J. Lipinski, Miniature and Precision Cameras, Iliffe & Sons, 1955


Further attempts at improving lens performance were frustrated by the inability to reduce the Petzval sum with the then known series of glasses. In 1886 Schott and Abbe introduced several new glasses in which the ratio of constringence, V,* and refractive index, N, were nearly the same; these glasses could be used to produce acrhomats with a considerably reduced Petzval sum. The stablization of these glasses led to a whole new series of symmetrical designs.