Contrast by Scattering 55 



of refraction would be entirely masked by scattering, i.e., objects 

 would behave like frosted glass, not like polished glass. But the 

 above mentioned dilemma can hardly be solved otherwise than 

 by assuming that very thin objects can in fact act as fairly regular 

 refractors. This assumption is supported by the theoretical esti- 

 mate, mentioned on p. 35, that 80-90 per cent of the electrons 

 pass without collisions of any kind through organic foils of the 

 order of 100 A thickness, and by the evidence collected in elec- 

 tron diffraction experiments. Further evidence has been found 

 recently by Hillier and Ramberg who could explain the fine 

 details of the Fresnel fringes at the edge of thin collodion mem- 

 branes only by assuming that an appreciable fraction of the 

 primary wave traverses the membrane as a coherent wave, but 

 with a phase retardation roughly as calculated from the above 

 mentioned refractive index. 



The discovery of phase contrast is too recent to say much 

 about at the present time. It appears that it w411 have to be 

 taken into careful consideration in the interpretation of almost 

 all electron micrographs of very thin objects. 



Some very interesting possibilities arise from the fact that 

 very thin objects can act, at least partly, as regular refractors 

 and retard electron waves by one quarter, or even one half wave 

 without destroying their coherence. This means that in addi- 

 tion to the macroscopic, man-made electromagnetic fields, we 

 may be able to use the microscopic inner fields of solids for 

 electron-optical purposes, thus adding a new chapter to electron 

 optics. The field of application is of course restricted by the 

 limit of about half a wave maximum retardation ; but there are 

 some interesting optical devices which operate with retardations 

 of a quarter wave only. Chief of these is the ingenious pJiase- 

 contrast microscope of Prof. Zernicke * which allows to see 

 the true shape of non-absorbing objects with refractive indices 

 only slightly different from those of the surrounding media in 

 their true shape. Experiments to translate this instrument into 

 electron-optical terms are in progress. 



* Prof. Zernicke, Phvsica 1, 689, 1934; C. R. Burch and J. P. P. Stock, 

 Journ. Sci. Instr. 19, 71, 1942. 



