Treatise on Light

Summary

Christiaan Huygens read the substance of this treatise to the Royal Academy of Sciences in Paris in 1678 and published it in French at Leiden in 1690 as Traité de la lumière, with a short discourse on the cause of gravity. The library text is Silvanus P. Thompson's literal English translation of 1912, complete, with the author's preface, Thompson's note and a table of matters; the many geometrical figures are marked but not reproduced. In the preface Huygens explains that his demonstrations cannot have the certainty of geometry, because here principles are confirmed by the conclusions drawn from them, yet agreement with many observations and with newly predicted effects can give a probability scarcely short of proof. Chapter I argues that light is a motion of some matter, since it comes from fire and can burn and since rays cross without hindering one another. Like sound in air, it must spread by spherical waves through a very fine ethereal matter of hard, elastic particles. Huygens relates Ole Römer's observations of the eclipses of Jupiter's moon, which show that light takes some twenty-two minutes to cross the Earth's orbit and so travels more than six hundred thousand times faster than sound. He then states the principle that bears his name: each particle reached by a wave becomes the centre of its own particular wave, and the visible wave is the envelope, the common tangent, of these. Chapter II derives the law of reflection, and Chapter III explains transparency and derives the law of sines for refraction on the assumption that light moves more slowly in glass and water than in air, reaching Fermat's principle of least time. Chapter IV treats the bending of light in the atmosphere, which makes the Sun visible before it rises. Chapter V, the longest, examines the strange double refraction of Iceland crystal, first described by Erasmus Bartholinus: Huygens explains the ordinary ray by spherical waves and the extraordinary ray by spheroidal waves, measures the crystal's refractions, and reports a further puzzle he cannot solve, that light passing through two crystals behaves differently as the second is turned, the effect later called polarization. He closes the chapter with conjectures about the crystal's internal structure from its cleavage. Chapter VI applies the wave construction to the shapes of lenses and mirrors, including Descartes's ovals and caustic curves. The book stands as the first sustained wave theory of light, a counterpart to the particle view later associated with Newton.

Top Lessons

• Confirmation by consequences

  Huygens admits in the preface that his principles cannot be proved as in geometry; they are verified by how well their consequences match the phenomena. His treatment of Iceland crystal shows the method at work: an hypothesis of spheroidal waves is tested against measurements and against new cuts of the crystal.

• Reasoning by analogy

  Huygens builds his account of light on the spreading of sound in air and the circles formed in water when a stone is thrown in. He also marks where the analogy fails, arguing that light and sound must travel in different media. A good analogy suggests a theory without replacing the evidence for it.

• Reporting what one cannot explain

  At the end of the Iceland crystal chapter Huygens describes a phenomenon with two crystals that his theory cannot account for and leaves it to others. The puzzle was later understood as polarization. His candour shows that honest science records its anomalies.

Verified Quotes

Medical Lens

The treatise is optical physics, not medicine, but it touches the physiology of vision at its starting point: Chapter I assumes, with the mechanical philosophy of the day, that sight is excited by a motion of matter acting on the nerves at the back of the eye, and the preface calls the refraction of light the foundation of the construction of our eyes. Like Galileo, Huygens uses one beat of the pulse as a rough unit of a second when computing the speed of light, a reminder that the pulse served as a common measure of time. Bartholinus, whose description of Iceland spar Huygens builds on, was a physician and professor of medicine at Copenhagen.

• Chapter I

  Sight is described as excited by the impression of a movement of matter on the nerves at the back of the eye, supporting the claim that light is a motion.

• Chapter I, on Römer's observations

  Huygens converts the speed of light into Earth diameters per second, equated with one beat of the pulse.