1. 3. M. Borish, Clinical Refraction , 3rd edn. Professional Press, Chicago. IL, USA. pp. 52 -61 (1975 ).
2. 4. D. J. Coleman, F. L. Lizzi, and R. L. Jack, Ultrasonography of the Eye and Orbit . Kimpton, London, UK, Chapter III. pp. 91 -141 (1977 ).
3. Steiger on refraction: a reappraisal.