528 The Electromagnetic Theory of Light [CH. xvm 



propagation in a dielectric to apply, provided we now replace K' by a 

 quantity K given by (cf. equation (574)) 



m 



or, separating real and imaginary parts, 



Using this value for K instead of that previously used ( 602), it is found 

 that the experiments referred to in 605 can be made to agree well with 



theory by giving a suitable value to ^. The quantities m and e are 



already known, but N is not. Thus these experiments give us the value of 

 N. Schuster* finds that N is comparable with the number of molecules per 

 unit volume in the metal. 



On substituting numerical values, it is found that for visible light the 



(wi \ 2 

 ~-\Tz P] * n ^ e Denominator in equation (591) is large in comparison 



with the term p 2 r 2 , instead of being negligible, as our former analysis 

 assumed it to be. There is now no difficulty in understanding why the 

 former analysis led to erroneous results. We see that for waves having the 

 frequency of light-waves, the resistance of the conductor, regarded as an 

 agency checking the free tiow of currents, is insignificant in comparison with 

 the inertia of the electrons. 



609. A further phenomenon has to be taken into account before the 

 theory gives a completely satisfactory account of metallic reflexion. We 

 have already considered the motion of the free electrons in the metal, which 

 carry the current : it is necessary also to consider the motion of the electrons 

 inside the molecules, which perform oscillations under the influence of the 

 waves of light. This part of the subject is beyond the scope of the present 

 work : the reader who studies the matter will find that this phenomenon is 

 capable of removing the difficulties which remain in the way of a satisfactory 

 electromagnetic explanation of metallic reflexion. 



CRYSTALLINE DIELECTRIC MEDIA. 



610. In all media in which light can be propagated, the magnetic per- 

 meability //, may be supposed equal to unity. Let us consider the propagation 

 of light, on the electromagnetic theory, in a crystalline medium in which the 

 ratio of the polarisation to the electric force is different in different directions. 



* Phil. Mag. 7, p. 151. 



