Ratio of τ_H, the optical depth derived from our Herschel observations, to τ_P, the opticaldepth derived from Planck observations (Planck Collaboration XI 2014) (grey points), with both τ values computed at 850 μm. The ratio is plotted vs. τ_P. Green pointscorrespond to low values of τ_H due to the convolution applied to the Herschel column density map (see text). Mean values ± one standard deviation of the grey points, excluding the green ones, are shown with the blue histogram. The dark green and the two red lines show the weighted mean of the ratio: 1.174 ± 0.040.