Recently, a couple of psychology European professors, Richard Wiseman (U of Hertfordshire) and Rob Jenkins (U of Glasgow) came up with this interesting optical illusion to honor Charles Darwin's birthday.
This illusion takes advantage of a phenomenon called an afterimage. As Darwin's grandfather was a research pioneer in afterimages, this tribute is well-thought-out, highly appropriate to the occasion, and actually pays tribute to two distinguished Darwins.
In case you're not familiar with the phenomenon, here's how it works. After making sure your display is quite bright, stare at the monkeys for around 30 seconds without blinking, then look at a blank white background. Blinking a couple of times helps clarify the afterimage.
Both Jenkins and Wiseman have done extensive research on visual perception (how our brains interpret what we see).