  Using birefringence techniques we have measured the critical exponents beta,
gamma, and delta in As-doped TbVO4, a structural realization of the
random-field Ising model where random strain fields are introduced by V-As size
mismatch. For pure TbVO4 we observe the expected classical critical exponents,
while for a mixed sample with 15% As concentration our results are beta=0.31
+/-0.03, gamma=1.22 +/-0.07, and delta=4.2 +/-0.7. These values are consistent
with the critical exponents for the short range pure Ising model in three
dimensions in agreement with a prediction by Toh. The susceptibility data
showed a crossover with temperature from classical to random field critical
behaviour.
