Noble metals show absorption in the blue or ultra-violet; to this is due the colour of copper and gold. As was pointed out by Mott, the main absorption seems due to internal photoelectric effect, with excitation of a d electron. This absorption could be analysed - as for the alkalis - as a transition between two bands, described by Bloch orbitals, here d and s. But we must take into account the correlation between the hole created in the nd shell and the Fermi electrons. We can assume that the hole is screened by a (n+1)s electron bound to it, so that absorption should begin for the frequency of formation of the excited nd9(n+1)s configuration of the monovalent ion. Agreement with experiment obtained using this model is satisfactory both in the pure metals and their alloys The same model may be used for x-rays, and optical absorption energy is related to the energy separation between the s and d parts of K and LII,III emission bands An order of magnitude for the van der Waals interaction energy between d shells could perhaps be deduced from the structure on the short wavelength side of the absorption peak.