MEASURING REFRACTION INDICES— UNPOLARIZED MICROSCOPES 179 



^0^0 in the object corresponds to the fringe ab outside it. The shift d 

 observed is the actual shift and shows the path difference at the point 

 of the object being considered. Likewise, to determine whether n > n' or 

 n < n', all that is needed is to observe in monochromatic hght an 



Fig. 6.13. For the object of Fig. 6.12, there is no break in the fringes. Fringes are 



continuously distorted. 



object whose index is known, and which enables to follow up fringe 

 distortions. The matter is readily solved by comparing the shift 

 direction of the fringes in both instances. 



2. DETERMINING THE REFRACTION INDEX FROM PATH DIFFERENCE 



MEASUREMENT 



The foregoing measurements show d = {n' — n)e. If e and n' are 

 known, n is educed as follows: 



n = n — 



(6.2) 



Let us assume that the diiference n' — n correlates a (5 value of ap- 

 proximately A/2 and the d error does not exceed //40. If e = 20 /^ 

 and assuming the measurement accuracy to be 1 /.i, the relative error 

 in the difference n — n is: 



A{n'-n) Ad Je 



01 



n—n 



