578 OPTICAL PHENOMENA. 



and in that direction, the ray is still polarized on emergence, but 

 the plane of polarization has turned through a certain angle in 

 the direction of the external currents of the coil that is, towards 

 the left of an observer who receives the ray. The rotation is 

 towards the right, on the contrary, if the light travels in the 

 opposite direction. 



Hence, on the hypothesis of circular vibrations, it is necessary 

 that during its passage through an active medium, one of the rays 

 should have got an advance of phase over the other equal to 

 twice the angle of rotation of the plane of polarization. For flint 

 glass and diamagnetic substances, the left circular rays gets in 

 advance when the propagation is in the direction of the lines of 

 force ; the reverse is the case for magnetic media. 



In any case the conception of two inverse circular rays which 

 travel through a medium with different velocities is not a mere 

 hypothesis; experiment shows, in fact, .that the refractive index 

 of a given circular ray differs according as it traverses the sub- 

 stance submitted to the action of magnetism in the direction of 

 the lines of force, or in the contrary direction. 



M. Cornu has shown, moreover, that in both cases the variations 

 in the indices are equal and of opposite sign to that which the ray 

 would have in the same medium when not under the action of 

 magnetism. 



598. This difference of phase may be variously explained ; it 

 may be assumed that the period is the same in both rays, but 

 the velocity of propagation is different ; or, that, while the velocity 

 of propagation is the same, the period is no longer equal for the 

 two rays, and is different for each of them from what it is in the 

 external medium ; or, lastly and what is, perhaps, most probable, 

 having regard to the ordinary laws of dispersion that the period 

 is modified as well as the velocity of propagation. 



It is in general impossible to conceive a permanent vibratory 

 state with a period different from that of the cause which produces 

 it; but, in the present case, the difficulty does not seem to exist, 

 if we assume that the medium which transmits light itself possesses 

 a rotatory motion in a determinate direction ; the period of the 

 relative motion would be the same for both rays, and the same as 

 in the external medium ; the advance of phase would be due solely 

 to th'e difference of the absolute motion, and precisely equal to the 

 half of this difference. 



In all cases when the two circular rays emerge from the medium, 

 they assume the same period and the same velocity of propagation, 



