Abstract

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 distinguishingb etweenm ineralsi n an immersionm ount. 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 ±0001 under routine controlled conditions, is similar to that of the conventional Becke line technique using microchromatic 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.

Introduction
Determination of refractive index
　Isotropic substqnces
　Anisotropic subsｔances
　Interpolation and extrapolation
Precision and limitations of the technique
Additional applications
Acknowledgments
References
Appendix 1
　Modification of the microscope
Appendix 2
　Alternative modes of focal masking
　　Apertural focal masking
　　Unilateral focal masking
　　Annular focal masking
　　Strip focal masking
Appendix 3
　Calibration of color perception

Fig. 1. Dispersion curves (refractive index vs. wavelength) for a typical glass and immersion liquids at a given temperature. Fig.2. Spectrum formed by a beam of white light at an inclined solid/liquid interface. Refractive indices of solid and liquid match in the region of the yellow wavelengths. (Dashed line is normal to the interface.) Fig. 3. Diagramatic representation of the ｆrmation of the colored image of an immersed fragment using central focal masking. Refractive indices of fragment and liquid match in the region of the yellow wavelengths, rays of which are blocked by the dot mask at the focal point of the objective lens. Fig. 4. Dispersion colors observed in focal masking, modified Cherkasov(1957). Colors may vary somewhat depending on spectral composition of the illumination. Fig. 7. Observed dispersion colors for various combinations of refractive index difference and dispersion difference between liquid and immersed fragment (adopted from Schmidt and Heidermanns, 1958.) Colors may vary somewhat depending on spectral composition of the illumination. Fig. 8. Plot of dispersion vs. refractive index for selected organic immersion liquids and inorganic solids. Values for solids taken from Winchell (1929, table V) and Winchell and Winchell (1964, p. 137.) Fig. 42. Representation of the formation of the dispersion color image using apertural focal masking when refractive indices of fragment and liquid match in the region of the yellow wavelensths. Fig. A3. Representation of the formation of the dispersion color image using unilateral focal masking when refractive indices of fragment and liquid match in the region of the yellow wavelengths. Fig. A4. Substage condensing system for annular focal masking: (a) vertical section in vibration plane of polarizer, (b) bottom view of substage annular opening. Fig. A5. Alternative forms of substage opening for strip focal masking: (a) simple strip opening, (b) flared opening for increased illumination. Wilcox（1983）による『Refractive index determination using the central focal masking technique with dispersion colors』から