  We present an extinction map of the Polaris molecular cirrus cloud derived
from star counts and compare it with the Schlegel et al. (1998) extinction map
derived from the far--infrared dust opacity. We find that, within the Polaris
cloud, the Schlegel et al. Av values are a factor 2 to 3 higher than the star
count values. We propose that this discrepancy results from a difference in
$\tau_{FIR}/ A_V$ between the diffuse atomic medium and the Polaris cloud. We
use the difference in spectral energy distribution, warm for the diffuse atomic
medium, cold for the Polaris cloud, to separate their respective contribution
to the line of sight integrated infrared emission and find that the
$\tau_{FIR}/ A_V$ of cold dust in Polaris is on average 4 times higher than the
Schlegel et al. value for dust in atomic cirrus. This change in dust property
could be interpreted by a growth of fluffy particles within low opacity
molecular cirrus clouds such as Polaris. Our work suggests that variations in
dust emissivity must be taken into account to estimate Av from dust emission
wherever cold infrared emission is present (i.e. molecular clouds).
