Collapsed cross sections for the four rim regions displayed in Fig. 2 (a total extent of 25′′), showing the variation of extinction, c, T_e (K) and N_e (cm^-3) with relative offset across the rim (solid lines). The dashed lines refer to the mean values across the rim region 1, to the NE. Left: cross sections for N_e from the mean value determined from the [S II]6716/6731 Å and [Cl III]5518/5538 Å ratios and T_e from the [S III]9069/6312 Å ratio and theoretical Balmer and Paschen line ratios computed for these values. Right: instead of measured T_e, an impulse function (Gaussian with FWHM 0.9′′) is used with the observed N_e to compute c from the H line ratios. The size of the temperature excursion (6000 K) was chosen to remove the trough in the extinction.