A new refractometer is described, based on the principle of the penetration of light into the rarer medium at total reflection. A transparent hemisphere with a slightly curved diametral plane makes point contact with the smooth surface of the sample to be measured. When viewed through the hemispherical surface, Newton’s black spot is visible. At the critical angle, the light is totally reflected at the hemisphere boundary, and the black spot diappears. The critical angle at which the spot is no longer seen can be translated into refractive index and measured with very simple equipment. Operation is unaffected by the index of a contact liquid and indices can be measured which are as high as the optical constants of the material available for the hemisphere. Measurements are described with a rutile hemisphere which renders the instrument useful to the index of the ordinary ray, n=2.61.
A. H. PFUND, "A High Index Refractometer," J. Opt. Soc. Am. 39, 966-968 (1949)