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August 29, 1891

THE PRISM-DIOPTRY VS. THE CENTRAD.Read in the Section of Ophthalmology, at the Forty-second Annual Meeting of the American Medical Association, held at Washington, D. C., May, 1891.

Author Affiliations


JAMA. 1891;XVII(9):343-346. doi:10.1001/jama.1891.02410870027001g

I have had occasion to remark before, in respect to this question, that what we need in the nomenclature of prisms is scientific accuracy, readiness and simplicity of practical application with as little innovation in established custom as possible. The prism dioptry, I believe, fulfills all these requirements. It takes as a standard prism one which causes a deflection of 1 cm. on a plane at one meter's distance, which is nearly what is now known as a one degree prism, and will thus entail no change in the present process of manufacture or in the plant of the manufacturers. It is scientific and accurate, every prism being precisely what its number indicates, and its power can be measured by any one having a centimeter rule and a meter measure. No complicated apparatus is necessary, though for the finer determinations and for certain combinations of sphero-and cylindro-prisms the elegant instrument