ELECTKO.N MlCKOSCOl'Y 



Mecklenburg and Malil produced much bet- 

 ter emission micrographs with thermionic 

 electrons. 



Secondary electrons for producing emis- 

 sion microscopy had been hrst used in the 

 middle thirties. A crude image had been 

 shown in 1933 by Zworykin, and a year or 

 two later Knoll had shown better results. The 

 best results to date are those of Mollcn- 

 stedtin 1953, who replaced the primary elec- 

 trons with ions for the excitation of second- 

 ary electrons. 



Photo emission as a source of emission 

 microscopy had been first used by Briiche 

 and by Pohl in 1933 and 1934. The method 

 has been further developed in the hands of 

 Grivet and Septier in 1956. 



The most spectacular emission microscope 

 is not a true microscope in the optical sense. 

 Field emission microscopy started with a 

 two-dimensional model of Johnson and 

 Shockley in 1930. Very soon thereafter, in 

 1935-37, Mueller developed the point emis- 

 sion microscope. This was followed in 1951 

 In' the invention of a field ion microscope 

 with a best achieved resolving power of about 

 2.7 A. 



A method derived from dark field micros- 

 copy is the so-called schlieren observation of 

 electrostatic and magnetic fields and other 

 perturbations of the optical medium. One of 

 the first observations of this kind was pub- 

 lished by Boersch m 1937, when he demon- 

 strated the dark field image of a vapor 

 stream. In 1943-44, von Ardenne deliber- 

 atel\^ produced schlieren conditions. Due to 

 the war, this paper was not known in the 

 United States, and it was independently dis- 

 covered by Marton in 1946-47 who with 

 Simpson and Lachenbruch applied this 

 method for quantitative evaluation of mag- 

 netic fields. 



As this review is hmited to the electron 

 microscopical aspects of electron optics, a 

 very brief enumeration of other de^•elop- 

 ments of electron optics may be sufficient. The 

 oscilloscope tube development was started 



in 1894 by A. Hess and in 1897 by F. Braun. 

 These early tubes used gaseous discharge 

 with cold cathodes. The hot cathode in the 

 cathode ray tube was introduced by Wehnelt 

 in 1903. The first proposals for the use of 

 cathode ray tubes as image transmitting ele- 

 ments were made in 1900-07 by Dieckmann 

 and Glage, as well as by Rosing. Storage of 

 the image now used in television pickup 

 tubes was first announced in 1908 by Camp- 

 bell-Swinton. Modern development is linked 

 to the names of Zworykin between 1925-33, 

 Round in 1926, Farnsworth in 1927, and 

 Henroteau in 1929. 



Mass spectrographs are another interest- 

 ing chapter of electron optics. They were 

 first conceived by J. J. Thomson in 1897; 

 180° magnetic deflection was fu'st used by 

 Classen in 1908. Ten years later, Dempster 

 used magnetic focusing; and in 1919, Aston 

 invented ^-elocity focusing. Modern mass 

 spectroscopy (developed in 1932-34) is linked 

 to the names of Herzog and Mattauch w^ho 

 developed the electron optics of mass spec- 

 troscopy devices, as well as Bainbridge, Bar- 

 ber, and Stephens. 



The development of electron optical 

 theory is linked to three names essentially: 

 Stoermer we mentioned earlier, but the for- 

 mal theory of axially symmetrical devices 

 was not developed until the years 1933-36. 

 Foremost are the names of Glaser and Scher- 

 zer: the first used an essentially Fermat- 

 Hamiltonian approach, whereas the latter 

 preferred the trajectory method. The first 

 linking of geometrical electron optical theory 

 to wave mechanics was done by Glaser in 

 1943. 



We now present a short description of 

 the development of physical electron optics. 

 Electron difTraction instrumentation for the 

 observation of crystallographic structures 

 has been greatly improved by the addition 

 of a magnetic lens to the diffraction instru- 

 mentation by Lebedeff, in 1931. Modern 

 diffractographs took advantage of the high 

 resolving power of combined electron optical 



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