73 The Microscopic Technique
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 utilizes 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 back 1895; its development, however was virtually halted in the 1940s because the development of the electron microscope was progressing rapidly. During the 1940s, electron microscopes routinely achieved resolution better than that poss ible 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 developmen t of new sources of x ray illumination. As a result, the brightness available today is millions of times tha t 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 microscope, x ray microscope 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 , is also sufficiently penetrating to image intact biological cells in ma ny 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.
实用口语:Nicole's day at school
大学新生常用口语:熟悉校园和同学
实用口语: Nicole's Close Election
20条地道实用英语句型(2)
实用口语:英语口语要素精选 18
实用口语情景轻松学:你知道怎么申请赴美签证吗?
地道口语:职场必备的五个简单句子
地道口语:用21句表达沮丧
实用盘点:赞美他人时必备口语
英语口语主题:交际英语热门话题47个(21--生日派对)
实用口语:关于兔子的英语口语
奥运会实用英语口语200句: 你可以乘火车去上海
奥运会实用英语口语200句:这是一个非常受人欢迎的目的地
实用英语:如何用英语砍价
口语情景对话:走遍美国精选 偷得浮生半日闲ACT 3 - 1
英语流行语:你跟谁“合得来”?
实用英语口语要素精选24
实用口语情景轻松学:说一说旅行要准备的东西
2011年实用口语练习:In the library 在图书馆
日常口语精华集1
疯狂口语要素精选 7
新东方英语口语开口篇:打招呼(3)
口语情景对话:走遍美国精选 二度蜜月ACT 3 - 2
实用口语情景轻松学:你能借我点儿钱吗?
新东方英语口语开口篇:Age 年龄(4)
2011年实用口语练习:今天你“团”了吗
实用口语情景轻松学:老外和菜农砍价时的地道英语对话
口语:“血肉之躯”用英语怎么说?
2011年实用口语练习:一起来找“茶”
实用口语情景轻松学:你最喜欢哪个季节?
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |