36 



The Electron Microscope 



inating beams, with a divergence of less than 10"*, down to a 

 few 10"^. In the absence of the specimen these beams fill only 

 a small fraction of the objective aperture.'^ 



Cross -over. 



Fig. 13. Trajectories in electron gun 



The limitation of the aperture by a physical diaphragm in 

 the objective is a comparatively recent improvement in electron 

 microscopy. It was introduced, in 1939, almost simultaneously 

 by E. Ruska and B. von Borries ^^ and by M. von Ardenne. Its 

 introduction resulted in about doubling the obtainable resolution. 

 The extremely fine bores are rather difficult to make, and certain 

 disadvantages result from it. After prolonged operation a semi- 

 conducting substance of unknown origin forms a deposit on the 

 diaphragm.-"* This acts like a very poor electrostatic lens and 



* Cf. J. Hillier and R. F. Baker, A Discussion of the IHuminating 

 System of the Electron Microscope, Jour. Appl. Ph^vs. 16, 469-483, 

 August 1945. 



