a type of mirror made from speculum metal (= a mixture of copper and tin) that was used in old teles

中文解释

（旧时望远镜中所使用的）铜锡合金反射镜 The Melbourne reflector was the last major telescope to be equipped with a speculum mirror. 墨尔本反射镜是最后一个装有铜锡合金反射镜的大型望远镜。