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Hermann-Hering illusion. The illusion is achieved by the observer staring at the small black square for 30 seconds. They then stare at the small white square below it. A moving grid pattern is seen overlaying the one already present. This demonstrates the way the eye receives images. Staring at the black square for that length of time burns an image of the grid onto the retina (back of the eye). When the eye is shifted this remains and is seen faintly moving on top of the real grid. Its movements are a consequence of the continual, small, rapid movements of our eyes.
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