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Welcome to stuff you missed in History Class, A production of I Heart Radio. Hello and welcome to the podcast, I'm Tracy B. Wilson, and I'm Holly Fry.

We have gotten various requests over the last several months to talk about people who discovered or accomplished something during some kind of a pandemic or quarantine. It's not surprising that we're getting requests like this. By really coincidence. Today's topic almost fits with that. Abu Ali al-Hasan Ibn Al Hasan in Al Haythem, whose name has also been Latinist as Alhassan or al-Hasan, made massive contributions to the world's understanding of light and vision through experiments that he did during a prolonged house arrest.

That house arrest took place in Cairo in the early 11th century. He was one of the polymaths of the Islamic golden age. And in addition to his work on optics, he also wrote about medicine and philosophy and astronomy, math, ethics on and on. He's been on my list for a really long time. And it was only after really getting into it that I was like, oh, this almost fits with this thing that people have been asking us to do for the last seven or eight months.

Hurray.

So the Islamic golden age was a period of intellectual, cultural and economic flourishing that spanned from about the 8th through the 14th centuries. We have talked about figures from this era a few times on the show before, including Ibn Sina, also called Avicenna, who was a physician and an astronomer, and Al Kurumi, who is known as the father of algebra, the Abbasid Caliphate, whose territory covered a lot of Northern Africa, the Middle East and Western Asia, selected Baghdad as its capital in the 8th century.

Baghdad was one of the wealthiest cities in the world, and it was ideally positioned to bring in scholars and texts from all around Europe and Asia, as well as from within the caliphate's territory. The Abbasid dynasty was also really strong and prosperous, pretty stable, so it could focus on and encourage learning and discovery rather than, for example, needing to put all of its resources toward defending itself from its neighbors.

Although Baghdad was the heart of all of this, artists, scientists and intellectuals, including Christian and Jewish scholars, spread all through the Muslim world during this era, and other cities and intellectual centers were founded as well. One was Cairo, established by the Fadiman caliphate as its capital in 1969. The Fadiman caliphate also built the Al-Azhar Mosque and University complex in Cairo not long after founding the city. Before this point, the Fadiman dynasty had controlled parts of northern Africa and Sicily, and the establishment of Cairo was part of its ongoing efforts to take control of what's now Egypt.

The Fadiman caliphate was part of the Ismaili sect of Shia Islam, while the Abbasid caliphate was Sunni. So when Cairo was founded, this was part of the Fadiman dynasty's efforts to try to expand both its territory and its religious influence and then the spread of Shia Islam.

Cairo was where Ibn Al Haytham would go on to do most of his intellectual and scientific work. But he was born in Basra, Persia, which is now Iraq around 1965 because of his birthplace. Sometimes you'll also see al-Masri included as part of his name. Some sources also suggest that there were really two different Ibn al Haitham because one early source list his first name as Mohammed. But it was also common to name newborn boys Mohammed and to add another name as the child got older, at which point most sources would refer to that new name.

We really don't know all that many details about Ibn Al Haytham biography, although he did write an autobiography that is not focused on the details of his life at all. It's more about his intellectual development. The two earliest accounts of his life date back to the 13th century and their brief, and also they contradict one another in various details.

It is generally agreed that he studied in Baghdad and that this included an extensive study of religion in this region of the world, was home to the Shiite and Sunni branches of Islam, as well as Christianity and Judaism. And then within those, there were also various divisions and sects. In studying all of this, Ibn Al Haytham came to the conclusion that none of these religions really expressed the truth and that the truth itself was its own religion. So he decided to devote himself to science.

To be clear, though, this wasn't a rejection of everything that might be associated with religion. He wrote, quote, I constantly sought knowledge and truth. And it became my belief that for gaining access to the effulgence and closeness to God, there is no better way than that of searching for truth and knowledge before making this decision.

He had trained for some kind of civil service position, perhaps as a judge or a mayor, but he left that position and immersed himself in the works of previous figures like Aristotle and Galen. He eventually established himself as a renowned scholar, one of the most widely repeated stories about Ibn Al Haytham. Life has to do with a project to try to control the waters of the Nile River. The river's annual flood cycle enriched the soil of the Nile River Valley with silt, but it could also be incredibly destructive.

And as the story goes, Al-Hakeem, who was at the time the caliph of the Fadiman dynasty, wanted to find a way to control the rivers floods through hydro engineering. Either he had heard of Émile Haythem reputation or Ibn Al Haytham had volunteered for this project.

Either way, though, but Al Haythem went to Cairo to start working on this.

The logical place to try to control the river. Was it the Nile first cataract near Aswan. But after Ibn Al Haytham got there, he eventually concluded that the project was just impossible. Again, there is some lack of clarity here, whether it was because the materials available were sufficient or because he thought damming the river would cause flooding upstream. That would destroy ancient temples and other monuments there. Or if it was because it was just beyond his abilities as an engineer, whatever happened, this project was a failure.

This really should not reflect on his ability, though it would be almost 1000 years before anyone managed to dam the Nile River.

Ibn Al Haytham was appointed to a post within Al-Hakeem administration. But before long he started to suspect that his life might be in danger. Al-Hakeem had come to power at the age of 11 and at this point he would have been in power for about 15 years. And during that time he had developed a reputation for being a zealous, unpredictable and cruel ruler. Under his rule, anyone who wasn't a Shiite Muslim faced religious persecution, and some of his other policies seemed simultaneously harsh and random, like ordering that all the dogs in a city be killed because he didn't like their barking.

Even Al Haythem had failed to find a way to control the waters of the Nile, and he feared that the punishment for that failure might be severe. So he feigned madness and he was placed under house arrest. He stayed there for about ten years until al-Hakim mysteriously disappeared while out for a walk one night in ten twenty one. At that point, Ibn Al Haytham used the work that he had been doing during those years, which we're going to get into in a moment to convince authorities that he was in his right mind and that he could be released.

Ibn Al Haytham stayed in Cairo after all of this, living near the university and mosque complex. He made a living copying manuscripts, which meant that he became really deeply familiar with the range of scholarly and religious texts that he was copying. And during his lifetime, he wrote at least 100 works of his own. Some sources say as many as two hundred fifty five of them have survived until today. And these works include writings on astronomy, meteorology, medicine and anatomy, especially the anatomy of the eye, including other subjects.

But the biggest and most recognized contribution was to optics. And we're going to get into that after we pause for a sponsor break.

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Most of the ideas that informed about Al Haithem work on vision and optics came from Greece, and I find some of the Greek world's ideas about how vision works to be delightful. They are indeed.

So, for example, several Greek philosophers, including Lupus, Democritus and Epicurus, favored the concept of intro mission or the idea that something that objects were emitting was entering the eye. Specifically, they were emitting. These were thought to be tiny, tiny replicas of objects that flew off of everything in every direction.

The entry of these Ebola into the eye was what allowed people to see. I'm just going to say, if you want to go Google this, make sure to include the word vision or eye after intermission, because it also has another meaning that is related to human reproduction.

One piece of evidence fighting for this idea of tiny replicas of things flying around was that sometimes when you look at a person's pupil, you can see a little tiny image of what they're looking at in their pupil. It's almost like a tiny, tiny version of that thing they're looking at might actually be inside of there. The logic is sound, if not supported by actual science. This idea did raise some questions, though, like if there were tiny replicas of everything in the world flying around all the time, wouldn't they collide with or pass through each other causing some kind of distortion or interference, even if these replicas were so thin as to basically be one dimensional?

If they're coming from every object in the world, that is still a lot of material floating around. And then once they got into the eye, how did that translate into actually seeing and then what happened to the Ebola after it traveled into the eye? How is it possible for giant objects like mountains and tiny objects like ants to both admit I dolar that were the right size to enter the eye? These are all natural questions and they go on and on and on.

Other philosophers, including Plato and Euclid, were proponents of a different theory called XtremIO. In an extradition, the eyes themselves were emitting something described as light or fire or pneuma, depending on who you're reading this emission from, the eyes struck objects. And that was what allowed the viewer to see them. One piece of evidence cited as support for this whole idea was the fact that sometimes animals eyes seem to glow in the dark, the idea being that this glow was something from their eyes that was allowing them to see.

Euclid's explanation for this concept included the idea that the emission took the form of a cone with the pointed part at the viewer's eye, which explains how objects look bigger when they are close up and smaller when they're farther away. Some philosophers also reasoned that this force being emitted from the eye interacted with a force that was also being emitted by the object. So sort of a hybrid of these two concepts.

Ptolemy supported Euclid's ideas on this with the addition that the pneumo was actually continuous. Gaylan supported the idea of extermination as well, believing that the pneuma originated in the brain and traveled down the optic nerve before going out through the eye. But as was the case with intermission, this idea of extra mission also raised a number of questions. As one example, how could something emitted by a person's I travel all the way to the stars and wouldn't that take a long time?

Why wasn't there a delay when looking at the night sky before the stars became visible?

The various philosophers who detailed their ideas of vision also wrote about visual phenomena that just didn't seem to make sense, like the way objects appear to be bent when you view them through a layer of still water, or the fact that the moon looks so much bigger when it's near the horizon, or iridescent bird feathers or plants or mirrors just in general. And the various ways that mirrors can distort the objects that they're reflecting. For the most part, these were interpreted as errors in vision, not as phenomena that had some kind of rational explanation on their own.

Ibn Al Haytham had access to many of these ideas, including a portion of Ptolemy's optics that had been translated into Arabic and commentaries that had been written in or translated into Arabic. This included work from some of his own contemporaries, including Ibn Saule, a Persian mathematician who built on Ptolemy's work to study Linzess and refraction.

Yeah, these these ideas and variations on them were just really the commonly understood explanation for how light and vision worked. During his house arrest, in addition to working from these texts, Al Haythem did experiments with light. He observes how it passed through different substances like water and smoke, and he also worked with lenses and mirrors. At some point, as the story goes, he walked into a darkened room where a tiny, tiny hole had formed in one of the window.

Shades light from that hole struck the wall on the opposite side of the room. And when he examined that spot of light, he realized he was seeing a tiny, inverted view of the world outside the window.

The term camera obscura would not be coined until the 17th century, but that's basically what this was. Ibn Al Haytham built a version of the room in miniature, essentially a pinhole camera, and incorporated that into his experiments as well. He used complex arrangements of candles to study how the images from the pinhole changed from one arrangement to the next.

To be clear, there was not a film involved in this. It was just the light that he was seeing and what he was seeing in that light. Based on all of this work in Binzel, Haythem drew several important conclusions. One was that light always travels in straight lines. He described the thinnest possible amount of such light as the least light, which Isaac Newton would later describe as a Ray. Ibn Al Haytham also concluded that light itself was invisible and colorless, but that its existence allowed people to see because of the way it interacted with the eyes, in addition to all the experimental work he did with the camera obscura.

He also cited the fact that looking directly at a bright light can be painful and can cause a person to see after images when they look away can also damage the eyes. Clearly, looking at light directly affected the eye.

Even Latham's explanation of vision was that invisible light reflected off of every point of every surface in every direction, and once that reflected light entered a person's eye, it allowed them to see. He also concluded that this reflected light was entering the eye at every angle, but that only the light that struck the lens straight on perpendicular to the lenses surface actually passed through the pupil to be used by the eye.

Each perpendicular ray of light made one point of a corresponding image on the back of the eye. The optic nerve carried that image to the brain where it was combined with the image from the other eye.

So there are some details of this that are not quite there. For example, the cornea and the lens of the eye work together to help focus the image that's being seen, rather than it being a matter of only perpendicular rays of light, making an image point by point on the back of the eye.

Even so, though, there is a lot about this explanation that is right or really close to right. And it is way, way more correct than intermission or extra emission, along with the detailed diagrams of the anatomy of the eye. Even Latham's explanation for how light and vision worked were the most accurate and most complete for more than 500 years after he developed all this.

We're going to talk about some of Ibn Latham's other work after we pause once again for a quick sponsor break. My name is Lowell Berlanti, and I created the podcast Prodigy to find the answer to a very complicated question can genius be created? I asked academics, researchers, scientists and the prodigies themselves to gain a better understanding of intelligence, skill acquisition and expert performance. So disregard all simple explanations because complex questions require complex answers. Listen to Prodigy every Thursday on the I Heart radio app, Apple podcasts or ever you get your podcasts.

Nearly 600 years after the invention of the printing press, the most important book in the history of the world has arrived.

There might be overstating things, stuff you should know, an incomplete compendium of mostly interesting things. It will change your life forever.

Well, that's not necessarily true. Most scientists agree that stuff you should know an incomplete compendium of mostly interesting things is proof that time travel is possible because that is the only way to explain how a book this impressive was possibly made and why that stuff you should know. An incomplete compendium of mostly interesting things will regrow hair, whiten your teeth and improve your love life.

That's just not at all. Right.

Well, the love life part, maybe if you find someone who thinks smart is sexy stuff, you should know an incomplete compendium of mostly interesting things available for preorder. Now at stuff you should know Dotcom.

Now, that is true. Given Al Haythem turned his study of light and vision into a seven volume work called the Treasury of Optics, the first three volumes were dedicated to the topic of sight, the next three to reflection and the last to refraction.

The volumes on sight also included, as we mentioned earlier, extensive work on the anatomy of the eye. And for about 600 years after he finished this work, this whole total seven volume thing was the most extensive, detailed and correct work on vision and optics in the world. The next major work on this subject was Johannes Kepler astronomy upas optical or optical part of astronomy, which came out in 16 04. And it wasn't until the 19th century that his explanation of binocular vision was really improved on.

But as we mentioned at the top of the show, like many other scholars of the Islamic Golden Age, Ibn Haythem was a polymath. In addition to his extensive work on optics, he also published treatises on astronomy, mathematics, medicine, all kinds of stuff. So we're going to hit some of the highlights. In addition to that seven volume work on optics, Ibn Al Haytham also published a lot of other work on light and human's perception of it. This included on the light of the moon, on the halo and the rainbow, on the shape of the eclipse and a discourse on light.

He also wrote on the subject of burning lenses, that is lenses that can focus light to the point that it can cause objects to ignite. Some of his writing on lenses also suggest that he understood their potential to magnify, although lenses made to actually do that weren't developed until the 13th century. He also discussed and wrote a solution for a problem that Ptolemy had first articulated in about the year 150, which is now known as al-Hasan Problem or al-Hasani billion problem.

Essentially, here's how it goes. Imagine a source of light and a spherical mirror. How can you find the exact point on the mirror that will reflect light into the eye of an observer? It's called the billiard problem because you can ask the same basic question using billiard balls and at exactly what point one would need to strike a ball in order for it to bounce from the edge of the table and strike another ball. Ibn Al Haytham solution to this problem involved a series of six geometric proofs.

Ebele Haythem also wrote on astronomy, including on the configuration of the world, and he combined astronomy and optics in the trace on the moon's face. At that point, people didn't really know what exactly caused the dark pattern that is on the surface of the moon when it's viewed from the Earth. But as was the case with how vision works, there were lots of ideas for that in the trace on the moon's face. In all, Haithem rules out the idea that these are shadows cast by features on the moon because such shadows would shift based on the angle of the light hitting the moon.

He also rules out the idea that they were a reflection of things on the Earth because those reflections would also change based on where on the earth you were when you observed the moon. They do not change that are the same from everywhere.

He concludes that the moon's dark areas are dark because they're made from a material that doesn't reflect as much light as the rest of it. Which is true. Yeah, this is a long time before again, like early. Sixteen hundreds is when people first observed the moon through a telescope. So he's way ahead of the game here.

Ibn Al Haytham also wrote about why it is that the moon looks so big when it's low on the horizon, concluding that thick vapors in the Earth's atmosphere might be a factor, but that it was mostly just a matter of perception. He also wrote about mathematics, preferring no theory and geometry over algebra in terms of geometry.

He wrote about squaring the circle, that is, figuring out how to create a square that would be the same area as a corresponding circle. It doesn't appear that he totally solved this problem. He wrote a treatise on the subject that still survives and doesn't include the solution. It refers to a forthcoming treatise that has not been unearthed yet at this point and may not have survived until today. He also wrote Doubts Concerning Ptolemy, which I just love as a title that analyzed the second century mathematicians major works and corrected what he saw as errors and inaccuracies in them.

In addition to work on optics and astronomy and other work on physics and math, Ibn Al Haytham was a practicing physician and surgeon, including developing a method for removing soft cataracts from the eye.

Using a hollow needle in the vast majority of his work email, Haythem followed a method that was really, really similar to the scientific method. As it's described today, he would make an observation. Form a hypothesis and then test that hypothesis using experiments. These experiments allowed him to gather data that he could analyze with that analysis, either proving or disproving the hypothesis. He was by far not the first person to include experiments in their work. But a lot of earlier scholars weren't so much as conducting an experiment as they were performing a demonstration.

They were kind of choosing their experiment in order to illustrate what they already thought was the right answer. Meanwhile, Haithem, on the other hand, was designing experiments to test a hypothesis. For that reason, sometimes he's called the father of the scientific method, or the first scientist, Ibn Al Haytham died in Cairo in about 10 40 at the age of roughly 74. In the decades after his death, the Turkish Seljuk dynasty invaded Baghdad and crusaders invaded the region around Cairo.

These and other factors may have kept Ibin Latham's works from being widely disseminated until the late 13th century. That was when mathematician Kamal el-Din Al Faricy published an interpretation of his work on optics in the 13th century. Someone also translated the Treasury of objects into Latin. That translation went on to influence the work of people like Roger Bacon, Yohannes Kepler, Leonardo da Vinci and Rene Descartes. Yeah, there were other translations later that we know who did them, but that first one is kind of a mystery.

But all Haythem work translated into Latin or summarized and interpreted was widely circulated in 13th century Europe, credited to this Latin ised name of Alhassan or al-Hasan. Medieval European scholars nicknamed him Ptolemaic Cecum. This or the second Ptolemy. He was well known enough that he's referenced in the 13th century poem, The Roman de la Rose. And then the Squire's tale from Geoffrey Chaucer's Canterbury Tales and the 14th century of an El Haythem work also started to be translated into Italian and affecting people's understanding of the world.

Their. In 2015, to mark the one thousand the anniversary of Emmental Haythem seven volume work on optics, UNESCO had the International Year of Light. This was a year of programs and events related to light, science and light technologies. A short film was made as part of this called 1001 Inventions of the World of Ibn al Haytham. It combines live action and animation. And it was Egyptian actor Omar Sharif's final film role for the International Year of Light.

Ibn Al Haytham story was also made into a National Geographic Kids book, and we will end with a quote from Ayman al Haythem from his doubts about Ptolemy. Quote, The Seeker after truth is not one who studies the writings of the ancients and following his natural disposition puts his trust in them, but rather the one who suspects his faith of them and questions what he gathers from them, the one he submits to argument and demonstration and not the sayings of human beings whose nature is fraught with all kinds of imperfection and deficiency.

Thus the duty of the man who investigates the writings of scientists. If learning the truth is, his goal is to make himself an enemy of all that he reads and applying his mind to the core and margins of its content, attack it from every side. He should also suspect himself as he performs his critical examination of it so that he may avoid falling into either prejudice or leniency.

Oh, the trickiest part. Right? That's true. That's yeah, I as I was, I loved this quote. And I also feel like in the world we're living in, we definitely see cases of people definitely doing the first part of questioning the work that has come before, but not as much. The second part about examining their own selves to make sure that there is not prejudice or leniency involved. Yeah. In what they're doing. Yeah. Oh, do you have a listener mail?

I do. This is from Jessica and Jessica wrote, Hi, Tracey and Holly. I was listening to your episode on The Demon Core, and you mentioned Operation Crossroads. Funnily, this wasn't the first time I'd heard of it. And the story behind why is kind of humorous. At one point I was going through old pictures with my grandfather of his time in the Second World War, serving in the Navy on the USS Columbus. We were looking at normal naval ship life.

And so we turn the page and there were multiple pictures of atomic mushroom clouds. Absolutely not. What I was expecting to find, especially when he told me that he himself had taken the pictures.

According to him, his ship was part of the process to determine what would still be functional on a ship after a nuclear bomb was detonated above it. And there's mushroom cloud pictures are from that experiment. He said that the men were taken off their ships. The bomb dropped and then brought back to the ships to inspect their area. Obviously, this is a horrible idea. And many of and he said many of his shipmates got cancer and he was lucky to be an electrician working at the bottom of the ship.

So his area was spared most of the radiation servicemen who were a part of Operation Crossroads have nifty little cards proclaiming it and he still has this card. I have no idea how to verify any of this story other than what little I can piece together online. So take the story, as you would all remembrances of a 92 year old man. But the pictures are still incredibly cool to have. Long story short, my family has these random pictures of a nuclear test and I have no idea if they should go to a museum or just stay in the family.

But I have attached the pictures because I felt like fellow history lovers. You might enjoy looking at this yourself. I just ask that you don't share them without giving credit to my grandfather. Thanks for being so entertaining at work. My co-workers and I love to hear your science related episodes the most because we are all analytical chemists and our complete nerds. Jessica. Jessica is the best, worst pick up line my co-workers have ever made was after listening to the Demon core episode.

It goes, Hey person, you're so hot. You make my Geiger counter melt.

Here's the thing, if you say that to someone and they get it, laugh and then maybe want to go on a date with you, you might have found your person.

Yeah, I mean, that's true. I just love this whole story.

I'm just like flipping through some family, you know, photos back during their service days. All of a sudden a mushroom cloud. What in the world? Oh, cool.

Thank you so much, Jessica, for sending this email.

The attached pictures were indeed very dramatic, and I would have found them quite startling if I were looking through them with my grandfather.

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