A Variable Phase-Contrast System for Microscopy

Abstract

PHASE-CONTRAST effects occur when the paths of the dioptric and diffracted rays between object and image differ sufficiently to produce some degree of interference. Were it possible to polarize the diffracted rays and dioptric rays at right angles to one another, a path difference could be introduced by passing them through a birefringent screen, the refractive index of which, being different for two perpendicularly polarized rays, would produce the effect of different optical thicknesses accordingly. Such a screen might be a mica plate causing a fixed difference of a quarter of a wave-length, or a compensator of the Babinet–Soleil type producing a variable difference.