Nebular attenuation inferred from Balmer decrements. The ratios of the different Balmer lines to Hβ, as measured from both G395M and PRISM spectra, are compared with the theoretical values set by Case B recombination (solid black bars) for T_e = 20 000 K and n_e = 600 cm⁻³ (consistent with electron temperature and density derived in Sect. 4). The ratios measured from both PRISM and gratings suggest negligible dust attenuation; the best-fit A_V values are consistent with zero within 1σ uncertainty.