6«2 



©N THE THEORY OF LIGHT AND COXOURS. 



the body, and at a very small distance from 

 it. So soon as the ray is past the body, it 

 goes right on." (Optics, Qu. 28.) 



Now tlie proposition quoted from the 

 Principia, even supposing it to be strictly 

 demonstrated, does not directly contradict 

 this proposition ; for it does not assert that 

 such a motion must diverge equally in all 

 directions ; and the admission of the term 

 " almost" is sufficient to invalidate the chain 

 of reasoning : neither can it with truth be 

 maintained, that the parts of an elastic 

 medium, communicating any simple motion, 

 must propagate that motion equally in all di- 

 rections. (Phil. Trans. 1800. lOg. .112.) All 

 tbat can be inferred by reasoning is, that the 

 marginal parts of the undulation must be 

 somewhat weakened, and that there must be 

 a faint divergence in every direction : but 

 whether either of these effects might be of 

 sufficient magnitude to be sensible, with re- 

 spect to light, could not have been concluded 

 from argument, if the affirmative had not 

 been rendered certain by experiment. 



As to the analogy with other fluids, the 

 most natural inference from it is this : " The 

 waves of the air, wherein sounds consist, 

 bend manifestly, though not so much as the 

 waves of water;" water being an inelastic, and 

 air a moderately elastic medium: but the ether 

 being most highly elastic, its waves bend 

 very far less than those of the air, and there- 

 fore almost imperceptibly. Sounds are pro- 

 pagated through crooked passages, because 

 their sides are capable of reflecting sound, 

 just as light would be propagated through a 

 bent tube, if perfectly polished within. 



The light of a star is by far too weak to 

 produce, by its faint divergence, any visible 

 illiimination of the margin of a planet eclips- 



ing it. Such a light has, however, often been 

 seen attached to the moon in a. solar eclipse, 

 as could not be attributed to a lunar atmo- 

 sphere only. What Newton here says, of in- 

 flection, h inconsistent, as Mr. Jordan has 

 already remarked, with some of his own ex- 

 periments. 



To the argument adduced by Huygens, in 

 favour of the rectilinear propagation of undu- 

 lations, Newton has made no reply; perhaps, 

 because of his own misconception of the na- 

 ture of the motions of elastic mediums, as de- 

 pendent on a peculiar law of vibration, 

 which has been corrected by later mathema- 

 ticians. (Phil. Trans. 1800. 116.) On the 

 whole, it is presumed, that this proposition 

 may be safely admitted, as perfectly consist- 

 ent with analogy and with experiment. 



Proposition iv. When an Undulation 

 arrives at a Surface which is the Limit of Me- 

 diums of different Densities, a partial Re- 

 flection takes place, proportionate in Force to 

 the Difference oftlie Densities. 



This may be illustrated, if not demon- 

 strated, by the analogy of elastic bodies of 

 different sizes. " If a smaller elastic body 

 strikes against a larger one, it is well known 

 that the smaller is reflected more or less 

 powerfully, according to the diflierence of 

 their magnitudes : thus, there is always a re- 

 flection when the rays of light pass from a 

 rarer to a denser stratum of t-ther;- and fre- 

 quently an echo when a sound strikes against 

 a cloud. A greater body striking a smaller 

 one, propels it, without losing all its motion: 

 thus, the particles of a denser stratum of 

 ether do not impart the whole of then- mo- 

 tion to a rarer, but in their eflort to proceedj 

 they are recalled by the attraction of the re- 

 fracting substance with equal force ; and 



