Optical depth measurement toward the H II region G19.075-0.287 (Wang et al. 2018). The plot shows the optical depth as a function of LSR velocity and was computed using Eq. (9). For some channels, the absorption spectrum saturates and the measured optical depth is a lower limit of τ = 1.6, which is indicated by the horizontal dotted line. The gray shaded area indicates the velocity range between 43 and 56 km s ⁻¹, where HISA features have been extracted.