Ancient Theories of Vision and Al-Kindi’s Critique of Euclid’s Theoryof Vision

Ika Putri

1 Introduction

Vision is a wide and complex subject in science. The human being had been surveyingoptical phenomena since the early advanced civilisation. The oldest mirrors and burninglenses which have survived were dated before 1500 B.C.. Around the same time Egyptianshad listed various eye diseases on papyrus, they even underwent several optical surgeries,though the results were nowhere near successful. And ca. 400 B.C. people in China wereexamining the nature of radiation, shadows and reflections ([5] p.1).So what were the reasons for the interest in vision already on such an early period? The eyeis the most important organ of sense and finding out a way to cure an eye disease is seen asan extraordinary achievement in the history of mankind. The early phase of the history ofvision is divided into three parts: The theories introduced by the ancient Greek philosophers,by the Roman and the Arabs. In the following chapters we will go into each theories in moredetail.

2 The Ancient Theories

The early advanced culture we found in present-day Greece was commonly known for theirreligious conception. Myths and sages characterise their history. Only after 500 B.C. peoplebegan with the attempts to understand the nature rationally ([2], p.16). Therewith the timeof Greek philosophy had started. It was divided into different disciplines such as politics ornatural sciences, which the optic is assigned to. In this chapter, we will discuss chronologicallyseveral concepts dealing with the mechanism of sight.

2.1 The Atomists

In the ancient Greek, where people believed in the existence of gods and supernatural powers,the atomists were the first who were able to deliver a neutral, non-religious considerations([3] p.23f.). By the time, there were several theories introduced:

• Democritus (ca. 460 B.C. - 370 B.C.) suggested that the air between the eye and theobject seen is “contracted and stamped” by the object itself and the observing eye ([5]p.2). The pressed air, still holding various colours of the object wanders and “appearsin the eye” ([5] p.2).

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Figure 1: Eye Construction according to Democritus ([6] p.69)

• Epicurus (ca. 341 B.C. - 270 B.C.) proposed that particles or atoms flow continuouslyfrom the object’s body into the eye. Nevertheless the body does not shrink becauseother particles will replace and fill in the empty space ([5] p.2).

Both theories above are based on the same principle, namely that an object can only beperceived if it directly comes into contact with the corresponding organ ([5] p.3). The lastpart was interpreted differently; the essential part of vision is for Democritus the pressed air,for Epicurus the activity of particles moving along and eventually entering the eye.In addition to the intromission theory - the (particles of the) object must enter the eye inorder to be visible - the atomists also studied the anatomy of the eye. According to Dem-ocritus, the eye is composed of water ([5] p.3) which is right. An eye, as shown on Figure 3,consists of fluid which is covered by two coats and which can travel along the hollow opticnerve ([6] p.65).Democritus stated too that there were four basic colours: White objects are “smooth”, “bril-liant”, and cast no shadows. The opposite colour is black; things of this colour are “rough”and “irregular”. The third colour is red, which symbolises “heat” . The last colour, green, hasa “solid” and “void” attributes. The other colours are produced by mixing the basic colourstogether ([6] p.65f.).

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2.2 Plato

Plato (ca. 427 B.C. - 347 B.C.) who lived around the same time as the atomists brought outanother theory of sight. His version happens to be a combination of the intromission theoryof the atomists and the extramission theory, which says that light proceeding from the eye isthe cause of vision. The second concept is going to be discussed later (see Euclid).Plato claimed that “a stream of light or fire” ([5] p.3) proceeds from the observing eye. Thisray has the same property as the sunlight and even the surface of the eye. Being in suchstate, the eye will only let light of the same kind pass through. The inner fire fuses thenwith the sunlight and form a “homogeneous body” ([5] p.5), located on a direct line with theeye. While this occurs, the targeted object lets “flame particles” ([5] p.6) off its body. Theconcept of the idea is very similar to the theory suggested by Epicurus. If the object is placedwithin the homogeneous body, the particles will be able to enter the eye and come in contactwith the soul ([5] p.5).In contrast to the intromission theory, which aforementioned that vision occurs in the eye,Plato argued that vision should happen in a sort of mediator which passes on the motion ofanything that comes to interact with the soul. In this case, the mediator is the fusion of theinner fire and the external light.In Plato’s teaching, colours are those flame particles flowing from the objects. If one comparethese with the particles of the visual ray, there are three different kind of colours: If the size ofboth particles are equal, one perceives a transparent object. If the flame particles are bigger,then one senses something cold and black. If they are smaller, than a sensation of hot andwhite will be present ([5] p.6).

2.3 Aristotle

The third theory of vision that became popular in the ancient Greece and later was developedby Aristotle (ca. 384 B.C. - 322 B.C.). Unlike his predecessors Aristotle relied more in hissenses and had more faith in the results of his own observations ([6] p.2). Apparently duringhis experiments he could not prove that the earlier theories, especially the atomists’ andPlato’s theory, were right. So, he stood against them and represented his own idea.He denied the idea supposing light as a “corpuscular emanation” ([5] p.6) which was part ofthe intromission concept. Aristotle was certain that light could not be a solid body since “itis neither fire, nor ... an emanation from any body” ([5] p.6) Then what is light? Aristotlepointed out that luminous bodies such as fire cause light. So it is actually not really a bodybut a mere immaterial actuality of the transparency ([5] p.7) Neither the extramission northe intromission theory was accepted by Aristotle as many other scientists who found outthat both ideas do not make sense: If visual rays proceed from the eye, then one should beable to see in any circumstances while keeping the eyes open. But you cannot see anythingin the dark place with your bare eyes. Besides, it is impossible for an object seen to shrinkand enter the eye. Furthermore Aristotle was against the alternative idea offered by Platobecause a satisfying explanation how the inner light can collide with sunlight and how bothlights form a body could not be found ([5] p.6).Aristotle’s theory of sight can be considered very advance by his time. He believed that(sun)light is reflected by an object and somehow then transmitted by a medium into the eye([5] p.12). This is the basic knowledge for the theory we know nowadays. Here, the mediumplays an important role: vision only occurs if a medium between the eye and the object exists.

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Figure 2: Eye construction according to Aristotle ([6] p.70)

If you bring an object directly in front of the eye, you won’t see it since there is no mediumin between. What could the medium possibly be? Aristotle stated that the medium shouldbe of something transparent, so that one can see an object through them ([5] p.7). Air, wateror other solid but transparent material are some examples of such medium. Colour is thenwhat lays on the surface of the object and it sets the transparent medium in motion ([5] p.8).This way, light can be transmitted into the eye. So, colour and medium have to interact witheach other in order for vision to occur.Like other scientist before him, Aristotle also studied the anatomy of the eye. He was known

to have cut up animal eyes to analyse its structure. He came to a conclusion saying that aneye consists of three coats covering a humour. In his study-sketches depicted on Figure 2.3above, the lens is not drawn. This is an indication that the eye he had dissected was alreadyin a post-mortem state.

2.4 Euclid and Ptolemy

After the death of Alexander the Great in 322 B.C. the empire started to fall apart; peopleleft their homeland. The Roman Empire took this opportunity and conquer the territory.The study of vision was then continued by roman philosophers.

Euclid A Greco-Roman who made a great contribution to the development of the visiontheory is Euclid (ca. 325 B.C. - 265 B.C.). He introduced a theory which deals only with the

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geometrical aspects of vision ([5] p.12).Euclid’s theory is called extramission since he assumed that visual rays emanate from the eye.However, he did not explain in his works why one can perceive things at all. Euclid describedthe visual perspectives, which the former theories lack of.Euclid’s “Optica” is based on the seven postulates he had set ([5] p.13f). These are listedbelow:

1. There are far-reaching rays proceeding directly from the eye.

2. The rays form together a cone which has its apex in the eye and the base at the limitsof your vision.

3. If the visual cone falls on a thing, it will be visible.

4. The bigger the angle, which the object is seen under, the bigger it will appear.

5. An object stricken by a higher visual ray will appear higher, and vice-versa.

6. The further right the visual ray that fall upon an object, the more to the right is theobject seen in the observer’s eye.

7. Things seen under more angles, i.e. the more visual rays encounter the observed object,the clearer they are perceived.

Although at a first glance Euclid seemed to disregard all other visual aspects, we can seethat the postulates above include other elements. The first three axioms explain the conceptsof visual ray that proceeds from the eye as we know it from Plato’s theory. Postulate 4 - 6describe how the size and position of the object seen depend on the angle, under which anobject is observed. And the last potulate deals with the clarity of the object: The further theobject, the bigger the visual cone and thus the less visual rays fall upon it and the less clearit will be perceived. Conclusively Euclid did not leave out other aspects completely.

Ptolemy A follower of Euclid’s idea happened to be the “greatest optician of antiquity” -Claudius Ptolemy ([5] p.15). He extended Euclid’s theory by adding the physical, physiologicaland psychological aspects.Ptolemy (ca. 100 - 175) agreed with Euclid’s postulate saying visual rays emanate from theeye in a form of a cone with the vertex at the eye and the base on the object seen. But hedid not accepted it as is; he added that a visual ray has the same nature as the external light(sunlight). The idea was adapted from Plato, who believed if both lights meet each other, ahomogeneous body is formed. Thus, visual light must be a consistent body.Euclid mentioned that there are gaps between the visual rays which explains why one cannotsee things clearly at some occassions. On the contrary, Ptolemy insisted that there exist onlya single visual ray emanating in form of a cone, otherwise it is impossible to see an entireobject at one time. Additionally he argued that rays only represent the geometry of sight,but not the reality itself, like Euclid seemed to have meant.Finally Ptolemy revived the theory of colour as Aristotle defined it. However he added thatcolour produces a modification (“passio”) in the visual cone, whereas Aristotle only statedthat colour cannot influence the visual cone. In his case it is the transparent medium withoutthe “presence and cooperation of the external light” ([5] p.16). Unfortunately some of his

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Figure 3: Eye construction according to Galen ([6] p.73)

statements remain unclear because Ptolemy’s first work which contains the explanation of hisideas did not survive.Ptolemy added two geometrical assumption to Euclid’s concept. First, within the visual conethe clarity of the object observed might differ depending on its position within the cone. Anobject located over the main axis is perceived more clearly than an object placed on theperiphery of the visual cone. Second, the apex of the visual cone is situated precisely at thecentre of the cornea and the rotation of the ocular eye ([5] p. 17).

2.5 Galen

A scientist of the late Roman Empire who studied the optical phenomena is Galen (ca. 133- 200). He concentrated his work in the structure of the eye. He was known to have dissectmonkeys and freshly slaughtered oxen to study the anatomy ([6] p. 66). Nonetheless, psy-chological and physical elements are to be found in his theory.An optical spirit called pneuma travels along the hollow optic nerves connecting the eye and

the brain. While being in the eye, pneuma comes in contact with the air surrounding theeye ande changes it to its nature. This way, the air has been converted into an “instrumentof soul” and became perceptive ([5], p.9). Galen has adapted this part from the Stoics, andadded his own contributions: All these take place exactly in the crystalline lens situated inthe middle of the eye. As a consequence, the lens is the vital instrument of vision. Cataracthad lead him to draw this conclusion. The cause of the disease lies between the lens and thecornea; if you removed it surgically you should be able to see again ([6] p. 69).

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Through his studies Galen was capable of gaining a nearly exact knowledge of the eye struc-ture. As we can see above, he was already capable of locating the lens. In addition to this, healso mentioned in one of his works the existence of retina, which had a netlike structure andenables pneuma to travel along the nerves and let the soul interact with images captured bythe eye. The cornea supposed to be an outgrowth of the eye which protects the inner partsof the eye ([6] p.89).

3 Al-Kindi and his critiques against Euclid’s theory

Around the 5th century The Roman Empire was defeated. And so the European supremacyin science and medicine came to an end. At the same time the Arabic culture started tospread and it reached its climax by the conquest of Spain in the 7th century. Thus the centreof science was translocated to the Orient ([2] p.30). Abu Yusuf Ya’qub Ibn Ishaq Al-Kindiwas one of the first moslems who devoted himself in transferring ancient Greek knowledgeto the arabic scholars. He translated important scientific and philosophical works of Greekphilosophers ([1] p.207). In the following, only his thoughts of optic will be discussed.

3.1 Defending the extramission theory

Al-Kindi did a great contribution in optic; his work “De Aspectibus” is influenced by greatancient philosophers like Aristotle, Galen and especially Euclid. He was in favour of Eu-clid’s extramission theory. He showed this by mentioning arguments speaking against theintromission and the other theories, such as described below:

• According to Theon of Alexandria, whose theory Al-Kindi accepted, the structure ofa sense organ should comply to its function. Take the eye as an example: an eye hasa globular shape and can move itself around. Apparently it is not designed to collectimages as the intromission theory suggests ([5] p.22). Consequently an eye has to moveand send something, namely visual rays, in order to perceive an object.

• The intromission as well as the Aristotelian theory assumed, as long as object particlesenter the eye, the object will be seen entirely at one time. But if you take a circle asan example and take a look at it from the edge, what you see is just a line. If you flipit and have a look from the front you will recognize it as a circle ([5] p.23). As opposedto this, the extramission thory is possible to explain such phenomena - what you see iswhat the rays fall upon.

The extramission theory can be combined easily with Galen’s concept, which Al-Kindi alsodid. He mentioned that a kind of visual spirit “meets the eye” and transform it, which ranalong a very long distance ([5] p.31). However this does not make Al-Kindi one of Galen’sfollowers. Al-Kindi never mentioned that the air thereby become percipient as Galen hadstated.

3.2 What’s wrong with Euclid’s visual ray?

Although Al-Kindi accepted fully Euclid’s opinion about the extramission theory, there aresome points concerning visual rays, which he had to disapprove:

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Figure 4: ([5] p.29)

• As stated by Euclid, visual rays are geometrical, one-dimensional lines ([5] p.24). How-ever, one-dimensional lines do not have width and length; they hit the object only onpoints. Since points do not have size, it is impossible to perceive points and the targetedobject will not be visible at all ([1] p.216). Therefore, Al-Kindi claimed, the visual raysmust be three-dimensional.

• It is mentioned in Euclid’s work that there suppose to be gaps between the visual raysand this is the reason why the sight grows weaker as the distance between the objectand the eye grows; the size of the gaps increases and so the section which should becovered. Al-Kindi once again disagreed with Euclid. If visual rays are separated byspaces, seeing an object completely is impossible; all you see are “spots” of your actualvisual field ([5] p.25). Thus, visual rays should be continuous to enable a completeperception.

3.3 Perception Sensitivity of visual rays

Al-Kindi also took his time observing the perception sensitivity within the visual cone. Euclid,or rather his followers, argued that it depends on the length of the ray: The shorter the raythe clearer the object will be perceived. Al-Kindi took this assumption for impossible: anobject put on the axis but farther away from the eye is perceived more clearly than an objectplaced nearer to the eye, but at the periphery of the cone.In order to explain this phenomena Al-Kindi has thought up another theory: Consider a

visual ray as candle light; two candles can lighten a room better than one ([5] p.28). Therefore,the more visual rays fall upon an object, the clearer it will be perceived. As shown in Figure4. the circle ADGB is the part of the eyeball that sends visual rays. From A, a visual raycovers the field TEH. The middle part B of the eyeball emits the visual ray ELZ. And fromG a visual ray proceeds and lightens IZK. As can be seen, the marginal sector of the visualfield is only covered by a single ray. The further you go to the middle LB, the more visualrays that cover the field. This way if an object is positioned nearer to the main axis LB of

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the visual cone, it will be perceived more clearly ([6] p. 28). This is due to the increasingnumber of the rays falling upon the object seen.

4 Conclusion

Now, the theories discussed above will be reviewed. The Atomists assumed that vision occurif the object ”touches” the sense organ. This happens either when the air between theorgan and the target is compressed or if particles of the object fly off and enter the eye.The second assumption was more popular; it was adopted and modified by many scientists.Plato improved the intromission theory mentioned above as following: A kind of gentle lightproceeds from the eye, then hit sunlight and both lights act as a connector between the eyeand the object. This way, the object’s particles are able to enter the eye. Aristotle disagreedwith both concepts and constructed a new one. Aristotle meant that light is reflected by theobserved object and then enters the eye. A medium inbetween is responsible for transmittingthe motion of the colour particles. Only then visual perception is possible. However, hedid not develop an entirely new idea because the colour theory was a modification of Plato’sinterpretation. Euclid noticed that the visual perspectives were never mentioned in the formerconcepts. So he established the seven postulates which explain the mechanism of sight. Buthe eventually left other aspects out of his work, leaving others to complete the visual theoryhe defined. Ptolemy was the one who attempted this successfully. The last ancient theory isintroduced by Galen. He mentioned of a visual spirit pneuma which travels along the opticnerve, connecting the eye and the brain in order for the soul to sense an object. Galen alsodid a great contribution to optic by delivering a detailed structure of the eye As one of theearliest Arabic philosophers Al-Kindi studied and translated the works of the ancient Greeks.He attempted to improve and introduce the ancient concepts to the mass. He adopted Galen’stheory of visual spirit and Euclid’s as well as Aristotelian concepts.As one can see, the understanding of eye mechanism is advancing bit by bit; at first thetheories were based on speculations. These then were extended and improved by youngerphilosophers whose considerations resulted more and more from objective observations. Thisway, the knowledge of optic has been enriched greatly through the history, until today.

References

[1] Adamson, Peter: Arabic sciences and philosophy - Vision, light and color in al-Kindi,Ptolemy and The ancient commentators, New York, 2006

[2] Ausbuttel,Frank M.;Bohning, Peter; Emer, Wolfgang: Grundwissen Geschichte, Klett2003

[3] Brieger-Wasservogel, Lothar: Klassiker der Naturwissenschaften: Plato und Aristoteles,Leipzig, 1905

[4] Hecht, Eugene: Optik, Oldenbourg, 2005

[5] Lindberg, David C.: Theories of Vision from Al-Kindi to Kepler, Chicago, 1976

[6] Wade, Nicholas J.: A Natural History of Vision, MIT Press, Cambridge, 1999

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