The coexistence curves of liquid (N,N-dimethylacetamide + heptane) have been determined within 15 K of the critical temperature by measurements of the refractive index n. The critical amplitudes B and the critical exponents beta have been deduced from (T, n), (T, x), and (T, phi) curves. Experimental values of beta are consistent with the theoretical prediction. The results have also been analysed to determine Wegner correction terms when beta and Delta are fixed at the theoretical values, and to examine the behaviour of the diameters of the coexistence curves. Molar mass-dependences of B and the critical volume fraction phi(c) have been derived from the classical theory without the limitation of phi(c) much less than 1, and tested against the experimental results.