  We present a study of the distribution of [O III] $\lambda$5007 and [O II]
$\lambda$3727 emission in the Narrow Line Region (NLR) of the Seyfert 1 galaxy
NGC 4151. While the NLR of NGC 4151 exhibits an overall structure consistent
with the unified model of Seyfert galaxies, narrow-band [O III] and [O II]
images obtained with the Wide Field and Planetary Camera 2 aboard the Hubble
Space Telescope reveal significant emission from outside the the emission-line
bi-cone. The [O III]/[O II] ratios are lower in these regions, consistent with
a weaker ionizing flux. We performed a photoionization modeling analysis of the
emission-line gas within a series of annuli, centered on the the central
continuum source, with inner radii from 13 to 90 pc. The gas is ionized by
radiation that has been attenuated by a relatively highly-ionized absorber
(HABS), which completely covers the central source, and a lower-ionization
absorber (LABS), which has a covering factor ranging from 0 to 1. We found that
the [O III]/[O II] ratios are well fit by assuming that, within each segment of
an annulus, some fraction of the NLR gas is completely within the shadow of
LABS, while the rest is irradiated by the continuum filtered only by HABS. This
suggests that the structure of the NLR is due to filtering of the ionizing
radiation by ionized gas, consistent with disk-wind models. One possible
scenario is that the low-ionization absorbers are dense knots of gas swept up
by a wind.
