BUT YRO- REFR ACTO METER. 



349 



A drop of the filtered fat is placed on the glass surface of the 

 lower prism, spread evenly over it, and the prism closed ; the 

 reflector is adjusted so as to reflect clear daylight or lamplight 

 through the prisms, and the refractive index in scale degrees is 

 read off. 



This instrument is extremely rapid, as a determination, in- 

 cluding reading of the temperature and scale degrees, does not 

 take more than a minute. After use, the instrument should be 

 cleaned by rubbing off the fat with a duster, and polishing the 

 prisms with a clean linen cloth slightly moistened with alcohol. 



Scale divisions may be converted into refractive indices by 

 Table LXXXII. 



TABLE LXXXII. 



There is a diiJerence in the refractive index depending on the 

 light used ; this is corrected in the instrument by making the 

 prisms of different kinds of glass, so that when used with butter 

 ordinary white light behaves as if it were simple light. Other 

 fats (and adulterated butters) may be tinged at the edge with 

 blue or red. In this case it is not easy to read the dividing line 

 accurately. The author is in the habit of using the sodium 

 flame, obtained by heating sodium chloride in a Bunsen burner, 

 as the source of light, and finds that absolutely sharp readings 

 can thus always be obtained. The readings with butters do not 

 diflfer, whether white light or sodium light be used. 



The refractive index varies 0"55 scale degree for each 1° C, 

 and can be corrected by means of this factor if the temperature 

 differs from that adopted as normal. 



For the correction of scale readings taken at any temperature 

 to any other temperature or to that adopted as a standard. Leach 

 and Lythgoe have devised a slide rule, but, except for small 



