1. Textbook of;Duke-Elder, W.S.;Ophthalmology,1938
2. Huggert, A. (1952). Acta paediat. (Uppsala), 41, 463.
3. Schmoger, R. (1955). Kinderdrztl. Prax., 23, 433.
4. Seissiger, J. (1929). Arch. Augenheilk., 100-101, 302.
5. Wagner, G. (1957). Klin. Mbl. Augenheilk., 131, 326.