-55- 301 



fornnilas; 



^n ~ 2m(b+c) If n = 2m 



= 2m(b+c) + 2b if n = 2m + 1 



U^ = 2m(b+c) if n = 2m 



= 2m(b+c) + 2c if n = 2m + 1 



The signs of the Images are given in the diagramo 

 Images with respect to the sphere will be 

 neglected. 



The first term in the expansion of f in powers 

 of a can be obtained by finding the potential at of 

 the infinite series of images P^, Q-, Pg, Q^* etc. Some 

 simple algebraic transformations lead to the formula 



^o = HTc^^^^^^^ ■ ^°S 2) 



where x = ° " t^ and 



f(x) = -^ --^ + -^ 

 l-x'^ Q-x"^ 25-x'^ 



It is to be noted that 



!£« = i£_^ [f(x) + xf'(x)] 



9h (b+c)"^ 



To facilitate comparison with previous results. 



