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.

The manufacture of optical glass--Raw materials--
Mixing--Furnaces and crucibles--Kilns for heating
pots--Transfer of pots from kiln to melting furnace--
Introduction of cullet and raw materials--The fining
process, difficulties and limitations--The stirring
process--The final cooling of the glass--Rough sorting
of the glass fragments--Moulding and final annealing
of the moulded glass--Grinding and polishing of plates
and discs for examination; smallness of yield obtained--
Difficulty of obtaining large blocks of perfect glass _p._ 223