Roșu Angel-Lucian
Gr. 357
Huygens` Traité de la Lumière
First written and presented in front of the Academie Royale des Sciences in Paris in
1678, Huygens` Traité de la Lumière was to see the light of print only twelve years later in
1690. The Treatise covers a broad range of light phenomena and explains them starting from
an assumption that considers light as propagating in waves that sets him apart from his
contemporary counterpart`s (Newton`s) emission theory of light. Even if on this matter (and
on many others) the theories of the two natural philosophers diverge, they also had a lot in
common like a firm background in Cartesian metaphysics which both outgrew (each in
different ways) or a common interest in creating a mathematical metaphysics based on
observation and experiment1.
The time the Treatise came about is very important since in the exact same period
both Huygens and Newton were working on their competing theories of light. In fact they
were aware of each other’s work and even met and discussed it in 1689 at a meeting of the
Royal Society. Huygens explicit aim in the Treatise was to explain certain properties of light
and more specifically the double refraction of certain type of crystal. But his more profound
interest was to depart from traditional geometrical optics in which the properties of light were
deduce from established principles towards an explanation of these principles based on certain
assumptions2.
In the preface he had written twelve years after the Treatise itself he expressed his
concern with the fact that his “demonstrations [..] do not produce as great certitude as those
of Geometry”3 since, unlike geometrical demonstrations which proceed by deduction from
“incontestable principles” towards their conclusions, whereas his principles can only be
verified by the conclusion deduced from them. Therefore, the principles he employed in his
demonstrations are not by far “incontestable” and can only be considered probable and the
more experiments confirm the correspondence between the principles and the phenomena the
higher the probability of the theory is.
1
Alan E. Shapiro, Huygens' 'Traité de la Lumière' and Newton's 'Opticks': Pursuing and Eschewing Hypotheses,
p. 223.
2
Shapiro (1989), p. 225.
3
Christiaan Huygens, Treatise on light, translated by Silvanus P.Thompson, Chicago University Press, 2005, p.
vi.
1
The first chapter of the Treatise is concerned with the propagation of light rays in
straight lines and here is the place where he establishes his wave theory of light. The
beginning of the chapter meets us with one of Huygens` general assumptions stating that the
properties of light, as well as Physics in general, can only be made comprehensible if it is
described in “terms of mechanical motions”. This principle, although strongly Cartesian, is
brought together with rejections of two Cartesian principles of physics: first, the principle
that considered light to travel on a path different from a straight line (which he elaborates
later) and second, the principle that light travels instantaneously.
Relating to Roemer observations on the satellites of Jupiter, Huygens computes that
the speed of light must be finite opening the road to a kinematic explanation of how the
movement of light works. In analogy with sound`s movement through air he develops a
corpuscular billiard-ball model of the propagation of light not through air but through a fine
ether. If Roemer observations are taken as proof of the finitude of the speed of light, than an
explanation of how this movement takes place is required and Huygens constructs a kinetic
system in which the collision of fine, elastic, ethereal corpuscles represent the medium of
propagation of light. This model allowed him to develop a way of explaining how the waves
of light (more specifically a wave front) can be explained as the conjoined effects of a
multitude of waves each with the center represented by every particle in the wave front
respectively. Having set these principles, the reminder of the treatise is meant to explain the
properties of light starting from them.
Huygens` Traité de la Lumière allows one to uncover some of the traits of his
scientific method. Some of these traits are common features of the philosophers nature of the
seventeenth century, like the mathematisation of physics, the importance of observation and
experiment, the “geometric method” of proof etc. but the major trait that separates Huygens`
Treatise from his contemporaries is his concern not with the certainty of his theory but with
its probability.
2
Bibliography:
Christiaan Huygens, Treatise on light, translated by Silvanus P. Thompson, Chicago
University Press, 2005
Wilbur Applebaum (ed.), Encyclopedia of the Scientific Revolution from Copernicus to
Newton, Garlan Publishing, 2000
Alan E. Shapiro, Huygens' 'Traité de la Lumière' and Newton's 'Opticks': Pursuing and
Eschewing Hypotheses, Notes and Records of the Royal Society of London, Vol. 43, No. 2,
Science and Civilization under William and Mary (Jul., 1989), pp. 223-247
3