Twenty-four still photographs are made into a continuous moving picture just because ___.A

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.

The visual sensor that is capable of distinguishing shades of color is called ___.

A.cones

B.color vision

C.rods

D.spectrum

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?” 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.

The passage may be taken from ___.

A.a news report

B.a textbook of food processing

C.a book of popular science

D.a manual of food irradiation

With cases of food poisoning increasing, ___.

A.food irradiation should be carried out with care

B.it is more urgent to irradiate foods

C.medical researches into treatment of the diseased should be strengthened

D.Americans are beginning to accept food irradiation

Irradiated food ___.

A.certainly loses its nutritive value

B.maintains its nutritive value no different from the nonirradiated

C.keeps its nutritive value better than canned food

D.is recommended as the best of all preserved foods

Gamma rays used to irradiate food ___.

A.are generally not strong enough to destroy contaminating organisms

B.do not bring about significant changes in the food itself

C.may destroy some of the nutrients in the food

D.should be submitted to FDA for approval

nirradiated food. Since the 1960’s researchers have studied irradiated food at microscopic levels to try to find such substances, called unique radiolytic products. After reviewing these studies, the FDA determined that compounds formed during irradiation are similar to substance found in nonirradiated foods and are not dangerous to consume. Destruction of microorganisms that cause illness is an important goal of irradiation. About 250 million cases of food poisoning or 1 per person—occur every year in the U.S., according to FDA estimates. Food poisoning can cause vomiting, diarrhea, fever, headache—and, occasionally, death. Because of the apparent safety of food irradiation, and the problems presented by contaminated food, scientific groups—including the American Medical Association, the World Health Organization, and the United Nations food and Agriculture Association—have voiced nearly universal support for the process. Worldwide, 38 nations have approved irradiation for 355 products. Like microwave ovens, food irradiation has aroused apprehension and misunderstanding. Yet it has been scrutinized more thoroughly than other methods of food treatment that we have come to regard as safe, and it appears to be a method whose time has come. Cobalt 60, besides irradiating food, is also employed to ___.

A.detect metallic flaws

B.run a nuclear reactor

C.cure cancer patients

D.strengthen concrete walls

Scientists perform careful experiments and do wide research because______.

A.they must learn to destroy all the insects that we need

B.they must be sure one insect pest is not traded for another

C.research keeps them from inventing new insecticide for the crops

D.research helps them find a way to kill all insects

Which statement does this article lead you to believe?

A.All changes are predictable.

B.Nothing ever changes in nature.

C.Nature is not always predictable.

D.Nature always serves man well.