text: Al-Kindi was the first major writer on optics since antiquity. Roger Bacon placed him in the first rank after Ptolemy as a writer on the topic. In the apocryphal work known as De radiis stellarum, is developed the theory "that everything in the world ... emits rays in every direction, which fill the whole world." This theory of the active power of rays had an influence on later scholars such as Ibn al-Haytham, Robert Grosseteste and Roger Bacon.
  Two major theories of optics appear in the writings of al-Kindi: Aristotelian and Euclidean. Aristotle had believed that in order for the eye to perceive an object, both the eye and the object must be in contact with a transparent medium (such as air) that is filled with light. When these criteria are met, the "sensible form" of the object is transmitted through the medium to the eye. On the other hand, Euclid proposed that vision occurred in straight lines when "rays" from the eye reached an illuminated object and were reflected back. As with his theories on Astrology, the dichotomy of contact and distance is present in al-Kindi's writings on this subject as well.
  The factor which al-Kindi relied upon to determine which of these theories was most correct was how adequately each one explained the experience of seeing. For example, Aristotles theory was unable to account for why the angle at which an individual sees an object affects his perception of it. For example, why a circle viewed from the side will appear as a line. According to Aristotle, the complete' sensible form of a circle should be transmitted to the eye and it should appear as a circle. On the other hand, Euclidean optics provided a geometric model that was able to account for this, as well as the length of shadows and reflections in mirrors, because Euclid believed that the visual "rays" could only travel in straight lines (something which is commonly accepted in modern science). For this reason, al-Kindi considered the latter preponderant.
  Al-Kindis primary optical treatise "De aspectibus" was later translated into Latin. This work, along with Alhazens Optics and the Arabic translations of Ptolemy and Euclid's Optics, were the main Arabic texts to affect the development of optical investigations in Europe, most notably those of Robert Grosseteste, Vitello and Roger Bacon.