MEASURING REFRACTION INDICES— POLARIZING MICROSCOPES 191 



visible fringe in the duplicated area. Of course, in the overlapped 

 area abed, the fringes are in alignment with the outer fringes. Then, 

 measurements can be carried out by setting the extended edge of the 

 object at 45° from the fringes (Figs. 7.14 and 7.15). Duplication is 

 less marked but the fringe shift 6' is readily observed. 



I zi=o 



Dobling 

 Fig. 7.13. Straight edge of a dephasing object at right angles to the direction of duplication. 



Fig. 7.14. Measurements when the extended 

 edge of an object is at 45° from the fringes. 



-^ !*-<?' 



Fig. 7.15. Measurements when the straight 

 edge of an object is at 45*^ from the fringes. 



Nevertheless, it is advisable, when these two types of objects are 

 involved, to have the fringes at right angles to the direction correspond- 

 ing to the larger dimension. It might occur that merely rotating the 

 object 90° from the positions shown in Figs. 7.11 and 7.13 would be 



