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2. Contact Lens Practice, edited by M. Ruben, M. Giullon (Chapman Hall Medical, London, 1994), p. 135.
3. Current concepts of multifocal intraocular lenses, edited by W.A. Maxwell, L.T. Nordan (SLACK Inc., Thorofare, 1991), p. 13.
4. A.L. Cohen, Bifocal Contact Lens Optics, Spectrum, June 1989, pp. 43-52.