1. Beetham, W. P. (1935). Trans. Amer. ophthal. Soc., 33, 413.
2. Coverdale, H. (1948). Brit. J. Ophthal., 32, 669.
3. Duke-Elder, S. (1930). Ibid., 14, 61.
4. Eadie, S., and Thompson, M. (1955). Ibid., 39, 90.
5. Ellman, P., Weber, F. Parks, and Goodier, T. E. W. (1951). Quart. J. Med., n.s. 20, 33.