p327fig08.47 Two classical examples of part motion in a moving reference frame illustrate the general situation where complex objects move while their multiplie parts may move in different directions relative to the direction of the reference frame.<BR>||  Two kinds of percepts and variations (Johansson 1950). Symmetrically moving inducers: each do moves along a straight path, each part contributes equally to common motion; Duncker wheel (Duncker 1929): one dot moves on a cycloid, the other dot (the "center") moves stright, unequal contributipon from parts;  If the dot is presented alone: seen as cycloid; if with center: seen as if it were on the rim of a wheel.