/values observed require sub–solar metallicity or shock excitation of \[\].

Next, we examine the behavior of \[\]/ versus \[\]/\[\] in figure \[fig:o3\_Ne\] (omitting for now NGC 5253, II Zw 40, and NGC 55 because of their low metallicity.) Optical line ratios are from the literature, and *ISO* observations of \[\]/\[\] are from @thornley. Galaxies with \[\]/$<0.5$ generally have line ratios consistent with the overplotted Starburst99/Cloudy models. With higher \[\]/, the scatter increases. Without accurately knowing the metallicity of each galaxy in figure \[fig:o3\_Ne\], it is difficult to judge how much of the scatter in \[\]/ versus \[\]/\[\] is due to the sensitivity of \[\]/ to metallicity rather than effective temperature.

According to the Cloudy models, metallicity alone cannot explain the line ratios of NGC 6240, IC 1623A, Arp 220, NGC 3690A, and NGC 7469 (and possibly NGC 972) in figure \[fig:o3\_Ne\]. Low metallicity and a upper mass cutoff of $30$  could together explain all but IC 1623A and NGC 6240. Alternatively, aperture mismatch, severe extinction, or shock excitation of \[\] could be at work. NGC 972 is not strongly centrally concentrated in optical emission line images, so the explanation may lie in aperture mismatch: the optical line ratios were measured with slitwidths of a few arcseconds, while the *ISO* neon lines were measured with a $14$ by $27$ aperture. The remaining discrepant galaxies all have very heavily obscured star formation regions, and it is likely that the discrepancy arises because the optical and mid–infrared spectra sample distinctly different regions along the line of sight. We also note that NGC 278 has extremely low \[\]/ for its measured \[\]/\[\]. Higher–spatial resolution mid–infrared spectroscopy (e.g., with SIRTF) may resolve this discrepancy. We will delay discussion of whether \[\]/\[\] is a reliable  diagnostic until § \[sec:midir-test\].

Next, we consider the optical helium and hydrogen recombination lines, which should for