The St Louis Arch
Probably the largest 'real' optical
illusion in the world is the St Louis arch above. Contrary to (very
persistent perception) the arch is exactly as wide as it is tall. This
classic optical illusion is due to our eyes automatically heightening a
shape that points upwards and thins as it rises. You can check that the
two yellow lines below have identical length.
Norman Fenton
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Making Sense of Probability: Fallacies, Myths and Puzzles