1. DUANE (1903).-Ophthal. Rec, Vol. XII, p. 420.
2. Proc. Optical Convention;EMSLEY, H.H.,1926
3. Visual Optics, London.
4. Skiascopy and its Practical Application to the Study of Refraction;JACKSON, E.,1896
5. KLEIN, M. (1942).-Brit. Ji. Ophthal., Vol. XXVI, p. 510.