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



tion. This and other attendant objections, have at the pre- 

 sent day led to the general admission of the aether being set 

 in motion by the passage of the earth through it. 



On this hypothesis the conclusion does not obviously or di- 

 rectly follow, but it may be shown to do so by an analytical 

 investigation. That the theoretical investigations on the un- 

 dulatory hypothesis, supposing the aether in motion, given 

 both by Mr. Stokes and Prof. Challis, are of the highest value 

 and importance, as evincing the capability of the undulatory 

 theory to surmount the difficulty here presented, must be on 

 all hands allowed, whatever may be thought of the points on 

 which the two distinguished mathematicians differ. 



The principle of Mr. Stokes's investigation is, in a word, 

 the analogy between sound and light; and the object is to 

 show that the luminiferous waves are propagated through cether 

 in motion, just as those of air constituting sound are propa- 

 gated through a mass of air in motion, and that in both cases 

 alike the impression reaches the organ just in the same man- 

 ner as if the medium were at rest. Hence some limitations 

 are introduced in the analysis ; the very natural supposition 

 that the aether near the earth acquires the same velocity as the 

 earth, and thus remains, relatively to it, at rest, involving the 

 condition that a certain expression {udx + vdy + wdz) is an 

 exact differential. 



Prof. Challis, on the other hand, discards these restrictions, 

 allowing this expression to be an exact differential or not, and 

 the motion of the aether any whatever: thus giving a greater 

 degree of generality to the investigation. 



But on the question whether we can legitimately infer that 

 the motion acquired by the aether must be precisely equal 

 to that of the earth, and whether, consequently, the more ge- 

 neral assumption be necessary for a completely satisfactory 

 conclusion, — or, on the other hand, whether the restrictions in 

 the analysis are such as, without impairing its full applica- 

 bility to the case, are more simple and natural, as well as on 

 other minor points, I do not here enter: the main question 

 with which we are at present concerned, is that which refers 

 to the necessity for such theoretical investigation at all, as re- 

 gards the complete explanation of the phaenomenon : on which 

 point also the two eminent disputants are at variance. 



If what has been before advanced be assented to, I conceive 

 while we deny the necessity for such an investigation as regards 

 the explanation of the facts, we must at least fully admit its 

 importance as regards the credit of the theory. 



We may illustrate the subject by taking a parallel case: 

 granting the laws of reflexion and refraction, and the unequal 



