INDEX OF REFRACTION

117

In other words, the part 6 has a refractive power which is greater
for red, smaller for violet, and the same for yellow as the cor-
responding powers of the combination of a and a'.

If this Amici prism is placed in the path of light which has
been refracted (and dispersed) by the prism-fluid system of the
refractometer, it will either add its dispersive effect to that
already produced or oppose the latter, according to the way in
which it is turned about the axis of the instrument but, no matter
what its position, the direction of the yellow ray is unchanged and
the reading for index of refraction is not altered, since this value
is usually stated in terms of yellow light. If the angle between
the planes of dispersion of Amici and refractometer prisms is such
that the dispersion of the latter is exactly neutralized, " compen-
sation" is effected and the border line between light and dark
fields becomes distinct and without color.

6--
D=-d D=-2cl
FIG. 34.—Five typical positions of the units of a double compensator, showing
net dispersive effect (JO) in the direction a&.
If two Amici prisms are used, as is the case in the Abb6
refractometer, and if they are mounted in such a manner as to
revolve in opposite directions, the dispersive effect of the com-
bination may be varied between the limiting values of + 2 d and
— 2 d, where d is the quantitative effect of one Amici. This
widens the range of possible compensation.