Δμ/μ derived from individual line pairs (52) which are separated by less than 110 Å and show a difference in sensitivity of more then 0.02. The errorbars reflect the combined positioning error of the two contributing lines. The weighted fit corresponds to Δμ/μ = (6 ± 12) × 10^-6. The filled squares graph 11 line pairs, selected to give the largest difference in sensitivity (≥ 0.02) towards variation in μ (see Table 5).