530 Royal Society : — Mr. G. F. Fitzgerald on 



of the right-handed ray is less within the medium than the left- 

 handed, or that the refractive index for right-handed rays is greater 

 than for left-handed in a medium that rotates light to the right. 

 Hence, from what Verdet has shown, it appears that, in a ferro- 

 magnetic substance, for a ray of light travelling from the south to 

 the north pole, the magnetic action is such as to make the refractive 

 index for right-handed circularly polarized rays less than for left- 

 handed ones ; for in this case the plane of polarization is turned 

 to the left, for it is a right-handed current that would produce the 

 magnetic force. 



By applying this to the case of light reflected from the south 

 pole of a magnet, we get what I believe to be the true explanation 

 of Mr. Kerr's interesting experiment. In like manner, as in the 

 case of a transmitted ray, I consider the incident plane-polarized 

 ray to be the resultant of two circularly polarized ones, one right- 

 and the other left-handed. Now, for the right-handed one, the 

 refractive index at the surface of the south pole of the magnet, 

 being a ferro-magnetic substance, is less than for the left-handed 

 ray. Hence if each of the two circularly polarized rays be sup- 

 posed to be the resultant of two plane-polarized rays, one polarized 

 in the plane of incidence and the other at right angles to it, the 

 intensities of these four rays being equal, it is evident that the 

 intensities of the pair of reflected rays corresponding to the left- 

 handed ray will be greater than the corresponding intensities of 

 those due to the right-handed ray. Hence the two rays which 

 were polarized perpendicularly to the plane of incidence, and 

 which originally destroyed one another, will, after reflection, have 

 a component in the direction of the vibration of the left-handed 

 ray after reflection. ]NTow, on account of the change of direction 

 of the ray on reflection, this latter is towards the right. This is 

 completely explained in M. Jamin's ' Cours de Physique,' vol. iii. 

 part 2, p. 674, where he shows that a ray the azimuth of whose 

 plane of polarization was originally towards the right is by re- 

 flection turned towards the left. Hence the result of reflection is 

 to furnish two rays, one polarized in the plane of incidence, and the 

 other at right angles to it. The phases of these rays will, in 

 general, be different ; for they differed by 90° before reflection, and, 

 except at the polarizing angle for iron, this difference of phase 

 would not be completely destroyed, so that the resultant would 

 generally be an elliptically polarized ray the direction of whose 

 major axis would make a small angle towards the right with the 

 plane of incidence ; and at the polarizing angle for iron this 

 ellipse would become a plane-polarized ray whose plane of pola- 

 rization was turned towards the right, which I understand to be the 

 direction in which Mr. Kerr observed it to be turned — although 

 from some ambiguity as to the meaning of right and left rotations 

 in a ray, arising from not specifying whether it is relative to the 

 direction in which the ray is goiug or in which it is observed, I 

 am not quite sure whether I understand Mr. Kerr correctly. 



