POLARIMETRY 167 



It is even better to incline the body of the microscope until 

 the tube is in a horizontal position, swing the mirror to one side 

 and project the beam of parallel light upon the polarizer. A 

 dark cloth thrown over the instrument and the head of the 

 observer prevents light from entering between the polarizer 

 and the tube of the microscope and any side light from entering 

 the eye. 



Upon looking into the microscope, the field will no longer be 

 dark gray or black. Turn the analyzer until the field again 

 acquires its maximum darkness and read the scale. The amount 

 of displacement to the right or left, as the case may be, is the 

 rotation of the solution. Dextrorotatory substances give a 

 smaller angle when the nicol is turned to the right, to obtain 

 maximum darkness, than when turned toward the left; while 

 laevorotatory substances will give the smaller angle when the 

 displacement from zero is to the left than when to the right. In 

 all cases a series of angle measurements should be made and the 

 average taken. It is obvious that in this series, the first measure- 

 ments must include rotation of the analyzer both to the right and 

 to the left. 



The specific rotatory power of a substance for yellow light, 



(a) D , is found from the equation («) D = — - — , where a is the 



angle of rotation found, c the number of grams of substance in 

 ioo cubic centimeters of solution and / the length of the polari- 

 zation tube employed expressed in decimeters. 



Since in most cases the specific rotatory power of a substance 

 is known, we may determine the per cent of the optically active 

 substance by dissolving a known weight of the material contain- 

 ing it in water, making the volume up to ioo cubic centimeters 

 and determining the angle of rotation a in the Bates tube. This 

 tube is ioo millimeters long. In the above equation all the 

 members will thus be known but c, i.e., the number of grams 

 of the active substance present in the mixture. Solving for c 

 will give the result sought. 



For further details the student is referred to the standard 

 works on the polarimeter and saccharimeter. 



