Quantity ] that shows (in color) the dependence for any wavelength and spectra line of the emergent Stokes V amplitude on the backlight illumination (vertical axis) and on the source function for V (horizontal axis) for an atmosphere with T_ν0 = 1. The medium becomes neutral along the dashed line. For other optical depths, the background colors remain qualitatively similar but the neutral medium takes place at the corresponding continuous lines.