  The dust extinction towards bright HII regions in NGC 598 and NGC 5457 has
been studied in detail by forming line ratios of Balmer and Paschen emission
lines covering a large wavelength range. Three homogeneous models of the
geometrical distribution of the emitting sources and the obscuring dust have
been tested. Only for low extinctions can the data be fit by a homogeneous
slab. For most of the observed HII regions the Witt et al.(1992) 'dusty
nucleus' model matches the observations equally well as the usual assumption of
a foreground screen, but the former implies much larger actual dust contents.
Spatial variations in the optical depth in V of the order 0.3 across a region
are found.
