The central focal masking ("dispersion staining") technique is convenient and effective for determining the refractive index of a microfragment by the immersion method and for distinguishing between minerals in an immersion mount. For most microscopes the only modification needed is the installation of a small opaque dot at or near the focal point of the medium power objective. White light illumination, stopped down to the angular aperture of the opaque dot, produced a dark field on which the image of the fragment is outlined in diagnostic dispersion color. Precision of refractive index determination by this technique, about + or -0.001 under routine controlled conditions, is similar to that of the conventional Becke line technique using monochromatic yellow illumination. However, it has the advantages that (1) near the match the direction and approximate amount of mismatch may be inferred from the dispersion color of the image alone without the need for manipulation of the focus, (2) at the match the microfragment is clearly visible, and (3) results are obtainable even in the presence of an appreciable amount of inclusions or specific absorption (body color) in the fragment.--Modified journal abstract.

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