Crossover behavior in He3 and Xe near their liquid-vapor critical point

We present a detailed discussion of the crossover from mean-field to Ising critical behavior upon approach of the critical point, both for He3 and Xe. By combining different sets of experimental data, we are able to cover an unusually large temperature range on either side of the critical temperature T_c. Below T_c, we thus can make an accurate comparison with a recent calculation for the crossover of the coexistence curve. For the regime above T_c, an analysis of the compressibility demonstrates that the crossover regime in He3 is unexpectedly widened by a subtle interplay between quantum and critical fluctuations.