  We demonstrate the implementation of a ring cavity to enhance the efficiency
of parametric four-wave mixing in rubidium. Using an input coupler with 95%
reflectance, a finesse of 19.6$\pm$0.5 is achieved with a rubidium cell inside.
This increases the circulating intensity by a factor of 5.6$\pm$0.5, and
through two-photon excitation on the $5s_{1/2}\rightarrow5d_{5/2}$ transition
with a single excitation laser, up to 1.9$\pm$0.3 mW of power at 420 nm is
generated, 50 times what was previously generated with this scheme. The
dependence of the output on Rb density and input power has been explored,
suggesting the process may be approaching saturation. The blue output of the
cavity also shows greatly improved spatial quality, combining to make this a
promising source of 420 nm light for future experiments.
