Composition and light scattering results in a three-phase equilibrium of a microemulsion system provide evidence for the existence of an apparent field variable related to composition. This variable, the water-over-surfactant ratio ($X$), takes the same value in the three coexisting phases. This three-phase equilibrium disappears by a critical end point which is approached by varying $X$. The dependences of the scattered intensity and the correlation length as a function of $X$ are described by power laws with Ising exponents.

• Received 31 January 1984

DOI:https://doi.org/10.1103/PhysRevLett.52.1895