14 



BRITISH OPTICAL GLASS. 



SOME NOTES ON ITS HISTORY, MANUFACTURE, 

 DEFECTS AND THEIR DETECTION. 



THE glass industry, as developed in Egypt, is of great 

 antiquity, and can be traced back for more than 4,000 years. 

 The insolubility of the material in water, its transparency 

 and general mechanical properties at ordinary temperatures, 

 and the changes which result with heating, give it a unique 

 character, which must have been recognised at a very early 

 date. The material was consequently manufactured for 

 working into vessels for use and ornament and, at a later 

 date, for windows. 



In the strictest scientific parlance, glass is an extremely 

 viscous liquid which, even at ordinary temperatures, slowly 

 changes its shape under the action of force and becomes 

 increasingly fluid as the temperature is raised. In the 

 absence of stress, it is isotropic, amorphous, and shows no 

 transition temperatures of melting and solidification, such as 

 characterise true solids. It is rather a highly cooled, 

 unstable liquid which, in the absence of suitable nuclei, has 

 escaped crystallisation. The separation of crystalline con- 

 stituents is always tending to occur ; and, at suitable elevated 

 temperatures, in a longer or shorter time, such crystalline 

 growths will appear, giving the phenomenon of devitrification. 



All but a very small fraction of the glass at present manu- 

 factured is used on account of its valuable properties as a 

 material which is rigid, hard, transparent or selectively trans- 

 parent to light, highly resistent to chemical action, and which 

 gradually softens on heating, thus becoming amenable to 

 change of shape by running, pressing, drawing or blowing. 



A small amount of glass is manufactured for the sake of 

 its optical properties of refraction and dispersion, occasionally 

 together with that of selective absorption; and when the 

 nature of the use to which this glass is put necessitates the 

 employment of a strictly homogeneous material, it is spoken 

 of as " optical " glass. 



HISTORY. 



The glass from which are fashioned the lenses and prisms 

 of telescopes, microscopes and other scientific instruments is 

 made in a different way from any other kind of glass. In 

 the case of the more common material, which is made into 

 sheet or plate glass, bottles, globes, etc., the substance is 



