ELECTUON MICKOSCOPY 



These studies started probably around 1929. ber with two-way motion of the specimen 



At least, this is the date of a patent applica- and an air lock, as well as a photographic 



tion on electrostatic lenses by I^oll. The chamber for internal photography of the 



fii-st publications on the study of electro- electron micrographs. This photographic 



static and magnetic lens elements date from chamber was also provided with an air lock. 



1931 when Davisson and Calbick published The first electron micrographs of biological 



a note on the focal length of an electrostatic objects in 1934 were made on material im- 



lens and Knoll and Ruska studied the be- pregnated with osmium salts because of the 



havior of magnetic lenses. Further studies of then prevalent notion that the electron 



electrostatic lenses were published by Briiche beam would destroy any biological material, 



and collaborators. Within a year, it was recognized that this 



Experimental electron microscopy had its impregnation was not absolutely necessary 



beginning in 1931. The formal beginnings are and that by reducing the total exposure time 



marked by the patent application of Riiden- and beam intensity one can achieve biologi- 



berg and the publication of a paper by Knoll cal pictures without destroying the material, 



and Ruska; however, the subject goes back This was possible because of the introduction 



at least three or four years earlier to discus- of the internal photography described above, 



sions m physics colloquia in the Berlin area During 1934, Ruska had also demon- 



as seen from a publication by Gabor. Gabor's strated that the electron microscope had a 



name should be mentioned here in another resolution somewhat greater than that of the 



respect too. The studies of Knoll and Ruska light microscope. These observations, com- 



on magnetic electron lenses were greatly bined with the theoretical prediction that the 



facilitated by the fact that an ironclad mag- electron microscope is able to reach a re- 



netic lens had been developed in 1926-27 by solving power exceeding that of the light 



Gabor. During 1931-32, the first instru- microscope by a factor of several hundred, 



ments which merit the name of electron led to increasing efforts toward an improve- 



microscopes had been built. The fii'st of these ment of the resolving powder of the electron 



probably is one built by Knoll and Ruska for microscope. 



the observation of emitting objects, which From the beginning, it was understood 



utilized magnetic lenses. A second was the that a good part of the mechanism of the 



instrument built by Briiche and Johannsen, image formation is due to scattering of the 



with electrostatic lenses, to study emission electrons within the transmission-type ob- 



phenomena. Before the end of 1932, Mar- jects. Both Ruska and Marton referred to a 



ton had a simple instrument using a magnetic scattering mechanism in their 1934 papers, 



lens to investigate emission phenomena, and In 1936, Marton made the first quantitative 



later transmission phenomena. During 1933, attempt to explain the image formation on 



two instruments were built. One was Knoll the assumption of multiple scattering by the 



and Ruska's revised and improved transmis- object. The same approach was used a couple 



sion instrument, using two magnetic stages of years later by von Ardenne. Very soon, 



and having a limiting magnification of however, it became obvious that the average 



12,000. IVIarton's second instrument also specimen of electron microscopy is much too 



used magnetic lenses for transmission ob- thin for multiple scattering. Consequently, 



servation, but it was simpler and its limiting ]\Iarton and Schiff in 1941 studied image for- 



magnification was of the order of 2,000. The mation assuming single scattering. In the 



first biological observations were carried out postwar years, single scattering calculations 



with Marton's two-stage instrument in 1934. were further improved by von Borries, Lenz 



Marton built in 1934 an improved two-stage and others, 



instrument, incorporating a specimen cham- In the meantime, efforts were under way 



1.56 



