CHAPTER VII 



Measuring Refraction Indices 



of Transparent Objects: Polarizing 



Interference Microscopes 



APPLICATION OF THE FULL- DUPLICATION METHOD 



Full-duplication polarizing interferometers are eminently suitable 

 for microscope measurement of the refraction indices of transparent 

 isotropic objects. As in all interference processes, measurements are 

 based on the previous determination of path diflferences. Two pro- 

 cedures may be used: 



(a) In the flat-tints method, the object and adjacent area exhibit 

 two uniform but different tints. The path difference between the light- 

 rays travelling through the object and those passing next to it can 

 readily be calculated by means of Newton's scale of colours and the 

 refraction index of the object educed therefrom. This process however, 

 only approximates the index value. 



(b) In the method using interference fringes in monochromatic- 

 light, the whole field of view is streaked with straight, parallel and 

 equidistant fringes. The fringes located in the imaged object are not 

 in alignment with the other fringes. The shift, reckoned as inter- 

 fringe spaces, shows the path difference between the rays traveUing 

 through the object and those propagating next to it. As previously, 

 the refraction index is educed from the shift. The potential accuracy 

 of the method is very high and depends on the precision with which 

 fringe settings are carried out. 



1. MEASURING PATH DIFFERENCES BY MEANS OF THE FLAT-TINTS 

 PROCESS. FULL-DUPLICATION PROCESS 



The following processes are applicable to objects of any shape but, 

 in order to simplify matters, the diagrammatic object in Fig. 6.1 is 

 again referred to. The object, whose refraction-index n is required, 



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