* I use the word ‘scientist’ to characterize the vocation of Huygens and many of his peers, although it does not appear in the English language until 1834.
* The Dutch stadhouder translates literally as place-holder, congruent with lieutenant in French. The title is conventionally anglicized as ‘stadholder’ and is similar in rank to a high steward or a governor. A stadholder might be the governor of one or more provinces. The seven provinces at this time were, working broadly northward, Zeeland, Holland, Utrecht, Gelderland, Overijssel, Friesland and Groningen. Their political assembly was termed the States General. Various other lands that did not fall within the provinces were directly governed, in the same way as overseas colonies, by the States General. Most of these lands joined the republic following the Treaty of Westphalia at the end of the Eighty Years War, becoming the province of North Brabant.
* Glass-making spread very slowly through Europe in part because the Venetian Republic imposed harsh penalties on workers who emigrated, in some cases even sending assassins after them.
* In a couplet of 1632, Constantijn Huygens also acclaimed Metius, although he was surely aware, too, of Lipperhey’s demonstration for the stadholder, which happened when his father was his secretary. Christiaan Huygens later went against both Descartes and his father when he credited Lipperhey with the invention of the telescope in his own later treatise on optics (Huygens ed. Worp, Gedichten vol. 2 236; OC13 438).
* As is also increasingly the case in the modern history of science, it is now recognized that the discovery was effectively made by somebody else entirely, working much earlier, much further east. The mathematician Abuˉ Sa‘d al-‘Alaˉ’ ibn Sahl employed the same geometric formula for making more efficient burning glasses in tenth-century Baghdad (R. Rashed, Isis, vol. 81 (1990) 464–91).
* Like the BuendÍa family in Gabriel García Márquez’s One Hundred Years of Solitude, the Huygenses confusingly make use of very few given names down the generations. The Christiaen Huygens here (1551–1624) is the father of the poet and diplomat Constantijn Huygens (1596–1687). Constantijn’s first son was also Constantijn (1628–97), the second was Christiaan (1629–95), named for his grandfather, who is my principal subject. I have used the earlier spelling Christiaen to refer to the grandfather, and Christiaan for the grandson. Where they are liable to be confused, I have occasionally added ‘the elder’ or ‘the younger’ to indicate which Constantijn is meant, although the family themselves used a variety of nicknames to keep things clear. To make matters worse, the Huygens wives of these three generations were all called Susanna or Suzanna, as was the elder Constantijn’s daughter, but here context is usually sufficient to avoid ambiguity.
* Hooft’s official title in Muiden.
* The apocryphal advice to ‘buy land, they’re not making it any more’, often attributed to Mark Twain, did not apply in the Dutch Republic. During the first half of the seventeenth century, the area of land available for farming within the borders of the province of Holland, for example, increased by nearly a third thanks to poldering (Helmers and Janssen 35).
* Following the accession of Charles I in 1625, Rubens was commissioned to paint the ceiling of the building with patriotic allegories, which were greatly admired by Huygens on a later visit, and became an inspiration for him when painters were commissioned for the Oranjezaal, the memorial room in the palace outside The Hague created on the orders of Amalia van Solms after the death of her husband in 1647.
* Ironically, the house was demolished in 1876, and replaced by the neo-Gothic Ministry of Justice building, which still stands.
* Hofwijck’s garden survives only above the waist. The legs were amputated by the arrival of the railway in the nineteenth century, and a later motorway makes further encroachments. It is ironic that good transport links were one of Huygens’s reasons for acquiring the site.
* Huygens’s Hofwijck bears sufficient similarities with Andrew Marvell’s ‘Upon Appleton House’, written in the same year, for some to believe that the two men must have compared notes (Huygens tr. Davidson and van der Weel 208). It is true that both poems use the idea of a walk through woods to the house, and both invoke Vitruvian man, although Marvell is obliged to place this conception in tension with his subject, an older house, a former priory, that lacked the requisite architectural ideality. Marvell also offers a lengthy history of Appleton House (impossible in the case of the new Hofwijck) and an account of the heroic deeds of its owner (which it would have been immodest for Huygens to include). It is possible that Huygens conducted Marvell around Hofwijck when the Englishman visited Holland in the 1640s. However, there is no evidence that the two men ever met, either in Holland or in England.
* The full theory behind this decidedly Archimedean question was not developed until the nineteenth century, building on the principle established by Huygens that, whatever its shape, the centre of gravity of a floating body will tend to lie as low as it can (OC11 83–92).
* The editors of Christiaan Huygens’s Oeuvres Complètes add: ‘one must attach [Huygens’s] name, rather than any other, to distinguish it from the more general Principle of the same name developed in recent times by Einstein’ (OC16 27).
* A very different philosopher, Baruch Spinoza, always preferred to grind lenses manually. See Chapter 12.
* It is interesting to compare the Huygens brothers’ lens-making with that of two other siblings, William and Caroline Herschel, polishing the mirrors for their reflecting telescope nearly a century later, which was ‘dirty, exhausting and monotonous work, for which they wore rough clothes and ignored ordinary household routines and niceties’ (quoted in Holmes 86).
* Huygens suggested that his ‘rejected nestling’ be reserved for private performance only, and for three centuries it was, until 1950, when its public premiere was given, appropriately in Antwerp, by the National Theatre of Belgium (Huygens ed. Hermkens 28, 21).
* In a further example of this seventeenth-century ‘sexting’, Huygens wrote barely a month later to Utricia Swann, who had gone to take the waters at a spa. His letter in English rather ungallantly expresses the hope that she will return with ‘a paire of gelegentheitjes fatt and plumpe, and such as I suppose they were a dozen yeares since’. The word that translates literally as ‘little opportunities’ clearly indicates Utricia’s breasts (quoted in Joby 231).
* The drops are still the subject of contemporary research. See, for example, H. Aben et al., ‘On the Extraordinary Strength of Prince Rupert’s Drops’, in Applied Physics Letters, vol. 109, 231903 (2016).
* Anagrams were also something of a Huygens family obsession. Christiaan’s father sometimes amused himself by working out the number of permutations of the letters of people’s names, and finding character-revealing anagrams for them. His demanding charge Amalia van Solms (Amelie de Solms in courtly French) became ‘sommeille d’aise’ and ‘sale de sommeil’ (‘comfortably dozy’; ‘dishevelled from sleep’). His own name, Constantinus Hugenius, he transformed into ‘Continuus, haut segnis’ (‘enduring, not at all slothful’) (KA 48 ff.240–1).
* A clue to the fact that the limit of resolution is always apt to be pushed further lies in my referring here to the ‘ring’ of Saturn. Nineteen years after Huygens’s discovery, better telescopes enabled Giovanni Cassini to observe that the ring was in fact split in two, hence the ‘rings of Saturn’ we think of today, although close-up images captured by spacecraft now show that this label too is a gross simplification.
* The anomaly of Saturn soon came to be regarded more positively. Following William Herschel’s discovery of the next outer planet, Uranus, in 1781, Joseph Banks wrote in excited anticipation of ‘what new rings, new satellites, or what other nameless and numberless phenomena’ it might reveal (quoted in Holmes 105).
* Although there is no written evidence that Huygens met Thuret until 1662, stylistic details of some of the clocks that Huygens developed in the late 1650s are characteristically French, and may have originated from earlier contact (Whitestone 2017).
* For example, it was later discovered that, as well as being the tautochrone, the cycloid is also the brachistochrone, the curve of fastest descent, a phenomenon of interest to winter sports enthusiasts.
* His father was rather more excited to see the first of his children married off. He gives a brilliantly colourful description of the ceremony in a letter to his friend Béatrix de Cusance, who had hoped to attend the wedding (OC3 67–72).
* Constantijn Huygens was perhaps not aware of the full horror of the story. Cromwell’s head was in fact severed from his rotting corpse summarily exhumed in January 1661, shortly after the Restoration, and more than two years after his death, probably from pneumonia related to chronic malarial fever.
† English dates have been converted to New Style to accord with those used in Huygens’s travel journal and letters. Evelyn gives 1 April.
* Of organizations pertinent to Huygens’s interests, the International Astronomical Union was established in 1919, and the International Union of Pure and Applied Physics in 1922. Their European equivalents were not set up until after the Second World War.
* As Huygens wrote to Moray in September 1661: ‘M. Hobbes is about as good a geometer as Jos. Scaliger. [the sixteenth-century French scholar who insisted that π was equal to the square root of ten]’ (OC22 71–3). In response, Moray sent Huygens a copy of Hobbes’s treatise on the nature of air. It was no better. Huygens replied: ‘In the dialogue of Monsieur Hobbes [in which he attempted to apply the methods of geometric proofs to the vacuum] I find nothing solid, only pure visions. It’s a mental error or because he likes to contradict what he doesn’t accept as real reasons in the effects of the vacuum given in Monsieur Boyle’s book . . . besides it is a long time since Monsieur Hobbes has lost all credit with me in the field of geometry’ (OC3 383–5).
* Huygens was admitted at the same time as Samuel de Sorbière – the first foreign fellows of the Royal Society, although their foreignness was never thought worthy of note until the society tightened its election rules in 1682 (Hunter 119).
* There are 1,200 cents in an octave, and thus 100 cents between each note in Stevin’s twelve-tone scale.
* The Royal Society agreed to employ Hooke as its salaried curator of experiments in November 1662, but his formal appointment was delayed until 1665 (Inwood 30).
* Huygens had reason for doubts on technical grounds, too. Experiments that he conducted in The Hague at this time revealed that two clocks mounted a few feet apart would begin to swing their pendulums in time with one another. This ‘sympathy’, as he termed it (OC5 246–9), Huygens first attributed to an effect of the air between the clocks, but he later realized that it was due to vibrations passed through the shared support (OC5 255–6). Unfortunately, he had gone public with his first thought, and so felt obliged to publish one of science’s first important retractions. ‘There is no shame in retracting what one has misunderstood,’ he reassured his father (OC5 301–2). The phenomenon remains a puzzle to scientists today. See for example Scientific Reports, vol. 6, article number 23580 (2016).
* Not that there were not disputes. One especially futile squabble concerned the Paris Observatory that Colbert had ordered to be built. The architect Perrault wanted a frieze of the zodiac carved into its walls, but the newly arrived astronomer Cassini judged the notion vulgar and unscientific (George 394).
* ‘Milk, and all that comes of milk, as butter and cheese, curds, etc., increase melancholy’, according to Robert Burton’s Anatomy of Melancholy. Bad news for many Dutch people (Burton, vol. 1 219).
* Huygens paraphrases Cicero: ‘It is not the business of a wise man to take part in politics.’
* In 1691 Leibniz wrote to a by then entirely sympathetic Huygens: ‘I like better a Leeuwenhoek who tells me what he sees than a Cartesian who tells me what he thinks’ (OC10 49–52).
* The word was only just beginning to acquire its now usual meaning as applied to scientists. One of the earliest examples cited of this usage in the Oxford English Dictionary is from the diarist John Evelyn, writing in 1662: ‘Hugens . . . so worthily celebrated for his . . . universal Mathematical Genius.’
* The Theban general Epaminondas died on the battlefield in 362 BCE. According to legend, a friend, commenting that he died childless, received the riposte that he was leaving behind him his daughters, his victories.
* Huygens presented this idea to the French Academy of Sciences in September 1678, a few months after the loss of a dozen French ships on rocks off the Caribbean island of Bonaire, which the French had tried to attack, having previously captured the Dutch colony of Tobago. ‘We do not see that Huygens makes any allusion to the fact that this accident was to our advantage,’ write the Dutch editors of the Oeuvres Complètes. ‘Considerations of this kind were not his department’ (OC22 707).
* Constantijn Huygens told Descartes: ‘Theologians are like swine; when you pull one by the tail, they all squeal’ (Descartes, vol. 3 676–9).
* Perhaps he even believed the harm could be physical. When Leibniz complained to Huygens that he could find no Cartesians interested in the philosophy of medicine, Huygens replied: ‘There are enough doctors who think they follow Cartesian philosophy, but they are the ones I would call last if I were in need’ (OC10 55–8).
* A canal-based public transport network linking many of the cities of Holland had been in place for several decades (de Vries 2006 14). Huygens tells us that the last barge from The Hague that stopped at Voorburg departed at 6.30 p.m. (OC9 346–7).
* When Huygens learned – from another source – that Newton had experienced a sustained attack of ‘phrenzy’ (a mental breakdown) in 1693, he carelessly shared the news with Leibniz (OC10 615–16). ‘The English, as it seems, had thought to conceal this accident but in vain,’ he wrote to his brother Constantijn. ‘Aside from his too earnest studies, it could be that the misfortune of a fire which he had, which carried off his Laboratory and some papers, has contributed to so disturb his mind, which is the worst of all ills that can come upon a man’ (OC10 617–18). Newton was, in fact, fully recovered by the time that Huygens received this intelligence, and Leibniz wrote back cordially: ‘It is to men like you, Monsieur, and him, that I wish a long life and good health, in preference to others, whose loss would not be so considerable speaking comparatively’ (OC10 639–40).