the silicon abundance determined from optical versus ultraviolet spectra in SDSS1228+1040 and GALEX1931+0117, for the latter also the oxygen abundances show this difference. There are at least three possible explanations:

*Uncertain atomic data.* This is a perennial problem, as there are many, and large differences in various compilations of atomic data. The [O]{} resonance lines in GALEX1931+0117 are perturbed by airglow, interstellar absorption and overlapping [Si]{} lines (see above), and the ultraviolet abundance determination rests largely on one excited line at 1152.1 Å. Similarly, the optical O abundance is measured only from the [[O]{}7777Å]{} triplet [@vennesetal10-1; @melisetal11-1]. However, our abundance measurements for Si use many lines in the ultraviolet. In the recent compilation by [@bautistaetal09-1] the authors combined several different computational methods, previous theoretical calculations by other authors, and experimental data into a “recommended” value for $\log$ gf. These values agree fairly well with the ultraviolet data from NIST that we have used. However, for the five optical lines they consider, the values are $0.25-0.30$dex smaller, though with errors as large as 0.3dex. Using these values would [*increase*]{} the abundance determined from optical spectra, contrary to what would be needed for a more consistent solution. In addition, in a recent 