REFLECTING TELES60PE* 3J 



yellow tinge, so, when these three colours are exhibited in a 

 eloud-like mixture, they shew an equality and due proportion 

 of their respective metals in the composition. When too 

 large a mass of the metal is cast together, its intense and 

 lasting heat calcines the surface so deeply, as (when exposed 

 to the air) to obscure the colours; so that a small quantity 

 will best serve to exhibit them. 



As to the method of casting the mIrrOrs, it has been di- Method of 

 rected, to leave the ingate, or superfluous part of the cast, c^^^^^S^ 

 so large, as to contain a quantity of metal, equal to that in 

 the mirror itself; which would occasion a great waste of 

 it, and render it not easy to cast, at once, more than one mir- 

 ror in each mould ; and even this might be done so injudi- L«ge ingai© ? 

 ciously, as not to afford security against a miscarriage of the 

 cast. But it will appear, that this great quantity of metal 

 and incommodious manner of casting it, are by no means ne- 

 cessary. However, a judgment cannot be formed, of what 

 may be the safest and most eligible method for casting the 

 mirrors, unless it be considered, what are the circumstances 

 attending this operation, in the case of malleable metals ; and 

 how the management of speculum-metal, in this respect, must 

 differ from that of them : since there must be peculiar difficulty 

 in casting, in sandi a metal more brittle than glass. 



When any fused metal is poured into the flask, the external Effects of the 

 parts of it, which are in contact with the mould, congeal and contraction of 

 harden sooner than the internal partSj and form a solid shell, ^"^^^ "^^^^ *' 

 filled with the rest of the metal, in a fluid state. This will, 

 consequently, remain in a state of greater expansion, from its 

 heat, than the external crust; and its particles will, in the 

 act of shrinking as it cools, recede from one another, as being 

 more easily separable, and cohere, on each side, with the par- 

 ticles already fixed and grown solid : by which means a va- 

 cuum will be formed in the middle, and this will be gradually 

 filled by the superincumbent metal, which has been later 

 poured in, and remains longer in a fluid stale. But, when 

 there is no more metal supplied, the void, which was in this 

 way latest formed, remains unfilled ; and then the shell of the 

 metal, adjacent to the vacuum, as yet remaining sofl, and 

 unable to bear the weight of the atmosphere, resting on it, 

 anks, and is pressed down into the vacuum ; bv which means, 

 D2 



