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 deft; 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 fight, 
is found from the equation (aju = where a is the 
angle of rotation found, c the number of grams of substance in 
lOO cubic centimeters of solution and I 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 loo cubic centimeters 
and determining the angle of rotation a in the Bates tube. This 
tqbe is loo 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. 
