Editing Ibn al-Haytham (section)

=== Theory of optics ===
{{See also|Horopter}}
[[File:Thesaurus opticus Titelblatt.jpg|thumb|upright|Front page of the ''Opticae Thesaurus'', which included the first printed Latin translation of Alhazen's ''Book of Optics''. The illustration incorporates many examples of optical phenomena including perspective effects, the rainbow, mirrors, and refraction.]]
Two major theories on vision prevailed in [[classical antiquity]]. The first theory, the [[Emission theory (vision)|emission theory]], was supported by such thinkers as [[Euclid]] and [[Ptolemy]], who believed that sight worked by the [[eye]] emitting [[Ray (optics)|rays]] of [[light]]. The second theory, the [[intromission theory]] supported by [[Aristotle]] and his followers, had physical forms entering the eye from an object. Previous Islamic writers (such as [[al-Kindi]]) had argued essentially on Euclidean, Galenist, or Aristotelian lines. The strongest influence on the ''Book of Optics'' was from Ptolemy's [[Ptolemy#Optics|''Optics'']], while the description of the anatomy and physiology of the eye was based on Galen's account.<ref>{{harvnb|Smith|2001|p=lxxix}}.</ref> Alhazen's achievement was to come up with a theory that successfully combined parts of the mathematical ray arguments of Euclid, the medical tradition of [[Galen]], and the intromission theories of Aristotle. Alhazen's intromission theory followed al-Kindi (and broke with Aristotle) in asserting that "from each point of every colored body, illuminated by any light, issue light and color along every straight line that can be drawn from that point".<ref name="{{harvnb|lindberg|1976|p=73}}.">{{harvnb|Lindberg|1976|p=73}}.</ref> This left him with the problem of explaining how a coherent image was formed from many independent sources of radiation; in particular, every point of an object would send rays to every point on the eye.

What Alhazen needed was for each point on an object to correspond to one point only on the eye.<ref name="{{harvnb|lindberg|1976|p=73}}." /> He attempted to resolve this by asserting that the eye would only perceive perpendicular rays from the object{{snd}}for any one point on the eye, only the ray that reached it directly, without being refracted by any other part of the eye, would be perceived. He argued, using a physical analogy, that perpendicular rays were stronger than oblique rays: in the same way that a ball thrown directly at a board might break the board, whereas a ball thrown obliquely at the board would glance off, perpendicular rays were stronger than refracted rays, and it was only perpendicular rays which were perceived by the eye. As there was only one perpendicular ray that would enter the eye at any one point, and all these rays would converge on the centre of the eye in a cone, this allowed him to resolve the problem of each point on an object sending many rays to the eye; if only the perpendicular ray mattered, then he had a one-to-one correspondence and the confusion could be resolved.<ref>{{harvnb|Lindberg|1976|p=74}}</ref> He later asserted (in book seven of the ''Optics'') that other rays would be refracted through the eye and perceived ''as if'' perpendicular.<ref>{{harvnb|Lindberg|1976|p=76}}</ref> His arguments regarding perpendicular rays do not clearly explain why ''only'' perpendicular rays were perceived; why would the weaker oblique rays not be perceived more weakly?<ref>{{harvnb|Lindberg|1976|p=75}}</ref> His later argument that refracted rays would be perceived as if perpendicular does not seem persuasive.<ref>{{harvnb|Lindberg|1976|pages=76–78}}</ref> However, despite its weaknesses, no other theory of the time was so comprehensive, and it was enormously influential, particularly in Western Europe. Directly or indirectly, his ''De Aspectibus'' ([[Book of Optics]]) inspired much activity in optics between the 13th and 17th centuries. [[Kepler]]'s later theory of the [[retina]]l image (which resolved the problem of the correspondence of points on an object and points in the eye) built directly on the conceptual framework of Alhazen.<ref>{{harvnb|Lindberg|1976|p=86}}.</ref>

Alhazen showed through experiment that light travels in straight lines, and carried out various experiments with [[lens (optics)|lenses]], [[mirror]]s, [[refraction]], and [[Reflection (physics)|reflection]].<ref name="auto">{{harvnb|Al Deek|2004}}.</ref> His analyses of reflection and refraction considered the vertical and horizontal components of light rays separately.<ref>{{harvnb|Heeffer|2003}}.</ref>

Alhazen studied the process of sight, the structure of the eye, image formation in the eye, and the [[visual system]]. Ian P. Howard argued in a 1996 ''[[Perception (journal)|Perception]]'' article that Alhazen should be credited with many discoveries and theories previously attributed to Western Europeans writing centuries later. For example, he described what became in the 19th century [[Hering's law of equal innervation]]. He wrote a description of vertical [[horopter]]s 600 years before [[Aguilonius]] that is actually closer to the modern definition than Aguilonius's{{snd}}and his work on [[binocular disparity]] was repeated by Panum in 1858.<ref>{{harvnb|Howard|1996}}.</ref> Craig Aaen-Stockdale, while agreeing that Alhazen should be credited with many advances, has expressed some caution, especially when considering Alhazen in isolation from [[Ptolemy]], with whom Alhazen was extremely familiar. Alhazen corrected a significant error of Ptolemy regarding binocular vision, but otherwise his account is very similar; Ptolemy also attempted to explain what is now called Hering's law.<ref>{{harvnb|Aaen-Stockdale|2008}}</ref> In general, Alhazen built on and expanded the optics of Ptolemy.<ref>{{harvnb|Wade|1998|pages=240, 316, 334, 367}}; {{harvnb|Howard|Wade|1996|pages=1195, 1197, 1200}}.</ref>

In a more detailed account of Ibn al-Haytham's contribution to the study of binocular vision based on Lejeune<ref>{{harvnb|Lejeune|1958}}.</ref> and Sabra,<ref name="{{harvnb|sabra|1989}}.">{{harvnb|Sabra|1989}}.</ref> Raynaud<ref>{{harvnb|Raynaud|2003}}.</ref> showed that the concepts of correspondence, homonymous and crossed diplopia were in place in Ibn al-Haytham's optics. But contrary to Howard, he explained why Ibn al-Haytham did not give the circular figure of the horopter and why, by reasoning experimentally, he was in fact closer to the discovery of Panum's fusional area than that of the Vieth-Müller circle. In this regard, Ibn al-Haytham's theory of binocular vision faced two main limits: the lack of recognition of the role of the retina, and obviously the lack of an experimental investigation of ocular tracts.

[[File:Alhazen1652.png|left|thumb|upright|The structure of the [[human eye]] according to Ibn al-Haytham. Note the depiction of the [[optic chiasm]]. —Manuscript copy of his [[Kitāb al-Manāẓir]] (MS Fatih 3212, vol. 1, fol. 81b, [[Süleymaniye Mosque]] Library, Istanbul)]]
Alhazen's most original contribution was that, after describing how he thought the eye was anatomically constructed, he went on to consider how this anatomy would behave functionally as an optical system.<ref>{{harvnb|Russell|1996|p=691}}.</ref> His understanding of [[Pinhole camera model|pinhole projection]] from his experiments appears to have influenced his consideration of image inversion in the eye,<ref>{{harvnb|Russell|1996|p=689}}.</ref> which he sought to avoid.<ref>{{harvnb|Lindberg|1976|pages= 80–85}}</ref> He maintained that the rays that fell perpendicularly on the lens (or glacial humor as he called it) were further refracted outward as they left the glacial humor and the resulting image thus passed upright into the optic nerve at the back of the eye.<ref>{{harvnb|Smith|2004|pages=186, 192}}.</ref> He followed [[Galen]] in believing that the [[Lens (anatomy)|lens]] was the receptive organ of sight, although some of his work hints that he thought the [[retina]] was also involved.<ref>{{harvnb|Wade|1998|p=14}}</ref>

Alhazen's synthesis of light and vision adhered to the Aristotelian scheme, exhaustively describing the process of vision in a logical, complete fashion.<ref>{{Cite journal|url=http://www.jstor.org/stable/3657357|title=Alhacen's Theory of Visual Perception: A Critical Edition, with English Translation and Commentary, of the First Three Books of Alhacen's "De aspectibus", the Medieval Latin Version of Ibn al-Haytham's "Kitāb al-Manāẓir": Volume Two|author=Smith, A. Mark|year=2001|journal=Transactions of the American Philosophical Society|volume=91|issue=5|pages=339–819|doi=10.2307/3657357|jstor=3657357|access-date=12 January 2015|archive-date=30 June 2015|archive-url=https://web.archive.org/web/20150630235046/http://www.jstor.org/stable/3657357?|url-status=live}}</ref>

His research in [[catoptrics]] (the study of optical systems using mirrors) was centred on spherical and [[Parabola|parabolic]] mirrors and [[spherical aberration]]. He made the observation that the ratio between the [[angle of incidence (optics)|angle of incidence]] and [[refraction]] does not remain constant, and investigated the [[Magnification|magnifying]] power of a [[Lens (optics)|lens]].<ref name="auto" />