MICROSCOPY 105 



images against a dark background or dark images against a light back- 

 ground. Boundaries within the object that differ only slightly in refrac- 

 tive index might not be visible in the ordinary microscope but are seen 

 easily with phase contrast. 



Phase contrast microscopy requires a good deal of experience and 

 study. Very faint boundaries or edges become readily visible, and it is 

 too easy to see structures that are not real. A wrong choice of the type of 

 phase modifying plate could give a completely erroneous impression of 

 the object. Skilled users, however, have seen and photographed parts 

 of living cells that would be completely invisible without the phase 

 contrast microscope. 



The polarizing microscope 



The use of the polarizing microscope yields information about ma- 

 terials with an ordered internal structure. Chemists use this microscope 

 to examine crystals because the crystal contains atoms or molecules 

 arranged in a definite order. Certain biological materials seem to have 

 similar definite arrangements, and information about this fine structure 

 can be obtained by studying the materials under polarized light. 



The Microscope: In the polarizing microscope, light is supplied 

 through a disk of polarizing material, such as high-grade "Polaroid," so 

 that the specimen is illuminated by light which vibrates in only one plane. 

 The disk of polarizing material, known as the polarizer, is mounted in or 

 adjacent to the condenser in such a manner that it can be rotated. A 

 similar disk of polarizing material, called the analyzer, is mounted behind 

 the objective or in the eyepieCe and also can be rotated. Scales marked in 

 degrees tell the direction of orientation of the polarizers and analyzers. 

 If the two are parallel to each other, the polarized light passes through 

 the analyzer easily, and the field appears bright (with no specimen on the 

 stage). If the analyzer is rotated 90°, it prevents the passage of the polar- 

 ized light, and the field will be dark. This is the condition known as 

 "crossed polarizers." Polarization microscopes are also fitted with a rotat- 

 ing stage so that the object can be rotated within the polarized illuminat- 

 ing beam. 



Effects of Materials Upon the Polarized Beam: Some kinds of ma- 

 terials have no effect upon polarized light regardless of the direction in 

 which the light passes through. The polarizing microscope will behave 

 the same as if the material were not in the light path. Such materials 



