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]