Source : Webster's Revised Unabridged Dictionary (1913)

Opeidoscope \O*pei"do*scope\, n. [Gr. ?, ?, voice + ? form +
   -scope.] (Physics)
   An instrument, consisting of a tube having one end open and
   the other end covered with a thin flexible membrance to the
   center of which is attached a small mirror. It is used for
   exhibiting upon a screen, by means of rays reflected from the
   mirror, the vibratory motions caused by sounds produced at
   the open end of the tube, as by speaking or singing into it.
   --A. E. Dolbear.