26 



devitrification frequently develops in localised patches, the 

 appearance of which is evident from photograph. 



Optical glasses usually develop devitrification as small 

 spherical particles, more or less evenly distributed throughout 

 the mass. These particles increase in size comparatively 

 rapidly at high temperatures, and at a rate appreciable in 

 the course of many years even at ordinary temperatures. 

 In exceptional cases the change may take place with great 

 rapidity, and a glass which is being stirred may, within a 

 minute or two, go practically solid, owing to sudden devitri- 

 fication having taken place throughout the mass, resulting 

 in blocks having the appearance of marble. 



Opalescence is a trouble which usually develops during the 

 slow cooling of glass when the founding is completed, and is 

 particularly liable to occur in glasses of high borate and 

 fluorine content. 



Imperfect durability is not quite so vital a trouble, in 

 that, if it is essential for a particular purpose to utilise glasses 

 of very special optical constants, such glasses, if not properly 

 durable, may often be protected in use by cementing between 

 some more durable components. 



From the point of view of the user of optical glass, the 

 chief defects are : 



1. Imperfection in annealing. 



2. Imperfection in homogeneity. 



3. Striae or " veins." 



(These defects distort the light to a greater or less 

 extent in its passage through the glass, thereby 

 affecting definition.) 



4. Bubbles or " seed." 



5. Bad metal or " feather." 



6. Flaws. 



7. Imperfect durability. 



I. Imperfection in annealing affects glass in two ways. 

 The more obvious effect is that it causes it to exhibit to some 

 extent the property of " double refraction " thus producing 

 a retardation of one part of the incident light relative to the 

 remaining part which is vibrating in a different plane. This 

 effect is not nearly so serious as has often been supposed to 

 be the case, and considerable departure from perfect annealing 

 could be tolerated, even in instruments of precision, but for 

 a concomitant effect produced in the glass by the stresses 

 which give rise to the phenomenon of bi-refringence. A block 

 of glass is always, to a greater or less extent according to the 



