Mr. G. G. Stokes on the Aberration of Light. 335 



which are not magnetic after the manner of iron, are diamay- 

 netic or magnetic after the manner of bismuth, we should be 

 led to conclude immediately that the optical action being con- 

 comitant with a certain mechanical action, it is at least pre- 

 sumable that this action is exerted upon the bodies, and not, 

 directly and immediately on the light which passes through 

 them. 



But if it happens, as in my experiments, either from the 

 relative weakness of my magnets or from the imperfection of 

 the methods which I have employed, or from other causes — 

 if it happens that the various kinds of glasses, distilled water, 

 the fatty bodies, &c, which are so sensitive to the optical 

 action, are nevertheless insensible to the mechanical action 

 of the magnetism, it would not be a reason to conclude that 

 magnetism acts directly upon the light itself; a conclusion 

 which, moreover, would only have a precise meaning in the 

 system of emission, for in the undulatory theory, which 

 seems at present so completely demonstrated, it is the aether 

 of the body submitted to the experiment which would be 

 modified by the magnetism, and it would doubtless be very 

 difficult to recognise whether it is modified without any par- 

 ticipation of the ponderable matter of the body with which 

 it is so intimately connected. 



LI I. On the Aberration of Light. By G. G. Stokes, M.A., 

 Fellow of Pembroke College, Cambridge*. 



T WISH to say a few words more on the subject of aberra- 

 " tion, to prevent misapprehension. It is evident from Prof. 

 Challis's last communication, that we differ merely as to the 

 phenomenon which we understand by the term " aberration 

 of light." When the position of a star has been corrected for 

 refraction, precession, and nutation, and proper motion if it 

 has any, let s be its mean annual place referred to the celestial 

 sphere, s x the point to which the star is referred by astrono- 

 mical measurement, and s 2 the point in which the sphere is 

 cut by the line along which the light comes from the star, 

 produced backwards, s 2 being corrected in the same manner 

 as s v It is shown by observation that s x is displaced from s 

 towards the point towards which the earth is moving, through 

 an angle equal to the ratio of the velocity of the earth to that 

 of light multiplied by the sine of the earth's way. This is the 

 phenomenon which I understand by the aberration of light, 

 and which it was the object of one of my former communis 



* Communicated by the Author. 



