Electromagnetic Rotation of the Plane of Polarization. 309 



and opposed to the molecular currents, whether the plane of 

 polarization of the incident light is coincident with, or at 

 right angles to, the plane of incidence. 



In all the experiments of Dr. Kerr the magnitude of the 

 rotation is very small — in favourable cases it amounts only to 

 a few minutes ; and Kerr has consequently made no measure- 

 ments, but has only estimated the amount of rotation in the 

 different positions and determined its direction. Some two 

 years ago I repeated Kerr's experiments. My experiments 

 completely confirmed all Dr. Kerr's statements ; with the 

 one exception that, upon reflection at the surface of a pole, 

 I obtained also a reversal in the direction of rotation at an 

 incidence of about 80°, when the plane of polarization of the 

 incident light and the plane of incidence were at right angles 

 to each other. This reversal seems to have escaped Dr. 

 Kerr. I have thus succeeded in bringing all the phenomena 

 of reflection at the side faces as well as at the poles of the 

 magnets under one point of view, from which possibly an 

 explanation of the same may result. 



That a rotation of the plane of polarization takes place at 

 perpendicular incidence upon the pole-surface of an electro- 

 magnet has been further confirmed by Gordon*. He obtained 

 a rotation of 2b v 45" upon reversal of the magnetizing current. 

 Then Hallf has shown that with cobalt and nickel also, 

 when they form the pole of a magnet, a rotation of the plane 

 of polarization in the direction opposite to that of the magne- 

 tizing current takes place. It may be further mentioned that 

 Hall has also examined whether the plane of polarization of 

 the light which traverses a very thin transparent film of 

 nickel is rotated. The transparent film was prepared accor- 

 ding to the method given by Wright, by deflagration of a 

 nickel electrode in a Greissler tube. Hall examined two nickel 

 mirrors, but was not able to perceive any rotation, and remarks 

 that " both these experiments on direct transmission have 

 been, we may say, quite negative." 



Lastly, we must mention also that Fitzgerald has endea- 

 voured to give a theoretical explanation of the rotation produced 

 by magnets. I shall further on briefly state Fitzgerald's 

 theory, and show that it is not in agreement with the obser- 

 vations. 



I have now recently measured the electromagnetic rotation 

 of the plane of polarization upon reflection from iron, cobalt, 

 and nickel, and completely confirmed my previous experiments 



* Physical Treatise on Electricity and Magnetism, ii. p. 261.^ 

 t Phil. Mag. 1881, oth series, xii. p. 171. 



