Field G4.18+35.79 (LDN 134). Upper frames: τ(250 μm) (frame a)) and the ratio τ(250 μm) /τ(J) (frame b)). The lower frames show the lower (frame c)) and upper (frame d)) limits of τ(250 μm) /τ(J) calculated as (τ(250 μm) + δτ(250 μm))/(τ(J)−δτ(J)) and (τ(250 μm)−δτ(250 μm))/(τ(J) + δτ(J)). The areas not covered by Herschel observations and regions with a SN below 0.5 have been masked. In frame a), the solid black contour and the dashed white contour correspond to τ(250 μm = δτ(250 μm) and τ(J) = δτ(J). The maps have a resolution of 180″ and τ(J) is derived using 2MASS data.