Circular polariscopy micrograph of urea crystals on glass. Image acquired on a Zeiss Axiophot microscope with a 2.5x objective.
The basic advantage of a circular polariscope over a plane polariscope is that in a circular polariscope setup we only get the isochromatics and not the isoclinics. This eliminates the problem of differentiating between the isoclinics and the isochromatics. http://en.wikipedia.org/wiki/Photoelasticity
Note: Isoclinics are the loci of the points in the specimen along which the principal stresses are in the same direction. Isochromatics are the the lines which join the points with equal maximum shear stress magnitude.