Variation of Stokes V in Na I D₁ when changing V₀ (see column titles) assuming ξ = 1.3, 0.3, and 0 for bottom, second, and third rows, respectively. Other parameters are T_ν0 = 5, B = 100 G, , I_c = 2 × 10⁻⁵ (cgs units), and α = 1. The background radiation is represented in the top row, with V₀ calculated from I₀ using the weak-field approximation. The V/I_c and V₀/I_c profiles have the same vertical scale.