330 Dr. J. Kerr on Rotation of the Plane of Polarization 



only to assume that the immediate optical effects of the four 

 operations (R, 1ST), (L, S) are similar in kind for all, and 

 similarly directed for those of either pair, but oppositely di- 

 rected for different pairs. R and L turn the plane of polari- 

 zation ; so therefore, according to this view, do N and S. R 

 and N turn the plane of polarization in one direction ; L and S 

 turn it in the contrary direction. But even from an optical 

 point of view there is still an important difference between the 

 mechanical operations and the physical ; for in the one case 

 (R or L) the full effect of the operation is impressed upon the 

 light before incidence, while in the other case (N or S) the 

 effect is impressed somewhere and somehow in the very pro- 

 cess of reflection. 



To get a more definite statement of this interpretation, con- 

 sider the pair of conspiring operations (R, X). In the case 

 of operation N, and to an eye which looks into the polar mir- 

 ror, the nominal direction of the magnetizing current round 

 the core is right-handed (9). In the case of operation R and 

 to the same eye, the direction of rotation of the plane of po- 

 larization, or the direction of rotation of the trace of that plane 

 upon the reflecting surface, is evidently left-handed (9). We 

 infer that a right-handed current gives a left-handed rotation 

 of the plane of polarization. And this completes the first ex- 

 perimental proof of the general statement made in art. 2. 



14. To test the truth of this ^iew of the facts, I thought of 

 three methods which appeared accessible : — first, to apply 

 each of the four operations (R, N), (L, S), and to characterize 

 them separately by definite compensating actions in the polari- 

 scope ; secondly, to apply the operations N and S in com- 

 bination with small permanent rotations of the second Nicol ; 

 thirdly, to return to the case of perpendicular incidence, 

 which I had already tried roughly without success. I shall 

 prepare the way for an account of the first of these methods 

 by a short mathematical discussion. 



Compensation of effects of operation R. 



15. Let the angle of incidence be about 75° ; and suppose 

 that the initial conditions are as in the first and second expe- 

 riments (12), and particularly that the direction OX of the 

 vibration is perpendicular to the plane of incidence. The ope- 

 ration R being now applied, and the incident vibration being 

 turned thus through a small angle X C = a, it is required 

 to find the character of the reflected light, particularly with a 

 view to compensation. The two rectangular components 

 (one in X) of the incident vibration (c in C) are 



