82 Lord Rayleigh on the 



observation*, require us to suppose that in a doubly- refracting 

 crystal the rigidity of the medium varies with the direction of 

 the strain; while, in order to explain the facts relating to the 

 intensities of reflected light, we have to make the inconsistent 

 assumption that the rigidity does not vary in passing from 

 one medium to another. A further discussion of this subject 

 will be found in papers published in the Philosophical Maga- 

 zine during the year 1871. 



If the dielectric medium be endowed with sensible conduc- 

 tivity, the electric vibrations will be damped; that is to say, 

 the light will undergo absorption, with a rapidity which Max- 

 well has calculated. By supposing the conductivity to be so 

 great that practically complete absorption takes place within 

 a distance comparable with the wave-length, we may obtain a 

 theory of metallic reflection which is not without interest, 

 although the phenomena of abnormal dispersion show that it 

 cannot be regarded as complete. 



For an isotropic medium at rest we have the equations 

 (Maxwell's ' Electricity and Magnetism/ §§ 591,598,607, 

 610, 611) 



u= P + dt> &c -> 00 



/=£ P ><^ CO 



i>=CP,&c. ; (3) 



p _ dF dV k /A . 



asB ^7- 7fe' &c -> ( 5 ) 



dE_dG 



dy dz 



a=/nu, &c, (6) 



4 ™=t-f> &c - ; ( 7 ) 



in which /, g, li are the electric displacements, p, q, r the 

 currents of conduction, u, v, w the total currents, P, Q, R the 

 components of electromotive force, K the specific inductive 

 capacity, C the conductivity, u, /3, 7 the components of mag- 

 netic force, a, b, c the components of magnetization, /u the 

 magnetic capacity, F, Gr, H the components of electrokinetic 

 momentum, and M* the electric potential. 



* Glazebrook, Phil. Trans. 1879. 



