It was found that the refractive index of heat-strengthened plate glass diminishes continuously from surface to center. As a consequence (1) prisms of such glass form spectral lines that are greatly curved, (2) plates of glass with plane parallel ends refract like cylindrical, concave lenses. This change of refraction is probably due to the condition of strain in the glass rather than to a relative change in the concentration of the component oxides.
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Hopfield et al. (1936) studied this question.