The Resolution Limit 61 



described in the previous chapter in connection with figure 15b. 

 Once this method was found, correction became possible by 

 screwing eight small iron screws into the gap between the pole 

 pieces, controlling the results all the time by means of Fresnel 

 fringes. This very laborious method led to brilliant results. 

 The resolution limit of the R.C.A. electron microscope was at 

 once reduced from about 20 to 10-8 A, which is only very little 

 more than the above calculated theoretical limit of 6 A. 



It may be mentioned that the above theoretical estimate with 

 geometrical addition of errors cannot lay claim to great accuracy. 

 More recent calculations by W. Glaser, using Born's method of 

 unified treatment of geometrical and dififraction errors, seem to 

 indicate that the estimate errs on the safe side, and that the 

 theoretical limit may be 3.5-5 A only. 



We must now check the assumption that the required con- 

 stancy of voltage can be realized. With C = 2, / = 0.3 cm and 

 V ^ 60 kv, equation (24) gives 4.5 X 10'^ for the optimum 

 aperture. This is very near to the value 5 X 10"^ used by von 

 Ardenne in taking the micrograph shown in figure 16. The 

 diameter of the objective aperture is 0.0027 cm, and the spheri- 

 cal aberration ds is 5.35 A. The relative fluctuation ^r-r- which 



according to equation (20) produces the same error is 0.002 per 

 cent. This degree of stability and an even better one has been 

 indeed realized in modern electron microscope circuits. ^^' -^ 



The formulas (25) and (26) were derived under the assump- 

 tion that no lens stronger than a certain lens of maximum power 

 can be realized because of saturation of the magnetic pole pieces. 

 This limitation does not exist in the case of the electrostatic 

 microscope, but it so happens that electrostatic lenses have very 

 nearly the same limit, though for an entirely different reason. 

 The power of an electrostatic lens is proportional to the intensity 

 of its electric field, and beyond a certain intensity flash-overs 

 occur through autoelectric emission. For the present the resolu- 

 tion of the best electrostatic microscopes is still considerably less 

 than that of the magnetic type (about 50 A). 



