Theory of the Electron Microscope 



145 



By proving the advantage of oblique illumination, Abbe's 

 theory proved to be of great heuristic value, as it led to the 

 wide angle condenser which was one of the most important 

 improvements in the history of the microscope. But in pointing 

 toward wide angle illumination, the theor}^ pointed beyond itself. 

 It was soon found experimentally that it is better to fill the 

 cone of the objective aperture entirely or almost entirely * with 

 light, than to use hollow cones with nearly marginal rays, and 

 here the simple assumptions of the original Abbe theory were 

 obviously not fulfilled. It was suspected that Abbe had over- 

 emphasized the importance of coherence, and this view received 

 strong support, in 1896, when Lord Rayleigh showed that if 

 the light source is sharply focused on the object, it will become 

 almost exactly equivalent to a self-luminous object, as different 



a) Centra? beam only. 



Intensity/. 



h) Central beam and Fi'rab order side beams' 



c) side bearns only. 

 Fig. 54 



* In practice it is best to fill the cone about two thirds with light, as 

 this gives better contrast than the full cone. 



