1. Anstis, G. R. and Cockayne, D. J. H. (1979), Acta Cryst., in press.
2. Boersch, H. (1947), Z. Naturforsch 2a, 615.
3. Born, M. and Wolf, E. (1964), “Principles of Optics”, Pergamon Press, London.
4. Bowen, D. K. and Hall, C. R. (1975), “Microscopy of Materials”, John Wiley and Sons, New York.
5. Cowley, J. M. (1975), “Diffraction Physics”, North Holland Publishing Company.