the compensator to give increased brightness. The principle is essentially 

 the same as for bright phase-contrast microscopy. The phase change, 0, 

 is the product of birefrigence and thickness, t, and is expressed as 



With the object in the position of increased brightness, one of its axes 

 (major or minor) will be in line with the slow axis of the compensator 



A O O 



Field of view 



Analyzer 



Specimen 



Polarizer 



Figure 11-25. Schematic Drawing Showing Variations in Darkness and 

 Brightness of an Anisotropic Object When Placed Between Crossed Polarizer 

 and Analyzer and Rotated ±45°. 



and is designated accordingly as the slow axis. The fast axis of the 

 object, therefore, lies at 90° to that of the slow axis. The sign of bire- 

 frigence is designated positive or negative depending on whether the 

 slow axis of the object is parallel or perpendicular to some previously 

 selected dimension or direction of the object. It is generally convenient 

 to take the long axis of cylindrical structures (e.g., muscle and collagen 

 fibers, chromosomes) as the axis of reference, or radial direction as in 



246 / CHAPTER 11 



