THEORY OF LIGHT AND COLOURS, g3 



land dafliing againft the bottom of the eye, fomething after the 

 manner that vibrations in the air caufe a fenfation of found by- 

 beating againft the organs of hearing. Now, the moft free 

 and natural application of thishvpothefis to the folution of phe- 

 nomena, I take to be this : that the agitated parts of bodies, 

 according to their feveral fizes, figures, and motions, do ex- 

 cite vibrations in the ether of various depths or bignefles, Particular deve- 

 which, being promifcuoufly propagated through that medium topemcnt j 

 to our eyes, effect in us a fenfation of light of a white colour ; 

 but if by any means thofe of unequal bignefles be feparated 

 from one another, the largeft beget a fenfation of a red colour, 

 the leaft or fhorteft of a deep violet, and the intermediate ones 

 of intermediate colours ; much after the mannner that bodies, as " m found, 

 according to their feveral fizes, fhapes, and motions, excite vi- 

 brations in the air of various bignelTes, which, according to 

 thofe bignefles, make feveral tones in found : that the Jargeft 

 vibrations are beft able to overcome the refiftance of a refract- Various refran- 

 ing fuperficies, and fo break through it with leaft refraction ; gl ' Ity : 

 whence the vibrations of feveral bignefles, that is, the rays of 

 feveral colours, which are blended together in light, muft be 

 parted from one another by refraction, and fo caufe the phe- 

 nomena of prifins, and other refracting fubftances; and that it Thin tranfpa- 

 depends on the thicknefs of a thin tranfparent plate or bubble, rent P late ** 

 whether a vibration fliall be reflected at its further fuperficies, 

 or tranfmitted ; fo that, according to the number of vibrations, 

 interceding the two fuperficies, they may be reflected or tranf- 

 mitted for many fucceflive thicknefles. And, fince the vibra- 

 tions which make blue and violet, are fuppofed fhorter than 

 thofe which make red and yellow, they muft be reflected at a 

 lcfs thicknefs of the plate : which is fufficient to explicate all 

 the ordinary phenomena of thofe plates or bubbles, and alfo of 

 all natural bodies, whofe parts are like fo many fragments of 

 iuch plates. Thefe feem to be the moft plain, genuine, and 

 neceflary conditions of this hypothefis. And they agree fo 

 juftly with my theory, that if the animadverfor think fit to 

 apply them, he need not, on that account, apprehend a divorce 

 from it. But yet, how he will defend it from other difficulties, 

 I know not." (Phil. Tranf. Vol. VII. p. 5088. Abr. Vol. I, 

 p. 145. Nov. 1672.) 



V To explain colours, I fuppofe, that as bodies of various Repetition of 

 fizes, denfities, or fenfations, do by percuflfion or other action the the «»7* 



G 2 excite 



