54 The Electron Microscope 



Phase Contrast 



One of the first photographs in which Fresnel fringes have 

 been observed is reproduced in figure 15c. In 1943, the author 

 received an original copy of it from Dr. V. K. Zworykin. On 

 closer investigation, the Fresnel-fringe interpretation appeared 

 somewhat doubtful. Only two fringes were visible, which may 

 have been just as well steps in the light distribution, as in the 

 case of chromatic fringes, with which they happened to coincide, 

 if it was assumed that the characteristic loss was that in carbon, 

 24 volts, and multiples of it. But' the main reason to question 

 Hillier's and Boersch's interpretation was that the fine fibers 

 showed detail which appeared incompatible with a defocusing 

 of the order of 10 ^a, which had to be assumed if the contour 

 phenomena were to be explained as Fresnel fringes. 



This dilemma was solved only quite recently. It appears that 

 in addition to contrast due to absorption and to scattering there 

 exists in electron microscopy a third : phase contrast. This is a 

 wave-optical phenomenon, well known in light microscopy. 

 Transparent objects, with a refractive index different from that 

 of the medium (for instance: thin glass fibers) are almost 

 invisible if the objective is sharply focused on them; but they 

 produce very strong contrast on defocusing, if the focal plane 

 of the objective happens to coincide with the focal points or 

 lines of the object. In other words, what the microscope will 

 be able to see in such a case, is not the object itself, but the 

 disturbance caused by the object in the original, more or less 

 plane wave-front. 



It is known from electron diffraction experiments that solids 

 refract electron waves with a refractive index which is 



n 



=v>+^-+»^ 



where V is the volt energy of the electron and Vi the iyincr 

 potential of the solid, about 12 volts for carbon. In electron 

 microscopy it was generally assumed, against the evidence of 

 diffraction experiments, that even in thinnest objects the effect 



