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The Collaborative International Dictionary of English v.0.48:

Opsiometer \Op`si*om"e*ter\, n. [Gr. ? sight + -meter: cf. F. opsiom[`e]tre.] An instrument for measuring the limits of distinct vision in different individuals, and thus determiming the proper focal length of a lens for correcting imperfect sight. --Brande & C. [1913 Webster]