32 



The Electron Microscope 



Source^ 



Condenser lensA 



m 



Object plane 

 Objective, 



1 



Intermecft'abe,. 



/ma^e. i — 

 Projector 

 Jens. 



I \ 



Photographic I 



/ 



/ \ 

 / \ 



4. 



Fig. 10 



fact, most of the technique of these early microscopes originated 

 in high voltage cathode ray oscillography, and the first instru- 

 ment of Knoll and Ruska was a slightly modified oscillograph. 

 (For the history of this development, cf. reference 2.) In a gas 

 discharge tube, it is almost impossible to keep the voltage varia- 

 tion below about 1 per cent; it was therefore a very essential 

 step when this was replaced by a high vacuum — hot cathode 

 discharge. 



The magnetic lenses very soon assumed a shape from which 

 later designs have departed only in minor points. Figure 12 

 shows details of the design of the objective in Ruska's microscope 



