  Our aim is to compare the infrared properties of big, ``classical'' dust
grains with visual extinction in the cloud L1642. In particular, we study the
differences of grain emissivity between diffuse and dense regions in the cloud.
The far-infrared properties of dust are based on large-scale 100um and 200um
maps. Extinction through the cloud has been derived by using the star count
method at B- and I-bands, and color excess method at J, H and Ks bands.
Radiative transfer calculations have been used to study the effects of
increasing absorption cross-section on the far-infrared emission and dust
temperature. Dust emissivity, measured by the ratio of far-infrared optical
depth to visual extinction, tau(far-IR)/A(V), increases with decreasing dust
temperature in L1642. There is about two-fold increase of emissivity over the
dust temperature range of 19K-14K. Radiative transfer calculations show that in
order to explain the observed decrease of dust temperature towards the centre
of L1642 an increase of absorption cross-section of dust at far-IR is
necessary.This temperature decrease cannot be explained solely by the
attenuation of interstellar radiation field. Increased absorption cross-section
manifests itself also as an increased emissivity. We find that, due to
temperature effects, the apparent value of optical depth tau(far-IR), derived
from 100um and 200um intensities, is always lower than the true optical depth.

Astrophysics