118 QUANTITATIVE AGRICULTURAL ANALYMX

The diagrams of Fig. 34 illustrate a few of the possible posi-
tions of the two elements of a double compensator, with the,
net effect of the altered system upon dispersion. In explanation
of these diagrams the combined dispersive effect in the direction
ab is represented by D and that of one unit in its own plane (repre-
sented by the arrows) by d. It is evident that the component
of dispersion in ab for any given position of a unit varies as the
cosin of the angle, 6, between ab and the plane of dispersion for the
unit.
Any conceivable value between 2 d arid — 2 d may be obtained
and compensation thus effected for any dispersion within this
range.
Butyro-refractometer.—The Zeiss butyro-refraetometor has
an optical system similar to that of the Abl>6 instrument.
The principal difference is in the fact that the telescope
and prism system of the former instrument are rigidly con-
nected so that the divided field cannot be shifted to bring
the line of division upon the central crossing of cross hairs.
Instead, a scale graduated in arbitrary degrees in fixed within
the instrument and the position of the bounding line is read
upon this.
As its name indicates, the butyro-refraetometer in designed for
use in dairy laboratories and its chief function is in the tenting
of butter. Therefore, instead of being provided with a com-
pensator the prisms of the instrument arc* "achromatized"
for pure butter so that this fat gives no dispersion. This con-
stitutes the basis for an additional qualitative test for butter,
since other fats used as substitutes will have different dispersive
power and the bounding zone in the field will therefore be
spectrally tinted.
Dipping Refractometer.—From an inspection of Fig. 32 it will
be seen that the essential parts of the AbW refraetometer, from
the optical standpoint, are the upper prism, the objective and an
eyepiece for viewing the field. (The latter in not shown in the*
figure.) The thickness of the liquid film is of no particular
moment and the prism might as well be immersed in a quantify
of the fluid. In the dipping refraetometer (Fig. 85) thin principle
is utilized. The entire instrument is in one rigidly built piece,
the prism being fixed at the lower end. The instrument it* hung