EXPLANATION OF ROTATORY POLARIZATION. 579 



and they reconstitute a rectilinearly polarized ray. The difference 

 of time that these two rays of velocities V and V" take to traverse 

 a thickness e of the medium is 



[ I 



-"v 7 



If T is the period of the ray in air, interference occurs on 

 emergence between two rays whose difference of phase on entering 

 was 



As these rays have the periods T' and T" in the medium, the 

 numbers m and m" of oscillations which they have made are 



=- and m"T" 



which gives 



The difference of phase on emergence is then 



/ , A f T / i i\ i /i i\-| 



(2) 27r( m" - m +- ) = 27rH ( + - ) -- ( + - ) 



\ T/ |_ V '\T" T/ V'\T' T/J' 



and the rotation of the plane of polarization 



(3) e= 



If V and X denote the velocity, and the wave length of the 

 ray in air, we may write 



(4) ^ r 



P P 2 



