Analysis by the Electron Microscope 



139 



and diffraction patterns can be taken without removing it from 

 the position in which it was micrographed. Under favorable cir- 

 cumstances such diffraction patterns can be obtained with any 

 conventional electron microscope with sufficiently wide physical 

 aperture. Hillier and Baker '"^ noticed sharp black lines in elec- 

 tron micrographs of minute crystals which on closer investigation 

 were shown to be images of crystallographic planes, very nearly 

 parallel to the optic axis. They appeared blank because they 

 reflected the beam outside the paraxial zone of the objective. 

 The trace of the diffracted beams could also be found at a certain 

 distance from the crystal because of spherical aberration, as 

 explained in figure 51. It is an obvious conclusion that such 

 evidence of the structure of the particles investigated could be 

 found in most electron micrographs, if the contrast were not 

 insufficient. It may be suggested that the contrast could be 

 greatly improved by cutting out the central beam by means of 

 a small shield or a thin wire, as indicated in figure 51. Electron 

 micrographs and diffraction patterns could be taken in immedi- 

 ate succession, the first without a shield, the second with one, 

 and with longer exposures. 



A second method for the identification of substances is based 

 on the characteristic line spectrum of energy losses, discovered by 

 Ruthemann.^^ J. Hillier'*^ has described a special electron micro- 

 scope in which this effect is utilized. By a two-stage magnetic 



Fig. 50. Diffraction pattern and electron micrograph of the same 

 sample of monohydrated aluminum oxide, taken with R.C.A. diffraction 

 adapter. 



