The manufacture of optical glass forms a very important section of the glass industry, and presents some of the most difficult problems the glass maker has to deal with. It is in this section of the glass trade that applied physical and chemical science becomes of the utmost importance to the manufacturer. The production of optical glass is impeded by any defects which become evident in the structure of glass when examined under a polariscope. The presence of any striae, seeds, or stresses within the structure of the glass disqualifies it for any important optical work. It is a difficult matter to get pieces of optical glass only a few inches in diameter of the right optical constant and refractive index that are homogeneous enough to allow of the light rays passing without some dispersion when set up for use. It becomes necessary, therefore, to achromatise one glass with another in the form of doublets to correct aberration. A high degree of transparency and durability is necessary in all optical glasses. The persistent evidence of stresses developed in the solidification of the glass upon cooling, even when the glass is slowly and carefully annealed, is a most difficult factor to deal with. In annealing optical glass, the various temperatures and time periods have to be delicately adjusted and controlled, or big losses result. Even then many efforts may be made before a suitable piece of glass is obtained, and the costs keep accumulating with each attempt, and some idea of the amount of labour involved in the undertaking to produce optical glass at once becomes evident. The use of decolorizers and impure materials is not permissible, on account of the absorption and consequent resistance to the passage of light rays. The annealing, instead of occupying one or two days, is sometimes extended over a course of ten or fifteen days, in order gradually to relieve any stress present. The pots in which the glass is melted may only once be used, as the glass is usually allowed to cool down gradually and undergo the process of annealing within the pot.