ƒƒ 



( --57 ) 



(§\:da\d, 



is the volume-integral of (S> , taken over tlie whole volume of t. 



We have already written for this i)itegral Ql' r, denoting bv ^'y a 

 certain mean value (§ 5). Hence, the fi)'i>f. part of the components 

 of the electric force resulting from tiie integration with respect to 

 tlie directions outside the cone A', becomes 



•" 2Th'J V'cos» ^ ■' 22'VJ V'cos» ^ 







2t 



= 1 -^- — d-/ (9) 



22'Vj V\'os» ^ ' 







The second part results from a similar integration of the second 

 vector 



Now it will appear further on, that we can onl}' determine the 

 exact value of those terms, in which tlie denominator contains the 

 first power of )■. We may therefore contine ourselves to such terms 

 in the whole course of our calculations. The cone over which we 

 have to integrate being of the order l/r, we may omit terms, which 

 already contain r in tiie denominator. It will be evident thei;efore 

 that instead of 



and 



Ave may take the values of these quantities, corresponding to that 

 wave-normal, in the meridian plane passing through OP, which lies 

 at the same time in the plane F. If ch' is a line-element of that 

 wave-normal, we have to consider the integrals 



I — -- — dz and I — — — dz' 

 J dz'^ J dz'^ 



which evidently are zero, yr being zei'O at the boundary of t. It 



appears in this way that we need not at all consider the second vector. 



^ 9. We now proceed to elfect the integration of the right hand 



members of the equations (8) so far as is necessary in order to 



obtain the terms with -. We shall lake the real parts of all expres- 



