Ancient Optics

Many ancient philosophers worked on explanations of our senses. Touch and taste were fairly easy as they involve direct contact, and people were able to figure out that smell is conveyed by vapors entering the nostrils and hearing was because of a noise-making object’s motion causing sound and the sound travels to the ears. Vision, on the other hand, was a bit trickier, for a couple of reasons: A. it is effective over vast distances, for example, the stars; and B. It is done in an effortless manner, the object does not need to do anything in order for itself to be seen. There were several prevailing theories:

Theories of Vision

Extramissionism

This theory was proposed by the geometers. They believe that just like touch and taste, the feeling body needs to actively reach in order to see. The extramissionists believe that the eye can emit a material called ocular rays which is a very fine, light, and straight material that is given off from our eye, and it travels with an infinite velocity. Aside from the velocity, the shape of which ocular rays consist is also described. In Euclid’s Optics, he thinks that the shape encompassed by these lines of sight is a cone with its vertex in the eye and its base at the limits of the object seen. The rationale for this theory, they propose, is that first, unlike sound, in which the object being heard needs to make effort to make noise, the object being seen is effortless, therefore sight should behave in an active fashion like touch. Secondly, even when there is no light in the environment, some people can still see in the dark, and most importantly, we can see the light in nocturnal animals' eyes. It was believed that the goddess Venus lit the fire in the animal’s eyes.

Intromissionism

This branch of the theory is called Intromissionism, which also believes in the traveling of material. However, as its name indicates, it suggests it is in the opposite direction—which is from the object to the eye. The most famous exponent of this theory was the Epicureans, the atomists. They suggest that every visible object emits a fine film called the eidola, which is in the same shape as its contour, to the eye. They posit that ordinary objects behave just like fire, only in a dimer way. In specific, the white wax part of the candle emits light just like its fire. They also think that mirrors act as an “eidola-trap” that catches and reflects the stream of films that travel through the air and makes them visible.

Fusion Theory

The third theory is a combination or mid-ground of the first two. Plato proposes that there are three different streams of “fire”: 1) one coming from the eye, 2) from a source of illumination, and 3) one from the seen object. He wrote in the Timaeus that it is a precondition for the sight that the first two streams fuse: “whenever there is daylight roundabout, the visual current issues forth, like to like, and coalesces with it and is formed into a single homogenous body in a direct line with the eyes”. Then, fiery particles from a colored object impinge on this fused body, causing the observer to see white or brightness if the atoms are smaller than those in the body, and black or darkness if the atoms are bigger (45b–46c, 67c–e). Plato thinks that the mirror is a perfect demonstration of his theory since it shows a fusion of the two currents.

Aristotle's theory

The last theory is brought up by Aristotle, which does not involve the movement of a certain matter. Aristotle noted that four factors need to be present together for vision to occur: a seeing eye, a transparent medium, a visible body, and some form of illumination. Later Stoic and Galen devised a similar theory that doesn’t involve the transportation of material, but involves pneuma, a soul-like substance that was believed to govern all physical activities in the world, and of which each individual soul was composed. It is believed that a person’s pneuma can assimilate with a section of air that is also cone-shaped with its apex at the eye and its base on the surface of the visible object. After the assimilation, the cone of air became a sensitive extension of the soul into the environment and works as a walking cane of the eye in sensation.

Explaination of colors

Atomists

The atomists think that just like anything else, colors can be explained by atoms and void. They suggest that “What is smooth is white; for if something is not rough, casts no shadow, and is not hard to penetrate, it is in every case bright ... Black is made from the opposite, from [sc. atomic] shapes which are rough, unequal, and dissimilar ... things made of larger shapes will be more red…Yellow-green is composed of both solid and void, its hue varying with the order and position of its particles.” (On the Senses 73–75). Every other color is a mixture of primary colors.

Plato

Similarly, Plato thinks the colors are also because of atoms. But instead of the shape of it (because he thinks the nature of atoms—fire, water, air, and earth—determines their shape, and color can only be fire atoms, therefore, tetrahedron), he suggests that colors are caused by different sizes and force levels of atoms. Namely, white was produced when smaller fire atoms “dilate” the visual current, and black, is caused by larger atoms that “contract” it. “Brilliance” and red are pseudo-primaries, produced by powerful fire atoms which force the eye to water.

Aristotle

Aristotle thinks the colors are all mixtures of black and white which are aspects of ordinary brightness and darkness. For the Stoics, finally, color was a kind of body—or, to be more precise, the visual aspect of pneuma, the material that holds each object together. When the visual cone comes into contact with an object, the object’s pneuma actually assimilates the air of the cone; this qualitative change then propagates backward until it passes into the eye and eventually the soul.

Theories in Real Life

Now we are familiar with the different theories, let's turn to a case study.

This is the famous temple of the Parthenon on the Athenian Acropolis. As we know, there are not many, if any, straight lines or right angles in the Parthenon but it sure looks like a giant rectilinear construction. For example, the base of the temple is actually domed and the four corners of the pedestal droop downward. The ancient architects used optical refinements in the building to make the illusion to avoid an optical “sagging” of the building’s middle, how would ancient philosophers explain that?

The Epicureans, who believed in eidola, thought that in order for an eidolon to travel to you, it will need to pass a great amount of air and the air produces collisions which force its impact to become weak, thus the perfectly straight lines were perceived as curved and thus we have to over correct it for the lines on the Parthenon’s to occur straight.

Stoics, however, went on to account for illusions and cases of misperception by invoking a qualitative degradation of the cone, caused either by the “weakness” of the pneuma within the eye, or the dissipation of its “strength” as it attempted to reach objects which lay too far off, but transform it to match our pneuma’s shape and thus causing the illusion.

Examples of Optics used within the Ancient World

The Nimrud Lens

In 1850, Austen Henry Larard discovered the Nimrud Lens at the Assyrian palace of Nimrud. This lens from the 8th century BCE has the ability to magnify objects viewed through it with perfect clarity up to seven times the size of the original object. With significant distortion, it can even magnify objects up to twenty times their original size! For reference, the magnification of an average magnifying glass is between two to three times the size of the original object, making this lens much more effective. However, there is little evidence that this lens was used to actually magnify objects

Face of an Egyptian Statue

Egyptian statues are famous for having eyes that seem to follow one around the room. Egyptians realized that some gemstones reflected light, with different materials used depending on what effect was wanted for the statues. The rocks acted as lenses, reflecting light so that your eyes would see them "lit up" no matter your location in the room. This gives an extremely eerie feeling as if being watched, and almost gives some life to the statue. It is not hard to believe that the Egyptians liked this effect.

3D Printed Recreation of a Roman Sundial

This Sundial was found in a village near Pompeii in the 18th century, but was only recreated to figure out how it worked in 2017. It is known as the "pork clock" since it bears a striking resemblance to the shape of a ham. The gnomen is in the shape of a pig's tail, and casts a shadow across the distorted grid. when held vertically. When the tip of the shadow is lined up with the vertical line that represents the current month, the place the shadow falls shows what solar hour it currently is. In the image above, it is four hours from sunrise or sunset.

Narrative: Lenses in the ancient world

If you were asked to think of a lens, you would probably think of a magnifying glass, or glasses. Many think that the term lens equates to having a degree of magnification, but this is simply not true. The term lens actually refers to any object that has the ability to bend and refract light while you look through it. Lenses serve many purposes nowadays, and served many purposes in antiquity. In the modern era, one of common use of lenses is to magnify objects, whether that is in a school science class, as glasses for people with myopia or hyperopia, or within a telescope. Another extremely common use of lenses today is as jewelry. Gemstones are praised for their beauty in how they seem to "sparkle". This sparkling is caused by them acting as a lens! Their brilliance refractive index determines how much light bounces around inside the gemstone. The more light bounces inside the stone, the more it seems to "gleam" and catch our eyes.
In the ancient world, lenses were also used as pretty things to look at. Many lenses have been found in temples, and many have been found as jewelry. Romans wore many gemstones, including opals, emeralds, diamonds, topaz, and pearls which are all refractive gemstones. These gems would be used to make earrings, bracelets, rings, brooches, necklaces, and diadems and even anklets. We see gemstones used in the statues to catch your eye, which was especially common in ancient egypt as gemstones for the eyes of statues make them seem to watch you as you walk by them, giving a creepy sensation. Unlike us however, there is not much evidence that the ancients used lenses for magnification. The evidence that supports their use is thatmany techniques they used for creating wax seals, and certain architectural techniques seem impossible without magnification. However there are two problems with this argument. First, the lenses that have been found are all quite unweildy, and thus would make doing intricate designs a real challenge. Second, since these techniques were so widespread, if they did require magnification, we would expect to have some recording of the use of lenses. But this is simply not the case. We do not have any writings that support this theory. In fact, the earliest evidence of magnification comes in the C.E., when a man named Seneca who read a scroll while peering through a bowl of water, which made the scroll larger.

There is one more use of lenses in the ancient world, that of creating fire. Copius amounts of lenses have been found that were likely used to burn things. These lenses would be carried in a pocket, and had a hole cut in the middle of them so that a rope could be used to carry them. The hole suprisingly did not lower the efficacy of the lenses. Some evidence for the ancients being well aware of the ability of focused light to cause fire comes from Aristophene's play the Clouds, in which there is a reference to seeing a lens burn some wax. Other evidence comes from a myth where Archimedes, being a genius, designs a massive mirror to reflect and focus sunlight at the sails of ships, burning them down.

Above: Painting of Archimedes burning the sail of a ship with a massive, concave mirror

Video: Click here here to see a demonstration of burning things with a lens!

Above: Ancient Roman Necklace

Interactive: Click here to virtually explore the pantheon!

Narrative: The Sun in Antiquity

The sun played an extremely important role for humans in antiquity. It gave life by allowing crops to grow, and defined a person's "day". People woke at sunrise and went to sleep at sunset. And, during those days, the sun would be used to tell how long they had left in their day. Sundials were invented sometime before 1500 BCE, but the exact time is unknown. In fact, there is some evidence of even earlier humans using shadows to tell time! Obelisks built as early as 3500 BCE acted like a proto-sundial, which split the day into morning and afternoon, and showed which days were longer and shorter. A sundial consists of three main parts. First, the gnomon is a branch that is pointed North. It casts a shadow that lands on the hour marker. This hour marker told people how many solar hours there were from dawn or until dusk. They are called solar hours since their hours were shorter in months with shorter days, and longer in months with longer days. Finally, there was some form of a dial, which changed what month the sundial would tell time for. Sundials were practically everywhere in antiquity. Over 500 sundials have been found in Rome alone. Most ancient sundials were fixed in place in public areas. One sundial could serve an entire community. There have even been portable sundials found, that could fit into one's pocket! In terms of accuracy, ancient sundials were not perfect, but generally, they were accurate at least within the half-hour, which is pretty good.

The sun likely also played an important role in symbolizing power and the heavens. The Roman Pantheon was a temple thought to have been built to all the gods, but it may have been more than that. The hole in the roof of the Pantheon, called an oculus, allows the sun to cast a singular beam of light, which perfectly lines up with the doorway on the vernal and autumn equinoxes. In addition to that, on April 21st, which is the traditional founding of Rome, the ray of light streams perfectly into the middle of the building. There is also evidence that the emperor had some connection to the sun, and there is evidence that the Pantheon served as not only a Temple but also an Imperial monument. Thusm, taken together, it seems like the Pantheon helped connect the Roman emperor to the gods, using the symbol of the sun.

Annotated bibliography

Hannah, Robert, and Giulio Magli. “The Role of the Sun in the Pantheon’s Design and Meaning.” Numen, vol. 58, no. 4, 2011, pp. 486–513. JSTOR, http://www.jstor.org/stable/23045866.

This paper explores the Roman pantheon in great depth, arguing that the key to understanding the role of the temple lies in understanding how the building interacts with the sun, and undertanding the apotheosis of the Roman emperor. The paper brings up two major pieces of evidence supporting the Pantheon's connection to the sun. First, the hole in the top of the roof of the pantheon allows the sun to cast a beam of light inside the building. This beam of light just so happens to be cast directly above the door on both the autumn and vernal equinoxes. Second, the design of the roof of the pantheon is extremely similar to several other imperial monuments, which were made to have a roof as a sundial. The article then links the idea of the deification of Roman emperors to the thought that the sun represents the heavens. The paper concludes by stating that evidence seems to support that the Pantheon helped connect the Roman emperor to the sun, and thus the heavens and gods.

Sines, George, and Yannis A. Sakellarakis. “Lenses in Antiquity.” American Journal of Archaeology, vol. 91, no. 2, 1987, pp. 191–96. JSTOR, https://doi.org/10.2307/505216.

This article starts with detailed analyses of three different lenses. The first two were found in a cave in Crete, and are compared with the third lens, which is from the palace of Knossos. The analyses include: usable level of magnification with perfect clarity, level of magnification with distortion, size and shape of the lenses, and any imperfections or flaws on the lenses. The article then pivots to argue that the similarity between lenses shows the widespread use of lenses in the ancient world, and gives several reasons that the ancient Greeks and Romans may have used lenses, including but not limited to: magnification for seals and architecture, starting fires, and simply being pretty to look at. The article finishes by presenting two opposing viewpoints and the evidence for the usage of lenses for magnification. The evidence that supports lenses used as magnification is that there is a large prevalence of seals in the ancient world and modern gem cutters have determined that the gemwork of the ancients seems impossible without the use of optical magnification. The problem is that there is no reference in anything from BCE that mentions using lenses as magnification, and lenses are mainly found in places where they would be able to be observed.

Thibodeau, P. “Ancient Optics: Theories and Problems of Vision. A Companion to Science, Technology, and Medicine in Ancient Greece and Rome”, 133-144. JSTOR, https://doi.org/10.1002/9781118373057.ch8

This is an essential paper for us to lay the ground. It surveys and covers the fundamental theories of optics. This article not only gives us a basic understanding of extramissionism, intromissionism, the fusion theory, and Aristotle's pneuma theory, but also explains colors and shapes. Moreover, it is a gate for us to explore the field and provides important sights for other curiosities such as mirrors and lenses.

Wright, M. T. “Greek and Roman Portable Sundials: An Ancient Essay in Approximation.” Archive for History of Exact Sciences, vol. 55, no. 2, 2000, pp. 177–87. JSTOR, http://www.jstor.org/stable/41134104.

This paper mentions that there were two main subtypes of portable sundials, one that could account for changes in latitude and one that could not. The one that could not account for changes in latitude was by far the more common one, which at first, one would espect to not be the case. Using a lot of complicated math, Wright finds that the difference in time detected when accounting for latitude versus when not is generally within a few minutes, with the most extreme differences being less than fifteen minutes. Thus, Wright concludes that this, combined with the fact that almost no ancient sundial subdivided hours, means that the Romans simply did not care if their time was slightly off. All sundials in antiquity were subject to error of their maker and user, and likely would be ever so slightly off, but a difference of ten minutes seems not to matter.

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