624 GENERAL THEORIES. 



As the functions ^ and 6 play no part in the periodical phenomena 

 if the medium is not a conductor, we shall have 



(33) 



a/ 2 a/ 



642. PROPAGATION OF A PLANE WAVE. Let us consider a 

 plane wave perpendicular to the z axis. We need only take into 

 account the component Z of the magnetic force parallel to this axis ; 

 in this case, if the field is constant, equations (33) reduce to 



/a 2 F V\ a 2 F 

 Kji ( + yZ )= , 



V a/ 2 a/ 



(34) 



Equations (2) and (12) of Arts. 567 and 572 give 



Y i 

 = + -= --- r-, 



ax i a 2 G 



From this follows 



