In Our Time - The Eye

Episode Date: February 27, 2014

Melvyn Bragg and his guests discuss the eye. Humans have been attempting to understand the workings and significance of the organ for at least 2500 years. Some ancient philosophers believed that the eye enabled creatures to see by emitting its own light. The function and structures of the eye became an area of particular interest to doctors in the Islamic Golden Age. In Renaissance Europe the work of thinkers including Kepler and Descartes revolutionised thinking about how the organ worked, but it took several hundred years for the eye to be thoroughly understood. Eyes have long attracted more than purely scientific interest, known even today as the 'windows on the soul'.With:Patricia Fara Senior Tutor of Clare College, University of CambridgeWilliam Ayliffe Gresham Professor of Physic at Gresham CollegeRobert Iliffe Professor of Intellectual History and History of Science at the University of SussexProducer: Thomas Morris.

This BBC podcast is supported by ads outside the UK. Every Sunday, we talk about the week's tech news on this week in tech. Hi, this is Leo Leport. inviting you to join me this week with Lisa Schmeiser, Dan Patterson, and Yanko Rekkers. We're going to talk about the new 49 megabyte web page. It's the standard, you know. We'll also talk about Elon Musk. You've got some spleenin to do and the Yassify filter, new from Nvidia.

That's this week on this week in tech. You'll find it at Twitter. or wherever you get your podcasts. Thanks for downloading the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash radio four. I hope you enjoy the program. Hello, in the collection of Cambridge University Library

is a modest notebook which belongs to Isaac Newton when he was a student. As well as notes about the mathematical textbooks he was reading, it contains the detail of some of his earliest experiments, One of these, which would horrify modern researchers, entailed inserting a knitting needle behind his eyeball to see what effect this had on his vision. Newton's dangerous experiments were part of a two-and-a-half-thousand-year quest to understand the nature of human vision.

Ancient thinkers were fascinated by the eye, believing it contained a mysterious, illuminating fire. In later centuries, it became known, in Leonardo's famous phrase, as the window of the soul. But establishing the structures and properties of the eye took many centuries and the work of some of the greatest figures of Western thought. With me to discuss the history of the I are Patricia Farrah, senior tutor of Clare College, University of Cambridge,

William Ailiffe, Gresham Professor of Physics at Gresham College, and Robert Eiliff, Professor of Intellectual History and the History of Science at the University of Sussex. Patricia Farah, who were the earliest thinkers to study the workings of the hour that we know about, and in what context did they undertake that? Well, the first theories that we really know about were by the pre-Socratics in 5th century BC in Greece. And one of the most famous of these was Empedocles.

And their basic idea was that there's some divine fire inside your eye that goes out, and it's almost like the feeling of touch. You stretch out your fingers to touch something and feel it and learn about it. And similarly, the fire goes out from your eye and enables you to learn about what it is that's in front of you. And you can see relics of that idea in one English. We talk about, we say something like his eyes flashed with anger or his eyes gleamed with appreciation.

You get similar imagery in Homer, although I think it probably meant something more real to Homer than it does to us. So this idea is called extramission. The idea that... Extramission. Extromission. The idea that something is going out being sent from Missou is the Latin for I send. So something is being sent.

out of your eye to grasp and perceive the outside world. There's some fairly obvious objections to that theory. One is it doesn't really explain why you can't see anything at night. Another problem is it doesn't really explain what happens when you look in a mirror, for example. So there are various difficulties. So over the centuries, people refined that theory. One of the most important people to do that was Plato, and he, as had many other people, noticed that things like the sun or a candle also give out their own light.

So he developed a more, sort of slightly more interactive theory. So there's a special divine light that comes out of your eye, and it interacts in some strange way with the ambient light. And there's a very nice image of that in the poem The Ecstasy by John Dunn, when he describes lovers' eye beams twisting together into a double-yceding. thread, and so Plato had this sort of thread notion that the fire in your eye is linked to the ambient light. And after Plato, there's Euclid, who was the student of one of Plato's students, and he worked out the geometry of optics, but he followed the platonic idea of extra mission, so he had the idea that

light comes from a cone, at the point of a cone, right in the middle of your eye, and spreads out. He worked out all the geometry, all the angles of incidents and reflection, but of course that geometry works exactly the same way if the light is coming into the eye as we now believe. So that was the one tradition of very strongly believing there's some sort of rays that come out of your eye. The other type of theory was introduced by the atomists, and they wanted tiny particles,

they had this idea that if you look at something, a tiny skin of atoms, perhaps just one atom, thick, creates something called the idelon, which is like the shape of the object, and it transmits through the air, and somehow mysteriously enters into your eye. So there's two different traditions.

Intramission, that's intramission, yes, when the idelon somehow comes to your eye. I mean, there's all sorts of problems about why doesn't the idelon lose shape when it goes through water, or how is it if you're looking at a mountain, you can get a whole mountain into your eye. So there are a lot of problems with that theory, too. But those are the two

basic opposing ideas of the ancient Greeks. Will Ayelif, what context was this discussion taking place in? Was this, again, part of the Greek's pursuit of pure knowledge for the sake of knowledge, or was there any practical reason behind it? There were both of those are correct. It looks experimentally, if you look at an animal by a fire at night, you see the eyes reflecting. And so it was very easy for them to imagine that the,

fire was coming out of the eye. And similarly, if you look at someone closely, you can see a reflection, an erect image of yourself and anything behind you in the cornea, which again would suggest that these little simulchre or idea were coming off objects and being implanted on the seat of vision. They were starting to introduce surgery for eye diseases at this stage, particularly couching for cataract. So in order to do this, they needed to have some theory of vision, so it was being driven by practical considerations as well. Did Aristotle's thought have any impact on this?

Aristotle's had completely changed the way people were thinking. He's very much a supporter of an intramission theory, but through a complicated process, he believed that the transparent medium instantaneously coagulated with the object that was being seen and somehow impressed an image on the organ of sight, which in those days was believed

to be the crystalline lens. Now, how would actually do this? So where does he stand in the extramission, intramission? He's very much on the intramission side of things, and in fact, his theories are going to become dominant. You can do the mathematics of rays of light, whether it's intermission or extramission.

So even the later writers Euclid can do very complicated maths. It doesn't matter whether it's intermission or extramission, but it does matter. when you're actually going to consider how sight is actually working. So the Aristotelian ideas are the ones that become dominant in later thought and particularly picked up in the early medieval period. I'm sorry to interrupt. You see as it were setting the right course,

setting the right line for future thinkers on this, a line that's most profitable to follow? Yes, and certainly his... Because we don't know about a man's influence stretched from, for about 1,500 years, and right into the middle age, his people are referring to him, So Aristotle says this, we will continue in that line, aren't they?

So do you think he was on the right lines? Well, absolutely, because the earlier medieval people were able to prove that extramission was incorrect. And they were also to show that skins coming off objects, and the objects didn't gradually disappear like ice, again meant that a thousand-year-old objects were still the same shape and size as there were a thousand years earlier, which they couldn't think they were shedding their skins all the time. So Aristotle becomes immensely influential.

And all of these theories, as Patricia was saying, still have echoes in our modern world. But as so often happened, then, the people who took it on, developed it, added to it and so on, was the, let's call it the Arab civilization from about the 8th and 19th, to the 12th, 13th century. The Islamic world took this up very cleverly and assiduously.

Could you describe that a little bit of a question? Absolutely. Well, when the court moved, to Baghdad, the influence of the Persian thinkers and philosophers from centuries before became very prominent in Arab thought, and many experimentalists were to come along and experiment with light rays. We have Ibn Sal who discovered what we now call Snells' law hundreds of years before it was recognised in the West, and this is how you can measure the mathematically the bending of a ray of light in a christmas.

structural structure. We have great translations done of the Greek works. De Aspectibus, translated again into Latin. And all of these thoughts are going on at this time. It's a very exciting time. There's practical experiments being

done as well. And in fact, some of the thoughts of Aristotle this coagulation go on and prefigure Descartes and Descartes that there is a solid but transparent medium with the balls actually of the atoms to transmit information. Let's turn the Arab world for a moment, though. Rob I live,

the most celebrated authority on the eye in the Islamic world was a scholar known Al-Hazan. What did he add to the knowledge? Well, Al-Hazan is arguably the greatest obstacle theorist of all time. He was born in 965 in the Common Era and was put under house arrest in about 2010 in the common era. And for the next 10 years, he engaged in a whole series of great optical experiments, some of them psychophysiological, dealing with sort of unseen movements of the eye, dealing with binocular vision. He did a whole series of experiments with mirrors. But perhaps his greatest achievement is really to try and understand how there is a one-to-one correspondence between what you see and what there is in the outside world.

Can you develop though? Yeah, sure. If you imagine the way these, people are thinking at the time most of the people in the Arabic tradition are arguing that from every point in every part of the world there is if you like a spherical array

of rays leaving at each point if that's true then all these rays should be coming in through the pupil in the eye and causing a kind of infinity of confusion in what you see

and what Al-Hazen did was to attempt a theory to explain why it is that we don't have confusion and why it is that we see things, as Will said earlier on, on the lens. Was this a convincing theory? It is convincing in its own right. I mean, it's essentially the idea that we only see rays that come into the pupil perpendicularly. So any other rays, any other light that doesn't come in perpendicularly or is refracted by the cornea, as they saw it, into the lens, is actually not seen at all.

Now, that's implausible, but his account does explain this one-to-one correspondence. His work wasn't as neat as this, but as we're brushing through two and a half thousand years, we move on. The medieval European thinkers, Gross Test and Roger Bacon, could be said to have developed, built on his work. Could you tell me how they did that?

Yes, I wouldn't say the gross test, builds that much on his work. In fact, Gross Test has got a very sophisticated and interesting metaphysical theory about the importance of light. If you like, light is the sort of primary energy of the universe. It's what causes and is the result of the great Big Bang in his theory. But what Gross Test does do is he improves, if you like, the Aristotelian theory of sensible forms. For Aristotle and his followers, it's sensible forms that are the things that emanate from objects and that come into the eye. Gross test cause these things species

and what they are remains somewhat problematic. But what he says is taken up by the other great theorist of the time, Roger Bacon. Who adds what? Well, Bacon is a great synthesis. He's somebody who really brings together in a slightly implausible way all the things we've heard so far. So he takes on board the,

the al-hazan intramission theory where rays come into the eye. He also is interested in the idea that Gross Test has that actually there are species that resemble the original thing seen that come into the eye and sort of implant themselves on the lens in the eye. But Bacon peculiarly also has an extramission theory and this is perhaps the most peculiar of the theories that people will hear today. But Bacon essentially says that although there are species of these objects,

it's coming into the eye, the soul, as it were, the soul of the body, which reaches to the lens, actually projects its own species outwards into the medium between the person and the thing seen. And as it were, in his words, it ennobles the medium to allow you to see things veridically, that is, as they really are. So, Bacon strives, Roger Bacon strives to bring together to synthesize all these previous theories. And his work is extremely influential through two of his followers. Picham and Wittalo in the 14th, 15th and 16th centuries, all three of those write books with the title de Perspectiva,

which is why they're known as the prospectivists. Patricia Farah, we haven't talked about anatomy yet. Can we talk to the... Sorry, can we discuss the great Renaissance anatomist Andreas Vassalius and what he brought to this accumulation of knowledge is coming from different pathways to discover more about the eye? Yes, Vasilius was by far the most famous Renaissance anatomist. Have we got a particular, can you give us a specific date here?

Well, his big book came out in 1453, the same year as the book in which Copernicus put the sun at the centre of the universe. So 1453 was a very, very, very, sorry, I mean 1543. 1543 was a very, very important year. So that was the date that he published his big book on anatomy, and he was a Flemish doctor who came down and who studied in Padua in Italy, in Italy. And the thing that he aimed to do was to overthrow previous theories, but also to overthrow the previous style of medicine. So one very important person that we haven't mentioned yet is

Galen. And Galen was originally a Greek surgeon, and then he went and worked in Rome for the Emperor. He carried out a lot of dissection. Most of his dissections were on baboons. He knew a lot of human surgery, because one of his jobs was to patch up the gladiators. And, and he carried out, and And he also did, for the eye, he did a lot of experiments on oxey's rather than on human eyes. And Galen was convinced that it's the lens, not the retina, it's the lens that's the seat of vision. And this idea amongst many others that Galen absolutely held sway right through into the 16th, even into the 17th century. And when Vasilius did his dissection, he tried to overthrow Galen's ideas. and he also tried to show that the way an anatomist should work

is by looking directly at the body. You should work from the body rather than from books. But for some reason he wasn't particularly interested in the eye. And when you look at his anatomy of the eye, it still shows traces of the Galenic influence. And the same is true of Leonardo da Vinci, who's the drawings he did of the eye are very Galenic with the lens right at the very centre.

Well, did Leonardo add much? He added a wonderful phrase window to the soul, but did he add much to the development of thought about the eye? He did, but unfortunately it's one of the very few areas where he was entirely wrong. Again, he was taking the Golanic view. He believed in the importance of the ventricles of the brain. He did some wax castings and confirmed that Galen was correct and that the first ventricle,

what we now call the lateral ventricles, was two connected together, and there were two others. So the idea was that vision or any other special sense came in. It went through a filter and then went into the first ventricle, the common sense, the communa sensor. Again, an idea that persists to the days. And then it goes for further processing and finally into memory, which is the final ventricle at the back.

This was ideas that have been developed by the early church. They were very important for the religious thinking. Why was that? Because they were moving the idea of the seat of the soul from the heart and putting it into the brain and the importance of the brain. And Leonardo describes a particular impassiva, which is the area that receives the sensations before they go into the ventricles. And this is an anatomical structure that's never been found before.

nor since. So unfortunately in this aspect, he's not of much help to us. Coming back to the great Franciscans, of course, when we look at Bobby Grosstead, of course he didn't have our Kassan because he hadn't been translated by Gerald Cremona at that time. Roger Bacon did have these. So we're

looking at thoughts that are coming in from the Arab world and changing the Western world and the way it's thinking and developing its ideas. You brought up that idea of influence, and going to Leonardo da Vinci, although, of course, we all absolutely adore him and are very fascinated by all his drawings. He had very little influence because everything remained in manuscript, and he never published it. So although he's a key figure for us, he was not a key figure at the time.

So I think when you consider the history of any sort of science or intellectual activity, looking at publications and how ideas were spread is absolutely crucial. One of the problems we have when we look at the eye and the brain is how dissections were done. The first day of dissections after prayers was when you actually took out all the rotting organs in the middle cavities, the lungs, the guts, the liver and the lights. You then did the dissection of the muscles. And then on the third day, you opened up the scalp, cut into the head, and then you anatomized the brain, which is why Vreselius's drawing of the brain is not an accurate representation. In fact, it wasn't until that great anatomist

and Gresham professor Christopher Wren, who was sometime latterly an architect, he did the first correct drawing of the human brain through pickling it and using wax injections of the blood vessels was able to prove Leonardo was wrong and there wasn't a retinatus miraculous which is present in animals,

which is why Galen had written about it, but it's not present. It's the circle of Willis and the Gresham College is moved to Oxford at this time to Wadham. There's a great synthesis of great thinkers, and Willis is there, and Wren actually does many of the drawings and antotopical things for Willis's great book. Rob Ilyne, if we're coming to the 17th century,

the Scientific Revolution, great figures, and I want you to talk about Johannes Kepler, but we must, I think, pay however brief attention to, we're hinted at by Will, the theological dimension to this. Theologians, many of whom were extraordinarily brilliant men within the canopy that they accepted as the intellectual perimeter of their lives.

They let there be light, God had created light, and so it had to go back to that. Did that have an influence on the way investigations were pursued? I think the eye is seen as the most spiritual of the sensory organs. it has to be said the organ which is most concerned with lust and it has to be conditioned, it has to be disciplined in a very important way, whether you're a Catholic or a Protestant.

The other issue, I think, is what we call natural theology and the way in which natural theology and Aristotelianism go together. The fact is that for Aristotelians, which is nearly everybody in this period, that there is such a thing as authentic seeing, if you are not mad or blind, God has created your lens, your eye, so that you will see things as they really are. It's as if the eye itself, let alone the soul or the person behind it, but the eye itself has been made by God with the purpose of seeing things veridically.

And in a sense, the wonderful construction of the eye, when you look at it, it is made to see, but also when you look at it and analyze it, it bespeaks a divine creator. Can we talk, Johannes Kepler, was he, well, let's talk about his work. He made a major contribution. Was he, in passing,

was he a believer, a religious man? Yes. Yes, right. So, Johannes Kepler. Well, you can make a big argument, I think, for saying that the first decade of the 17th century is the most important decade in the history of science.

Galileo makes his discovery of the telescope. Kepler himself writes his great book, Astronomy, Nova in 609. But Kepler himself makes this totemic extraordinary intervention with his work, which is a, it's

a hymn, a Pian to Wittalo in 604. And what... Who came from Roger Bacon? Yeah, well these are the Franciscans. Picham and Wittalo are these two people who were the first one, what Picham knew Bacon, but their works

are the most influential in the scholastic medieval period. Kepler goes to work with the great Danish astronomer Tico Brachey in 1600 and he's interested in looking at Bracki's observations and he wants to know whether you can make corrections for refraction because he knows that light is refracted through the atmosphere and what this takes him to is a long, deep analysis of the structure of the eye and there are two sources that are very important for him.

One is a work on anatomy by a man called Felix Platter and Platter in a sense shows definitively that the the prospectivist account of the role of the lens is false. The prospectivists had argued that the lens was attached to the retina and that anything that knocked the lens would through motion or some other means influenced the way you saw things. Platter himself suggested that the retina was the seeing part of the eye but Kepler also, for reasons we'll go into,

thinks that's not true. The other source is the natural magic tradition of Gian Battista Delaporta. And Delaporta uses this object called the camera obscura, this box with a hole in it that displays images and it's very important for Renaissance painters. And what Kepler does is he modernizes Al-Hazan's work on the eye. And he shows that if you consider seriously the role of the lens, in refracting light that comes in through the pupil,

that lens will always refract one point in the outside world to a corresponding point on the retina. So in a sense, Kepler does something quite remarkable. What he also shows is that there is an inverted left to right, upside down image painted, as he says, as it were in a picture on the retina itself. Patricia, Patricia Varrett, you spoke earlier that whatever Leonardo did, he didn't publish so he didn't have much effect.

Kepler did publish, do we take it that, Rob was just saying, this was perhaps the most influential time of the scientific revolution, as may be, but did we take it, do we take it that this was now in the open among scholars who were interested in this sort of thing, and he began to move forward

this study in a more purposeful way? Yeah, I think it's, in a way, it's a transition between optics, what we would now call optics, being part of astronomy, because in the Aristotelian cosmos, the laws of light were unique in that light behaved the same

on the earth as it does out in the heavens and the laws for the earth and the heavens were mostly very different in Aristotle's cosmology and light was the same in both of those spheres and so the whole study of optics was by astronomers and Kepler for example

it was after Kepler that it was people like Descartes and Newton who started to found what we would call a science of optics and optics started to become a speciality in its own right So can refer first to Descartes? Descartes sort of picked up the Aristotelian idea that Will was talking about earlier, that light is in some way due to changes in pressure in a medium.

And he did a lot of work on the rainbow. And then when Newton, there's a huge transition round about the time of Descartes and Newton, because now Newton started focusing on colour. And colour is something we've scarcely mentioned up till now. So for them, the important focus of study was why is it that we see colours? We're going to come to that, Patricia. But how we said everything you want to say about Descartes in the briefer time available? Descartes is always thought of as a very theoretical armchair philosopher,

but he writes that he went down to the local butcher's shop and bought an ox's eye and started dissecting it. And he's very important in the history of vision because he has, the Cartesian view of the world, that mind and body are completely separated. And there's a marvellous drawing that he did at a cross-section an eye, and there's a little man with a beard sitting behind the eye looking in at the image on the retina.

And it's though somehow you can completely separate your body and the image on your retina from your mind, your brain, which is somehow looking in. So it's this idea that in your eye you can form a very objective view of the world, one in which your mind is completely absent. Well, Alive, so far we've been talking about people trying to understand it, but there was a parallel line which would do with people treating it surgically,

treating the eye, surgically and improving it practically or trying to, anyway. Can you give us some idea? Well, treating it in this Aristotelian and Galenic theory, We've got an idea now that light and something in the eye are coming together to cause sight in the lens and the animal spirit is coming down the optic nerve into the eye and somehow interacting with light to create the image in the lens.

Now we now know that cataract is caused by the lens becoming opaque and we know that cataract surgeries is removing the opaque lens and creating clarity for the image to now be formed back on the retina. But in their day, they didn't have a concept. Obviously, you can't be removing the lens, because that's the seat of vision, and the people could see afterwards.

So they believed when they were cutting into the eye and removing this opaque material, it was a collection of some fluid in front of the organ of sight, the lens, between that and the cornea. So it made sense when they were putting a sharp instrument into the eye, wiggling it about, and pushing down the cataractor's lens. For them, they were clearing some sort of coagulatory. white fluid in front of the organ of sight.

And in fact, it's not really until Brissot in the 1700s who dissected a soldier's eye who had been couched where he found the cataractors lens in the bottom of the eye, which proved that in fact Kepler was correct, that an image was being formed on the retina, the retina is the important seat of sight, and in fact that the lens is a focusing mechanism, not the organ of vision. Well, these methods of surgery, how are you? effective were there? Were people blinded by them as often as they were cured by them? Do we have enough records? There are

records. It depended on when you look. The very early results within the first few days were totally remarkable and miraculous. For example, when Bach had his cataract depressed, he was able to see for a few days, but unfortunately he developed inflammation and eventually went blind. And so subsequently died after a second contract operation. When the data we do have is from the Indian survey where couching was performed very frequently.

Couching is where we put the needle into the eye and depress the lens. It is still used by non-qualified surgeons in parts of Africa and in parts of India to this day. It's a very ancient technique. It was certainly effective and there were many people who were couched who went on and lived many, many years with sight when they'd previously been blind. We have to remember it was unfocused sight because they didn't have any mechanism of correcting this until the invention of spectacles

and then to work out how to make extremely powerful positive lenses. So we can look at Howeth, for example, and look at the cataract spectacles there, which are plus six, which would have given him some vision after he'd had his eye depressed. Can I come to you, Rob, to tell us about Newton, his investigations into the eye

can you can you bad to tell us about the experiments he he undertook on himself this young strange man can I just before that just very briefly go back to what Patricia said because one of

Descartes's sources is Kepler and the the thing that joins Kepler and Descartes does Kepler completely in terms of the anatomy and of the eye and and how the picture is painted on the back of the retina,

Descartes is interested in binocular vision. But the interesting thing I think with both of those is that the price of the better understanding or the cost of the better understanding of the mechanism of the eye is that the nature of vision becomes completely opaque to deliberately use the pun. It's impossible to know how it is that we actually see things, even though we now know how the eye works.

It's something to do with the optic nerve and the brain, but we don't know quite what. Now Newton addresses this question, I think, immediately. He goes straight to the heart of the matter, and he finds that the best way of determining what is due to the mechanism of the eye, the outside world, and what is due to the soul, is to actually do these experiments on his own eye. And they are extraordinary, unprecedented, unrivalled experiments.

Well, as you said earlier on, he sticks things in his eye. He stuck a needle, he called a bodkin, behind his eyeball and press it, to do what? First of all, he, well, what he's interested in doing is producing colours by simply deforming the eyeball. So when you see the images, and we have these on the front of the Newton Project website, when you see these images, what he's doing is, first of all, he puts a brass plate underneath his eyeball as close to the backbone as he can get it, and that's not fine enough. So he puts a bodkin, which in Shakespeare is a dagger, but for Newton,

it's a kind of an implement used in leather. And he keeps pulling his eye in a number of different places. But he also does more extraordinary things. Like he looks at the sun for a number of minutes. We know that he did it through a coloured prison, but it's still damaging to his eye. And he had to stay in his bedroom with the curtains drawn for a number of days. But the most extraordinary thing is he repeats the experiments

because that is the kind of guy who he is. but he's trying to work out how colours are produced. What is due to the outside world and what is due to, as it were, the mind, the soul or the brain? Do you think he's... Patricia, you take that up, yes. I think, of course, these experiments that Newton did on himself sound extraordinary and make everybody feel quite squeamish.

But it was very common for people to experiment on themselves. There was the idea that if you could use your body as an instrument and if you were a gentleman, you had somehow privileged relationship with the world, and your testimony could be relied on to tell the truth. So, for example, when electricity was invented during the 18th century, people were always charging themselves up and putting electrodes in all sorts of very uncomfortable places.

Or similarly with chemistry, people are always sort of sipping and sniffing and trying things out on themselves. So I think in the context of the period, it wasn't quite so extraordinary as it sounds to us now, although it's definitely not one to do at home. And of course, even today, if you were to rub your eye vigorously, you would see lights, which we call phosphines. And essentially, Newton was just producing them

with rubbing a bit harder than you should do. This was also kind of difficult for people because it was thought to be evidence for the extra emission theory. Because if you rub your eye, you're seeing lights inside. There may be an internal fire. So experiments have done where people were rubbing their eyes to create these phosphines, and someone else was looking into the eye

and then noticing no light was coming out. So they realised then that this was something happening internally and not being transmitted out. Rob, Rob, Aleph, the surgery and the development of understanding of the eye was also, we've talked little about theology, but it also entered into philosophy, didn't it? What did we see? What was the world like?

And a good example of it was a Molyneur problem, written by written to John Locke I let it listen to John Locke could you briefly tell us about that John Locke is the outstanding, a friend of Newton by the way and the outstanding empiricist philosopher

of his time that is a man who argued that everything we know everything about us is gained through the senses ourselves and knowledge of everything including ourselves the first edition of his great work essay concerning human understanding appeared in 1689.

And Locke, at that point, put forward some ideas about vision and sense and whether what we see in the world is learned, how much of it is due to learning. And he asked, he was very interested actually also in the way that babies acquire vision. And in 1693, William Molyneux, who's a Dublin natural philosopher, wrote him another letter he'd written to him before, but posed this famous question, which is, is if somebody had been blind from birth and had learned to distinguish various objects by touch, let's say a cube and a sphere, roughly the same size, and if they were brought to sight again, would they simply by vision alone be able to tell by looking at these objects which was a cube

and which was a sphere? And both of them answered in the negative. That is, for different reasons, they said that you have to learn to be able to see. And this is a very, very complex issue. and it's a wonderful issue that modern philosophers find and indeed neurophysiologists and surgeons find very interesting. But the interesting historical issue is that very quickly after this, I think coincidentally, there was a chance to experimentally test whether somebody who was brought back to sight again

could actually see these objects and tell the difference between them. And this is William Cheseldon's famous cataract, well, couching of a 13-year-old boy. It took place over a year because he did one eye and then the next eye, published his results in 1728. And it's a famous description. Many people think it's the most famous case study in medicine before the early 20th century.

But Cheseldon seemed to argue in the negative again. That is that without learning in the case of this boy, he could not tell the difference between a sphere, and a cube just by sight alone. And that took us into a completely different area. I'd like to go back to colour perception, Will Aleph. From Newton on, colour perception became, people were intrigued by it and tried to develop it.

And we're working towards people beginning to know about the structure of the eye, beginning to know about the lens, the retina, and so on. But now we're talking about colours. Yes, and of course the interest here is of, both what colours are, what is the nature of light, which stems back to pseudo-dionysian light metaphysics and the transparency of light coming from the perfect light of God and being transferred into lumen

and making opaque stones such as Lappas Lazarus into blue glass, and blue having this particular colour. The oldest blue glass in the world, of course, is at sight in St Peter's Church in Durham. the abbey built by Bishop Chugherst, Saint-Denie, was filled with blue glass. Very, very important colour. But actually how it was seen and what it was and what it was composed of is very difficult. In fact, colour doesn't exist in the real world.

It's an imaginative construct by our brains. colour is the differential reflection of lights at different wavelengths from surfaces and we evolved receptors that maximally pick up these vibrations at different wavelengths and we compare them and that comparison allows our brain to generate a colour knowing that it becomes very difficult to retrospectively go back and start to understand what was Newton's colour theory, what was Gertes' colour theory, where does Turner fit into this? Well, essentially what we're looking at is the nature between

what actually colour is in the outside and what colour is being perceived. And if we drop the colour theories broadly into those categories, we look at Gertrater saying that colour is something to do with me, it's a perception that I am generating from the outside world. And the optical, they're saying colour is something that is present in the world. we can break white light and put it into a prison.

I'm really sorry, we have to shift it. There's obviously seven programs in this, but I think that takes us there. Briefly, Patricia, can you talk about the colour blindness and the work of John Dalton, how that fits into what we're saying? I don't know how true this story is, but there is a story that John Dalton, who was a very devout Quaker, went off to church one day,

and he was wearing bright red stockings, and he thought he'd brought brown stockings, and he got told off by his brother for misbehaving. And that led him into a whole series of experiments where he looked at flowers and other things by candlelight. And he discovered that he and his brothers saw colours rather differently from how everybody else does. And so he was the first person to sort of analyze and explore this phenomenon.

And it became known as Daltonism right through until the middle of the 19th century when Brewster coined the term colourblind, because he said it was disrespectful to Dalton's memory that he should only be remembered by something that was defective in his eyes. And as late in the 1990s, they operated on Dalton and they discovered what the deficiency was that he had. We can't do this programme without mentioning,

without mentioning the German Vili Kuna, who added greatly to the story. Yes, well, Vinikuna was working on the retina and what the nature of vision was at the retinal level. Visual purple had been disdain. discovered by bolts. He again had the capillarian view that the eye could be locked on as a camera. And what he tried to do was to use the eye to take photographs, essentially. And he had a rabbit, and he killed the rabbit,

and he immediately fixed the retina and found an image of the bars of the cage and the window of his laboratory in Heidelberg. He was then invited to a hanging, and immediately after death, fixed the eye of the person who was hanged, and says that he could see an image on the back. So they were developing the retina as a camera, which led to the great misperception that that's in fact how the eye is working.

And people actually believe that these images were there and in fact comes to the inquest of Chapman where the jury firm actually says, did you take photographs of her eyes to identify Jack the Ripper photographed on the back of the retina and, of course, in Ulysses and so on. Well, thanks for rushing that. I'm sorry I rush you at the end, but needs must. Thank you for Prussia Farah, Rob Aleph, Will Ayeliff. Next week we'll be talking about the first century Roman slave Spartacus. Thank you for listening. There are many more Radio 4 arts and discussion programmes to download for free. Find these on the website at BBC.co.uk.uk slash radio 4.