UC San Diego, USA, Université Paris Descartes, France
(© 2011 Stuart Anstis & Patrick Cavanagh)
A rotating figure 8 made of two overlapping rings is ambiguous. Small spots painted on the rings can resolve the ambiguity, forcing them to look like an 8 or like 2 rings. Even without any spots, if the brightness of the intersections where the rings overlap makes the rings look transparent, they slide. If not, they stick.
Illusions from rotating rings,
