Illusory smoke and dazzling fog

It is well known that a flat ellipse rotating in the frontoparallel plane a ppears, after brief inspection, as a rigid circular disc tilting back and f orth in a 3-D space. We here report that rotation of a grey-shaded ellipse on a white or on a black background produces the compelling illusion of a d ark smoke or a dazzling fog (depending on the conditions of the background) moving in front of a completely white or completely black tilting disc. Th e fog effect disappears when there is a luminance contrast all along the pe rimeter of the ellipse.,An experiment is reported showing that the effect c an be experienced in static conditions only to a limited extent and mostly in the 'dazzling' version, and that relative movement between the contours of the figure and the shaded area is crucial to the occurrence of the effec t, while the occurrence of a depth effect is not.