Rotation of the Plane of Polarization of Light. 531 



Also from the fact that there are exceptions* to the rule that rota- 

 tions are positive for diamagnetic and negative for ferro-magnetic 

 substances, neutral chromate of potash being diamagnetic, yet pro- 

 ducing a negative rotation, I should be rather inclined to deduce 

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

 transparent, from Mr. Kerr's experiment. 



It would be quite easy to deduce the difference of the refrac- 

 tive indices 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 MacCullagh's or Cauchy's formula 

 for the intensities of the reflected rays ; and these are so compli- 

 cated that it is hardly worth while going through the calcula- 

 tions, as the effect Mr. Kerr has observed seems only barely obser- 

 vable. 



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

 substances, organic solutions, and quartz ; but the amounts in 

 these cases would be entirely beyond the range of observation of 

 our present instruments ; for in quartz, for instance, the difference 

 of the refractive indices of the two circularly polarized rays is only 

 0-00008. 



Observations confirmatory of the foregoing Explanation. 



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

 made some experiments in confirmation of it. The instruments, 

 with the exception 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 be- 

 tween the edge and the face to prevent their being drawn together 

 when the magnet was set in action. Two Mcol's prisms were so 

 placed that a horizontal beam of light traversing one of them was 

 reflected down the other by the polished face from that part of its 

 surface which was opposite the edge. 



A beam of sunlight was now transmitted through the apparatus 

 and observed on emerging from the second Nicol. The following 

 results were thus obtained : — When the light was polarized by 

 the first Nicol, either in or perpendicularly to the plane of incidence, 

 and when it had been extinguished by the analyzer, as soon as the 

 electro-magnet was set in action the light immediately reappeared. 

 On now slightly moving the analyzer the light could be partly ex- 

 tinguished ; but no motion of the analyzer could make the field as 

 black as it had been before the magnetism was excited, thus con- 

 clusively proving that what was produced was an elliptically 

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



2M2 



