Probing Extragalactic Disks---Limits on the Size and Mass of the ``Local'' Lyman Alpha Disk toward 3CR 196

Optical observations of the QSO 3CR 196 reveal the presence of low-ionization absorption (Ca II, Fe II, Mg I, Mg II, and Mn II) at a redshift of 0.43685 +/- 0.00005, ~60 km s^-1^ away from the 21 cm absorption feature previously reported. Since the radio emission is predominantly from extended lobes, the absorber must be at least large enough to cover the optical QSO and part of one or both of the lobes. This leads directly to a lower limit on the size of the absorber of about 13 h^-1^_50_ kpc. Under a plausible set of assumptions, this size limit, coupled with the observed velocity difference between the radio and optical absorption, leads to a lower limit on the mass of the absorber of about 5 x 10^9^ M_sun_. The relative elemental depletions are similar to those reported for other absorption-line systems containing Ca^+^ and for the interstellar medium toward ζ Oph.