626 GENERAL THEORIES. 



The equations (37) give 

 y Z^ 2 ?ryZ 



m 



2/A 47T 2/xA 2 ' 



p i / mZK i /' 



~~jn / V' +y -9?"my> 



VK?'\ 8,r VK/x'V 4,r* 



As the term in y is very small, we may take the square root as 

 an approximation, and we get finally : 



87T 2 



We draw from this the following conclusions : 



i st. When a polarized ray travels along the direction of a 

 magnetic line of force, the plane of polarization is rotated in a 

 direction which depends on the sign of y. 



This is the phenomenon discovered by Faraday, with the 

 inversion for magnetic bodies observed by Verdet. 



2nd. The velocity of propagation is increased by the electro- 

 magnetic action ; but this effect is no doubt too feeble to be 

 made evident. 



If A and V are the wave length and the velocity of the 

 ray in vacuum, when it is withdrawn from the magnetic action, 

 and n the refractive index on passing into the medium in question, 

 we have A = A and V = V . The time which this ray takes in 



traversing a thickness e of the medium is = ; the rotation 6 



v v o 

 which the plane of polarization experiences is 



