206 KINETIC THEORIES OF GRAVITATION. 



cipitatioD, mucb after the manner that vapors are eoudensed into water. 

 . . . . Thus 2^Gr]ia])s may all thmgs he originated from cether. . . ." 



"In the second place, it is to be supposed that the (rther is a vibrat- 

 ing medium like air, only the vibrations far more swift and minute; 

 those of air made by a man's ordinary voice succeeding one another at 

 more than half a foot or a foot distance, but those of a?ther at a less 

 distance than the hundred-thousandth of an inch. And as in air the 

 vibrations are some larger than others, but yet all equally swift, (for in 

 a ring of bells the sound of every tone is heard at two or three miles^ 

 distance in the same order that the bells are struck,) so I suppose the 

 aitherial vibrations differ in bigness but not in swiftness."* 



]S!"ewton had in 1G72 controverted the supposed opposition of his views 

 to the action of the aether by answering : " 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 sether, 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 of the air cause a sensation of sound by beating against 

 the organ of hearing.' Now the most free and natural application of 

 this hypothesis to the solution of phenomena 1 take to be this : That 

 the agitated parts of bodies, according to their several sizes, ligures, 

 and motions, do excite vibrations in the sether of various depths or 

 bignesses, which being promiscuously propagated through that medium 

 to our eyes, effect in us a sensation of light of a white color ', but if by 

 any means those of unequal bigness be separated from one another, the 

 largest beget a sensation of a red color, the least or shortest of a deep 

 violet, and the intermediate ones of intermediate colors, 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 vibra- 

 tions are best able to overcome the resistance of a refracting superficies, 

 and so break through it with the least refraction ; whence the vibrations 

 of several bignesses, that is the rays of several colors which are blended 

 together in light, must be parted from one another by refraction, and so 

 cause the i^henomena of x)risms and other refracting substances ; and 

 that it depends on the thickness of a thin transparent plate or bubble 

 whether a vibration shall be reflected at its further superficies or trans- 

 mitted; so that, according to the number of vibrations interceding the 

 two superficies, they may be reflected or transmitted for many succes- 

 sive 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 i)lienomena of those plates or bubbles, and 



* History of the Roj'al Society of London, by Thomas Birch, 1757, 4 vols, quarto, 

 vol. iii, pp. 249-251. 



