( 736 ) 



The first vector lias again the same dii'ection as the electric force 

 in plane waves whose normal coincides with the direction Ave are 

 considering; its components along the axes of symmetry are therefore 



2 T' V' cos {> ■' 2 T' V cos» ' 2 T^ V' cos » 



Now 5Sa:' , is of the order P and tiie integration is to be effected 

 over a solid angle of the order /. Tims, confining onrselves to 

 directions in a single plane passing through OP, we may regard as 

 constants the quantities a, V and cos i>, assigning to them the values 

 they take in the plane F. 



We determine an arbitrary direction in the plane passing through 

 OP by the angle S which it makes with OP and its azimuth ■/ 

 with respect to a fixed plane also passing through OP. Then 

 dia = sin S d^ d'f^. 



Now we have for the direction considered 



S''' 



the integral being extended to the portion inside t of a plane G, 

 passing through P jierpentlicularly to that direction. If q is the 

 normal drawn from towards G, we have 



q =: r cos 5, 

 \dq\ ^ r sin S d^, 



givmg 

 and 



1 



dio =: — \dq\ d-/^, 



«' = 2^Jt^,-^'''/*''I'''!- 



Here for each particular value of /> tbe latter integral is to be 

 extended to all values that can be given to S or q. Further 



Tift,. \dq\ =. f\dq\ fe' da^C C(Sl'da\dq\, 



whereas 



do \dq\ '■ 



is the element of volume of an infinitely small cylinder whose upper 

 and lower base are formed respectively by one of the surface 

 elements of G and of an infinitely' near plane G', the generating 

 lines of the cylinder being perpendicular to G. It follows from 

 this that 



