Integrating by parts in (II), we get J e = ikr d7 pc-11 / d (e r - ay= **rJ** Z d y - r / 1 dY, in which the integrated terms at the limits vanish, Z being finite only within the region T.

' S 'e ' ',l0"0 t !**rJ**,GheJt,, seveld ?

Each of the four co-ordinates, n', x', y are functions of %, **rj**, x, y; and if it be assumed that the field of view and the aperture be infinitely small, then, n, x, y are of the same order of infinitesimals; consequently by expanding ', ii', x', y in ascending powers of E, rt, x, y, series are obtained in which it is only necessary to consider the lowest powers.