Coexistence curves for {xC(6)H(5)NO(2) + (1 - x)CH3(CH2)(10)CH3} have been determined within 10 K of the critical temperature by measurements of refractive index n. The critical amplitude B and the critical exponent beta have been deduced from (T,n), (T,x) and (T,phi) curves, where x and phi are mole fraction and volume fraction, respectively. The values of beta are consistent with the theoretical ones. The experimental results have also been analysed to examine the Wegner correction terms and the behaviour of the diameter of the coexistence curve in various units. The coexistence curves have been successfully described by a combination of the Wegner equation and the expression for the diameter.