Critical phenomena occur in pure fluids, superfluids, and in binary liquid mixtures having a miscibility gap ending in a critical point of solution. Of the three, only the binary liquid mixtures are capable of dissolving a wide range of solutes, which makes them ideal for studying critical phenomena in multi-component systems. Using 632.8 nm light, we have measured the turbidity associated with critical point opalescence in the binary liquid mixtures, nitromethane + ethylene glycol, and nitromethane + 1-nonanol. Equilibrium in both cases is described by three intensive thermodynamic variables, namely, temperature, pressure, and one composition variable. In both cases the turbidity diverged toward infinity as a function of the temperature in the critical region. Upon addition of a small amount of 1-dodecanol to the nitromethane + ethylene glycol mixture and a small amount of ethylene glycol to the nitromethane + 1-nonanol mixture, however, the equilibrium requires four intensive variables for its description, namely, temperature, pressure, and two composition variables. The turbidity data in both of these cases maintained a finite value as a function of temperature in the critical region. We have also searched for a critical temperature effect in the turbidity in the binary mixture of 1-nonanol + ethylene glycol. Here the refractive indices of the two liquids making up the mixture are nearly equal. In this mixture, the turbidity was found to be a smooth function of temperature, including temperatures in the critical region. Given this non-scattering property, the mixture of 1-nonanol + ethylene glycol presents the possibility for studying critical point fluctuations using optical probes whose performance might otherwise be degraded by opalescence.