  A high finesse optical cavity can be used to search for cosmic axions in the
mass range 10^{-6}< m_a <10^{-4} eV. Either a two-arm or a single-arm cavity is
suitable and in either case the signal as resonant sidebands imposed on the
carrier. Assuming for the local axion density the usual figure of 500 MeV/cm^3
[8], the KSVZ axion line [4] g/m_a = 0.4 Gev^{-2}, can be reached over the full
mass range in a one year search.
