90 CARBOHYDRATES. 



ment. The slightest deviation of the analyzer from this position 

 causes one-half of the field to appear darker and the other half 

 lighter. There are thus presented to the eye two lights to be 

 compared, and the instrument is thus very sensitive. Monochro- 

 matic light must be used. In some instruments the circle is 

 divided both into degrees and sugar units, or percentages. The 

 scale is read by means of a vernier and lens (n). Before using the 

 instrument the observation tube is filled with water and placed in 

 position between the analyzer and polarizer. If the instrument 

 is properly adjusted, the zero mark on the vernier will correspond 

 with the zero point of the scale when the two halves of the field 

 are equally illuminated. The tube is then filled with the solution 



FIG. 82. The Laurent shadow polarizing sacchari meter. 



to be tested and again placed between the analyzer and polarizer, 

 when, if it is an active substance, the plane of the polarized ray 

 coming from the analyzer will be turned to the right or to the left 

 in passing through the solution, and one-half of the field will be 

 lighter than the other. The amount of rotation of the plane of 

 the polarized ray will be proportioned to the amount of the active 

 substance in the solution. It will now be necessary to rotate the 

 analyzer (h) to the right or to the left, so that the two halves of 

 the field will again appear equally illuminated. When this has 

 been accomplished we may read oflf on the vernier the degrees 

 of the circle through which the analyzer has been rotated. In 

 this way the amount of rotation of the polarized ray is deter- 

 mined. 



