It is necessary that speculum metal should possess, in 

 the highest attainable degree, the qualities of whiteness, 

 brilliancy, and resistance to tarnish. Lord Oxmantown has 

 found that these conditions are best satisfied in the definite 

 combinations of four equivalents of copper to one of tin ; or 

 by weight, 32 and 14*7 nearly. Metals differing from this 

 by a slight excess of either component, are, when first 

 polished, scarcely less brilliant, but are dimmed so rapidly 

 that the lapse of a few days produces a sensible difference. 

 On the other hand, some large specula of the atomic com- 

 pound have been lying uncovered for years, without ma- 

 terial injury to their polish. 



But this compound is brittle almost beyond belief; a 

 slight blow, or even the application of partial warmth, will 

 shiver a large mass of it ; though harder than steel, its sur- 

 face is broken up with the utmost facility, and it has a 

 most energetic tendency to crystallize. The common pro- 

 cess of the founder fails with it, except for masses of very 

 limited magnitude, as the cast cracks in the mould, and the 

 subsequent difficulties of the annealing are such, that it has 

 been a very general practice to use an alloy lower (contain- 

 ing more copper) than the atomic standard. Even Sir 

 WiUiam Herschel was obliged to yield to this necessity. It 

 appearsfrom a letter of Smeaton, (Rees' Cyclopaedia, Art. Tele- 

 scope,) that for his 20 feet mirror of 19 inches aperture, the 

 composition was 32 copper to 12*4 tin ; and that for the 40 

 feet it was even lower. Yet two out of three attempts to cast 

 this huge speculum failed. 



Lord Oxmantown at first endeavoured to evade the diflS- 

 culty, by constructing a speculum in pieces, soldering plates 

 of fine metal to a back of a peculiar brass, ascertained to 

 have the same expansion ; and has completed one of thirty- 

 six inches aperture and twenty-seven feet focal length, 

 which performs very well on stars below the fifth magnitude. 



