1. Agar, A. W. (1960). Brit. J. Appl. Phys., 11, 185.
2. Agar, A. W.(1973). Private communication.
3. Airy, G. (1835). See Preston, T. (1928). Theory of Light, Macmillan, New York, p. 324.
4. Allan, P., Crellin, E. B., and Bevis, M. (1973). Phil. Mag., 27, 127.
5. Andrews, K. W., Dyson, D. J., and Keown, S. R. (1971). Interpretation of Electron Diffraction Patterns, Hilger, London.