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 37.8 and in one case reported by Mai and 
