Contrast by Scattering 49 



course, even better to take the photographs through a fiher which 

 absorbs the fluorescence and transmits only the primary uhra- 

 violet Hght. 



It is evident that the beneficial eflFects of the chromatic and 

 spherical aberration exist only so long as the illumination of the 

 object can be considered as homogeneous and undififused. A 

 thick supporting membrane v^^ould destroy the high resolution 

 altogether. The same is true of the resolution of details in thick 

 objects. For instance, density differences in the membrane of the 

 bacillus will remain invisible, which differences could be easily 

 resolved if it were possible to spread out the membrane without 

 the bacillus. This fact is of importance in the interpretation of 

 micrographs. 



It may be added, that the beneficial effects of lens defects 

 turn into the reverse also in electron microscopes w^ith dark field 

 illumination. In these instruments, objects are shown up not in 

 the negative by the electrons which have dropped out of the 

 background illumination (missing electrons), but in the positive 

 by the scattered electrons themselves. This is at least one of the 

 reasons why such instruments were so far rather unsuccessful.^^ 



Contour Phenomena ; Fresnel Diffraction 



The chromatic fringes mentioned in the last section are very 

 often masked by a much more striking contour phenomenon : 

 the Fresnel diffraction fringes. These were discovered, in 1940, 

 independently by J. Hillier ^- and H. Boersch.'*^* 



In light optics, Fresnel diffraction arises if a part of a parallel 

 or almost parallel light beam is cut off by the edge of some dark 

 object. Instead of the more or less sharply defined shadow, one 

 finds at some distance behind the object a sort of oscillatory 

 transition between light and darkness, with several maxima and 

 minima. This phenomenon can be very easily observed in light 

 microscopes of only moderate magnification. If the microscope 

 is sharply focused on the edge of the object, the contour appears 

 sharp, but as soon as the objective is focused a little ahead or 

 behind the object, the edge appears surrounded by fine fringes of 



