Expervnenis and Obftrmlions on Ligki. 83 



fuppofed to appear between the two lafl: iieareft the concourfc of the edges ; but obferva- 

 tion X, remarks, that this light is really the light of the laft pair of fringes neareft 

 to the concourfe of the edges exhibiting the appearance and colours which (hould belong to 

 thefe fringes under fuch circumftances, and from the overfight difcovered with refpeft to 

 this pair of fringes, from the accuracy required in the meafurements of thefe flvadowy 

 parts, from the aQual parallelifm of the lines of the fringes on one fide to, and the ap- 

 parent probability that they will crofs and cut the afymptotic lines on the other fide, 

 queftions the accuracy of the determinations refpefting thefe lines. Obfervation y., (hows 

 how by ufing a triangular piece or cone of lead, or the points of pins of various fizes, 

 inftead of the parallel-fidcd pieces or cylinders of obfervation S, all the appearances of that 

 obfervation are at onte produced at the edges of the triangle or cone. 



In obfervation eleventh, Newton examines the fringes made by differently coloured 

 prifmatic lights, and finding the fringes made by red light alone formed at greateft, by 

 violet at fmalleft, and by intermediate colours at intermediate diftances, as if all thofe 

 lights were mixed in the white, and early feparated at the time of inflexion, concludes 

 that the colours arife not from any modifications imprefled upon the rays of light by the 

 hair, but from fimple feparation by various inflexions of the feveral forts of rays. To this 

 mode of explanation another is oppofed in obfervation v, namely, that the different rays 

 when apart are, indeed, infleftftl in the fame manner to make fringes of various colours, 

 as are rays of the fame colour when being at once varioufly feparated from white light 

 they make, together with the reft of the colours, party-coloured fringes. This, however, is 

 attributed not to fimple feparation alone, but to modification at the time of feparation, 

 which gives to each portion of the white light thus feparated a permanent condition, 

 diftinft from all the feparated portions, and the original portion out of which they were 

 feparated. 



Separating by inflexion a fmall portion of white light into various colours, and then re- 

 turning the various rays back again to the fame point under the fame circumftances, or 

 fuppofing them fo returned, they will unite into the original portion, or fuppofing them 

 returned without other change to different points, they will move in parallel lines to each 

 other, pveferving their diftinft chara£l:ers; or if now a fecond time returned without 

 further change, coming in the fame parallel dire£tion, they will be inflefted differently. 

 Thefe circumftances evince a change and permanent conftitution, fuperinduced upon each 

 of thefe portions, and by which they are rendered diftindt from each other, and from the 

 original portion until their re-union into the fame, as a portion fimilar to that from which 

 they virere derived. Thefe fmall portions of white light are its rays, which pafs through 

 the intervals between the particles of bodies, and coloured rays are always formed there- 

 from by fimilar modifications, produced by the aftions of the particles of other bodies at 

 the fame time, modifying as well as feparating them. The very mode of diftribution of 

 the white light argues the adion of a principle, modifying as well as feparating, rather 



M 2 than 



