132 



METHODS OF ANALYSIS. 



(b) ABBE'S REFRACTO METER. OFFICIAL. 



The refractive index is conveniently determined by the Abbe refractometer, 

 the latest model of which, with water-jacketed prisms, is shown in fig. 4. To 

 charge the instrument open the double prism AB by means of the screw head 

 v and place a few drops of the liquid on the prism, or, if preferred, loosen the 

 set screw and pour a few drops of the liquid into the funnel-shaped aperture 

 between the prisms. Then close the prisms firmly by tightening the screw head. 

 Allow the instrument to stand for a few minutes before 

 the reading is made, so that the temperature of the 

 liquid and the instrument will be the same. 



The method of measurement is based upon the obser- 

 vation of the position of the border line of total reflec- 

 tion in relation to the faces of a prism of flint glass. 

 Bring this border line into the field of vision of the 

 telescope by rotating the double prism by means of the 



alidade J in the 

 following man- 

 ner : Hold the 

 sector S firmly, 

 move the alidade 

 forward from the initial posi- 

 tion, at which the index points 

 to no=1.3, until the field of 

 vision is divided into a light 

 and a dark portion. The line 

 dividing these portions is the 

 " border line." This, as a rule, 

 will not be a sharp line, but a 

 band of color for which correc- 

 tion is made by rotating the 

 screw head M until a sharp, 

 colorless line is obtained. The 

 border line should now 

 be adjusted so that it 

 falls on the point of inter- 

 section of the two cross hairs. 

 Read the refractive index of 

 the substance directly on the 

 %cale of the sector. Check the 

 correctness of the instrument 



by means of the quartz plate which accompanies it, using inonobroinonaphthalene 

 and make the necessary correction in the reading. 



(C) 



BUTYRO-REFRACTOMETEB.O PROVISIONAL. 



Place the instrument (fig. 5) in such a position that diffuse daylight or any 

 form of artificial light can be readily obtained for illumination. Supply through 

 nozzle D a stream of water of constant temperature. Then open the prism cas- 

 ing by giving to the pin F a half turn and carefully clean the surface of the 

 prism. Melt the fat and filter, allowing the first two or three drops to fall on 

 the surface of the prism contained in casing B. Filter oils if they are turbid. 



Wiley, Principles and Practice of Agricultural Analysis, 8 : 339. 



