Many diverse observations of the interstellar medium have provided evidence for significant ionization of the diffuse gas which is outside of bright, localized, well defined regions of ionized hydrogen. Recent reviews on the subject refer to this warm, ionized gas as one of the four major phases of the interstellar medium. Dr. Reynolds' observing program at the University of Wisconsin has become the primary source of information about the warm, ionized phase of the interstellar medium (ISM). The objective of the project is to investigate the physical state of the ISM and the mechanisms which ionize and excite the gas through the study of faint emission lines from the gas. A high resolution Fabry-Perot spectrometer has been specially developed for this investigation.
