The Principles of Ray Optics Before the Wave Theory of Light
Although the central physical image of optics in the late 18th and early19th century presumed light to consist of particles governed by forces (the emission theory), no quantitative results with empirical consequences ever resulted from that assumption, with the partial exception of Laplace’s expression for the relation between index of refraction and density. Nevertheless, Malus and then Biot in France, as well as Brewster in England, did generate testable claims that both Biot and Brewster insisted were independent of the emission theory. Adopting Thomas Young’s terminology, we will call the ray-based theory deployed by these investigators (and others before and after them) the selectionist theory of light. That theory presumed the independent existence and persistence of a countably-finite set of physical rays, whatever such things might actually consist of. Wave optics altogether rejected any such notion, with the consequence that results obtained by Malus, Biot and Brewster could no longer be accepted – though in the principal case of partial reflection, whose theory was due to Malus, no difference could at the time be detected between his formulae and the ones produced by Fresnel.