  The isotropic-to-nematic transition in a two-dimensional fluid of hard
needles is studied using grand canonical Monte Carlo simulations, multiple
histogram reweighting, and finite size scaling. The transition is shown to be
of the Kosterlitz-Thouless type, via a direct measurement of the critical
exponents eta and beta, of the susceptibility and order parameter,
respectively. At the transition, eta=1/4 and beta=1/8 are observed, in
excellent agreement with Kosterlitz-Thouless theory. Also the shift in the
chemical potential of the nematic susceptibility maximum with system size is in
good agreement with theoretical expectations. Some evidence of singular
behavior in the density fluctuations is observed, but no divergence, consistent
with a negative specific heat critical exponent. At the transition, a scaling
analysis assuming a conventional critical point also gives reasonable results.
However, the apparent critical exponent beta_eff obtained in this case is not
consistent with theoretical predictions.
