In Section 3 we have already underlined the uncertainties related to the reddening corrections we must apply to take into account the presence of dust in spiral galaxies. First of all the assumption made about the reddening curve, assumed to be that of our Galaxy.
We have seen that the most widely used method to estimate
the dust content is based on the relative strengths
of the lower Balmer lines, 
/
.
By taking the gas emission lines
as dust tracer one assumes that the emission from the gas comes
from the same place as the emission from the ionizing stars,
which generally is not the case, since the gas in a spiral galaxy
is situated in a plane.
In general, we must admitt we have only a meager knowledge
of the real distribution of dust in emission-line galaxies.
It seems quite likely that it is patchy and irregular, and
thus that the assignment of a single value
for the ``extinction'' is only a crude first approximation.
Neglecting at first order these uncertainties
one can write
the effect of reddening on the ratio /
as :
where C is the measure of the amount of reddening (E(B-V) =
0.77 C), f(
) is the reddening curve, I(
) indicated
the (unreddened) intrinsic flux and F(
) the flux measured.
Note that only the difference in optical depths at the
two wavelengths enters in this equation; so the correction depends on
the form of the interstellar extinction curve (with arbitrary
normalization) and on the amount of extinction (C).
Table 4: Standard interstellar extinction curve.
If the Whitford reddening curve as parameterized by Miller and Mathews (1972) is used (see Table 4) one obtains
For the intrinsic flux ratio for H II region-like objects
we adopted the case B Balmer recombination decrement
I()/I(
)=2.85 for 
K and 
.
For AGN we must taken into account that
the harder photoionizing spectrum results in a large transition
zone, or partly ionized region, in which 
coexists with 
and free electrons. In this zone collisional
excitation is also important in addition to recombination collisional
excitation.
The main effect of collisional excitation is to enhance
. The higher Balmer lines are less affected
because of their large excitation energies and smaller excitation
cross sections. For this reason I(
.
