burg ‘to leave out the last paragraph of the hypothesis, where I mention Mr. Hooke and Grimaldi together.’ ‘If you have opportunity (Newton added, p. 387) pray present my service to Mr. Hooke, for I suppose there is nothing but misapprehension in what has lately happened.’
This paper ‘about colours’ was the last separate memoir published by Newton on optical subjects. His various papers were collected in the ‘Optics,’ published in 1704, and to those which we have mentioned were added his researches on the colours of thick plates (bk. ii. pt. iv.) and on the diffraction or inflexion of light (bk. iii.). It will be convenient, therefore, to summarise in this place Newton's views on optics, and his position with regard to the theory which might account for his observations.
Two theories have been proposed to account for optical phenomena. Descartes was the author of one of these, the emission theory, which supposes light to consist of small particles shot out by the luminous body; Hooke, though his work was very incomplete, was the first to suggest an undulatory theory. In his ‘Micrographia,’ 1664, p. 56, he asserts that light is a quick and short vibrating motion, ‘propagated every way through an homogeneous medium by direct or straight lines extended every way, like rays from the centre of a sphere. … Every pulse or vibration of the luminous body will generate a sphere which will continually increase and grow bigger just after the same manner, though indefinitely swifter as the waves or rings on the surface of water do swell into bigger and bigger circles about a point on it.’ On this hypothesis he gave an account of reflexion, refraction, dispersion, and the colours of thin plates. His reasoning was, however, utterly vague and unsatisfactory, and he convinced few of the truth of this theory. Newton followed. He may have known of Hooke's theories. The copy of the ‘Micrographia’ in Trinity College Library has the inscription ‘Trin. Coll. Cant. A. 1664,’ and below in a different hand, ‘Ex dono Mgri Gale huius Colleg. Socij.’ It may well have been used by Newton, for among the Portsmouth MSS. of early date are some extracts from the work. Still there was nothing in Hooke's theories but hypotheses unsupported by fact, which would have no charm for Newton. It is claimed for him, and that with justice, that he was the true founder of the rival theory, the emission theory. In Descartes's hands that theory was a vague hypothesis. Newton deduced from it by rigid dynamical reasoning the laws of reflexion and refraction; he applied it with wondrous ingenuity to explain the colours of thin and of thick plates and the phenomena of diffraction, though in the process he had to assume the existence of a mechanism which he must have felt to be almost impossible—a mechanism which in time, as it was applied to explain other and more complex phenomena, became so elaborate that, in the words of Verdet, writing a hundred years later, ‘Pour renverser ce pénible échafaudage d'hypothèses indépendantes les unes des autres, il suffit presque de le regarder en face et de chercher à le comprendre.’ But though Newton may with justice be called the founder of the emission theory, it is most unjust to his memory to state that he fully accepted it as giving a satisfactory account of optics. When he first began his optical work he realised that facts and measurements were needed, and his object was to furnish the facts.
Hooke's hypotheses were right: light is due to wave-motion in an all-pervading ether. But the discovery a century later of the principle of interference vaguely foreshadowed by Hooke (Micrographia, p. 66) was needed to remove the difficulty which Newton experienced. Newton called repeated attention to the difficulty which, unless removed, rendered the rejection of Hooke's theory inevitable. Thus, in reply to Hooke's criticism of his first paper in 1672, he wrote (Phil. Trans. vii. 5089, November 1672): ‘For to me the fundamental supposition itself seems impossible—namely, that the Waves or Vibrations of any fluid can, like the rays of Light, be propagated in straight lines without a continual and very extravagant spreading and bending every way into the quiescent medium where they are terminated by it. I mistake if there be not both experiment and demonstration to the contrary. … For it seems impossible that any of those motions or pressions can be propagated in straight lines without the like spreading every way into the shadowed medium.’
Nor was there anything in the controversy which took place about 1675 to shake Newton's conviction that Hooke's ‘fundamental supposition’ was impossible. Hooke had (18 March 1675) read his paper describing his discovery of diffraction (Posthumous Works, p. 186). He had announced it two years earlier, November 1672 (Birch, Hist. of Roy. Soc. iii. 63). There is no doubt that this was an original discovery, and not, as Newton seemed to imply soon after, a theory borrowed from Grimaldi. But Hooke's paper did not remove the difficulty, nor was there anything more satisfactory in the lectures which he delivered as Gresham professor in 1680–2; in these he supposed the velocity
