By means of a microphotometer the principal reflectances at normal incidence have been measured, in air and in oil, on oriented sections (OOOl), (1010) and (1120) of cinnabar. Measurements were made at intervals of 10 nm from 400 nm to 850 nm. From 550 nm a Nicol prism was used as the polariser; as monochromator a glass prism was used. Dispersion curves are given for the reflectances (in air and in oil) and also for the values of the refractive index and the absorption coefficient
