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You stare at waterfall for a minute or two, and then shift your gaze to its surroundings. What you now see appears to drift upward. These optical illusions occur because the brain is constantly matching its model of reality to signals from the bodys sensors and interpreting what must be happeningthat your brain must have moved, not the other; that downward motions is now normal, so a change from it must now be perceived as upward motion. The sensors that make this magic are of two kinds. Each eye contains about 120 million rods, which provide somewhat blurry black and white vision. These are the windows of night vision; once adapted to the dark, they can detect a candle burning ten miles away. Color vision in each eye comes from six to seven million structures called cones. Under ideal conditions, every cone can see the entire rainbow spectrum of visible colors, but one type of cone is most sensitive to red, another to green, a third to blue. Rods and cones send their messages pulsing an average 20 to 25 times per second along the optic nerve. We see an image for a fraction of a second longer than it actually appears. In movies, reels of still photographs are projected onto screens at 24 frames per second, tricking our eyes into seeing a continuous moving picture. Like apparent motion, color vision is also subject to unusual effects. When day gives way to night, twilight brings what the poet T.S. Eliot called the violet hour. A light levels fall, the rods become progressively less responsive. Rods are most sensitive to the shorter wavelengths of blue and green, and they impart a strange vividness to the gardens blue flowers. | 
