198 



HENKY A. ROWLAND 



It is seen that all these effects are such as would happen were the 

 electric current to be rotated in a fixed direction with respect to the 

 lines of magnetic force, and to an amount depending only on the mag- 

 netic force and not on the current. This fact seems to point imme- 

 diately to that other very important case of rotation, namely the rota- 

 tion of the plane of polarization of light. For, by Maxwell's theory, 

 light is an electrical phenomenon, and consists of waves of electrical 

 displacement, the currents of displacement being at right angles to the 

 direction of propagation of the light. If the action we are now con- 

 sidering takes place in dielectrics, which point Mr. Hall is now investi- 

 gating, the rotation of the plane of polarization of light is explained. 



I give the following very imperfect theory at this stage of the paper, 

 hoping to finally give a more perfect one either in this paper or a 

 later one. 



North Pole above. 



North Pole below. 



Let $ be the intensity of the magnetic field, and let E be the original 

 electromotive force at any point, and let c be a constant for the given 

 medium. Then the new electromotive force E' will be 



and the final electromotive force will be rotated through an angle which 

 will be very nearly equal to c>. As the wave progresses through the 

 medium, each time it (the electromotive force) is reversed it will be 

 rotated through this angle; so that the total rotation will be this quan- 

 tity multiplied by the number of waves. If ^ is the wave-length in air, 

 and i is the index of refraction, and c is the length of medium, then 



the number of waves will be and the total rotation 



The direction of rotation is the same in diamagnetic and ferromag- 

 netic bodies as we find by experiment, being different in the two; for it 



