450 



On the Plane of Polarization of Light. 



[Dec. 21, 



one of tliem 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 IN'icol. 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 extinguished ; but no motion of the 

 analyzer could make the field as black as it had been before the magnetism 

 was excited, thus conclusively proving that what was produced was an 

 elliptically polarized ray, as T had anticipated. When the light was re- 

 flected from a south pole the plane of polarization was rotated to the 

 right of the observer, which is the direction of rotation assumed in my 

 explanation. 



I next covered a portion of the polished face with gold leaf, as Professor 

 Barrett had suggested ; and now the hght reflected from this diamagnetic 

 substance Avas unaffected by the magnetism, as I had also anticipated. 

 I exhibited all these effects to Mr. Stoney, who entirely confirmed my 

 observations. 



Received November 25, 1876. 



The angle of incidence in the experiments described above was about 

 60°. If the incidence were either perpendicular or grazing, the theory 

 which I have proposed would lead to the conclusion that the angle 

 between the major axis of elliptic polarization and the original plane of 

 polarization would vanish. If, accordingly, the observation can be made 

 at a perpendicular incidence, and if the Xicol's prisms be so placed as to 

 extinguish the light before magnetizing the iron, then on exciting it light 

 ought to reappear, as it does at oblique incidences ; but the field should 

 not become darker on moving the analyzer. 



I attribute great weight to the verification of my theory arising from 

 the fact that the polarization of the reflected ray is found by experiment 

 to be in general elliptic, and also from the fact that there is no appreciable 

 effect when gold, a diamagnetic substance and therefore feeble, is sub- 

 stituted for iron. 



Since communicating my paper, I learn, through Professor Stokes, that 

 when Mr. Kerr's paper was read before Section A of the British Associa- 

 tion, both he and Sir W. Thomson spoke of the possibility of connecting 

 Mr. Kerr's result with a powerful double refraction of the same kind as 

 the feeble doable refraction shown by transparent substances under the 

 influence of magnetism. It is a connexion of this kind which I have 

 endeavoured to demonstrate. 



