Liquid crystal is an intermediate state between liquid and solid. In crystalline phases, molecules possess orientational order and three dimensional positional order, while in isotropic phases the molecules lack (long-range) order. In the various liquid crystalline phases, the molecules are in between two extremes. Most liquid crystal molecules are large and anisotropic. Some ternary lyotropic system as tetradecyltrimethyl ammonium bromide + decanol + water (TTAB+decanol+H2O) can show different mesophase and nematic phases. The micellar aggregates that form such nematic phases can take different shapes. The uniaxial nematic phases have been shown to be prolate (calamitic-N-c) and oblate (discotic-N-d) micellar aggregates dispersed in water. The biaxial nematic phase (N-BX) appears to be an intermediate phase along the border between the two uniaxial nematic ones. In this study, we present the first optical observation on the thermal behaviour of refractive index and on the optical sing of a lyotropic nematic system formed by TTAB+decanol+water.