1891.] of Light at the Surface of a Magnetised Medium. 77 



series, the light was reflected from a polished plate of soft iron laid 

 upon the poles of a horseshoe electromagnet, so that the lines of 

 magnetic force were parallel to the reflecting surface. In both 

 series of experiments, it was found that, when the circuit was closed, 

 so that the reflector became magnetised, the reflected light exhibited 

 certain peculiarities, which disappeared when the current was off. It 

 was also found that the effects of magnetisation were, in most cases, 

 reversed when the direction of the magnetising current was reversed ; 

 that is to say, if the intensity of the reflected light was strengthened 

 by a right-handed current, it was weakened by a left-handed one. 



In these experiments, a metallic reflector was employed, and con- 

 sequently the results were complicated by the influence of metallic 

 reflection ; it therefore seems hopeless to attempt to give a complete 

 theoretical explanation of these experiments until a satisfactory 

 electromagnetic theory of metallic reflection has been discovered ; and 

 this, I believe, has not yet been done. 



There are, however, several non-metallic substances (such as strong 

 solutions of certain chemical compounds of iron), which are capable, 

 when magnetised, of producing an effect upon light ; and the theore- 

 tical explanation of the magnetic action of such substances upon 

 light is accordingly free from the difficulties surrounding metallic 

 reflection. I have accordingly, in the present paper, attempted to 

 develop a theory which is applicable to such media. 



The theory, which is due to Professor Rowland, is founded upon the 

 following considerations : 



It was proved by Hall* that, when a current passes through a con- 

 ductor which is placed in a strong magnetic field, an. electromotive 

 force is produced, whose intensity is proportional to the product of 

 the current and the magnetic force, and whose direction is at right 

 angles to the plane containing the current and the magnetic force. 

 Professor Bowlandf has assumed that this result holds good in a 

 dielectric which is under the action of a strong magnetic force ; 

 accordingly the general equations of electromotive force become 



(1), 



where , /3, 7 are the components of the external magnetic force, and 

 C is a constant which depends upon the physical constitution of the 

 medium. 



Writing p\ = C, &c., it follows that, if the medium is isoti'opic, 

 the equations of electric displacement are of the form. 



* < Phil. Mag./ March, 1880. 

 t Ibid., April, 1881. 



G 2 



