Supermicroscopes 



35 



Fig. 12. Design of magnetic objective 



(E. Ruska, 1934) 



If these electrons are ultimately accelerated by v volts 



11,600 



the root mean square divergence is 



82 



— \^ 11,600 ^ J 



In the case of a tungsten cathode with T = 2,600° K and 

 V = 60,000 volts this is very nearly 2.\0r^. 



Whenever the cathode is imaged on the specimen, the di- 

 vergence cannot be less than this value, though it can be more, 

 as a consequence of lens errors and space charge effects. But 

 this divergence, small as it appears, is of the order of the aper- 

 tures which can be allowed in uncorrected objectives. Numerous 

 measurements, in particular those by R. F. Baker, R.C.A., dem- 

 onstrate that the resolving power and contrast improve very 

 appreciably if the illuminating angle is reduced considerably 

 below this limit. This, however, is possible only if the current 

 density at the specimen is reduced by defocusing below the maxi- 

 mum value which can be obtained by focusing the cathode or 

 the cross-over directly on it, or if part of the beam is cut out 

 by an aperture in the center of the condenser lens. Such arrange- 

 ments have proved very successful, and the modern tendency 

 in electron microscopy is to operate with almost parallel ilium- 



