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From The Collaborative International Dictionary of English v.0.48:
Opeidoscope \O*pei"do*scope\, n. [Gr. ?, ?, voice + e'i^dos 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. [1913 Webster]