VOL. VII.] PHILOSOPHICAL TRANSACTIONS. 17 



mental part of it, is not against mc. That fundamental supposition is — " That 

 the parts of bodies, when briskly agitated, do excite vibrations in the aether, 

 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, some- 

 thing 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 applica- 

 tion of this hypothesis, to the solution of phacnomena, I take to be this: that the 

 agitated parts of bodies, according to their several sizes, figures, and motions, 

 do excite vibrations in the aether of various depths or sizes, which being 

 promiscuously propagated through that medium to our eyes, effect in us a sensa- 

 tion of light of a white colour; but if by any means those of unequal sizes 

 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 sizes, which, ac- 

 cording to those sizes, make several tones in sound: that the largest vibrations 

 are best able to overcome the resistance of a refracting superficies, and so break, 

 through it with least refraction ; whence the vibrations of several sizes, that is, 

 the rays of several colours, which are blended together in light, must be parted 

 from one another by refraction, and so cause the phaenomena of prisms and other 

 refracting substances: and that it depends on the thickness of a thin transparent 

 plate orbubble, whether a vibration shall be reflected at its further superficies, or 

 transmitted; so that according to the number of vibrations, interceding the two 

 superficies, they may be reflected or transmitted for many successive thicknesses. 

 And since the vibrations which make blue and violet, are supposed shorter than 

 those which make red and yellow, they must be reflected at a less thickness of 

 the plate: which is sufficient to explicate all the ordinary phaenomena of those 

 plates or bubbles, and also of all natural bodies, whose parts are like so many 

 fragments of such plates. 



These seem to be the most plain, genuine and necessary conditions of this hy- 

 pothesis: and they agree so justly with my theory, that if the animadversor 

 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. For to 

 me the fundamental supposition itself seems impossible ; namely, that the waves 

 or vibrations of any fluid can, like the rays of light, be propagated in straight 

 lines, without a continual and very extravagant spreading and bending every way 

 into the quiescent medium, where they are terminated by it. I mistake if there 

 be not both experiment and demonstration to the contrary. And as to the 

 other two or three hypotheses which he mentions, I had rather believe them 



VOL. II. D 



