投射镜 meaning in English

projecting mirror
projectoscope

To get a projection focal length of 61 centimeters requires a glass sphere whose diameter is a whopping 2 . 4 meters
要得到焦距61公分的投射镜，玻璃圆球的直径得大到2 . 4公尺。
My finding about the unlikely focal length of the putative projection mirror has a second implication , even more constraining to the hockney theory
既然设想中投射镜的焦距不太可能实现，我的发现还延伸出第二个推论，更使霍克尼的理论受限。