Drie bollen
Referring to the topmost stacked sphere Escher said he had tried to suggest three-dimensionality as powerfully as possible. But, he added, the sphere is only a projection on a sheet of paper. If you cut out the shape it is a flat disc, just like the one at the bottom of the stack and the top side of the halved sphere in the middle.
Kunstenaar: Maurits Cornelis Escher