A microscope in which the source of illumination is a stream of electrons emanating from a tungsten cathode in a high vacuum and accelerated by a strong electric impulse (300 kv). The electrons are focused by a series of magnetic fields that function as lenses in the same way as glass affects waves of visible light, i.e., the electron stream curves as it passes through the magnetic field. Such lenses were developed in Germany by Busch, Knoll, and Tuska in the 1930s, and were adapted to microscopy by Zworykin and Hillier at the RCA Laboratories in the early 1940s, when the first commercial instruments were produced. The electron microscope is characterized by extremely high resolving power due to the ultrashort wavelength of electronic radiation—a small fraction of an angstrom unit (Å). Resolution of 2 Å is possible, which permits determination of the structure of macromolecules (DNA) and even observation of large atoms (uranium). Instruments of the scanning transmission type with a resolving power of 0.05 Å were developed at the University of Chicago. Two kinds of electron microscopies are in general use: the transmission type, in which the electrons penetrate the specimen, and the scanning type introduced in 1970, in which the electrons, condensed to a fine beam, repeatedly traverse the surface of the specimen, producing a three-dimensional contour effect by means of secondary electrons emanating from the specimen itself. Pictures of astonishing accuracy have been obtained, especially of surface structures, a matter of great importance in the study of catalysis and other critical phenomena in both industry and the biological sciences. A unique combination of these techniques is the scanning transmission electron microscope (STEM), by means of which colored motion pictures of uranium atoms on a thin-film carbon substrate have been obtained.See Optical Microscope; Resolving Power; Ultramicroscope; Field-ion Microscope.
