  We investigate the wavenumber scale of Fe I and Fe II lines using new spectra
recorded with Fourier transform spectroscopy and using a re-analysis of
archival spectra. We find that standards in Ar II, Mg I, Mg II and Ge I give a
consistent wavenumber calibration. We use the recalibrated spectra to derive
accurate wavelengths for the a6D-y6P multiplet of Fe II (UV 8) using both
directly measured lines and Ritz wavelengths. Lines from this multiplet are
important for astronomical tests of the invariance of the fine structure
constant on a cosmological time scale. We recommend a wavelength of 1608.45081
{\AA} with a one standard deviation uncertainty of 0.00007 {\AA} for the
a6D9/2-y6P7/2 transition.
