ERRATA. 



SECTION A. 



p. 40, par. 13, 1. 12 from beginning of par. 



Tor (00) = (22) = 



read (00) = (22) = (02) = (20) = 



p. 46, par. 25, line5 9-12 from beginning of par. 



Tor Hence we easily . . . . c being the curvature 

 read Hence ire find A, = - Z-^ {a^&zfi'Z'"). 



Expanding this determinant with the aid of Eresnel's formulae and the expressions for Z, Z', etc., 

 given in par. 52, we find A, = Z~*(r'a^ 



