Graphical output of the IRAF/IDL Bonn pipeline obtained from the analysis of the FORS 2 data of the hydrogen lines of HD 23478. The top panel presents the derivative of Stokes I. The regions used to calculate the magnetic field are marked by a thick blue line centred at zero. In the middle panel, the top spectrum presents the Stokes I spectrum arbitrarily normalised to the highest value, the middle red spectrum corresponds to the Stokes V spectrum (in %), while the bottom blue spectrum is the N spectrum (in %). The green asterisks drawn on the Stokes V and N spectra mark the points that have been removed by the sigma clipping. The pale blue strip drawn behind the N spectrum shows the uncertainty associated with each spectral point. As a double-check of the statistical consistency of the data and uncertainties, the thick green bar on the left side of the N spectrum shows the standard deviation of the N profile. In the bottom left panel, we present the linear fit used to determine the magnetic field value using Stokes V (i.e. ⟨B_z⟩). The red solid line shows the best fit. From the linear fit we obtain ⟨B_z⟩ = −1477 ± 95 G. In the bottom right panel, we present the same as in the bottom left panel, but for the null profile (i.e. ⟨N_z⟩). From the linear fit, we obtain ⟨N_z⟩ = 30 ± 64 G.