171 



read on the scale (J). At the same time the temperature is read 011 the 

 thermometer. 



The instrument should be tested first with water and its accuracy es- 

 tablished thereby : Ref. Ind. of water at 20° being 1.3330. The substance 

 to be examined should be at about the same temperature at which the 

 readings are to be made. Therefore waiting a few minutes after applying 

 the liquid to the prism, it could be considered that this is at the same 

 temperature. 



Tables have been prepared for converting the refractometer read- 

 ings to dry substance or per cent, of moisture. To some of these pre- 

 vious reference has been made. Hugh Main, in the International Sugar 

 Journal of 1907, Vol. 9, page 481, gives a table covering this. The readings 

 are to be made at 20° C. Geerligs has published a table also for dry 

 substance from the refractive index. The temperature of the reading is 

 28° C. and he has also prepared a table for corrections for other tem- 

 peratures than 28°. These are now given: 



