Comparison of sheet with polished plate--Raw materials for sheet--Furnaces: various forms of tank furnaces-- Blowing process--Gathering, forming the gathering on blocks, forming the shoulder of the cylinder, blowing the cylinder, opening the end of the cylinder, detaching cylinder from pipe--Cutting off the “cap”--Splitting the cylinder--Flattening and annealing--Cutting and sorting sheet-glass--Defects of sheet-glass--Variations of the process--Attempts to produce “sheet” glass by rolling--Sievert’s process--Direct drawing processes-- The American process for drawing cylinders--Fourcault’s processes--Difficulties and limitations--Crown glass-- The blowing process--Limitations _p._ 149 CHAPTER XI. COLOURED GLASSES. Definition of coloured glass--Physical causes of colour-- Colouring substances: copper, silver, gold, carbon, tin, arsenic, sulphur, chromium, uranium, fluorine, manganese, iron, nickel, cobalt--Range and depths of tints available--Intensely coloured glasses--The process of “flashing”--Character of “flashed” glass--Colours produced on glass by painting: use of coloured “glazes” as paints--Ancient stained glass and modern glass-- Technical uses of coloured glass, photography, railway and marine signals _p._ 178 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 CHAPTER XIII. OPTICAL GLASS.