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PASSAGE 7

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the

correct answer to each of the questions

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 microscopes 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 time, 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 properties will make possible

investigations that will complement those performed with light- and electron-based instruments.

Question 1. 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

Question 2. 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.

Question 3. The word "minuscule" in the passage is closest in meaning to

.

A. circular

B. dangerous

C. complex

D. tiny

Question 4. Why does the author mention the 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