\[innercoldens\]).\
(3) $R_{out}$ is the radius at which the C$^{18}$O column density was measured in the outer parts of the core (see section \[outercoldens\]).\
(4) $N_{H_{2}}^{out}$ is the column density inferred from C$^{18}$O(1-0) observations at $R_{out}$ (see section \[outercoldens\]).\
(5) $I^{MIR}_{zodi}$ is the zodiacal light emission in the mid-IR (see col. \[8\] of Table \[sources\]).\
(6), (7) $I_{back}^{MIR}$ & $I_{fore}^{MIR}$ are the mid-IR background and foreground intensities as deduced from Eq. (\[itau\]) and the values of the central and outer column densities, using the additional constraint that $I_{fore}\geq I_{zodi}$ (see section \[intensities\]).\
$^{\dagger}$The values of $N_{H_{2}}^{flat}$ and $<N_{H_{2}}>_{13\arcsec}$ given in Table 2 of WMA99 are too large by $\sim$15 % and $\sim$ 70 %, respectively.

\[mm\]

Modelling of the mid-infrared absorption {#modelling}
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In order to derive information about the column density structure from the mid-IR intensity, we have used a simple geometrical model: each dense core is embedded within a lower-density parent molecular cloud. On the line of sight to the core, we measure i) a background intensity arising from the rear side of the parent cloud that is attenuated by the absorption from the core, and ii) a foreground intensity arising from the front side of the parent 