Slopes k = Δτ(250 μm)/Δτ(J) for all cases with uncertainties δk/k< 0.1. The black, blue, and red symbols correspond to values derived for the full τ(J) range and for data below and above the limit of τ(J) = 0.6. The values of τ(250 μm) have been derived from SPIRE data without the subtraction of the local background. Neither τ(250 μm) nor τ(J) has been corrected for the expected bias.