  We have investigated the Faraday rotation of in-plane magnetized bismuth -
substituted ferrite garnet films grown by liquid phase epitaxy on (100)
oriented gadolinium gallium garnet substrates. The Faraday spectra were
measured for photon energies between 1.7 - 2.6 eV. To interprete the spectra,
we use a model based on two electric dipole transitions, one tetrahedral and
one octahedral. Furthermore, the Faraday rotation sensitivity was measured at
2.3 eV, and found to be in good agreement with the theoretical predicitions. In
particular, we find that the sensitivity increases linearly with the bismuth
content and nonlinearly with the gallium content.
