The coexistence curves of (T, n), (T, phi) and (T, Psi) (n and phi are refractive index and volume fraction, respectively; Psi is defined as Psi = phi/[phi + phi(c)(1 - phi)/(1 - phi(c))]) for a ternary microemulsion [(phi(AOT-water) - (1 - phi)octane] at constant pressure and a constant MOLAR ratio (40.8) of water to AOT have been determined within about 10 K from the critical temperature T-c by measurements of refractive index. This system can be regarded as a pseudobinary mixture. The critical exponent beta has been deduced from (T, n), (T, phi), and (T, Psi) coexistence curves within 1 K above T-c. They were 0.320, 0.325, and 0.320, respectively, and were consistent with the 3D Ising value. The experimental results in a temperature range of (T - T-c) < 5 K also have been analyzed to obtain critical amplitudes and the Wegner-correction terms, to examine the diameters of the coexistence curves, and to discuss the goodness of density variables for constricting order parameters.