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.
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 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 surroundings. They therefore observe the effect of a shift in the color of illuminating on both, and adjust accordingly.
The eyes can distinguish several million graduations of light and shade of color. Each waking second they flash tens of millions of pieces of information to the brain, which weaves them incessantly into a picture of the world around us.
Yet all this is done at the back of each eye by a fabric of sensors, called the retina, about as wide and as thick as a postage stamp. As the Renaissance inventor and artist Leonardo da Vinci wrote in wonder, “Who would believe that so small a space could contain the images of all the universe?”
1.Visual illusions often take place when the image of reality is ___.
A.matched to six to seven million structures called cones. B.confused in the body’s sensors of both rods and cones. C.interpreted in the brain as what must be the case. D.signaled by about 120 million rods in the eye.
2.The visual sensor that is capable of distinguishing shades of color is called ___.
A.cones B.color vision C.rods D.spectrum
3.The retina send pulses to the brain ___.
A.in short wavelengths B.as color pictures C.by a ganglion cell D.along the optic nerve.
4.Twenty-four still photographs are made into a continuous moving picture just because ___.
A.the image we see usually stays longer than it actually appears. B.we see an object in comparison with its surroundings. C.the eyes catch million pieces of information continuously. D.rods and cones send messages 20 to 25 times a second.
5.The author’s purpose in writing the passage lies in ___.
A.showing that we sometimes are deceived by our own eyes. B.informing us about the different functions of the eye organs. C.regretting that we are too slow in the study of eyes. D.marveling at the great work done by the retina.
第49篇答案：CADAB
第50篇：(Unit 13,Passage 2)
Art is considered by many people to be little more than a decorative means of giving pleasure. This is not always the case, however; at times, art may be seen to have a purely functional side as well. Such could be said of the sandpaintings of the Navaho Indians of the American Southwest; these have a medicinal as well as an artistic purpose.
According to Navaho traditions, one who suffers from either a mental or a physical illness has in come way disturbed or come in contact with the supernatural—perhaps a certain animal, a ghost, or the dead. To counteract this evil contact, the ill person or one of his relatives will employ a medicine man called a “singer” to perform a healing ceremony which will attract a powerful supernatural being.
During the ceremony, which may last from 2 to 9 days, the “singer” will produce a sandpainting on the floor of the Navaho hogan. On the last day of the ceremony, the patient will sit on this sandpainting and the “singer” will rub the ailing parts of the patient’s body with sand from a specific figure in the sandpainting. In this way the patient absorbs the power of that particular supernatural being and becomes strong like it. After the ceremony, the sandpainting is then destroyed and disposed of so its power will not harm anyone.
The art of sandpainting is handed down from old “singer” to their students. The material used are easily found in the areas the Navaho inhabit; brown, red, yellow, and white sandstone, which is pulverized by being crushed between 2 stones much as corns is ground into flour. The “singer” holds a small amount of this sand in his hand and lets it flow between his thumb and fore-finger onto a clean, flat surface on the floor. With a steady hand and great patience, he is thus able to create designs of stylized people, snakes and other creatures that have power in the Navaho belief system. The traditional Navaho does not allow reproduction of sandpaintings, since he believes the supernatural powers that taught him the craft have forbidden this; however, such reproductions can in fact be purchased today in tourist shops in Arizona and New Mexico. These are done by either Navaho Indians or by other people who wish to preserve this craft.
1.The purpose of the passage is to ___.
A.discuss the medical uses of sandpaintings in medieval Europe. B.study the ways Navaho Indians handed down their painting art. C.consider how Navaho “singer” treat their ailments with sandpaintings. D.tell how Navaho Indians apply sandpainting for medical purposes.
2.The purpose of a healing ceremony lies in ___.
A.pleasing the ghosts B.attracting supernatural powers C.attracting the ghosts D.creating a sandpainting
3.The “singer” rubs sand on the patient because ___.
A.the patient receives strength from the sand B.it has pharmaceutical value C.it decorates the patient D.none of the above
4.What is used to produce a sandpainting?
A.Paint B.Beach sand C.Crushed sandstone D.Flour
5.Which of the following titles will be best suit the passage?
A.A New Direction for Medical Research B.The Navaho Indians’ Sandpainting C.The Process of Sandpainting Creation D.The Navaho Indians’ Medical History
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