1881.] the Electromagnetic Theory of Light. 157 



Similarly 



[K^dx K^zj'dt (5) ' 



. f 1 dg 1 dh) da ... 



^[^dz-K-Jyrdt (6) - 



If u, v, w are the components of the current in the dielectric 

 medium 



. dc db 



*"»"-*-« (7) - 



etc., 



V 



Then 



d 2 f_ du 



de~dl 



1 d (dc db 

 47r//, dt\dy dz 



1 fd 2 / d 2 f[ I d 2 g 1 d"A 



]_1 



Put 



pK t ==-.ete. 



a 



Then the general equations of electric displacement are 



eft 2 " \dy 2 + dz 2 ) • dxdy ° dzdx W ' 



d Ll-tf( d Ll+ d l9\ _:ji_^L ~. JlL (q\ 



dt~ \dz^ dxV dydz dxdy w * 



eft 2 ~° \dx* + dyV a dzdx dydz l U; ' 



a, 6, c are the principal velocities in the crystal and must not 

 be confused with a, b, c the components of magnetic induction. 

 Maxwell uses the same symbols for both. 



Now suppose we are considering a plane wave whose equation 

 at time t is Ix + my + nz— Vt = 0, and let \, fj,, v be the direc- 

 tion cosines off, g, h. 



