CHAPTER XII.

OPTICAL GLASS.

Nature and properties of optical glass--Homogeneity-- Formation and removal of striæ in solutions and in glass--Transparency and colour--Absorption of light in “decolourised” glasses--Refraction and dispersion-- Definitions--Refractive index, dispersion, medium dispersion, the quantity ν--Specification of optical properties in terms of certain spectrum lines--Table of typical optical glasses and their optical constants-- Crown and flint glasses--Relation between refraction and dispersion in the older and newer glasses--Work of Abbé and Schott--Applications of the new glasses-- Non-proportionality of dispersion in different types of glass--Resulting imperfections of achromatism-- The relative partial dispersions of glasses--Pairs of glasses giving perfect achromatism not yet fully available--Constants of Schott’s telescope crown and flint--Narrow range of optical glasses, consequent limitations in lens design--Causes of these narrow limits--Possible directions of extension--Chemical stability of optical glasses--Double refraction in optical glass arising from imperfect annealing _p._ 205