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As early as 1900 it was known that an axially symmetric magnetic field could converge a beam of electrons, and not long after it was found possible by such means to form actual electron images of an emitting source. The work of H. Busch, published in 1926 and 1927, unified experimental results by showing that there is an analogy between the behavior of light rays and electron beams, and that the imaging action on electrons of cylindrically symmetric magnetic or electric fields is similar to that of a glass lens on light. With the possibility of electron lenses comes the possibility of producing the electron counterpart of many optical systems. Moreover, when the fact that periodicity may be associated with moving particles was verified experimentally, it was realized that the magnitude of the resulting wave would allow electron analogs to possess much greater ultimate resolving power than corresponding light systems. Thus, in 1932, Borries and Ruska were able to predict that if the beam of light in a compound microscope were replaced by an electron beam, and the optical system were replaced by corresponding electronic elements, it should be possible to construct an electron microscope, which, by virtue of the extremely small wave length of high velocity electrons, should possess an ultimate resolving power surpassing that of the light microscope by several orders of magnitude.
Research Engineer, Research Laboratories, RCA Manufacturing Co., Camden, N. J.