Validation of our iterative method for the full non-LTE transfer problem (compare with Fig. 3c of Hummer 1969). The normalized source function is displayed as a function of frequency at different line center optical depths within the atmosphere: namely at τ = 0, 1, 10, 100, 10³, and 10⁴. For comparison, we also show the corresponding CFR source function (constant with frequency) as dashed lines.