Platonicae quaestiones

Besides, a right line, whether great or little, preserves the same rectitude; but as to the circumference of a circle, the less it is, the crookeder it is; the larger, the straighter. Therefore if a convex superficies stands on a plane, it sometimes touches the subject plane in a point, sometimes in a line. So that a man may imagine that a circumference is made up of little right lines.