. <H«)

scmetbingjtfter tbe tnanner, that Vibrationi in thu /lir caqse a Sensation tf Soundby beruhig agrtinfi tbe Organs of Hearing. Now, the most free and natura 1Application of this Hypothesis, to tbe Solution of Phatnonjena, I take to bcrhis: That the agita ted parte of Bodies, according to their several Sizes, Fi-gures, and Motions, do excite Vibrations in the dEther of various Depths, orBignesses, which being promiscuouily propagated through that Medium toour Eyes, cffect in us a Cenlation of Light of a White Colour ; but if by a-ny means thole of unequal Bignesies be (eparated from one another, the largestheget a Sensation os' a Red Colour j the lealt or shortest, of a deep Violet ;and the intermediate ones, of Intermediate Colours; much after the mannerthat Bodies, according to their several Sizes, Shapes, and Motions, exciteVibrations in the Air of various Eignenes, which, according to those Bignesies,make several Tones in Sound: That the largest Vibrations are best able toovcrcome the resistance of a Refracting Superficies, and so break through it withleaft Refractjon j whence the Vibrations of several Bignesies, that is, theRays ofleveral Colours, which are hlended together in Light, must beparted from oneanother by Rcfraction, and so cause the Phaenomena of Prjsms, and other Re-frafting Substances: And that it depends .on the Thicknesi of a thin transiparent Piate or Babble, whether a Vibration lhall be Reflected at its furtherSuperficies, or Transinitted ; so that, according to thcNumber of Vibrationsintercediug thetwo Superficies, theymay be Reflected or Transinitted forma-ny f'ucceili.ve Thickneües. And since the Vibrations which make Blue andViolet, are siupposed ihortet than those which make Red and Yetlow, theymust be Reflected at a leß Thicknesi of the Plate; Which is sufficient to ex-plicate all theordinary Phaenomena of those Plates or Bubbles, and also of allnatur ai Bodies, whofc parts are like so m@ny Fragments ofsuchPlates.

These leern to be the most plant, genuine, and necesiary Conditions of thisHypothesis. And they agree so justly with my Theory, that if the Animad-Ibr think fit to apply them, be need not, on that Account, apprehend a Di-voreefrom it. But yet bow he will defend it from otficr Difficulties, I kcownot. For,to me the Fundamental supposition it sollftems impoifible, namely, th«tbe Waves or Vibrations of any Fluid, can, like the Rays of Light, be propa-gated in streight Lines, without a Continua! and very extravagant spreadingand bending every way into the Quiescent Medium, wbere they are termi-nated :by it. I mistake, if there be not both Experiment and Demonstrationso the contrary. And as to the other two or three Hypotheses, which hementions, I had rather believe thenvsubject to the like difliculties, than süfpectthe Animadversor should seiest the worst for his own.

What I have seid of this, may be eafily applyed to all other MechanicalHypotheses, in which Light is 'suppo sed to bc caused by any Preüion or Mo-tion whatsoever, excited in tbe Ätther by the agitated parts os LuminousBodies. For, it seems impoitble, that any of those Motions or Prestions canbe propagated in Streight Lines, without the like spreading every way intothe Shadowed Medium, on which they border. But yet, if any Man canthink it posiible, he must at least allow, that those Motions, or Endeavours toMotion, caused in the fEther.by the several parts of any Lucid Body, that dif-