5. Summary

A sharp laminar edge appears as a dark line bordered by asymmetric fringes when examined in direct illumination under a microscope. The theories of Lummer and Rieche as also Sur, based on the assumption of an abrupt change of phase in the emerging wavefront, fail to account for this asymmetry. The Becke phenomenon—the appearance of a bright line close to the dark line corresponding to the “edge” when the microscope is put out of focus—merely represents the strongly asymmetric character of the pattern in the out-of-focus position as well. The simple geometrical explanation of the phenomenon as usually given is shown to be inadequate as it fails with normal parallel light. It is postulated that the phase change caused by the edge is not abrupt but is gradual and extends over a part of the emerging wavefront. The theory of microscopic vision as applied to such a deformed wavefront shows the pattern as observed in focus to be asymmetric. The same postulate also explains the asymmetry in the out-of-focus position—the Becke Phenomenon. Thus the asymmetric microscopic appearances both in and out-of-focus are explained as due to asymmetric diffraction effects at the edge. An estimate of the smallest path retardation that may be detected under direct illumination is also made.