I20 FUNDAMENTALS OF S U BMIC RO S C O PI C MORPHOLOGY I 



admiration on realizing how the resolving power has been increased 

 by a factor of about a hundred by the discovery of the electron 

 microscope ! 



The small aperture of the objective coils is responsible for the great 

 focal depth of electron optical images. This depth determines the ratio 

 between the layer thickness in the object imaged sharply and the re- 

 solving power. In the ordinary microscope with large aperture the 

 focal depth is only about i, which means that a section of several [x 

 thickness can be analyzed into successive optical sections at different 

 levels by means of the fine adjustment. In the electron microscope this 

 ratio is about looo. This is a drawback in the spatial analysis of the 

 object, but it is a very valuable aid to the sharp focusing of the image 

 and to obtaining stereophotographs (Ardenne, 1940b; Muller, 

 1942a; Heidenreich and Matheson, 1944). 



The similarities and the dissimilarities between ordinary and elec- 

 tron microscopy are listed in Table XIIL 



TABLE XIII 

 PROPERTIES OF ORDINARY AND ELECTRON OPTICAL IMAGES 



The electron microscope may be changed into an apparatus produ- 

 cing electron diffraction spectra if the objective current is turned oft", 

 and the projective lens is removed (E. Ruska, 1940). The electron 

 diffraction diagrams obtained have the appearance of X-ray diagrams ; 

 they are only formed if a crystal lattice is present in the object. 



