Transparent Bodies by the action of Magnetism. 509 



Experiments on Heavy Glass. Thickness 40 millims. 



Experiments on Bisulphide of Carbon. Thickness 44 millims. 



The law just established explains a circumstance which I had 

 occasion to notice in my former researches ; when the ray of 

 light is parallel to the direction of magnetic action, the adjust- 

 ment of the apparatus may be deranged to a sensible extent, and 

 consequently the angle « made to vary from 3 to 4 degrees, 

 without altering the rotation of the plane of polarization to any 

 appreciable extent. 



If we adopt the theoretical notions of Fresnel with respect to 

 the rotation of the plane of polarization, we must imagine the 

 polarized ray, falling perpendicularly upon the transparent sub- 

 stance which is submitted to the influence of magnetism, trans- 

 formed into two rays circularly polarized, and in opposite direc- 

 tions, and propagated with unequal velocities. If the velocities 

 of propagation be represented by v and v 1 , it follows from the 



law above enunciated, that the expression - t varies propor- 

 tionally to the cosine of the angle enclosed between the direction 

 of the luminous rays and that of magnetic action. 



I shall confine myself to this theoretical remark, and for the 

 present abstain from any further reflections on the law I have 

 just established. 



