1876.] 



the Plane of Polarization of Light. 



449 



the resultant would generally be an elliptically polarized ray the direc- 

 tion of whose major axis would make a small angle towards the right 

 with the plane o£ incidence ; and at the polarizing angle for iron this 

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

 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 

 going or in which it is observed, I am not quite sure whether I under- 

 stand Mr. Kerr correctly. Also from the fact that there are exceptions* 

 to the rule that rotations are positive for diamagnetic and negative for 

 ferro-magnetic substances, neutral chromate of potash being diamagnetic, 

 yet producing a negative rotation, I should be rather inclined to deduce 

 the direction of the rotation that would be produced, if iron were trans- 

 parent, from Mr. Kerr's experiment. 



It would be quite easy to deduce the difference of the refractive in- 

 dices of iron for the two circularly polarized rays if we knew the amount 

 by which the plane of polarization is turned ; but it would be necessary 

 to employ MacCuUagh's or Cauchy's formulae for the intensities of the 

 reflected rays; and these are so complicated that it is hardly worth while 

 going through the calculations, as the effect Mr. Kerr has observed seems 

 only barely observable. 



Similar effects must, of course, occur in the cases of diamagnetic sub- 

 stances, organic solutions, and quartz ; but the amounts in these cases 

 would be entirely beyond the range of observation of our present instru- 

 ments ; for in quartz, for instance, the difference of the refractive indices 

 of the two circularly polarized rays is only 0*00008. 



Received November 23^ 1876. 



Ohservations confirmatory of the foregoing Eooplanation. 



Since sending my explanation of Mr. Kerr's experiment 1 have made 

 some experiments in confirmation of it. The instruments, with the ex- 

 ception of the electro-magnet, which was kindly lent to me by Mr. Teates, 

 are the property of Trinity College, Dublin, and were placed at my 

 disposal by Professor Leslie. 



The electro-magnet I used is of the horseshoe pattern, with movable 

 soft iron armatures, a face of one of these being well polished. The 

 magnet was placed vertically, and the armatures were arranged on the 

 poles so that the polished face was vertical and a vertical edge of the 

 other armature parallel and very close to this face. A folded piece of 

 paper was inserted at the top between the edge and the face to prevent 

 their being drawn together when the magnet was set in action. Two 

 Mcol's prisms Avere so placed that a horizontal beam of light traversing 

 ^ Unless, indeed, these are due to the nature of the solvent. 



