Olivier Morizot

ABSTRACT In his ‘Theory of Light and Colours’, presented to the Royal Society in November 1801, Thomas Young defended a mechanical explanation of the coloured fringes observed outside of the shadow of an opaque object – the so-called ‘colours by inflection’ – that was based on the hypothesis of an ethereal density gradient surrounding all material bodies. However, two years later, he publicly rejected that hypothesis, without giving much detail of his reasons. Although Geoffrey Cantor has demonstrated the crucial role of mechanical and astronomical arguments in explaining the withdrawal of this fundamental hypothesis long ago, the purpose of this article is to draw deeper attention to a set of experiments performed by David Brewster on the inflection of light, described in a letter he addressed to the Royal Society in January 1802, but was retained by William Herschel, and finally was neither read before the Society, nor published in its Philosophical Transactions. For the hypothesis that will be evaluated here is that these unpublished experiments of Brewster’s were eventually known to Young through the mediation of Herschel, and eventually played a significant role in Young’s rejection of his ether distribution hypothesis.