Schematic view of lines of sight of fixed length l_los inferred from the analytical model described in Appendix C, and corresponding contributions to the histogram of the normalized column densities N_H ∕l_los and N(H₂)∕l_los. Bottom panels: the white stars correspond to all lines of sight while the black stars correspond to the specific cases illustrated above. Any line of sight is intercepting several components of constant density n_H. Diffuse components () have a size ; dense components () have a distribution of sizes . Only components with densities larger than are molecular (see main text). The red star in the bottom panels indicates the mean value of N_H ∕l_los and N(H₂)∕l_los computed over a large sample of lines of sight (white stars), hence the expected mean molecular fraction. The dotted line indicates a fully molecular medium.