KIDNEY ULTRASTRUCTURE 



that the energy losses are all the so-called 

 characteristic energy losses and are very 

 well defined. In this case, we will have an 

 object represented in the image plane by two 

 circles of least confusion, one* corresponding 

 to the primarj^ energy and the one to the 

 primary energy minus the characteristic 

 energy loss. This will result, in principle, in 

 a doubling of the images, but as the differ- 

 ence in primary energj^ and primary energy 

 minus characteristic energy are relatively 

 small (of the order of 10-15 electron volts), 

 the resulting doubling will be of the order of 

 a few angstroms and will be hardly noticea- 

 ble. 



Furthermore, the effect will depend upon 

 the relative cross sections for the elastic and 

 inelastic event. If these two are widely dif- 

 ferent, then one of these circles of confusion 

 will be predominant and the other will be 

 disappearing in the background. The situa- 

 tion is changed if we consider not the charac- 

 teristic event but a continuous energy dis- 

 tribution of inelastically scattered electrons. 

 Such inelastically scattered electrons exist 

 and have been measured extending over an 

 energy range of several hundred electron 

 volts below the primary energy. Although the 

 cross section is relatively low, the wide 

 energy distribution will contribute to a wash- 

 ing out of both the resolution and the con- 

 trast in the final image. 



It is interesting to see in detail what the 

 effect may be in approximately spherical 

 objects. For very thin and small objects, a 

 loss of contrast induced through the chro- 

 matic aberration of the inelastically scattered 

 electrons is negligible. If the aperture of the 

 illuminating beam is very small (a ill. < 10~^ 

 rad), the loss of contrast may be 15 % as de- 

 fined by the scattering process alone (phase 

 contrast is favored by a very small illuminat- 

 ing aperture). For objects of medium thick- 

 ness (i.e., objects whose thickness corre- 

 sponds approximately to the mean-free path 

 of the electrons in the object) and for large 

 objective apertures, a good part of the in- 



elastically scattered electrons will be con- 

 centrated in the image and therefore the loss 

 of contrast may l)c quite marked. 



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B64, 449 (1951); Nat. Bur. .Stand. Circular 



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L. Marton 



KIDNEY ULTRASTRUCTURE 



The mammalian kidney is a highly vascu- 

 larized organ and the contact between its 

 capillary bed and its functional units, the 

 nephrons, is extensive. Each nephron con- 



163 



