Degree of polarisation (in %) as a function of wavelength (in nm) for both simulations and observations. The measurements are the same as in the first panel of Fig. 9 and represent the very centre in orange, the off-centre region in green, and the whole central region in red. The simulated spectra are shown in a continuous blue line for a grain ratio of 1:1.5 and dashed line for a ratio of 1:2, with corresponding optical depth and dilution fraction parameters to best fit the very-centre (orange) data points.