OPTICAL ROTATION

125

tube of solution is then placed in position and the analyzer is
turned so that its plane of polarization lies in the new plane,
which has been rotated by the solution from its original position.
The magnitude and direction of the angle, a, through which the
analyzer has been turned, is noted and from this and from the
length of column and the known concentration of the solution
the specific rotation is calculated.

In the more common case the specific rotation of the substance
is already known and the concentration of the active substance
in the solution is the factor in question. For example, the per
cent of cane sugar in a syrup is to be determined. A definite
weight of the syrup is diluted to a definite volume and the angle
of rotation produced by a column I dm in length is determined.
The specific rotation of sucrose is given as +66.5. We have
then the equation (from Eq. (1), page 123):

66.5 =

100 a,
cl

or c

66.5 r

The length, Z, is known and the angle, a, is determined by
observation. The concentration, c, of sugar in the solution,
as well as the concentration of sugar in the original syrup, is then
easily calculated.
Construction of Polarizer and Analyzer.—In the most common
form of this instrument the polarizer and analyzer are of identical
construction. It is a well known fact that when a ray of light
falls perpendicularly upon certain faces of a crystal of Iceland
spar (natural, crystallized calcium carbonate) the light is broken
into two rays which are unequally refracted, so that when any
object is viewed through such a crystal two images are observed.
What is equally important is that these two rays are polarized in
planes perpendicular to each other. .
The Nicol Prism.—This is made by cutting a crystal of Iceland
spar into two wedge-shaped pieces and grinding the faces in such
a manner that when these pieces are cemented together one of the
plane-polarized rays may pass through while the other will be
reflected to the side of the prism and there absorbed by a black-
ened surface. In Fig. 38 the incident ray, W, is double-refracted
and at the dividing surface between the two parts of the
crystal, the "ordinary" ray, 0, is reflected to the side of the prism