Definition: Perception of an object or illustration that does not correspond to the visual stimulus itself. * Also known as visual illusion, optical illusion.
Illuminating illusions
There are some dozen or so well-known optical illusions, rejoicing under names such as Hering-Hermann's Grid (appearance of darkish spots in a grid of white lines between black squares), Schroeder's Staircase (a line illustration looks like a staircase seen from below, no, it doesn't, it's a staircase from above, no, maybe it's from below) and Poggendorf's Illusion (a diagonal line with a square over the middle does not appear continuous). They all tell us something about the way the retinal cells are 'hard-wired' to extract information -- sometimes erroneously, when we set out to fool them.

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