Hello there ! I am writing here, because I could not find any good explanation in the literature and on the web Going straightaway to the point, my question is connected to Ewald's sphere and so called the limiting sphere. I red that limiting sphere is such a construction that appoints the detectable reflects by rotating the single crystal (Ewald's sphere) about rotating axis, it means rotation about tangent to the Ewald's sphere. As a result of this rotation we should obtain toroid, but for more( it is said- "some") rotations about several tangent we get limiting sphere with diameter two times greater than Ewald's one. I don't see it. What does " some" mean and what does it depend on how many rotations we have to do to obtain a limiting sphere ?