CHAPTER 6 



THE ORIGIN OF CONTRAST IN 

 ELECTRON MICROSCOPIC IMAGES 



Contrast by Scattering; A Result of Lens Defects 



IN ordinary light microscopy the contrast arises mostly from 

 different absorption in different parts of the specimen. 

 Differences in refracting power and in fluorescence can be also 

 utilized. The first electron micrographs were photographs of 

 thin wires, metal foils with holes, and the like, that is, objects 

 which showed great differences in electron absorbing power. 

 This was thought a necessary condition for obtaining pictures, 

 and Marton,^*^ who was the first to apply the electron micro- 

 scope to the investigation of organic cells, impregnated them 

 with osmium salts. It appears that F. Krause (ref. 16, p. 55-60) 

 was the first to discover that this was not necessary. In 1936, 

 Marton ^^ carried out the first theoretical investigation of the 

 origin of contrast. He showed that scattering of electrons rather 

 than absorption is the basis of electromicroscopic image forma- 

 tion. This general result is now well established. Marton's 

 theory gave a satisfactory account only of the imaging of thick 

 objects, or of objects resting on thick supporting membranes. 

 It appears that most writers on the subject have applied it rather 

 uncritically also to small objects and thin membranes, that is to 

 say to the sort of objects usually employed to test the definition 

 of electron microscopes. An important step was made, in 1939, 

 by J. Hillier who pointed out certain effects arising from the 

 chromatic error of electron lenses. It will be shown in the 

 following that without these errors which most authors consider 

 as regrettable imperfections, the high performance of the un- 

 corrected electron microscope could not have been achieved. 



40 



