How Many Colours?

Abstract

Isaac Newton’s first optical paper (published in the Philosophical Transactions in February 1672) was controversial: Newton argued for a new theory of light and colour when no one else thought the old one was inadequate, and moreover he claimed certain truth for his new theory! A debate followed, in which Newton defended his claims against the objections of optical heavy weights such as Robert Hooke and Christiaan Huygens. One major sticking point between Newton and his critics concerned the number and division of colours. Newton argued that there is an indefinite number of ‘primary’ colours, but Hooke and Huygens objected to this inflated ontology. Each critic argued, for different reasons, that there were only two original colours. I examine Newton’s responses to these objections. I argue that they are revelatory of Newton’s unique methodology: a mathematico-experimental approach that eschewed ‘hypotheses’ in favour of ‘theories’. I also show that we should read Newton’s claim that there are an ‘indefinite’ number of colours in epistemic terms. Nowadays, Newton’s first optical paper represents a landmark in the science of optics. Its exploitation of the correspondence between refraction and spectral colour, provided a new approach to the study of light. And its views on the properties and nature of light, set a new agenda for the field.