CARBOHYDRATES 31 



be found in textbooks or books of reference. By observing the 

 rotation cc of a solution of a known sugar and substituting this 

 value in the above equation it is possible to calculate the amount 

 of the given sugar present. On the other hand, if we have 

 a solution containing a known amount of an unknown sugar, 

 it is possible to calculate its specific rotation, which usually will 

 identify the substance, especially if used in conjunction with 

 other tests. For this purpose we may solve our formula for c 



100. oc 



C = 1. [ cc ] 20 

 D 



The actual observation of oc is made with a polariscope, an 

 instrument in which light is polarized and passed through the 

 solution to be examined. The apparatus is so constructed that 

 it is possible to measure the amount of rotation produced by 

 the solution. The accompanying diagram indicates the struc- 

 ture of a Laurent polariscope. 



fc c d e f g 



FIG. 1. DIAGRAM OP LAURENT POLARISCOPE. 



A. Source of light, sodium flame. 



B. Plate cut from a crystal of potassium bichromate. In place of this, a flat-sided cell 



containing a solution of potassium bichromate may be used to insure absence of 

 other than yellow light. 



C. Lens to make the rays of light parallel. 



D. Nicol prism, called the polarizer, which polarizes the ray. 



K. Quartz plate covering a portion of the field to produce half shadow 



F. Tube containing solution to be studied. 



G. Second Nicol prism called the analyzer. 

 H. Lenses for focusing. 



I. Eye of observer. 



The Nicol prism is a device for polarizing light, and getting 

 rid of one of the two rays into which the original ray is split. 

 A crystal of calcite is sawed through diagonally, and the two 

 pieces stuck together with a thin layer of Canada Balsam. 



On entering the prism, light is polarized and split into two 

 diverging rays. From the diagonal surface, one of the two 

 polarized rays is reflected to the side, the other passes on through 



