Reverse Masks

Two masks of faces are mounted side by side—one is concave, the other convex. Viewed from a distance, the masks look two-dimensional. From experience people know that faces are convex. As the viewer walks back and forth in front of the masks though, one seems to move. Parallax refers to the rule that more distant objects move less. Since the nose of this mask moves the least, it would seem to indicate a concave face. Experience overrides this conclusion and the result is the observer sees a moving face, rather than a concave one.