Newton, Isaac (1642–1727)

2. Experimental philosophy in the light and colours debate

Newton’s 1672 paper on light and colours reported only a small fraction of the optical experiments he had conducted. The debate it initiated concerned what the reported experiments had established. According to Newton, these experiments had conclusively shown that the oblong shape of the image cast by sunlight that has passed through a round hole and has then been refracted by a prism is caused by sunlight’s consisting of rays that are refracted in different degrees by the prism. (The correspondence between these different refrangibilities and different colours led Newton to invent the first reflecting telescope, which eliminates the problems of chromatic aberration that had marred the refractive telescopes of the era.)

Hooke, interpreting Newton as claiming that the experiments established a corpuscular theory of light, insisted that a wave theory could account for the results just as well. Newton responded that the hypothesis that light is a body was put forward only as a conjecture suggested by the experiments, and not as part of what he claimed to have been established by them. He granted that Hooke’s wave hypothesis could explain the conclusion the experiments had established; but this conclusion spoke of light only abstractly as ’rays’ propagating in straight lines from luminous bodies, with no commitment to any specific ’mechanical’ hypothesis.

His Dutch contemporary, Christiaan Huygens argued that Newton had failed to show the nature and difference of colours because he had offered no ’hypothesis by motion’ to explain them. Newton responded that he ’never intended to shew, wherein consists the nature and difference of colours, but only to shew that de facto they are original and immutable qualities of the rays which exhibit them’ (1958: 144).

Newton’s contemporaries had trouble understanding his attempt to construe light rays abstractly in a way that would allow experiments to decide claims about them – this, independently of any mechanical account of light. In his replies, Newton outlined how, according to his experimental philosophy, diligently establishing properties of things by experiment takes precedence over framing hypotheses to explain them. Yet he also made clear that the propositions he regarded as conclusively established by experiment were nevertheless subject to correction based on detailed criticism of the experimental reasoning that had established them or on further experimental results challenging them (see Optics §§1–2).