OV THE THEOnV OF LIGHT AND COLOURS, 



621 



weakened its attractive forccj or have con- 

 tracted its inflecting atmosphere. In other 

 circumstances, the lateral divergence might 

 appear to increase, instead of diminishing, 

 the hreadth of the beam. It is said that a 

 beam of light, even passing through a va- 

 cuum, is visible in all directions, and if the va- 

 cuum were as perfect as it is possible to make 

 it, the experiment would afford a strong ar- 

 gument against the projectile system. 



The whole of the phenomena described 

 by Grimaldi, under the very proper denomi- 

 nation " diffraction," afford us examples of 

 the deviation of light from rectilinear motion, 

 nor have we the slightest evidence that an 

 attractive force is concerned in producing 

 these effects ; on the contrary the experiment 

 already mentioned, in which the refractive 

 density of the substance concerned appears 

 to be indifferent to the result, renders the 

 supposition of such an inflecting force ex- 

 tremely improbable. 



As the subject of this proposition has al- 

 ways been esteemed the most diflicult part 

 of thcundulatory system, it will be proper to 

 examine here the objections which Newton 

 has grounded upon it. 



" 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 straigiit lines, 

 without a continual and very extravagant 

 spreading and bending every way into the 

 quiescent medium, where they are termi- 

 nated by it. I mistake, if there be not both 

 experiment and demonstration to the con- 

 trary." (I'hil. Trans. VTI. 5089. Abr. I. 14G. 

 Nov. lf)72.) 



" Motus omnis per fluidum propagatus di- 

 rergit a recto tramile in spatia immota." 



" Quoniam medium ibi," that is, in the 



middle of an undulation admitted, "densiu* 

 est,quam in spatiis hinc inde,dilatabitscse torn 

 versus spatia utrinquc sita, quani versus pul- 

 suum rariora intervalla; eoque pacto — pulsus 

 eiidemfere celeritate sese inmedii partes qui- 

 escentes hinc indc relaxare debent; — ideoque 

 spatiumtotinn occupabunt. — ^Hoc experimur 

 in sonis." (Princip. Lib. II. Prop. 42.) 



" Are not all hypotheses erroneous, in 

 which light is supposed to consist in pression 

 or motion, propagated through a fluid me- 

 dium? — If it consisted in pression or motion, 

 propagated cither in an instant, or in time, it 

 would bend into the shadow. For pression or 

 motion cannot be propagated in a fluid in 

 right lines, beyond an obstacle which stops 

 part of the motion, but will bend and spread 

 every way into the quiescent medium which 

 lies beyond the obstacle. — The waves on the 

 surface of stagnating water, passing by the 

 sides of a broad obstacle which stops part of 

 them, bend afterwards, and dilate them- 

 selves gradually into the quiet water behind 

 the obstacle. The waves, pulses, or vibra- 

 tions of the air, wherein sounds consist, bend 

 manifestly, though not so much as the waves 

 of water. For a bell or a cannon may be 

 heard beyond a hill, which intercepts the 

 sight of the sounding body ; and sounds arc 

 propagated as readily through crooked pipes 

 as straight ones. But light is never known 

 to follow crooked passages, nor to bend into 

 the shadow. For the fixed stars, by the in- 

 terposition of any of the planets, cease to be 

 seen. And so do the parts of the sun, by 

 the interposition of the moon, i\Iercury, or 

 Venus. The rays, which pass very near to 

 the edges of any body, are bent a little by 

 the action of the body; — but this bending is 

 not towards but from the shadow, and is 

 performed only in the piissagc of the ray by 



