The effect of dispersion on the interference figures of crystals

Abstract

Crystallographers are familiar with the fact that the colour-lines in the figures shown by crystalsections between crossed nicols in the polarisation microscope often deviate to a notable extent from the so-called "isochromatic lines" discussed in the textbooks of physical optics. The difference is attributed to the dispersion of the optic axes, and its character enables the type of dispersion to be determined (for details, see for example Tutton's "Crystallography"). Though the phenomenon is thus well known and of considerable importance in practical work with crystals, I have found no record of any attempt to determine theoretically the form of the true isochromatic lines for any specified dispersion. Possibly it has been thought that the task would be too complicated and laborious to be worth undertaking. It may, therefore, be worth while to point out a fairly simple way of approaching the matter.