38 The Electron Microscope 



Hon. If f changes but the position of the lens does not change 

 appreciably, this effect is very nearly the same as if / had been 

 kept constant and the lens shifted along the axis, and the result 

 will be a radial shift of extra-axial points. If it were possible 



to make the coil current J strictly proportional to VV, both 

 effects could be avoided. But this is practically impossible, as 

 lens coils have very high inductivities and J cannot be made to 



follow \/V instantaneously. Therefore, the only possibility is to 

 keep both J and V constant, and if the variation of f is to be kept 

 within given limits, it is convenient to prescribe for J tolerances 

 half as wide as for V. In the best modern magnetic microscopes 

 these tolerances are about 0.002 per cent for V and 0.001 per 

 cent for J. 



It can be said that the development of the magnetic electron 

 microscope, from about 1934 onward, went exactly parallel to 

 the improvement in stabilization of the driving circuits. By 1940, 

 this development had reached a stage at which further improve- 

 ments in the stabilization, very difficult anyhow, would have 

 brought no corresponding improvements in the performance of 

 the microscopes. This progress was obtained, however, at the 

 price of very elaborate and expensive auxiliary equipment of 

 the magnetic microscope. 



Electron microscopes, operating with electrostatic lenses only, 

 are free from this necessity. The power of an electrostatic lens 

 can be written approximately 



1 /U\2 



j=const.(^yj (19) 



where U is the potential difference between the electrodes of the 

 lens. It has been assumed that there are in it only two different 

 electrode potentials. By making U ^ V this becomes a constant. 

 This means that one group of the lens electrodes must be con- 

 nected with the cathode, the other with the accelerating potential 

 V. Such fixed focus or unipotential lenses were first used by 

 Briiche and his collaborators in the A. E.G. laboratory, from 

 1932 onward. H. Mahl i^ utilized them, in 1939, for the first 



