PLA^E WAVES. 613 



In like manner, the equations analogous to (13) give, 





We see already that the electrical perturbation is also in the 

 plane of the wave, and perpendicular to the electromagnetic dis- 

 turbance, for if we have Y = (that is if the electromagnetic 

 disturbance is parallel to the x axis), we shall have u' = 0, and the 

 electrical disturbance will be parallel to the y axis. 



For a non-conducting medium, equation (12) and the analogous 

 relations to the other co-ordinates give 



- , 

 < 2 o) 



from this follows 



1J7 2 "" 



= _ _ 



a/*~K/*&*' a/ 2 ~ ~w' 



7) 2 H 



The integral of these latter equations is 

 (22) H 



an expression in which A and B are functions of z. This quantity 

 H is therefore constant, or varies proportionally with the time. 

 In any case it does not intervene in the propagation of periodical 

 phenomena, 



