Formation height above the 500 nm continuum level as a function of the line-cord index. Magenta dots and curve: FeI 630.1 nm line; blue dots and curve: FeI 630.2 nm line. Full lines show the uncorrected results, the discontinuity of the slope at index 8 is an artifact due to measurement difficulties in the contrast inversion layer. Dots show the results corrected for the inversion layer discontinuity, as explained in the text.