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II. On the Constitution of the Luminiferous JEther, viewed 

 xvith reference to the phcenometwn of the Aberration of Light. 

 By G. G. Stokes, M.A., Fellow of Pembroke College, 

 Cambridge*. 



IN a former communication to this Magazine (July 1845), 

 -*■ I showed that the phenomenon of aberration might be 

 explained on the undulatory theory of light, without making 

 the startling supposition that the earth in its motion round 

 the sun offers no resistance to the aether. It appeared that 

 the phenomenon was fully accounted for, provided we sup- 

 posed the motion of the aether such as to make 



udx -f vdy -f wdz (a.) 



an exact differential, where u, v, w are the resolved parts, 

 along three rectangular axes, of the velocity of the particle of 

 aether whose co-ordinates are x, y, z. It appeared moreover 

 that it was necessary to make this supposition in order to ac- 

 count in this way for the phenomenon of aberration. I did 

 not in that paper enter into any speculations as to the physi- 

 cal causes in consequence of which (a.) might be an exact 

 differential. The object of the present communication is to 

 consider this question. 



The inquiry naturally divides itself into two parts: — First, 

 In what manner does one portion of aether act on another be- 

 yond the limits of the earth's atmosphere? Secondly, What 

 takes place in consequence of the mutual action of the air and 

 the aether ? 



In order to separate these two questions, let us first con- 

 ceive the earth to be destitute of an atmosphere. Before 

 considering the motion of the earth and the aether, let us take 

 the case of a solid moving in an ordinary incompressible fluid, 

 which may be supposed to be infinitely extended in all direc- 

 tions about the solid. If we suppose the solid and fluid to be 

 at first at rest, and the solid to be then moved in any manner, 

 it follows from the three first integrals of the ordinary equa- 

 tions of fluid motion, obtained by M. Cauchy, that the motion 

 of the fluid at any time will be such that (a.) is an exact dif- 

 ferential. From this it may be easily proved, that if at any 

 instant the solid be reduced to rest, the whole of the fluid will 

 be reduced to rest likewise; and that the motion of the fluid 

 is the same as it would have been if the solid had received by 

 direct impact the motion which it has at that instant. Prac- 

 tically however the motion of the fluid after some time would 

 differ widely from what would be thus obtained, at least if the 



* Communicated hv the Author. 



