This article contains the results of optical measurements for 43 porcelain bodies made of kaolin, feldspar and quartz, or kaolin, feldspar and alumina.
Each material was ground to pass through a sieve with 10000 meshes per sq. cm. The members in the systems kaolin-feldspar-quartz and kaolin-feldspar-alumina were compounded and formed into small cones.
After firing to 1460°C and preparing thin sections, their microstructures were observed.
Thin sections of the system kaolin-feldspar-quartz consisted of glass, quartz, mullite, yellow spots, and pore, while those of the system kaolin-feldspar-alumina contained crystalline alumina in addition. The glasses were heterogeneous even in same thin section, Solubility of mullite in the glasses is discussed.
Their refractive indices were determined by the double variation method originated by Mr. R. C. Emmons and improved by Mr. T. Sueno. The relation between the refractive index and the chemical composition is shown by tables and triaxial diagrams. General tendency is that quartz reduces the index while alumina and kaolin act to increase it.

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