Detection of dichroism with the microscope

TWO simple, sensitive, visual methods are described for the detection of dichroism in microscopic objects. 1. The object is oriented at +/- 45 degrees between crossed polars, and either the polarizer or the analyser is rotated through an equal angle alternately clockwise and anti-clockwise. A dichroic object will brighten as the polar is rotated in one direction and darken as the polar is rotated in the other. If the optic axis of the object lies north-east to south-west and the transmission axes of polarizer and anaiyser respectively east-west and north-south, a positively dichroic object will darken as the polarizer is rotated anti-clockwise, or the analyser is rotated clockwise. 2. A formally similar method, which avoids the need to rotate either polar, is to orient the object at +/- 45 degrees between crossed polars, and insert a Nakamura plate between the polars in an optical plane conjugate with the specimen. Dichroism is indicated by inequality of the apparent brightness of the specimen on either side of the dividing line of the plate. Both methods are more sensitive than visual techniques traditionally used in microscopy. The sign of the dichroism of polished smears of dye particles is discussed. It is suggested that polishing in the dry state orients individual dye molecules, while polishing of a damp smear orients dye molecule aggregates. The relation of dichroism to the colour of stained histological structures, as seen either between crossed polars or with bright-field illumination, is discussed, and the elementary optical theory of dichroism is summarized.