Optical depth at 9.7 μm, τ_9.7 versus logarithmic of the hydrogen column density at X-rays, N_H. N_H in units of cm^-2. Symbols as in Fig. 6. Dot-dashed lines are the CLUMPY model predictions for i = 0° and N_o =  [5,25] clouds. Dotted lines show how the curves evolve for the minimum and maximum number of clouds with a probability higher than 10% of intercepting the LOS. The coloured thick line show how the curve evolves when the viewing angle changes from i = 0° (black) to i = 80° (yellow). The dotted lines are the model that includes the Poisson probability of detecting a number of clouds along the ray (see text).