Measurements of the Faraday rotation and magnetization of terbium-doped Hoya FR5 glass and cerium-doped Hoya FR4 glass have been made as a function of temperature T in the range 4.2 K -10 K and of magnetic field H of up to 80 kG at the $Ar^+$ laser wavelength of 514.5nm. The saturations of magnetization and Faraday rotation above H/T> $5kG.K^{-1}$ can be analyzed by the quantum theory of paramagnetism. Calculated parameters show that the large Verdet constant of $Ce^{3+}$ glass is due to the effective $4f ightarrow5d$ electric dipole transition effect and that of $Tb^{3+}$ glass is due to the magnetization effect.effect.