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406 ILLUSTRATED WORLD
The Colorimeter
The light in the upper right corner throws a ray into the
tube above which contains the standard sample, and
another ray into the tulje below which contains the calcite
prism and quartz disk.
Nevertheless, there have been constant
disagreements in this matter of cottonseed
oil grading, with claims for rebates
based upon allegations that the product
did not correspond to specifications, and
so forth. An appeal was made finally to
the Bureau of Standards, which was
asked to test the glasses. It undertook
the task, and found that the glasses in
different boxes did not match. In many
instances the differences between those
supposedly alike—as indicated by their
labels—were greater than the differences
over which refiners and dealers had been
disputing.
In order to assist the cottonseed oil
chemists and dealers out of these
troubles, the Bureau of Standards conducted,
at the request of and in co-operation
with the Society of Cotton Products
Analysts, an extensive investigation of
the color of cottonseed oil for the purpose
of devising a satisfactory and practicable
method of grading the oil by its
color. The transmission and absorption
of light of different colors by many different
samples of oil are being deter­
mined by the spectrophotometer and by
photometers with selected color screens.
The color of these samples also is specified
and recorded by means of an instrument
called the Arons Chromoscope.
One feature of this investigation has been
the design and construction by the
Bureau's experts of a new instrument
based on the same principle as the Arons
Chromoscope, but embodying several improvements
and especially adapted to
measure the color of the cottonseed oil
of commerce.
The essential feature of the instrument
is a combination of two Nicol prisms with
a plane disk of quartz crystal. These
prisms have the property of polarizing
light, that is, all vibrations passing
through them are thrown into one plane.
The quartz disk will cause this plane to
revolve, optically speaking, and the
colors of which the light is composed are
thereby split up, because all of them do
not respond equally to the optical effect
of the disk of quartz.
It is not possible here to give an adequate
description of the colorimeter. But
it will suffice to say that the standards of
color it establishes are absolute and invariable,
being expressed in terms of the
angle between the principal planes of the
Nicol prisms and the thickness of the
quartz disk. The thicker the disk, the
more it will twist the plane of light rays
coming through it. One looks through
the eye-piece and sees a bright circle, onehalf
of wdiich shows the color of the sample,
while the other half shows a color
which can be slowly changed and made
to match it by rotating the Nicol prism.
When both halves match perfectly, the
circle is all of one color. The reading
of the circle on the instrument together
with the thickness of the quartz plate
then furnishes a definite specification of
the color.
It wdll be observed that color specifications
of this kind can be preserved in the
form of a few simple figures, so that
color standards can be maintained without
depending upon the permanence or
"fastness" of colored materials.