During the Middle ages Islamicate scholars analysed, studies, criticised and developed a wide range of academic disciples that they had adopted from their Greek, Persian, Chinese, and India predecessors before passing them back into Europe during the twelfth-century Scientific Renaissance. One of the disciples where their endeavours had the biggest impact was in the science of optics.
As we saw in an earlier episode, as opposed to the popular cliché, the Ancient Greeks propagated a wide range of theories of vision ranging from the Atomist intromission theory, over the Platonist combined extramission/intromission theory, the pure extramission theory in the geometric optics of Euclid, Heron, and Ptolemaeus, to the Aristotelian intromission theory and finally the Stoic pneuma based theory shared by Galen. All of these reached the medieval Islamic society in translation and each of them found their critics, supporters, and propagators.
Already in the ninth century Abū Yūsuf Yaʻqūb ibn ʼIsḥāq aṣ-Ṣabbāḥ al-Kindī (c. 801–873), an Arabic Muslim polymath, who was born in Kufa, in what is now south-central Iraq, the son and grandson of the governor. Originally educated in his home town he moved at some point to Baghdad to complete his education and where he would go on to serve three ‘Abbāsid Caliphs.
His interests were wide-ranging and he is said to have written at least two hundred and sixty books, which, as is often the case, have mostly been lost. Of major interest for his contributions to optics is his De radiis stellarum, a work that only exists in Latin translation, the Arabic original being lost. Here al-Kindī presents a central element of his general philosophy:
It is manifest that everything in this world, whether it be substance or accident, produces rays in its own manner like a star … Everything that has actual existence in the world of the elements emits rays in every direction, which fill the whole world.[1]
Of course, given this general statement optics with its light rays and visual rays is a central area for al-Kindī. He wrote several works on optics of which On the Causes of Differences in Perspective or De aspectibus, to give it its Latin title, is the most important but like De radiis stellarum, the Arabic original is lost. In this work al-Kindī comes down in favour of the Euclidian theory of geometrical optics with its pure extramission theory of vision but not without criticism. To start he summarises the various alternative theories he has inherited from antiquity:
Therefore I say that it is impossible that the eye should perceive its sensibles except [1] by their forms travelling to the eye, as many of the ancients have judged, and being impressed in it, or [2] by power proceeding from the eye to sensible things, by which it perceives them, or [3] by these two things occurring simultaneously, or [4] by their forms being stamped and impressed in the air and the air stamping and impressing them in the eye, which [forms] the eye comprehends by its power of perceiving that which air, which light mediates, impresses in it.[2]
One is obviously the atomists, two is Euclid and Ptolemaeus, three is clearly Plato, and four is the mediumistic theory of Aristotle. Through argument al-Kindī eliminates all but Euclid by attacking the basic principle of intromission. He argues that a circle viewed edgewise, should in an intromission theory still appear as a circle but in reality it appears as a straight line:
Therefore it remains that the power proceeds from the observer to the visible objects, by which they are perceived . this power proceeds from the eye in straight lines and falls only on the edges of the circles, perceiving them as straight lines.[3]
Having established that Euclid is the only valid model of perception he now takes him to task. He presents six propositions at the beginning of his work that demonstrate that luminous rays are rectilinear, although he is not intending to replace Euclid’s visual rays with luminous rays. He also differs from Euclid on the constitution of the visual cone. Whereas Euclid conceives it to consist of single rectilinear rays, al-Kindī sees it as a continuous whole. He goes further and argues that rays issue in all directions from every point on the surface of the eye. He bases this claim on the analogous behaviour of external light. al-Kindī argued that light reflects from every point on an object in every direction. He appears to have been the first to explicitly state this simple concept which would go on to be an important element in theories of vision and optics in general. Although Euclid’s extramission theory of vision would prove to be wrong in the long run al-Kindī’s De aspectibusremained popular amongst Islamic scholars and together with his De radiis stellarum would have a major impact in Europe following the twelfth-century Scientific Renaissance.
al-Kindī’s Arabic, Nestorian Christian, contemporary Ḥunayn ibn ʾIsḥāq al-ʿIbādī (808–873), who was born in al–Hirah, near Kufa in what is now south-central Iraq, but moved to Baghdad where he worked as a translator and physician. Ḥunayn ibn ʾIsḥāq had studied medicine under Yuhanna ibn Masawaih (c. 777–857), a Persian or Assyrian, East Syriac Christian physician, the first to write in Arabic over ophthalmology and the student would come to outperform his teacher in this area of medicine. His medicine is principally Galenic, who was for the Arabic physicians the “Prince of Physicians”, so it comes as no surprise that his ophthalmology is basically Galenic and his theory of vision Galenic and Stoic. He wrote two works on ophthalmology, Ten Treatises on theEye and the Book of the Questions on the Eye.
In his Ten Treatises on the Eye, Ḥunayn ibn ʾIsḥāq gives a detailed description of the structure and function of the eye that closely parallels that of Galen.
His theory of vision is also that of Galen, which he specifically choses over alternatives, he writes:
We say: the object of vision can be seen only in one of the following three ways: [i] by sending out something from itself to us by which it indicates its presence so that we know what it is; [ii] by not sending anything out but remaining steady and unchanged in its place; then the faculty of perception goes out from us to it, and we recognise what it is through this medium; [iii] by there being another thing … intermediate between us and it; it is this which gives us information about it, so that we learn what it is. And we shall now see which of these three [theories] is the right one.[4]
Alternative one covers both the intromission theories of the atomist and Aristotle, which Ḥunayn rejects with the old argument, how can a perceived mountain enter the eye? The second alternative covers the extramission theories of Euclid and Ptolemaeus, which Ḥunayn also dismisses thus:
It is not possible that the visual spirit extends over all this space [between the eye and a distant visible object] until it spreads round the seen body and encircles it entirely.[5]
The third alternative turns out to be that of Galen and the Stoic in which pneuma coming out of the eye triggers the air that already exists between the object and the eye creating a connection along which the visual perception takes place. This is according to Ḥunayn the right one.
Ḥunayn’s Ten Treatises on the Eye was very widely read both in Islamicate culture and later in Latin translation in medieval Europe. It was in the latter case for many people their introduction to the theories of Galen, whose own work was first translated into Latin much later.
The work of both al-Kindī and Ḥunayn ibn ʾIsḥāq were widely read and highly influential and both of them dismissed the intromission theory of vision of Aristotle. However, Aristotle had two heavyweight champions, who defended and propagated his theory in Ibn Sīnā (980–1073), Latin Avicenna, and Ibn Rushd (1126–1198), Latin Averroes, probably the two must influential medieval, Islamic philosophers. I have included brief biographical sketches of both in the episode on Islamic theories of motion so I won’t repeat myself here.
Ibn Sīnā, who was incredibly prolific, wrote about the theory of vision is a number of still extant works including the Kitab al-Shifa (The Book of Healing, also known as Sufficientia), Kitab al-Najat (The Book of Deliverance), Maqala fi ’l-Nafs (Epistle or Compendium of the Soul), Danishnama (Book of Knowledge), and Kitab al-Qanum fi ‘l-Tibb (Liber canonis of Canon of Medicine).
Ibn Sīnā doesn’t so much defend Aristotle’s intromission theory of vision as demolish the extramission theory in its various forms. I’m not going to go into detail, just say that his arguments are convincing. His main argument is that the rays going out to the object do not perceive the object but the object is perceived by something returning to the eye. This being the case we perceive by something entering the eye so we don’t need the rays going out from the eye. Against the Galenic theory he basically argues convincingly that either air as a medium can convey perception or it can’t and if it can it doesn’t need to be activated by pneuma. This naturally leaves him with just Aristotle’s theory as acceptable. Both Ibn Sīnā and Ibn Rushd take over the basic Galenic structure and function of the eye from Ḥunayn.
Ibn Rushd is, of course the most avid Aristotelian during the Islamic Middle Ages, which earned him the title of “The Commentator” when his works were translated into Latin. He refutes the theories of visual perception of Euclid, Ptolemaeus, Galen, and al-Kindi arguing that they would all imply the ability to see in the dark. He also says that an extramission theory would imply that the eye produces enough rays to fill a hemisphere of the world every time somebody opened their eyes which was just absurd. In general, Ibn Rushd is more concerned with what happens to the image once it enters the eye, which is physiology and/or psychology and not physics, so doesn’t concern us here.
We now turn our attention to Abū ʿAlī al-Ḥasan ibn al-Ḥasan ibn al-Haytham, Latin Alhazen or Alhacen, (c. 965–c. 1040), a Persian or Arabic mathematician, astronomer, and physicist, who was born in Basra and spent a large part of his life in Cairo. Ibn al-Haytham is one of the most important figures in the history of optics before the seventeenth century and he worked a revolution in the discipline. In his From Sight to Light: The Passage from Ancient to Modern Optics (University of Chicago Press, 2015) Mark Smith titles his chapter on Ibn al-Haytham Alhacen and the Grand Synthesis, which is a pretty good summary in five words.
Amongst Ibn al-Haytham’s extant works eleven deal wholly or partially with aspects of optics. Amongst the no longer extant works another six deal with the topic in one way or another. However, there is one work that is in the history of optics dominant and that we will look at briefly here, that is his Kitab al-Manazir (Book of Optics) which was translated into Latin by an unknown translator in the twelfth century as De aspectibus or Perspectiva.
Ibn al-Haytham rejects the extramission theories basically taking over the arguments of Ibn Sīnā. He also notes that strong light entering the eyes causes pain and prolonged staring at strong light sources produced after images when the eyes are returned to the dark, so the eyes are sensible to light, which comes in, not goes out. Interestingly and also very important to the future development of optics, although he dismisses the extramission theory he doesn’t dismiss the geometric optics of Euclid and Ptolemaeus. He accepts their cone of vision, and as we will see even utilises it himself, but on the condition that their rectilinear rays are merely geometrical constructs and not real visual rays. Thus, making Euclid’s and Ptolemaeus’ geometrical optics independent of the extramission theory. A seemingly trivial but highly significant redefining.
Having followed Ibn Sīnā in dismissing the extramission theory he doesn’t follow him in adopting Aristotle’s intromission theory but develops an entirely new one. Adopting al-Kindī’s theory that every point on an illuminated object reflects light rays in every direction, Ibn al-Haytham states that it is these reflected light rays that transmit the colour and luminosity of the object to and into the eye. This is truly a radically new concept. The theories of Plato, Aristotle, and the Stoic all required the presence of light to facilitate visual perception but Ibn al-Haytham says quite simply that all it requires is light, anything else is superfluous.
Ibn al-Haytham sees a problem with his intromission theory, if light rays are meeting the eye from every possible direction how does the eye form a distinct image of the viewed object? He offers up a fairly refined solution to this problem. Firstly, although the structure of the eye that he adopts is that of Galen/ Ḥunayn for Ibn al-Haytham the surface of the cornea, in his model, is a perfect sphere. He then hypothesises that only those rays that meet the surface of the eye perpendicularly can actually enter the eye. All the other light rays slide or veer off.
He justifies this with an analogy. He says, consider an iron ball thrown at a wooden plank. If it hits the plank perpendicularly it rebounds or if thrown hard enough breaks the plank, If the ball hits the plank at an angle it slides or veers off. Ibn al-Haytham argues perpendicular rays are strong and penetrate the eye, whereas rays that meet to eye at an angle are weak and veer off.
Because his cornea is perfectly spherical this means that all the perpendicular rays meet at the centre of this sphere and this is where the image of the object is formed. The rays coming from the viewed object to the centre of the sphere form a visual cone like that of Euclid and Ptolemaeus but with the rays going from object to eye and not from the eye to the object. This explains or justifies his retention of their geometric optics. Alongside making vision purely based on light this justification of a geometric optics within an intromission theory is Ibn al-Haytham’s second major contribution to the evolution of optics.
You will often come across the claim that Ibn al-Haytham established his theory of vision experimentally and empirically, this is simply not true. The theory of vision is argued entirely philosophically without any experimentation involved. The experiments appear first in the later chapters of his Kitab al-Manazir where he deals with the mathematics of reflection and refraction, in both cases building on and extending the work of Ptolemaeus in his Optics.
Because of these false claims, Ibn al-Haytham, like Galileo, is often credited with being the inventor of modern science, or the inventor of empirical experimental science, or the inventor of the scientific method, or the inventor of mathematics based science, all of which claims are total rubbish. It is in particular rubbish because almost everything he did was a copy and extension of the empirical, experimental work done by Ptolemaeus. There are even people who make these claims for both Ibn al-Haytham and Galileo! Are they really one and the same scientist cursed to travel through time inventing modern science over and over again?
In the section on reflection Ibn al-Haytham describes a very complex and sophisticated experimental set up to investigate reflection in plane, concave, and convex mirrors. As already noted these experiments are more complex version of the ones that can be found in Ptolemaeus’ work, so not as ground-breaking as they are very often painted. However, having described in great detail the set up and how it supposedly worked Mark Smith has the following to say:
Indeed, given its obvious unfeasibility as actually described–with all the planes perfectly aligned and all measurements perfectly reproduced–the test appears to have been an elaborate thought experiment designed to confirm what Alhacen already took for granted, that is, that light reflects at equal angles. The experiment is therefore intellectually but not physically replicable.[6]
Ibn al-Haytham does, however, go on to subject the topic of reflection to a detailed, very accurate, high level mathematical analysis.
As already mentioned following on to his analysis of reflection Ibn al-Haytham now handles the topic of refraction, once again taking Ptolemaeus as his inspiration and role model. Once again we get a complex experimental set up and once again, this time for different reasons, Mark Smith doubts whether they were ever carried out:
We are therefore led to raise the same doubt about feasibility that we did with the reflection experiment, and in this case the doubt is deepened by Alhacen’s failure to acknowledge the problem posed by critical angle for the tests for refraction from glass to air and glass to water. In short, there is good reason to believe that he did not carry out the experiment as described, which helps explain his failure to provide any values. That in turn raises serious doubt about the experiment’s replicability and, therefore, its “modernity.” Furthermore, its originality is questionable in that it is clearly based on Ptolemy’s experimental derivation of the angles of refraction.[7]
As with the section on reflection there is extensive mathematical analysis.
Although Ibn al-Haytham building on the work of others very clearly laid the foundations of modern optics, it would be a mistake to think that his work immediately established itself as the go to theory of the discipline. The rival theories of al-Kindī, Ibn Sīnā, and Ibn Rushd continues to have their supporters almost all the way down to the seventeenth century.
I have now sketched the full spectrum of theories of vision presented by scholars during the Islamic Middle Ages. All of these theories would be translated into Latin during the twelfth century and as we will see in a later episode would have a major impact.
[1] David C. Lindberg, Theories of Vision: From Al-Kindi to Kepler, University of Chicago Press, 1976, p. 19
[2] Lindberg, pp. 21-22
[3] Lindberg, p. 23
[4] Lindberg, p. 38
[5] Lindberg, pp. 38-39
[6]A. Mark Smith, From Sight to Light: The Passage from Ancient to Modern Optics, University of Chicago Press, 2015, p. 199
[7] Smith, p. 218