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Certain patterns confuse your eyes and brain, causing you to misjudge the size of a circle, or the length of a line. Though hundreds of optical illusions have been recorded and studied over the last century, researchers still do not always agree on why these patterns cause errors in perception. Scientists are fascinated by illusions because, by figuring out how the eye and brain perceive them, the normal workings of the visual system can be better understood. As Purkinje, a noted 19th-century Czech physiologist, put it: "Deceptions of the senses are the truths of perception."

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