resolved by the object and oscillating in mutually perpendicular planes 

 are exactly "crossed" or at right angles with the axes of the analyzer. 

 Both polarized components of the recombined ray coming from the ob- 

 ject are partially transmitted by the analyzer, with the result that the 

 object appears maximally bright. If n^ and n-j are the refractive indices 

 associated with the fast and slow ray, respectively, the quantity n-j — «i 

 is the birefrigence. For strongly birefrigent crystals such as calcite, the 

 birefrigence may be as high as 0.01. The birefrigence of biological ma- 

 terials is considerably less than this value, being about 0.0025 for living 

 muscle fibers. 



i*-^'i--^' 



Figure 11-24. Schematic Drawing Showing the Splitting of a Beam of 

 Plane-polarized Light by an Anisotropic Body into Two Components Which 

 Oscillate in Mutually Perpendicular Planes. Note that the slow component 

 (S) is retarded x/4 in velocity relative to the fast component (F). 



To determine the sign of birefrigence (i.e., positive or negative) of 

 an object it is necessary to know its slow and fast axes of transmission. 

 This is determined by use of a compensator, which is simply a birefrigent 

 material whose slow and fast axes of transmission are known. With the 

 object rotated to its position of maximum brightness (±45° angle with 

 respect to analyzer) between crossed polarizer and analyzer, the com- 

 pensator is inserted below the analyzer in the body tube and rotated. 

 A position is reached where there occurs an enhancement or re-enforce- 

 ment of brightness of the object. At this setting, the slow ray of the 

 compensator (the direction of which is known) is oscillating in the same 

 plane as the slow ray of the object. The enhancement of brightness 

 results from the summation of the individual retardations of the object 

 and compensator, i.e., the slow ray of the object is further retarded by 



SURVEY OF CYTOLOGICAL TECHNIQUES / 245 



