OPTICAL ANALOGY 



345 



current supplies for the lenses have a constancy of better than 0.002 

 per cent for the objective lens, 0.004 per cent for the projection lens, 

 and 0.02 per cent for the condenser lens (Zworyken, Hillier, and Vance 

 [1941]). This stability of the current is necessary if one wishes a resolu- 



tion of 10 cm. 



Mycobacterium 



tuberculosis ( human ) 



x 21,000 



Fig. VIII-22. An electron image of a human tuberculosis bacterium (original 

 magnification was 42,000 X) obtained with an RCA electron microscope. (By 

 courtesy of V. K. Zworykin, RCA Research Laboratories.) 



Optical Analogy 



A diagrammatic sectional drawing of an electron microprojector 

 incorporating the essential features of a hot-cathode electron source with 

 magnetic field electron lenses is shown in Fig. VIII-23. The train of 

 magnetic lenses conforms to the standard design of the two-stage hot- 

 cathode magnetic lens electron microprojector. 



The electron gun G at the top is corona shielded. Its electron beam 

 can be focused by the rack-and-pinion movement F of the anode on the 

 condenser stop. Its cross-sectional area at this point is used as the 

 electronic source. The condenser lens C focuses this beam on the 

 specimen placed in the 0.12-sq-mm cavity of the object stage at S. 

 The first-stage image, after a cross-over of the beam, is focused in the 

 first-image plane I. 



