  It is generally agreed that interstellar dust grains consist of two main
components, namely, silicates and graphites. Some models, like MRN model,
assume these grains to be homogeneous spheres following a power law size
distribution. This paper presents, in the framework of Mie theory, a
parametrization of extinction spectrum curves of the silicates and the
graphites separately in terms of frequency and the minimum and maximum of sizes
in the distribution. Analytic expressions in ultraviolet and far-ultraviolet
are presented for both types of grains.The values of maximum and minimum sizes
for which these equations are valid have been identified. These equations can
be useful in a number of situations involving silicate and graphite grains.
