i8 
Dr. Young’s Lecture on 
HYPOTHESIS III. 
The Sensation oj different Colours depends on the different fre- 
quency of Vibrations , excited by Light in the Retina. 
Passages from Newton. 
“ The objector’s hypothesis, as to the fundamental part of it, 
“ is not against me. That fundamental supposition is, that the 
“ parts of bodies, when briskly agitated, do excite vibrations in 
“ the ether, which are propagated every way from those bodies 
“ in straight lines, and cause a sensation of light by beating 
“ and dashing against the bottom of the eye, something after 
“ the manner that vibrations in the air cause a sensation of 
“ sound by beating against the organs of hearing. Now, the 
“ most free and natural application of this hypothesis to the 
“ solution of phenomena, I take to be this : that the agitated 
“ parts of bodies, according to their several sizes, figures, and 
“ motions, do excite vibrations in the ether of various depths 
“ or bignesses, which, being promiscuously propagated through 
u that medium to our eyes, effect in us a sensation of light of a 
“ white colour ; but if by any means those of unequal bignesses 
“ be separated from one another, the largest beget a sensation 
“ of a red colour, the least or shortest of a deep violet, and 
“ the intermediate ones of intermediate colours ; much after 
“ the manner that bodies, according to their several sizes, 
“ shapes, and motions, excite vibrations in the air of various 
“ bignesses, which, according to those bignesses, make several 
“ tones in sound : that the largest vibrations are best able to 
f< overcome the resistance of a refracting superficies, and so 
“ break through it with least refraction ; whence the vibrations 
