586 A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



The experiments of Knoblauch * on electric induction through crystals seem 

 to shew that a, b and c may be different. 



The inequality, however, of X, p., v is so small that great magnetic forces 

 are required to indicate their difference, and the differences do not seem of 

 sufficient magnitude to account for the double refraction of the crystals. 



On the other hand, experiments on electric induction are liable to error 

 on account of minute flaws, or portions of conducting matter in the crystal. 



Further experiments on the magnetic and dielectric properties of crystals 

 are required before we can decide whether the relation of these bodies to mag- 

 netic and electric forces is the same, when these forces are permanent as when 

 they are alternating with the rapidity of the vibrations of light. 



Relation between Electric Resistance and Transparency. 



(106) If the medium, instead of being a perfect insulator, is a conductor 

 whose resistance per unit of volume is p, then there will be not only electric 

 displacements, but true currents of conduction in which electrical energy is 

 transformed into heat, and the undulation is thereby weakened. To determine 

 the coefficient of absorption, let us investigate the propagation along the axis 

 of x of the transverse disturbance G. 



By the former equations 



by (A), 



dtf~ 



If G is of the form 



G = .->** COB (qx + nt) (96), 



we find that 



27^n_2^F 



......----.................... I ' i I. 



p q pi 



where V is the velocity of light in air, and i is the index of refraction. The 

 proportion of incident light transmitted through the thickness x is 



'* (98)- 



* Philosophical Magazine, 1852. 



