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.
School where lessons focus on leadership
That would be stretching it
Mining company blamed for blast
French town lures Chinese tourists to 'revolution road'
'Fearless Felix' breaks the sound barrier
Tablet makers diversify to tackle Apple
Colombia's 'queen of cocaine' shot dead at 69
Vanishing languages saved for posterity
58 people drown after refugee boat sinks
Survey shows Chinese workers stressed out
HIV sufferers to receive mental counseling
Xi slams Diaoyu 'purchase'
看过老友记的花絮吗?
Heat vs Clippers tickets to go on sale
Toy firm set to play abroad
Funds earmarked for relief
Philippines, Muslim rebels agree landmark peace deal
Trophy makes Serena hungry for more
French cartoons fuel Muslim anger
Chinese go online to buy latest iPhone
US embassies tighten security
Foreign minister summons Japan's envoy
Writers win copyright lawsuit against Baidu
Virus compared with SARS not found in China
Obama needs to tread carefully in Chicago strike
Overloaded trucks at fault for bridge collapse
Getting the elderly to eat well
Large shoes to fill?
'Pandas' hot to globe-trot
Forgotten bunker provides glimpse of wartime Hanoi
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |