Expcrmcttts and ^hfervat'tons on Light, S i 



Obfervation f diftinftly ftates all the plisenomena that occur at the parallel edges of 

 two knives in a beam of parallel light approaching towards each other in the fame plane. 

 At firft appear two borders of infleded light, one at each edge, fimilar each' to the one 

 before obferved at the fingle edge. In thefe borders, and of this light, are made two fets 

 of fringes, one at each edge, which beginning in the form of flender white ftreaks and dark 

 intervals on the edges of the borders neareft to the knives become broader, and co- 

 loured, and by degrees occupy the whole light between the edges, forming two complete 

 fets of fringes, each compofed of the colours before defcribed in obfervation 0, and be- 

 ginning in the centre of the pafling light. TJ^efe fringes grow broader, then vanifli by 

 pairs, until at laft only one pair, with a darl^ interval between them, remains, which 

 decreafe and vanifti upon the contact of the knives. From this ftatement it appears, that 

 Newton was miftaken in confidering this laft pair of fringes as ftreams of light diftinfl 

 from the fringes, and as being the very borders thcmfelves of infletled light obferved at the 

 edge of a fingle knife, out of which they are indeed formed, and of which they really 

 are only an inconfiderable part. In this cafe each edge by infleiSring gives the light pafling 

 neareft to it a due divergency, and the other forms fringes therein. 



In obfervation ? a very fmall hole, ^V of an inch wide, being ufed, gives a due diver- 

 gency to the light, and, therefore, the two edges, when even at a diftance, form fringes 

 to themfelves of an arrangement of colours, the Inverfe of thofe in the preceding obferva- 

 tion inftead of borders of white light, but upon their nearer approach break up and diflblve 

 thefe, and by their more powerful mutual attraftions form fringes in a contrary order, 

 fimilar to thofe of obfervation e, thus exhibiting, under different circumftances, two dif- 

 ferent fets of fringes. 



Obfervation v\, fhews that even in different diftant planes the fame phenomena, with 

 due change of circumftances, are produced at the edges of two knives. 



Obfervation S, by placing parallel fided plates of lead, of different breadths, in a 

 divergent beam of light paffed through a very fmall hole, produced not only the two ufual 

 fets of fringes at their external edges, but alfo a fecond double fet within the fhadow 

 beginning from the centre thereof, both fets of fringes growing broader as the breadth of 

 the lead was diminifhed, and the fringes in the fhadow vanifhing by pairs, and at laft en- 

 tirely difappearing. Thefe fringes are the coloured ftreaks obferved by others on the 

 fhadows of fmall bodies^ Cylinders alfo of fucceffively reduced diameters down to the hair 

 of the firft obfervation, produce the fame appearances, and even with the fame cylinder or 

 plate of a due fize, the fame appearances may 'be produced by diftance or obliquity of 

 obfervation. 



In Newton's eighth obfervation the edges of the knives being inclined until they met, 

 and the beam pafEng through a hole.^ of an inch wide, at a very fmall diftance from the 

 knives the fringes appeared at, ran along the edges and met, and where they met ended \ 

 but at a much greater diftance they appeared to grow broader as they approached, then 

 met and crofTed each other, and then appeared broader than before, whence he concluded, 

 Vol. IV — May 1800. M that 



