Observations of WD 2105−820 obtained with FORS1 on 2004-08-10 UT 05:01 = MJD 53 227.209. The top panel shows the observed flux F (black solid line, in arbitrary units, and not corrected for the instrument response), the P_V = V / I profile (red solid line centred about 0), and the null profile N_V (blue solid line, offset by −2.75% for display purpose). The null profile is expected to be centred about zero and scattered according to a Gaussian with σ given by the P_V error bars, which are represented with light blue bars centred about −2.75%. The regions used for field measurement are marked with green bars above and below this spectrum. The slope of the interpolating lines in the bottom panels provides the mean longitudinal field from P_V (left bottom panel) and from the null profile (right bottom panel) both calculated using only the H Balmer lines. The corresponding  ⟨ B_z ⟩  and  ⟨ N_z ⟩  values are 9770 ± 843 G and −11 ± 868 G, respectively.