82 THEORY OF LIGHT AND COLOURS. 



Vibrations com- " In the fccond place, it is to be fuppofed, that the ether 13 



of found. * ° e " a v ib rat ing medium like air, only the vibrations far more 



'• fwift and minute ; thofe of air, made by a man's ordinary 



'5 voice, fucceeding one another at more than half afoot, or a 



'? foot diflance; but thofe of ether at a lefs diitance than the 



" hundred thoufandth part of an inch. And, as in air the vi- 



" brations are fome larger than others, but yet all equally fwift, 



" (for in a ring of bells the found of every tone is heard at two 



'/ or three miles diftance, in the fame order that the bells are 



" ftruck), fo, I fuppofe, the ethereal vibrations differ in big- 



% nefs, but not in fwiftnefs. Now, thefe vibrations, befide 



*l their ufe in reflection and refraction, may be fuppofed the 



" chief means by which the parts of fermenting or putrifying 



" fubflances, fluid liquors, or melted, burning, or other hot 



"bodies, continue in motion." (Birch. Vol. III. p. 251. 



Dec. 1675). 



—— reflexion, " When a ray of light falls upon the furface of any pellucid 



refraction 5 < t D0C i V) an( j i s there refracted or reflected, may not waves of 



*' vibrations, or tremors, be thereby excited in the refracting 



" or reflecting medium ? And are not thefe vibrations propa- 



" gated from the point of incidence to great diftances ? And 



" do they not overtake the rays of light, and by overtaking 



" them fuccefTively, do not they put them into the fits of eafy 



'f- reflection and eafy tranfmiflion defcribed above }" (Optics, 



Qu. 17). 



and the alter- " Light is in fits of eafy reflection and eafy tranfmiflion, be- 



nate fits. <t f ore [\_ s incidence on tranfparent bodies. And probably it is 



'* put into fuch fits at its firfl: emiffion from luminous bodies, 



" and continues in them during all its progrefs." (Optics, 



Second Book, Part III. Prop. 13.) 



HYPOTHESIS III. 



Tlte Senfation of different Colours depends on the different frequency 

 of Vibrations, excited by Light in the Retina. 



Paffagesfrom Newton. 

 Hypoth. III. " The objector's hypothefis, as to the fundamental part of 



Colour depends ; t> ; s not againft me. That fundamental fuppofition is, that 

 of the echlr e e aT y the P arts of bodies, whenbrifkly agitated, do excite vibrations 

 vibrations. in the ether, which are propagated every way from thofe bo- 

 dies in ilraight lines, and caufe a fenfation of light by beating 



and 



