Lenses and Waves

1677-1679 –WAVES OF LIGHT 187
Descartes presented his tendency theory of light in Le Monde (written
1630-1632, published posthumously in 1664) and Principia Philosophiae
(written 1641-1644, published 1644). Light was central in his system of
natural philosophy, the nature of light being ultimately connected with the
essence of the cosmos. The full title of Le Monde was Le Monde ou Traité de la
Lumière. Descartes envisaged a system of natural philosophy founded solely
on mathematical principles. 74 Quantity was the only thing to be investigated
about material substance and it was subject to the laws of motion. According
to Descartes, the cosmos is completely filled with matter, which is manifest
in three kinds or elements. The third element, composed of the bulkiest
parts, constitutes the visible objects around us, like the earth, the planets and
comets. The first and finest element makes the Sun and the stars; the second
consists of spherical particles and makes the heavens. 75 All three elements
have their share in the explanation of light: the bodies of the first element
produce light, the second element makes up the medium that propagates it,
and reflecting and refracting bodies are made of the third element. Descartes
argued that the Sun and Stars, in rotating about their axes, exert a radial
tendency (‘conatus’) upon the Heavens which instantaneously spreads
outward along straight lines. This tendency is light, and Descartes explained
its properties in a discussion of circular motion. 76
In Traité de la Lumière Huygens did not waste his breath on the tendency
theory. He only mentions it once, to reject it on the basis of just one
argument:
“… [Descartes] has light consist in a continual pressure, that only tends to movement.
As this pressure cannot act at once from two opposing sides, against bodies that have
no inclination whatsoever to approach, it is impossible to comprehend what I have just
said of two persons who mutually see each others’ eyes, nor how two flambeaus can
illuminate each other.” 77
Huygens did not bother to criticize the mechanistic underpinnings of the
tendency theory in detail. Although we can figure that Huygens would reject
the ‘raisons de mechanique’ employed, he regarded instantaneous
propagation as the decisive problem of the theory. It returns unremittingly
whenever mention is made of Descartes. To Huygens the speed of light was
necessarily finite.
74 Descartes, Principles, [76].
75 Descartes, Principles, [110].
76 Descartes, Principles, [111-118]. For a detailed discussion see: Shapiro, “Light, pressure”, 243-266.
77 Traité, 20. “… Descartes, qui fait consister la lumiere dans une pression continuelle, qui ne fait que
tendre au mouvement. Car cette pression ne pouvant agir tout à la fois des deux costez opposez, contre
des corps qui n’ont aucune inclination à s’approcher; il est impossible de comprendre ce que je viens de
dire de deux personnes qui se voyent les yeux mutuellement, ni comment deux flambeaux se puissent
éclairer l’un l’autre.”