Solid bodies are of two sorts: the one has the surface curvilinear, oval, or spherical; the other has ​several surfaces, or sides producing angles, either regular or irregular. Spherical, or oval bodies, will always appear detached from their ground, though they are exactly of the same colour. Bodies also of different sides and angles will always detach, because they are always disposed so as to produce shades on some of their sides, which cannot happen to a plain superficies^[1].