Ferrous chloride is an antiferromagnet (AF) below its Neel temperature TN, but it may be driven into a saturated paramagmetic (SP) state by an external magnetic field applied along the c-axis. This AF-SP transition is first order below and continuous above a tricritical temperature Tc, a classic example of metamagnetism. With a pulsed dye laser, piezoelectric modulator, and a unique gated amplifier, we monitored the scattering of circularly polarized light by the mixed AF-SP phase below Tc. Above Tc the optical density of a sharp absorption line changes with the spin ordering; because the transition couples to spin fluctuations, the optical density maximizes in the SP phase. We have used these optical techniques to map the thermodynamic phase diagrams of FeC12 and FeC12 doped with cadmium, which to our knowledge is the first study of a randomly disordered critical system.