Results of a Monte Carlo simulation: the mean optical depth and its standard deviation for a sample of sources chosen randomly with a probability distribution proportional to ρ(r,z), (points+error bars). For comparison, the (solid line) shows the optical depth calculated with the help of the integral of Eq. (7) with l = 0 and χ = 0. Top panel: results for the Mexican hat vertical profile with h = 117 pc. Bottom panel: results for the exponential vertical profile with h′ = 88 pc.