ON PHYSICAL LINES OF FORCE. 



513 



X is the wave-length of the undulation in the substance. Now if A be the 

 wave-length for the same ray in air, and i the index of refraction of that ray in 

 the body, 



* = J (162). 



Also v, the velocity of light in the substance, is related to V, the velocity of 

 light in air, by the equation 



o = J (163). 



Hence if z be the thickness of the substance through which the ray passes, the 

 angle through which the plane of polarization will be turned will be in degrees, 



a 180 

 = qz 



TT 



(164); 



or, by what we have now calculated, 



(165). 



In this expression all the quantities are known by experiment except r, the 

 radius of the vortices in the body, and s, the density of the luminiferous 

 medium in air. 



The experiments of M. Verdet* supply all that is wanted except the deter- 

 mination of Z in absolute measure ; and this would also be known for all his 

 experiments, if the value of the galvanometer deflection for a semi-rotation of 

 the testing bobbin in a known magnetic field, such as that due to terrestrial 

 magnetism at Paris, were once for all determined. 



* Annales de Chimie et de P/tyrique, s6r. 3, Vol. XLI. p. 370. 



VOL. I. 



65 



