In 1912 Bowen1 produced pure sodium nepheline artificially and measured its optical propoerties; he also showed that it could take into crystal solution as much as 35 per cent of the materials of anorthite; with increase in the tenor of these constituents he showed that the index, N0, remained constant at 1.537, while Ne increased from 1.533 to 1.539, thus changing the optic sign from minus to plus. In 1917 he showed2 that NaAlSiO4 can intercrystallize at a high temperature with all proportions of KAlSiO4. In 1931 Bannister and Hey3 made it clear that at low temperatures no more than about 30 per cent of KAlSiO4 is stable in NaAlSiCO4. In 1933 the writer suggested4 that (in harmony with atom for atom explanation of crystal solution) CaAl2Si2O8 does not intercrystallize with NaAlSiO4, but that its components, CaAlAlO4 and SiSiO4, do so. Bowen himself reported in 1912 that NaAlSiO4 can take into crystal solution at least as much as 5 per cent of SiSiO4; so far as known to the writer no tests of the solubility of CaAlAlO4 in NaAlSiO4 have been made. It seems probable that if it is soluble it would increase the density and refractive index. Is it because SiSiO4 would leave a vacancy in the structure and therefore lower the density and index that the combination of these two does not change the index (for the ordinary ray)?