  We are engaged in a comprehensive program to find reliable elemental
abundances in and to probe the physical structure of the Orion Nebula, the
brightest and best-resolved H II region. In the course of developing a robust
extinction correction covering our optical and ultraviolet FOS and STIS
observations, we examined the decrement within various series of He I lines.
The decrements of the 2^3S-n^3P, 2^3P-n^3S and 3^3S-n^3P series are not in
accord with caseB recombination theory. None of these anomalous He I decrements
can be explained by extinction, indicating the presence of additional radiative
transfer effects in He I lines ranging from the near-IR to the near-UV. CLOUDY
photoionization equilibrium models including radiative transfer are developed
to predict the observed He I decrements and the quantitative agreement is quite
remarkable. Following from these results, select He I lines are combined with H
I and [O II] lines and stellar extinction data to validate a new normalizable
analytic expression for the wavelength dependence of the extinction. In so
doing, the He+/H+ abundance is also derived.
