Christiaan Huygens was born in The Hague on April 14th, the Saturday before Easter, 1629. His father, Constantijn Huygens, was a diplomat; he was secretary to the Stadholder Frederik Hendrik, Prince of Orange. In the young republic of the United Netherlands the Huygens family had risen to high social rank because of its services to the house of Orange; Christiaan's grandfather had been a secretary to Prince William the Silent.
Christiaan, together with his brother Constantijn junior, was educated by private tutors, but in several subjects their father instructed them himself. At the age of 16 Christiaan went to Leiden to study law and mathematics. Two years later he went to a newly founded college in Breda where he stayed two years. A year later, in 1650, the stadholder William II died. There was to be no stadholder until 1672; the influence of the Huygens family diminished and there was no post available for Christiaan in the diplomatic service. He was young, well off, and without a job. He studied, occupying himself primarily with mathematics, but also with mechanics and optics. His first publications appeared, dealing with mathematical subjects. In 1655 he visited Paris for the first time. Then there was a turning-point in Huygens's career. He made an invention and a discovery which assured his fame as a scientist: he invented the pendulum clock (1656), and he discovered the ring of Saturn (1655-1656). The pendulum clock became known quickly; Huygens published a small book about it and obtained patents. In 1659 he published the discovery of the ring. By 1660 Huygens was famous.
Two further trips to Paris were made in 1660-1661 and 1663-1664. Huygens resumed earlier contacts and made new ones. He also visited London and became a member of the newly found. Royal Society. In these years his fame increased, and he was recognized as the greatest mathematician and natural philosopher in Europe. Understandably, then, it was Huygens who was invited, as primus inter pares, to organize the Academie des Sciences in Paris, which was founded in 1666.
Thus began a period during which Huygens belonged to France rather than to the Netherlands. He lived in spacious apartments in the Royal Library; the king awarded him a high salary. He proposed a programme for the Academie inspired by Bacon, and he participated actively in many experiments. The character of his work changed. He remained interested in mathematics, axiomatic mechanics, and technical questions, but now he a took up more fundamental questions of the explanation of the phenomena of nature. He worked out a theory about the cause of gravity and a wave theory of light.
The Paris period lasted till 1681; it was interrupted twice by longer stays in Holland, both times for reasons of health. In 1672 war broke out between France and Holland. Once more a Prince of Orange, William was appointed Stadholder. Christiaan's father and his brother got important positions and played a considerable role in the diplomatic and military defence against the French attack. Nevertheless, Christiaan stayed in Paris in the service of the king of France; he even dedicated his great book about the pendulum clock, Horologium Oscillatorium (1673), to the king. A curious position, the more so because Huygens was certainly not indifferent to the cause of the Netherlands.
Strictly speaking, Huygens's period as member of the Academie did not end in 1681. He returned then to the Hague for reasons of health although he expected to return to Paris after a few months. But the illness lasted longer, and in 1683 Colbert died. Colbert had been an important protector of the Academie, and he was also a friend of Huygens. Because of his death and because of the changes in the political climate in France (the revocation of the Edict of Nantes was near) Huygens hesitated about going back. Nor was he encouraged in any way to return to France. So he stayed in Holland, again without an official function.
Now he found time to publish his theories of gravity and light; the Traite de la lumiere appeared in 1690. Huygens turned to mathematical questions again, but other things, such as clocks and their use, also kept his interest.
In 1694 Huygens became ill once more, and this time he did not recover, he died on the eighth of June 1695.
But who was the person Christiaan Huygens hidden behind these external events of his life and career? Christiaan grew up in the stately house' of the Huygens family on the "Pleein" in The Hague. The family moved there when Christiaan was eight. The removal was a sad affair because Christiaan's mother died at that time from the effects of the birth of her daughter Suzanne. There were five children, of whom Christiaan was the second; an aunt supervised the household. The grandeur of the house reflected the high status of the family, and Christiaan grew up there to belong to the jeunesse doree of Holland. In his letters we read about love affairs, festivities, wedding proposals, etc. But Christiaan did not join in very much. In 1650 (he was then 21) he described a wedding party in Amsterdam: "my greatest pleasure was to look at the folly of the young people of Amsterdam, which seems to me excessive and unbearable."
An earnest youth-the well-known portrait of the family by Hanneman also gives us that impression. It was painted in 1640, when Christiaan was eleven years old. A beautiful boy, of somewhat weak constitution and of medium height. A very gifted child; he learned arithmetic very young, he had a fine musical sense, he spoke Latin fluently at the age of nine and learned other lan-guages. A son to be proud of, especially for a father like Constantijn Huygens, who himself took a great interest in the arts and sciences, who corresponded with Mersenne, who received Descartes as his guest and was also a poet and a musician.
Mon Archimede-my Archimedes-so Constantijn called his son. The com-parison shows the father's pride, but it is also a very apt comparison. Already at an early age Christiaan had shown that combination of deep mathematical insight and feeling for technical constructions which Archimedes also must have had. Huygens was skilful in practical things; at the age of thirteen he built himself a lathe and in 1655, together with his brother, he started to grind lenses.
In the years before his first journey to France Christiaan published his first books on classical mathematical subjects. During the same period he also wrote extensive treatises, bringing them to the point where they were almost ready for publication. But he did not speak much about them, and in the end he never did publish them. The most famous study which remained unknown to the scientific world too long in this way was the treatise on collision. But there is also a magnificent work on floating bodies written in 1650. On the cover of the manuscript Huygens noted later that he doubted the usefulness of the piece, and that most of it could be thrown into the fire-which, fortunately, did not happen. In these years Huygens also started a treatise on dioptrics, on which he continued to work during the rest of his life. All the time he considered the work almost ready for publication, but not quite. A curious, but characteristic reticence; we can find several other instances of this in Huygens's work. Nevertheless, his first mathematical publications brought him a reputation; he received letters of praise, which he answered with a well-formulated modesty, evidently enjoying the approval he received.
Huygens felt at home in the cultural and scientific circles in Paris. He liked the style, and he fitted in like a born Frenchman. In 1657, as the gallant older brother who had seen much of the world, he wrote a letter to a friend of his sister Suzanne. The friend had seen a portrait of Suzanne, drawn by Christiaan. She liked it; Christiaan had promised to make a copy for her, but he was late in fulfilling the promise. This is the letter:
Mademoiselle, It is not indifferent to me to be on good terms with the most beautiful and lovable person in the world. And apart from that, I am a man of my word. although perhaps you have imagined the contrary.. Therefore, now that I am set to go with all the family to Zulichem, I wanted to acquit an old debt by sending you this profile of my sister.
Zulichem, or Zuylichem, was the estate of the Huygens family, a castle on the river Waal. The Huygenses did not go there often; Christiaan found it a boring place. The letter on the portrait of Suzanne continues:
Although now there appear two eyes, you should not think that it is not the same profile as the one you saw here and which I promised to send you. But believe me, this change has come to it as bit by bit and insensibly it began to turn around, so that with time you may expect that it will 1.k straight at you, and maybe the lips will begin to move, because you see that the color has already become much more lively than it was.
And if that does not happen, I still hope that you will be good enough to keep it as it is. I ask as reward only that you revoke all the insulting things you have said about me in my absence and that, if perhaps I shall have the pleasure of seeing you one of these days in Bolduq, I shall not be exposed to your reproach.. There is nothing in the world that I would value more-as there is nothing so precious to me as the honour of your good graces and than to be, Mademoiselle, your very humble and most obe-dient servant, Christiaan Huygens of Zulichem. My sister kisses your hands.
A gay and elegant letter. We do not know who the young lady was because we have only the draft of the letter. And curiously enough, Huygens was not satisfied with it. He rewrote the part about the face that is alive and turns, and the final version of the letter is definitely less graceful. I find this characteristic for Huygens: his style is elegant, but there is always a certain formality and de-liberation, a tendency to reconsider and polish, and the effects of that tendency were not always favorable for his work.
Huygens certainly enjoyed his elegant contacts. He never married, but he was not indifferent to female charm, and he liked the company of women. In 1661 he was much impressed by the beauty of Marianne Petit, the daughter of one of his Paris correspondents. He visited her often and he drew her portrait. Later, in the period when he lived in Paris, he liked to be with the attractive daughters of the Dutch merchant Caron, liking Suzette Caron especially. Later she married a Frenchman. After the revocation of the Edict of Nantes she settled in Holland and often visited Huygens-remarkably often, according to his family.
Unfortunately, the portraits which Huygens made of women are lost. The only portrait we have by him is a portrait of his father; it is rather stiff. But there are drawings by Huygens of landscapes which show that he was a skilful draughtsman.. He also was a good musician. Artistic sense, elegance, easy contacts in international society, these are natural for a member of the Huygens family. But the family was close to politics and diplomacy too. Here Huygens did not keep up the tradition. He followed political developments attentively but from a distance. When in 1650 the Stadholder threatened to use military force to compel Amsterdam to obey his commands Christiaan wrote to his brother: "I myself, Spero meliora and I patiently wait for the end of the whole affair without letting it in any way bother me." He kept the same attitude, under much more dramatic circumstances, in the years 1672-1673 when, despite the French offensive against the Netherlands, he stayed in Paris in the service of the French king.
A quiet, friendly and composed man—so the portraits also show him. They are stately portraits but the mouth shows some irony and I see a trait of wonder in the eyes. Was he really so calm and controlled? He could be angry, he could be obstinate—for instance in the matter of patents. He could be bitter and feel insulted, for instance by the French attitude which prevented him from returning after 1681. In 1686 he voiced this bitterness M a letter to his brother about a book which Von Tschinthausen had dedicated to Louis XIV. The book was called Medicina mentis et corporis, medicine for the mind and the body, and Huygens remarked that Louis certainly needed the one as well as the other.
And there were the illnesses. In 1670-1671 Christiaan was so ill that one feared for his life. In 1675,1681, and 1694 the illness returned. We do not know what it was, but it is certain that it entailed deep depressions and, especially in the last months before his death, feelings of desolation and despair.
After a difficult sleepless night in 1694 Huygens noted between his mathematical calculations a Latin poem describing how the forces of the reason and the mind dwindle when the body weakens, how then inevitably melancholy arises and one cannot any longer trust one's own rational judgement. It seems as if, during Huygens's periods of illness, feelings found an outlet which normally remained hidden behind rationalism and sedateness.
Huygens's family saw in these depressions the signs of fear and repentance over his liberal attitude towards religion. Yet Huygens did not change his religious views; he remained a member of the Protestant Church, but in his rationalistic approach to theology the idea of God lost more of its content than was acceptable to the ministers of that church. Till his father's death, in 1687, Huygens lived in the parental house, after-wards in lodgings in The Hague. In summer he was often at Hofwijck, the family's country house in Voorburg not far from The Hague. His last years were lonely, especially if compared with the Paris period, but fruitful. Huygens worked hard, published, and took up lens grinding again. His scientific and technical work must have been a source of strength for him, a fixed point from which h could combat the fits of depression and the basis for his attitude to life.