Achievements of Electron Microscopy 



91 



Fig. 32. Tobacco mosaic virus. Magnification X 21,000 



make it practically certain that this filament was formed by 

 extrusion during the development process. The extrusion starts 

 from a spot of the silver bromide grain when the latent image, 



• . 





\r*^ 3f^- KIT'- »■-*!, ■T^, -^ * • ♦* 



Fig. 33. Photographic grains. Magnification X 125,000 



(By courtesy of the Eastman Kodak Research Lab.) 



formed by a few hundred silver ions was produced by the 

 exposure. This spot is determined by a minute speck of sulfur 

 in the gelatin of the emulsion. The micrograph at the left is 

 especially remarkable, as the silver threads in the Lippmann 

 emulsion are only about five atoms thick. They are still appreci- 

 ably above the contrast limit of the R.C.A. microscope. 



In metallography and other surface investigations, the trans- 

 mission type electron microscope cannot be used directly. A 

 great number of ingenious methods have been worked out to 

 make film replicas faithfully reproducing the ups and downs of 

 the surface, and thin enough to be used in transmission (500- 



