MEASURING REFRACTION INDICES— POLARIZING MICROSCOPES 211 



tints method (full duplication and differential process), even if the 

 tint varied considerably from one point to another of the object, 

 a small area can be isolated so that its colour does remain virtually 

 constant and detectable. The same applies to the fringe-shift process. 

 Fringes may be substantially deformed, thus making for less accurate 

 performance, but not restraining apphcation of the process. Detecting 

 a dark fringe portion and measuring the shift in relation to the outer 

 fringes of the object will achieve the same result. 



The accuracy mentioned in the above examples can readily be 

 improved, provided due regard is given when adjusting the aperture 

 of the incident light-beam (see Chapter VIII, § 2). 



