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.
© Webster 1913.