HOW THE GLASS IN LENSES IS MADE 



stirring the molten glass with a rod of fireclay, to 

 ensure a thorough mixing of the components. This 

 led to considerable improvement and the method 

 has survived to the present day. 



The first real advance in the manufacture of 

 optical glass, was due to the ingenuity of two 

 Germans, Abbe and Schott, who lived at Jena. 

 Jena glass became famous for the manufacture of 

 lenses, so much so that a stupid idea still prevails 

 in many quarters that only the Germans can make 

 good optical glass. To give them their due it is 

 good but quite recent events have shown the world 

 that the Britisher can make better. The two 

 German scientists used new chemicals in making 

 [their glass and they succeeded in producing a sub- 

 stance which possessed hitherto unheard of pro- 

 perties. In what is now known as the older crown 

 and flint glass the dispersion and refraction in- 

 creased with the density, that is to say, the heavier 

 the glass the more it scatters and bends light rays 

 passing through it. With the new methods, glass 

 is made which scatters the light rays very little, 

 though bending them considerably and vice versa. 

 The ordinary crown glass is composed of silicates 

 of calcium and sodium or of calcium and potassium 

 or a mixture of both and it is possible to make it 

 colourless and free from defects, but its optical 

 properties are never so valuable as those of Jena 

 glass. The most important components of the 

 newer glass are the oxides of Barium, Magnesium, 

 Aluminium, Zinc and Boron. 



285 



