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microscope
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History
The single-lens microscope may date from as early as the 15th century, but the first truly powerful lenses were probably made by Anton van Leeuwenhoek (1632-1723). His single-lens microscopes were capable of magnifying up to 300 times. With them he discovered microorganisms, thereby founding the science of microbiology and providing the basis for the germ theory of disease. Probably the greatest of the early microscopists, however, was the Italian Marcello Malpighi (1628-1694), who is generally regarded as the founder of histology.
The compound microscope, which has two lens systems, was developed toward the end of the 16th century. However, the single-lens microscope continued to be widely used until the 19th century, when improvements in optical design and glass technology made the compound microscope a practicable instrument.
Light microscopes continued to be refined, with the development of the phase-contrast microscope, for example. However, the next major advances were instruments that used electrons instead of light – the transmission electron microscope (TEM), invented in the early 1930s, and the scanning electron microscope (SEM), invented in the mid-1960s.
Light microscopes
Compound microscopes are the most widely used microscopes. They have two lens systems – the objective and the eyepiece – which are mounted at opposite ends of a tube called the body tube. There is also a stage to hold the specimen, a light source, and an optical condenser. The maximum practicable magnification of an ordinary light microscope is limited by the wavelength of light to about 1,500 times.
Electron microscopes
Transmission electron microscopes (TEMs) are similar to light microscopes, except that they use a beam of electrons instead of light, and electromagnetic "lenses" instead of glass ones. Furthermore, because electrons are invisible, the image must be formed on a fluorescent screen or photographic film. Electron microscopes allow much higher magnifications than light microscopes. Modern TEMs can magnify up to about five million times, enabling viruses and large molecules (such as DNA) to be seen.
The scanning electron microscope (SEM) works in a different way from the TEM. SEMs have a lower maximum magnification (approximately 100,000 times) than do TEMs. However, unlike TEMs, SEMs provide three-dimensional images. This makes SEMs particularly valuable for applications such as studying the surface structure of cells and tissues.
Other microscopes
Phase-contrast and interference microscopes are types of light microscopes with modified illumination and optical systems that make it possible for unstained transparent specimens to be seen clearly. These microscopes are particularly useful for examining living cells and tissues.
Another instrument, the fluorescent microscope, is used to study the chemical composition of cells. In fluorescence microscopy, a specimen that has been selectively stained with fluorescent dyes is illuminated with ultraviolet light, which makes the stained parts glow.
Operating microscopes are low-powered compound microscopes with several modifications. They do not have a stage, and the illumination system is arranged to shine light down on to the living tissues rather than up through the specimen.
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