In Our Time - Optics

Episode Date: March 1, 2007

Melvyn Bragg and guests discuss the history of optics. From telescopes to microscopes, from star-gazing to the intimacies of a magnified flea. As Galileo turned his telescope to the heavens in the early 1600s, Kepler began to formulate a theory of optics. The new and improving instruments went hand in hand with radical new ideas about how we see and what we see. Spectacles allowed scholars to study long into the evening (and into old age), while giant telescopes, up to 100 feet long, led to the discovery of planets and attempts to map the universe. The craze for optical trickery swept Europe with enthusiastic amateurs often providing valuable discoveries. But this new view of the world through a lens raised questions too – how much can you rely on the senses, on what you see? The further into space you can spy, the larger and more unmanageable the universe becomes. At the same time, the microscope was utterly transforming the world close at hand.So how did these developments inform ideas of knowledge? If new methods of scientific observation support an empirical approach, what does this mean for divine, innate reason?With Simon Schaffer, Professor in History and Philosophy of Science at the University of Cambridge; Jim Bennett, Director of the Museum of the History of Science and Fellow of Linacre College at the University of Oxford; Emily Winterburn, Curator of Astronomy at the National Maritime Museum

This BBC podcast is supported by ads outside the UK. What makes people want to believe in aliens? I'm Tristan Redmond, one of the hosts of the Global Story podcast from the BBC. Donald Trump last week announced that he'd be releasing the US government's UFO files. So why the renewed interest in life out there? And what deeper spiritual meaning might people be searching for? Check out the global story. We are serious journalists on BBC.com.

or wherever you get your pods. 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, from telescopes to microscopes, from stargazing to the revelation of a magnified flea. Today we'll be discussing the history of optics.

As Galileo turned his telescope to the heavens in the early 1600s, Kepler began to formulate a theory of optics, The new and improving instruments went hand in hand with radical new ideas about how we see as well as what we see. Spectacles allowed scholars to study long into the evening and into old age, while giant telescopes up to 100 feet long led to the discovery of planets and attempts to map the universe. The craze for optical trickery swept through Europe,

with enthusiastic amateurs often providing valuable discoveries. But this new view of the world through a lens raised questions too. How much can you rely on the senses on what you're going to? you see. The further into space you can spy, the larger and more unmanageable the universe seems to become. At the same time, the microscope was utterly transforming the world close at hand. So, how did these developments inform ideas of knowledge? If new methods of scientific observations support an empirical approach, what does this mean for divine inspiration or innate reason? Joining me to discuss the history of optics, Simon Schaffer, Professor in History and Philosophy of

Science at Cambridge University. Jim Bennett, director of the Museum of the History of Science, and fellow Blinica College at Oxford University, and Emily Wintervern, curator of astronomy at the National Maritime Museum. Simon Schaffer, we know, or we think we know, that ancient civilizations were using basic lenses. But what level was that on? I think there's a very interesting question that the whole long history of optics raises for us,

which is what's the relationship between technique and theory? What's the relationship, in this case, for example, between having available transparent crystal or jewels which could be used somehow as lenses, glass spheres full of water, which could certainly magnify images. All these are pretty familiar in a lot of ancient and medieval civilizations. But what's also striking is that it's as though those technologies came too soon

since, in theory, in the texts that have come down to us from the Greeks and the Romans and from many of the great Arabic writers, the problems of these technologies are not treated front and centre. Rather, what one tends to see, and here I think we have to begin with the great Muslim scholars

of about a thousand years ago from cities like Baghdad and Basra and Cairo who wrote masterpieces of optical theory, which explained how light streams through the universe, which explains the different models of how we see. Perhaps we see because rays enter our eyes. Perhaps we see because something streams out from our eyes. But it's very unusual indeed to see those, as it were, cosmological explanations

of what's going on in sight and in light, being linked technically to the way in which bits and pieces of optical devices work. It is strange, isn't it, that with the... the ability to magnify, which was there we think from the Babylonians and so and so forth, and the necessity for military operations on a scale which often demanded seeing where the enemy was, whether it's sea or land, that sort of thing, these things were not developed earlier, this long gap before it got going. I mean, you could have done with a telescope on the Roman wall, for instance, couldn't you, it have helped an awful lot. So is there any explanation why it's still,

it's very low level for so long? I think there are two different kinds of explanations. here. I mean, one is a social explanation. It's very unusual in history for scholars and craftsmen intimately to work together. And I think one of the distinguishing features of the European Renaissance and what we sometimes call the scientific revolution is precisely the intensity and distribution of those kinds of collaborations. It's very rare in culture for the world. of the mind and the work of the hands to be seen as fitting together obviously. What might seem obvious to us is not at all obvious elsewhere or else when. But there's a second kind of problem, which is much more interesting, really, which is that what burning glasses and water-filled

spheres and crystal lenses show you is not just that you can see things bigger or further or better, but that you can be deceived. So for every optical device, which shows you, shows you something advantageous. You're in the same gesture, showing that you can't trust what you see. And the paradox, I think, of optics is that it's both the most reliable of the senses, and it's the main sense that deludes us. Right. So can I now come back on track with these Arab scholars, Al-Kindi and Al-Hassan, in the 9th and 10th centuries?

And can you just reiterate what they brought to the table? I think the great Arabic writers of the 9th and 10th century working both in the Valley of the Tigris and the Euphrates and in the Valley of the Nile under the Abbasid and Fatimid Caliphates are drawing on extraordinarily complex and rich traditions. Not only the magnificent translations of Greek and late Roman material, but also, as their geographical situation might suggest, they clearly had access along the long trade routes to the east to Indian and Indian. Chinese material and sitting as it were at the crossroads of civilization with very important state patronage from the Caliphate and with a mastery of really the most advanced geometrical techniques they were in an unparalleled and really unprecedented position to formulate the basic principles of optics as they were then known. Jim Bennett we heard in what Simon said about the link between the hand and the mind. Let's at a touch.

an extremely useful phrase and the mechanics of what we see. Can we move on to the practicality of the development of instruments and something that is very dear to all of us spectacles which we know we're around in the 1200s but we see in a painting in 1304 or something like that. So there they are.

Can you tell us how they developed and why that was important? Well, it's surprising how little we know about that and that's relevant to Simon's point about the practical and the theoretical because we become aware of them in the latter part of the 13th century because, as you say, we start seeing portraits and people quite proudly having their spectacles.

So clearly something has happened and some practical development has taken place and there are spectacle makers. This begins with the correction of long-sightedness. So that's an evident problem. People are used to being able to work well at close work. Their livelihoods, their interests and so on are all bound up with close work.

As life progresses, they find this more and more difficult, having to hold things further and further away, that they need a solution. So there's a clear problem. There's also an area of practice, these lenses and magnifying spheres and so on, which looks as though it might be able to help. And somehow that comes together in the provision of spectacles with convex lenses, that's to say lenses that are fatter in the centre and thinner at the edges, that help people prolong their working lives.

then the concave lenses, that's more difficult. It's more difficult in two ways. They're more difficult to make, and it's also more difficult to fit them to the needs of your customer, if you're a spectacle maker. So that takes a little longer. Mid-15th century, we know of there being concave lenses being used in spectacles in Florence, for instance.

So there, if you're thinking about instruments, you have the ingredients in a spectacle-maker's shop of the early telescope, which has a convex lens and a concave lens. But generally for a telescope, you wouldn't have the quality or indeed the sort of focal lengths that you need for a telescopic effect. So as part of your answer to the Roman soldier who doesn't have a telescope, the ingredients aren't there.

And it takes a while before the spectacle makers are making them for correcting vision with the kind of range of performance that you need so that someone might put it together and notice a telescopic effect. Emily Winterverne, when Galileo, got the lenses from Holland and massively improved them and turned the telescope up to the sky that marked a change

and can you tell us where he got the lenses from and what he did with them? He hears about this invention through letters I think so he doesn't have a practical example, the invention of the telescope so he doesn't have a practical example to work from but he works from these sort of instructions

and he builds his own telescope and he increases so the Dutch version I think magnifies by about three or four times he makes one that first magnifies eight times, then up to about 20 times, by grinding and polishing his own lenses. And so he greatly magnified this telescope. Why did he, as it were, it had been used then to see the enemy coming across the lines or ships on the horizon or that sort of thing?

Do we know why he turned it to the skies? He does first of all suggest a military use, and he sends it off to the Senate, and one of his telescopes and says, look, you could use this for military use, and he gets a professorship in exchange. Then later on he uses it, and he views various things in the sky, and he looked at Jupiter,

and he saw, gradually, over several days, that you have satellites moving round Jupiter, so he has four satellites that he identifies as moving round Jupiter, so rather than moving round the earth. And so that's the first thing. He also looks at the moon and sees that that's not quite as perfect as you might expect,

or that you can see with the naked eye. One of the things that you notice when you look through these telescopes is that you can't actually see very much at a time, you can see sort of quarter of the moon at a time. How did these results go down? I mean, who were people very excited by? Was much notice taken of them? Much notice is taken now because of his place in the history of this.

But then, at the time, people liked the idea of having a telescope and he made quite a bit of money by making and selling these telescopes. But who took it up? Did it enter into a sort of body of conversation and of knowledge of Dan? One of the things that he points out is that he doesn't invent the telescope, but

that's a sort of unnamed craftsman that does it by chance, and he's the one who actually kind of finds a use for it and makes it an instrument that's interesting. I mean, one of the things that people found difficult with it was that it was so new that when he was saying, oh, I can see all these things through the telescope, people couldn't actually replicate it straight away. It took a while for those telescopes to be distributed,

and so people couldn't necessarily. believe what he was saying because they couldn't see it for themselves straight away. At the same time, Jim Bennett, or more or less the same time, and this is the great coming together of this, at the same time Galileo is doing that, making observations, noting them down. Kepler is making theoretical strides forward. Can you briefly tell us about that and what his contribution was? At last we're going to bring this together.

You've been hoping what happened. Kepler does indeed approach this as a mathematician. and looks at the geometry of the situation and publishes a treatise in 1611 about the theory of the telescope. Kepler does too, and as well as that important step in bringing theory to bear into this area of practice, there are two important things that Kepler does. First of all, he shows us that if we were going to build a perfect telescope,

we wouldn't begin where we were. We wouldn't be using lenses with spherical curvatures. they're the only ones that anyone can make. That's the only things that spectacle makers can grind and polish, but they don't bring light to a proper focus. What you need, Kepler says, is other shapes, hyperbolic surfaces and so on. That's all very well in theory. So again, you have this mismatch, but people can't make them.

But nonetheless, that's important for what happens in the 17th century because the way to deal with Kepler's point is to have... We talk to the early 17th century. Yes, a very slight curvature, a very long focal length lens. So telescopes become... unmanageably long as a way of dealing with Kepler's problem of what's called spherical elaboration. The other thing that Kepler does is that he invents a new sort of telescope. And here you have this idea of a theoretician coming in and working out a new way of doing things.

And this is a telescope with a convex object glass, the way Galileo says. But instead of a concave eye piece, which Galileo has, Kepler has a concave eye piece. So a positive, what we would call a positive objective and a positive eye piece. The problem with that for everyday use is that you get an image that's upside down. And that doesn't seem like a good idea intuitively. You know, you look at... And Simon was talking about illusion and delusion in the telescope. Well, here's an illusion, certainly, if things turn it upside down.

It takes a little while, but astronomers realize it doesn't really matter as far as they're concerned. If things are inverted in the telescope, that doesn't really matter. And this is a much better kind of telescope for them, because, as Emily says, the Galilean telescope is a very small field of view. This one has a larger field of view. it has the advantage also that you can turn it into a telescopic site on a measuring instrument. And astronomy in this period is all about measurement. It's all about position. So you get new dimensions of use for the telescope thanks to Kepler's work.

Both better telescopes, no longer, and telescopes used for measurement in what would be thought of as proper astronomy. As I understand it, when Kepler started to make these jobs, he didn't have a telescope. He himself didn't have a telescope. That's right. And so where is he getting, is he going back to Al-Kindian al-Hazan? Yes.

I mean, when Kepler begins riding on optics, he does so, let's remember, as an astrologer. Yeah. Kepler is one of the great, I think, professional astrologers of early modern Europe. He says at one point, astrology works to control human character the way peasants tie up their pumpkins to make them turn into different shapes. it isn't the stars that make us grow, but the character that we have is dominated by the stellar aspect at our birth. That's part of Kepler's motivation for being interested in light. He was also an extraordinarily well-equipped scholar.

He goes back to medieval optical texts, especially the medieval Christian Latin versions of and commentaries on Al-Hazen and Al-Kindy. And one of the things that he finds in the Al-Hazen text, remarkably, is an examination of the structure of the human eye. Yes, this is what I want to get to. And it's a principle in a certain sense that runs right through the history of science that whenever a new instrument comes along, people think that it explains something about our bodies.

That's why we think our brains are computers. And in exactly the same way, Al-Hazen, who is one of the first people to describe the pinhole camera, and to suppose that a pinhole camera is very like the way in which images are formed in general. Kepler, through his Latin sources, seizes on this and experimentally shows that that's right, that the human eye works like a pinhole camera.

Because it had been thought that light streamed out of the eye. The dominant model for millennia, if not centuries, was that we see, because there are rays streaming from our eyes. And we still talk that way. We still say that people have a penetrating gaze. And we still, or at least some of us, when young, bought x-ray specs, which are objects based on that theory. But it's from the renaissance that the dominant model of what's called intramission,

that's to say we see because something enters our eye, becomes dominant. But that theory, again, I think, comes too soon. I think what's fascinating about Kepler's model is not just that he assumed, more or less that we see because something is coming into our eye, he actually experimented on how that worked. He scraped off the back of an ox's eyeball and looked through the lens. And what he saw is what you see in a pinhole camera.

So there's a strong analogy, going back to what Jim was saying about the Keplerian telescope with two convex lenses, between this puzzle of the trick, of actually getting the wrong image, getting the image upside down, and what would happen in the human eye if we didn't have proper lenses?

Kepler decisively shows, it's picked up by many 17th century authors, that the image is formed at the retina and not at the front of the eye, and it's the lens of the eye that inverts the image, so you'll be delighted to know we all see things the right way up.

Well, that's a really thing, I think. Emily Winterman, can you tell us the problems with the early telescopes. What problems were they having and how did they address these problems? Well, one of the problems was just the quality of the glass.

I mean, the glass that Galileo was using if you think of historic houses, the glass on the windows there, it was sort of bubbly and a kind of slightly greenish tinge because of the iron in it. So the glass gradually improves. The other problems were, as I said, the aperture, so

you're seeing only a tiny, tiny field of view. It means it's very difficult to actually find what you're looking for when you pointed at the sky. I mean, once you found the moon, you can only see a quarter of it at a time. But actually finding it in the first place is quite difficult when you're looking through an aperture that small. One of the solutions to spherical

aberration was to have bigger and bigger telescopes. The other problem was chromatic aberration. So white light is made up for lots different colours. Those different colours focus at a slightly different distance from the lens, and so that can give you a kind of blurred image.

And that was solved sort of in the mid-18th century, first by a guy called Chester Moore Hall and then that was taken up very quickly by John Dolland, whose successes now form Dolland and Aitchison, the High Street opticians. But John Dolland developed that idea of basically an achromatic lens, so using two different kinds of glass to sort of compensate for the chromatic aberration in each.

Can we turn to the contributions assignment of Isaac Newton? Can you briefly tell us what he brought to the development? Newton was extremely interested in the Keplerian tradition, in the history of optical illusions, and in whether what we see is really what's there. He did a series of heroic and almost suicidal experiments on his own eyes, which one should not try at home, involving putting wood and needles somewhere near your eyeball, let's not go there.

The reason why he did these things when he was in his 20s as a student at Trinity was because he was fascinated by the difference between, between what we see that is objectively there in the world and what we see, but which is actually a result of our fancy or our imagination. And he very rapidly, I mean, in a few months, it's absolutely dramatic, he came to see that one of the areas where the difference between illusion and reality is most obvious

is in the making of colour. And if he could only get hold of glass of a very high quality without striations or bubbles or streaks or cracks, he could show, he reckoned, that white light, which up till then had been assumed to be primitive and original, was in fact compound, that it was made of seven different colour-making rays, and that when you pass a beam of white light

through a very fine glass prism, you can split the white light into those seven different rays. And he's the first person to use the word spectrum, that's to say ghost to describe the striated, coloured image that you then see. And the fact that he decides to call it a ghost reminds us of the, as it were, this is very anachronistic, psychological origin of his experiment.

But what he then immediately demonstrates, again, as Emily showed nicely, is that if a different colour-making ray bends at a specific angle through glass, you cannot bring all the rays in white light to the same focus. Therefore, Newton said, there is a limit to the improvement of magnification in telescopes using lenses. You must use mirrors. And he designs a magnifying device, what we now call a Newtonian telescope, which uses a concave mirror to magnify. And it's that telescope, which was made in London under Newton's instructions, that gets Newton elected to the Royal Society.

and that provides the occasion for his first publications. So what's very interesting in retrospect is that Newton is kind of ushered onto the public stage of 17th century English science, not as the supreme mathematician, which he already was, and not as the author of a magnificent theory of gravitation, but as someone who brought together a theory of light with a better telescope, just as Kepler had.

And I think that's a very interesting sign, of what counted as status at the time that Newton was working? We haven't less time. It's rather fitting that we have much less time to talk about microscopes. We've been telescopically involved for quite a while now, but we're going to tackle microscope, Simon, so if we can briefly... Why don't we start with Robert Hook,

who was a contemporary rival with Newton? So we're talking about the same time, and what he did with microscopes and where it goes from there. So we'll have to get a bit of a move on with microscopes. Well, it was very closely linked. and you see this in Robert Hook's work in the middle of the 1600s exceptionally clearly, with natural history. What one did with these tiny lenses,

sometimes, as in the Dutch case, using just a single spherical bead of glass, which itself is so tiny one can hardly imagine using it, to see what living beings are truly made of. Why is that an interesting question in the 1600s? It's an interesting question because it might unlock God's purposes in the living creation. And what Hook's work of the 1660s showed, beyond any controversy, really, was that the works

of nature are in almost all cases more elegant than the works of art, so that if you really want a sharp edge, look at a fly, look at a louse, don't look at the edge of a razor blade, which looks like a cross-section through the Alps. Secondly, that there are objects in the world, especially in the living world, that we could not have predicted that there's a sense in which the users of the microscope were represented as being so many new Columbuses, opening entirely new worlds beneath and unimaginably beyond our vision. And above all, which runs right through from the 1600s onto the 1800s really, the great puzzle of the microscope, which is you're being shown things that you can't calibrate. In the

case of a telescope, you can see that it works because it shows you a ship in recognisable shape only much nearer than it really is. In the case of a microscope, you have nothing with which, really, to compare what it is that you're seeing. And there's a problem, therefore, of authority, of trickery and of illusion that I think underlines the whole history of optical theory and optical instruments and becomes massively present, really in the early 1800s, when you get a whole range of. A whole range of, of new optical devices, kaleidoscopes, cameras, and so on, which trade on the trickseiness of what we see through our eyes and through lenses.

Jim Bennett, can we bring in the ideas again here and the ideas of Descartes? Yes, I mean, Simon's talked about the positive side of the natural history element in microscopy, the wonders of the very small, but there's a problem to that as well. There's another side to that coin because it's also a huge distraction from another of who's great missions, which is to show you that you can put what's called the mechanical philosophy on an empirical basis. That's to say you can take this Cartesian idea that the world is all tiny particles, microscopic particles. It's a very fundamental notion in science that in order to understand better, we have to go below the superficial appearance. You have to go to this invisible tiny realm,

from which everything ultimately stems. But the more you look at fleas, you find their little animals, the more you look at even smaller animals. They're still little animals. there seems to be no way to get to these atoms or particles. So certainly the motivation for microscopy is benefited by this theory of the truth of the very small. But what you find is that things are very interesting. And for all Hook's attempts to focus us on the mechanical substrate, he keeps getting distracted,

and his audience keep getting distracted by how amazing it is to look at all these little bugs. and that's where a microscopy goes. This way of seeing a world, was this a threat to establish religion? The very common idea, certainly in English science in the 17th century, it's an idea expressed by Francis Bacon, that when Adam and Eve started eating fruit, their eyes dimmed. And with the new optical devices of the 1600s, we could get back to the state that Adam and Eve were in.

So there's a wonderful writer called Joseph Glanville in the 1660s. He was a colleague of Robert Hook in the Royal Society, who says Adam needed no sense. spectacles. He could feel his own blood circulate. He could feel the earth move around the sun. We unfortunately can't do any of those things. But with these new optical instruments, we could. So, on the one hand, optical instruments aren't, especially microscopes, in a sense, aren't really delivering because they're not showing us the atoms and corpuscles that we think the world is made of. But on the other hand, there is a theological sense that they might get us back to the state

we were in in the Garden of Eden. There's a lot we haven't done. I'm going to end on this spectroscope. Benet, shadowing this conversation has been scientists, thinking about the properties of light itself. Now, spectroscopes seem to be able to define the composition of size. Can you briefly say something of the spectroscopy? We haven't got time to get photography, electricity, I'm very sorry, other programmes. Well, people get very interested in the nature of light in the 19th century. It's a whole Newtonian programme of work there.

And a lot of people realise that flames of different colours are related to the composition of the material in the flame. So we might be able to use that in a reverse. where we might be able to have a process of chemical analysis, which uses the prism, Newton had shown as an analytical tool, to look at the light coming from incandescent objects. And that's wonderful for astronomy.

That transforms astronomy, because astronomers realise that they're surrounded by incandescent objects. The stars are incandescent objects. And although we've always thought of light from the star as simply telling us where the star is, simply telling us the position, suddenly it can tell us the composition of the star, the chemical composition.

we never realized that that was encoded in the light and that changed astronomy forever. Well, thanks for doing that at such a rapid pay. I'm very great. We didn't do wholly what we intended to do, but what we did do, I enjoyed very much. I'm sure everybody else said it was terrific. Thank you very much, Jim Bennett, Emily Winterburn,

Simon Schaffer, and thanks for listening. We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programmes about history, science and philosophy at BBC.com.com.com. Radio 4.