192 



PROGRESS IN MICROSCOPY 



adequate. Such is not the case for the extended object shown in Fig. 7.1 1 

 for both images are overlapped at abed and the shift hardly observable 

 in the two areas aa^ccx and bboddi (Fig. 7.16). As regards the straight 

 edge of a phase-shifting object it is unobservable (Fig. 7.17) since 

 duplication is invisible as it parallels the straight edge of the object. 



Fig. 7.16. Extended object parallel to the duplication. 



Therefore, fringes must be paralleled to the duplication and the 

 object's larger dimension be perpendicular to the fringes. This is the 

 most convenient and rehable arrangement when precise measurements 

 are required. 



In polarizing interference microscopes, the fringes are usually 

 paralleled to duplication and all the operator has to do is to set the 



Fig. 7.17. Straight edge of an object parallel to the duplication. 



object in the proper direction this being readily achieved with a little 

 practice. 



There are several methods for paralleling fringes to duplication. 

 Several of these are shown later. For the time being, this adjustment 



