1997年8月托福考试阅读理解全真试题

　　Each advance in microscopic technique has provided scientists with new perspectives on the function of living organisms and the nature of matter itself. The invention of the visible-light microscope late in the sixteenth century introduced a previously unknown realm of single-celled plants and animals. In the twentieth century， electron microscopes have provided direct views of viruses and minuscule surface structures. Now another type of microscope， one that utilize x-rays rather than light or electrons， offers a different way of examining tiny details， it should extend human perception still farther into the natural world.

　　The dream of building an x-ray microscope dates to 1895， its development， however， was virtually halted in the 1940 s because the development of the electron microscope was progressing rapidly. During the 1940 s electron micro- scopes routinely achieved resolution better than that possible with a visible-light microscope， while the performance of x-ray microscopes resisted improvement. In recent years， however，interest in x-ray microscopes has revived， largely because of advances such as the development of new sources of x-ray illumination. As a result， the brightness available today is millions of times that of x-ray tubes， which， for most of the century， were the only available sources of soft x-rays.

　　The new x-ray microscopes considerably improve on the resolution provided by optical microscopes. They can also be used to map the distribution of certain chemical elements. Some can form pictures in extremely short times， others hold the promise of special capabilities such as three dimensional imaging. Unlike conventional electron microscopy， x-ray microscopy enables specimens to be kept in air and in water，which means that biological samples can be studied under conditions similar to their natural state. The illumination used，so-called soft x-rays in the wavelength range of twenty to forty angstroms （an angstrom is one ten-billionth of a meter）， is also sufficiently penetrating to image intact biological cells in many cases. Because of the wavelength of the x-rays used，soft x-ray microscopes will never match the highest resolution possible with electron microscopes. Rather， their special pro- perties will make possible investigations that will complement those performed with light- and electron-based instruments.

　　30.What does the passage mainly discuss？

　　（A） The detail seen through a microscope

　　（B） Sources of illumination for microscopes

　　（C） A new kind of microscope

　　（D） Outdated microscopic technique

　　31.According to the passage， the invention of the visible-light microscope allowed scientists to

　　（A） see viruses directly

　　（B） develop the electron microscope later on

　　（C） understand more about the distribution of the chemical elements

　　（D） discover single celled plants and animals they had never seen before.

　　32.The word "minuscule" in line 7 is closest in meaning to

　　（A） circular

　　（B） dangerous

　　（C） complex

　　（D） tiny

　　33.The word "it" in line 10 refers to

　　（A） a type of microscope

　　（B） human perception

　　（C） the natural world

　　（D） light

　　34.Why does the another mention me visible light microscope in the first paragraph？

　　（A） To begin a discussion of sixteenth century discoveries.

　　（B） To put the x-ray microscope in historical perspective

　　（C） To show how limited its uses are

　　（D） To explain how it functioned

　　35.Why did it take so long to develop the x-ray microscope？

　　（A） Funds for research were insufficient.

　　（B） The source of illumination was not bright enough until recently.

　　（C） Materials used to manufacture x-ray tubes were difficult to obtain

　　（D） X-ray microscopes were too complicated to operate.

　　36.The word "enables" in line 30 is closest in meaning to

　　（A） constitutes

　　（B） specifies

　　（C） expands

　　（D） allows

　　37.The word "Rather" in line 38 is closest in meaning to

　　（A） significantly

　　（B） preferably

　　（C） somewhat

　　（D） instead

　　38.The word "those" in line 40 refers to

　　（A） properties

　　（B） investigations

　　（C） microscopes

　　（D） x-rays

　　39.Based on the information in the passage， what can be inferred about x-ray microscopes in the future？

　　（A） They will probably replace electron microscopes altogether.

　　（B） They will eventually be much cheaper to produce than they are now.

　　（C） They will provide information not available from other kinds of microscopes.

　　（D） They will eventually