so Dr. Young's Lecture on 
“ or face the retina, being such refracting superficies, when the 
“ rays impinge upon them, they must there excite these vibra- 
“ tions, which vibrations (like those of sound in a trunk or 
“ trumpet) will run along the aqueous pores or crystalline pith 
<c of the capillamenta, through the optic nerves, info the. senso- 
“ rium ; — and there, I suppose, 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 observed." (Birch Vol, III. p. 262. 
Dec. 1675.) 
“ Considering the lastingness of the motions excited in the 
“ bottom of the eye by light, are they not of a vibrating nature ? 
iC — Do not the most refrangible rays excite the shortest vibra- 
“ tions, — the least refrangible the largest ? May not the.harmony 
“ and discord of colours arise from the proportions of the vibra- 
“ tions propagated through the fibres of the optic nerve into 
“ the brain, as the harmony and discord of sounds arise from 
“ the proportions of the vibrations of the air ?" (Optics, Qu. 
l6, 13, 14.) 
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 vibrations 
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 undulation, it 
