rsive powers of gases are, however, generally comparable
with those of liquids and solids.

_Dispersive Power._--In order to find the amount of dispersion caused
by any given prism, the deviations produced by it on two rays of any
definite pure colours may be measured. The angle of difference between
these deviations is called the dispersion for those rays. For this
purpose the C and F lines in the spark-spectrum of hydrogen, situated
in the red and blue respectively, are usually employed. If [delta]F
and [delta]C are the angular deviations of these rays, then [delta]F -
[delta]C is called the mean dispersion of the prism. If the refracting
angle of the prism is small, then the ratio of the dispersion to the
mean deviation of the two rays is the dispersive power of the material
of the prism. Instead of the mean deviation, 1/2 ([delta]F + [delta]C),
it is more usual to take the deviation of some intermediate ray. The
exact position of the selected ray does not matter much, but the
yellow D line of sodium is the most convenient. If we denote its
deviation by [delta]D, then we may put

_Dispersive power_ = ([delta]F - [delta]C)/[delta]D (1).

This quantity may readily be expressed in terms of the refractive
indices for the three colours, for if A is the angle of the prism
(supposedly small)

[delta]C = ([mu]C - 1)A,
[delta]D = ([mu]D - 1)A,
[delta]F = ([mu]F - 1)A,

where [mu]C,[mu]D,[mu]F are the respective indices of refraction. This
gives at once

_Dispersive power_ = ([mu]F - [mu]C)/([mu]D - 1) (2).

The second of these two expressions is generally given as the
definition of dispersive power. It is more useful than (1), as the
refractive indices may be measured with a prism of any convenient
angle.

By studying the dispersion of colours in water, turpentine and crown
glass Newton was led to suppose that dispersion is proportional to
refraction. He concluded that there could be no refraction without
dispersion, and hence that achromatism was impossible of attainment
(see ABERRATION). This conclusion was proved to be erroneous when
Chester M. Hall in 1733 constructed achromatic lenses. Glasses can now
be made differing considerably both in refractivity and dispersive
power.

_Irrationality of Dispersion._--If we compare the spectrum produced by
refraction in a glass prism with that of a diffraction grating, we