Comparison of the optical depth, τ, of dust (black solid lines) to that of para- and ortho-H₂ (blue and cyan solid lines, respectively, upper-left panel), H (orange solid lines, upper-right panel), CO (green solid lines, bottom-left panel), and N₂ (purple solid lines, bottom-right panel) at N(H₂) = 10²¹ cm⁻². The H₂ optical depth is shown for model ℋ and H₂ o:p = 3:1. The H, CO, and N₂ optical depths have been computed by assuming constant abundances and the photoabsorption cross sections presented in the Leiden Observatory database (https://home.strw.leidenuniv.nl/~ewine/photo/; see also Heays et al. 2017).