102 The Electron Microscope 



R 65 130 260 520 1040 cm2gauss2volt-i 



1.54 0.81 0.50 0.34 0.26 



D 



c 

 D2 



7.6 2.4 1.12 0.68 0.51 cm-2 



Do 

 f_ 



fo 



C_ 

 C. 



Cf 



1 1.414 2 2.828 4 



1 0.74 0.65 0.62 0.67 



1 0.63 0.59 0.71 1.06 



1 0.47 0.38 0.43 0.71 



(Cf)o 



Consider a typical objective lens with D = 0.25 cm, Hmax = 

 10,000 gauss with V = 100,000 volt. This gives R = 62.5, i.e., 

 it corresponds very nearly to the first column of the table. We 

 take now this lens as unit, and refer the other columns of the 

 table to it. Assuming that Hmax is kept constant, these cor- 

 respond to proportional increase of the linear dimensions in the 



ratios V2, 2, V2 X 2 and 4, as shown in the table below the 

 line. It may be seen that both / and C decrease first, go through 

 a minimum, and increase again. The most important parameter, 

 the product Cf, which figures in equation (24b), has a minimum 

 at twice increased dimensions, but this is so flat that even a 

 three to four fold increase gives appreciably the same gain.* 



As the dimensions of the lens increase, starting from small 

 values, the object must be approached to the center of the 

 field, and finally, at large values of R, it has to be positioned 

 beyond the maximum, if it is to be focused at infinity. The 

 optimum is reached when the object coincides with the maximum 

 of the field, as was first pointed out by W. Glaser. Such an 

 arrangement has been also considered in careful detail by L. Mar- 



* The connection between lens strength and rcsokition has been dis- 

 cussed in detail by V. E. Cosslett, "The variation of resolution with 

 voltage in the magnetic electron microscope," Proc, Phys. Soc, 58, 

 443 (1946). 



