Sir David Brewster has shown how the rigid features of a white bust
may be made to move and vary their expression, sometimes smiling and
sometimes frowning, by moving rapidly in front of the bust a bright
light, so as to make the lights and shadows take every possible
direction and various degrees of intensity; and if the bust be placed
before a concave mirror, its image may be made to do still more when it
is cast upon wreaths of smoke.