408 Mr. R. T. Glazebrook on the Molecular 



+ VV=0, . (23) 



B 1 



These or the original equations (19), if we put — = — jj, and 



X the impressed electromotive force = 0, and substitute 



2llG 

 for , 87rG, agree with Mr. Fitzgerald's equation. If 



we consider that the magnetic force is parallel to z and that 

 our wave is travelling in that direction, Mr. Fitzgerald has 

 shown that the wave must consist of two circularly polarized 

 waves travelling respectively with velocities v 1 and v 2 , where 



1 27rCy 



and 



^ _ _JL 27rC 1 7 



So that the rotation of the plane of polarization produced by 

 a length I of the substance is 



AttCMi 2 / ^dt\ J 



-Y^-V- X dx) l > 



i being the refractive index, V and X the velocity and wave- 

 length in air, and M the magnetic force. The rotation of unit 



A (~1 3 J 



length for unit magnetic force will be ^ 2 if we neglect — . 



V pA- a A, 



According to Mr. J. E. H. Gordon (Phil. Trans. 1877), the 

 value of this quantity for the thallium-ray in carbon disulphide 

 is 3-047 x 10- 5 about. Also 



\ =5-349xl0- 5 , 



V = 3xl0 10 , 



V ^=75; 



p is the density of the medium — of course not the dielectric, 

 but the aether. According to Maxwell (" Physical Lines of 

 Force," Phil. Mag. April 1861), this is of the same order as 



fi, being equal to ~ if the vortices are circular, and fju is 



nearly unity. Thus 



- 3- 047 xl0~ 5 x 75 



C== 4tt? ; 



and is of order 1Q _ 5 Qr 10 _ 6> 



