It is also shown that only when the gratings are cylinder or plane ones, the second order sagittal anastigmatic circularfield and meridional non defocus flat field spectral images can be formed.
还指出仅当柱面或平面光栅时，才能形成二阶子午无离焦平场和弧矢无象散圆场谱面。
2
In a dusty field in Kitui, eastern Kenya, farmers are being taught how to construct small, semi-circular barriers of earth that control the flow of water, slowing its run-off.