Edléns polarization formula is found to fit the high-l solar infrared emission lines attributed to Mg I to the accuracy of the high-precision observations. For several previously unidentified weak lines, assignments are made, a few to Mg. I. With some revision of laboratory energy levels, a more precise ionization limit for Mg I is determined to be 61 671.056 (10)cm^-1. The fine structure of the two strongest lines at 811 and 818 cm^-1 is found to consist of 4 components spaced about 0.001cm^-1 apart with nearly equal intensities. Their behavior in the penumbra magnetic field is well described by the Paschen–Back effect. Their line profile, polarization, and directional properties are discussed.