676 iiEPOET— 1893. 



reflecting sethereal superficies puts it into a vibrating motion, that physical super- 

 ficies being by the perpetual appulse of rays always kept in a vibrating motion, 

 and the ether therein continually expanded and compressed by turns, if a ray of 

 light impinge on it when it is much compressed, I suppose it is then too dense and 

 stiif to let the ray through, and so reflects it ; but the rays that impinge on it at 

 other times, when it is either expanded by the interval between two vibrations or 

 not too much compressed and condensed, go through and are refracted. . . . And 

 now to explain colours. I suppose that as bodies excite sounds of various tones> 

 and consequently vibrations in the air of various bignesses, so when the rays o-f 

 light by impinging on the stiff refracting superficies excite vibrations in the ether, 

 these rays excite vibrations of v.arinus bignesses ; . . . therefore, the ends of the 

 capillamenta of the optic nerve which front or face the retina being such refract- 

 ing superficies, when the rays impinge on them they must there excite these vibra- 

 tions, which vibrations (like those of sound in a trumpet) will run along the 

 aqueous pores or crystalline pith of the capillamenta through the optic nerves 

 into the sensorium (which light itself cannot do), and there, I suppose, affect th* 

 sense with various colours, according to their bigness and mixture — the biggest 

 ■with the strongest colours, reds and yellows ; the least with the weakest, blue» 

 and violets ; the middle with green ; and a confusion of all with white.' 



The last idea, the relation of colour to the bigness of wave-length, is put even 

 more plainly in the ' Opticks/ Query 13 (ed. 1704) : — * Do not several sorts of rays 

 make vibrations of various bignesses, which according to their bignesses excite 

 sensations of various 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 whole is but a development of a reply, written in 1672, to a criticism of 

 Hooke's on his first optical paper, in which Newton says : ' It is 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,' he continues, 

 * my hypothesis has a much greater affinity with his own [Hooke's] than he seems 

 to be aware of, the vibrations of the ether being as useful and necessary in this as 

 in his.' 



Thus Newton, while in the ' Opticks ' he avoided declaring himself as to the- 

 mechanism by which the fits of easy reflexion and transmission were produced, has 

 in his earlier writings developed a theory practically identical in many respects with 

 modern views, though without saying that he accepted it. It was an hypothesis ; 

 one difficulty remained, it would not account for the rectilinear propagation, and it 

 must be rejected till it did. 



Light is neither ether nor its vibrating motion ; it is energy which, emitted from 

 luminous bodies, is carried by wave motion in rays, and falling on a reflecting 

 surface sets up fresh waves by which it is in part transmitted and in part reflected. 

 Light is not material, but Newton nowhere definitely asserts that it is. He 

 ' argues the corporeity of light, but without any absolute positiveness.' In the 

 ' Principia,' writing of his particles, his words are : ' Harum attractionum baud 

 multum dissimiles sunt Lucis reflexiones et refractiones ; ' and the Scholium con- 

 cludes with : ' Igitur ob analogiam quae est inter propagationem radiorum lucis et 

 progressum corporum, visum est propositiones sequentes in usus opticos subjungere ; 

 interea de natura radiorum (utrum sint corpora necne) nihil omnino disputans, 

 sed trajectorias corporum trajectoriis radiorum persimiles solummodo determi- 

 nans.' '■ 



No doubt Newton's immediate successors interpreted his words as meaning that 

 he believed in the corpuscular theory, conceived, as Herschel says, by Newton, and 



' The reflexions and refractions of light are not very unlike these attractions. 

 Therefore, because of the analogy which exists between the propagation of rays of 

 light and the motion of bodies, it seemed right to add the following propositions for 

 optical purposes, not at all with any view of discussing the nature of rays (whether 

 they are corporeal or not), but only to determine paths of particles which closely 

 resemble the paths of rays. — Principia, lib. i., sect, xiv., prop. 96, Scholium. 



