MEASURING REFRACTION INDICES— POLARIZING MICROSCOPES 197 



using the fringe-shift method, the shifts K^ and K.^ are detected, 

 respectively, in the media n[ and n'.. Whence the two equations: 



e 



'1 _ •• ■•:: ^ 



from which are derived both index and thickness: 



n',K,-u[K, {K,-K.}X 



n=—r. 77— e= ; ;-. (7.10) 



Kx—K.2 «i — "2 



4. MEASURING THE REFRACTION INDEX OF LIQUIDS 

 (FULL-DUPLICATION PROCESS) 



Kofler propounded a method based on employing glass powders 

 whose indices are known (cf. Chapter XI, § 5). A specific powder is 

 immersed in the liquid to be investigated. Observations are carried 

 out on a stage provided with an adjustable heating device. The micro- 

 scope objective has a low aperture and the condenser is diaphragmed 

 to provide coherent light. Shifting slightly the microscope settings 

 evinces a luminous fringe, called the "Beck line", which surrounds 

 every glass particle. As the microscope is moved away from the 

 specimen, the Beck line is moving towards the higher refracting index 

 medium. Conversely, as the microscope nears the specimen, it moves 

 towards the lower refracting index medium. Observing the Beck line 

 enables one to determine which way the temperature is to be adjusted 

 for the glass particles to vanish, i.e. to obtain equal indices. At the 

 temperature considered and the wave-length used, the hquid has the 

 same index as the glass particles. 



Interference processes enable measurement of the refraction index 

 without any temperature adjustment. The procedure is to measure 

 the specimen at its own temperature. Were the temperature to vary, 

 the relevant refraction-index changes are followed up. E. E. Jelley 

 has made use of multi-wave interferences. The more readily applied 

 polarizing interference methods will now be discussed. 



Using a micro-chamber 



The 2 cover-glass plates, d and d', whose index n is known, are 

 secured to the object-slide a (Fig. 7.23). The liquid (whose index n is 

 to be measured) is poured in a minute space provided between the 

 two plates. This arrangement is covered by the object-slide b. The 

 two plates, d and d', need not touch either a ot b and only the thick- 



