14
LIGHT AND GRAVITY
The invasion did not begin well. Lying off Hellevoetsluis, Prince William’s fleet of forty-eight warships, twelve fireships and more than two hundred transport ships was at first delayed in its departure for the English coast and then forced back to port by storms with many vessels damaged. The horses taken aboard for the army’s march to London had been incorrectly stalled facing the bulwarks, and many of their heads had been stove in by the ships’ motion, and so the carcasses had to be dumped overboard. Travelling on the prince’s vessel, Christiaan’s brother Constantijn alone lost five animals, and the order went out to round up replacements from wherever they could be bought. Fortunately, the 50,000 printed copies of William’s declaration of his claim to the English throne that the fleet carried aboard remained intact.
A small pen-and-ink sketch, presumed to be a self-portrait, shows Constantijn Huygens shortly before the 1688 expedition to England, with a smooth, untroubled face emerging from amid the abundant curls of his periwig, wide-set eyes and the ghost of a smile, a man at ease and perhaps even a little smug in his important secretarial office. A medallion profile in ivory made in 1690 while he was gathering maps and equipment in London in preparation for the army’s further campaign in Ireland reveals a rather different figure, stout and jowly, tight-lipped and proud, staring coldly ahead. The two years away from home were often testing and trying, but the prince’s secretary has the look of one who has carried out his duties well, and is as secure as anybody can be who holds a position at court.
In the frantic days as the fleet was being readied again, and everybody waited for an easterly wind, Constantijn suffered from a fever and bad dreams. The Dutch were on the brink of a momentous action that would, if all went well, secure the safety of the republic from French ambitions. In England, the campaign would come to be known as the ‘Glorious Revolution’, a term all but coined by Huygens himself as the army waited, when he noted down the words of William’s field marshal, the Prince of Waldeck, that they were embarking upon ‘a great and glorious enterprise’.
The armada – four times the size of the one Spain had launched against England a hundred years earlier – finally got away on 11 November. The ships flew down the Channel before a ‘Protestant wind’ passing Dover and the Isle of Wight, and dropped anchor in Torbay four days later. ‘We landed at a village called Braxton [Brixham],’ Huygens wrote in his diary. ‘It is very simple. Its few decayed houses are built of stone – hewn from rocks found both on the coast and inland – and roofed with slate . . . We dined this evening on a very tough fricassé of lamb. I wrote a letter to my wife.’
The Dutch army, 21,000 strong including Huguenot, Scots and English volunteers, set forth nervously at first, uncertain as to whether they might encounter English troops loyal to King James II. But they met little resistance. Huygens noted ‘the high mountains and deep valleys’ of the Devon countryside.
The roads were incredibly bad, all made of stone and strewn with loose rocks, and very dirty and muddy to boot. Along the way we saw country folk everywhere, as on the previous day. Women and children shouted, ‘God bless you,’ showering us with good wishes. They gave the Prince and our men apples, and an old woman with a bottle of mead wanted to pour a glass for His Highness.
In Exeter, they found the people ‘polite’ but ‘extremely frightened’ because James had issued a warning not to deal with the Dutch invaders. Huygens had the delicate task of writing to the bishops of Exeter and Bath ‘asking – but with threats added – to have our troops billeted in their cities’. Prince William addressed the people and gave reassurances, and what little resistance his men encountered soon melted away. It became clear there would be no counter-attack.
The Dutch sent a message ahead that English troops should remove from London in advance of William’s arrival. Rain turned to hail and the mud grew thicker as they marched on through Dorset and Wiltshire, but there were a few highlights on the route. Huygens admired the newly erected Italianate town hall in Abingdon. William made a point of seeing the paintings at Wilton House, although Huygens did not accompany him, preferring to hurry on to Salisbury ‘to warm myself’.
They reached the royal castle at Windsor on 24 December. Huygens marvelled at the sight of the River Thames in the splendid weather, and inspected the royal apartments with their tapestries and ‘many good paintings by Titian’. Often in England, when William was in the mood to chat, he spoke with Huygens, a kenner of art like his father, about the paintings in the royal collection at Windsor and at Whitehall and Hampton Court. The following evening, Huygens called on the aged Isaac Vossius, a scholar and family friend from The Hague who had published a treatise on the nature of light before moving to England and becoming the prebendary at Windsor. He found the old man in his library where
he complained about the impoliteness and gruffness of the English, with whom he spoke little, he told me. He said that he was afraid that if His Highness took the crown, he would lose the goodwill of the people. Then he began to talk in his own manner about physics, about the seeming circles on the moon, as he postulates in his book, though without sound reasoning.
Christiaan was relieved to hear of his brother’s safe arrival in England, especially after all the difficulties at the outset of the expedition. ‘Now your arrival in London is awaited with impatience, and the reception that will be given to the Prince which will doubtless be a marvellous thing to behold,’ he wrote. Aware of French interests, Christiaan was keen to hear as soon as possible ‘how everything will be set up and run both there and here’, and anxious that Dutch soldiers be sent home, ‘now that you no longer need them, lest we are the recipients of some great insult as we go about protecting our neighbours’.
Alone in The Hague, Christiaan was clearly missing his sibling’s companionship and some back-and-forth with a scientific equal. Cassini’s recent claim to have sighted two further new satellites of Saturn at the Paris Observatory still rankled with him while he remained unable to see them with his own equipment. ‘When shall we work together again on large lenses?’ he asked plaintively. Heedless of his brother’s burden of court duties, he urged that when Constantijn reached London he should waste no time in visiting a particular lens-grinder as well as Robert Boyle and other members of the Royal Society. He clearly regarded his brother as the advance guard for the trip he hoped to make himself in the spring. ‘I would wish to be in Oxford [he meant Cambridge], only to make the acquaintance of Mr. Newton, of whom I greatly admire the fine inventions I see in the work [the Principia] he has sent me.’
Constantijn and the Dutch forces finally arrived at Whitehall Palace on 27 December: ‘many bonfires were lit’. Over the next few months he did find time to make some scientific connections. He visited the shop of Thomas Tompion, the leading watch-maker. He had a case made for the Campani telescope that he had carried with him for astronomical recreation during the campaign. He bought books and enquired about the latest Transactions of the Royal Society, which was late to the press. The Dutch were horrified to find that the only printing operations in England were in London, Oxford and Cambridge, in sharp contrast with the situation in the Dutch Republic, where every little city had its own press.
Huygens was officially William’s secretary for Dutch affairs, but it was often hard to compartmentalize the work or to get the prince’s attention. Before they left Holland, he had been entrusted with the English seals, but he had to relinquish these to the secretary for English affairs once they were in England, which caused him some disquiet. At times, there was an absurd dimension to the competitiveness between the secretaries, for example when it came to obtaining a favourable camping pitch while on campaign, or on one occasion that Huygens ruefully records when a rival’s coach pushed ahead of his own in order to move closer to the prince.
Sometimes Huygens was able to use the minor distinctions in court duties to work his way out of an awkward situation. In February 1689 he was approached by men seeking to put off the execution of George Jeffreys, James II’s Lord Chancellor, the notorious hanging judge of the Bloody Assizes held after the Monmouth Rebellion against the Stuart king. He was offered 500 pounds, which he declined to accept, saying it was an English matter. He primly told the men that in Holland such behaviour would be seen as corruption.
That same month saw the first session of Parliament held before Prince William. ‘The Lords and Commons came to His Highness to thank him for his care and trouble in delivering them from popery and slavery, and asked him to continue governing,’ Huygens wrote. The joint coronation with his English wife, Mary, the Protestant daughter of James II, took place on 21 April. All that spring Huygens suffered from colds and gout. He grew dissatisfied with his cramped quarters at Hampton Court and with his salary, which he felt would be insufficient to support his family when they followed him over. He cannot have been so badly off, however, as he consoled himself by buying art for his master and for himself, including a book of human movement studies that he took to be by Leonardo da Vinci. His relationship with William remained companionable, though, and occasionally the prince would ask after his secretary’s family or his health. When Huygens complained about the damp weather, William commiserated: ‘It is the air here . . . Do you not long for home sometimes?’
Huygens was clearly on good terms with William, but he was not as favoured as some. Rumours of homosexuality at court arose in England after the 1688 invasion, fuelled by Jacobite partisans. Although William and Mary produced no heir, it is known that Mary suffered many miscarriages. Their relationship, which had been cool at first upon their marriage in 1677, grew into one of true love. But William did have male favourites. He was distraught when, upon arriving in London, he learned of the death of one close friend, Gaspar Fagel, who had helped to orchestrate anti-Jacobite propaganda in advance of the invasion: ‘I am losing the greatest friend that I can have in this world,’ he wrote.
However, William’s most faithful companion was Hans Willem Bentinck, a page who was soon promoted to chamberlain and thereafter to more senior advisory and diplomatic positions. When the prince contracted smallpox in 1675, Bentinck lay in bed with him, on doctor’s advice, in an effort to cure him; the prince recovered but Bentinck caught the disease in turn. This selfless act earned William’s lasting gratitude, and Bentinck was showered with favours and later given the title of Duke of Portland. In England, however, another page, Arnold van Keppel – charming, vivacious and twenty years Bentinck’s junior – caught William’s attention when his horse broke its leg. When Keppel was promoted, Bentinck felt rejected and eventually asked to be relieved from the service of the court.
William’s sexuality had never been an issue in the Netherlands (where sodomy was a capital offence, but prosecutions were very rare). Nor was it a great scandal at court or even sufficiently noteworthy for Huygens to write about in his diary, where it seems that the subject simply held no interest for him. The evidence in any case amounted to no more than a habit of effusively emotional greetings revealed in hand-kissing and letters – William signed his letters to Bentinck with endearments such as ‘I am always till my last sob yours’. Besides, other factors may have guided the king’s judgement. Huygens observed that Bentinck sometimes came across as ‘too Dutch’, whereas Keppel could be relied upon to behave like an Englishman on difficult occasions.
Others experienced more severe pressures, which contributed further to the stresses of Huygens’s job. One of the new king’s first actions was to appoint John Temple (the son of Sir William Temple, who had been Charles II’s special ambassador to the Netherlands) to be his secretary of war. But Temple had gone to the king to confess that he did not feel up to the job. Huygens recorded the rest in his diary:
The king had encouraged him and told him that if he hired the old clerks, he would learn the trade very soon. Then Temple left in a pair of oars on the river, and had himself rowed to London Bridge. When he arrived there, he took out a paper or a letter, together with a shilling for postage. When he was under the bridge, he said, Adieu, watermen, and threw himself in the river and drowned.
According to other sources, Temple had stopped off on his final journey downriver to call on various friends, who found him ‘very Melancholy and Discontented, or at least somewhat disturbed and troubled of Mind’. He had then switched boats and ordered the new skipper to shoot the bridge, where he leapt out into the rough water. His body was retrieved two days later a little way downstream at Pickle-Herring Stairs. The suicide was a reminder – hardly needed in Huygens’s case – that the service of Prince William was onerous in many ways.
On 11 June 1689 Constantijn’s wife and son arrived in Harwich along with his brother. Christiaan had felt abandoned in The Hague, not only by family but also by his nation now that the stadholder was in England. He had prophesied to Constantijn: ‘It is only England that will profit in the end from this great revolution, and the only advantage that will redound to us [Dutch] is, as I think, that we should otherwise have fallen into greater misfortunes.’ Above all, perhaps, he regretted that Prince William had ‘so little fondness for studies and the sciences’. If matters had been otherwise, he might have had the hope of royal patronage such as he had enjoyed in France, and a little more scientific companionship. As things stood in The Hague, there was ‘not a single soul to talk to about things of that nature’.
His principal reason for going to London, then, was not to bask in the aura of the Dutch court abroad, and he was not sorry to miss William’s coronation, the second such occasion that he had managed to avoid in London. Instead, he wished ‘to see some old friends, apart from those who have recently passed away, and what is being done in the way of sciences, in London and in Oxford and Cambridge or wherever I am sufficiently known’.
It was to prove a short but significant visit. He travelled to Greenwich, where John Flamsteed showed him the observatory and his instruments. He shared a laugh with Robert Boyle, who told him a story about a man who claimed to have made an ounce of gold out of lead by using various powders, and who had subsequently been arrested in France. On 22 June he attended a meeting of the Royal Society at Gresham College, where he read parts of his works on light and gravity. Among the audience was Isaac Newton.
Newton was in his pomp, forty-six years old, with his masterwork, Principia Mathematica, published just two years before. But Huygens, at sixty, was still the acknowledged senior figure, the most respected physicist in Europe. Newton had sent a personal copy of Principia to Huygens upon publication (it was hand-delivered by Edmond Halley). This was not just good manners. Newton knew that Huygens was one of very few recipients of the work in a position to offer useful comments on it. Furthermore, the ground had been prepared by Fatio de Duillier, who, as an informed acquaintance of both men, was in an ideal position to mediate a correspondence between them.
Huygens had devoured the Principia at his Hofwijck retreat, weighing the merit of its new ideas against his own, still unpublished, theory of gravity influenced by Cartesian vortices. He duly responded with various suggestions and a list of errata, which he hoped might be incorporated into a second edition. Newton graciously acknowledged the comments, but they were not such as to cause him to revise his basic thinking. In his private notebook, meanwhile, Huygens wrote: ‘Vortices destroyed by Newton. Vortices of spherical motion instead.’
Huygens felt unable to embrace Newton’s work in toto. He was uneasy about the Englishman’s dynamical approach, based on unseen forces, including the strange centripetal force that held celestial bodies in orbit. Surely any force had to be accompanied by matter in motion. How could the force of gravity be transmitted through empty space, as Newton’s theory stipulated, without the aid of vortices? He was critical, too, of Newton’s reliance on empirical evidence. For him, it was not enough to say that a hypothetical force explained all kinds of motion, and that it was consistent with rigorous mathematical exposition. He still wanted to know where the force actually came from. Even Newton had no answer for this, and to Huygens the omission seemed a step backwards, perhaps even towards occultism. On the other hand, the Principia offered persuasive explanations of many phenomena that had troubled natural philosophers. For instance, Newton’s deduction that the Earth was ellipsoidal in shape – an idea that Huygens had entertained independently – neatly explained the pendulum anomalies that had dogged the sea trials of his marine clocks. Furthermore, Huygens recognized the mastery of Newton’s (still unpublished) method of calculus, and it was this above all that convinced him that his theory of gravitation must be essentially correct. For Huygens as for Newton, mathematics was the ultimate arbiter of truth.
Despite the two men’s differing philosophies, it was an affable meeting. If there was any sense of noblesse oblige on Huygens’s side, or of triumphalism on Newton’s, nobody thought to note it down. In his Gresham College lecture, Huygens had set out his explanation of the double refraction of Iceland spar, and it was this novelty that drew them into conversation afterwards, causing Newton to concede that the Principia was not yet complete. It was a meeting of minds rarely equalled even under the auspices of the Royal Society. The men met at least twice after this and began to correspond directly.
The relative standing of the two great physicists at this time is illustrated by something that happened the following month. Though he had long been Lucasian Professor of Mathematics at Cambridge and a fellow of Trinity College, as well as having been recently elected as one of the two members of Parliament for the university, Newton nurtured an ambition to be the provost of King’s College. This required a petition to the king himself. Through his brother, the king’s secretary, Christiaan Huygens was able to provide access for his new friend. On 9 July Newton called on Christiaan, who was with Constantijn at Hampton Court, and the following day Christiaan, Newton and Fatio de Duillier were granted an audience with William. The king announced that he was prepared to support Newton’s nomination, but the idea was later rejected by the fellows of the college – an unprecedented action, after which royal prerogative was never again used in this way. Though it may not have had the desired outcome, this moment of scientific internationalism shows that it is Newton who is put in Huygens’s debt, and not the other way round as modern observers might assume.
A few weeks after this episode, Christiaan set sail from Gravesend on a slow but eventful crossing of the North Sea.
24 Wednesday. Embarked in the morning on the ship Briel, the same which had carried the King of Holland and England . . . Saturday 27, sighted since morning 3 vessels approaching close to us thought to be French. All prepared for combat, the passengers except 2 or 3 each taking a carbine and a bandoleer. The women went down in the rope hole and were there for more than 2 hours. Finally with my telescope I began to discern Orange, white and blue banners, and it was realized a little later that these were vessels from Amsterdam.
He arrived safely in The Hague on 30 August.
Constantijn continued to exchange scientific news with Christiaan, although he was now mainly preoccupied with the forthcoming military expedition to Ireland, which was now ‘only too certain’. That autumn and winter, they discussed Leeuwenhoek’s microscope observations and his notion of what Christiaan called the ‘infinite inclusion of animals and plants’, that is, the idea that each creature contains the seed of its successor and so on ad infinitum, so that no truly new living creatures are made in the world. Constantijn visited lens-grinders and watch-makers, and heard Thuret speak at the Royal Society, claiming to have invented a new marine clock. On one occasion, he was amused to find himself accosted by a Scottish milord in the royal antechamber who wanted to know if it was his father or his brother who had invented the pendulum clock. But the political upheaval made life difficult both in Holland and in England. Christiaan sometimes thought it would have been better for both countries if James had been allowed to rule his kingdom in peace. In Holland he noted ‘a great cooling towards all the finer things’.
In June 1690, Constantijn Huygens rode with the king’s army to Chester, from where they sailed via the Isle of Man to Carrickfergus Bay. On a long stone bridge on the way to Belfast, he was ‘disconcerted to see a large crowd of poor and miserable people, men, women and children, bad-tempered and looking very ugly and unhealthy’. In Belfast itself, the king took lodgings at a grand house ‘which had very bad paintings’ while Huygens lodged in an alehouse. He noted that the people ‘do not want to admit to being Irish, and say that they are Scots who came over’.
The progress that had been largely bloodless through England was to turn remarkably bloody in Ireland. William’s army marched south from Newry to Dundalk for Dublin, and on 10 July they spotted James’s troops, which they estimated to be up to 20,000 strong, assembled on the far bank of a bend in the River Boyne. An exploratory salvo from the Jacobite side found its mark. Huygens wrote: ‘The King was hit by a cannonball, which stripped his coat, waistcoat and shirt and scorched his skin. He was bandaged in a hollow-way, making no appearance of discomfort from the first, and saying only, “That should not have gone further”.’
The following day, Huygens rose early, uncertain about the king’s intentions. ‘Nobody spoke with any assurance about the passage of the river,’ he observed despondently. Nevertheless, a plan was hatched. William’s men waited for low tide and then waded across the river, firing as they advanced. James’s troops appeared to retreat, but gathered on a hilltop and rushed down in a counter-attack. This manoeuvre was repeated until Dutch reinforcements arrived, whereupon the Jacobites fled ‘so fast that our cavalry could not overhaul them’.
James fled to France and his army of mostly untrained soldiers deserted. William’s men continued their march south through the ramshackle villages, plundering as they went. Huygens noted that ‘our English soldiers committed various atrocities to women and wretches’, while a Huguenot commander freely told Huygens that he had ‘chopped off both of a girl’s hands, gouged out her eyes, and left her lying there’. Protestant Dubliners rushed out of the city to greet the advancing troops. Huygens was pleasantly surprised by Dublin. ‘The houses are fairly good, compared with most cities in England . . . The shops are rather poorly stocked and the people clad worse than in England, but very much in their own way. The womenfolk are reasonably pretty.’
William’s Irish campaign would drag on for another year yet, with some far bloodier battles ahead. But Constantijn Huygens’s war was over. He sailed from Dublin to Chester on 13 August, and was back in London just over a week later.
Christiaan Huygens’s many-times postponed work on light finally appeared in print at the beginning of 1690. Published together with the (also delayed) Discours de la Cause de la Pesanteur (‘Discourse on the Cause of Gravity’), the Traité de la Lumière (‘Treatise on Light’) was his first major work written in French rather than Latin, an indication of the increased prestige of French science. He made the change from his decades-old working title of Dioptrics to the more ambitious Traité de la Lumière just before the work went to press, at the urging of his French colleagues. This was an acknowledgement of the expanded scope of the final text, but also a signal that Huygens was ready to move on from Kepler and, especially, from Descartes, both of whom had produced works called Dioptrics. The contents dealt with the sun and other sources of light, the speed of light, reflection, refraction and the design of optical media, and yet still gave half its pages over to the new discoveries related to Iceland spar.
The preface to the treatise was notably modest. ‘One may ask why I have been so slow in bringing this Work to light,’ Huygens wrote with disarming candour. He answered his own question by explaining that he had begun to write in Latin, in which language he felt he could describe the science better, before switching to French. ‘I finally judged it better that this text should appear as it is than to let it run the risk, in waiting longer, of being lost.’ He confessed that some of the experiments he described had not produced results quite as crisp and clear as the geometrical analysis implied, and referred, with obvious pain, to ‘where I leave difficulties without solving them’. But this was a new, relaxed and almost carefree Huygens, who was at last prepared to publish what he had and let others be his judge.
Perhaps he was emboldened by Newton, who could not give an ultimate explanation of the cause of colours, and was not especially concerned to do so. Huygens summarized his theory of light, still largely based on Cartesian principles of collisions occurring in the ether, with the almost offhand statement that ‘light expands successively in spherical waves, & how it is possible that this expansion happens at such high speed, that experiments, & celestial observations must investigate’. In fact, Huygens had long been comfortable in principle with such uncertainties. In 1673 he had written to Pierre Perrault with this eminently quotable axiom: ‘I do not believe we know anything with complete certainty, but everything probably and to very different degrees of probability.’ What was new was that he had overcome his fear of the effect it might have on his reputation to commit uncertain findings to cold print. Cajoled by Leibniz, Huygens also abandoned the classical method of argument of earlier works such as Horologium Oscillatorium, based on a succession of theorems and proofs, in favour of a more modern approach, which made the Traité in some respects more forward-looking even than Newton’s Principia.
Huygens sent out the treatise in his customary fashion to his international scientific colleagues. He sought to use the offices of his brother serving William III in London to distribute copies in England, but although his letter of instruction arrived, at first the books, sent along with a consignment of horse-blankets, did not. This was eventually discovered to be because the emissary entrusted with the task, a royal table-dresser named, appropriately enough, L’Orangeois, ‘was found having tried to smuggle through Customs a piece of Holland cloth and some lace which he had hidden all round his naked body’. Soon enough, though, copies were dispatched to Newton, Halley, Boyle, Fatio de Duillier and John Flamsteed, the first Astronomer Royal. ‘You will see that I have in several places marked the divergence of my feelings from those of Mr. Newton,’ Huygens noted to Fatio de Duillier, ‘as I have found myself obliged to do in order to support my Theory, but I have not made them of such a kind that I think he will take them in bad part.’ On the continent, Leibniz, Pierre Bayle and Johannes Hudde also received copies of the treatise. Another recipient was the French Protestant inventor Denis Papin, who had collaborated with Huygens on the design of a gunpowder-operated engine when they had both been living in Paris. Following the revocation of the Edict of Nantes, he had fled to Marburg, where he was about to build the first prototype steam engine.
Newton responded warmly to the Traité, calling it ‘perfectly beautiful and worthy of the author’, in a comment passed on to Huygens by Fatio de Duillier. Clearly, much of the tension between the two great optical scientists dating from the time of their argument about colour had dissipated with their meetings in London. Newton was at work on what would become his own Opticks, but where their theories differed, they sought to avoid conflict. Huygens was opposed to all colour theories, and had purposely omitted to include anything in the Traité about colours, concerning which both Newton and Hooke had such strong ideas. He told Leibniz: ‘I have said nothing about colours in my Treatise on Light, finding this topic very difficult; above all because of the many different ways in which colours are produced.’ For his part, Newton took Huygens’s discussion of Iceland spar as a cue to begin his own experiments with the material, but he made no mention of them – or of Huygens’s substantially correct analysis – when his Opticks finally appeared in 1704, some years after Huygens’s death.
Huygens sent a further copy of his treatise to the philosopher John Locke, who had returned to England with the ‘Glorious Revolution’ following five years living in exile in Amsterdam and Rotterdam. Locke had made contact with Huygens during that time, seeking his confirmation that the mathematics in Newton’s Principia was sound. (Huygens assured him it was.) Locke and Huygens found common ground in disagreeing with Newton’s model of gravitational attraction, in which neither was able to accept the notion of force required to act at a distance through a vacuum. In return, Locke sent Huygens his own newly published An Essay Concerning Human Understanding. Huygens read it ‘with much pleasure, finding there a great sharpness of mind, with a clear and agreeable style, which not all those in that country possess’.
Huygens’s theory of light took its place in a lineage of theories of this puzzling yet most apparent of natural phenomena stretching back to antiquity. Geometers such as Euclid had been unable to decide whether the eye was the originator or the receptor of the light signal. Sometime after 1000 CE Al-Hazen in Cairo separated reflected and refracted rays into vertical and horizontal components, which greatly assisted in the geometric analysis of light. In the fourteenth century William of Ockham and his followers speculated that light might travel like a wave. Leonardo da Vinci also proposed a wave mechanism, while Huygens’s closer contemporaries Francesco Grimaldi and Evangelista Torricelli conducted early experiments on light diffraction, a wave-like behaviour easily observed in water.
In 1801 the British physician Thomas Young demonstrated that interference patterns produced by the diffraction of light through closely spaced slits were analogous to the patterns generated by the combination of water waves from multiple sources. This development at last provided conclusive evidence in favour of Huygens’s wave theory, with Young adding the correct surmise that colours corresponded to light of different wavelengths. Later, Young suggested that the wave action was transverse to the direction of travel of the light rather than longitudinal like sound in air. A few years later, in 1815, a French engineer named Augustin Fresnel, apparently unaware of the work of either Young or Huygens, independently developed his own purely wave-based model of light. The Huygens Principle is now often known as the Huygens–Fresnel Principle.
The long hiatus has played out badly for Huygens’s lasting reputation. As Newton’s stature grew, wave theories of light were soon forgotten. Young even experienced abuse in Britain for daring to suggest that Newton might not be right about everything. Confirmation that Huygens’s theory was correct had to await the advent of optical instruments suited to experiments at scales close to the wavelength of light. Huygens himself did not even live long enough to see his theory effectively ousted by Newton’s misleading corpuscular theory.
Huygens drew. He drew to help guide his scientific thinking and to express new concepts and give form to inventions; he drew to communicate his ideas to others; and he drew for amusement. Though never quite so much or quite so well as his older brother Constantijn, Christiaan sketched and tinted and painted throughout his life: designs for telescopes and microscopes and what he saw through them; the shifting shapes that eventually resolved themselves into the ring around Saturn; Venus in its phases; striped Jupiter; and Mars, recording the first impressions of surface features, including the south pole and the dark blotch of Syrtis major.
He drew designs for lens-grinding equipment and carriages, fountains and pumps. Some drawings reveal the essence of unrealized inventions – for example, a shoe with a sprung sole for a more comfortable walk, or the same idea scaled up as a device to be fitted to ships’ bottoms to float them if they ran aground.* He drew pendulums and the gear mechanisms of clocks. Sometimes a hand emerges from a ruffled sleeve to grip some manually operated device. He drew harps and recorders and keyboards. The margins of his paper and the interstices between the strings of numbers, staves of music, anagrammatic codes and scribbled notes, often running vertically as well as horizontally on the page, are filled with calligraphic doodles and shading exercises. Occasionally the face of an attractive girl emerges from the chaos of data. He drew the Dutch landscape – trees, cottages, windmills, canal boats, the beach at Scheveningen, the house at Hofwijck. He even drew Death, in the series of comically posed skeletons to be projected using his magic lantern.
It was inevitable that the Huygens brothers should grow up to become at least competent draughtsmen. The Dutch Republic was soaked in ink and paint. Mathematicians such as Simon Stevin developed painters’ sense of perspective into a science of optical projection. In turn, optical accuracy became an ‘ethical imperative’ for many artists, a means of exciting in the viewer both wonder and a sense of insight. At the same time, advances in printing technology made it easier to include detailed illustrations in books. One of the most alluring and persuasive images in a scientific text was the representation of vortices produced by an unknown woodcut artist for Descartes’s Principia Philosophiae of 1644. The ethereal vortices, so essential to Descartes’s theory of matter in motion, but conceptually hard to grasp, were given compelling visual form in a set of illustrations showing the solar system with the sun and the planets in orbit around it. With its key features labelled with letters of the alphabet, the illustration was clearly diagrammatic, but it also seemed to be a natural image – a honeycomb of cells, each carefully shaded with pinpoint dots arranged like lines of longitude to give an impression of actual presence and bulk, and all of them together filling space like a mass of soap bubbles or onions packed in a box. Significantly, it was the elder Constantijn Huygens who advised Descartes to use woodcuts rather than copper engravings, because they could be placed directly in the text, where they could best amplify the Frenchman’s theory.
Christiaan Huygens’s facility made drawing an important part of his scientific thinking. He often articulated his ideas visually, and understood, like Descartes, that a visual image could be a useful weapon in his rhetorical armoury. This was especially the case with his many drawings of Saturn’s ring, which not only resolved the blurry shapes seen around the planet into a convincingly pure and simple form, but also explained its appearance and disappearance during different phases of the planet’s orbit. His colleagues realized the persuasive potential of his diagrams very early on. In August 1656, for example, when theories still abounded concerning the nature of what surrounded Saturn, Gilles de Roberval urged Huygens to divulge his interpretation of the latest observations, adding: ‘a figure will assist the imagination; and, if need be, you will do it easily’.
When Huygens employed a simile, it was often visual, too. Light is compared to cannonballs and arrows on the one hand and to water waves on the other. When he discussed the idea of the relativity of motion, arguing that not only objects might be in motion but also the space around them, he introduced the analogy with men in moving barges, even deleting the word ‘space’ from his description and replacing it with ‘boat’ to make his argument more convincing. When describing the double refraction of Iceland spar, he asked his readers to imagine light falling on a ‘toothed surface’ like an array of prisms and thus being split off in two directions.
Huygens’s visual mentality also helps to explain his abiding fondness for the geometrical exposition of physical phenomena over algebraic methods, even when, during the latter part of his career, the latter became more potent with the advent of the differential and integral calculus of Newton and Leibniz. Unlike Huygens, Newton and Leibniz had little training in Greek geometry, and were always more comfortable using equations to express line and form, although Newton did admire Huygens’s proofs enough to want to find his own ways of representing the methods of calculus in geometrical terms.
This is not to say that Huygens avoided more abstract methods. His wave theory of light surpassed those of Hooke and others because of the sophistication of its mathematical description. But even when algebra was his working tool, geometry would often remain his preferred mode of presentation, because it gave an instant sense of physical relationships that many were unable to discern from algebraic formulae alone.
All this puts Huygens firmly on the side of the visualizers in a perennial debate in physics. This most mathematically expressible of sciences has long nursed a division between those who think visualization is a helpful means of interpreting physical concepts that lie beyond the realm of the visible, and those who prefer abstract mathematical methods, and fear that visualizations may be erroneous in themselves and, worse, might lead scientists further astray in their thinking. The split widened at the beginning of the twentieth century with the advent of quantum mechanics, whose weird revelations of the invisible world of the atom led some to demand new visual images to assist interpretation.
There is no doubt that mathematics has the upper hand in the development of physical theories, but images remain hard to resist. Albert Einstein’s famous ‘thought experiments’, involving a train speeding through a storm with its lightning flashes and observers here and there, gave his abstruse ideas about relativity an approachable face that he found helpful not only for himself, but also when explaining his concepts to other scientists and the broader public. It is no surprise to learn that Einstein praised Huygens for coming so close to the concept of relativity himself when he presented his very similar image of bargemen on the Holland canals.
Painters need light and astronomers and microscopists need light. Is it more than coincidence that these disparate pursuits prospered together in the Dutch Republic? Of course, the wealth and confidence of the new country, and its peculiar, self-imposed requirement to display that wealth, if at all, then in quiet, unobvious ways, must remain the major impetus for its astounding art and science. But all this work has its media and its materials, and light is chief among them. Not light in the fundamental physical sense, of course, which we know to possess certain absolute and universal properties, but the ambient light, light that has been filtered through a local atmosphere, light that illuminates a local geography, light that belongs to the place.
The abundance of sand for glass-making in the dunes of Zeeland and Holland may have provided the essential raw material for the lens-grinding innovators of the telescope and microscope. Was there also some special quality of the Dutch sky, as frustrated Italian astronomers muttered when Huygens saw Titan and they didn’t? Is there – was there – a Dutch light?
The artists of the Dutch Republic had no thought of being bathed in a special light. For them, light was more usually discussed as a representation of the force of moral rectitude derived from Calvinist theology. The idea of light and optical devices such as mirrors were frequently invoked in the titles of pamphlets issued for the purpose of illuminating heresies and corruption. In 1668, for example, the Amsterdam brothers Adriaan and Johannes Koerbagh, one a physician, the other a preacher, wrote a polemic, ‘Een ligt schijnende in duystere plaatsen’ (‘A Light Shining in Dark Places’) attacking Reformed Church dogma; Adriaan was put in the workhouse, where he perished the following year. An antinomian tract with the same title by one ‘Christianus Constans’ distributed in 1710 was banned by the States of Zeeland. The author was in fact one Grietje van Dijk, a scholar of Hebrew and cult leader who rejected the authority of the church. Even men like Huygens, whose concern was to understand and exploit light in a strictly physical sense, would have been aware of the importance of this other light.
Dutch painting lapsed from fashion during the eighteenth century, its moralizing intent no longer well understood, its quotidian subjects and pedestrian landscapes ill-matched to new decorative tastes. The English portraitist Joshua Reynolds in his Journey to Flanders and Holland, recording travels made in 1781, made no particular observation about the quality of light in the country he was passing through, and entirely neglected to mention many of the artists we prize today for their handling of the light, such as Jan Vermeer, Jacob van Ruisdael and Meindert Hobbema. His attitude towards the Dutch masters was notably equivocal. He did acknowledge Rembrandt’s handling of light, but he compared other Dutch artists unfavourably with their Renaissance forebears and with recent painters of the French baroque, and occasionally expressed the wish that they had been born in Italy, where they might have exploited their talents to better effect.
Many French artists, including Corot, Courbet and Manet, visited the Netherlands during the middle years of the nineteenth century in order to see for themselves the great paintings of the ‘Golden Age’. Claude Monet made the journey too, but told his friend Camille Pissarro he would have no time for museums because he meant to paint. In June 1871, on the outbreak of Franco-Prussian War, he travelled to Holland on his way to London, and then back through the country again on his return, when he stayed at Zaandam a few miles north of Amsterdam. There, over a period of four months, he produced some two dozen canvases, mainly river scenes with windmills, bridges and sailing vessels. In his Archives de l’impressionisme, the critic Lionello Venturi wrote: ‘This contact with reflections from the waters, which suggested to him the analysis and reconstitution of tones, renews his manner. This is already the definitive style of the Impressionist period: the effect of light is complete and perfect.’ Monet wrote to Pissarro that Holland was more beautiful than people had said, and that in Zaandam alone there was enough to paint for a lifetime. Henry Havard, a French art historian whom Monet accompanied for a time in Zaandam, observed more precisely: ‘The sky above and the water below which remirrors the air are both silvery white or of an extremely pale azure.’ In Monet’s canvases, the houses dotted along the rivers provided contrasting oranges and reds. Monet returned to Holland twice, visiting Amsterdam in 1874 and painting the bulb fields in bloom round Rijnsburg and Sassenheim in May 1886.
Another Frenchman, the writer Edmond de Goncourt, visited Amsterdam ten years before Monet, in September 1861. He found, as he noted in the celebrated journal that he kept with his brother, ‘a country at anchor, an aqueous sky: rays of sun that appear to have passed through a jug of brackish water’. He saw the same light shining forth from Rembrandt’s The Night Watch, ‘a warm, vibrant ray of sun’ that the artist had seized and twisted so as to fall vertically from above and splash across the assembled figures.
The sun shines from above but the weather comes from the west over the long, low coast fronting the pettish North Sea. The land is constantly exposed to the prevailing westerly winds and the damp Atlantic airstream. The atmosphere is cleansed at regular intervals by showers, its ‘daily deluge’, as Andrew Marvell put it. An endless parade of clouds populates the sky. The illumination is alternately soft when the air is laden with moisture or razor-sharp after rain. The low northern light etches the line of the horizon across flat fields and the rooflines in the town. Sometimes the sun is so low that it catches the underside of the clouds with a fierce luminosity, and a bonus radiance is reflected groundward.
No light is constant, however. Because of the unstoppable breeze, any sky is soon transformed. It is surely a defining characteristic of Dutch landscape painting that so many works capture this sense of the weather having just changed, or being about to change, in a way that simply never occurred to artists labouring under the cerulean skies of southern Europe. The rain-washed bricks in Vermeer’s View of Delft show such a moment. So do the ragged hems of clearing black clouds in Ruisdael’s winter landscapes. The popular subject of a breach in a dyke was inevitably also accompanied by departing storm clouds – the damage done, man’s lesson hopefully learned, and in the light in the sky, God’s vague promise never again to visit the same terror on the land.
Are we going to be scientific about this? Any theory must be testable. Is Dutch light truly unique? Is it demonstrably different from other light? It is easy to believe that it is different from the harsh light of Florence or Madrid. But what of other centres of painting? Venetian painters developed a softer style with a greater emphasis on colour and less on outline. Is that connected with the abundance of water there? A similar case might be made for other schools, such as St Ives on the Cornish coast or the Norwich school on the edge of the Norfolk Broads, a waterland created by the same peat-digging practices that carved out the ‘hol-land’ or hollow country that now lies well below sea level behind the protecting dunes.
Perhaps the test is not to think about other places but about other times. In the 1970s, the German artist Joseph Beuys provocatively suggested that the characteristic light that suffuses Dutch painting had been lost forever owing to the poldering of the Zuiderzee, once a substantial bay of the North Sea, during the middle decades of the twentieth century. His underlying supposition was that it is not just the impinging radiant light of the sun that makes the light of the country. Consider the path of a ray of sunlight. It passes through the atmosphere, where it encounters any condensed moisture. The tiny water droplets that make up this aerosol scatter a fraction of the light in all directions. The rest of the light carries on and strikes the ground. If it strikes water, much of this incident light may be reflected upward to pass a second time through the moist atmosphere, where yet more of it will be scattered by the aerosol of water droplets. The exceptionally high proportion of diffuse light produced by the sun’s rays taking this longer path, so the thinking goes, was once the secret ingredient of the Dutch painters. In the riverine landscapes of Albert Cuyp, for example, a luminous haze hanging low over the water is a characteristic feature, and is often the brightest band of colour lying across the canvas.
No people have physically manipulated their topography as much as the Dutch. From the beginning of the seventeenth century, the provinces set about systematically claiming land from the sea, using dykes and drainage pumps to create polders. Thousands of square kilometres of land were created, displacing the large inland seas that once lay around the major cities, such as the Haarlemmer Meer, the Leidse Meer and the Zoetermeer near The Hague (which were already the product of human activity, having been enlarged by digging peat for fuel during medieval times). Constantijn Huygens the elder played a part in this. In August 1633 he bought a parcel of land recently drained from a former lake known as De Waert near Alkmaar. Even though a network of new canals was created at the same time, these would not have compensated for the loss of shining water elsewhere. These works, industriously pursued in many provinces, and continuing into modern times, have undoubtedly had the effect of reducing the area of water within Dutch borders. Perhaps the ambient brightness of the air has fallen in parallel.
However, there are difficulties with Beuys’s theory from an optical and geographic point of view. The Zuiderzee mostly did not lap the shores of those parts of the Netherlands most celebrated for their painters. The province that was most greatly affected by these changes during the seventeenth century was Holland. And here, the cities that were home to the greatest artists – Delft, Leiden, Haarlem, Amsterdam – had already lost many of their surrounding waters by the time that they were working. So even here, in the critical place at the critical time, the quality of the light cannot have been suddenly altered from what it always was by changes made to the watery landscape.
Identified and celebrated only long after these old masters’ work had been rediscovered, it seems that the phenomenon of ‘Dutch light’ is more the product of this art than its vital stimulus, and it is the admiration of later generations of artists seeking to copy their techniques that has embedded the idea in the public consciousness.