The optical properties of rubber are discussed under four main headings: absorption, scattering, refractive index, and double refraction. A general survey of the results of previous work in each of these fields is given. Some of the possibilities for future investigation are indicated. New experimental data are presented for the transmission values of milled pale crepe and a soft vulcanized rubber compound at wave‐lengths from 400 to 750 m&mgr;. The results of some measurements on scattering are also included. The variation of refractive index with wave‐length is given, as calculated from unpublished measurements of the angles of minimum deviation of prisms of unvulcanized and of vulcanized rubber.