OpeidoscopeO*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.