OPTICAL ROTATION

127

intensities and these brighten as the middle section darkens.
Here again, the field of uniform intensity is seen at the zero
angle.
Light Source. Many of the forms of polarization apparatus
are constructed for monochromatic light of a specified wave
length. White light cannot be used with such an instrument
hecause its component rays suffer different rotation of their
polarization planes, according to their wave lengths, the shorter
waves being rotated to the greatest degree. Sodium light is
most commonly used for this purpose as it contains rays from a
very narrow band in the spectrum and it is therefore nearly
homogeneous. A sodium light is easily produced by placing
any suitable sodium compound in a non-luminous flame. So-
dium carbonate or recently fused sodium chloride is writable for
this purpose. The salt is placed in a platinum spoon or fused
into a bead in a loop of platinum wire, or Home similar device may
be employed.
Quartz Wedge Compensation: The Saccharimeter.It has
been stated in the preceding paragraph that the rotation of the
planes of polarization varies for light of different wave lengths.
If white light is used to illuminate the ordinary polarimeler the
effect of interposing an active substance in the path of the rays
is a dispersion of the various polarization planes. This in
analogous, in a manner, to the dispersion of white light by
refraction and it was noon in the discussion of the rofraetomctor
(page 116) that this dispersion could be corrected by optical
means without altering the refraction of a given ray, such as the
yellow one,
The quartz wedge compensator for the polarimetcr makes
possible the two of white (polychromatic) light. Quartz in optic-
ally actives and it occurs in both dextro- and lacvo-rotatory forum,
the angle of rotation of sodium light at 20° for a plate .1 mm
thick being ±21.72°. If absolutely similar platen of right and
left rotating quartz should be placed between polarizer mid
analyzer, the net effect would be zero rotation. If the thickness
of either one of these could be varied at will the effect of the
combination could be made either right or left rotating, within
certain limits, and this effect might be made such as to com-
pensate (neutralize) exactly the rotation of a solution which