Opacity spectral indices as a function of the fraction of molecular gas along the line of sight. a)β_mm versus f_H2. b) β_FIR versus f_H2. The points correspond to the thin Galactic disk, |b|^< _~1° or τ₃₅₃ ≥ 4 × 10^-4. Here f_H2 is estimated assuming that the dust opacity in the molecular phase is twice that of the atomic medium, R = 2, and using X_CO = 2.1 × 10²⁰ cm^-2 (K  km s^-1)^-1.