The data of Part I are examined in the light of accepted theories. The specific refractive index increment ñ of most polymer solutions lies between −0.2 and +0.2 ml./g., although larger values can obtain in circumstances wherein the scattering unit is unusually large, e.g., solutions of partially neutralized polyacids the units of which contain the gegenions. ñ depends on the indices of solvent n1 and polymer n2. Among common solvents, water and 1-bromonaphthalene are capable of affording high positive and negative values, respectively, for n. The Gladstone-Dale rule applies rigorously to pure and mixed solvents, but the Lorenz-Lorentz expression is preferable for evaluating n2. Results of current theories applied to mixed solvents and copolymers are summarized. In the former, the true molecular weight M is determined by using ñ and the variation of solvent index with composition. For a copolymer of monomers A and B, M as well as Ma and Mb are obtainable by using ñ, ña, and ñb. Dispersion is expressed as (ñ)λ = (ñ)436[D′ + D″/λ2] at a wavelength λ, and dispersive constants D′ and D″ are evaluated for some solutions. ∂ñ/∂T is generally 3.2 (±2.3) × 10–4 ml./g./°C. and changes very little with λ. When ñ increases with M, the limiting characteristic value is derived (at 1/M = 0) from a plot of ñ versus 1/M. ñ can be determined to a maximum accuracy of 1% by using n2 calculated from the Lorenz-Lorentz equation and the experimental partial specific volume.