On Ihe Iriflexioii, Reflfxion, and Colours cf Light. gf^ 



tRe bows with the regularity in which they always appear, nor could tlie colours be in the 

 order above mentioned from the different refrangibility of the rays : it will alfo be obvious 

 to any one who tries the thing, that the preiVure only increafes the bright.iefs and breadth 

 of the bows, but does not form them. The true folution of the difficulty feenis to be this • 

 The rays which enter the pupil areinfleded in their paflage through the fibres which extend- 

 over the cornea, and which are very minute, but op^ke ; by thefe they are decompounded 

 into fnnges, having the red outermofl: and the violet innermoll ; and the fringes formed by 

 each fibre, being joined together, form the bow. How then does the prelTure enlarge and 

 vivify them ? The fibres are naturally extended over the furface of a fpherical fegment : 

 when this furface is comprefl-ed into a plane circle, they are condenfed into a much lefs 

 fpace, and confequently brought nearer to one another; the rays are therefore move in- 

 flefted and feparated than before. If this explanation be true, it will follow that the like 

 bows may be produced by fmall hairs like fibres placed near one another ; and this I found 

 perfeaiy confiftent with faft : the bows are in this cafe brighter than in the other; and 

 the fmall hairs on a hat or the hand made them brighter than any other I have tried; A 

 circumftance which I obferved in both cafes feems to Ihew clearly the identity of the 

 caufes ; the white fpace which reached from the interior bow to the flame, was fpeckled or 

 mottled in a manner which cannot be eafily defcribedj but which any one will perceive 

 upon trying the experiment, 



8. The laft of thefe phenomena which I {hall mention is the celebrated one obferred by 

 Sir Ifaac Newton, namely the rings of colours with which the focus of a concave glafs mir- 

 ror is furrounded. Sir Ifaac made feveral moft ingenious and accurate experiments 

 to inveftigate their nature*; and finding their breadth to be in the inverfe fubduplicate 

 ratio of the mirror's thicknefs, he concluded that they were of the fame nature and original 

 with thofe of thin plates defcribed by him f. The Due de Chaulnes purfued thefe expe- 

 riments with confiderable fuccefs : he found that the rings were brighter, the nearer to the 

 perpendicular the rays were incident ;. and that if, inftead of a concave glafs mirror, a metal 

 one was ufed, with a fmall piece of fine cambric, or reticulated filver wire, firetched before 

 it, the colours were no longer difpofed in rings, but in ilreaks of the fame fiiape with the 

 intervals between the threads : hence he concludes that they are owing to inflexion ; that 

 in paffing through the firit lurface they are infleaed, and condenfed by the fccond $, 'l am 

 not, I own, quite f.ulsfied with this account of the matter: that they are produced by in- 

 flexion the Duke's experiments put l.cyond doubt ; but that they (hould be formed in pac- 

 ing through the firlt furi".ce, and reflt-acd by the fecond, is quite inconfiltent with the 

 ratio obferved by their breadth, this being greater in the thinneft glafs, and alfo with the 

 order of the colours. Befides, all the coloured images, which fall on the back fide of the 

 mirror, will be (by what we before found, when fpeaking of flexibility §) relleacd into a; 

 white focus ; fo that, upon the whole, there appears every reafon to believe that the ring* 

 are formed by the firlt furf..ct, out of the light which, after reflexion from the fecond fur- 

 face, is fcattered, and pafl.s on to the chart. It will follow, ift, that a plane mirror makes 

 thcai not ; for the regularly reflected light, not being thrown to a focus,. mixes with the 



•Optics, Book ii. Part IV. f Book ii. Parts I. and II. 



} Mun. dc I'wAcadtinie pour PAmitc 1755. § Pan II. Obf. (. of thi. piper. 



dccoai* 



