Utilizing the Biot-Fresnel construction in reverse, the magnitude and sign of the optic axial angle of a biaxial crystal are determinable from the stereogram of the extinction curves. Refinements in the determination of the extinction curves include the use of conoscopic observations, and of crystal cylinders instead of irregular fragments or entire crystals. Direct 2V measurements on some of the material investigated by this method show that this indirect and graphical approach permits the location of each optic axis to within ¼° of its true position.