78 DAIRY ANALYSIS 



Arrange the mirror to reflect either daylight or the 

 light of a lamp through the prisms, and observe the 

 point on the scale where the dark shadow comes, 

 focusing the eye-piece if necessary ; make two or 

 three observations till the position of the shadow is 

 constant, and read the temperature at the same time. 

 The thermometer may be graduated either in centi- 

 grade degrees, or in the normal reading for butter fat 

 at the temperature of the water ; if of the former kind, 

 both readings are noted, and the refractometer readings 

 corrected to a standard temperature by multiplying 

 the difference between the observed temperature and 

 the standard temperature by 0.55, and adding or sub- 

 tracting the result, as the temperature is higher or 

 lower than the standard, to the refractometer reading. 

 If the thermometer is of the latter type, the observed 

 reading of the refractometer is subtracted from the 

 reading of the thermometer, and the result is expressed 

 in degrees less than the standard. 



Table VIII gives the standard readings for each five 

 degrees centigrade. 



TABLE VIII 



The average figure for butter is 46 at 35 C. or a 

 little below, and margarine gives about 54. Genuine 

 butter sometimes gives a reading higher than 47, and 

 the limits found are 43.5 to 49. 



A standard solution (normal flussigkeit) is supplied 

 with the instrument, and the scale should be adjusted 

 from time to time with this ; a point is marked on the 



