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Greek philosophy on optics broke down into two opposing theories on how vision worked, the [[intromission theory]] and the [[emission theory (vision)|emission theory]].<ref>[http://www.stanford.edu/class/history13/earlysciencelab/body/eyespages/eye.html A History Of The Eye] {{webarchive|url=https://web.archive.org/web/20120120085632/http://www.stanford.edu/class/history13/earlysciencelab/body/eyespages/eye.html |date=2012-01-20 }}. stanford.edu. Retrieved 2012-06-10.</ref> The intromission approach saw vision as coming from objects casting off copies of themselves (called eidola) that were captured by the eye. With many propagators including [[Democritus]], [[Epicurus]], [[Aristotle]] and their followers, this theory seems to have some contact with modern theories of what vision really is, but it remained only speculation lacking any experimental foundation.
[[Plato]] first articulated the [[Emission theory (vision)|emission theory]], the idea that [[visual perception]] is accomplished by rays emitted by the eyes. He also commented on the [[parity (physics)|parity]] reversal of mirrors in ''[[Timaeus (dialogue)|Timaeus]]''.<ref>{{cite book|title=A manual of greek mathematics|author=T.L. Heath|publisher=Courier Dover Publications|isbn=978-0-486-43231-1|pages=181–182|year=2003}}</ref> Some hundred years later, [[Euclid]] (4th–3rd century BC) wrote a treatise entitled ''[[Euclid#Other works|Optics]]'' where he linked vision to [[geometry]], creating ''geometrical optics''.<ref>{{cite book |author=William R. Uttal |title=Visual Form Detection in 3-Dimensional Space |url=https://books.google.com/books?id=rhVOVKp0-5wC&pg=PA25 |year=1983 |publisher=Psychology Press |isbn=978-0-89859-289-4 |pages=25– |url-status=live |archive-url=https://web.archive.org/web/20160503123615/https://books.google.com/books?id=rhVOVKp0-5wC&pg=PA25 |archive-date=2016-05-03 }}</ref> He based his work on Plato's emission theory wherein he described the mathematical rules of [[perspective (graphical)|perspective]] and described the effects of [[refraction]] qualitatively, although he questioned that a beam of light from the eye could instantaneously light up the stars every time someone blinked.<ref>{{cite book|author=Euclid|title=The Arabic version of Euclid's optics = Kitāb Uqlīdis fī ikhtilāf al-manāẓir|editor=Elaheh Kheirandish|publisher=New York: Springer|year=1999|isbn=978-0-387-98523-7|url-access=registration|url=https://archive.org/details/arabicversionofe0000eucl}}</ref> Euclid stated the principle of shortest trajectory of light, and considered multiple reflections on flat and spherical mirrors.
[[Ptolemy]], in his treatise ''[[Ptolemy#Optics|Optics]]'', held an extramission-intromission theory of vision: the rays (or flux) from the eye formed a cone, the vertex being within the eye, and the base defining the visual field. The rays were sensitive, and conveyed information back to the observer's intellect about the distance and orientation of surfaces. He summarized much of Euclid and went on to describe a way to measure the [[angle of refraction]], though he failed to notice the empirical relationship between it and the angle of incidence.<ref name=Ptolemy>{{cite book |title=Ptolemy's theory of visual perception: an English translation of the Optics with introduction and commentary |author=Ptolemy |editor=A. Mark Smith |publisher=DIANE Publishing |year=1996 |isbn=978-0-87169-862-9}}</ref> [[Plutarch]] (1st–2nd century AD) described multiple reflections on spherical mirrors and discussed the creation of magnified and reduced images, both real and imaginary, including the case of [[chirality]] of the images.
[[File:Ibn Sahl manuscript.jpg|thumb|right|upright|Reproduction of a page of [[Ibn Sahl (mathematician)|Ibn Sahl]]'s manuscript showing his knowledge of [[Snell's law|the law of refraction]] ]]
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