Optical depth of the C-H stretch band at 3.4 μm vs. visual extinction A_V. The data obtained from this work are shown as red squares. The black filled circles are from McFadzean et al. (1989), Adamson et al. (1990), Sandford et al. (1991), Pendleton et al. (1994), Chiar et al. (2000), Chiar & Tielens (2001), Chiar et al. (2002), Rawlings et al. (2003), and Dartois et al. (2004). The various points associated with the Galactic centre sources have been slightly offset from each other in A_V for clarity (see text for details). Left: the optical depths are τ_tot; i.e., they include the contribution of the 3.47 μm band. Right: the optical depths are τ_3.4, due solely to the diffuse ISM (the estimated contributions of the 3.47 μm absorption have been subtracted). In both plots, the dashed line corresponds to the ratio A_V / τ = 250 determined by Pendleton et al. (1994) in the local ISM. See the text for more details about the evaluation of the extinction toward S7, S10, and S11.