The expression usually quoted for the extinction of light transmitted by a solution (the turbidity) is `Einstein's scattering formula' which contains two terms. It is shown here that in addition to these two terms which are due respectively to the excess scattering of the solute over the solvent and (essentially) to the pure solvent, there is a third term depending both on a natural `excess' polarizability of the solute and on the polarizability of the solvent. This term is unlikely to alter estimates of molecular weights from the turbidity by more than a few tens of molecular weight units so is important only for solutions of small molecules. The expected corrections agree very well indeed with published experimental data on carbon disulphide in carbon tetrachloride and diethyl ether in benzene, and thereby offer some experimental evidence on the unresolved question of the validity of a factor of refractive index in the theoretical expressions for turbidities. The corrected formula may make it possible to investigate by light scattering the dependence of the isothermal compressibility of solutions on the concentration of solute. The concept of an `excess' molecular polarizability for solute molecules in a solvent and scattering light is justified for small solute and solvent molecules.