ON THE THEORY OF LIGHT AND COLOURS. 



617 



those plates or bubbles, and also of all natu- 

 ral bodies, whose parts are like so many frag- 

 ments of such plates. These seem to be most 

 ])1ain, genuine, and necessary conditions of 

 this hypothesis. And they agree so justly 

 with my theory, that if the animadversor 

 think fit to apply them, he need not, on that 

 account, apprehend a divorce from it. But 

 yet, how he will defend it from other difficul- 

 ties, I know not." (Phil. Trans. VII. 5088. 

 Abr. I. 145. Nov. 1672.) 



"To explain colours, I suppose, that as 

 bodies of various sizes, densities, or sensa- 

 tions, do, by percussion or other action, excite 

 sounds of various tones, and consequently 

 vibrations in the air of different bigness; so 

 the rays of light,by impinging on the stiff" re- 

 fracting superficies, excite vibrations in the 

 ether, — of various bigness; the biggest, 

 strongest, or most potent rays, the largest 

 vibrations; and others shorter, according to 

 their bigness, strength, or power: and there- 

 fore the ends of the capillamenta of the optic 

 nerve, which pave or face the retina, being 

 such refracting superficies, when the raj's 

 impinge upon them, the}' must there excite 

 these vibrations, which vibrations (like those 

 of sound in a trunk or trumpet) will run 

 along the aqueous pores or crystalline pith 

 of the capillamenta, through the optic nerve 

 into the sensorium ; — and there, I suppose, 

 affect the sense with various colours, accord- 

 ing to their bigness and mixture; the biggest 

 with the strongest colours, reds and yellows ; 

 the least with the weakest, blues and violets; 

 the middle with green ; and a confusion of 

 all with white, much after the manner that, 

 in the sense of hearing, nature makes use of 

 aerial vibrations of several bignesses, to ge- 

 nerate sounds of divers tones ; for the ana- 



VOL. II. 



logy of nature is to be observed." (Birch. III. 

 262. Dec. 1675.) 



" Considering the lastingness of the mo- 

 tions excited in the bottom of the eye by 

 light, are they not of a vibrating nature? — 

 Do not the most refrangible rays excite the 

 shortest vibrations, — the least refrangible the 

 largest? May not the harmony and discord 

 of colours arise from the proportions of the 

 vibrations propagated through the fibres of 

 the optic nerve into the brain, as the har- 

 mony and discord of sounds arise from the 

 proportions of the vibrations of the air f" 

 (Optics, Qu. 16, 13, 14.) 



Scholium. Since, for the reason here 

 assigned by Newton, it is probable that the 

 motion of the retina is rather of a vibratory 

 than of an undulatory nature, the frequency 

 of the vibrations must be dependent on the 

 constitution of this substance. Now, as it 

 is almost impossible to conceive each sensi- 

 tive point of the retina to contain an infinite 

 number of particles, each capable of vibrat- 

 ing in perfect unison with every possible 

 undulation, it becomes necessary to suf)pose 

 the number limited ; for instance, to the 

 three principal colours, red, yellow, and blue, 

 of which the undulations are related in mag- 

 nitude nearly as the numbers 8, 7, and 6 ; 

 and that each of the particles is capable of 

 being put in motion less or more fprcibly, 

 by undulations difliering less or more from 

 a perfect unison ; lor instance, the undula- 

 fions of green light, being nearly in the ratio 

 of 64j will afl'ect equally the particles in uni- 

 son with yellow and blue, and produce the 

 same efl'ect as a light composed of those two 

 species: and each sensitive filament of the 

 nerve may consist of three portions, one for 

 each principal colour. Allowing this state- 



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