Source : Webster's Revised Unabridged Dictionary (1913)

Tachistoscope \Ta*chis"to*scope\, n. [Gr. ?, superl. of ? swift
   + -scope.] (Physiol.)
   An apparatus for exposing briefly to view a screen bearing
   letters or figures. It is used in studying the range of
   attention, or the power of distinguishing separate objects in
   a single impression.

Source : WordNet®

tachistoscope
     n : scientific instrument used by psychologists; presents visual
         stimuli for brief exposures [syn: {t-scope}]