Abstract
We present here what we believe to be the first experimental observation of the singular behaviour of the refractive index of a critical microemulsion in the vicinity of the liquid-liquid phase separation. As predicted by two-scale factor universality, and confirmed by independent constant-pressure heat capacity measurement, such anomaly cannot be explained by the well-known heat capacity one. Moreover, in spite of very large values of the correlation length in such samples, singular local-field correction does not seem to play the main role in the observed behaviour. Such a result confirms the assumptions generally used in the descriptions of critical binary mixtures near their consolute points.