Very much connected with this treatise is another of Ibn Sahl's works, this one on burning mirrors. In it he addresses the question of how to design not just mirrors but lenses that will focus incoming light rays at a given distance. He distinguishes between the cases in which the incoming rays originate from a source such as the Sun, which may be considered to be at an infinite distance, or from a source at a finite distance. Ibn Sahl considers both the theoretical and the practical aspects of this problem, which in the case of lenses demands consideration of refraction. And he states a geometrical relation between incident and refracted rays that, rewritten in modern trigonometric notation, is equivalent to the Law of Refraction, although it does not involve the notion of the refractive index of a medium.