The observations of Abell 36 were obtained with FORS1 on June 3, 2005. The top panel shows the observed flux F (black solid line, in arbitrary units, and uncorrected for the instrumental response), the P_V = V / I profile (red solid line centred on about 0), and the null profile N_V (blue solid line, offset by  −0.75% for display purposes). The null profile is expected to be centred on about zero and scattered according to a Gaussian of width σ given by the P_V error bars, which are represented by light blue bars centred on about  −0.75%. The slope of the interpolating lines in the bottom panels gives the mean longitudinal field from both P_V (left bottom panel) and the null profile (right bottom panel) both calculated using the H Balmer and metal lines. The corresponding  ⟨ B_z ⟩ and  ⟨ N_z ⟩ values are  −33 ± 158 G and  −420 ± 161 G, respectively.