Diffraction by a Rectangular Aperture

The diffraction pattern produced at infinity by a rectangular aperture in parallel light has been already investigated (Lord Raleigh's Scientific Papers, Vol. III, p. 80, or R.W. Wood's Physical Optics, 1911 Edn., p. 195). In that investigation, however, an assumption is made, namely, that the effective portion of the wave is confined to a very small portion at the centre and consequently the effect due to obliquity is neglected. As will be seen below, this problem can be solved more generally.