  We report the first use of a ring cavity to both enhance the output power and
dramatically narrow the linewidth ($<1\,$MHz) of blue light generated by four
wave mixing in a rubidium vapour cell. We find that the high output power
available in our cavity-free system leads to power broadening of the generated
blue light linewidth. Our ring cavity removes this limitation, allowing high
output power and narrow linewidth to be achieved concurrently. As the cavity
blue light is widely tunable over the $^{85}$Rb 5S$_{1/2} \,\,F=3$
$\rightarrow$ 6P$_{3/2}$ transition, this narrow linewidth light would be
suitable for second-stage laser cooling, which could be valuable for efficient
$^{85}$Rb BEC production.
