CHAPTER 7 



THE RESOLUTION LIMIT OF THE 

 UNCORRECTED ELECTRON MICROSCOPE 



THE resolution limit of an electron microscope is defined 

 as the smallest distance at which two small particles of 

 sufficient scattering or absorbing power will appear as 

 two, before merging into one. Figure 16 is a reproduction of a 

 brilliant test photograph obtained by M. von Ardenne,^^ in 

 1940. It shows particles of colloidal gold, deposited from an 

 extremely dilute solution on a thin membrane of collodion. It 

 can be seen that two particles at a distance of 30 A could just 

 be separated ; this, therefore, is the resolution limit. At the same 

 time, the photograph gives also a fair estimate of the detection 

 limit, as the diameter of the gold particles was about 10 A, and 

 this was just sufficient for good visibility. 



Von Ardenne's resolution limit of 30 A was the best achieve- 

 ment, in 1940, equaled in the same year by the R.C.A. In 1939, 

 the best resolution stood at 50 A, obtained by H. O. Miiller, a 

 collaborator of Ruska,^^ and by L. Marton and collaborators.^^ 

 By the end of 1943, the limit was improved to 24 A by V. K. 

 Zworykin and J. Hillier,^* while E. Ruska and his collaborators 

 achieved about 18-20 A. No further progress was made until 

 1946, when J. Hillier and E. G. Ramberg in a brilliant investi- 

 gation eliminated the lens astigmatism which until then stood 

 between the practical and the theoretical limit, and brought the 

 resolution down to 8-10 A. 



We have seen that there are three kinds of limitation for the 

 resolving power, due to : spherical aberration, diffraction and 

 fluctuations of the driving voltage. The last is often called 

 chromatic error but it is better to avoid this expression, as w^e 

 have seen that a part of the chromatic error is beneficial. All 



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