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.The magnitude of the errors introduced by birefringence was studied by t ests of a strongly birefringent compound under a broad variety of conditions. T o show the influence of conditions upon accuracy, two examples may be cited. Employing a 20 X , N.A. 0.40 objective, plane polarized li ght with its vibration parallel to the horizontal crosshair, and ordin ary oblique illumination from the side, the error in determining the extraordinary refractive index of mercuric cyanide is +0. 0056, but employing a 10 X , N.A. 0.25 objective, light with its vibration parallel to the horizontal crosshair, and double-d iaphragm illumination, oblique from the side, the error is less than 0.0001. CONTENTS P age

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