410 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



about 14 /*, so that the theoretical limit of resolution of the lig'ht micro- 

 scope is about 1/4 [x. This means that no optical system using visible 

 light, no matter how perfectly constructed, can separate points less 

 than 14 /* apart. 



The electron microscope makes use of the fact that electrons travel- 

 ing at high speeds have a wave as well as a particle character, and 

 the very short wavelengths associated with electrons streaming down 

 the tube of an electron microscope permit a much higher degree of 

 resolution, so that points as close as 6 A ^ can still be separated. As 

 soon as paleontologists began to investigate shell structure with the 

 electron microscope, the reason for earlier confusion became appar- 

 ent — most of the fundamental building miits employed by organisms 

 in constructing shells are so small that they cannot be resolved in the 

 light microscope. However, they are easily resolved and observed 

 in the electron microscope. 



Along with the advantages of this powerful new tool come some 

 difficulties which do not trouble the light microscopist. First of all, 

 it is impossible to observe shell material directly in the electron micro- 

 scope, for most crystalline materials, such as calcium carbonate, are 

 opaque to the beam of electrons streaming down the tube of the micro- 

 scope. To get around this difficulty, replicas of the shell material are 

 prepared. The replica is a film of carbon showing the details of the 

 shell surface, but is so thin that the electron beam passes through 

 it and produces an image of it on the viewing screen of the microscope. 

 A replica is prepared in the following manner: The specimen to be 

 replicated is placed in a vacuum chamber (called an evaporator) and 

 the air is pumped out until a high vacuum is obtained. Then a strip 

 of metal wire or carbon is heated to incandescence by passing an electric 

 current through it. The atoms in the glowing carbon tend to "boil 

 off" and, since there are no air molecules for them to collide with, fly 

 out in straight lines from the source. As a result, an extremely thin 

 film of carbon coats everything inside the evaporator. The delicate 

 carbon film is usually freed by dissolving the specimen in acid very 

 slowly. The film is floated off on water and picked up on support 

 screens used in electron microscopy. Since the film was deposited as a 

 stream of carbon atoms it reproduces surface features of the specimen 

 in minute detail, and the term replica is a well-chosen description. One 

 thing must constantly be kept m mind when studying replicas in the 

 electron microscope — they represent only the appearance of the surface 

 that was replicated, and do not penetrate to reveal internal structure. 



' The Angstrom unit, written A, is the standard unit of measure in the size range of 

 atoms and molecules. 1 A is 1/10,000 /t. 



