great illusion this one
. it has to with the fact that our visual perception is partly (largely ?) based on perception of contrast. the mechanism hereof lies in the eye
and brain; the activation of the cells (rods & cones) in the eye, and subsequent activation of cells (neurons) in the brain has one very cool
similarity: activated cells inhibit / depress the activation of their neighbours! long story short; the net effect is that when you're looking at an object, the
perceived light intensity depends on its surroundings. a.k.a. what you see is not necessarily what you get
...well, it was something like that anyway