Post-classical history

CHAPTER 9
Roger Bacon and the Science of Light

In 1277, the bishop of Paris had launched his condemnation of 219 philosophical opinions in an effort to resolve the dispute over Aristotle’s impact on theology. The repercussions were felt well beyond the bounds of his diocese. Just a few weeks later, the archbishop of Canterbury, Robert Kilwardby (d. 1279), issued a much shorter list of heretical statements that he banned from the university of Oxford. Like Pope John XXI, who had ordered the bishop of Paris to settle the row over Aristotle at Paris, Kilwardby was a formidable philosopher in his own right.1 His brief set of condemnations looks more like a settling of scores with his academic rivals than an attempt to crack down on debate at Oxford. In any case, his actions do not seem to have attracted much notice. His successor but one as archbishop, John Peckham (d. 1292), another scholar of considerable note, had to write to the university to ask if they had a copy of Kilwardby’s condemnations as he couldn’t lay hands on one himself. He eventually had to ask the bishop of Lincoln to lend him a copy.2

Although it was not as celebrated as Paris, Oxford’s university had already produced several thinkers of the first rank by 1277. Many of its brightest stars migrated across the channel to lecture at Paris. The shared religion of western Europe, as well as widespread knowledge of Latin, meant that medieval scholars formed a single international intelligentsia that was more closely knit than it has ever been since. Unfortunately, despite a common language among the elite, Europeans still spent plenty of time fighting each other – and new technology could help them do it.

The Physics of War

Medieval western Europe benefitted from the advantages that are enjoyed by large-scale empires without the disadvantages of unified secular control. The combination of political fragmentation with religious and cultural unity meant that scholars within Christendom could exercise a great deal of freedom. No secular ruler could control them and they enjoyed the overarching protection of the Church. Likewise, a theologian like William of Ockham (c.1287–1347), who fell out with the Pope, could join the Emperor’s party for protection. Other scholars could travel around to wherever the Church or their careers took them. Thomas Aquinas was brought up in Italy but made his name in Paris, while his longstanding opponent Siger of Brabant came from the Netherlands. Pope John XXI was Spanish and Albert the Great German.

Catholics saw themselves as a unit that was collectively called Christendom. Despite the frequent conflicts between the nobility and royalty, everybody accepted that it was, in fact, unacceptable to make war on fellow Christians. This was one of the reasons why no single ruler was ever able to dominate in the way Charlemagne had done. There was still a Holy Roman Emperor, but he had enough trouble holding together the fissiparous Germans and Italians to contemplate conquering France, England or Spain. The popes had no desire for a single political ruler to rival their own spiritual power and made it their priority to prevent the Holy Roman Emperor from ever achieving such supremacy. This was not difficult because all the other crowned heads of Europe were squarely behind limiting the Empire’s influence.

An effective way to control the martial ardour of the European nobility was to send them off on a crusade. The crusades were church-sanctioned wars but otherwise leadership was firmly in the hands of secular lords. Today, we tend to remember the ill-fated expeditions to the Middle East. These crusaders failed to conquer Islamic Palestine permanently, and they probably caused more damage to the Christian Byzantine Empire when they sacked its capital Constantinople in 1204. Less well known and more successful were the northern crusades launched against the remaining pagans of the Baltic region, the last of whom converted to Catholicism only in 1386.3

Secular rulers found the clergy extremely useful. After all, they were literate, urbane and well-travelled. Kings employed them as councillors and ambassadors, while anyone who could afford a tutor for their children would hire a cleric. As we have seen, the church even had to pass legislation to prevent monks and priests from practising as doctors. More surprisingly, siege engineers were sometimes in holy orders.4 While the clergy were not supposed to shed blood, this law was very loosely applied. Dropping large boulders on people from afar appears to have been acceptable behaviour. As priests were more likely than most to have the necessary mathematical and engineering knowledge required to accurately direct the fire of siege machines, those willing to perform this function were in great demand. If the enemy were infidels or heretics, it is unlikely that the priest in question would have had many scruples about carrying out this task.

The weapon of choice for knocking down walls was the trebuchet. This enormous machine, first reported in western Europe around 1200, consisted of a long wooden beam pivoted close to one end.5 This meant that one end of the beam was much further away from the pivot than the other. From the short side, a box of rocks or some other heavy counterweight was attached. The far end of the long side was extended even further by having a sling attached to it. Into this sling was inserted the stone that the trebuchet would fire. For a large trebuchet, the shot could weigh 300 pounds and be propelled into a target 500 feet away.6 To fire the weapon, the operator winched the counterweight into the air by pulling down the long side of the beam. When the counterweight was released, it fell due to gravity and the beam shot up to a vertical position. The sling attached to the end of the beam unleashed its load towards the target. No one knows who invented the trebuchet but whoever they were, they had a good understanding of how a lever works. The differential lengths of the beam on each side of the pivot meant that the sling was travelling at the highest possible speed at the moment when it released the stone. Likewise, the lever made it possible to haul the heavy counterweight back into the air for the next firing.

7. A large trebuchet

The lever is the sort of practical mathematics that the trebuchet’s inventor probably had only an instinctive feel for, even though the correct numerical formula to calculate its effects had been known by the ancient Greeks. Unfortunately, the Greeks’ ideas about the dynamics of a stone as it flew through the air, or about the flight of an arrow, were much wider of the mark. This is worth a brief explanation because advances in understanding of the motion of projectiles were among the great achievements of medieval science.

In his Physics, Aristotle explained that there are two kinds of motion, which he called ‘natural’ and ‘violent’.7 Natural motion means falling by the action of gravity. Aristotle explained that this was ‘natural’ because falling objects were inherently striving to reach their proper place. The proper place for an object depended on its weight. Rocks are heavy, so they want to be as low down as possible. Water is less heavy and so is content to rest between the rocks below and the air above. Fire is lightest of all, so it rises even through the air. The Greeks speculated that an invisible region of fire formed a boundary between the atmosphere and outer space, rather like the popular conception of the ozone layer today. Thus, according to Aristotle no force of gravity was necessary. It was the natural desire of objects to occupy their proper place that caused them to fall or rise.

Violent motion was any kind of movement apart from a gravitational one. Thus, when you lift a rock, the motion is violent, but when you let go the subsequent fall is natural. One surprising implication of Aristotle’s ideas was that the two kinds of motion were completely incongruent. That is, it was thought to be impossible for an object to move naturally at the same time that it was being forced violently. This doctrine gives rise to a very strange result for anything that is thrown, for example a trebuchet’s boulder. Because violent and natural motion could not co-exist, the boulder must in theory keep going in the direction in which it has been fired until it slows down and finally stops. At that point, it drops out of the sky. This means that a projectile will move in straight lines rather than the curved path that modern science predicts. Opinion in the Middle Ages was divided, but there is no doubt that Avicenna and those who followed his thinking took this idea completely literally.8 The effect of this would be much like the fate of a cartoon character who has inadvertently run over the edge of a cliff. As long as the poor creature’s momentum keeps him moving forward, he does not fall, but when he comes to a halt he drops like a stone.

Historians have long been puzzled about how anyone could believe that a projectile could travel in a straight line and then drop out of the sky. After all, experience should have taught otherwise. But experience can be misleading. Bowmen were well aware that they could shoot straight at a target for maximum accuracy or fire into the air for maximum range. Those under a hail of arrows would have noted that they came from above and, under the circumstances, no one would have bothered to measure the exact angle of incidence. The trebuchet also propelled its rock into the air and, by the time this landed, it had lost a good deal of its forward momentum to air resistance. It would have appeared to those under attack that the projectiles were coming from above.

By the mid-thirteenth century, Aristotle’s Physics was a central part of the undergraduate curriculum at Paris, Oxford and most other universities. We can be sure that Aristotelian ideas about motion had an equally wide currency. And the fact that clerics were capable of being employed as siege engineers provides us with a link between the academic realm of the universities and the military camp. Directing the siege train is probably what a certain Peter the Pilgrim (fl.1269) was doing with the army of Charles of Anjou (1225–85) while it was besieging the city of Lucera in southern Italy during 1269.9 The city, which was defended by a group of Muslim mercenaries, put up a considerable show of resistance but eventually capitulated.10

We actually know very little about Peter the Pilgrim and we cannot even be sure that he was a cleric, although given his literacy and education, it is almost certain that he was. He obviously found the siege long and tedious, a feeling shared by many of his fellow soldiers. To while away the time he decided to write a short treatise on what he called ‘the indubitable but hidden power of the lodestone concerning which the philosophers have hitherto given us no information.’11 Lodestone is a natural magnetic iron ore called magnetite. Magnetism was, of course, the archetypical occult property and did not really fit Aristotle’s materialist philosophy that forbade action at a distance. It was perfectly obvious that a magnet could affect another object even though the two were not touching. Rather than just worry about the theory, Peter decided to empirically investigate how magnets behave.

Peter was the first to realise that magnets have polarity – north and south. He found that this was always the case even if they are cut into pieces. As for the compass, he correctly deduced that the needle must be attracted towards some giant magnet whose influence could be felt by compass needles throughout the world. He guessed that the heavens themselves were magnetic and that compasses pointed towards the pole star rather than the earthly magnetic north pole. Another of his mistakes had extremely positive repercussions. Because he believed that the whole universe, which was spherical in shape, was a cosmic magnet, he deduced that a sphere would be the best shape for one. This is untrue, as a bar magnet is considerably stronger, but the idea would be picked up in the sixteenth century by a man who believed that occult forces could overthrow the Aristotelian laws of physics. That was William Gilbert (1540–1603), whom we will meet in chapter 18. Peter the Pilgrim’s work on magnets had a long and profitable future because it came to the notice of a man who was much more famous than he was. This contemporary praised Peter as ‘a master of experiment’ and an expert on alchemy, surveying and military tactics.12 The tribute came from Roger Bacon (1214–92), probably England’s most renowned medieval scholar.

The Life of Friar Bacon

Roger Bacon was born into a well-to-do family from the west of England who sent him to Oxford University when he was in his teens. Shortly after 1230, he travelled to Paris where he was lecturing on Aristotle by 1237. Ten years later he resigned his position and devoted himself to private study. He spent his time and plenty of money (some £2,000 by his own reckoning) on alchemical research.13 He seems to have had little interest in high living, but alchemy was notoriously expensive and soon he had run out of cash.

This was not an unusual situation for a student to be in, and many were forced to leave university because they were unable to pay the fees. Luckily a solution was at hand. The Dominicans and Franciscans were actively recruiting scholars because they needed trained preachers to carry out their evangelising missions. Joining these orders might not sound very attractive to us, as it involved living in complete poverty and in obedience to the commands of superiors. The advantage was that the friars had their own priories at Oxford that provided food and board. More importantly, the orders paid the university’s fees for it to educate its members.14 We know that Bacon was devoutly religious and in about 1257 he signed up with the Franciscans who ensured that he could continue his academic career.

As a Franciscan, he was moved between the friaries attached to the universities of Paris and Oxford. While he was at Paris, he met Cardinal Guy le Gros de Foulques who later became Pope Clement IV (d. 1268). Bacon made such an impression that after he was elected Pope, Clement asked him to send his prescription for Christian reform. The result of this request was the Opus Major (or ‘Larger work’) which Bacon compiled in a frantic year of activity. He also managed to run off the Opus Minor and Opus Tertium(the ‘Lesser Work’ and ‘Third Work’ respectively) during the same period as summaries of the Opus Major that he subsequently expanded with much original material. Unfortunately, Clement was dead before he received the massive tomes he had commissioned. It is difficult to know what he would have made of them if he had ever had a chance to read them.

A recent plausible suggestion for why Clement asked Bacon to write up his ideas was that they shared a concern about the end of the world.15 In the Opus Major, Bacon writes: ‘I am writing on account of the perils which happen and will happen to Christians and to the Church of God through unbelievers and most of all through the Antichrist.’16 Certainly, fears of an imminent apocalypse would explain the urgency that drove him to produce such an enormous body of writing in a single year. The main thrust of Bacon’s work is the defence of Christian truth. There is no hint in anything Bacon wrote that he had even slightly sceptical religious views. His concern was that the Church was not using the full armoury of the sciences to further the spread of Christianity. He was particularly concerned that Jews and Muslims should be converted before the doomsday, which he saw rapidly approaching.

In chapter 6, we saw how the university of Paris had initially banned Aristotle’s books about nature before finally accepting them as the bedrock of philosophy. Bacon was writing at the time that this dispute was in full swing. Therefore, we should read his threeOpera as part of the case for using the natural science of Aristotle, not to mention mathematics and linguistics, in Christian education. He expounds in enormous detail exactly how the sciences can be of aid to religion.17 Doubtless, Bacon was also interested in natural philosophy for its own sake. However, this in itself hardly makes him a subversive critic of religion. He held firmly to the idea, then so common among thinkers, that science was the handmaiden of theology.

Oxford’s Franciscans already had a strong academic reputation before Bacon joined them because Robert Grosseteste (c.1170– 1253) taught theology at their priory between about 1230 and 1235.18 Grosseteste had risen from humble beginnings to become head of the university and would later become bishop of the immense diocese of Lincoln. His story illustrates that particularly brilliant men from poor families could use the universities as a path to advancement far beyond their initial standing. Their families could expect to benefit from this, especially if the cleric in question did not take his oaths of celibacy too seriously and had a mistress and children. As far as Grosseteste was concerned, however, the oaths were binding and he became a feared opponent of clerical corruption. Early in his career, he suffered from a near-fatal illness that caused him to give up the trappings of worldly priesthood for the discipline of the Franciscans (although it is unlikely that he ever actually joined the order). We know very little about his early life, but he had probably been awarded his Master of Arts degree at Oxford before 1200 when the writer Gerald of Wales (1147–1223) commended him as skilled in the liberal arts.19 He was made head of the university in 1214, at least according to a longstanding tradition,20 and then appointed to the see of Lincoln in 1235. After he became a bishop, Grosseteste’s devotion to his ecclesiastical duties led to plenty of political disputes with more worldly clergy.

Although he ended his career as a bishop and theologian, many of Grosseteste’s early works are about natural philosophy. He wrote his books on nature in his youth before his brilliant mind turned to what he thought were higher theological matters. Grosseteste did not yet have access to all the newly translated books by Aristotle, but he made considerable progress nonetheless. An important aspect of his thought was his frequent references to experimentum. This is the Latin word from which we get our modern term ‘experiment’, but a more accurate translation would be ‘experience’. So, when natural philosophers wrote about how experimentum informs us of certain properties of nature, they are referring to what they have observed and not to controlled experiments that they have performed.21 The distinction is very important and follows from the common belief among ancient and medieval philosophers that natural phenomena could not be expected to perform in a laboratory in the same way as they did in the wild. For Grosseteste, to study nature meant having to observe, passively and unobtrusively, to see how things happened in the real world.

The Legend of Friar Bacon

Roger Bacon was next in the line of natural philosophers, after Grosseteste, who would ensure that Oxford had an illustrious reputation for the subject throughout the late Middle Ages. Notwithstanding the importance of his work, Bacon’s reputation today actually hinges on two misconceptions. First, we hear, his writing has a peculiarly modern flavour, with references to experiments and future inventions like cars and planes. Second, there is a persistent myth that because he was ahead of his time, he got into serious trouble with the Church.

We should deal with the second of these allegations first. According to several of the standard biographies, the Franciscan authorities imprisoned Bacon for ten years late in his life. For those looking for evidence of the conflict between science and religion, this was a prime example of clerical intolerance. Some historians had no doubt that the Church incarcerated Bacon for his dangerous scientific opinions. For others, it was his sympathetic view of both astrology and alchemy that doomed him to a dungeon. Today, a fresh look at the surviving sources show that it is difficult to prove Bacon’s imprisonment happened at all, let alone that it was caused by his dangerous scientific views.

The origin of the story is the Chronicle of the 24 Ministers General of the Franciscans dating from about 1370, a full century after his alleged arrest. This document claims that Bacon was a master of theology and imprisoned for unspecified ‘suspect novelties’.22As we know that Bacon never qualified as a theological master, it is hard to give this account much credence. Furthermore, the controversy in which the Chronicle implies Bacon was involved had nothing to do with science. Rather, a sect of extremely ascetic Franciscans was stirring up trouble. These men were convinced, like Bacon, that the world was about to end and that the Church should, forthwith, divest itself of all property in imitation of the poverty of Christ. The material riches of the medieval church are legendary and bishops certainly had no intention of living as beggars. If Bacon had been a supporter of these spiritual Franciscans – and, given the enormous piety and millennialism evident in his writings, this is plausible – he could have got into a great deal of trouble.23 However, the allegation that Bacon’s science led to his imprisonment finds no support in the historical record.

His reputation as a futurologist and experimenter has a stronger foundation in fact. In chapter 1 we saw how the early medieval period had been an era of rapid technological advance, and this trend continued through the following centuries. Among the most important inventions to reach Europe in the Middle Ages was the Chinese discovery of gunpowder. The earliest reference to this explosive substance in the West comes from a work of Roger Bacon which describes firecrackers, a popular amusement in China. A passage in the Opus Tertium reads:

There is a child’s toy of sound and fire made in various parts of the world with powder of saltpetre, sulphur and charcoal of hazelwood. This powder is enclosed in a packet of parchment the size of a finger. This can make such a noise that it seriously distresses the ears of men, especially if one is taken unawares, and the terrible flash is also very alarming. If an instrument of large size were used, no one could stand the terror of the noise and flash. If the instrument were made of solid material then the violence of the explosion would be much greater.24

It is clear from the last line that Bacon could see how the toy could be adapted to become a weapon. He might have heard about firecrackers from Franciscan missionaries who, like Marco Polo (1254–1324), were taking advantage of the trade routes opened up by the Mongol Empire. The conquests of Genghis Khan (1162–1227) had imposed a bloody peace on central Asia which made travelling from Europe to China practicable for the first time since the fall of the Persian Empire in the seventh century.

Some of Roger Bacon’s work reflects the spirit of inventiveness that permeated Europe. In a well-known letter called On the Marvellous Power of Artifice and Nature, he speculates on ideas like flying machines and horseless carriages:

It is possible that a car shall be made that will move with inestimable speed and the motion will be without the help of any living creature … It is possible that a device for flying shall be made such that a man sitting in the middle of it and turning a crank will cause artificial wings to beat the air after the manner of a bird’s flight.25

Today, we know that Bacon’s idea for a flying machine would be completely ineffectual, but Victorian historians proclaimed him as a genius ahead of his time on the strength of this document. A more modest view is that he was equipped with an active imagination. We should remember how much he got wrong as a result of the shared attitudes of his era. Where Bacon was unusual was in his interest in the work of craftsmen. Most medieval scholars had no time for technology and handiwork. This was partly a reflection of the views of Greek philosophers, like Plato and Aristotle, who thought that any kind of trade was beneath the dignity of intellectuals. This is an area where Christianity provided a useful counter to pagan chauvinism. After all, Jesus himself had been a carpenter. But old prejudices were slow to die out and most university-educated men did not involve themselves with trade.

In his Opus Major, Roger Bacon tried hard to convince the Pope of the importance of ‘experimental science’, but this also does not have quite the meaning we might expect. A large element of Bacon’s thought was clearly magical and the experimental work he did carry out appears to have been largely devoted to alchemy, the pursuit of which had swallowed up so much cash. Although he probably did carry out a good deal of meddling in magical practices, he was not putting forward the kind of research programme that today we would recognise as scientific.

It is possible that the association of practical skills with alchemy and magic helped frighten off university scholars. Bacon himself ended up with a reputation as a magician. In the sixteenth century, he was thought to be a necromancer of the same kind as Faust. Inevitably, he was said to have acquired Gerbert’s brazen head, which we last encountered in the hands of Albert the Great. The magic head supposedly gave its name to Brasenose College in Oxford, and some say that the brass doorknocker preserved in the college dining hall is the same one that once belonged to Bacon.

On a more positive note, Robert Record (1510–58), an important Tudor mathematician and writer, credited Bacon with the invention of a glass ‘in which men might see things that were done in other places’.26 This sounds like magic, but Record went on to explain that Bacon had used his knowledge of optical theory and natural philosophy to build the glass. In fact, Bacon never says he actually produced such a device, but he did suggest that it was possible to do so. ‘From an incredible distance we might see the smallest letters …’ he wrote, ‘so also might we cause the sun, moon and stars to descend in appearance here below and similarly to appear above the heads of our enemies.’27 Nevertheless, Record’s comment certainly proves that the idea of the telescope had been around long before it was officially invented in 1608.

Divine Light

Roger Bacon’s ruminations on the magnifying qualities of lenses did not arise in isolation. By the mid-thirteenth century, Arabic and Greek books on the science of light were available in Latin. Bacon digested this daunting body of work and produced an impressive synthesis of all the advances up until his time.

Many of his fellow Franciscans were also fascinated by light. To Robert Grosseteste, it was divine. When God created the world, his first words were ‘Let there be light.’ This implied that light was a fundamental property of the universe. Grosseteste imagined it as emanating from God, filling the universe with his glory so that his presence was everywhere. Divine light did not just illuminate the physical world; it also had a metaphysical component that allowed mankind to ‘see’, as in comprehend, the mysteries of the faith. Thus, Grosseteste believed that understanding light would also tell him something important about God.28 Unfortunately, he was writing before Europeans had access to the relevant Greek and Arab knowledge. Without a proper appreciation of what had gone before, he could make little progress beyond inspiring the next generation of scholars.

Among undergraduates studying physics, optics is one of the subjects with which they seem to encounter the most difficulty. They might be reassured to learn that the nature of light posed similar problems for even the most brilliant of minds in the past. The central problem was to explain how humans can see. Leaving aside psychological questions about the subjective experience of vision, natural philosophers wanted to understand the way in which the eye organises rays of light coming from every direction into a coherent image. By rights, we should not expect to see anything except a formless blur, because incoming rays from different angles should all interfere with each other and prevent us from forming a picture. Yet somehow our eyes reinterpret this chaos into a representation of the world around us.

Vision certainly left the Greeks stumped. Some of them thought we must see objects as whole entities. Thus, if we are looking at a dog, we receive an invisible signal of ‘dogness’ into our eye that transmits the whole image to our minds.29 How each signal could fly though the air without getting mixed up with others was harder to explain.

Other Greek thinkers, especially Euclid and Ptolemy, tackled light from a different perspective. They had figured out that light travels in straight lines and that mirrors reflect light rays according to a fixed law. This meant that they could start using their skills in geometry to draw diagrams showing how rays propagated.30 But this left wide open the problem of how the eye produces a coherent image from disparate rays. Euclid apparently solved it by suggesting that the eye launched rays outwards which interacted with the environment. This makes no difference to the law of reflection, which works in either direction. He may have imagined that the rays emitted by the eye bounced off objects and returned via the same path. This is quite an insight, because it is very similar to the way in which both sonar and radar detection work. In both cases, pulses are sent out which are deflected when they encounter something large and dense enough to form an obstruction. Some of the deflected pulse returns to the detector so that a picture of the environment can be formed. Bats actually do ‘see’ by listening for the echo of their high-pitched squeaks, and they can navigate through complete darkness as a result. So as a model for vision, Euclid’s emission theory is nothing like as far-fetched as it sounds; it is just not right for humans.

Euclid’s theory allowed him to use geometry to model the reflection and refraction of light. But his idea had many problems. The most pressing was that luminous objects, like the sun or a torch, were obviously giving out light. This contradicted the theory that we see by rays emitted from our eyes. This led Bacon, following Aristotle and the work of Muslim scholars, to reject Euclid’s theory but without abandoning his geometrical work on reflection.

Unfortunately, he was unable to make progress with the central problem of vision: how the eye sorted light rays, coming from different directions, into a coherent image. The answer to this problem that he adopted, which had been developed by the Muslim scholar Alhazen (965–c.1039), was to assume that only light rays that penetrated the eye head-on went into making up the image that we see.31 Any light rays that hit at a glancing angle were so weakened, he said, that they were unable to impose themselves on the retina. Of course, this raised the question of why rays that arrive just a few degrees from perpendicular do not produce a weak image that would give us blurred vision. The theory demanded that any ray that strayed even slightly from straight-on should be completely disregarded. As such, although it was the best idea available at the time, later theorists were never entirely happy with it.

Roger Bacon’s work on optics gives the lie to some of his rhetoric about the importance of experimental science. Although he seems to have played around with lenses, he betrays no sign of a systematic investigation of light’s properties. His theories look like bookwork and an attempt to join together the various authorities into a coherent whole.32 He was not even prepared to completely abandon the theory that our eyes send out rays; since it was supported by respected Greeks like Euclid, Bacon had to find a place for it.

The subject of optics, known as ‘perspective’ in the Middle Ages, attracted several other thirteenth-century natural philosophers. John Peckham, the archbishop of Canterbury shortly after Robert Kilwardby had been, was a Franciscan like Bacon and shared his fascination with light. He wrote his own textbook on optical theory, largely taken from Alhazen, which remained on university syllabuses until the sixteenth century.33 Bacon’s own work also informed a massive treatise by the Polish theologian Witelo (fl.1250–75) who was working in Rome. This combined all the Greek and Arab ideas together with the insights of his own era.34 Witelo’s book remained the last word on perspective until the sixteenth century when, as we will see in chapter 18, it formed the foundation of modern optical theory.

Perhaps Bacon would have solved the mystery of vision had he lived a little longer, because just as he reached his last days, a new invention appeared in Italy that provided the missing link – spectacles. The way that the eye can resolve an image is through its lens, which refracts all of the light rays emanating from a particular point and focuses them onto a single spot on the retina. Spectacles, with their own lenses, might have given Bacon the clue he needed.

The earliest mention of spectacles is found in guild regulations from Venice dating from 1300.35 In 1306, the Dominican Giordano of Pisa noted in a sermon that he was preaching in Santa Maria Novella in Florence:

It was not twenty years since there was discovered the art of making spectacles that help one see so well; an art which is one of the best and most necessary in the world. And that is such a short time ago that a new art that had never before existed was invented. I myself saw the man who discovered it and practised it, and I talked with him.36

So the evidence places the invention of spectacles in the late thirteenth century. Several writers patriotically sought to name the inventor as a denizen of their own cities, but these sources are extremely unreliable.37

The first spectacles were reading glasses with convex lenses (those that bulge in the middle and are thinner around the edge). These require much less precise grinding than the concave lenses needed to correct short-sightedness, besides also being easier to manufacture in other respects.38 Spectacles meant that monks and other scholars could continue working even after their eyesight began to deteriorate with age. And as we shall see in the next chapter, mechanical clocks started to tick in England and Italy at about the same time. These inventions catapulted medieval Europe into first place in the race to become the most technologically advanced civilisation on earth. Although he did not know it, medieval man had already surpassed China, Islam and the ancient world.

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