1. ASHTON, N., and COOK, C. (1953). British Journal of Ophthalmology, 37, 193.
2. and LANGHAM, M. (1951). Ibid, 35, 718.
3. Upsala Lak. Foren;BLIX, M.;Forh,1880
4. Quoted by Koby;FINCHAM, E.F.;Bd; In von Helmholtz, " Handbuch der physiologischen Optik,1930
5. KOBY, F. E. (1928). Rev. gin. Ophtal., Paris, 42, 293.