The spectral-transmission curve of a film of dielectric powder having uniform particle size shows a very pronounced minimum corresponding to the Mie scattering maximum. The wavelength of this “Mie minimum” changes as the refractive index of the medium surrounding the particles is changed. If a film of dielectric powder is prepared and the wave-lengths of the Mie minima are determined when liquids of different refractive index are flowed around the particles then, if the wave-length values are plotted against the refractive indices of the surrounding media, the points fall on a straight line that intersects the axis at the refractive index of the powder, thereby providing a method for measuring the refractive index of powders above the range of immersion liquids. A method, using the data for the Mie minima, is described for determining the optimum particle size for white paint pigments. This method is illustrated with data on zinc sulfide and titanium dioxide.
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