‖Di‐op″tre (?), n. [F. See 2d Dioptric.] (Optics) A unit employed by oculists in numbering glasses according to the metric system; a refractive power equal to that of a glass whose principal focal distance is one meter.
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Webster's Dictionary 1913
Webster's Revised Unabridged Dictionary, C. & G. Merriam Co., 1913.