Historical and Contemporary Theories of Sight


Sight is one of the most stimulant senses in our body. When we are born, assuming everything went well, we open our eyes to experience our world. But throughout our history, we theorized how we are able to see. What causes us to see? What phenomenon gives us this gift? Many great minds tried to figure out these questions, even though a lot of these people were wrong, it set a path for future scientist to base their work and try to find the correct answer. We come to know that light is the main component of sight; the eye is able to bend it and produce color and images.

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It is important to understand the past in order to understand the present. One of the first theories would be the Emission theory. Of course it’s been replaced, but it is a significant part of the history of optics… At the time, a challenging theory was the “Intro-mission” approach. Both these theories helped led a path for scientist to understand the true understanding. It allowed many scholars to choose sides and try to prove the other wrong.

Then came modern theorist, first being Hermann Von Helmholtz. And like many theories, it was challenged by Ewald Hering theory “opponent process”. Throughout the 1800’s, many scientists argued theories about color and sight. One of the biggest topics was the theory of color; many scientists came up with theories and team up with other scientist to try to figure out how color is created. In the 1900’s the Gestalt theory raised many questions by people who study the science of vision.

This essay is to answer, how the human eye operates by analyzing past and modern theories about sight. It is important to know how our body works, especially one of the most important sense, sight.

Historic Theories

The Atomists

In ancient Greek, many people believed in gods and supernatural powers, but then there were the Atomists. They were the first who were able to come up with neutral, non-religious ideas. Democritus, who lived between 460 B.C .E– 370 B.C.E, stated that the air between the eye and the object that is seen is “contracted and stamped” by the object and the eye that observed it. The air holds the various colures of the object and appears in the observing eye. Then there was Epicurus, (341 B.C.E -270 B.C.E), stated that particles flow from the object into the eye, but the body would not shrink because other particles will replace the empty space. Both of these theories are based on the same principle, the object can only be seen if it directly comes into contact with an observing eye. For Democritus, the essential part of vision is air. For Epicurus, the presence of particles is the most essential part. Democritus discoveries were pretty close to right. He stated that there were four basic colors, white, black, red and green. Democritus was right about two of the colors.


Another theorist, who lived around the same time as the Atomists, is Plato (427 B.C.E. – 347 B.C.E). Plato’s theory combines the intromission theory, like the Atomists, and the emission theory, which states that light hitting the eye is the cause of vision. Plato stated that a ray comes from the observing eye, so the light will only let the same ray to pass through. The inner light in the eye fuses with sunlight to form a “homogeneous body”, located directly with the eye. The object being observed lets of “Flame Particles”, similar to the concept by Epicures. If the object being observed is placed within the homogeneous body, the particle has the ability to enter the eye and soul, casing sight. Stated by Plato, colors come from the flame particles from an object. When compared to the ray, there are three different types of color. If the size of the object’s particle is equal to the size of the ray, then the object is transparent. If the particles are bigger, then it is a dark color. If the particles are smaller, then it’s a light color.


Aristotle (384 B.C.E – 322 B.C.E), relied on his senses and put his faith in his results of his observations. Aristotle could not prove the theories of the Atomists and Plato, so he went against them and came up with his own theory. Aristotle strongly believed that light could not be solid because it was not fire or a physical object. So what is light? Aristotle observed that objects, such as fire, can produce light on its own. Light must not be a physical material, but an immaterial transparent particle. To Aristotle, the emission and intromission theory did not make sense. If our eyes produce casual rays, then we should be able to see at any situation. But we cannot see with our eyes closed or in a very dark place. Aristotle disagreed with Plato because he could not find an explanation how light can collide with sunlight. Aristotle’s theory of sight was considered advance for its time, sunlight is reflected by an object and then transmitted through a medium to the eye (which is basic knowledge for modern theories). The only way vision can occur, is when a medium is in between the eye and an object. If you put an object in front of your eyes, you most likely won’t be able to see it (or slightly see it) due to there not being a medium in between the eye and object. What is this medium? Aristotle believes that this medium must be transparent, so we can see through it and see the object. Color lies on the object and allows motion to the transparent medium. This allows light to be transmitted to your eye. Vision occurs when color and medium interact with each other. Aristotle was also interested by the anatomy of the eye. He concluded, after cutting eyes of animals that the eye consists of three coasts covering a humor.

Euclid and Ptolemy

Euclid theorized about the geometrical aspects of vision. Euclid came up with the emission theory, since it involved visual rays that derive from the eye. But unfortunately, he cannot explain why one can perceive things. However, he does describe the visual perspectives. Euclid’s optics theory is based on his seven posits.

There are indefinite rays coming from the eyes
The rays form a cone of which the vertex is located at the eye and the base is located at the limit of your vision.
Things that fall on the cone becomes visible
Objects seen at a larger angle appear larger
Objects seen at a higher visual ray come out higher
The further right an object is stricken by the visual ray, the more right the object is seen
Objects seen under more angles are observed more clearly

The first three rules help explain concepts of visual rays that comes from the eye like Plato theorized. Rules 4 – 6 explains how the size and position of an object is depended on the angle being observed. The last rule explains the clarity of an object; the further the object, the bigger the visual cone is. This causes less visual rays upon the object, causing a less clear image.

One of the greatest followers of Euclid was Claudius Ptolemy. Ptolemy continued Euclid’s theory by adding psychological, physical and physiological to his theory. Ptolemy agreed with Euclid’s rules stating visual rays emerges from the eye is a shape of a cone. Ptolemy added that a visual ray has the same aspects as sunlight. The idea came from Plato’s teachings, which states that when both visual rays and sunlight hit, they will form a homogeneous body. Which means visual light must be a consistent body. Euclid mentioned that there are a medium in between the visual rays, which illustrates why someone cannot see clearly at certain times. Ptolemy disagreed with Euclid on that statement; Ptolemy stated that there is only a single visual ray emerging in the shape of a cone. It will be impossible to see an entire object at one time. He also argues that rays only illustrated the geometry of sight, not reality itself, like Euclid seem to think. Ptolemy also brought back Aristotle’s theory of color. Ptolemy added that color produces a modification in the visual cone; Aristotle only explained that color cannot affect the visual cone. In Aristotle situation, it is the transparent medium without the existence of an external light. But sadly, most of Ptolemy work is unclear due to the loss of Ptolemy work. Ptolemy created two geometrical assumptions to Euclid’s theories. First, the clarity of an observed object with the visual cone might vary depending on its position. An object located placed over the main axis is recognized more clearly than an object located in the perimeter of the visual cone. Second, the pinnacle of the visual cone is situating directly at the center of the cornea.


Galen, a scientist from the Roman Empire, studied the structure of the eye. Galen would dissect monkeys and oxen to study their anatomy. Galen was able to psychological and physical elements for his theory from these experiments. Pneuma, an optical spirit, travels along the optic nerves connecting the eye and the brain. While in the eye, pneuma meets with air surrounding the eye and changed to match its nature. Because of this, the air converts into an instrument of soul, and becomes perceptive. Galan adapted this theory from the Stoics, Galen also has his own ideas; these entire pneuma take place in the crystalline lens located in the middle of the eye. As a result, the lens is the vital appliance of vision. Impaired vision leads him to this conclusion. The cause of impaired vision falls between the lens and cornea; if removed, you are able to see again. Galen was able to almost fully understand the knowledge of the eye structure from his studies. Galen was able to locate the lens of an eye and mentioned the existence of the retina, which allowed pneuma to travel through nerved and allows the soul to interact with the images grabbed by the eye. The cornea’s purpose was to be a protective layer for the inner parts of the eye.

Modern theorist

Young – Helmholtz

Thomas Young and H.V. Helmholtz developed the trichromatic theory of color. The theory is based on the observation of the mixing of colors and states that we can create new colors by just mixing the three primary colors; blue, red and yellow. Through many experiments, Helmholtz and Young found out that the vision of color relies on three receptors located in the retina. Each receptor has different spectral sensitivities to wavelengths. The three cones wavelengths are short, medium and long. Each having a specific wavelength and peaks of light absorption; “long (560nm), medium (530nm), and short (420nm)”. The tree receptors are stimulated by light at different degrees and patters which will result in the formation of a color. Which allows us to determine what color would form if the lights from different wavelengths are combined due to the reaction of each receptor. One of the main supporting evidence for Young – Helmholtz’s trichromatic theory was a color matching experiment. This experiment found that a wavelength in one filed is paired by altering the amount of three different wavelengths to one another.

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