Mr. mudge on the ConftruCtion, See. 
297 
The befl compofition for the fpecula of reflecting telefcopes. 
The perfection of the metal of which the fpeculum 
fhould be made confifts in its hardnefs, whitenefs, and 
compaCtnefs; for upon thefe properties the reflective 
powers and durability of the fpeculum depend. And 
firft of the hardnefs and whitenefs of the metal. There 
are various compolitions recommended in smith’s Optics, 
all which have however their feveral defects. Three 
parts copper and one part and one-fourth of tin will 
make, he fays, a very hard white metal ; but it is liable 
to be porous. This, however, is an imperfection which 
I fhall prefently fhew the method of preventing ; but the 
permanent fault of it, and which I have myfelf expe- 
rienced, is, that it is not hard enough. The fpeculum of 
a reflecting telefcope ought to have the utmoft poflible 
hardnefs, compatible with its being operated upon by the 
tool. 
It is to be obferved, that ever fo fmall a quantity of 
tin added to melted copper deftroys its perfeCt malleabi- 
lity, and at the fame time produces a metal whiter and 
harder than copper. As the quantity of tin is increafed, 
fuppofe to a fifth or fourth part, the metal becomes 
whiter, ftill harder, and confequently more friable. If 
the quantity of tin be further increafed to a third of the 
Vol. LXVII. Q q whofe 
