29 : 8/ Microscopy 



555 



would so absorb the electrons that no useful image could be obtained. 

 For electron microscope studies of biological materials, a small round 

 metallic screen is used to support the specimens. These screens are 

 about I inch in diameter and have approximately 400 wires per inch. 

 The electron beam passes through the holes in the screen. The object 

 can be moved around so that different parts are viewed just as in the 

 conventional light microscope. When the beam strikes a wire of the 

 screen, no electrons come through; the open areas between wires, 

 however, are large compared to the field of observation of the electron 

 microscope. 



§18 



UPtr 





fef 



Figure 12. Bacterial flagella at 41,700 x (left) and 73,400 x 

 haying the external contour of a counterclockwise double 

 helix. After L. W. Labaw and V. M. Mosley, "Periodic 

 Structure in the Flagella and Cell Walls of a Bacterium," 

 Biochirn. et Biophys. Acta 15: 325 (1954). 



For particulate suspensions, such as bacteria, phages, or mitochondria, 

 a thin plastic film is placed over the screen and then the suspension is 

 dried on the film. It is necessary to dry all the material because the 

 electron beam in the electron microscope operates only in vacuum. In 

 some studies, the specimens are shadowed with metallic atoms ; in others, 



