MEASURING REFRACTION INDICES— POLARIZING MICROSCOPES 199 



APPLICATION OF THE DIFFERENTIAL METHOD 



Polarizing interference microscopes, based on the differential me- 

 thod, are suitable for measuring refraction indices of transparent 

 isotropic objects. These instruments are not so commonly used as 

 the full-duphcation types, but they should be used whenever feasible 

 owing to their simple construction. As in the full-duplication method, 

 measurements are based on a prior determination of the path difference. 

 Either the flat-tints or the fringe-shift method is appHcable to such 

 determinations. All the following measurements can be made with 

 the microscopes shown in Figs. 3.25, and 3.28. Provided that the 

 duplication brought about by W2 in the image P' be feeble, the mi- 

 croscope, shown in Fig. 3.20, is also suitable. 



5. MEASURING PATH DIFFERENCE USING THE FLAT-TINTS METHOD 



(DIFFERENTIAL PROCESS) 



As previously, and in order not to over-elaborate the present work, 

 we will consider a diagrammatic transparent object (Fig. 7.24). In the 

 areas b^ and ^i, i.e. to the left of BB' and the right of CC, there is 

 the same medium of index n', and another medium of index n in the 

 area between BB' and CC . Such media are separated by flat surfaces 



Fig. 7.24. Diagrammatic transparent object BCB'C. 



perpendicular to the plane of Fig. 7.24 which are projected along BB' 

 and CC . Their angle in relation to the object-slide is B. The object, 

 whose index is being measured, consists of the n index medium com- 

 prised between BB' and CC . The microscope is illuminated in white 

 light. Let us assume n < n . Prior to travelling through the specimen, 

 the wave-surface is the plane surface-wave E^ (Fig. 7.25). After travelling 

 it is deformed and becomes the wave-surface H^. The areas Bbo 

 and Cbx are lagging in relation to the area B'C {u < li). Owing to 

 the birefringent system in the interference microscope, the wave I is 

 duphcated. The two waves, E^ and E., are in the image plane 



14 



