722 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



In the case of volatile fluids or others which suffer by exposure to 

 air, the apparatus is modified as shown in fig. 196. A cap M is fitted 

 on to the lower end of the refractometer and then closed by a screw 

 lid D ; thus a prism of the given fluid is obtained and the illumination 

 is now through the side of a suitable trough. 



The limit of error in the value of » is + 3*7 units of the fifth 

 decimal place. 



Fig. 195. 



Zeiss' Differential Refractometer. — The object of this instrument, 

 recently reconstructed, is the direct determination of the refractive 

 difference of two fluid prisms of equal refractive angle set up behind one 

 another but sloped in opposite ways. The deviation of such a double 

 prism is directly proportional to the. difference of the refractive indices 

 of the two fluids ; it is dependent only on the difference of the indices, 

 not on the refractive index itself. The method is practically without 

 limit as regards the magnitude of the index, and is therefore applicable 

 for liquids of high and low index as well as for gases and vapours. The 



