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Excurse to the History of Image Concept and Vision: from Pythogars to Kepler

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Excurse to the History of Image Concept and Vision: from Pythogars to Kepler EXCURSE TO THE HISTORY OF IMAGE CONCEPT AND VISION: FROM PYTHOGARS TO KEPLER I Igal Galili H P The Hebrew University of Jerusalem T S Jerusalem 2010 EXCURSE TO THE HISTORY OF IMAGE CONCEPT AND VISION: FROM PYTHAGORAS TO KEPLER Igal Galili the Hebrew University of Jerusalem e-mail: [email protected] Keywords: Optical image, light ray, vision, eidola, extramission, intromission, image transfer Abstract We address teachers of high school physics course with this excurse which deals with the concept of image in physics. The excurse started from the classical Greece where scientists suggested several theories of optical image. These theories provided qualitative accounts for the phenomenon of optic image. These were holistic theories that considered the optical image of an object as a whole. Hellenistic theory of Euclid introduced visual rays that scan the world around the observer and thus supported vision. During the mediaeval period, the progress was made by Alhazen–a distinguished Arabic scholar who disintegrated image into many points each transmitted to the eye by light rays: point by point by means of a single ray. The further development was reached during the Renaissance, during which the new understanding suggested the similarity of human eye to a camera obscura, leaving as an open problem the issue of an inverted image. Eventually, the problem was resolved during the scientific revolution by Kepler in Germany. It was understood that human cognition presents a separate stage in the process of vision, during which the inversion of an image takes place in cognitive interpretation. By displaying the whole history of optic image – from holistic image to the image comprised by light flux – we make emphasis on several features of the theory of vision, which shed light on the nature of science. This includes considering scientific progress that takes places in a discourse between competitive theories. Physics theory seeks a differential mechanism that explains the considered phenomenon and points to an experiment that may reproduce this phenomenon. Physics knowledge grows in a cumulative process although knowledge accretion is a simple growth of a number of details but modification of the central idea and thus exchange of the theory through a conceptual change. Researches on students' knowledge revealed certain similarity of students' accounts for vision with the old and obsolete in science ideas from the past: the contest between intromission and extramission understandings of vision, between holistic and differential understanding of image creation, between the image transferring by light flux versus that by single rays. The similarity between the central ideas suggests using old ideas and the debate in science resulted in their rejection to cause remedy of common misconceptions in contemporary students. Thus, the historical excurse to the history of image becomes relevant in teaching and learning optics at school. ******************************************************************** 2 Excurse to the History of Image Concept and Vision And God said, let us make man in our image, after our likeness: and let them have dominion over … every creeping thing that creepeth upon the earth. (Genesis 1:261) Thou shalt not make thee [any] graven image, [or] any likeness [of any thing] that [is] in heaven above, or that [is] in the earth beneath, or that [is] in the waters beneath the earth. (Deuteronomy 5:82) So we all were instructed, first, that each of us, humans, presents an image of somebody, and second, that we are prevented from making the same, that is, from producing an image of something. This is a very heavy statement, which at least implies to know what image is and which way it is given to our perception through vision… It however appeared to be not a simple matter to know what image is. To understand it we will follow its history. ******************************************************************** I. Understanding of vision Introduction Reconstruction of an object appearance by means of light constitutes optic imagery and is a subject of optics – the theory of light and vision. The idea of optic image was central for people's understanding of light and vision in the course of a long history. Understanding of image, its origin and the ways of creation occupied human mind before science was established. Myths and tales incorporated various images but the inquiry of science regarding image is unique in the kind of knowledge it provides and the way that the knowledge was established. This we will demonstrate here. The inquiry in light and vision were inherently interwoven from the beginning of Natural Sciences (Natural Philosophy) in classical Greece. It was only much later, not before the scientific revolution of the 17th century that they split into physics of light and psychophysics of vision as we know them on our days. In accordance, our tracing the growth of image understanding will mingle the major ideas invented by people to account for light with those of vision. 1 http://www.bible-history.com/kjv/Genesis/1/ 2 http://www.bible-history.com/kjv/Deuteronomy/5/ 3 Excurse to the History of Image Concept and Vision Hellenic Science The first scientists of the Hellenic cultural period in Greece provided at least four competing theories of vision. The first one was introduced by the school of Pythagoras. It stated that an internal fire is radiated by observer's eyes, reach the observed objects and cause the observer to see them (Fig. 1). For the direction of activity in the process of vision, this theory was termed extramission. Soul Internal Fire Object Observer Direction of Action Figure 1. Schematic representation of the Pythagorean extramission theory of Vision We see here the interpretation of vision as a sense of touch in the observer, in parallel with other human senses: smell, hearing, etc. This understanding might be related to the much earlier understanding of light in the Ancient Egypt as we may observer in their drawings of more the thousand years old. As we see in figure 2, rays of light coming from the sun terminate with hands that symbolize the ability to touch the illuminated objects. Pythagorean extramission theory of vision continued this perception of touch but this time by the vision flux coming out of the observer eyes. This understanding looks reasonable. Indeed, the initiative of vision definitely belongs to the observer who turns his eyes to the object to see it. We "focus" our sight on the object. Strangely enough for us, the adherents of this understanding reasoned also by the convex surface of eyes: other senses like 4 Excurse to the History of Image Concept and Vision hearing and smelling show a kind of concave surface. So, since eye possesses convex surface it must radiate, not adsorb. They also appealed to the observation of animals, especially cats, who are known by their sparkling sights at night. The opponents of this theory, however, argued by questions "why don’t we see at night? or "Why do we see immediately after opening eyes the distant stars?" Both questions had no answer within this theory. The second theory was due to the school of Atomists of Democritus. They stated that each body around us produced a sort of its replica, an image (eidolon) (Fig. 3). Comprised of atoms of that object, eidola leave it traveling in all directions in space until entering the eye of an observer, causing the effect of vision. Apparently, this theory should be considered as intromission theory since the process of vision presumed entering some physical entity into the eye of the observer. Eidolon Soul Object Observer Direction of Action Figure 3. Schematic representation of the Atomists’ intromission theory of vision The opponents of this theory, argued by questioning, "how the eidola of big objects around, such as mountains, succeed to enter into a small eye" or "why do not we see images of things on walls?" Moreover, the same question, "why don’t we see at night?" remained without answer within this theory too. The third theory is usually referred to Empedocles Empedocles (a distinguished philosopher from Sicily) and Plato in Athens. This approach in a way synthesized the Plato two previous ideas, suggesting understanding of vision as resulting from a meeting of three fires (Fig. 4). Firstly, the "pure fire" of the ambient daylight "not admixed with other primary bodies". In contrast with flame, it "flows off flame, 5 Excurse to the History of Image Concept and Vision and does not burn but gives light to the eyes". Secondly, "internal fire" – the visual stream of the same kind as daylight, contained in the eyeball and capable of issuing out in a stream (emanating from the pupil) towards the object seen by the observer. Thirdly, the "external fire" radiated off by the observed objects and related to their colors. It is comprised of particles proportioned to those of the visual current, so as to yield sensation.3 Unlike the previously mentioned theories, in the theory of Empedocles-Plato, the vision process takes place only under the influence of light ("pure fire diffused in the air by Sun") interwoven with the internal fire of the observer. Ambient light somehow induces the radiation of the fire by the objects. Light fire Soul Internal Fire External Fire Object Observer Direction of Action Figure 4. Schematic representation of the Empedocles-Plato’s hybrid theory of Vision. The forth theory was also extremely creative. It was the intromission theory of vision due to Aristotle himself4. He distinguished color – an inherent feature of objects – from light, which he understood as a non-material trigger for the medium (air, glass, water) to be transparent and allow vision (Fig. 5). It is the color of the observed object, in view of Aristotle, that Aristotle causes a special compression in the medium between the object and observer which reaches the observer and is perceived by his eyes.
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