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    When we can see well, we do not think about our eyes very often. It is only when we cannot see perfectly that we come to see how important our eyes are.

    People who are nearsighted can only see things that are very close to their eyes. Many people who do a lot of close work, such as writing, reading and sewing（缝纫）, become nearsighted. Then they have to wear glasses in order to see distant things clearly.

    People who are farsighted（远视的）suffer from just the opposite problem. They can see things that are far away, but they have difficulty reading a book unless they hold it at arm's length. If they want to do much reading, they must get glasses too.

    Other people do not see clearly because their eyes are not exactly the right shape. This, too, can be corrected by glasses. Some people's eyes become cloudy because of cataracts（白内障）. Long ago these people often became blind. Now, however, it is possible to operate on the cataracts and remove them.

    When night falls, colors become fainter to the eye and finally disappear. After your eyes have grown used to the dark, you can see better if you use the sides of your eyes rather than the centers. Sometimes, after dark, you see a small thing to one side of you, which seems to disappear if you turn you head in its direction. This is because when you turn your head, you are looking at the thing too directly. Men on guard duty sometimes think they see something moving to one side of them. When they turn to look straight at it, they cannot see it any more, and they believe they were mistaken. However, this mistake happens because the center of the eye, which is very sensitive(敏感的) in daylight, is not as sensitive as the sides of the eyes after dark.