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About twenty years ago was inaugurated the reform in the numbering of ophthalmological lenses upon the basis of their actual strength as determined by their focal length. Before that time lenses were generally taken at a face value, which represented, in such diverse units of length as were locally employed, the radius of curvature to which each was ground. The reform met opposition on the ground that lenses had to be made by grinding to surfaces of given radii, that the radii gave a sufficiently accurate indication of the focal strength, that there would be large financial loss in abandoning the old system and adopting the new, with its less familiar units—in short, that the change, even if theoretically desirable, involved more practical difficulty than actual advantage. Yet the positive advantages of the reformed system, arising in part from its metric character, but also largely from its acknowledged accuracy of
RANDALL BA. THE REFORMED NUMERATION OF PRISMS AND THE CENTRAD AS THE UNIT.Read in the Section of Ophthalmology, at the Forty-second Annual Meeting of the American Medical Association, held at Washington, D. C., May, 1891.. JAMA. 1891;XVII(9):341–343. doi:10.1001/jama.1891.02410870025001f
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