swift and minute. … In the fourth place, therefore, I suppose light is neither æther nor its vibrating motion, but something of a different kind propagated from lucid bodies. To avoid dispute and make this hypothesis general, let every man take his fancy. Fifthly, it is to be supposed that light and æther mutually act upon one another.’ It is from this action that reflection and refraction came about. To explain colour Newton supposes that the rays of light impinging on a reflecting surface excite vibrations of various ‘bignesses’ (waves of different length, we should say), and these, transmitted along the nerves to the brain, affect the sense with various colours according to their ‘bigness,’ the biggest with red, the least with violet. Thus ‘Optics,’ query 13 (ed. 1704): ‘Do not several sorts of rays make vibrations of several bignesses which, according to their bignesses, excite sensations of several colours … and particularly do not the most refrangible rays excite the shortest vibrations for making a sensation of deep violet, the least refrangible the largest for making a sensation of deep red?’
The above is but a development of the reply to Hooke's criticism of 1672 (Phil. Trans. vii. 5086), in which Newton says: ‘'Tis true that from my theory I argue the Corporeity of Light, but I do it without any absolute positiveness, as the word perhaps intimates, and make it at most a very plausible consequence of the doctrine, and not a fundamental supposition.’ ‘Certainly’ my hypothesis ‘has a much greater affinity with his own than he seems to be aware of, the vibrations of the æther being as useful and necessary in this as in his.’
Thus Newton, while he avoided in the ‘Optics’ any declaration respecting the mechanism by which the ‘fits of easy reflexion and transmission’ were produced, had in his earlier papers developed a theory practically identical in many respects with modern views, though without avowedly accepting it. The something propagated from luminous bodies which is distinct from the ether and its vibratory motion is energy, which, emitted from those bodies, is carried by wave motion through the ether in rays, and, falling on a reflecting or refracting surface, sets up fresh waves, by which part of the energy is transmitted, part reflected. Light is not material, but Newton nowhere states that it is. In the ‘Principia’ his words are ‘Harum attractionum haud multum dissimiles sunt Lucis reflexiones et refractiones,’ and the scholium concludes with ‘Igitur, ob analogiam quæ est inter propagationem radiorum lucis et progressum corporum, visum est Propositiones sequentes in usus Opticos subjungere; interea de naturâ radiorum, utrum sint corpora necne, nihil omnino disputans, sed Trajectorias corporum Trajectoriis radiorum persimiles solummodo determinans.’
No doubt Newton's immediate successors interpreted his words as meaning that he believed the corpuscular theory of light, conceived, as Herschel says (Encycl. Metropolitana, p. 439), ‘by Newton, and called by his illustrious name, in which light is conceived to consist of excessively minute particles of matter projected from luminous bodies with the immense velocities due to light, and acted on by attractive and repulsive forces residing on the bodies on which they impinge.’ Men learnt from the ‘Principia’ how to deal with the motion of small particles under definite forces; the laws of wave motion were less clear, and there was no second Newton to explain them. As Whewell states (Inductive Sciences, vol. ii. chap. x.), ‘That propositions existed in the “Principia” which proceeded on this hypothesis was with many … ground enough for adopting the doctrine.’ A truer view of Newton's position was expressed in 1801 by Young, who writes (Phil. Trans. 12 Nov.): ‘A more extensive examination of Newton's various writings has shown me that he was in reality the first that suggested such a theory, as I shall endeavour to maintain; that his own opinions varied less from this theory than is now almost universally supposed; and that a variety of arguments have been advanced, as if to confute him, which may be found nearly in a similar form in his own works.’
The later editions of the ‘Optics’ contain some additional queries. The double refraction of Iceland spar had been discussed at a meeting of the Royal Society on 12 June 1689, at which Newton and Huyghens were present. Newton's views were first given in print in 1706 in the Latin edition of the ‘Optics,’ query 17. In the second English edition (1718) this became query 25. In this query Newton rejected Huyghens's construction for the extraordinary ray, and gave an erroneous one of his own. The succeeding queries expressed more definitely than elsewhere the view that rays of light are particles. Thus query 29: ‘Are not rays of light very small bodies emitted from shining substances?’ In the advertisement to the second edition Newton, in the case of a speculation about the cause of gravity, gave the reason for putting it in the form of a query, that he was ‘not yet satisfied about it for want of experiments.’
