1. Abbe, E. (1873), Archiv Mikroskopische Anat. 9, 413.
2. Beissner, R.E., R.L. Bond, W.W. Bradshaw, and W.L. Anderson, J.O.S.A. 60, 741A, 1970.
3. Becherer, R. and Thompson, B.J. (1965), Unpublished results (see Thompson 1970, 1972).
4. Bragg, W.L. (1939), Nature, London 143, 678.
5. Bragg, W.L. (1942), Nature, London 149, 470.