20 



of the glass on cooling. If we consider, say, a sphere of 

 glass which is cooling from a temperature at which it is 

 appreciably plastic, we can realise how these internal forces 

 arise. 



The outer portions of the sphere cool and harden first, 

 whilst the inner parts are at a higher temperature. As 

 cooling proceeds, the inner parts in turn reach a lower 

 temperature and endeavour to contract; this contraction 

 is, to some extent, prevented by the outer layers which are 

 already comparatively hard and rigid, and, in consequence, 

 a condition of tension is set up in the inner portions of the 

 sphere, which produces in the outer layers a balancing state 

 of compression. If any minute bubbles are present in the 

 glass, high tension does not develop, because the air bubbles, 

 under the diminished pressure, expand considerably and 

 relieve the stress. A large mass of glass which has been 

 quickly cooled always exhibits bubbles of considerable size 

 near the centre; but when subjected to a slow annealing 

 process these bubbles of rarefied gas shrink to a negligible 

 size. 



It may be seen, therefore, that the forces set up in a 

 block of glass on cooling are due to the fact that the state 

 of cooling necessitates a falling temperature gradient from 

 the centre outwards ; and that, as the mass acquires rigidity 

 under this unequal temperature distribution, forces are set 

 up in the mass when the ordinary temperature, uniform 

 throughout, is reached. The perfection of annealing is, 

 therefore, dependent on the time which is allowed to elapse 

 during the cooling operation, and the amount of strain 

 present can only be reduced so much as to be harmless and 

 can never be entirely avoided. 



Small plates of glass, moulded lenses, etc., may be cooled 

 comparatively rapidly without serious stress being developed ; 

 but the largest blocks, telescope discs, etc., must pass through 

 the operation known as " fine annealing," in order to reduce 

 the stress to an amount which is not harmful in the optical 

 instrument. 



In this process, the glass blocks are reheated to a tempera- 

 ture at which incipient softening occurs. All stress origin- 

 ally present in the glass becomes removed by slight flow of 

 different parts of the mass; and the rate of cooling of the 

 furnace is regulated to a great nicety so that harmful stresses 

 shall not be reintroduced during cooling. This operation 

 occupies some weeks; and, for exceptionally large pieces of 

 glass, it may require months. 



