  By radiation transfer models with a realistic power spectra of the projected
density distributions, we show that the optical properties of grains are poorly
constrained by observations of reflection nebulae. The ISM is known to be
hierarchically clumped from a variety of observations (molecules, H I,
far-infrared). Our models assume the albedo and phase parameter of the dust,
the radial optical depth of the sphere averaged over all directions, and random
distributions of the dust within the sphere. The outputs are the stellar
extinction, optical depth, and flux of scattered light as seen from various
viewing angles. Observations provide the extinction and scattered flux from a
particular direction.
  Hierarchical geometry has a large effect on the flux of scattered light
emerging from a nebula for a particular extinction of the exciting star. There
is a very large spread in both scattered fluxes and extinctions for any
distribution of dust. Consequently, an observed stellar extinction and
scattered flux can be fitted by a wide range of albedos. With hierarchical
geometry it is not completely safe to determine even relative optical constants
from multiwavelength observations of the same reflection nebula. The geometry
effectively changes with wavelength as the opacity of the clumps varies. Limits
on the implications of observing the same object in various wavelengths are
discussed briefly.
  Henry (2002) uses a recipe to determine the scattered flux from a star with a
given extinction. It is claimed to be independent of the geometry. It provides
considerably more scattering than our models, probably leading to an
underestimate of the grain albedos from the UV Diffuse Galactic Light.
