Vision Science in Middle Ages
The fall of the Roman empire in the 5th century embarked an end of classical Greco-Roman civilization, and consequently European studies in vision science declined severely. While most of Europe suffered through the dark periods during the Middle ages (lasting approximately 500 A.D. - 1400 A.D.), Islamic civilization reached its heights during the Middle ages. By the 7th century, the Islamic empire spanned from modern day India all the way to Spain. This led to a new era in history in vision science as many Muslim scholars translated ancient Greek and Roman discoveries and developed their own theories of how vision works. The following sections describe theories developed by various Arab scholars on vision.
Al-Kindi (801 A.D. - 873 A.D)
Al-Kindi was a great early Muslim philosopher. He is known to have written some 250 books. He also made several contributions to vision science. He defended Euclid's extramission theory and rejected intromission theory of earlier Greek philosophers. He argued against intromission theory by arguing that the eye's spherical structure suggests it is not designed to collect images. Rather, motion of eyes indicate eyes to have to move something, which Al-Kindi assumed were Euclid's visual rays emanating from eyes. Although Al-Kindi agreed with Euclid's general idea, he refined some of his concepts on visual rays. First, Al-Kindi claimed that visual rays could not be one-dimensional lines as Euclid theorized because one-dimensional lines would only hit the object at points. And since points have no size, the object would be invisible no matter how many lines hit the object at different points. Instead, Al-Kindi argued that visual rays must be three-dimensional so that rays would be able to illuminate the object. Second, Al-Kindi rejected Euclid's idea that the reason eyes get weaker is because gaps develop between visual rays as the distance between eyes and objects increases. He stated that if there any gaps between visual rays, objects would never appear smooth. Thus, visual rays must be continuous.
Al-Kindi's second contributions were on theory of perception. Al-Kindi conjectured that an object put on eye axes but far away is seen more clearly by eyes than an object that is put nearby eyes but on peripheral sides. Had Al-Kindi known about fovea and its ability to focus objects more clearly than peripheral retina, he would have championed this idea.  
Hunayn ibn Ishaq (808 A.D. - 873 A.D.)
Living around the same period as Al-Kindi, Hunayn described vision in great detail. His book Ten Treatises on the Eye was the first systematic textbook of opthalmology, described the structure of the eye, eye diseases and their treatments, and various remedies and their effects on eyes in extraneous detail. For instance, he described the lens as "white, transparent, luminous, and round." He also described treatments for conjunctivitis, cataracts, and trachoma. Furthermore, he described vitreous humor, retina and its connection to the optic nerve, as well as cornea, conjunctiva, and uvea. However, in describing function of the eye and its parts, Hunayn based much of his theory on mystical ideas similar to Galen's idea of pneuma, rather than physiological studies. For Hunayn, each part of the eye was a being in its own nature, and each part connected to two other parts of the eye in a very consistent order. Hunayn believed the eye to be a highly harmonious ordered structure. In his mind, each part of the eye had its purpose, and each part existed to serve its purpose. The lens, for instance, existed to survey the outside world. Thus, there was not any part of the eye that was not required for proper function.  
Alhazen (965 A.D. - 1039 A.D.)
Alhazen, another Muslim polymath, is regarded as the father of optics. His book Book of Optics explained modern intromission theory of vision, which was proven correct through his experiments. The key to Alhazen's success on developing a correct theory was his reliance on observations and his ability to design and run careful experiments, and develop new hypotheses based on results of experiments. He essentially created the modern scientific method on which all science is dependent upon today. He could be called the first modern scientist.
Alhazen debunked several old ideas of vision. In debunking extramission theory, he reasoned that when our eyes look at brighter objects such as the sun, they suffer and get injured. This would not occur if eyes were emanating light themselves! Thus, light emanates from the object seen by the eye, not by eyes themselves. Alhazen also stated that light from the object is emanated in all directions. Alhazen developed six conditions upon which light enters eyes and brings about perception. First, there must be a certain distance between the eye and the object. Second, the object must be within the visual field of an eye. Third, the object must either be a self-illuminating body or be able to be illuminated by light coming from another object. Fourth, light from the visible object must be able to hit the crystalline lens. Fifth, the medium between the object and the eye must be transparent. Sixth, the object must be dense and opaque. Otherwise, light would simply pass through the object and the object would be transparent. Already, we can see that Alhazen had developed the laws of reflection, refraction, and transmission. Alhazen also denied the existence of visual rays by conjecturing that there was no evidence for them. In describing how each area of the object is distinguished, Alhazen considered each area as a point from which a single ray emanates in a directions. Thus, light emanates from all points of the object and these rays move towards the center of the observer's eye, which then detects the object.
Alhazen further contributed to optics by describing reflection from curved and plane mirrors and refraction of rays. His Book of Optics was translated from Arabic to Latin in 12th and 13th centuries and was widely used in the European and Islamic world. In fact, his book was the standard for optical theory until Newton published Principia Mathematica. 
- ↑ Lindberg, David C.: Theories of Vision from Al-Kindi to Kepler, Chicago, 1976
- ↑ Lindberg, David C.: Alkindi's Critique of Euclid's Theory of Vision. Isis. Vol. 62, No. 4, (Winter, 1971), pp. 469-489
- ↑ Eastwood, Bruce Stansfield. The Elements of Vision: The Micro-Cosmology of Galenic Visual Theory according to Ḥunayn Ibn Isḥāq. Transactions of the American Philosophical Society Vol. 72, No. 5, (1982)
- ↑ Meri, Josef. Midieval Islamic Civilization. Taylor and Francis, (2006).
- ↑ Lindberg, David C.: Alhazen's Theory of Vision and Its Reception in the West. Isis. Vol. 58, No. 3, (Autumn, 1967)