必威体育精装版2013年12月六级阅读(适中难度).docVIP

(1)You stare at waterfall for a minute or two, and then shift your gaze to its surroundings. What you now see appears to HYPERLINK /w/driftdrift 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 HYPERLINK /w/blurryblurry black and white vision. These are the windows of night vision; once adapted to the dark, they can HYPERLINK /w/detectdetect a candle burning ten miles away. Color vision in each eye comes from six to seven million structures called cones. Under ideal conditions, every HYPERLINK /w/conecone can “see” the entire rainbow HYPERLINK /w/spectrumspectrum 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 HYPERLINK /w/opticoptic 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 HYPERLINK /w/impartimpart a strange vividness to the garden’s blue flowers. However, look at a white shirt during the reddish light of sunset, and you’ll still see it in its “true” color—white, not red. Our eyes are constantly comparing an object against its surroundi