1) G. W. Gray: Molecular Structure and the Pro-perties of Liquid Crystals (Academic Press, New York, 1962); A. Saupe: Angew. Chem. internatl. Edit. 7 (1968) 62; G. H. Brown et al.: Liquid Crystals (Gordon and Breach Science Publisher, New York, 1967, 1969); I. G. Chis-tyakov: Soviet-Phys.-Uspekhi 9 (1967) 551; G. H. Brown and W. G. Shaw: Chem. Revs. 57 (1957) 1049.