  We present a model for the diffuse interstellar dust that explains the
observed wavelength-dependence of extinction, emission, linear and circular
polarisation of light. The model is set-up with a small number of parameters.
It consists of a mixture of amorphous carbon and silicate grains with sizes
from the molecular domain of 0.5 up to about 500nm. Dust grains with radii
larger than 6nm are spheroids. Spheroidal dust particles have a factor 1.5 - 3
larger absorption cross section in the far IR than spherical grains of the same
volume. Mass estimates derived from submillimeter observations that ignore this
effect are overestimated by the same amount. In the presence of a magnetic
field, spheroids may be partly aligned and polarise light. We find that
polarisation spectra help to determine the upper particle radius of the
otherwise rather unconstrained dust size distribution. Stochastically heated
small grains of graphite, silicates and polycyclic aromatic hydrocarbons (PAHs)
are included. We tabulate parameters for PAH emission bands in various
environments. They show a trend with the hardness of the radiation field that
can be explained by the ionisation state or hydrogenation coverage of the
molecules. For each dust component its relative weight is specified, so that
absolute element abundances are not direct input parameters. The model is
confronted with the average properties of the Milky Way, which seems to
represent dust in the solar neighbourhood. It is then applied to four specific
sight lines including the reflection nebula NGC2023. For these sight lines, we
present linear and circular spectro-polarimetric observations obtained with
FORS/VLT. Using prolate rather than oblate grains gives a better fit to
observed spectra; the axial ratio of the spheroids is typically two and aligned
silicates are the dominant contributor to the polarisation.
