What PNe could tell about extinction inside external galaxies
ABSTRACT With the extinctions measured in PNe of the Galactic Bulge we had verified that the classical small clouds model for extinction in the solar neighbourhood is easily generalized to the entire Galaxy. I use this model to explore the behaviour of extinctions of PNe observed in an external galaxy. With a resonable number one can derive the opacity of the extinction disk. A larger number would be required to constrain the ratio of the scale heights of PNe and extinction. The measurement of the H$\alpha$/H$\beta$ ratio of PNs in disk galaxies provides a most useful probe to learn about the interstellar medium in external galaxies.