ANALYTICAL ELECTRON MICROSCOPY 147 



connections to the DuBridge and Brown (1933) amplifier and the 

 photocell B battery are made thi'ough a shielded flexible cable, which 

 also carries the galvanometer leads. The amplifier is powered by- 

 storage cells ( 12 volts) . A Leeds and Northrup type R galvanometer 

 is used. The entire apparatus is mounted in a dark room, and black 

 felt is used to prevent stray light of the apparatus from reaching 

 the photocell. 



Procedure. The image of the lamp filament is sharply focused 

 on the center of the cardboard screen with the dark-field diaphragm 

 removed and the slide in place. It is convenient to place a 12 power 

 convex lens in front of the lamp housing to enlarge the image to 

 about 4 in. The dark-field diaphragm is inserted, and, with the room 

 almost completely dark and the felt curtains in place to eliminate 

 stray light, the image of the ash is focused on the screen with the 

 axial adjustment of the mechanical stage. With the other adjust- 

 ments of the stage, the desired area is brought to the center of the 

 screen and the photocell box is moved so that the aperture is 

 properly set. Then the shutter is pulled and the galvanometer deflec- 

 tion is observed. The reading obtained when the clear glass of the 

 slide is placed in the optical field is substracted from the first 

 deflection, and the resulting figure is proportional to the quantity of 

 ash on the area taken when this quantity is small. Similar data 

 obtained for other areas enable a comparison of relative amounts 

 of ash without regard to the absolute quantities involved. 



F. ANALYTICAL ELECTRON MICROSCOPY 



The higher resolving power of the electron microscope has 

 enabled finer morphological differentiations in biological material 

 than were hitherto possible with the best optical microscopes. How- 

 ever important this advantage in other fields, as used today its 

 purely morphological value limits its contributions to cyto- and 

 histochemistry. However, a beginning has been made toward the 

 use of an analytical electron microscope, not merely as a means of 

 obtaining high magnifications, but as a tool for the identification 

 and localization of certain metallic elements in biological prepara- 

 tions. To date only calcium and/or magnesium can be identified and 

 localized. These elements can be detected with a sensitivity of about 

 1 X 10"^^ g- per kilogram wet weight of tissue (muscle). For the 



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