ELECTROSTA TIC ELECTRON-OPTICS 



27 



1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 



f 22.5 



y 



[^ 



y 

 y 



r 



^ 



/■ 



y 

 y 



.' 

 y 



-^ -y 



,^— ^ ^ 



^^: -^^— 



0.40 

 V2 



Fig. 18 — Principal focal distances — concentric tubes. 



