Discovery of his Theory of Colours. 117 



the capillamenta of the optic nerve, which pave or face the 

 retina, being such refracting superficies, when the rays impinge 

 upon them, they 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 sup- 

 pose, affect the sense with various colours, according 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 generate 

 sounds of divers tones ; for the analogy of nature is to be ob- 

 served." (Birch, vol. iii. p. 262, December 1675.) 



" Considering the lastingness of the motions 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 harmony and discord of sound arise from the propor- 

 tions of the vibrations of the air?" (Optics, Qu. 16, 13, 14.) 



After these quotations from Newton, Young brings out his 

 hypothesis of colour-sensations under the following 



" Scholium. Since, for the reason here assigned by Newton, 

 it is probable that the motion of the retina is rather of a vibra- 

 tory than of an undulatory nature, the frequency of the vibra- 

 tions must be dependent on the constitution of this substance. 

 Now, as it is almost impossible to conceive each sensitive point 

 of the retina to contain an infinite number of particles, each 

 capable of vibrating in perfect unison with every possible un- 

 dulation, it becomes necessary to suppose the number limited ; 

 for instance, 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 

 capable of being put in motion, less or more forcibly, by undu- 

 lations differing less or more from a perfect unison ; for in- 

 stance, the undulations of green light, being nearly in the ratio 

 of 6^, will affect equally the particles in unison with yellow 

 and blue, and produce the same effect 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." 



An attentive perusal of the above quotation will show that 

 Young's hypothesis imagines each sensitive point of the retina 

 to contain particles capable of vibrating in perfect unison to 

 those vibrations causing three principal colours (red, yellow, 



