Sketch is not to scale for the adopted geometry in MB thin-shell approach in the region B (W half) of NGC 588. The line of vision indicates the orientation of the detectors used to acquire the data. Black points represent the positions of the modelled annuli plotted in Fig. 1. We also show the model-resulted optical spectrum for some annuli to illustrate the radial variation of [O ii], [O iii], and Hβ across the nebula. Notice that in Fig. 1, photons only cross once in the shell although annuli look concentric.