104 MICROSCOPY 



jects are easily recognizable. Sometimes when the contrast is high, the 

 bright or dark interference bands around small objects are so obvious 

 that they become annoying. If the contrast is insufficient, the diffracted 

 rays will be weak, the direct rays will seem too bright, and the object 

 will be difficult or impossible to see. Phase contrast is basically a means 

 of balancing the direct and the diffracted rays to increase the visibility 

 of the interference fringes. 



Fig. 8-13. Hypothetical birefringent structure. 

 Polarized light passes through such a structure 

 more rapidly in some directions than in others. 



As as rule, the direct rays and the diffracted rays travel to the image 

 level by paths of different length and therefore arrive out of phase with 

 respect to each other. One of the features of the phase contrast micro- 

 scope is a modification of these phase relationships to provide a more 

 favorable image. In addition, the direct rays are commonly reduced in 

 intensity so that diffracted rays contribute relatively more to the image. 

 The diagram in Fig. 8-12 shows one common means of achieving this 

 balance. The ring-shaped opening (annulus) in the condenser causes the 

 illumination of the object by a hollow cone of light. The undiffracted 

 rays pass through the objective to the "phase plate" where they are modi- 

 fied. Diffracted rays pass through other parts of the objective lens and 

 phase plate. Various types of plates are available to produce bright 



