CHAPTER 14 



CHEMICAL AND STRUCTURAL ANALYSIS BY 

 THE ELECTRON MICROSCOPE 



ELECTRON microscopy possesses no equivalent of color 

 discrimination. This puts it at a certain disadvantage 

 against light microscopy in which substances can be often 

 identified by their characteristic color or dyed with selective 

 dyes. Different substances under the electron microscope are 

 distinguished only by their density, or more exactly by their 

 electronic density which is almost the same, except if they have 

 characteristic shapes, as in the case of crystals. This natural 

 disadvantage has been more than compensated by several recent 

 improvements which promise to make the electron microscope 

 the most powerful tool for micro-analysis. 



In 1942, Hillier, Baker and Zworykin '^ equipped the Type B 

 electron microscope of the R.C.A. with a diffraction adapter 

 which has become a standard component of the new universal 

 E.M.U. model. The adapter is a unit of the same size and inter- 

 changeabable with the projection lens. It contains a specimen 

 chamber in to which the object can be introduced through an 

 air-lock. Diffraction diagrams can be taken within 2-3 minutes 

 of taking a micrograph of the same object. The ray diagram is 

 shown in figure 49. The objective lens forms a reduced size 

 image of the cross-over which acts as source for the diffraction 

 camera. The previous projection lens has been divided to contain 

 two coils, the upper of which acts as a projection lens when 

 electron micrographs are taken, but is de-energized for diffrac- 

 tion photographs. Instead, the lower coil is used as a lens for 

 focusing the diffraction images on the photographic plate. This 

 arrangement allows considerably higher intensities and shorter 



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