Lenses and Waves

230 CHAPTER 6
specific conception of the corpuscular nature of light, questions like these
could be answered in a more or in a less straight-forward manner.
Broadly speaking, corpuscular conceptions of light can be divided in two:
emission and medium conceptions. That is, light is either propagated matter
or an action propagated through matter. The first maintained the primacy of
the light ray in a rather natural way. The rectilinearity of light rays seems to
follow directly from the view of a moving particle. The most prominent
exponent of the emission conception was Newton, who considered its
purport more thoroughly than anybody else. Apart from his explicit
refutation of medium conceptions - in particular waves - he carefully
considered his own understanding of light. At least in his early years he
thought of light in terms of atoms, but soon developed a precise definition
of a light ray that covered his emission conception without being dependent
on it, as well as carefully determining the relationship between a geometrical
ray and a physical ray, where a physical ray not necessarily is the
mathematical line of geometrical optics. 70 When he later reconsidered the
papers and letters he had published in 1672 in the Philosophical Transactions, he
added a footnote where he once again went into the details of the question
whether light is a “body” or “the action of a body”. 71
Medium conceptions of light marked a more decisive break with
traditional geometrical optics. A light ray came to be seen as the effect of
the propagation of light, not as its essence. This implied abandoning the idea
that a ray has much intrinsic physical significance, as Buchwald explains, and
he adds that few at that time were willing to do so. 72 In a medium conception
rectilinearity requires explanation. Significantly, Descartes, who originally
formulated the idea that light consists of an action propagated without
transport of matter, evaded the question. 73 The same can be said for Hooke
and Barrow who turned Hobbes’ pulse theory into what in essence was an
emission conception of moving rods. Huygens, who adopted his medium
conception in an almost a priori manner, took the problem seriously. In the
first chapter of Traité de la Lumière, he put great stock in demonstrating that,
and how, waves are capable of producing rectilinear light rays.
The choice for an emission or a medium conception determined the way
in which refraction could be conceptualized. In an emission conception, one
must account for the fact that a transition to a different medium results in an
instantaneous change of direction. The conception of refraction as a surface
phenomenon rooted in perspectivist analyses. Kepler (himself neither a
medium nor an emission theorist, to be sure) had explicated this by his
notion of surface density. In his final analysis of refraction, he tried to
analyze the interaction between the two-dimensional surfaces of light and
70
Shapiro, “Definition”, 206-208.
71
Cohen, “Missing author”, 23-26.
72
Buchwald, Rise, 5.
73
See also: Shapiro, “Light, pressure”, 254-260.