Left panel: Idealized cartoon illustrating the mixed configuration of dust, stars, and gas. Right panel: The same but with the foreground screen configuration. The amount of dust in the screen in the right panel is assumed to be roughly half of the total dust shown in the left panel. While the infrared τ₁₆₀ penetrates the full dust column along the line of sight, the optical depth inferred from the Balmer decrement only captures the dust between the stars that ionize the gas and the observer.