INTRODUCTION Light lances through the leaded panes and strikes the polished black-and-white tiled floor of the room, imposing one grid at an angle upon another. It crooks as it meets the window surface and crooks again after it has passed through to resume its original direction. It dances and twists, dodges and swerves round imperfections in the old glass. Here and there, tiny blemishes improvise lenses and prisms, which produce magnified and distorted flecks and gobbets of light on the floor, and sometimes a tiny rainbow. The brightest pattern of light is the one cut by the sharp shadows of the lead in the window. This falls off to one side because of the sun’s position in the sky. But there is another pool of light, too. Directly below the window, a hazy bluish gleam rises from the tiles, which is the reflection of the light of the sky. The room is large and bright and was originally used for dinners and musical entertainments. It has windows on three sides, but is even brighter than you’d expect for that. It is almost like being outdoors, which seems odd at first because overhead is not air but a heavy beamed ceiling. Then you realize that this ceiling is almost glowing with light itself, light from yet another source, coming from below the horizon and projected upward through the same window by the water in the moat that surrounds the Huygens house, known as Hofwijck. This estate – three miles south-east of The Hague – is where Christiaan Huygens lived after the death of his father, the poet and diplomat Constantijn Huygens, until his own death just eight years later, in 1695. When Constantijn built Hofwijck some fifty years earlier, he wrote that he wanted it to appear ‘as if by night, / It grew, like a mushroom revealed in the light’. And so, amid its calm, reflecting waters and with the elevated motorway now roaring too close by, it does, even today. Here, Christiaan finalized treatises on the nature of light and gravity that summed up his prodigious contribution to physics. Here, he set up his telescopes in the spacious grounds and began to speculate about life on other planets. Christiaan Huygens was Europe’s greatest scientist during the latter half of the seventeenth century, until the rise of Isaac Newton, by whom he has been largely eclipsed, especially in English-speaking parts of the world. This is an unfair judgement of the history of science, for Huygens’s achievement exceeds that of Newton in some important respects. A maker as well as an observer and a thinker, he added to both theoretical and practical science in astronomy, optics and mechanics. A supremely talented mathematician, Huygens tackled problems in fields as varied as geometry and probability theory, and became the first to employ mathematical formulae in the solution of problems in physics, the methodology that has become the basis of all modern science. Two centuries before it would become generally accepted, he proposed a wave theory of light. He was first to describe the concept of centrifugal force. Using telescopes of his own design and manufacture, he discovered Saturn’s ring system and its largest moon, Titan. He estimated the size of Mars and the distances to many stars. He found a way to incorporate the pendulum to create more accurate clocks, realizing the vision of Galileo before him. His innovations in optical instrumentation and timekeeping are still in use today. Huygens also had range: he was a fine draughtsman, a skill of use not only in designing mechanical and optical devices but also in representing to the world the planetary phenomena he observed through his telescopes, although he was not above making sketch portraits to flatter girlfriends, or little landscapes of the countryside in which he found himself. He was a proficient musician, joining in with performances wherever he went. Occasionally, a few notes of a melody or a lyric for a song are to be found in the margins of his scientific notes. But he also wanted to bring his mathematical science to bear in music, proposing a division of the octave into thirty-one notes, foreshadowing musical innovations of the twentieth century. More lasting in significance were his efforts to elevate science itself in Europe, both in the Dutch Republic and, more especially and surprisingly, in France, where he was instrumental in the establishment of the French Academy of Sciences. He was also an early fellow of the Royal Society of London, becoming a personification of the potential for science to transcend national borders. This was not always how he chose to appear to the world, however. When Huygens returned from Paris to The Hague in 1671, he sat for his portrait to be painted by Caspar Netscher, who had already painted several other family members. Netscher’s small oil painting demonstrates the artist’s mastery in representing fine fabrics. Huygens looks out with wide eyes from amid an ocean of silk and lace. He is at the height of his powers, and yet there is still something about him of the pretty child he once was. If we are in search of a display of learning – perhaps a table nearby with scientific instruments and papers of calculations lying casually on top – then we will have to look elsewhere. This is above all a man of fashion and extravagant taste. But he was, too, the prototype of a modern scientist. Although he explored many topics, and shuttled between them opportunistically, rather than pursuing what would today be called a programme of research, he conducted his investigations with diligence and rigour, even if, like many others at the time, he did not always rush to publish his findings. His employment of mathematics as well as his awareness of the importance of criteria of reproducibility, verifiability and falsifiability – the understanding that experiments should be repeatable in order to demonstrate their truth, and that experimental results that fail to support a hypothesis must lead to rejection of the hypothesis – reveal the essential seriousness of his project. His topics were well chosen as ones where a breakthrough might realistically be achieved. He did not wander off the track into the realms of superstition, which is more than can be said for some of his contemporaries. So dedicated to learning was he that, on his first visit to London in 1661, he turned down the chance to attend the coronation of King Charles II in order to observe the more interesting transit of Mercury. This all-round proficiency Huygens undoubtedly perfected with the help of Constantijn, the father who first doted on him and then stood in awe of his son’s talents, introducing him as ‘my Archimedes’ to Descartes and other illustrious visitors to his household. The long-lived Constantijn was a strong moral and intellectual influence for almost the whole of Christiaan’s life – so much so that when he eventually died at the age of ninety, Christiaan, by then fifty-eight years old, morosely had himself painted in the garb of an orphan. Poet, composer, diplomat, architect and artist, Constantijn was no less remarkable a figure than his son, which is why I devote substantial parts of this book to him. Born in 1596, he served as the secretary to a succession of stadholders, as governors of provinces of the Dutch Republic were known. Constantijn was what the Dutch called a kenner – not an amateur, a mere lover of his studies and pastimes, but one who has sweated to know them completely so that he has become a master himself, even though he will never need to rely on these skills to earn his living. So it was that Constantijn Huygens learned to draw and paint from one of the best draughtsmen of the day, and was able to put his first-hand expertise to good use when he identified the raw talent of the young Rembrandt, whose work he believed might fittingly adorn the stadholder’s court in The Hague. Constantijn effectively launched the artist on his career, although their friendship petered out in a trail of truculent correspondence as an increasingly bumptious Rembrandt chased his fees. The elder Huygens was also enough of an architect to take a leading role in designing his own houses. He slotted easily into the circle of leading Dutch poets, and documented his long life in verse. When, for instance, he proposed that a grand avenue be built from The Hague to the sea at Scheveningen, he wrote a poem about that, too. He composed hundreds of musical works and played many instruments. Unsurprisingly, his diplomatic missions attained a cultural dimension that did him no harm, and long before he was forty he had been knighted both in England, where he had pleased James I with his lute-playing, and in France, where it was his literary accomplishments that impressed Louis XIII. With these diverse interests, it is no surprise that learn that Constantijn was absorbed by scientific questions, too. The quality of his engagement with science presents an instructive contrast with that of his son, which is another reason to spend some time with him here. If Christiaan was the prototype of a modern scientist, then Constantijn was an exemplar of a foregoing type known as the ‘curioso’ – the man who wishes to know more about natural phenomena but who does not always ask the right questions or have the necessary equipment to answer them. This paternal fascination was surely formative for his children. Constantijn and his wife Susanna would have four sons and a daughter. The kenner instinct was inculcated in all of them, but especially in the two older sons: the eldest, another Constantijn, became a skilled grinder of lenses while Christiaan learned how to construct the intricate mechanisms for his own horological inventions and optical devices. Perhaps, too, their father’s pathologically impaired vision was a factor in the path that the boys chose. The younger Constantijn eventually succeeded his father in a diplomatic career by becoming secretary to the stadholder William III, and accompanied him on his triumphal journey through England in 1688, to join his wife Mary II on the English throne. Like Samuel Pepys, Constantijn also kept a diary that recorded every aspect of his personal and political life, from sexual gossip to the progress of the ‘Glorious Revolution’. The Huygens family milieu in The Hague, then, is essential to an understanding of who Christiaan Huygens was able to become. This was a household not only where Descartes and Rembrandt were among the guests, but also one close to the power and influence of the government of the Dutch Republic. All this provided introductions, but Christiaan’s brilliance did the rest, as he found his place in a European intellectual firmament that included Blaise Pascal, Pierre de Fermat, Marin Mersenne, Jacob Bernoulli, Robert Hooke, Robert Boyle, John Wallis, Giovanni Cassini, Thomas Hobbes, John Locke, Gottfried Leibniz and Isaac Newton. Was there something about the light? That’s what everybody wondered about the painting of the Dutch ‘Golden Age’. The sun was softer, the colours less harsh, the contrasts less violent than in Tuscany or Madrid. The windows were larger, the interiors less dim. Dutch light led artists to revel for the first time in the domestic scale of daily life, with its tranquil landscapes, unpretentious rooms and polite encounters. By the early seventeenth century, Dutch artists were already benefiting from the scientific understanding of perspective, and knew about the camera obscura, a device whereby an image of a scene could be projected through a small hole in a wall or other barrier onto some kind of screen. This novelty became creatively interesting when it was discovered that the exterior scene could be optically altered by placing a lens in the hole, allowing artists to compress impossibly wide panoramas onto canvas. It almost seemed as if they were able to trap light itself, releasing it again between gilded frames, as visions of the world made new in glowing quadrature. But the scenes they captured and the light they worked with – those were natuurlijk, natural, that is to say both devoid of artifice and composed of nature. They sprang from local earth and air and water. Theirs was art of its place. Did the sciences also benefit by native circumstance? We know the unsurpassed art of the Dutch seventeenth century: the landscapes of Ruisdael, the portraits of Rembrandt, the interiors of Vermeer. But the science was its equal, and we should not be embarrassed to think of the two together. As the Dutch painter and art scholar Samuel van Hoogstraeten decreed in 1687: ‘The Art of Painting is a science for representing all the conceptions or impressions which the whole of visible nature can offer and for beguiling the eye with outline and colour.’ And what tool was more essential to the beguiler than an ability to handle light, the light that illuminates visible nature, the light that permits us to see it? Light is surely the common factor that unites the interests of art and science. Much of the science of this period is directed towards understanding that light. Willebrord Snel of Leiden measured the size of the Earth and restated the law of refraction that still bears his name as Snell’s law. Then in nearby Delft, and in Alkmaar and Middelburg, Anthoni Leeuwenhoek, Johannes Swammerdam and Cornelis Drebbel made some of the first investigations using microscopes of their own design. The first telescope was demonstrated from a tower in The Hague. There followed, in addition to Huygens and his family, many other Dutch grinders of lenses and assemblers of experimental optical devices, not least the philosopher Baruch Spinoza, who earned his modest keep as a lenzenslijper after he was expelled from his religious community in Amsterdam. Spinoza, the child of immigrant Jews from Portugal, was fortunate to be born in the relatively liberal Dutch Republic. But others came purposely for its freedoms. The most notable of all was René Descartes, who, in 1628, fled religious turmoil in France for the intellectual liberty of the new Dutch universities, and then to find a quiet place to write his philosophical masterwork, Discourse on Method. Many regard this treatise, in which Descartes introduces his famous cogito argument, ‘I think, therefore I am’, as the foundational work of modern philosophy. But Descartes saw it rather differently, as the theoretical preamble to a series of works concerning the nature of the world in all its aspects, including La dioptrique, his exploration of the physics of optics, the nature of light and the anatomy of the eye. Both of these great philosophers, too, were guided by the light. The light made the United Provinces a place for looking. The liberal, inquisitive times permitted it. And the place insisted upon it: no shadows on level ground. What things might you wish to see? Look out first of all. There might be danger in these times of recurrent war and tenuous peace. Across the flat landscape, you might notice approaching strangers or enemy soldiers. At sea, distant ships, unfamiliar flags and wrecking sand bars just pricking the surface of the brine. Danger, perhaps, and opportunity, too. Remote shores to claim for trade and empire. Any place on a map or a chart or, better still, not marked on one yet. A place as yet unseen. Visualize it. Is a hoped-for gain not called a prospect? Now look around. What is this Dutch country? Of what is it made? Water. So much water, flat zees and meers, sometimes gleaming, sometimes dark when they are scuffed by the wind. Light always changing. Flat fields with ditches and dykes all the way out to the low horizon, with here and there perhaps a few stands of trees roughly shaken by the breeze. Scoured beaches, open heaths, scrubbed towns and villages identifiable from afar by their church spires. Or look up. You might nurse the ambition to grasp the unreachably distant, the heavens, the stars and planets and the voids in between them. Can you make darkness visible? What is out there? Look down. Look in. Look closely – it is the first time any human has been able to do so – at nature’s tiny miracles, seeds, insects, moulds, swimming animalcules. Do you dare to look at the human body? Your own in a mirror larger and more perfect than any you have known before, or somebody else’s (poor wretch) at a surgical praelector’s anatomy lesson. Both of these spectacles, the one domestic and private, the other theatrically public, were becoming technically feasible and socially acceptable as shame and superstition about the human body were swept away. You might be curious about the origins of life, and wish to spy on your potential descendants in semine or in utero. Keep looking if you can bear to. At our dirt and dust. At human waste. What is lost of us when we spit and defecate? In the seventeenth century, it’s suddenly all there to be seen. The very farthest thing. The very smallest thing. Things of beauty and wonder. Cosmos and microcosmos. Then you might wish to see the quality: the fineness of the weave, the clarity of the diamond, the skill of the artist with the brush. The quotidian, the ordinary, the small: the eye of the needle, the end of the thread, the missing stuiver or duit, the bottom of a pocket, the beginning of a hole. You might need help to read the written word, as Papa Constantijn did. You might long to see again the things you once could see unaided. Look about you. Regard your fellow citizens. How well they are doing! You might even wish to peer into your neighbours’ rooms, if only to be sure they have nothing to hide. And they will tempt you with their large polished windows. ‘The country is flat,’ explains the novelist Cees Nooteboom, which leads to the extreme visibility of people, and that in turn has become visible in their conduct. The Dutch don’t go towards each other, they come up against each other. They drill their radiant eyes into one another, and weigh their soul. There are no hiding places. Not even their houses. They open their curtains, and consider it a virtue. Go on, take a peek, have a snoop. Those words are Dutch, too. All of these things are better discerned by using the right optics. Thanks to the new dissectors prepared to press sharp steel into slimy viscera, it became understood that the lens of the human eye is not in fact the organ of sight, only its enabler, a kind of secretary, sorting incoming information into order for executive processing by the brain. If your own cornea, aqueous humour and lens are not up to the job, then you will need the assistance of additional lenses in the form of spectacles or a magnifying glass. To see new worlds, you will need new instruments: a spyglass or a perspective tube for the great and distant; a thread-counter or a ‘flea glass’ for the small and close. There was nowhere better equipped to manufacture these devices than the tech hubs that were the thriving cities of the Dutch provinces. Then who, but an artist, do you summon to record the images you see in the eyepiece? Unless, of course, you are enough of an artist to do the job yourself, which Christiaan Huygens certainly was. Christiaan Huygens offers so much: he made major discoveries in astronomy and physics; he showed how mathematics could be used to describe the operation of nature, without which modern science could not function; he was an inventor and fabricator of ingenious devices. He clearly has a story that needs telling. To say that it is necessary to reclaim him from the margins of history ought to be putting it too strongly, since his work is so thoroughly enmeshed with that of better-known major scientific figures. Yet even many historians of science have contrived to ignore him. And who else has heard of Huygens now? Would it not be good to be reminded why he would have been ‘the greatest scientist of his generation’, in John Gribbin’s words, ‘if he had not had the misfortune to be active in science at almost exactly the same time as Isaac Newton’? This kind of amnesia is not uncommon in the anglophone world. Yet there is something not merely neglectful, but actively unjust about the way in which Huygens’s reputation has been effectively annihilated by the towering presence of Newton. Writing about Huygens therefore involves taking a stance. The historiography of science requires that those whose theories are overtaken by better theories shall be forgotten. But this is not quite what happened in Huygens’s case. His discoveries for the most part still stand – your analogue wristwatch runs using his mechanism; you understand that light travels as a wave. It is rather that Newton radiates such a dazzling light that Huygens is simply outshone. So how to tell Huygens’s story? Should I pretend that Newton did not exist? That might be possible. Huygens worked in the Dutch Republic and in France. Tempting, perhaps, to bring down a fog in the English Channel so that the islands of Britain cannot be seen and to screen out English science for the duration. But Huygens and Newton were contemporaries: Newton was thirteen years Huygens’s junior, and outlived him by thirty-two years. More to the point, they were aware of one another’s work. They corresponded and even met. In fact, Huygens was one of the few people whose scientific opinion Newton valued. That certainly would not do. Perhaps, therefore, I should position Huygens in opposition to Newton. But then it would still be Newton’s story. Huygens’s story would be the antithesis, an un-story. He would be the one who didn’t explain gravity, who didn’t use calculus, who didn’t split light with a prism. That telling would insist on a kind of parity between these discoveries and Huygens’s own – centrifugal force, mathematical formulae, light as a wave – which would be tedious as well as tendentious to read. Or, the scientifically minded might interject, I could just write objectively the story of Huygens’s life and bring Newton in where he is relevant and leave him out where he is not. Wouldn’t that be best? Why complicate matters? Certainly, Christiaan’s life seems to have a convenient arc. By the age of seventeen, in 1646, he had demonstrated a mathematical prowess that brought him to the notice of the greatest practitioners of the day. In 1655, Saturn’s moon, Titan, and that planet’s mysterious ring became his calling card. In 1658, he presented his design for a more accurate clock to the Dutch state. In 1659, he diagrammed the antecedent of the slide projector, which he called a ‘magic lantern’. The late 1660s he spent largely in Paris, consolidating his professional relationships. After further work on light, establishing that it must have a wave-like nature, and new experiments with telescopes and microscopy, as well as improvements to clock mechanisms, he published major treatises on time in 1673 and on light in 1690. From his prodigious youth, then, he advanced with luck and skill (fortune favours the prepared mind, as a later scientist said), seizing his opportunities to make some surprising discoveries, settling down in dogged pursuit of other goals, before achieving a remarkable synthesis in maturity with his major works, and finally withdrawing into a period of eccentric decline. That would make a satisfying narrative, would it not? But closer inspection reveals this to be a gross simplification. Huygens’s life was nothing like as well shaped as this storyline suggests. The world gets in the way: he pursues many projects at once; they stop and start; he is here (The Hague), he is there (Paris); he is well, he is ill; there is peace, there is war. All this makes it impossible to tell Huygens’s story as a simple sequence of events. Besides, Huygens has other stories. He is not only the overshadowed genius. He is the polymath – curious enough and competent enough to make progress in several different fields at once. This story has allure, especially in these days when it is hard enough to be an expert in one tiny area of specialization and polymathy seems like a dream from a world before some kind of Fall. He is the diligent correspondent, the networker, the diplomat, too. Another of Huygens’s stories clearly revolves around his extraordinarily attractive family: his prolific, ubiquitous and apparently immortal father – the poet, composer and right-hand man of the stadholders – and his older brother, also called Constantijn, who succeeded their father to accompany William III of Orange on his peaceful invasion of England, and recorded the adventure in a remarkable diary. If John Donne, the doyen of English metaphysical poets, had fathered both Isaac Newton and Samuel Pepys, one could not have devised a more propitious family constellation. And there is more. Christiaan is the internationalist, heroically ignoring national differences and distinctions because they have no meaning in his world. He is a key figure – perhaps the key figure – in the construction of scientific research as an international project during the seventeenth century. This is a rather different story. Early modern science was mostly an enterprise of brilliant individuals who enjoyed the patronage of enlightened rulers here and there. Before the emergence of many nation states, it was certainly not organized at a national level. This is what makes Huygens’s years in Paris truly remarkable. Huygens’s brilliance as a mathematician and astronomer was readily acknowledged by most of the leading French natural philosophers, and he responded with alacrity to the initiative of Louis XIV’s influential minister Jean-Baptiste Colbert in 1666 to establish a scholarly academy to advise on the scientific improvement of the French state. Without Huygens – and one or two other cosmopolitan intermediaries such as Henry Oldenburg at the Royal Society in London – science might have remained for rather longer a rudderless amusement confined within isolated courts. Holland, as a land with neither a monarch nor even, for much of Huygens’s career, a stadholder, nevertheless prospered during the republican interregnum, owing to colonial conquests and the expansion of maritime trade. This economic growth brought with it a kind of freedom unknown elsewhere. Huygens could only have envisaged a more open way of doing things. Yet the Dutch Republic at this time could never have furnished its own scientific centre, because of the intense rivalry among its cities and provinces. Huygens was fortunate to find Paris so open to him, and France was fortunate to find him. This achievement is all the more remarkable when one remembers that the second half of the seventeenth century, when Huygens flourished, was a period of great religious and political turmoil and almost constant warring between European nations. The ferment accelerated the spread of new ideas, as well as raising practical obstacles to intellectual progress. At different times, the English philosophers Thomas Hobbes and John Locke, both well known to the Huygens family, found themselves respectively in exile in Paris and Amsterdam, for instance, while Dutch-trained painters such as Peter Lely and Godfrey Kneller sought work in England where patronage became more abundant. Huygens sought English patents for his maritime clock even as the Dutch navy sailed up the River Medway to torch the English fleet at Chatham in 1667. And he continued to work with French colleagues when Louis XIV invaded the Low Countries five years later, a campaign during which the Dutch were driven to flood their own precious farmland as a defence against the French advance. Two factors, then, explain why Huygens has a strong claim to be regarded as the leading actor in ‘the making of science in Europe’: first, his bringing of mathematical rigour to the description of physical phenomena; and second, his resourcefulness in developing institutional frameworks for scientific research on the continent. It is impossible to imagine progress in science today without either of these foundations being in place. As Huygens is purported to have said: ‘The world is my country, science my religion.’ While wandering among the dunes that guard Dutch cities against the incursions of the North Sea, I began to realize that Huygens’s geographical location might have a more material relevance to his work. It was these dunes that provided the sand for the glass from which lenses could be ground. Did this explain the extraordinary flowering of optical science that happened here, dominated by Huygens but preceded by Snel, and by the numerous claimants to have developed the telescope, and then followed by the microscopists Leeuwenhoek and Swammerdam, and by the philosophical lens-makers Descartes and Spinoza? Perhaps it was true, as Constantijn Huygens had written, that ‘The Lord’s benevolence shines from every dune’. This suggested another kind of story. Not the old one of the lone genius with his flashes of brilliance striking out of nowhere, but instead one of an investigator deeply engaged with his place, for whom light and sand were raw materials. This is the world I hope to describe in Dutch Light. I have followed in Huygens’s footsteps through the Netherlands and to Paris and London. I visited his houses, or more often the sites where they once stood. I sat in the grounds at Hofwijck where he sketched, and watched indoors as the sunlight tracked across the chequered floor. I peered at – and occasionally through – the few surviving instruments that he built and used. I inspected his portraits for clues to his personality. My principal source has inevitably been the twenty-two volumes of Christiaan Huygens’s Oeuvres Complètes, compiled with extraordinary labour by an international team of scientists and historians through two world wars from 1888 to 1950. Here, I could hear the authentic voice of the scientist. And in their letters, diaries and poems, I could hear the voices of his father and siblings. For a closer personal connection, I turned to Christiaan’s original notes in the library at the University of Leiden, which revealed additional facets of Huygens’s character in their dense tapestry of working sketches and calculations, showing how he pursued many thoughts – mathematical, mechanical, astronomical and musical – at the same time. There, I held in my hand Christiaan’s little sketch of Saturn and its ring (trying to ignore the wobbly pencilled line that the editors of the Oeuvres Complètes had crudely looped around it in order to select it for illustration). The perfect circle of the planet’s outline was precisely inked, as were the edges of the ellipse that girdled it. The space around the planet, and crucially also the spaces between the planet’s surface and the inner edge of the ring, were delicately shaded in a dove-grey wash of ink, and the whole was hemmed in by busy handwriting. Huygens had made this beautiful little drawing in 1659. My voyage of discovery has not been obviously epic. Rather, it has been an interior journey, into a world of luxury and leisure, but also into a world of curiosity, seriousness and purpose. A modest world, in its way, but not a smaller one. Like a ‘Dutch interior’ painting, it turns out to contain everything.