Hold your hand outstretched with one hand twice as far from your eyes as the other. Make a casual judgment about which hand looks bigger. Most people see the hands as about the same size, and many see...

Hold your hand outstretched with one hand twice as far from your eyes as the other. Make a casual judgment about which hand looks bigger. Most people see the hands as about the same size, and many see the nearer hand as slightly bigger. Very few people see the hand as four times as big, but by the inverse-square law, the nearer hand should appear twice as tall and twice as wide. Twice times twice means four times as big. That’s four times as much of your visual field as is occupied by the more distant hand. It is likely that your belief that your hands are the same size is so strong that your brain overrules this information. Try it again, only this time overlap your hands slightly and view them with one eye closed. Aha! Do you now see more clearly that the nearer hand is bigger? This raises an interesting question: What other illusions do you experience that can’t be checked so easily?