Alhazen, the name by which Abū ʿAlī al-Ḥasan ibn al-Ḥasan ibn al-Haytham (Basra, c. 965 – Cairo, 1039) was known in medieval Europe, was an Arab physician, philosopher, mathematician, physicist and astronomer.
He was one of the most important and brilliant scientists of the Islamic world (and generally of the beginning of the second millennium). He is considered the initiator of modern optics. He was also called al-Baṣrī (of Basra), al-Miṣrī (the Egyptian), Avennathan and Avenetan, Ptolemaeus secundus but, more than anything else, he was known precisely as Alhazen, a corruption of his nasab “Ibn al-Ḥasan.” An asteroid, 59239 Alhazen, was dedicated to him.
Originally from the areas of Mesopotamia (present-day Iraq), he grew up there studying religion and learning about the sciences through the teachings of local clerics, between Basra and Baghdad.
The son of a wealthy dignitary, his studies were initially directed toward careers that today might be called public administration; he was also appointed vizier for the province of Basra, and his qualities began to emerge, give him some notoriety and introduce him to the theorizing of classical culture in the Mediterranean area. One of his first “encounters” with classical science led him to meet Aristotle.
He moved while still young to Egypt, where he would operate for the rest of his days. He arrived there, according to one version (a legend to some historians), at the invitation of Imām al-Ḥākim of the Fatimid dynasty, who, having heard of his extraordinary talents, invited him to design a system for regulating the waters of the Nile, which caused the well-known floods; according to other versions, it would appear that Ibn al-Haytham had drawn up a design on his behalf, probably for a dam. Having arrived at al-Janadil, south of Aswān, with a very large team of technicians and workers financed for him by the Imām-caliph, he encountered difficulties that some indicate were technical, others financial, and had to give up the project.
Returning to the capital he had to suffer the contemptuous humiliation of al-Ḥākim who, disavowing his “professional” qualities, that is, accusing him of not possessing the qualities of a scientist, assigned him a position as a clerk-we would say today-of concept. Fearing his wrath, however, because al-Hākim was an eccentric tyrant who distinguished himself, yes, by constant and important patronage, but also by cold cruelty, Ibn al-Haytham feigned madness for a dozen years, until the Imām”s violent death (1021).
During this period he had the opportunity to travel (he seems to have visited Islamic Spain and Syria where – according to speculation that has no confirmation, however – he allegedly lived), while it is certain that he nevertheless settled in Egypt, in its capital (near the al-Azhar mosque) where the alleged insanity did not prevent him from being admitted to studies and teaching at that same mosque which, as today, functioned as a university. He also established a personal library whose size, for the time and for Alhazen”s location, was impressive: it was said to be second only to that of the Dār al-Ḥikma (House of Wisdom), erected by the Fatimid Imām.
In Cairo, thanks to the advantages offered by the capital”s very lively cultural activity, he studied science in depth in the theories developed by Greek scholars, translating a large number of works into Arabic and thus delivering to the Islamic world, at the very time when the flowering of the sciences was at its most flourishing, a documentary and informative contribution of the greatest importance.
He restored some lost works to all mankind: The Conics of Apollonius of Perga was in eight books, of which the last one was lost. Ibn al-Haytham was able to deductively rework (and continue the reasoning of the previous books) the missing book, giving it a draft that was entirely compatible with the original possible.
But translations (among which Euclid”s Elements and Ptolemy”s Almagest are notable) also introduced him to personal speculation on many of the subjects he analyzed, resulting in insights and reformulations that would remain of paramount importance for many centuries. The bulk of his studies is collected in 25 essays on mathematics and 45 papers on physics (he is credited with the first, substantial estimate of the thickness of the atmosphere) and metaphysics, as well as his autobiography of 1027.
It was especially in optics that his research produced outstanding results. Studying Euclidean optics, he enunciated theories on perspective, of which he focused his interest on the three fundamental points (the point of view, the visible part of the object, and illumination), reformulating the geometrical models that described their relationships.
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Demolition of old theories on optics
In later eras he would be considered the leading exponent of the “Arab school” of optics, not least because his studies were of considerable influence in demolishing old theories about the nature and diffusion of visual images: in antiquity, light was believed to be a subjective (and therefore relative) elaboration of the human psyche.
Later they had begun to speak of “peels” (or “èidola”) by claiming that particles of each observed object (sort of “shadows” that reproduced its shape and colors) detached themselves from the object to reach the human eye (although this theory could not explain the access to the eye of the “shadows” of large mountains except by assuming a mysterious progressive dimensional reduction en route).
This theory was followed by that of “visual rays,” for which the analysis of the blind man”s taking in visual information and deriving it with a stick was supposed to explain that the eye would be equipped with a kind of “sticks” with which to strike the visible world and derive its optical information. The theory was exposed to the arguments of those who objected that this would not explain the lack of night vision (or in the absence of light), would not explain what is now known as refraction, and, above all, did not explain how the human eye could “touch” with its supposed sensory sticks such distant objects as the Sun and stars.
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The Arabic school of optics peels
Of the Arabic school of optics, ibn al-Haytham is generally considered the first and greatest, brilliant exponent. It was thanks to his studies that new hypotheses could be formulated, which were also fresh due to a lack of cultural inertia, and that the study of these subjects had a chance to constitute itself into a “school,” destined to train a number (for the times very relevant) of specialized scholars.
One element that attracted his attention was the persistence of retinal images, together with the painful sensation procured by the observation of sources of intense brightness, such as the Sun. If in fact, was his reasoning, it really was the eye that was “searching” with rays or sticks for the object, there could have been no persistence of images during the albeit rapid closing of the eyelids (whereas this rapid movement is commonly imperceptible precisely because of the persistence of the image – we now know – on the retinal fundus). Moreover, if the eye, a sense organ, really handled visual information autonomously, it would not “touch” the “burning” Sun and no other troublesome source, procuring neither pain nor glare.
Having thus demolished the theory of visual rays, Alhazen went back to the theory of peels, this time assuming that the acquisition of light information was, yes, due to an external agent, but that this agent did not release “shadows,” traveling in the form of “peels” specifically in the direction of the observer”s eye, but rather “peels,” emitted by the object in all directions. For this, he had to deal with a hypothesis of rudimentarily particle-like decomposition of each of the observed objects, and attribute to each infinitesimal component of each object the ability to emit rinds in every direction.
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The genius of particle decomposition consisted in the first monition (elaborated in a form, it should be noted, exquisitely logical) of an embryo of the corpuscular theory: from each object, indeed from each of the very small component parts of the object, “luminous information” (rinds) would be detached, which would reach the eye, pass through the lens, penetrate the pupil, and cross the eyeball, stopping at the bottom. Then, for each object, for each particle of it, of all the rinds emitted in all directions, only one could have hit the cornea normally (i.e., according to a straight trajectory perpendicular to the plane of the cornea), crossed it and reached its destination. The uniqueness of the peel avoided duplication of images and confusion on the retina of each particle, allowing for orderly vision.
To this theory the scientist added as a corollary the hypothesis that there were two types of foreshortenings, some “normal” (fully seconding his theory) and others “irregular.” While the normal ones would regularly reach the retina proceeding in a straight line and with finite speed, the others would be stopped by refraction and rejected, denying vision of certain parts of objects. Of refraction he was moreover sketching theoretical rudiments, having carried out experiments on transparent (glassy) objects of spherical or cylindrical shape, and of reflection and absorption he was about to devote himself to deeper studies.
On the retina, the regular slivers (one from each of the particle components of the object) would stop to provide the visual information that, together with the other regular slivers that had arrived at their destination, would allow reconstruction of a general information about the object that had emitted them. Thus, the image would have been the result of the reception-perception of the sum of the rinds emitted by each particle of the object, sorted by the eye into a finally comprehensible vision.
Having extensively studied the anatomy of the eye, and having thereby developed a deep familiarity with the theories of Galen (from whom he had learned about the cornea and tunics), Ibn al-Haytham realized (well before the notion became generally accepted) that the peels, as they traversed the globe (in the then only supposed rectilinear trajectory), would arrange themselves on the retina in reverse order, as in fact happens: the resulting image on the retina is indeed upside down, and Ibn al-Haytham had guessed this with simple geometry diagrams.
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The search for the sensorium
Having no better elements, and not being able to accept that the image would be turned upside down (since man sees it “correctly”-nowadays we know, however, that this is not the case), but nevertheless firmly grounded in the awareness of the value of his theory, he resolved to look for the “sensorium,” that is, the nerve that transmits information to the brain, at a point in the trajectory of the peels that was reached prior to the point of “turning upside down” (the center of the eyeball).
And in front of the center of the globe were the uninfluent liquid, the pupil hole and the only transparent but solid element, the lens. It was in this therefore that Alhazen deduced the sensorium must be found and thus the correct image must be gathered.
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The specialty of sunlight
Consideration of the characteristics of illumination, now undoubtedly attributed to the effect of sunlight, coupled with consideration of the painful sensations brought about by direct observation of the maximum star, led Alhazen to hypothesize that something emanated from the Sun (perhaps not quite zest in the sense he had already identified) capable of causing the emission of “ordinary” zest by objects struck by sunlight.
He therefore intuited a kind of force, of energy emitted by the Sun (but did not come to a precise definition of it), strong enough to arouse the production of visual information from objects and too strong for the eye, which of such peels was to receive, not produce.
This sort of radiation allowed him to hypothesize that color was the effect of a secondary radiation, emitted by colored objects that had been solicited by a primary agent, such as sunlight; he went so far as to hypothesize, first, that sunlight illuminated the Moon and that the Moon reflected it back to Earth.
Synthesizing, ibn al-Haytham introduced the hypothesis that (as would later be developed by corpuscular theory) vision depended on an external agent (the lumen, an innovative concept with respect to lux) and that the information provided by the lumens was actually a flux of material particles emitted by objects.
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The camera obscura and optical illusions
The study of the flipping of the image inside the eyeball due to passing through the narrow pupil hole gave Alhazen the cue to develop the first ever study of the camera obscura. The scientist described with great anticipation and exactness the mechanism of image flipping as it passed through a hole and stopped at the bottom of the chamber.
Optical illusions were also dealt with extensively by ibn al-Haytham, citing them countless times in his works and using them to analyze the possible influence of the human psyche in the formation of error. The prevailing view of the time was that the eye tended to be fallacious, as the result of vision was expressed through the non-objective filter of each observer”s individuality, in the absence of technically “cold” feedback. But ibn al-Haytham”s inclination was in favor of the highly subjective nature of vision.
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The spread of Ibn al-Haytham”s theories.
It took a long time for Europe to become acquainted with Ibn al-Haytham”s studies. Hindering their rapid dissemination was the cultural and linguistic distance of the Western world from the Arab world, and political and religious distances were not helpful: for while Islam encouraged science and its dissemination, the church hindered it. A compendium of his studies was translated in 1270 by the Polish monk Vitellion, who under the overall title of “De Aspectibus” collected together other works such as the “Epistle on Light” and the “Book of Optics,” which was known in the West under the title of Alhazen”s Perspective.
The Arab scientist”s theories certainly challenged the established traditions in the theory of the peels, but-perhaps also because of the many general cultural implications-they did not unhinge them: instead, a kind of mediating theory between the old and the new hypotheses, called the “species theory,” was arrived at. In this, the peels became “species,” which left the object as a result of an external agent, reaching the eye thanks to some visual rays that the eye would emit to catch them.
Even studies on refraction and the camera obscura, such as those on the flipping of images in the eyeball, were not immediately received, but either lazily proceeded to the sometimes skeptical reconstruction of the paths followed by Ibn al-Haytham or followed the studies already begun while ignoring the contribution of the Basra scientist; Leonardo himself hypothesized (in contrast to the Arab) that even within the eye there was a flipping of the image similar to that of the Leonardian camera obscura.
It would be Abbot Francesco Maurolico of Messina, much later, who would reevaluate Alhazen”s insights, although he remained among his contemporaries very isolated and little considered; Maurolico perfected the idea of the multitude of signal-emitting points, calling them geometric rays. It was later with Kepler, inspired by the Arab and Maurolico, that Alhazen”s innovations served as the basis for the development of modern theory.
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The Hockney-Falco Thesis
At a scientific conference in February 2007, Charles M. Falco speculated that Ibn al-Haytham”s work on optics may have influenced Renaissance artists.