An analysis of the molecular effusive flow pattern through a narrow rectangular slit orifice is given in terms of the arrival flux on a parallel detector plane. It is shown that the advantage of having an orifice in the form of a long narrow slit in terms of a pronounced peaked incident beam at the detector plane is only achieved at detector plane separations less than the length of the orifice slit. At separations of the detector plane larger than twice the slit length the arrival flux distribution asymptotically approaches that of an infinitesimal orifice source of arbitrary shape. This is in contrast to recent calculations of flux density versus off‐axis detector position reported by Perram and Dorko in Rev. Sci. Instrum.54,275 (1983).