APPENDIX 



DIFFRACTION THEORY OF THE 

 ELECTRON MICROSCOPE 



THE diffraction theory of microscopic vision deals with 

 Hmitations inherent in the wave nature of hght or elec- 

 trons, and assumes an optical system perfect in the geo- 

 metrical sense. While in light optics this assumption is fully 

 justified, it is not even approximately true in electron optical 

 systems ; therefore, we must never forget that we are dealing 

 with a high degree of idealization. 



Objects can be self-luminous, or illuminated by an external 

 source. In self-luminous objects every point is supposed to emit 

 radiation independently of all others, and, as the phases of the 

 light emitted by them are distril^uted at random, the resulting 

 intensities in the image will be the same as if only one point 

 emitted light at a time, and the elementary intensity components 

 were summed up. Therefore, the theory of the microscope for 

 self-luminous objects is essentially contained in the Airy figure, 

 which is the intensity distribution corresponding to a single 

 luminous point, shown for reference in figure 52. It is produced 

 by diffraction of a spherical wave at the edge of the objective 

 aperture. 



Non-self-luminous objects, or as it is more correct to say in the 

 case of transmission type microscopes, the interstices between the 

 objects, receive light from the same elementary sources, there- 

 fore, the waves issuing from these interstices are coherent, i.e., 

 capable of interference. The importance of this fact was first 

 noticed by Ernst Abbe, in 1870. Abbe published the resolution 

 limit w^hich bears his name, the elementary considerations which 

 led to it, and some supporting experiments, in 1873, but he never 

 published the mathematical details of his theory. This was done 



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