Refractive index determination

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, produces 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 :0.ff)l under routine controlled conditions, is similar to that of the conventional Becke line technique using monochromatic yellow illumination. However, it has the advantages that (l) 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. Besides providing a useful means for refractive index determination, focal masking permits rapid distinction among constituents in a mixture and an estimation of their proportions. As a teaching aid the focal masking technique provides a convincing demonstration of the manner of image formation and resolution in the microscope. American Mineralogist, lolume 68, pages 1226-1236, 1983 using the central focal masking dispersion colors fragment and liquid is shown at once for all grains in the field of view by the dispersion colors of their images. The technique is useful further in revealing variations of refractive index within grains, the extent of which may be translated into terms of zoning of chemical composition. In the immersion method by focal masking, advantage is taken of the fact that the wavelength dispersions of refractive index of the common organic immersion liquids are appreciably greater than those of most inorganic solids of similar index. A typical example is illustrated graphically on Figure I for a glass that, at a given temperature, matches an immersion liquid for the orangeyellow light of the standard Fraunhofer DJine wavelength (589.3 nm) at refractive index 1.534. It is seen that for shorter wavelengths (the greens, blues, and violets of the spectrum) the refractive index of the liquid is higher, and for longer wavelengths (the oranges and reds) it is lower than that of the glass. It may be noted also on Figure I that the higher liquid (no : 1.538) matches this glass at wavelength near 650 nm, whereas the lower liquid (no = 1.530) matches near 520 nm, and that the match for liquid no = 1.522 falls far outside the visible spectrum. Figure 2 represents the essential behavior of axially parallel rays of white light passing through a fragment 0003-004)u83/ | | t2-1226$02.0