76 



The Electron Microscope 



tional features have probably remained unaltered in the commer- 

 cial model which has just appeared on the market at the time 

 of writing. 



A full view of the small microscope has been already shown 

 in figure 17. Figure 23 is a slightly simplified section of it. The 

 main dififerences from the large model will become evident from 

 a comparison with figure 18. Starting from the cathode end, 

 we note first of all the absence of the condenser lens. This has 

 been replaced by electrostatic focusing, by impressing different 

 potentials on the cathode shield which in figure 23 is called the 

 grid. If the grid is at cathode potential, the cross-over will very 

 nearly coincide with the object which is now illuminated by a 

 bundle of fairly wide divergence. This Inmdle is not cut off, as 

 the new microscope contains no physical objective aperture. It 

 contains only a small diaphragm between the electron source 



FLUORESCENT SCREEN 



PHOTOGRflPHID EMULSION 



PROJECTION LENS POLE PIECES 



OBJECTIVE LENS POLE PlECEb 



SPECIMEN STR6E 



SHIELDING flPERTURt 



GRID 



FILAMENT 



FILAMENT flOjUSTMENT 

 INSULATING 6UN SUPPORT 



Fig. 23. Section of R.C.A. electron microscope, desk model 



