29 : 6/ Microscopy 549 



rejects all light whose electrical vector (plane of polarization) is that 

 determined by the polarizer. Under these conditions, the polarizer 

 and analyzer are said to be crossed ; the background will appear dark, 

 as will any homogeneous isotropic object. 



In contrast, most fibers, all helices, and all asymmetric carbon atoms 

 are optically active ; that is, they rotate the plane of polarization of light 

 passing through them. Optically active crystals or fibers have one (or 

 more) preferential direction (s) called the optic axis {axes). If the plane 

 of polarization of the incident light is either parallel to or perpendicular 

 to the optic axis, it will not be rotated. A maximum rotation of the 

 plane of polarization occurs when the optic axis is perpendicular to the 

 direction of propagation of the light and at 45° to the plane of polariza- 

 tion. 



Materials so oriented that they rotate the plane of polarization will 

 appear bright, or at any rate gray against a dark background. The 

 action of the optically active materials may be considered as splitting 

 the incident beam into two beams polarized at right angles to each other 

 and traveling with different velocities through the sample. One of these 

 beams, the ordinary beam, obeys the ordinary laws of refraction and 

 has an index of refraction n . The other beam, the extraordinary 

 beam, travels through the sample with a different velocity. In terms 

 of its velocity, one may define an index of refraction n e . Optically 

 active materials are said to be birefringent, the degree of birefringence 

 being given by 



r = n e - n 



Because T is different from zero, there will be a phase difference 

 between the ordinary and extraordinary beams when these are combined 

 by the analyzer. Most biological samples are so thin that the phase 

 difference is very small. It may be enhanced by a number of methods. 

 One of these introduces an additional difference of phase through the 

 use of a compensator plate located below the analyzer. This brightens 

 and colors the background and also emphasizes the phase change intro- 

 duced by the birefringent material in the specimen. 



Almost all biological samples are birefringent. Accordingly, the 

 polarizing microscope has been widely used since its commercial intro- 

 duction about 1945. It made possible observations such as of the form of 

 the chromosomes, the spindles, and the mitotic figures in living, dividing 

 cells. Various additional methods have been introduced to increase 

 its utility further through combining polarization and interference- 

 contrast microscopes. However, the resolution of the polarizing micro- 

 scope is never better than that of the bright-field light microscope. 



