  Recent experiments in the original cuprate high temperature superconductor,
La$_2-x$Ba$_x$CuO$_4$, have revealed a remarkable sequence of transitions [1].
Here we investigate such crystals with Kerr effect which is sensitive to
time-reversal-symmetry breaking (TRSB). Concurrent birefringence measurements
accurately locate the structural phase transitions from high-temperature
tetragonal to low temperature orthorhombic, and then to lower temperature
tetragonal, at which temperature a strong Kerr signal onsets. Hysteretic
behavior of the Kerr signal suggests that TRSB occurs well above room
temperature, an effect that was previously observed in high quality
YBa$_2$Cu$_3$O$_{6+x} crystals [2].
