Question

During the early nineteenth century, the manufacture of high-grade optical glass for astronomical telescopes was severely constrained by the inability to eliminate density variations, known as striae, in large molten batches of crown and flint glass. Pierre-Louis Guinand, a Swiss artisan, revolutionized optical glass production by introducing a stirring mechanism constructed of fireclay cylinders into the molten crucible. This technique disrupted the thermal stratification and chemical heterogeneities that typically produced light-refracting imperfections upon cooling. Recognizing the potential of Guinand's innovation, the Bavarian entrepreneur Joseph von Utzschneider recruited Guinand to the Benedictbeuern glassworks, where he collaborated with the physicist Joseph von Fraunhofer. While Guinand focused primarily on mechanical homogenizing methods, Fraunhofer systematically analyzed how chemical compositions affected refractive index and dispersion. Crucially, Fraunhofer discovered that adding precise quantities of lead oxide to flint glass batches not only lowered the melting point but also heightened chromatic dispersion, whereas adjusting potash concentrations in crown glass stabilized optical clarity. However, Fraunhofer's paramount technical breakthrough—the precise mapping of dark absorption lines in the solar spectrum—relied explicitly on Guinand’s fireclay-stirred discs, which provided unprecedented homogeneity in large-diameter lenses.

According to the passage, Fraunhofer’s discovery regarding the addition of lead oxide to flint glass batches indicated that this substance produced which of the following effects?