Lenses and Waves

1690 - TRAITÉ DE LA LUMIÈRE 229
mathematics. 68 However, by that time establishing the properties of rays was
not a straightforward a matter as it had used to be in traditional geometrical
optics. With the rise of corpuscular thinking the ray no longer was a selfevident
physical concept. Properties of rays now needed further justification,
beyond the realm of visible phenomena or everyday experience. Huygens
and Newton recognized the full import of these new questions and were the
first to directly face them.
The matter of rays
Kepler can be said to have sharpened the question after the nature of rays
and their properties. In perspectivist accounts these were answered only
loosely, by an appeal to analogies between the motion of rays and that of
bodies. With his rigorously realist reading of mathematical description,
Kepler thought that the causes of rectilinearity, reflection and refraction
ought to be contained in their measure. In his theory, this implied
considering the interaction of incorporeal surfaces with the surfaces of
diverse media. In the case of refraction this indeed led to a quasi-physical
analysis of refraction on a microscopic level, as we have seen in section 4.1.2.
In Paralipomena, Kepler explicitly distinguished the mathematical ray from the
physical ray and, in a note on what he calls the ‘fourth kind of light’ meaning
light communicated by the interaction with bodies, he can be said to have
put the question after the physical nature of light propagation on the
agenda. 69 He did so in the first place, however, by the general reorientation of
perspectiva into optics: from a theory of vision to a theory of the behavior
and properties of light.
As contrasted to his achievements in geometrical optics proper, however,
Kepler’s ideas on the physics of light were little referred to later. Besides the
Renaissance idiom of his thinking, the conduct of Descartes seems to have
blocked the view on Kepler. Not only did he conceal the inspiration he had
drawn from him, more importantly, he gave a radical twist to the
perspectivist-cum-keplerian understanding of the behavior of light rays.
Descartes’ mechanistic interpretation of perspectivist causal analyses of the
laws of optics, turned these into material interactions. By the same token a
good deal of traditional conceptualization was channeled into seventeenthcentury
theories of light. Of old, geometrical optics had been geometry
applied to matter, the matter of light rays. Descartes now raised the question
of what matter these rays were and how this could explain their behavior.
Still, the question of the nature of the light ray no longer was a simple one.
Hobbes’ concept of a line of light indicates that the once natural
identification with a geometrical line no longer was valid. Questions arose
concerning the relationship between light and the geometrical line, whether it
somehow expressed the nature of light, or whether it was the route of the its
propagation, or merely an abstraction of some kind. Depending on one’s
68 Dijksterhuis, “Once Snel breaks down”.
69 Kepler, Paralipomena, 37 note (KGW2, 46) in particular; Kepler, Paralipomena, 35 and note (KGW2, 31)