MICROSCOPY 107 



and the polarizing microscope would not be used to measure it. Instead, 

 we would use an instrument called a polarimeter, which is similar in 

 principle but has provision for a much thicker layer of the material to be 

 examined. 



The electron microscope 



The electron microscope employs a beam of electrons instead of a 

 beam of Hght. These electrons, which are produced by a heated filament, 

 can be focused into a beam by an electrostatic or a magnetic field. The 

 electrons behave as if they had a frequency and a wavelength, but this 

 wavelength is much shorter than the wavelengths of visible light. The 

 "optical" parts of the electron microscope are analogous to those in the 

 light microscope, consisting of an electrostatic or magnetic "objective 

 lens" and a similar "projector lens." The electron beam passes through 

 the object, where electrons are either transmitted or scattered in various 

 directions, depending on the nature of the material in the object. The 

 transmitted electrons are brought to focus on a photographic plate in a 

 pattern corresponding to regions of high transmission or high scattering 

 in the object. Since the wavelength of the electrons is short, the resolu- 

 tion is greater than that available in the light microscope. 



Biological materials offer difficulties in electron microscopy, but the 

 solution of these problems has permitted pictures showing exceedingly 

 fine details of structure. Most recent biology books contain excellent 

 examples. Even though these show great detail, much is lost in the 

 printing process; the original photographs are truly magnificent. 



The electron beam must operate in a vacuum, which means that the 

 biological material must be dry (and therefore dead). Exceedingly thin 

 layers of material must be used, and usually, since biological materials 

 are relatively ineffective in electron scattering, atoms of metal are added 

 to increase contrast. 



Earlier electronmicrographs were prepared by drying a thin film of 

 biological material and then "shadowing" with metal atoms. In a vacuum 

 chamber the metal atoms are "sputtered" off a heated coil from a position 

 above and to the side of the biological material. The metal atoms form a 

 thick layer on one side of any raised places in the biological material 

 and a thin "shadow" behind these spots. The resulting electronmicro- 

 graph provides a three-dimensional effect more or less like an aerial 

 photograph, with alternations in light and shadow. 



