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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, theirdoi:10.1180/minmag.1958.031.241.05 fatcat:ikbvhqtz4zbwxedz7zlb66nzpm