THEORY OF LIGHT AND COLOURS. 91 



can it with truth be maintained, that the parts of an elaftic. 

 medium communicating any motion, muft propagate that mo- 

 tion equally in all diredions. (Phil. Tranf. for 1800. p. 109— 

 1 12.) All that can be inferred by reafoning is, that the mar- 

 ginal parts of the undtriation muft be fomewhat weakened, 

 and that there muft be a faint divergence in every direction ; 

 but whether either of thefe effects might be of fufficient mag- 

 nitude to be fenfible, could not have been inferred from argu- 

 ment, if the affirmative had not been rendered probable by 

 experiment. 



As to the analogy with other fluids, the moft natural infe- Sound deflefts 

 rence from it is this : f The waves of the air, wherein founds J™ lhan wave * 

 " confift, bend manifeftly, though not fo much as the waves 

 " of water;" water being an inelaftic, and air a moderately 

 elaftic medium ; but ether being moft highly elaftic, its waves 

 bend very far lefs than thofe of the air, and therefore almoft 

 imperceptibly. Sounds are propagated through crooked pal- Crooked paf- 

 fages, becaufe their fides are capable of reflecting found, juft ' a S es * 

 as light would be propagated through a bent tube, if perfectly 

 poliflied within. 



The light of a ftar is by far too weak to produce, by its 

 faint divergence, any vifible illumination of the margin o a 

 planet eclipfing it ; and the interception of the fun's light by 

 the moon, is as foreign to the qneftion, as the ftatement of 

 inflection is inaccurate. 



To the argument adduced by Huygens, in favour of the 

 rectilinear propagation of undulations, Newton has made no 

 reply ; perhaps becaufe of his own mifconception of the na- 

 ture of the motions of elaftic mediums, as dependent on a 

 peculiar law of vibration, which has been corrected by later 

 mathematicians. (Phil. Tranf. for 1800, p. 116.) On the 

 whole, it is prefumed, that this proposition may be fafely 

 admitted, as perfectly confift ent with analogy and with expe- 

 riment. 



(To be continued.) 



VIII. Remarks 



