Phototropic glass for multifocal spectacle-glasses

摘要

Glasses are provided which may be made phototropic and which have a thermal coefficient of linear expansion between about 43 and 58x10-7/ DEG C. and an index of refraction nd over about 1.60. Such glasses can be employed as near portion glass material in spectacle lens manufacture; thus, such glasses can be fused with known phototropic for portion glass material, for example, of the borosilicate type, to make multifocal glasses useful in spectacle lenses and spectacles.