42 MILK AND DAIRY PRODUCTS PT. i 



reading of the refractive index must be done at a certain 

 temperature, viz., 17*5 C. If a drop of the ether-fat solution is 

 put into the narrow space between the prisms, it is seen on 

 looking into the tube that a part of the field of view is bright, 

 whilst the other part is quite dark. The dividing line between 

 the dark and bright fields is very sharp. This is because a 

 portion of the rays of light which enter the prisms is entirely 

 refracted on passing through the ether-fat layer owing to the 

 manner of arranging the prisms. Only some of the rays con- 

 tinue their way into the tube, so that part of the field of view 

 is illuminated and part not. By means of the micrometer 

 screw (6r), the dividing line between the bright and dark portions 

 of the field is brought exactly on to a division of the scale. On 

 reading off this division, a measure of the refractive index is 

 obtained, and this varies according to the amount of fat 

 dissolved in the ether. The greater the amount of fat, the 

 greater the index of refraction. The thermometer which is 

 used in the refractometer was specially designed by Wollny, and 

 the point which corresponds to 17 *5 C. is taken as 0. If the 

 mercury stands at 0, it is only necessary to read off the 

 number on the scale of the refractometer to get the refractive 

 index. When the thermometer is higher than 0, the number 

 of tenths of a division on the thermometer has to be added to 

 the divisions of the refractometer scale, whilst for temperatures 

 below a corresponding subtraction has to be made. If, for 

 example, the refractometer scale reads 49'1 and the thermo- 

 meter + 0*3, the correct reading is 49*4. 



Pulfrich l has constructed a special apparatus for supplying a 

 stream of water of constant temperature, the arrangement of 

 which is shown in Fig. 14. 



For use with the refractometer, special glass tubes are made 

 (see Fig. 15, on left) which have a mark at 30 c.c. In these 

 tubes the milk is mixed with the caustic potash solution and 

 ether. 



The milk is measured by means of a self-filling pipette of 

 30 c.c., similar in construction to the one shown on the left of 

 Fig. 15, and on the right of the same figure is shown a 

 specially-constructed pipette for measuring the amount of 

 ether required. 



1 Zeitschrift fur Instrnmentenknnde, Jan. 1898, 



