252 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



ring on the barrel of the refractometer until the line of 

 demarcation between the dark and light areas is sharply 

 defined and colorless. When this line falls on one of the 

 divisions of the scale it is merely necessary to note the 

 number, which is then the refraction number. When the 

 line falls between two divisions of the scale the scale is 

 then moved towards the border line by turning the thumb 

 screw of the micrometer below the ocular until the scale 

 division which was nearest to the border line on the dark 

 side is exactly on it. The figure opposite the indicator 

 on the micrometer is then noted and is added to the 

 figure of the scale division as a decimal. For example, 

 if the figure opposite the scale division nearest to the 

 border line on the dark side was 38 and the micrometer 

 was rotated to 7 in moving the scale division to the 

 border line, then the refraction number is 38.7 The 

 refraction number can be converted into the refractive 

 index by means of a formula furnished with the refrac- 

 tometer, but this is not necessary. 



Before beginning the examination of a number of 

 samples of milk, the refractometer should be tested with 

 distilled water, which at 17.5 C. should give a reading 

 of 15.5. The water bath is made to hold 12 beakers 

 at one time so that when a number of samples are to 

 be examined there will be an opportunity for each one 

 to reach the required temperature without delaying the 

 examinations. When the prism of the refractometer is 

 removed from one sample, or from the distilled water, 

 it should be wiped dry with a soft linen cloth before being 

 introduced into another. 



The refraction number of normal market milk is in 

 most cases between 38 and 40.5. In rare cases it may 

 be as low as 37.3 and in one case reported by Mai and 



