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We investigate the wavenumber scale of Fe I and Fe II lines using new spectrarecorded with Fourier transform spectroscopy and using a re-analysis ofarchival spectra. We find that standards in Ar II, Mg I, Mg II and Ge I give aconsistent wavenumber calibration. We use the recalibrated spectra to deriveaccurate wavelengths for the a6D-y6P multiplet of Fe II (UV 8) using bothdirectly measured lines and Ritz wavelengths. Lines from this multiplet areimportant for astronomical tests of the invariance of the fine structureconstant on a cosmological time scale. We recommend a wavelength of 1608.45081{\AA} with a one standard deviation uncertainty of 0.00007 {\AA} for thea6D9/2-y6P7/2 transition.

Wavelengths of the 3d6(5D)4s a6D - 3d6(5D)4p y6P Multiplet of Fe II (UV 8)