  The absolute density of SD radicals in a supersonic jet has been measured
down to $(1.1\pm0.1)\times10^5$ cm$^{-3}$ in a modestly specified apparatus
that uses a cross-correlated combination of cavity ring-down and laser-induced
fluorescence detection. Such a density corresponds to $215\pm21$ molecules in
the probe volume at any given time. The minimum detectable absorption
coefficient was quantum-noise-limited and measured to be
$(7.9\pm0.6)\times10^{-11}$ cm$^{-1}$, in 200 s of acquisition time,
corresponding to a noise-equivalent absorption sensitivity for the apparatus of
$(1.6\pm0.1)\times10^{-9}$ cm$^{-1}$ Hz$^{-1/2}$.
