Using the technique of optical mixing spectroscopy, we have observed the Brownian movement and measured the diffusivity of proteins within whole, intact human and bovine lenses. The magnitude and the temperature dependence of the protein diffusivity in bovine and normal human lenses implies that cold cataract is the result of a first-order phase separation of the protein-water mixture in the lens. The magnitude of the diffusivity of proteins within cataractous human lenses indicates the presence of large aggregates of proteins.