116

QUANTITATIVE AGRICULTURAL AXALYXIS

light and dark in the field of observation will be shifted. It is
the observation of the position of this boundary that constitutes
the determination of index of refraction by means of this instru-
ment. The relative positions of telescope and prism system can
be altered at will The prism system is tipped until the bounding
line lies upon the cross hairs of the telescope. The index of
refraction is then read on an outside scale.

Dispersion.—The refractive effect at the surface separating
transparent media varies according to the wave length of the light,

shorter waves being most re-
fracted. The result of using
polychromatic light in a sys-
tem like* that just discussed
will therefore bo that the* light
and dark fields will be sepa-
rated by a colored zone of
HJX ctral tints, instead of by a
sharp, uncolored line. It then
becomes necessary to use
monochromatic light or else
to introduce Home device for
correcting the dispersion. In
the Abb6 instrument a "com-
pensator" provides this cor-
rection,

The Compensator.—T h e
dispersive effect of different
transparent media is not a
definite function of the index
of refraction for monochro-
matic light. For example, two media might have* the same
indices of refraction for yellow light but quite different indices
for violet light. This principle is utilized in the construction
of the Amici prism. In Fig. 33 the parts «, a' are prisms of
crown glass and 6 is of flint glass. The angles and the dispersive
powers of these prisms are so related that when they are cemented
together the combined effect is to allow the. yellow (Di) ray of
entering polychromatic light to pass through with its direction
unchanged, while rays of all other wave lengths are refracted.

FIG. 33.—-Single Amici prism, an used in
dispersion compensators.