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As part of a larger effort to investigate the line-emitting properties of starburst nebulae, we have undertaken a comparative study of two nearby emission nebulae associated with high-mass star formation. The Orion nebula (M42) is classified as a ``classic'' H~{\sc ii} region. It is approximately 3~pc in size and is powered mostly by EUV photons from the central Trapezium cluster of O and B-type stars. The emission bubble N70, in the Large Magellanic Cloud, is approximately 95 pc in size with a nearly spherical shell-like morphology. The nebular emission is powered by an uncertain combination of EUV photons, intense winds, and supernova blast waves from the central population of high-mass stars.
Using the Goddard Fabry-Perot etalons with the Rutgers Fabry-Perot imaging system on the CTIO 1.5-m telescope, we have obtained narrowband images (FWHM $\sim$6\AA) of M42 and N70 in the light of H$\alpha$ $\lambda$6563, [N~{\sc ii}] $\lambda$6584, [S~{\sc ii}] $\lambda\lambda$6717,6731, and (for N70) [O~{\sc iii}] $\lambda$5007, along with the corresponding red and blue continua. In M42, the central 0.5 pc is characterized by low [N~{\sc ii}]/H$\alpha$ and [S~{\sc ii}]/H$\alpha$ flux ratios ($\approx$~0.1--0.2 and $\approx$~0.01--0.03, respectively). Higher flux ratios are evident along the ionization front ([N~{\sc ii}]/H$\alpha$ $\approx$~0.4, and [S~{\sc ii}]/H$\alpha$ $\approx$~0.06) and in the neighboring Herbig-Haro objects ([N~{\sc ii}]/H$\alpha$ $\approx$~0.35, and [S~{\sc ii}]/H$\alpha$ $\approx$~0.1). By contrast, N70 shows much higher [S~{\sc ii}]/H$\alpha$ flux ratios which smoothly increase with radius --- from $< 0.3$ near the center to $> 1.0$ towards the outer filamentary shell. These ratios, when combined with generally low [N~{\sc ii}]/H$\alpha$ flux ratios of $< 0.25$, are difficult to explain with either pure photon or shock ionization models. EUV photons must contribute a significant fraction of the inner nebula's ionization, because the [O~{\sc iii}] emission shows a strong concentration towards the central hot stars. The contending ionization processes in the outer filaments are evaluated in terms of the available data.
