Lilac chaser
The lilac chaser is a visual illusion, also known as the Pac-Man illusion. It consists of 12 lilac (or pink, rose or magenta), blurred discs arranged in a circle (like the numbers on a clock), around a small black, central cross on a grey background. One of the discs disappears briefly (for about 0.1 seconds), then the next (about 0.125 seconds later), and the next, and so on, in a clockwise direction. When one stares at the cross for about 5 seconds or so, one sees three different things:
A gap running around the circle of lilac discs;
A green disc running around the circle of lilac discs in place of the gap;
The green disc running around on the grey background, with the lilac discs having disappeared in sequence.
The chaser effect results from the phi phenomenon illusion, combined with an afterimage effect in which an opposite, complementary, colour—green—appears when each lilac spot disappears (if the discs were blue, one would see yellow), and Troxler's fading of the lilac discs.
