CHAPTER 4 

 ELECTRON MICROSCOPES WITHOUT LENSES 



THESE devices, also called simple microscopes, utilize in a 

 very direct and ingenious way the fundamental advantage 

 of electron optics, the possibility of realizing enormous 

 refractive indices. Everybody knows that the mercury column 

 in a thermometer appears somewhat enlarged by the refraction 

 of the glass. If we had a glass with a refractive index of say a 

 hundred, the thermometer would look very nearly like a solid 

 rod of mercury. In electron optics such refractivities and even 

 larger ones can be realized, and this fact is the basis of the two 

 types of simple microscopes which are without counterpart in 

 light optics. 



In the cylindrical microscope, invented by R. P. Johnson and 

 \\\ Shockley,^- a thin emitting filament is stretched in the axis 

 of a cylindrical glass envelope which is coated with a fluorescent 

 powder. This is shown in figure 7. The anode can be a wire 

 spiral in contact with the tube wall, but this is not essential. It 

 is known from the theory of cathode ray tubes that the usual 

 fluorescent powders are good secondary electron emitters, and 

 will maintain themselves very nearly at anode potential, provided 

 that the anode is not too far off or shielded. The electrons move 

 outwards radially in very nearly straight lines and produce a 

 strongly magnified projection of the emitting patches of the 

 filament surface on the tube wall. 



The spherical type, shown in figure 8, was first used by E. W. 

 Miiller.^-^ The fluorescent envelope is spherical, and the cathode 

 is a wire which ends in a hemisphere with a very small radius, 

 approximately in the center of the bulb. The main difference 



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