Correlations between opacity ratio τ (250 μ)∕τ (J) and column density (panel a) or POL-2 polarisation fraction (panel b). The data are at 40′′ resolution and sampled at half-beam steps. The points coinciding with the point sources marked in Fig. 14 are marked with plus signs. Data for column densities N(H₂)< 2 × 10²² cm⁻² are plotted with open symbols and for these the p_mas estimates may be inaccurate.