12 



The Electron Microscope 



selves increasingly with the distance from the axis. Of these, 

 only the coma increases with the first power of extra-axial 

 distance, therefore, this is the only one of importance in micro- 



FiG. 5. Chromatic aberration 



scopes in which the field is seldom more than a few degrees of 

 arc. The coma is illustrated in figure 6. A lens, corrected for 

 spherical aberration but not corrected for coma, images extra- 

 axial points in the shape of an arrowhead pointing toward the 

 axis. The point of the arrow corresponds to the ray which passes 

 through the center of the lens ; all other rays strike the screen 

 at a greater distance from the axis. Rays passing through coaxial 

 circles of the lens strike the screen again in circles ; these are, 

 however, not concentric, but fit between two straight lines which 

 pass through the point A' and form an angle of 60°. This is the 

 characteristic coma figure which increases linearly with the 

 distance from the axis. 



It is possible to correct electron lenses for coma alone, but 

 this is of little interest as long as the spherical aberration cannot 



Fig. 6. Coma 



