1. Bergmann, G. (1976), Physics Rep. 276, 161.
2. Bergmann, G. and Koepke, R. (1976), private communication.
3. Bernal, J. D. (1960), Nature (London) 185, 68.
4. Bernas, H., Travese, A., Zawislak, F. C., Chaumont, J. and Dumoulin, L. (1980), J. Physique 41, C8 — 859.
5. Bernas, H. and Nedellec, P. (1981), Nuel. Instr. and Math., 183, 845.