Huygens’ Traité de la lumière and Newton’s opticks: pursuing and eschewing hypotheses

Writing in December 1688 to his brother Constantyn, who had only recently arrived in England with the court of William III, Christiaan Huygens expressed a wish to be in England, ‘ only to make the acquaintance of Mr. Newton whom I exceedingly admire for the beautiful inventions that I found in the work [the Principia] that he sent m e’ (1). Less than six months later, on 12 June 1689,.the two met in London for the first time at a meeting of the Royal Society which has by now passed into legend. ‘Mr Hugens of Zulichen’, according to the Society’s Journal Book, ‘being present gave an account that he himself was now about publishing a Treatise concerning the Cause of Gravity, and another about Refractions giving amongst other things the reasons of the double refracting Island Chrystall’. While ‘Mr Newton considering a piece of the Island Chrystall did observe ’ and describe the two sorts of refraction (2). It is a commonplace to observe the irony of Huygens lecturing Newton on gravity and Newton lecturing Huygens on double refraction. Ironies do abound in the relation of the two titans, but we should take the event for what it was: the two had chosen to challenge each other directly and defend each his own way of doing science. To be sure, they had much in common, a genuine mutual esteem; a firm grounding in Cartesian natural philosophy that they both outgrew, though in different ways; and the aim of creating a mathematical physics based on experiment and observation. Yet where Huygens remained faithful to Descartes’ requirement that all physical explanations be in terms of contact action between particles of matter, Newton transcended the confines of the mechanical philosophy and introduced attractive and repulsive forces;