Upper panel: Observed mean values of V²=(c·Δλ_w/λ₀)² versus the inverse atomic mass 1/μ for the four slit positions marked in Fig. 1. The dotted orange line represents a linear fit through all lines, and the solid orange line is used as reference for width excesses of the spectral lines. The diamonds for Fe II(b) and Mg b separate the close Fe II(a) and Na D values. The orange bar indicates the uncertainty of the opacity broadening of Hγ. Lower panel: Enlargement of the range of the metal lines. The position of Fe II(a) is slightly shifted to separate it from Fe II(b).