322 Dr. J. Kerr on Rotation of the Plane of Polarization 



heavy glass &c. But I think that the following facts bear 

 with equal or greater force the contrary way. 



(3) The enormous differences (in relation to magnetic force) 

 between iron and steel on the one side, and Faraday's transpa- 

 rent diamagnetics on the other. 



(4) The effects obtained by Verdet in his application of 

 Faraday's magneto-optic method to the salts of iron. The 

 strongest instance is that of the perchloride. A dense solution 

 of perchloride of iron in wood-spirit gives a rotation of light 

 contrary to, and nearly twice as great as, that given by heavy 

 glass under the same conditions. 



(5) The known laws of metallic reflection, particularly the 

 fact that silver, zinc, steel, and other metals are distinguished 

 from each other in a perfectly definite manner as reflectors, 

 each metal having specific relations to the principal component 

 vibrations (perpendicular and parallel to the plane of incidence) 

 with reference both to change of phase and change of am- 

 plitude. 



The preceding facts were sufficient to suggest a plan of pro- 

 cedure, as well as to give me a strong expectation of success. 



During the month of August last, in the course of some 

 careful experiments in the direction thus indicated, I obtained 

 several interesting results which appeared conclusive. Soon 

 afterwards I gave a description of the experiments before the 

 British Association. Since that time I have made one or two 

 additional observations, and have got rid of a serious error into 

 which I had fallen in my first view of the facts. In this paper 

 I propose to give an account of all my principal experiments 

 and views upon the subject. And first, for future reference, 

 I shall lay down the sum of the results in one sentence. 



The new Fact. 



2. When plane-polarized light is reflected regularly from 

 either pole of an electromagnet of iron, the plane of polariza- 

 tion is turned through a sensible angle in a direction contrary 

 to the nominal direction of the magnetizing current ; so that 

 a true south pole of polished iron, acting as a reflector, turns 

 the plane of polarization righthandedly. 



Apparatus and Arrangements. 



3. The Magnet. — This is an upright horseshoe electro- 

 magnet, and a very good instrument, I think, of its size. Only 

 one limb of the horseshoe is used at a time, the current being 

 sent through one of the coils, and the observations being made 

 on the enclosed core. Each of the cores is a solid cylindrical 

 bolt of soft iron, 10 inches long and 2 inches in diameter, 

 which is therefore the diameter of each polar surface. Each 



