- 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.

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