$£ THEORY OF LIGHT AND COLOURS. 



excite founds of various tones, and confequently vibrations in 

 the air of different bignefs; fo the rays of light, by impinging 

 on the ftiff refracting fuperficies, excite vibrations in the ether, 

 of various bignefs ; the biggeft, ftrongeft, or mod potent rays, 

 the largeft vibrations j and others ihorter, according to their 

 bignefs, ftrength, or power : and therefore the ends of the ca- 

 pillamenta of the optic nerve, which pave or face the retina, 

 being fuch refracting fuperficies, when the rays impinge upon 

 them, they mutt there excite thefe vibrations, which vibrations 

 (like thofe of found in a trunk or trumpet) will run along the 

 aqueous pores or cryftalline pith of the capillamenta, through 

 the optic nerves, into the fenforium ; and there, I fuppofe, 

 affect the fenfe with various colours, according to their bignefs 

 and mixture ; the biggeft with the ftrongeft colours, reds and 

 yellows ; the leaft with the weakeft, blues and violets ; the 

 middle with green; and a confuiion of all with white, much 

 after the manner that, in the fenfe of hearing, nature makes 

 ufe of aerial vibrations of feveral bigneffes, to generate founds 

 of divers tones ; for the analogy of nature is to be obferved." 

 (Birch, Vol. III. p. 262, Dec. 1675.) 



94 Confidering the Iaftingnefs of the motions excited in the 

 bottom of the eye by light, are they not of a vibrating nature ? 

 Do not the moll refrangible rays excite the fhorteft vibrations, 

 — the leaft refrangible the largeft ? May not the harmony and 

 difcord of colours arife from the proportions of the vibrations 

 propagated through the fibres of the optic nerve into the 

 brain, as the harmony and difcord of founds arife from the 

 proportions of the vibrations of the air?" (Optics, Qu. 

 16, 13, 14.) 

 Scholium. The Scholium. Since, for the reafon here afligned by Newton, 

 parts of the re- j t ls p r0Da bi e that the motion of the retina is rather of a vibra- 

 bTbly cap^We°o"f tory than of an undulatory nature, the frequency of the vibra- 

 vibrating in uni- t j ons mu ft foe dependent on the conftitution of this fubftance. 

 mited number of Now, as it is almoft impoflible to conceive each fenfitive point 

 colorific mo- of the retina to contain an infinite number of particles, each ca- 

 t,ons ' pable of vibrating in perfect unifon with every poffible undu- 



lation, it becomes necelfary to fuppofe the number limited, for 

 mftance, to the three principal colours, red, yellow, and blue, 

 of which the undulations are related in magnitude nearly as 

 the numbers 8, 7, and 6 ; and that each of the particles is ca- 

 pable of being put in motion lefs or more forcibly,, by undula- 

 tions 



