APA Dictionary of Psychology

n. a device consisting of two disks, one in front of the other, that rotate in opposite directions. The front disk is opaque with four slits through which a highly distorted image on the back disk is viewed. Although the image on the rear disk is both rotating and distorted, it will appear normal and stationary when viewed through the front disk if both disks are rotated at the appropriate speed. [first constructed by Belgian physicist Joseph Antoine Ferdinand Plateau (1801–1883)]