  We use a Sagnac interferometer to measure the dispersive and absorptive
properties of room temperature Rubidium vapor on the D_2 line at 780.2 nm. We
apply a pump beam such that the resulting Lambda system exhibits
Electromagnetically Induced Transparency. Using a "biased alignment" technique
we demonstrate a direct and robust method of measuring the rapid variation in
the refractive index. Such a "slow-light" Sagnac interferometer is ideally
suited to precision measurement applications such as magnetometry and inertial
sensing.
