46 MILK AND DAIRY PRODUCTS PART 



with the water-saturated ether after the lower prism has been 

 returned to its original position. The refractometer scale 

 ought now to read 20'6 if the thermometer is at 17 '5 C. 

 When taking these readings, the dividing line must be brought 

 exactly on to the nearest division of the refractometer scale 

 with the aid of the micrometer screw (G) (see Fig. 13). To the 

 number of whole divisions on the scale there must now be 

 added the number of tenths of divisions on the scale of the 

 micrometer screw. If the conditions have been found to be 

 correct for the ether, then the determination of the index of 

 refraction of the fat-containing ether can be made. After 

 carefully cleaning and drying the opposed faces of the prisms, 

 the prism box is closed and one of the sample tubes taken from 

 the water-bath, where it has been kept at 17'5 C. The cork is 

 then removed, and a small glass tube some 10 cm. long, open at 

 both ends and with an internal diameter of 3 4 mm., is dipped 

 into the layer of ether. A few drops enter the tube in this 

 way, the upper end of which is then closed with the finger and 

 the tube withdrawn. By applying the lower end of this narrow 

 tube to the mouth of the canal leading into the space between 

 the prisms and removing the finger, the ether flows down, and 

 the reading can be made immediately afterwards. If, for 

 example, the scale reads 49 and that on the micrometer screw 

 0'7, then the total would be 49*7 divisions; and suppose the 

 thermometer to read 0*2, then the real number would be 

 49-5. 



On referring to Naumann's tables (Table III in the Ap- 

 pendix), the percentage amount of fat corresponding to the 

 above number can be seen ; it is 3 '44 per cent. 1 



The refractometer is adjusted for a yellow sodium flame, but 

 it can be used without error for ordinary daylight or for lamp- 

 light. In the table below are given the indices of refraction 

 corresponding to the divisions of the refractometer scale. The 

 refractometer can also be used for determining the refractive 

 index of other liquids, provided the refractive index lies between 

 1*33 and 1'42 (e.g. milk serum). 



1 On the proposal of M. Ripper, Zeiss has so arranged the scale of his 

 refractometer that Naumann's tables are put alongside the ordinary scale, and 

 this admits of the percentage of fat being read directly. The dividing line of 

 the total reflexion crosses both scales. Corrections for temperature must 

 naturally be made as before. 



