108 MICROSCOPY 



More recently it has been possible to photograph internal details of 

 biological structures. The tissue to be examined is chopped and then 

 fixed in a solution of osmic acid (osmium tetroxide). The cells are 

 killed, and, at the same time, metallic osmium atoms are deposited in 

 certain parts of the cells. The fixed material is embedded in plastic and 

 sectioned on a special microtome. The slices produced must be thinner 

 (< 1/a) than those used for light microscopy. These thin slices are sup- 

 ported on a screen analogous to the glass slide and placed in the elec- 

 tron microscope. Direct viewing is possible if the electron beam is focused 

 on a fluorescent screen, or a photograph can be made. 



The theoretical limit of resolution of the electron microscope is in 

 the vicinity of 2 X 10~^" cm, compared to a theoretical limit for the 

 light microscope of about 2 X 10~^ cm. This calculation, how^ever, as- 

 sumes an electron "lens" of high N. A. In practice, magnetic and elec- 

 trostatic lenses have large aberrations, and only low values of N. A. 

 can be used. The practical limit of resolution for the electron micro- 

 scope is about 1 m/Lt, or 10"'^ cm. This limit is still a considerable im- 

 provement over that found in the light microscope. 



The interpretation of electronmicrographs requires some caution. The 

 biological material has been subjected to a variety of treatments and of 

 course is no longer alive. It is entirely possible that fixing, staining with 

 metal, and sectioning have introduced "artifacts," or apparent features 

 which are not real. The tissue will have been sliced in many different 

 planes, and the selection of a particular spot to photograph depends on 

 the microscopist's idea of what the preparation ought to look like. None- 

 theless, so many fine pictures showing exquisite detail have appeared 

 recently that the electron microscope has had a profound influence upon 

 biological research. 



SELECTED REFERENCES 



Chamot, Emile Monnin, and Clyde Walter Mason, Handbook of 

 Chemical Microscopy, 3rd ed., vol. I. New York: John Wiley & 

 Sons, Inc., 1958. This volume presents a better discussion of gen- 

 eral microscopy and special methods than some of the books on 

 general microscopy. An especially detailed treatment of polarization 

 microscopy is included. 



Clark, George L., ed.. The Encyclopedia of Microscopy. New York: 

 Reinhold Publishing Corporation, 1961. Complete coverage of all 

 the various topics of microscopy. 



