Islamic Civilization

Islamic Optics

The solution to Greek optics came in stages as is well explained by Lindberg.[1] Early Islamic followers of Galen held the view that vision occurred through a ray, which issued from the eye towards the object, and either by touching the object or compressing the intermediate air conveyed an impression of the object on the eye.[2] Hunayn ibn Ishaq (d.877) somehow sides with Galen. In his Ten Treatises and Book of the Questions on the Eye, he makes a more systematic version of Galen's theory, and provides a more advanced ocular anatomy. Al-Kindi, on the other hand, attacks the intromission theory because of its incompatibility with the laws of perspective.[3] Al-Kindi does not fail to make a critique of Euclid either, but fails to formulate the ultimate theory of optics.[4] Still his work, preserved in Latin  translation, was to have great influence on Roger Bacon and other Western men of science.[5] More doubts about Greek theories of vision arose with Al-Razi  in his lengthy treatise Kitab fil’l Kaffiyat al-Ibsar…, which, from its title (Book on Properties of  Sight Where it is Proved that Sight Does not Occur by Means of Rays Issuing From the Eye and the Propositions of Euclid’s Work on Optics are Refuted,) clearly reasserts the refutation of both Euclid and Ptolemy on visual sights, and puts forward the foundation for the doctrine that sight occurs by means of light rays issuing from a source of light.[6] 

It was Ibn al-Haytham, who contributed most to the resolution of the problem of vision, and set up the foundations for modern optics. Ibn al-Haytham, according to Sabra, saw that a correct theory of vision had to combine the ‘mathematical' approach of Euclid and Ptolemy with the ‘physical' doctrine favoured by the natural philosophers.[7] Lindberg notes how the five strands, cited above, converge in Ibn al-Haytham.[8] From neo-Platonism through Al-Kindi, Ibn al-Haytham gained his concept of a power propagated from all natural bodies, and in the case of light this power consists of forms imposed on the medium by the presence of a visible body. Galen's views on the anatomy of the eye and his identification of the crystalline lens as the sensitive organ are reproduced with little change by Ibn al-Haytham, whose intromission theory of sight also owes something to both the Atomists and the Aristotleians, whilst his geometrical optics follows Euclid and Ptolemy.[9] However, Lindberg points out, Ibn al-Haytham’s optical system is by no means simply a composite of these earlier traditions, he took the ancient materials at his disposal and constructed out of them a new edifice.[10]

Ibn al–Haytham’s most influential work is Kitab al-Manazir, translated into Latin  as De aspectibus or perspectiva (The Book of Optics) by Gerard of Cremona in the 12^th century is lost in the original; but the first three books (on direct vision) were edited in 1983,[11] and translated into English (London 1989) by Sabra. The basic element of the Perspectiva, of course, is its theory of direct vision; it may thus be of value to describe this theory in general terms as Lindberg does.[12] The medieval Latin version begins with an attack on the ancient theory of visual rays. The emission theory (Euclid and followers), maintained that a ray issues from the eye and proceeds to the object of vision where its termination constitutes the act of vision. To Ibn al-Haytham it contradicts a number of fundamental observations, such as that an observer looking at a bright light may have the sensation of pain, and an afterimage remains when he shifts his gaze to a darker place. These and other observations imply that the eye is the recipient of an action coming from without, which may occasion pain and injury if sufficiently strong, and which leaves its impress in the eye after the eye turns away.[13] He, thus, concludes that:

‘Light issues in all directions opposite any body that is illuminated with any light [and of course, also opposite any self luminous body]. Therefore when the eye is opposite a visible object and the object is illuminated with light of any sort, light comes to the surface of the eye from the light of the visible object.'[14]

Ibn al-Haytham equally resolves the problems of the intromission theory by explaining physical contact between the object and observer through the intromitted rays, and through its visual cone explained the perception of shape (which had been ignored by Aristotle) and accounted for the laws of perspective. Ibn al-Haytham incorporates in his theory the mathematical achievements of Euclidean visual cone, and the anatomical and physiological tradition of Galen.[15] 

Ibn al-Haytham’s greatest merit is in his reliance on experimentation. As Hill notes, he rejects the axiomatic approach of his predecessors, whereby postulates are assumed to be self evident, and any experiments just meant to reinforce axioms.[16] Equally, Ronan points out how Ibn al-Haytham breaks with the Greeks, his work citing no authority ‘but the authority of empirical evidence.'[17] In this, Ibn al-Haytham is not particularly innovative, but simply pursuing his Muslim predecessors’ path. Al-Kindi, who flourished more than a century before Ibn al-Haytham, who came from the same part of southern Iraq  as him, and who also wrote consistently on optics, surely had a great impact on him in this respect. Al-Kindi, in relation to the Greek deficiency in experimenting, in the second optical treatise on burning mirrors or rays, recalls Anthemius’ report on how ships were set aflame by burning mirrors during a naval battle:

‘Anthemius should not have accepted information without proof… He tells how to construct a mirror from which twenty four rays are reflected on a single point, without showing how to establish where the rays unite at a given distance from the middle of the mirror’s surface. We, on the other hand, have described this with as much evidence as our ability permits, furnishing what was missing, for he has not mentioned a definite distance.’[18]

Following on the same path as his Muslim predecessors, Ibn al-Haytham is concerned principally with the origin of the first principles and their justification, the first step in any scientific investigation, and even more, he is very much aware of the fallibility of sense-perception.[19] Omar  also comments how Ibn al-Haytham recognises that:

‘It is part of ‘human nature' to employ the senses, fallible as they are, as ‘the tools' for studying nature. Thus man, lacking any other alternative short of scepticism, if he is to study nature through the senses, can only conduct it by perfecting these tools ‘to the maximum degree that this is possible.'[20]

It was Ibn al-Haytham’s consistent and constant attempt to perfect those ‘tools', Omar  adds, which, more than any other single factor, was responsible for this 11^th century physicist's development ‘of the experimental method to unprecedented levels of sophistication, and its deployment in roles foreign to those of the ‘experiments' of his predecessors.’[21]

Omar  goes on to reproduce some of Ibn al-Haytham’s experiments, including drawings and figures.[22]

The other merit of Ibn al-Haytham, according to Russell, is his capacity to resolve complex issues into independent yet closely interrelated simple investigations, subjecting every problem to a quantitative analysis of its variables under strictly controlled conditions.[23]

Lindberg comments that it is not possible to generalise on the results or the display of wealth of geometrical techniques employed in Ibn al-Haytham’s Perspectiva, which was far superior to earlier optical works, and no optical treatise composed before the 17th century rivalled it mathematically.[24] His Kitab al-manazir impacted upon Roger Bacon (13^th century), Witelo, da Vinci and Kepler.[25] His studies of the refraction of light and his laws of refraction were relied upon by both Kepler and Descartes.[26] Kepler, in fact, took up where Ibn al-Haytham left off.[27]

Ibn al-Haytham’s ‘revolutionary’ breakthroughs are, however, far from being revolutionary. It is important to note with Rashed,[28] that when historians analyse Ibn Al-Haytham’s dioptrics, for instance, they refer only to Ptolemy, which gives the impression that Ibn al-Haytham was preceded by ‘a vacuum reaching back to Ptolemy[29] and followed by another vacuum up to al-Farisi.’[30] In truth, Ibn al-Haytham relied to great extent on other Muslim writers on the subject who flourished just prior to him, and one amongst such writers was Ibn Sahl (fl. 984 at the court of Baghdad .) In his Discourse on Light Ibn al-Haytham referred explicitly to Ibn Sahl and recalled some of the latter’s ideas on the transparency of media and refraction.[31] Ibn al-Haytham also copied Ibn Sahl’s opuscule entitled: Proof that the Celestial Sphere is not Completely Transparent.[32] Rashed also points out that Ibn Sahl was the first to have studied lenses.[33] When Ibn Sahl had completed his examination of burning mirrors, both parabolic and ellipsoidal, Rashed notes, he considered hyperbolic plano convex lenses and the hyperbolic biconvex lenses, and also stated what is called Snellius’ law centuries before the latter.[34] Ibn Sahl’s contribution to optics was, thus, quite substantial, and not just in terms of the subjects he addressed, but also coming just a few decades before Ibn al-Haytham, from the same land of Iraq  (Ibn al-Haytham came from Basra ). The closeness of the two scholars in space and time was bound to induce in Ibn al-Haytham’s mind a powerful spirit of research on a similar topic, and also armed him with scientific facts, which served him decisively.

Ibn al-Haytham other accomplishments, to a very large extent, were also built on his Muslim predecessors who studied the same subjects, and that can be found amongst others with Al-Kindi, for instance. His treatise On the Burning Glass exhibits a profound and accurate conception of the nature of focussing, magnifying, and inversion of the image, and of formation of rings and colours by experiments.[35] In his treatise ‘The Shape of the Eclypse,' Ibn al Haytham makes an attempt to explain the crescent image cast by the partially eclipsed sun through a small round aperture.[36] Ibn al-Haytham also wrote on the rainbow, the halo, and spherical and parabolic mirrors, and fixed the height of the atmosphere at the equivalent of about ten English miles.[37] More of Ibn al-Haytham’s accomplishments are explained by Schramm, but in German.[38]

Lesser known than Ibn al-Haytham is a writer on optics of Mamluk Egypt : Al-Qarafi (d. 1285) who was a faqih (a scholar in law) of the Maliki persuasion, and who wrote mostly on theology. However, one scientific work of his is on optics: Kitab al-Istibsar fima tudrikuhu’l absar (The Revelation of What the Eyes May Perceive).[39] In al-Qarafi’s day, the majority of the problems related to optics were studied under the titles of optics and perspective; the optical phenomena appearing in the atmosphere were discussed in books on meteorology, other optical problems were studied in connection with astronomy. In al-Qarafi’s work (Revelation etc), questions of all three kinds are studied in one book.[40] Al-Qarafi’s preparation of this book was initiated by the five questions sent by the Emperor of Sicily , Frederick II, to King al-Kamil.[41] In the book mentioned, Al-Qarafi investigates fifty questions, two of these are said by him to be the Emperor’s questions (Questions 25 and 30 in al-Qarafi’s book).[42] The first of these seeks the reason why Canopus appears larger when on the horizon, and Question 30 deals with the explanation of some kind of hallucination.[43] Wiedemann dealt with al-Qarafi’s treatment of the problem of mirages,[44] (which is the topic of his 37^th and 38^th problems) and his explanation of the rainbow.[45] In Al-Qarafi’s explanation of the colour of the rainbow, we read:

‘In vapours the colour of the sun is always red, as the colour of the sunset sky and the colour seen after the morning twilight show us; because this red colour is composed of the (colour of the) light of the sun and that of the vapour; (it is not a body, therefore, that causes the red colour in the rainbow). The damp vapours are partly dense, namely the ones far from the earth which are stiffened into stones because the cold existing in these high regions; and (the damp vapours are) partly rare in the lower locations because they are far away from these cold regions and because the vapours ascending from the earth are heated through the effect of the heat in the earth. So the dense vapours appear almost black, very nearly sky blue (asmanjuni), the rare ones appear pure (clear) without blue. The colour next to the red is black. It is a rule, however, that if black and red mix together yellow is produced. There are, therefore, four colours on the rainbow: red, yellow, sky blue, and the pure colour. There are two (types) of colours, the colour of the vapours and that of the sun; and then the colour composed of these two.’[46]

After al-Qarafi, the two other illustrious figures of Islamic optics were Qutb al-Din al-Shirazi,[47] and Al-Farisi. The latter repeated and improved Ibn al-Haytham’s experiments on the camera obscura (al-Qamara or al-beit al-muzlim), and also observed the path of the rays in the interior of a glass sphere, hoping to determine the refraction of solar light through raindrops. His findings enabled him to give an explanation of the formation of the primary and secondary rainbows.[48]