  It is known that the diffuse H-alpha emission outside of bright H II regions
not only are very extended, but also can occur in distinct patches or filaments
far from H II regions, and the line ratios of [S II] 6716/H-alpha and [N II]
6583/H-alpha observed far from bright H II regions are generally higher than
those in the H II regions. These observations have been regarded as evidence
against the dust-scattering origin of the diffuse H-alpha emission (including
other optical lines), and the effect of dust scattering has been neglected in
studies on the diffuse H-alpha emission. In this paper, we reexamine the
arguments against dust scattering and find that the dust-scattering origin of
the diffuse H-alpha emission cannot be ruled out. As opposed to the previous
contention, the expected dust- scattered H-alpha halos surrounding H II regions
are, in fact, in good agreement with the observed H-alpha morphology. We
calculate an extensive set of photoionization models by varying elemental
abundances, ionizing stellar types, and clumpiness of the interstellar medium
(ISM) and find that the observed line ratios of [S II]/H-alpha, [N II]/H-alpha,
and He I 5876/H-alpha in the diffuse ISM accord well with the dust-scattered
halos around H II regions, which are photoionized by late O- and/or early
B-type stars. We also demonstrate that the H-alpha absorption feature in the
underlying continuum from the dust-scattered starlight ("diffuse galactic
light") and unresolved stars is able to substantially increase the [S
II]/H-alpha and [N II]/H-alpha line ratios in the diffuse ISM.
