gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1964.033.266.12 A New Universal Stage Extinction Technique: The Determination of 2V When Neither Optic Axis Is Directly Observable Tocher F. E. B.Sc., Ph.D. Department of Geology and Mineralogy, University of Aberdeen 9 1964 33 266 1038 1054 Copyright © 1964, The Mineralogical Society 1964 Summary

A new universal stage extinction technique is presented. It is based on the special case of coplanar and conical extinction curves where an optic axis lies on the wave-normal locus. It may be used in favourable circumstances to determine 2V when neither optic axis is directly observable.