Geometrical effects of a spherical shell. Left panel: visualization of the radial profile that would provide the projected column number density distribution in the central panel along a cut through the bubble center. Right panel: dependence of the apparent shell width (red) and the limbbrightening factor (black) on the geometric shell width. The observed [C II] and CO (3 → 2) shell widths and limb brightening factors are overplotted. The error bars indicate the variation of both quantities along the observed ring.