  In this work we realize a narrow spectroscopic feature using a technique that
we refer to as magnetically-induced optical transparency. A cold ensemble of
$^{88}$Sr atoms interacts with a single mode of a high-finesse optical cavity
via the 7.5kHz linewidth, spin forbidden $^1$S$_0$ to $^3$P$_1$ transition. By
applying a magnetic field that shifts two excited state Zeeman levels, we open
a transmission window through the cavity where the collective vacuum Rabi
splitting due to a single level would create destructive interference for probe
transmission. The spectroscopic feature approaches the atomic transition
linewidth, which is much narrower than the cavity linewidth, and is highly
immune to the reference cavity length fluctuations that limit current
state-of-the-art laser frequency stability.
