SCIENCE PHOTO LIBRARY SCIENCE PHOTO LIBRARY
Optical illusion, 19th-century artwork. Array of white circles on a black background, the starting point for a demonstration of an optical illusion. The circles appear to be hexagons depending on how they are viewed, and this illusion persists even when the image is replaced with a grey screen. The illusion, which varies between the right and left eye, is thought to be caused be after-imaging effects in the brain, where curves are interpreted as corners and vice-versa. Artwork from the 13th volume (first period of 1894) of the French popular science weekly 'La Science Illustree'.
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