It is of the greatest importance that the polariscopes and all
apparatus used in the work shall be carefully and accurately adjusted
and graduated, and upon a single and uniform system of standardization.
Recent investigations of the polarimetric work done in the customs
branch of the Treasury Department have shown that a very considerable
part of the want of agreement in the results obtained at the different
ports was due to a lack of uniformity in the standardization of the
instruments and apparatus.
_(a.) The Polariscope._--There are many different forms of this
instrument used. Some are adapted for use with ordinary white light,
and some with monochromatic light, such as sodium ray. They are
graduated and adjusted upon various standards, all more or less
arbitrary. Some, for example, have their scales based upon the
displacement of the polarized ray produced by a quartz plate of a
certain thickness; others upon the displacement produced by an
arbitrary quantity of pure sucrose, dissolved and made up to a certain
volume and polarized in a certain definite length of column. It would
be very desirable to have an absolute standard set for polariscopic
measurements, to which all instruments could be referred, and in the
terms of which all such work could be stated. This commission has
information that an investigation is now in progress under the
direction of the German imperial government, having for its end and
purpose the determination of such data as will serve for the
establishment of an absolute standard. When this is accomplished it
can easily be made a matter of international agreement, and all future
forms of instruments be based upon it. This commission would suggest
that the attention of the proper authorities should be called to the
desirability of official action by this government with a view to
co-operation with other countries for the adoption of international
standards for polarimetric work. Until this is done, however, it will
be necessary for the Internal Revenue Bureau to adopt, provisionally,
one of the best existing forms of polariscope, and by carefully
defining the scale of this instrument, establish a basis for its
polarimetric work which will be a close approximation to an absolute
standard, and upon which it can rely in case of any dispute arising as
to the results obtained by the officers of the bureau.
For the instrument to be provisionally adopted by the Internal Revenue
Bureau, this commission would recommend the "half shadow" instrument
made by Franz Schmidt & Haensch, Berlin. This instrument is adapted
for use with white light illumination, from coal oil or gas lamps. It
is convenient and easy to read, requiring no delicate discrimination
of colors by the observer, and can be used even by a person who is
color blind.
This form of instrument is adjusted to the Ventzke scale, which, for
the purposes of this report, is defined to be such that 1° of the
scale is the one hundredth part of the rotation produced in the plane
of polarization of white light in a column 200 mm. long by a standard
solution of chemically pure sucrose at 17.5° C. The standard solution
of sucrose in distilled water being such as to contain, at 17.5° C. in
100 c.c., 26.048 grms. of sucrose.
In this definition the weights and volumes are to be considered as
absolute, all weighings being referred to a vacuum.
The definition should properly be supplemented with a statement of the
equivalent circular rotation in degrees, minutes, and seconds that
would be produced by the standard solution of sugar used to read 100°
on the scale. This constant is now a matter of investigation, and it
is thought best not to give any of the hitherto accepted values. When
this is established, it is recommended that it be incorporated in a
revision of the regulations of the internal revenue relative to sugar,
in order to make still more definite and exact the official definition
of the Ventzke scale.
The instruments should be adjusted by means of control quartz plates,
three different plates being used for complete adjustment, one reading
approximately 100° on the scale, one 90°, and one 80°.
These control quartz plates should have their exact values ascertained
in terms of the Ventzke scale by the office of weights and measures by
comparison with the standard quartz plates in possession of that
office, in strict accordance with the foregoing definition, and should
also be accompanied by tables giving their values for temperatures
from 10° to 35°.
_(b.) Weights._--The weights used should be of solid brass, and should
be standardized by the office of weights and measures.
_(c.) Flask._--The flasks used should be of such a capacity as to
contain at 17.5° C. 100.06 cubic centimeters, when filled in such a
manner that the lowest point of the meniscus of the surface of the
liquid just touches the graduation mark. The flasks will be
standardized to contain this volume in order that the results shall
conform to the scale recommended for adoption without numerical
reduction of the weighings to vacuo. They should be calibrated by the
office of weights and measures.
_(d.) Tubes._--The tubes used should be of brass or glass, 200 and 100
millimeters in length, and should be measured by the office of weights
and measures.
_(e.) Balances._--The balances used should be sensitive to at least
one milligramme.