1. Hl. de Vries, Acta Cryst. 4, 219 (1951).
2. J. Pergason, Liquid Crystals, Gordon and Breach, New York (1967)5 p. 89. This work was presented at the 1st Int. Conference on Liquid Crystals, Kent, Ohio, August 1965.
3. W.J. Harper, Liquid Crystals, Gordon and Breach, New York (1967), p. 121.
4. J.R. Hansen and R.J. Schneeberger, IEEE Trans. Electron Devices, Vol. ED-15, 896 (1968).
5. C.J. Gerritsma and P. van Zanten, Mol. Cryst. 15, 257 (1971).