Summary: This paper describes a development for the universal stage of the extinction curve method for determining the orientation of the optical indicatrix in biaxial crystals. The extinction curve may be defined as the locus of the poles of the extinction directions when the crystal is rotated between crossed polarizers about an arbitrary but fixed axis perpendicular to the microscope axis.

When two or more extinction curves, or parts of them, are plotted on a stereographic projection, their intersections give the directions of the axes of the indicatrix, of which at least two are directly accessible. The main application of this new orthoscopic method is to provide a means of refining the orientation of the indicatrix obtained by this or any other method. The accuracy that can be obtained for minerals of low birefringence such as the felspars is considerably greater than with the conventional orthoscopic method.

A complete account is given of the details of the plotting procedure.