(Hg,Cd)Te was the first mixed crystal which allowed the study of the transition from a wide gap semiconductor (CdTE) to a narrow-gap material without change of the optical transitions forming the interband edge. Now with (Pb,Sr)Se a material with rocksalt structure is available offering the same possibility. As an example, a critical re-examination of interband and intraband contributions to the refractive index is given. In the narrow-gap lead salts a maximum of n(E) near E_g was found earlier and attributed to the rapid change of absorption coefficient at the interband absorption edge. On the basis of measurements of the real absorption edge slope, as determined both by band structure and by broadening, a study was made connecting for the first time the experimental absorption edge parameters with the n(E) maximum.