  A good example of how al-Kindi applied mathematics to other fields is his use of geometry in optics (see further Lindberg 1971, Rashed 1997, Adamson 2006). On this subject al-Kindi followed the tradition inaugurated by Euclid, and carried on by Ptolemy and others, in which geometrical constructions were used to explain phenomena such as visual perspective, shadows, refraction, reflection, and burning mirrors. This procedure implies that light and vision can be formalized as geometrical lines, an implication that al-Kindi and his sources embrace by claiming that vision occurs when “rays” emitted from the eyes along straight lines strike a visual object. Likewise, objects are illuminated when a light source emits light rays that strike the objects’ surfaces. Aspects of al-Kindi’s account anticipate that of Ibn al-Haytham, who some decades later would be the first to explain vision accurately.