The Development of Newton's Theory of Fits

Abstract

This paper traces the development of Newton's theory of “fits,” designed primarily as an explanation of the phenomenon of periodic rings created by light, and developed in Bk. II of his Opticks. We can recognize three distinct stages in the development of Newton's explanation of this phenomenon: the aethereal vibration hypothesis, the corpuscular vibration hypothesis, and the theory of fits. My aim is to show what led Newton through these successive explanations, and why he ultimately settled on the theory of fits in the Opticks. Alan Shapiro has argued that Newton avoided committing himself to the corpuscular vibration hypothesis in the Opticks because of his methodological precept of abstaining from hypotheses that lack sufficient demonstration. In addition, I highlight Newton's observations concerning the partial reflection of light and his experiments involving thick glass plates, which may provide a further reason why he avoided committing himself to the corpuscular vibration hypothesis.