30 THEORY OF LIGHT AND COLOURS. 



it. I miftake, if there be not both experiment and demon- 

 stration to the contrary." (Phil. Tranf. VII. 5089, Abr. I. 

 146. Nov. 1672.) 



" Motus omnis per ftuidum propagatus divergit a reclo tra- 

 mite infpatia immota." 



f< Quoniam medium ibi" in the middle of an undulation ad- 

 mitted, " denfius eft, quam infpatiis hinc inde, dilatabit fefe tarn 

 rerfus fpatia utrinque fita, quam verfus pulfuum rariora inter- 

 valla ; eoque patio — pulfus eadem fere celeritate fefe in medii par- 

 tes quiefcentes hinc inde relaxare debent ; — ideoque fpatium totujn 

 occupabunt. — Hoc experimur in fonis." (Princip. Lib. II. 

 Prop. 42.) 



u Are not all hypothefes erroneous, in which light is fup- 

 pofed to confift in preffion or motion, propagated through a 

 fluid medium ? — If it con lifted in preffion or motion, propa- 

 gated either in an inftant, or in time, it would bend into the 

 fhadow. For preffion or motion cannot be propagated in a 

 fluid in right lines beyond an obftacle which ftops part of the 

 motion, but will bend and fpread every way into the quiefcent 

 medium which lies beyond the obftacle. The waves on the 

 furface of Stagnating water, paffing by the fides of a broad 

 obftacle which ftops part of them, bend afterwards, and di- 

 late themfelves gradually into the quiet water behind the ob- 

 ftacle. The waves, pulfes, or vibrations of the air, wherein 

 founds confift, bend manifeftly, though not fo much as the 

 waves of water. For a bell or a cannon may be heard beyond 

 a hill, which intercepts the fight of the founding body ; and 

 founds are propagated as readily through crooked pipes as 

 ftraight ones. But light is never known to follow crooked 

 paflages, nor to bend into the fhadow. For the fixed ftars, 

 by the interpofition of any of the planets, ceafe to be feen. 

 And fo do the parts of the fun, by the interpofition of the 

 moon, Mercury, or Venus. The rays which pafs very near 

 to the edge-, of any body, are bent a little by the action of the 

 body ; but this bending is not towards but from the fhadow, 

 and is performed only in the paffage of the ray by the body, 

 and at a very fmall diftance from it. So foon as the ray is 

 paft the body, it goes right on. 5> (Optics, Qu. 28.) 

 Anfwcrstothe Now the propofition quoted from the Principia does not 

 Newtin" 5 ^ dire % contradia this propofition ; for \t does not aflert that 

 fuch a motion muft diverge equally in all directions ; neither 



can 



