1. Nye, J. F., in Physical Properties of Crystals (Oxford Univ. Press, London, 1964).
2. Wooster, W. A., in Crystal Physics (Cambridge Univ. Press, Cambridge, 1949).
3. Shubnikov, A. V., in Principles of Optical Crystallography (Consultants Bureau Enterprises, Inc., New York, 1960).
4. Szivessy, G., in Handbuch der Physik, 20 (Springer, Berlin, 1928).
5. Tatarskii, V. B., Kristallografiya, 9, 451 (1964). [Sov. Phys.-Crystallog., 9, 377 (1964).]