Today is the birthday (1629) of Christiaan Huygens FRS, a Dutch physicist, mathematician, astronomer and inventor, who is widely regarded as one of the greatest scientists of all time and a major figure in the scientific revolution, even though his name is not a household word these days. In physics, Huygens made groundbreaking contributions in optics and mechanics, while as an astronomer he is chiefly known for his studies of the rings of Saturn and the discovery of its moon Titan. As an inventor, he improved the design of the telescope with the invention of the Huygenian eyepiece. His most famous invention, however, was the pendulum clock in 1656, which was a breakthrough in timekeeping and became the most accurate timekeeper for almost 300 years. Because he was the first to use mathematical formulae to describe the laws of physics, Huygens has been called the first theoretical physicist and the founder of mathematical physics. Huygens is one of the giants whose shoulders Newton stood on to be able to see so far.

In 1659, Huygens was the first to derive the now standard formula for the centripetal force in his work *De vi centrifuga*. The formula played a central role in classical mechanics and became known as the second of Newton’s laws of motion. Huygens was also the first to formulate the correct laws of elastic collision in his work *De motu corporum ex percussione*, but his findings were not published until 1703, after his death. In the field of optics, he is best known for his wave theory of light, which he proposed in 1678 and described in 1690 in his *Treatise on Light*, which is regarded as the first mathematical theory of light. His theory was initially rejected in favor of Isaac Newton’s corpuscular theory of light, until Augustin-Jean Fresnel adopted Huygens’ principle in 1818 and showed that it could explain the rectilinear propagation and diffraction effects of light. Today this principle is known as the Huygens–Fresnel principle.

Huygens invented the pendulum clock in 1656, which he patented the following year. In addition to this invention, his research in horology resulted in an extensive analysis of the pendulum in his 1673 book *Horologium Oscillatorium*, which is regarded as one of the most important 17^{th}-century works in mechanics. While the first part of the book contains descriptions of clock designs, most of the book is an analysis of pendulum motion and a theory of curves.

In 1655, Huygens began grinding lenses with his brother Constantijn in order to build telescopes to conduct astronomical research. He designed a 50-power refracting telescope with which he discovered that the ring of Saturn was “a thin, flat ring, nowhere touching, and inclined to the ecliptic.” It was with this telescope that he also discovered the first of Saturn’s moons, Titan. He eventually developed in 1662 what is now called the Huygenian eyepiece, a telescope with two lenses, which diminished the amount of light dispersion.

As a mathematician, Huygens was a pioneer on probability and wrote his first treatise on probability theory in 1657 with the work *Van Rekeningh in Spelen van Gluck*. Frans van Schooten, who was the private tutor of Huygens, translated the work as *De ratiociniis in ludo aleae* (“On Reasoning in Games of Chance”). The work is a systematic treatise on probability and deals with games of chance and in particular the problem of points (the division of stakes when there is no clear winner). The modern concept of probability grew out of the use of expectation values by Huygens and Blaise Pascal (who encouraged him to write the work).

The last years of Huygens, who never married, were characterized by loneliness and depression. As a rationalist, he refused to believe in an immanent supreme being, and could not accept the Christian faith of his upbringing. Although Huygens did not believe in a supernatural being, he did hypothesize on the possibility of extraterrestrial life in his *Cosmotheoros*, which was published shortly before his death in 1695. He speculated that extraterrestrial life was possible on planets similar to Earth and wrote that the availability of water in liquid form was a necessity for life.

This recipe for a pie filled with brie, pears, and eggs is a little before Huygens’ time, but it is an interesting challenge and can yield excellent results. Fruit and cheese can make superb combinations. It comes from *Eenen nyeuwen coock boeck* (A new cookbook), written by Gheeraert Vorselman and published in Antwerp in 1560. The recipe is more than a little vague, but can be made serviceable.

*Een keesgheback*

Legget in coppen kese van Brij ende harde eyeren tsamen gestooten met peren ende hier toe neemt men suker ende heel doyeren van eyeren.

*A Cheese Pie*

*Put some Brie cheese and hardboiled eggs, mashed together, with pears in a pie. Add sugar and whole egg yolks.*

Not much to go on, I admit. It looks like a version of quiche. That is, take a pie shell and fill it with a mix of sliced pears and hardboiled eggs and Brie mixed together. Beat egg yolks (and sugar), and pour over the pie filling. Bake until the crust is golden and the eggs are set.