Source : Webster's Revised Unabridged Dictionary (1913)

Chronoscope \Chron"o*scope\, n. [Gr. ? time + -scope.]
   An instrument for measuring minute intervals of time; used in
   determining the velocity of projectiles, the duration of
   short-lived luminous phenomena, etc.

Source : WordNet®

chronoscope
     n : an instrument for accurate measurements of small intervals
         of time