Information on anisotropy of the local field of light wave in nematics is obtained mainly from refractivity measurements. Such investigations, supplemented by those concerning thermal dependence of density rho(T), allow for determining optical polarizability anisotropy of nematogenic molecules, as well as their orientational order parameter. This paper discusses, on the basis of extensive experimental data, the following problems : applicability of approximate relations between refractive indices of nematic liquid crystals (NLCs) and their molecular parameters for describing optical properties of nematic mesophase has been estimated; anisotropy of the local field factor F-k has been investigated, as well as its influence on calculated anisotropy of molecular polarizability. Influence of various extrapolation procedures on calculated molecular parameters of the NLCs has been investigated.