Electron Microscopes IVitlwut Lenses 



27 



metals. Their cathode was a thin wire of the metal to be investi- 

 gated, etched to a fine point, approximately hemispherical, with a 

 curvature radius of about 2.5 X ^^^' cm. As the bulb radius was 

 5 cm, the magnification was about 200,000. The pointed wire 

 was welded to a filament, as shown in figure 8, which enabled 

 the sample to be heated during observation. Figure 9 is an 

 example of the many beautiful patterns obtained in this investi- 

 gation. It shows the emitting patches of a minute single crystal 

 of nickel and the changes which are produced in them by heat 

 treatment. 



Fig. 8 



On w^atching these microscopes in action, one can often see 

 very small bright points moving hesitatingly about on the surface. 

 Some of these are probably caused by single ions, crawling about 

 on the crystal and producing increased local emission. To make 

 a single ion visible is an achievement unmatched up to now by 

 any of the more complicated microscopes. 



