Il6 FUNDAMENTALS OF SUBMICROSCOPIC MORPHOLOGY I 



able property, however, of possessing at the same time the character 

 of waves. They can be deflected by crystal lattices and, like X-rays, 

 give rise to interferences. Hence an electron ray represents a corpus- 

 cular ray and a wave train at the same time! The wavelength X of 

 electron rays depends on the voltage applied to the cathode tube; A is 

 inversely proportional to the square root of the voltage. In the case 

 of light waves, the velocity of propagation in vacuo is independent of 

 the wavelength. This does not apply to electron rays, for, besides 

 lowering the wavelength, an increase in voltage also results in a greater 

 velocity of the electrons. This velocity may become as high as io^° 

 cm/sec, i.e., 1/3 of the velocity of light. Since electron microscopy 

 operates with very high tensions, the electrons are "rapid", i.e., rich 

 in energy. At a tension of 57 kV the wavelength amounts to about 

 5.10"'° cm = 0.05 A (BoRRiES and Ruska, 1939a). This is one 

 twentieth of the wavelength of hard X-rays (about i A) and one two 

 hundredth of the wavelength of soft X-rays (about 10 A). In spite of 

 this extremely small ^^avelength and in contrast to X-rays, electrons 

 have no penetrating power, as the electrons are already totally ab- 

 sorbed by layers of solid substances of a thickness of o.i /x. When 

 passing through an object, they lose part of their energy and leave 

 it with a somewhat smaller velocity, i.e., with a changed wavelength 

 depending on the energy loss in the object. This means that the elec- 

 tron beam, originally monochromatic, becomes polychromatic, and 

 images from electron lenses show not only spherical but also chromatic 

 defects as light microscopic images do. 



The electron microscope. Since the resolving power of the microscope 

 depends on the order of magnitude of the wavelength of the light 

 used, one might expect great improvement in the resolving power of 

 an X-ray microscope as compared to the ordinary microscope. That 

 dream could not be realized, because lenses for X-rays do not exist. 

 The possibility of focusing electron rays has, however, made the 

 construction of a short-wave microscope feasible. (Martin 1938; 

 BoRRiES and Ruska 1939b; Ardenne, 1940a, b; Zworykin, 1940, 

 1941 ; Zworykin, Hillier, and Vance, 1941 ; Borries, 1941 ; Induni, 



I945-) 



The electron microscope operates according to the same principle 

 as the ordinary microscope. The light source is replaced by a source 

 of electrons. Usually this is a hot cathode, but Induni (1945) has also 



