Il8 FUNDAMENTALS OF SUBMICROSCOPIC MORPHOLOGY I 



magnitude o.oi /< = loo A) transmit electron rays. In this case the 

 imao-e formation is due to the fact that many of the electrons are de- 

 fleeted from their rectilinear trajectory by the atoms in the object, in 

 much the same way as a small celestial body which enters the sphere 

 of attraction of a star. Now if the object lens possesses a small aperture, 

 the electrons which are deflected do not reach the image and the object 

 appears darker than the background. Since heavy atoms deflect elec- 

 trons more strongly than do light ones, metallic colloid particles appear 

 darker than organic particles, which often furnish a very faint contrast. 

 It is possible to enhance the contrasts by introducing heavy atoms such 

 as iodine (Husemann and Ruska, 1940), osmium (OsOJ or tungsten 

 (phosphotungstic acid; Hall, Jakus and Schmitt, 1945) as "electron 

 dyes". It must be emphasized that the comparison with "dyes" is not 

 strictly correct because the absorption of electrons is very slight. If 

 there is appreciable absorption, e.g. in thick sections, organic objects 

 are instantly burnt by the high energy released by the captured 

 electrons. Therefore, preparations for the electron microscope must 

 be so thin that the electron absorption is negligible. As indicated 

 above, the contrast observed is due to scattering. 



The electron scattering comprises different phenomena. In the 

 lirst place there is the coherent diff'raction of the beam in much the 

 same way as in the ordinary microscope. The coherent light of the 

 diffracted rays is apt to interfere and to furnish a uniform image when 

 these rays are collected by a lens. However, the scattering of electrons 

 which causes the contrast in the electron microscope is incoherent, i.e. 

 the deflected rays are no longer able to interfere with each other and 

 to be focused at the proper place in the image screen. Most of these 

 aberrant electrons are scattered elastically, when the ray is deviated 

 by some atom nucleus without loss of energy. But there is also inelastic 

 scattering whereby the electron loses some of its energy, and then 

 not only is it deflected from its original path, but its velocity is slowed 

 down at the same time, so that the wavelength of the ray is increased. 

 This corresponds to a chromatic error. The geometric and chromatic 

 aberrations of the scattered electron cause an indistinct blurred image 

 if they reach the objective. They are therefore screened off" by a 

 narrow diaphragm (Fig. 80); consequently, the more incoherently 

 electrons are scattered by an object, the darker it must appear on the 

 image screen owing to this loss of electron light. In order to obtain 



