8. Final Years

For the rest of his life, Huygens remained in the Netherlands, an internationally famous scientist. He continued his research and publications to the end, living off the wealth of his family.

Initially he lived in his father’s house in The Hague, but after his father’s death, in 1687, he moved to the country house Hofwijk in nearby Voorburg, and there he spent his remaining years. After 1687 there was talk now and then of marriage, but Huygens never took that step.

During this period, Huygens occupied himself with further improvements of his pendulum clock, hoping to adapt it for maritime use, so that it could offer a solution to the problem of determining longitude at sea. The trials were, however, disappointing. He also turned his attention again to optics, and these studies led to a new theory of light, which he published in 1690 under the title Traité de la Lumière. With his theory, Huygens could explain the puzzle of the double refractive properties of Iceland spar (calcite).

In the meantime, a new generation of scientists had arisen, among which Isaac Newton and Gottfried Wilhelm Leibniz, with new ideas and methods. Huygens viewed their discoveries with great interest but also critically. In 1689, Huygens once again visited England, where, among others, he met Newton, whose magnum opus, the Philosophiae naturalis principia mathematica had appeared two years earlier. Huygens enjoyed the trip greatly. Unfortunately, he left no record of his discussions with Newton, with whom he had fundamental philosophical differences.

In 1695 Huygens’s health deteriorated rapidly. On 9 July of that year, after having drawn up a will, he passed away. He left his papers to the university of Leiden, where they remain to date. His instruments and telescope lenses remained in the possession of the Huygens family until 1754 when the collection was broken up at a public auction.

7. Scholar of International Renown

In Paris, Huygens drew up the research agenda of the Académie. He himself made important contributions, especially in the area of mechanics, and several of his discoveries were discussed in the meetings of the Académie. Among his most important works of this period was the Horologium Oscillatorium of 1673, in which he derived the laws of the pendulum and proposed an improved pendulum clock. Huygens was one of the central figures in Parisian scientific circles and became an international authority.

He worked hard, and apparently felt at ease in this situation, but he began to experience health problems. In 1669 he became so ill that people were worried that he might die. In order to recuperate, he returned to his family in The Hague, where he remained from September 1670 until June 1671. When, in 1681, illness threatened his life for a second time, he again returned to The Hague.

When the French King declared war on the United Dutch Provinces in 1672, there were no repercussions for Huygens’s position. His family occupied important positions in the administration of the new Dutch stadtholder, Prince William III of Orange, who quickly emerged as Louis XIV’s arch enemy. But Christiaan remained at his post in Paris. He even dedicated the Horologium Oscillatorium to the French King.

In the course of time, the climate in France became less tolerant, however. The freedom of religion of the Protestants were progressively restricted, a trend that was to culminate in the revocation of the Edict of Nantes in 1685, and foreigners of Protestant lands increasingly became the subjects of suspicion. On the advice of friends familiar with the conditions in Paris, Huygens, who had been in The Hague because of illness since 1681, finally decided not to return to Paris.

6. The Académie Royale des Sciences

In 1666 Huygens’s choice of a career as an investigator of nature was rewarded in spectacular fashion. The French King, Louis XIV, invited him to Paris to take on the leadership of the scientific academy the King wished to found. This was a great honour, which was also financially rewarding: the King promised Huygens a yearly salary of 6000 livres.

Huygens himself had been present at the cradle of the Academy. With several Parisian colleagues, such as the astronomers Adrien Auzout and Pierre Petit, he shared his dissatisfaction with the level of scientific research in Paris. On one hand, too many incompetent cooks spoiled the broth, and on the other there was a lack of funds and means. Meetings therefore often got stuck in endless and aimless discussions. Several influential men therefore urged the King to found a formal organization for science.

Such academies, or learned societies, were a common phenomenon at the time. They were local societies that usually sprang up spontaneously and promoted learning and the arts. And they existed in any number of areas and with various aims: purification of a language, poetry, etc. For the rising new sciences, they were of vital importance, precisely because science did not yet have a clear social place and function: only under protection of such groups could science carve out a social niche.

The French Académie Royale des Sciences was not the first scientific academy in Europe. But because the French King thought that a scientific academy would fit in well with his designs to give government more prestige and authority, and therefore brought to bear his full resources (including finances) on it, it was the first academy where something approaching professional science was practiced. There was, of course, also a disadvantage: the academy was strongly dependent on the political desires of the monarch.

5. A Gentleman of Independent Means

Christiaan spent the next several years for the most part in The Hague, but he kept in contact with the learned world across Europe. In 1660-61 and 1663-64, he again went on journeys and spent several months in Paris and London. In The Hague, he lived in the family home. He never married — a condition not unusual for scholars in the seventeenth century.

These years after his university studies, years without a position, were the most productive of his career. His studies produced spectacular discoveries in several areas, some of which he published during this period, and some of which saw the light of day much later.

In the first instance, Huygens made a name especially as a mathematician, an area in which he continued to work for the rest of his life and in which he reached great heights. Most of this work is esoteric and difficult for non-specialists. But one part of it is more accessible: his calculations of the likelihood of winning or losing in games of chance. Huygens was one of the founders of the study of probability.

Besides mathematics, throughout his life Huygens had a love of making devices and instruments. In some cases these were toys, such as the magic lantern invented by him; in other cases they were serious scientific instruments. Together with his brother, he ground and polished lenses and made telescopes and microscopes. His mathematical studies of lenses produced important results, which remained unpublished for the time being. He did, however, publish the discoveries he made with his telescopes. In 1656 he announced that he had found a moon of Saturn, and in his Systema Saturnium (1659) he published the solution to the puzzle of Saturn’s remarkable and always changing appearance: the planet is surrounded by a ring. With these discoveries about Saturn, Huygens achieved international recognition as a scientist.

Huygens also devoted attention to mechanics — here again in connection with his tinkering with instruments. In 1656-57, he invented the pendulum clock. At the time, he published only a brief description; a more exhaustive treatment came much later. His thoughts on more fundamental mechanical problems — collision, centrifugal force — remained for the most part unpublished.

4. Huygens’ Stay in Paris

When the successor of Frederick Henry, Prince William II of Orange, died at a young age, in 1650, the western provinces of the Republic (whose independence from Spain had recently been officially recognized at the Peace of Westphalia) decided they no longer needed a stadtholder as military leader. The followers and officials of the House of Orange were therefore pushed aside, and this meant that the influence of father Constantijn Huygens diminished drastically. He could thus do little to help his sons obtain suitable high office. Christiaan, therefore, remained without an income-producing position, but because the family was sufficiently rich, he could now actually devote himself fully to his beloved studies.

In 1655, Christiaan went on a voyage to Paris, accompanied by his younger brother Lodewijk and two of their cousins. In those days, such a journey was a customary way for young men of well-to-do families to round off their education. They saw something of the world and learned the customs of foreign lands and courts. They also formed acquaintances with nobles, statesmen and other influential people in other parts of Europe, ties that would be useful to them in their later careers.

Christiaan not only spent his time visiting curiosities and paying courtesy visits: in Paris he made the acquaintance of the most important mathematicians of France and participated in the salon discussions about the new discoveries that were made in science. He formed a friendship with the poet Jean Chapelain and with the astronomer Ismael Boulliau. It was probably at this time that his decision to seek fulfilment in his life in science, and not in administration or belles lettres became fixed.

3. At the Illustrious School in Breda

After two years at Leiden, Christiaan was transferred by his father — this time with his younger brother, Lodewijk — to the new “Illustrious School” at Breda, which had just been founded by the Prince of Orange (Frederick Henry), and of which his father was one of the trustees. Here, Christiaan continued his legal studies from March 1647 to August 1649.

There is little doubt that Law was not Christiaan’s own choice: his earlier studies had awakened in him a love of mathematics and science. In his social circle, these subjects were considered more as hobbies appropriate for a gentleman of leisure — and which were therefore taught at university — but which should not become a serious occupation. One could not make a living with research, and practical mathematics was the domain of humble “mathematical practitioners” and therefore unsuitable for a person of Huygens’s social standing. With training in Law, one could hope to attain high administrative positions, and it is therefore understandable that father Huygens pushed his sons in this direction. However, partly because of the new ideas of Descartes, Christiaan saw the study of mathematics and natural philosophy (i.e. physics) as the key to a new world, to the pursuit of which he felt he could devote his life. Throughout his studies at Breda, he remained in contact with his Leiden mathematics professor, Van Schooten.

2. At Leiden University

On 11 May 1645, Christiaan and his brother Constantijn inscribed their names in the student rolls of the university of Leiden (17 km north of The Hague). At that time, universities generally offered higher degrees in only three subjects, Law, Theology, and Medicine. The boys’ father wanted them to study Law, so that they would be able to continue the family tradition of high government service.

The brothers therefore matriculated as Law students, but, like all students, they began by following the general, introductory, Liberal Arts curriculum considered the essential prerequisite for more advanced studies. These Liberal Arts took on various forms in different universities, but in general they involved the study of Classical authors and their opinions in the areas of History, Moral Philosophy, and Natural Science; they also included mathematics. Christiaan took the courses of the professor of Mathematics, Frans van Schooten, a friend of the famous French mathematician and philosopher René Descartes (who was living in the Dutch Republic and on occasion visited the Huygens family). Descartes had developed an entirely new approach to natural science, and young Christiaan became thoroughly grounded in these new concepts that profoundly affected his later scientific work.

1. Education at home

Together with his older brother, Constantijn, Christiaan was taught by his learned father and by specially appointed tutors. When he was eight, a live-in tutor, a certain Abraham Mirkenius, was hired, whose main task was to teach the brothers Latin, the international language of learning. Their father instructed them in music and arithmetic. This training was so successful that, as father Constantijn related, Christiaan began composing his own music when he was nine, and could then already converse in Latin with his brother. In the following years, he also studied geography, prosody, logic, Greek, French, and Italian, while learning to play the lute, viola, and clavichord.

When he was fourteen, Christiaan became interested in drawing and mechanics. He quickly taught himself to copy printed pictures and built little models of devices he read about. “He then spent all his free time in making little windmills and other models, including even a lathe, which in this year [1643] he had managed to put together well enough to enable him to begin making some things with it.” He soon had his own wood-turning lathe. (His new live-in tutor, Hendrik Bruno, had little patience with these hobbies.) The next year, Christiaan began formal lessons in dancing and horseback riding. At this time, too, a mathematics tutor, Jan Jansz. Stampioen was hired for Christiaan and his brother Constantijn.