Generalised theory of interference and its applications. Part III. Interference figures in transparent crystals

Abstract

The geometric method of specifying states of polarisation by points on the Poincare sphere is used for giving a unified and physically intelligible approach to the interference phenomena displayed by crystalline plates in parallel or convergent light-under general conditions when the polarising and analysing states are linear, circular or elliptic in form. Examples discussed are the spiral figures exhibited in convergent light (a) by the basal section quartz between a circular polariser and linear analyser (or vice-versa) and (b) by two superposed basal sections of left- and right- quartz between crossed nicols. The Airy's spirals observed in the latter case are interpreted as the 'isogyres' of the optically inactive plate to which the combination is equivalent.