A new optical method has been developed for the measurement of ac magnetic susceptibilities. The method is based on the modulation of the Faraday effect by a variable frequency ac magnetic field, applied parallel to the light passing through an optically transparent sample. The present system operates between 2 Hz and 200 kHz but the method should be capable of covering an even wider frequency range. Calibration of the frequency response is achieved by measuring a Van Vleck paramagnet (thulium aluminum garnet), which is predicted to have a frequency independent susceptibility up to ∼1 THz. Preliminary results for dysprosium aluminum garnet at various fields are presented.