434? Prof. Powell on the Theory of the Aberration of Light. 



But this question may appear to some perhaps superfluous; 

 the above explanation, it may be said, only requires to have it 

 shown that the two rays do actually assume the same direction 

 in space by whatever means; and this is all that is necessary to 

 support the inference that the two objects must be referred to 

 the same direction. Those who agree in this view will then 

 merely consider any further discussion of this particular ques- 

 tion as irrelevant to the complete explanation of the fact, 

 though they may admit it as a question of curiosity. 



Others however may not be satisfied without a further ex- 

 planation ; and whether that just offered be considered fully 

 satisfactory or not, I confess it appears to me that some con- 

 siderations of the kind are necessary for showing that the star 

 and wire ought to be referred to the same direction. 



The question raised as to what that direction is, also seems 

 to be answered at the same time, if the foregoing view be ad- 

 mitted. 



The impression produced is clearly that which results from 

 the concurrence of two impressions, the one of the ray from 

 the wire, the other of that from the star ; the one simple, the 

 other arising from its motion of propagation together with a 

 motion of translation in the eye, but whose effect coincides 

 with the former: and a Keries of such compound impressions 

 come in succession continuously in the direction of the axis of 

 the tube to the eye. 'this circumstance appears to me deci- 

 sive of the question, that both objects must be seen in the 

 direction of the axis. 



The explanation however was held to be independent of 

 this question. It was said it would be equally valid should it 

 be contended (as, for example, on any physiological or other 

 grounds) that the eye received the impression not in the di- 

 rection of the axis, but in some other, as for instance that of 

 the diagonal. 



And when such a doubt was started, and when it was con- 

 tended that the above explanation was independent of any such 

 question, or that the introduction of it was superfluous, a diffi- 

 culty was raised in many minds; it was construed as if it tended 

 to overset all the received notions as to the direction of vision, 

 and to substitute something of an abstruse and theoretical 

 kind ; and the question raised, whether the star or the wire is 

 seen displaced, led to serious objection, especially as coupled 

 with the use of the term aberration applied to terrestrial 

 objects. 



But on the above view of the matter, the direction of vision 

 does not appear an irrelevant consideration ; and the question 

 seems clearly answered/ Thus I conceive we arrive at a pre- 

 cise statement of the case, and at the same time cut off a con- 



