206 PROGRESS IN MICROSCOPY 



served equates 1/5 of the inter-fringe space. Then K= 1/5 whence: 



0-546 

 n = 1-527- -—- - 1-515. (7.19) 



45-5 



Provided that K be measured with an accuracy of 1/40 of the inter- 

 fringe space and that d = 20" be known within 1°, the index n is de- 

 termined with an accuracy within two units of three places of decimals. 



When the object's slope is unknown 



The procedure is the same as described in § 3. Provided it is 

 feasible, the object of slope d is immersed seriatim in two media whose 

 indices, n[ and «o, are known and, in accordance with the fringe- 

 shift method, the shifts K^ and Ko are evinced, in the media /;[ and n',, 

 respectively, whence the two equations: 



n = n, — ; n = «..— 



' dtand - dtanO 



from which are educed both index and slope: 



n',K^-n[K, {K,-K.^a 



n = tan B = —-, . (7.20) 



K^~K. {fi[-n'.)d 



8. MEASURING THE REFR.\CTION INDEX OF LIQUIDS 

 (DIFFERENTIAL PROCESS) 



With a micro-chamber 



The index n of the prismatic plate b^BB' is at a known angle 

 to the plate a^a. (Fig. 7.35). One drop of the liquid of the index n 



(2) (II 



1 

 Jb2 



w^ 



02 



■n^-:::-v;:B,y>V'V g. 



Fig. 7.35. Micro-chamber for index measurements (liquids) in the ditlerential process. 



to be measured is dropped in the angle BB'ox and the plate set upon 

 the interference microscope stage, whereupon the shift K of the fringes 

 observed in the area (1) in relation to those observed in the area (2) 

 is measured. The formula (7.18) gives //. To follow the fringes from 



