128

QUANTITAT1VE AGRICULTURAL ANALYSIS

is placed in the instrument and whose rotation is to be measured.
In such a case both polarizer and analyzer might be made as
rigid, stationary parts of the instrument, the only adjustable
part being one of the quartz plates. This possibility of adjust-
ment is accomplished by cutting one of the plates diagonally,
making two wedge-shaped pieces which may be thrust past one
another by means of an appropriate screw, the magnitude of the
effect being noted upon a scale.
Now it happens that the rotation dispersion of quartz for white,
or other polychromatic, light is nearly identical with that of
cane sugar in solution. Since, in using this instrument, the
quartz wedge combination will always be adjusted to be equal in
rotatory power to that of the solution being investigated, but
in the opposite direction, it will also bo true that the dispersion of
the sugar solution will be nearly compensated by the opposite,
but otherwise nearly equal, dispersion of the quartz system.
Because of these relations the instrument constructed in this

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i

fig. 39.——Diagrammatic ropronontatioii of th<» oRs<»nf,ial purt.H of a quartz wedges
mtcoharimotor, having double romponHating winifrcH.
manner is known as a "saccharinieter." If used with other solu-
tions than those of cane sugar the polarization dispersion could
be compensated only approximately, at best, and readings of the
angle of rotation could not be correct. In such a cane it would be
necessary to use sodium light or a selective light filter.
The relations of the optical parts of the quartz wedge saeehari-
meter are shown diagrammatieally in Fig. 39.
Light Filter for Use with the Saccharimeter.—The quartz
wedge system fails to give exact compensation for the rotation
dispersion of sugar solutions arid in order to avoid slightly high
readings it is necessary to absorb a part of the blue and violet
waves from white light, an those suffer the greatest dispersion.
The International Commission for Unifying Methods of Sugar
Analysis adopted the suggestion of Bryan^1 that white light shall
^1J. IntL Eng. Chwn., 6, 107 (19.13).