sentences:  Effects in the film were created using stage machinery, pyrotechnics, substitution splices, and dissolves.The mirror sequence in the film is based on a routine that had long been popular in music halls.The effect was not produced with a real mirror, which would have reflected the studio windows and the camera; instead, there were two actors on the set, one of whom mimicked the other's gestures from the opposite side of the imaginary "glass."