The Collaborative International Dictionary of English v.0.48:

Tachistoscope \Ta*chis"to*scope\, n. [Gr. ta`chistos, superl. of
   tachy`s 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.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

tachistoscope
    n 1: scientific instrument used by psychologists; presents
         visual stimuli for brief exposures [syn: tachistoscope,
         t-scope]