We have carried out a high-resolution x-ray-scattering study of the critical fluctuations in the nematic phase associated with the nematic-smectic-$A$ transition in N-$p$-cyanobenzylidene-$p$-octyloxyanilene (CBOOA). It is shown that for $8\times {10}^{-5\mathrm{}}\lesssim \frac{T}{T_c}-1\mathrm{}\lesssim 2\times {10}^{-2\mathrm{}}$ the transverse and longitudinal correlation lengths diverge with identical exponents, that is, $|{\nu }_L-{\nu }_T|<\mathrm{}0.03\mathrm{}$. As one approaches $T_c$ the system appears to crossover from helium- to mean-field critical behavior at a reduced temperature of ∼${10}^{-3\mathrm{}}$.

• Received 6 June 1977

DOI:https://doi.org/10.1103/PhysRevLett.39.352