448 Mr. G. F. Fitzgerald on the Rotation of [Dec. 21, 



This is equivalent to two circularly polarized rays, one right- and the 

 other left-handed (as regards the observer). After passing through the 

 medium the ray is still plane-polarized, but the plane of its polarization 

 is turned, say, to the right (as regards the observer). Hence of the two 

 circularly polarized rays, that which is right-handed must have had its 

 phase accelerated with respect to the other during its passage through 

 the medium. In other words, the right-handed ray has performed a 

 greater number of vibrations, and therefore has a smaller wave-length 

 within the medium than the left-handed ray which has the same periodic 

 time." This is the same as saying that the velocity 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 Yerdet 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 maJce 

 the refractive inclesc 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. ISTow, 

 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 supposed 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 eiddent that the in- 

 tensities 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 ra}^ Hence the two rays which were polarized perpendi- 

 cularly 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. JN'ow, 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 reflection 

 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 



