Accuracy of microscopical methods for determining refractive index by immersion

The accuracy of the immersion m ethods used in petrography for determining refractive indices is limited by the sensitivity of the criteri on of match. In a series of tests, the sensitivity of three m ethods of ascertaining match was investigated as a function of objective aperture and size and shape of particle. The followin g data will illustrate the way in which experimental det a ils, represented in the literatu re as equally good, can influence the accura cy of a res ult. A single determination of the refractive index of potassium alum has a proba ble error originating in the cri terion of match which is as small as 0.00008 when the d oublediaphragm method and an obj ective, 3.2 X, N.A. 0.11, arc employed, or as large as 0.00104 with central illumination and an object ive, 45 X , N .A. 0.85. The similar error w ith potassium chloride ranges between 0.00014 and 0.00125, d epending upon conditions. All methods are more sensitive with objectives of low numerical aperture.