Scatterplots of data for the selected lines of sight (Left: extinction; Right: submillimetre emission). Polarization degree p_V and intensity P_S were debiased with the Modified Asymptotic method (Plaszczynski et al. 2014). We note the variable that has the larger error in each plot: in the visible, τ_V (x axis); in the submillimetre, P_S (y axis). In the left panel, the line (blue) represents the “classical” upper envelope, p_V = 0.0315 τ_V (Serkowski et al. 1975). This upper envelope has been transferred to the right panel using the derived value for R_{S /V}.