Source : Webster's Revised Unabridged Dictionary (1913)

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