ABSTRACT
The continuous drawing of conic curves requires not only the invention of an instrument, or of a mechanical system capable of drawing these curves, but also the elaboration of concepts in the theory of conics capable of explaining and controlling the use of this instrument. Ibn Sahl conceives of a mechanical system founded on the properties of the foci and the directrix to trace only the three conic sections. Al-Quhi invents the perfect compass to trace all the lines that he calls 'measurable, qiyasiyya'. Ibn Sahl's contemporary, al-Quhi, invents another instrument that makes possible the continuous drawing of conic curves, as well as the circle and the straight line. This chapter is concerned about the geometrical construction by the intersection of conics. Ibn Sahl's systematic study of burning mirrors and of plano-convex and biconvex lenses, and also of the anaclastic properties of the three conics are also included in the chapter.
