This apparatus consists of three wires, each bent to resemble transverse waves. The wires are mounted in a rectangular brass box that was placed in front of the lens of a projector. The top two wires are identical, but positioned so that their shadows appear to move in opposite directions as a crank on the side of the box is turned. As they do this, the crests and troughs of the waves alternately lineup and overlap. The corresponding “interference” of the two waves is seen in the changing shape of the third wire.