Vision Science in Classical Antiquity

Introduction

Classical antiquity refers to the golden age of Greco-Roman civilization that lasted approximately from 800 B.C. to 500 A.D. The birth of philosophy in ancient Greece played an important role in the development of science of optics. Ancient Greek and Roman philosophers developed optics as one of the branches of the natural sciences. Understanding mechanisms of vision and light was very important to Greek and Roman philosophers because vision was considered the most important sense, and the idea of curing an eye disease was considered even more miraculous than it is today. Because Greek and Roman philosophers were also highly religious, they attempted to explain vision not just rationally but in a manner that was compatible with their religious and spiritual ideas. Some of the ancient theories of vision and optics described below sound very strange to us when we study them, but to Greeks and Romans of the time, these theories were remarkable achievements of their philosophers as they brought them closer to understanding their world. The following sections describe theories developed by various Greek and Roman philosophers on vision.

Democritus (460 B.C. - 370 B.C.)

Democritus developed the idea that in order for human eye to see an object, the object must physically come into contact with the eye. This idea is referred to as intromission theory. The object accomplishes this by pressing the air between the eye and the object. The pressed air then holds the color of the object and travels to the eyes where it comes into contact with them, resulting in vision. Democritus also studied the anatomy of the eye and figured that the eye is composed of water, and water not only fills the eye but can also travel through the optic nerve located in the back of the eye. ^[2]

Democritus also claimed that there are four basic colors: white, black, red, and green. The rest of colors are produced by mixing these four basic colors. ^[3]

Right image: Democritus' eye constructions.

Epicurus (341 B.C. - 270 B.C.)

Epicurus' theory was similar to Democritus' theory in a way that he also claimed that the object seen must come into contact with the eye. However in Epicurus' opinion, vision occurs when particles from the object travel to eyes and contact them rather than pressing the air. Although this would result in gradually shrinking the object, the object does not shrink because particles from surroundings fill empty spaces in the object. ^[2]

Plato (427 B.C. - 347 B.C.)

Plato took intromission theory developed by Democritus and Epicurus and enhanced it with a new theory called extramission theory, which states that vision occurs when light comes out of the eye and hits objects outside. Thus, eyes are light producers. Plato imagined light from a ball of fire emanating from the eye and combining with sunlight to hit the object seen. When this occurs, object releases "flame particles," similar to the idea of Epicurus. The colors that we see are those flame particles. Flame particles come in different sizes, which give rise to variations in color. Bigger particles generally produce darker colors, while smaller particles produce lighter colors.^[2]

Aristotle (384 B.C. - 322 B.C.)

Aristotle did not believe his predecessors' theories to be right because he could not prove them by his experiments. Instead, he decided to develop his own theories. Aristotle was much more rational in forming his ideas and relied more on observations than imaginations. He claimed that only luminous objects such as fire produce light. He described light as being something immaterial. He also claimed that sunlight is reflected from objects and reflected light hits our eyes for vision to occur. The medium that light travels through is something transparent so that eyes can see through. This theory is the basis of modern optics. Aristotle did not accept either extramission or intramission theory, stating that eyes could not have possibly produced light. Otherwise, we would be able to see even in a dark room! Aristotle also stated that it is impossible for an object to shrink and enter the eye as there was no evidence of this occurring.^[2]

Euclid (325 B.C. - 265 B.C.)

Euclid, the great geometrician, explained geometrical aspects of vision. In explaining his ideas, he assumed that light emanates from eyes. His book Optica explains characteristics of light in which he stated several postulates. His postulates claim that visual rays emanate from the eye and if rays hit an object, the object will become visible. These visual rays form a cone with apex of cone present in the eye. His other postulates explained that the size and position of the object depends on angles of rays hitting the object. The closer the object to the eye and the more it is in line with eye angle, the more rays would hit the object and the brighter the object would appear. ^[2]

Ptolemy (100 A.D. - 175 A.D.)

Ptolemy agreed with Euclid that light rays emanate from the eye. He also added that Plato's idea that visual rays are same as sunlight, in effect enhancing Plato's idea of eye light combining with sunlight. Ptolemy also touched on the idea of resolution by adding to Euclid's ideas that clarity of objects seen depends on where the object is put in the eye cone. He also argued that the apex of visual cone is situated in the center of cornea.^[2]

Galen (133 A.D. - 200 A.D.)

Galen was a great eye anatomist. He made the most precise eye drawings of the time by dissecting monkey eyes. He also formulated a very strange but interesting idea called pneuma. Pneuma is an optical spirit that connects the eye with the brain by traveling through the optic nerve. He described retina as a structure that allows pneuma to travel along nerves and let soul interact with images captured by the eye. He also explained that the cornea helps protect the eye. Galen turned lens into a centerpiece of the eye by arguing that pneuma in lens contacts air surrounding the eye such that surrounding air becomes eye-like. He even argued that cause of cataracts lies between lens and cornea and removing this area would allow one to see again.^{[4] [5]}

Right image: Eye constructions according to Galen.

Vision Science in Middle Ages

Introduction

The fall of the Roman empire in the 5th century embarked an end of classical Greco-Roman civilization, and consequently European studies in vision science declined severely. While most of Europe suffered through the dark periods during the Middle ages (lasting approximately 500 A.D. - 1400 A.D.), Islamic civilization reached its heights during the Middle ages. By the 7th century, the Islamic empire spanned from modern day India all the way to Spain. This led to a new era in history in vision science as many Muslim scholars translated ancient Greek and Roman discoveries and developed their own theories of how vision works. The following sections describe theories developed by various Arab scholars on vision.

Al-Kindi (801 A.D. - 873 A.D)

Al-Kindi was a great early Muslim philosopher. He is known to have written some 250 books. He also made several contributions to vision science. He defended Euclid's extramission theory and rejected intromission theory of earlier Greek philosophers. He argued against intromission theory by arguing that the eye's spherical structure suggests it is not designed to collect images. Rather, motion of eyes indicate eyes to have to move something, which Al-Kindi assumed were Euclid's visual rays emanating from eyes. Although Al-Kindi agreed with Euclid's general idea, he refined some of his concepts on visual rays. First, Al-Kindi claimed that visual rays could not be one-dimensional lines as Euclid theorized because one-dimensional lines would only hit the object at points. And since points have no size, the object would be invisible no matter how many lines hit the object at different points. Instead, Al-Kindi argued that visual rays must be three-dimensional so that rays would be able to illuminate the object. Second, Al-Kindi rejected Euclid's idea that the reason eyes get weaker is because gaps develop between visual rays as the distance between eyes and objects increases. He stated that if there are any gaps between visual rays, objects would never appear smooth. Thus, visual rays must be continuous.

Al-Kindi's second contributions were on theory of perception. Al-Kindi conjectured that an object put on eye axes but far away is seen more clearly by eyes than an object that is put nearby eyes but on peripheral sides. Had Al-Kindi known about fovea and its ability to focus objects more clearly than peripheral retina, he would have championed this idea. ^{[1] [2] [10]}

Hunayn ibn Ishaq (808 A.D. - 873 A.D.)

Living around the same period as Al-Kindi, Hunayn described vision in great detail. His book Ten Treatises on the Eye was the first systematic textbook of opthalmology, described the structure of the eye, eye diseases and their treatments, and various remedies and their effects on eyes in extraneous detail. For instance, he described the lens as "white, transparent, luminous, and round." He also described treatments for conjunctivitis, cataracts, and trachoma. Furthermore, he described vitreous humor, retina and its connection to the optic nerve, as well as cornea, conjunctiva, and uvea. However, in describing function of the eye and its parts, Hunayn based much of his theory on mystical ideas similar to Galen's idea of pneuma, rather than physiological studies. For Hunayn, each part of the eye was a being in its own nature, and each part connected to two other parts of the eye in a very consistent order. Hunayn believed the eye to be a highly harmonious ordered structure. In his mind, each part of the eye had its purpose, and each part existed to serve its purpose. The lens, for instance, existed to survey the outside world. Thus, there was not any part of the eye that was not required for proper function. ^{[3] [4] [6]}

Right image: Anatomy of the eye according to Hunayn.

Alhazen (965 A.D. - 1039 A.D.)

Alhazen, another Muslim polymath, is regarded as the father of optics. His book Book of Optics explained modern intromission theory of vision, which was proven correct through his experiments. The key to Alhazen's success on developing a correct theory was his reliance on observations and his ability to design and run careful experiments, and develop new hypotheses based on results of experiments. He essentially created the modern scientific method on which all science is dependent upon today. He could be called the first modern scientist.

Alhazen debunked several old ideas of vision. In debunking extramission theory, he reasoned that when our eyes look at brighter objects such as the sun, they suffer and get injured. This would not occur if eyes were emanating light themselves! Thus, light emanates from the object seen by the eye, not by eyes themselves. Alhazen also stated that light from the object is emanated in all directions. Alhazen developed six conditions upon which light enters eyes and brings about perception. First, there must be a certain distance between the eye and the object. Second, the object must be within the visual field of an eye. Third, the object must either be a self-illuminating body or be able to be illuminated by light coming from another object. Fourth, light from the visible object must be able to hit the crystalline lens. Fifth, the medium between the object and the eye must be transparent. Sixth, the object must be dense and opaque. Otherwise, light would simply pass through the object and the object would be transparent. Already, we can see that Alhazen had developed the laws of reflection, refraction, and transmission. Alhazen also denied the existence of visual rays by conjecturing that there was no evidence for them. In describing how each area of the object is distinguished, Alhazen considered each area as a point from which a single ray emanates in a directions. Thus, light emanates from all points of the object and these rays move towards the center of the observer's eye, which then detects the object.

Alhazen further contributed to optics by describing reflection from curved and plane mirrors and refraction of rays. His Book of Optics was translated from Arabic to Latin in 12th and 13th centuries and was widely used in the European and Islamic world. In fact, his book was the standard for optical theory until Newton published Principia Mathematica. ^[5][8]

Vision Science during Renaissance

Introduction

Renaissance was a period of European rebirth that lasted roughly from 14th through the 17th century. New ideas developed by Islamic scientists and philosophers were very influential in bringing Europe out of its dark ages. Renaissance scientists used ancient Greek and Roman ideas on vision as well more recent ideas of Muslim scholars, especially Alhazen's ideas of optics and scientific method, to develop their own theories. The rise of rational thinking and the use scientific method was the key to the success of these Renaissance thinkers. The following sections describe theories developed by various European scholars during the Renaissance period on vision.

Kepler (1571 A.D. - 1630 A.D.)

Kepler, the great astronomer, also contributed to theory of vision and optics. Being very religious, Kepler believed light was the essence of God. For Kepler, light was something that connected the spiritual world with the material world. Kepler also attempted to relate light with the soul. He believed that light is associated with life through soul. He thought that God added light to the material world so humans could interact with God not only spiritually but also physically. And it was the eye that interacts with the spiritual essence of light.

Kepler's greatest contribution to vision science was on function of the lens. Kepler was the first person to propose the idea that the lens focuses images onto the retina. This idea would later be proved correct by Descartes. ^[1]

Leonardo da Vinci (1452 A.D. - 1519 A.D.)

Leonardo's success in vision science lies in the fact that he was a good observer. One of his major achievements was to show that light passing through a tiny hole forms an inverted image. Leonard made a small hole in a heavy paper and in a dark room observed an image cast on the wall from light passing through the small hole. He noticed the images was inverted. This was a landmark experiment at the time as it showed that the idea that light travels in straight lines is accurate. This allowed Leonardo to conjecture that the pinhole could be a simple model for the eye. In the eye, the pupil could serve the purpose of the pinhole and the back part of the eye could serve as a screen or wall where the image is projected. And the eye's pinhole was even better than a paper pinhole because the pupil was not static but instead could contract or expand and hence regulate the amount of light that goes into the eye. Leonardo observed this phenomena carefully, observing that the size of pupil is smaller during daytime and larger during nighttime. ^[2]

Right image: Leonardo's demonstration of refraction by cornea.

Descartes (1596 A.D. - 1650 A.D)

Although Descartes spent most of his career developing philosophical ideas, he made a hugely significant contribution to vision science by proving Kepler's idea that the lens focuses image onto the retina. In an experiment, Descartes surgically removed the eye from an ox and scraped the eye's retina to make it transparent. He then decided to hold the ox eye in front of the window so that the eye was looking outside the window. To Descartes' surprise, he saw an inverted image of the outside world projected onto the retina! Hence, not only did Descartes prove Kepler right but also discovered that the lens forms an inverted image onto the retina.

Newton (1643 A.D. - 1727 A.D)

Newton's contributions to optics, and perhaps to science in general, were of greater importance than any other scientist before him because it enabled humanity to think about light and vision in a completely new way. His book Optiks was one the most revolutionary books published in history and still enlightens modern science. His most famous discovery in optics is that white light (coming from the sun) is composed of several different colors, which could be separated into individual colors if white light is passed through a prism. This discovery was not only a more thorough explanation of refraction but also shed light on how various colors are produced by different objects. Newton decided to investigate further on color. He observed that color light does not change its properties when it reflects, refracts, or is transmitted through different objects. This led Newton to develop his theory of color, which states that the color of objects depends upon their interaction with colored light, and that objects themselves do not generate color.

Newton embarked on a completely new journey in the field of optics when he first proposed the idea of wave-particle duality of light. Newton realized that light was such a phenomenon that only considering it as a wave or only considering it as particles could not explain all of its characteristics. To Newton, light could be composed of particles (now called photons) as its particle nature explained how particles of light could be refracted by accelerating towards the denser medium as they moved from a less dense to a more dense medium. At the same time, light could be made of waves because Newton could explain the idea of diffraction better using waves. Newton had already entered the realms of quantum mechanics and even though quantum mechanism does not agree with Newton's ideas, Newton was the one who gave birth to this field. Newton's ideas would later greatly influence future ideas on interaction of light and eyes and their implications for visual processing. ^[3]

Right image: Artist's conception of Newton separating sunlight into color components.

References

1. Lindberg, David, C. The Genesis of Kepler's Theory of Light: Light Metaphysics from Plotinus to Kepler. Osiris. 2nd Series, Vol. 2, (1986).
2. Ackerman, James, S. Leonardo's Eye. Journal of the Warburg and Courtauld Institutes. Vol. 41, (1978).
3. Shapiro, Alan, E. The Evolving Structure of Newton's Theory of White Light and Color. Isis. Vol. 71, No. 2, (Jun., 1980),
4. Lindberg, David C.: Theories of Vision from Al-Kindi to Kepler, Chicago, 1976
5. Lindberg, David C.: Alkindi's Critique of Euclid's Theory of Vision. Isis. Vol. 62, No. 4, (Winter, 1971), pp. 469-489
6. Eastwood, Bruce Stansfield. The Elements of Vision: The Micro-Cosmology of Galenic Visual Theory according to Ḥunayn Ibn Isḥāq. Transactions of the American Philosophical Society Vol. 72, No. 5, (1982)
7. Meri, Josef. Midieval Islamic Civilization. Taylor and Francis, (2006).
8. Lindberg, David C.: Alhazen's Theory of Vision and Its Reception in the West. Isis. Vol. 58, No. 3, (Autumn, 1967)
9. Wade, Nicholas J.: A Natural History of Vision, MIT Press, Cambridge, 1999
10. Putri, Ika. Ancient Theories of Vision and Al-Kindi’s Critique of Euclid’s Theory of Vision. Isis.