

THEORY OF LIGHT AND COLOURS. g£ 



Third Book, Obf. 5 . and they fully anfwer the defcription of 

 this propofition. 



Let the concentric lines in Fig. 1. (Plate V.) reprefent the Linear explansf 

 contemporaneous fituation of fimilar parts of a number of fuc- tl0n * 

 cefiive undulations diverging from the point A ; they will alfo 

 reprefent the fuccetfive fituations of each individual undula- 

 tion : let the force of each undulation be reprefented by the 

 breadth of the line, and let the cone of light A B C be admit- 

 ted through the aperture B C ; then the principal undulations 

 will proceed in a rectilinear direction towards G H, and the 

 faint radiations on each fide will diverge from B and C a? 

 centres, without receiving any additional force from any inter- 

 mediate point D of the undulation, on account of the inequa- 

 lity of the lines D E and D F. But, if we allow fome little 

 lateral divergence from the extremities of the undulations, it 

 muft diminifh their force, without adding materially to that of 

 the diffipated light ; and their termination, inftead of the right 

 line B G, will aflume the form C H ; fince the lofs of force 

 muft be more confiderable near to C than at greater diftances. 

 This line correfponds with the boundary of the fhadow in 

 Newton's firft obfervation, Fig. 1 ; and it is much more pro- 

 bable that fuch a diftipation of light was the caufe of the in- 

 creafe of the fhadow in that obfervation, than that it was ow- 

 ing to the action of the inflecting atmofphere, which muft have 

 extended a thirtieth of an inch each way in order to produce it ; 

 efpecially when it is confidered that the fhadow was not di- 

 minifhed by furrounding the air with a denfer medium than 

 air, which muft in all probability have weakened and con- 

 tracted its inflecting atmofphere. In other circumftances, the 

 lateral divergence might appear to increafe, inftead of dimi- 

 nifhing, the breadth of the beam. 



As the fubject of this propofition has always been efteemed Obje&ions of 

 the moft difficult part of the undulatory fyftem, it Will be pro- Newt °n to the 

 per to examine here the objections which Newton has ground- ftem ofTighu" 

 ed upon it. 



" To me, the fundamental fuppofition itfelf feems impofli- 

 ble ; namely, that the waves or vibrations of any fluid can, 

 like the rays of light, be propagated in ftraight lines, without 

 a continual and very extravagant fpreading and bending every 

 way into the quiefcent medium, where they are terminated by 



it. 



