单词Newtonian telescope
网络: 牛顿望远镜；牛顿式天文望远镜；牛顿式望远镜
Mirror 4 is just a flat mirror like Newtonian telescope"s secondary mirror , to give convenience for eyes .
镜4只是一块平面镜，就象牛顿式望远镜的副镜一样，方便观测。