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As the scissor movement, conic cornea and spheric aberration, as recognized by the retinoscope, are so difficult of demonstration, except in the individual patient, the writer has had made three lenses which will illustrate these conditions respectively, when placed in front of his schematic eye; and thus the beginner in retinoscopy may have the opportunity to see, know and study these important and interesting manifestations, before proceeding direct and in comparative ignorance to his patient.
Figure 1, is a plano-concave cylinder of 2 D., mounted in a cell of the trial case and to one half of its plane surface is cemented (at the same axis) a plano-convex cylinder of 4. D., thus making a combination lens, one-half of which is a -2 D., and the other half a +2 D. Placing this lens with its axis at 180 degrees, before the schematic eye at emmetropia (zero) and the observer
THORINGTON J. LENSES FOR THE STUDY OF THE SCISSOR MOVEMENT, CONIC CORNEA AND SPHERIC ABERRATION WITH THE RETINOSCOPE.. JAMA. 1897;XXIX(25):1261-1262. doi:10.1001/jama.1897.02440510023001k