  We have measured the coexistence curve of the binary liquid mixture
n-heptane+nitrobenzene near its consolute point using an optical method. In
particular, the critical exponent beta describing the coexistence curve was
measured for this system. Previous experimental values of beta for
n-heptane+nitrobenzene were higher than the typical theoretically calculated
value, an unusual, although not unique, occurrence. In an effort to study this
discrepancy, we have used an improved experimental apparatus for our
measurements. We have taken special care to minimize temperature gradients and
maximize the temperature stability of our thermal control system. We have also
exploited features of a known optical method to analyze, thoroughly, sources of
systematic errors. We measured an apparent value of beta as 0.367+/- 0.006 and
by a careful study of the known sources of error we find that they are not able
to remove the discrepancy between the measured and the theoretical values of
beta. We also measured the critical temperature of the system at Tc=291.80+/-
0.02 K (18.65 C).
