The “singer” rubs sand on the patient because ___.A.the patient receives strength from the

The purpose of a healing ceremony lies in ___.

A.pleasing the ghosts

B.attracting supernatural powers

C.attracting the ghosts

D.creating a sandpainting

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.

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.

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