Note 8 — Chapter II · THE MODERN TELESCOPE

This was probably due not only to unfavorable climate, but to the fact that Herschel, with all his ingenuity, does not appear to have mastered the casting difficulty, and was constrained to make his big speculum of Cu 75 per cent, Sn 25 per cent, a composition working rather easily and taking beautiful, but far from permanent, polish. He never seems to have used practically the SnCu4 formula, devised empirically by Mudge (Phil. Trans. 67, 298), and in quite general use thereafter up to the present time.