The propagation of light in absorbing biaxial crystals

Abstract

The phenomena shown in convergent light by a section plate of iolite cut normal to an optic axis are discussed and illustrated. Of particular interest among the photographs reproduced are those taken (a) between circular polariser and crossed circular analyser, demonstrating the existence of the two singular axes ( a right-circular vibration alone being propagated unchanged along one singular axis, and a left-circular vibration alone along the other. (b) with a circular polariser alone, confirming the theoretical prediction in part I that the singular axis where the incident circular vibration can be propagated unchanged should appear darker than the other; the state of the elliptic vibration emerging from the brighter singular axis was also qualitatively in accord with theory. Examination between elliptic polariser and crossed elliptic analyser demonstrated that along any general direction in an (optically inactive) absorbing biaxial crystal, two similarly rotating elliptic vibrations can be propagated unchanged-the major axes being crossed but not coincident with the principal planes of refraction.