英语六级的真题
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 body’s sensors and interpreting what must be happening—that 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.
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