Optical depths in disk model with the disk scale height h = 325   pc. Points represent the observational data averaged over longitude within the range out to l =  ± 5^°. Top panel: the result for the rotation curve A (thick line) and after subtracting the gas contribution (thin line). Bottom panel: comparison of optical depths for different model rotation curves: A (solid line) and B (dashed line) and, in addition, the rotation curve based on the McMillan data (dotted line) (without gas subtraction and with R_° = 8.29   kpc preferred by McMillan).