CHAPTER 5 



ELECTRON MICROSCOPES OF THE 

 TRANSMISSION TYPE (SUPERMICROSCOPES) 



THE electron microscopes in which the image is formed by 

 means of electron lenses can be conveniently divided into 

 two classes : for self-luminous and for non-self-luminous 

 objects. A self-luminous object in electron optics is a cathode; 

 it may be thermionic, photo-electric, radio-active, or a secondary 

 emitter. The choice of objects is therefore rather restricted, but 

 there are also other factors which restrict the importance of this 

 type of instrument. In the other type of microscope, the image 

 is formed by electrons emitted by a separate cathode, which 

 strike the object after having been accelerated to a rather high 

 velocity. Only transmitted electrons can be used, as true elastic 

 reflection is a very rare process. Electrons which reemerge by 

 diffusion with reduced velocity cannot be distinguished from sec- 

 ondary electrons, and microscopes which utilize these belong 

 strictly speaking to the first class. Such reflecting microscopes, 

 the electronic counterparts of the metallurgical microscope, were 

 up to now rather unsuccessful, and they are not likely to have a 

 great future, the more so, as we possess now in the scanning 

 microscope (to be discussed in a later chapter), a fully satisfac- 

 tory tool for surface investigations. 



The first electrostatic microscope for self-luminous objects 

 was constructed by E. Briiche and H. Johannson,^^ and started 

 operating in 1932, almost simultaneously with the first super- 

 microscope. In the following, we shall deal only with the second 

 type, not only because of its wider scope and greater general in- 

 terest, but also because the microscope for self-luminous objects 

 appears to have completed its development, about the year 1936. 



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