( 734 ) 



2'"^ If P lies outside these planes 

 a. For positive values of z 



iit€,'x 'tv, 

 2 T' F' 

 b. For negative values of s' 



inz' 

 _ , i JT iili T '■ tT, 



ViV = e «to. 



9 T' IT'' 



The Z'-component of the electric force consists of two parts, one 

 of which corresponds to Sy , the other to S',/. Having already omitted 

 the }"'-corap0nent, we shall take only the first part, €\'i, of the 

 2r' -component and add the second part 'S-'2 to the F'-component 

 afterwards. Then by tiie third equation of (4) 



It appears tVoni this that outside the tangent planes @;' and Sj- 



are connected with each other in the way they always are in the 



case of plane waves. We may therefore represent the electric force 



g , 



by if & is the angle between this vector and the corresponding 



cos & 



dielectric displacement in a system of plane waves. Finally we have 



the following equations for the components of the electric force 



along the axes of symmetry 



g, 1= e ^ dvi, 



2T'V'cos» 



_ i.^€'r -^'i^ } (8) 



^•» r= e aw, , 



2T'F»ro«^ ; 



where «, ^ and y are the direction cosines of the electric force with 

 respect to the axes of symmetry. For negative values of z' the same 

 formulae will apply, provided that z' be replaced by — z'. 



§ 7. In the preceding equations the symbols Sr, (Jy and S'^ were 

 used for the (small) electric force, produced by a single element of 

 the integral (1) in a point P, lying at a given distance r from the 

 origin 0. We have seen that the expression for this small electric 

 force took a diiferent form according to the point P lying or iiot 



