MEASURING REFRACTION INDICES— POLARIZING MICROSCOPES 195 



Therefore, the ultimate image of the object and the Savart fringes 

 are seen concurrently. Any change in the optical path in object, 

 i.e. within the image, shifts the fringes of the Savart S. 



-i- 



r^i7 



/WN 



0. 



•^rr^'y^ 



■T 





Fig. 7.21. Fringes parallel to the duplic- 

 ation (Wollaston). 



Fig. 7.22. Fringes parallel to the duplic- 

 ation (Savart). 



3. DETERMINING THE REFRACTION LNDEX FROM PATH-DIFFERENCE 

 MEASUREMENT (FULL-DUPLICATION PROCESS) 



As in all interference methods, ihe above processes evince an 

 optical path, i.e. the product of an index by a thickness. If n is to 

 be measured, e must be determined and vice versa. Let us assume e 

 to be known. 



Flat-tints method 



When b > a and n > n\ the equation (7.5) yields: 



77 = n' + {c—a)je . 



Other instances are easily derived from the equations (7.5). 



Since e and the index n' of the medium encompassing the object, 

 are known, merely measuring c and a enables one to educe the index 77 

 of the object. 



