We extend and investigate the spheroidal model of interstellar dust grains
used to simultaneously interpret the observed interstellar extinction and
polarization curves. We compare our model with similar models recently
suggested by other authors, study its properties and apply it to fit the
normalized extinction A(λ)/AV and the polarizing efficiency
P(λ)/A(λ) measured in the near IR to far UV region for several
stars seen through one large cloud. We conclude that the model parameter
Ω being the angle between the line of sight and the magnetic field
direction can be more or less reliably determined from comparison of the theory
and observations. This opens a way to study the spatial structure of
interstellar magnetic fields by using multi-wavelength photometric and
polarimetric observations.Comment: 11 pages, 4 figures and 4 tables, To appear in MNRAS (added