the instant of observation ; and that it is not the same ray 

 which would have come to us at a previous instant, though it 

 is one parallel to it (or in the case of a nearer body, differing 

 only by its parallax), is indeed evident, but does not explain 

 the fact, that the point indicated by the telescope differs from 

 the real place of the star; and where and how, it would still 

 be asked, does the deviation take place ? It is probable that 

 the more discerning students would have been led to some 

 answer to this question, from considering attentively the ne- 

 cessity which obviously exists, and indeed on which the whole 

 turns, for referring the star to some fixed •point, that is to a 

 point fixed relatively to the observer, though really in motion 

 with him and the earth, such as the cross wire of the tele- 

 scope. 



Now in some even of the earliest explanations offered, it is, 

 I think, clear that at least a near approach to a more precise 

 statement of the case, including this essential idea, is distinctly 

 made. In Bradley's original paper f it is worthy of notice 

 that he specially introduces the consideration of the direction 

 of the tube of the telescope, and the light passing along its 

 axis, as essential to the conception of the case (p. 647). But 

 besides this, in his lectures at Oxford (which commenced in 

 * Astron, § 2831. f Philosophical Transactions, 1728. 



