place of a single sheet of glass, two or more sheets may be employed.
"In the drawings, Fig. 63 represents a plan view, and Fig. 64 an elevation, of a portion of the mirror, designed to show its graduated opacity.
"a is a stage. It may be in a lecture-room or theater. b b are the seats for the audience in front of the stage. c c is a small room—eight or ten feet square and eight high will often be sufficiently large; but it may be of any size. It may advantageously be raised and approached by two or three steps from the stage a.
"d is a vertical mirror, passing diagonally across the chamber c and dividing it into two parts, which are exact counterparts the one of the other. The mirror d is so mounted that it can be rapidly and noiselessly moved diagonally across the chamber in the path represented by the dotted line d′ and be withdrawn whenever desired. This can conveniently be done by running it in guides and upon rollers to and from a position where it is hidden by a screen, e, which limits the view of the audience in this direction.
"In consequence of the exact correspondence of the two parts of the chamber c, that in front
