XII. ELECTRON MICROSCOPY 389 



is not as satisfactory as direct observation through a lij2;ht micro- 

 scope. In the electron microscope, however, the angular aperture 

 of the objective is usually very small, less than 0.01 radian. Thus, 

 in the electron microscope the depth of field is correspondingly large 

 and may be several microns for work requiring moderately good re- 

 solving power. This is actually an important advantage for the 

 electron microscope since, for any specimen suitable for examination 

 in the electron microscope, the entire field of view is in focus even 

 though it may have considerable depth. As a result of this and 

 because of other technical reasons electron micrographs, that is, 

 photographically recorded electron microscope images, usually con- 

 tain more information than the images made visible by means of a 

 fluorescent screen. This also makes it possible to obtain stereoscopic 

 images providing three-dimensional pictures of the specimen at the 

 highest magnifications. Unfortunately, because all techniques in 

 light microscopy have been developed with these limitations in mind, 

 it will be necessary to develop new techniques before the full useful- 

 ness of the electron microscope in this extra dimension can be 

 realized. Nevertheless, it is a point that the present or prospective 

 users of an electron microscope should keep in mind, because very 

 often, even wdth available techniques, it is possible to obtain micro- 

 graphs with the electron microscope that show a manifold improve- 

 ment over those obtained with the light microscope, even though the 

 extremely high resolving power of the electron microscope may be 

 only partly utilized (Fig. 4). 



One of the most important differences in the results provided 

 by the light and electron microscopes is found in the structures of 

 the images themselves. In the light microscope the image exists 

 by virtue of the fact that different points of the specimen are different 

 in a number of their physical properties — such as refractive index, 

 selective absorption, and scattering power for light — and is the result 

 of a superposition of the effects of these differences. In the electron 

 microscope, on the other hand, the situation is much simpler. The 

 image is monochromatic and consists only of intensity variations 

 resulting from differences in the electronic scattering power of differ- 

 ent points of the specimen. While this simple relationship is seriously 

 disturbed by the lens aberrations and by the diffraction of electrons 

 by crystalline particles in the specimen, it leads nevertheless to a 

 more direct interpretation of the image. 



