What you can see: This illusion, the Kaleidoscopic motion and velocity illusions, was among the finalists in the 2007 Best Illusion of the Year Contest, submitted by Peter van der Helm. It shows a cogwheel, partly behind an inner wheel and partly in front of the outer wheel, that rotates with constant velocity. If it exactly fills the space between the stationary wheels, it may look as if the middle wheel pulsate, jolt, or accelerate, and the inner stationary cogwheel may wiggle. van der Helm suspects that the effects are probably by colour assimilation and/or ambiguous figure-ground segregation.

What can you do? By default a red wheel rotates between dark blue wheels. You can change between ten such colour combinations using the up/down buttons. Each color combination exists twice, with the second always having a transparency of 75%. What influence do the colour combinations have on the perceptibility of the effect? For me the effect is clearly weakened by transparency.

What else can you do? The speed of rotation can be changed with the speed-slider or with the right/left buttons. How does this influence the effect? Does the stacking of the wheels have any influence? With pressing the button o you can determine whether the middle wheel (1) is centered above the outer wheel, (2) centered above the inner wheel or (3) above both wheels.

Related topics: Wagon Wheel Effect
References:
van der Helm, P
. (2006). Kaleidoscopic motion and velocity illusions. Vision Research 47/4,pp. 460-465.
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