MEASURING REFRACTION INDICES— POLARIZING MICROSCOPES 209 



If ^ = 25//, u' = 1-527, / = 0-546//, then n - 1-509. Measuring 

 K with an accuracy of 1/40 of the inter-fringe space and if /• is known 

 with an accuracy within r/40. then /; is deiermined within 5 units of 

 the fourth place of decimals. To determine whether n > /?' or n < /?', 

 a known liquid must be measured. 





Fig. 7.38. Index measurement of a melted eutectic. 



The photograph (Fig. 7.38) shows the method apphed to the de- 

 termination of the index of a melted eutectic. The two unmelted 

 substances which formed the eutectic are visible at the top and bottom 

 of the photograph. The fringe method applied here shows that the 

 fringes in the melted eutectic are not in alignment with the fringes 



