Number density of H⁻ vs. optical depth log τ_Ross for three different abundances of Mg. The blue line (A(Mg) = 7.58) corresponds to the solar abundance of Mg, the red line to the Mg abundance in Pisces II 10694, and the black line (A(Mg) = 5.54) to the Mg abundance expected for a normal metal-poor star with [Fe/H] = −2.6. The molecular bands and the metallic lines are mainly formed between log τ_Ross = −2 and −1, a region where the number density of H⁻ is strongly affected by the Mg abundance.