The electron microscope, which is not limited by the powers of optical lenses and light, permits greater magnification and greater depth of focus than the optical microscope and reveals more details of structure. Instead of light rays it employs a stream of electrons controlled by electric or magnetic fields. The image may be thrown on a fluorescent screen or may be photographed. It was first developed in Germany c.1932; James Hillier and Albert Prebus, of Canada, and V. K. Zworykin, of the United States also made notable contributions to its development. The scanning electron microscope, introduced in 1966, gains even greater resolution by reading the response of the subject material rather than the direct reflection of its beam. Using a similar approach, optical scanning microscopes achieve a resolution of 400 Angstroms, less than the wavelength of the light being used. Finally, the scanning tunnelling microscope, invented in 1982, uses not a beam but an electron wave field, which by interacting with a nearby specimen is capable of imaging individual atoms; its resolution is an astounding one Angstrom.
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