158 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777. 



The best composition for the specula of reflecting telescopes. — The perfection 

 of the metal of which the speculum should be made, consists in its hardness, 

 whiteness, and compactness ; fur on these properties the reflective powers anil 

 durabihty of the specuUuTi depend. And first of the hardness and whiteness of 

 the metal. There are various compositions recommended in Smith's Optics, all 

 which have however their several defects. Three parts copper and one part and 

 one-fourth of tin will make, he says, a very hard white metal ; but liable to be 

 porous. This however is an imperfection which I shall presently show the 

 method of preventing ; but the permanent fault of it, and which I have myself 

 experienced, is, that it is not hard enough. The speculum of a reflecting 

 telescope ought to have the utmost possible hardness, compatible with its being 

 operated on by the tool. 



It is to be observed, that tlie smallest quantity of tin added to melted copper 

 destroys its perfect malleability, and at the same time produces a metal whiter 

 and harder ihan copper. As the quantity of tin is increased, suppose to a 5th 

 or 4th part, the metal becomes whiter, still harder, and consequently more 

 friable. If the quantity of tin be further increased, as to a 3d of the whole 

 composition, it will then have its utmost whiteness ; but at the same time will 

 be rendered so exceedingly hard and brittle, that the finest washed emery on 

 lead or brass will not cut it, without breaking up its surface ; and the common 

 blue stones used in grinding the speculum will not t(juch it. Mr. Jackson, a 

 mathematical instrument maker, and a most excellent workman, told me^ that 

 the tin was increased to the above proportion in his metals ; but that they were 

 so exceedingly hard, that it cost him an infinite deal of pains, and a journey of 

 200 miles, to find out a stone of sufficient hardness to cut it, and whose texture 

 at the same time was fine enough not to injure its surface. I have seen several 

 of his finished metals ; they were indeed perfectly hard and white ; but the kind 

 of stone with which he ground them he kept a secret. 



After many experiments with various proportions of tin and copper, by gra- 

 dually increasing the former, I at last found that 14-i- ounces of grain-tin to 2 lb. 

 of good Swedish copper, made a beautiful white and very hard metal ; so hard 

 indeed, that the stones would but barely cut it, and washed emery on brass or 

 tin but just grind the surface without breaking it up; whereas the proportion of 

 tin being increased by the addition of only another half ounce, the former in- 

 convenience immediately took place. This therefore is the maximum in point of 

 hardness. 



Tliis composition, though complete in the former respects, was, as well as 

 Dr. Smith's, subject every now and then to be porous ; sometimes indeed I suc- 

 ceeded in casting a single metal, or perhaps 2 or 3 without this imperfection ; at 

 other times, and most frequently indeed, they were attended with this defect. 



