ince only with solutions that are
entirely clear and bright can accurate polarimetric observations be
made.

When a sufficient quantity of the clear liquid has passed through the
filter, the 200 mm. observation tube is filled with it. The 100 mm.
tube should never be used except in rare cases, when notwithstanding
all the means used to effect the proper decolorization of the
solution, it is still too dark to polarize in the 200 mm. tube. In
such cases the shorter tube may be used, and its reading multiplied by
two. The zero deviation must then be determined and applied to the
product. This will give the reading which would have been obtained if
a 200 mm. tube could have been used, and it only remains to apply the
correction determined by the use of the control plate as previously
described.

Example:

Solution reads in 100 mm. tube 47.0
Multiplied by 2 2.0
----
Product 94.0
Zero reads plus 0.3 0.3
----
Solution would read in 200 mm. tube 93.7

Reading of control plate 90.4
Sugar value of control plate 90.5
----
Instrument too low by 0.1
Add 0.1 to 93.7
----
Correct polarization of solution 93.8

Before filling the tube it must either be thoroughly dried by pushing
a plug of filter paper through it, or it must be rinsed several times
with the solution itself. The cover glasses must also be clean and
dry, and without serious defects or scratches. Unnecessary warming of
the tube by the hand during filling should be avoided; it is closed at
one end with the screw cap and cover glass, and grasped by the other
end with the thumb and finger. The solution is poured into it until
its curved surface projects slightly above the opening, the air
bubbles allowed time to rise, and the cover glass pushed horizontally
over the end of the tube in such a manner that the excess of liquid is
carried over the side, leaving the cover glass exactly closing the
tube with no air bubbles beneath it, and with no portion of the liquid
upon its upper surface. If this result is not attained, the operation
must be repeated, the cover glass being rubbed clean and dry, and the
solution ag