QUANTITATIVE ESTIMATION OP SUGAR. 



269 



halves of the double plate at m appear of the same color to the observer, who 

 looks from v through the telescope introduced at e. By appropriate rotation 

 of the Nicol s prism at v, a bright rose color is preferably selected fn this position 

 the telescope must be so adjusted that the vertical dividing line of the double 

 plate is plainly visible. Thus adjusted the instrument is ready for use The 

 tube, 10 cm long, is filled with the fluid to be examined, which must be perfectly 

 clearshould it contain albumin, this must be removed by boiling and filtering - 

 and the tube is introduced into the apparatus between m and n. By rotating the 

 Nicol s prism at v, the rose-red color is again brought into view. Then the com- 



FIG. 98. The Soleil-Ventzke Saccharimeter. 



pensator at g is turned until both halves of the field of vision are exactly of the 

 same color. When this has been done, the number of divisions the zero-mark of 

 the vernier has been moved to the right in the case of albumin to the left 

 can be read directly from the scale. The number of divisions read off shows 

 directly the number of grams of the rotatory substance in 100 cu. cm. of the 

 fluid. Turbidity that persists in spite of filtering often disappears after addition 

 of a drop of acetic acid or a few drops of a solution of sodium carbonate or lime- 

 water, with subsequent filtration. For a description of other apparatus employed 

 for the same purpose the polaristrobometer of Wild, the polarimeter of Zeiss, 



