ELECTRON MICKOSCOl'Y 



Electron 



Beam 



Electron 



/O 



o 



-3020 



Beam 



\/ 







B ^-^ 



20fO 



21 10 



1 1 So 



OOOJ 



loTo 



(a) (b) 



Fig. 14. Schematic representation of micro-diffraction, a) diffraction pattern ob- 

 tained from a single crystal particle, b) ring pattern obtained from a collection of 

 colloidal particles. 



Fig. 15. Micro-diffraction pattern from a col- 

 loidal particle of silver iodide, a) selected area of 

 particle, b) micro-diffraction pattern from this se- 

 lected area. 



such as finite aperture of illumination, etc. 

 play a part in line broadening. 



Dark Field Image Analysis. A useful 

 complement to micro-diffraction experiments 

 is dark-field image analysis. This method was 



developed by Mollenstedt and others (35, 36? 

 37) and has principally been applied to 

 lamellar crystals. The technique consists of 

 isolating a single spot on the diffraction 

 pattern by means of an aperture in the plane 

 of the objective diaphragm, from which the 

 part of the electron beam, focused as the 

 spot, passes through the small aperture; 

 the latter is then moved aside from the 

 normal position on the axis into the position 

 for dark field. 



Thus a dark-field image can be obtained 

 which corresponds to the portion of the crys- 

 tal containing the lattice planes from which 

 diffraction was actually taking place. In this 

 manner each spot of a single crystal diffrac- 

 tion pattern can be studied individually and 



138 



