594 Pltr.cmt'ia deptndeiit on the Rcjleslon of Light. 



IC Is attrafted, and KC repelled, but not fo much as to reduce them to parallclifm and 

 whitenefs ; an image being formed narrower, and lefs coloured, than when LC is moved 

 fo far round that KC is attracted, and IC defle£led or repelled. If LC is moved round, fo 

 that the mirror is concave, then KC is repelled and IC attradted, as before, unlefs the 

 curvature be confiderable ; and then KC and IC are both repelled, and an image formed 

 in the cattjlic by reflexion. In Obf. 3. we found that certain irregularities in the furface of 

 the refle£lor caufed the images to be in the inverted order of colours. How docs this hap- 

 pen .? In fig. 1 3. let^/,/f, er, r'l, and \h, reprefent the fe£lions of the convex fibres on the 

 furface of the refleftor, and let the ray AB be reflefled from j/", feparated into B;- red and 

 Bt> violet ; then if AB was fo inclined to ef, that Br and Bv fell upon cr, the fide of the 

 fibre next to ef, and a little larger than tf, it is evident that Bi- will be reflefted into vV, and 

 Br into rR, and an image VR will be formed, having the violet outermoft, and the red 

 iniiermoft, the intermediate colours being in their order from V to R. Laftly, it is evident 

 that the greater the angle of incidence is, the longer will be the image, and the farther fe- 

 parated its colours j for which reafon the farther the images are from the fliadow, the lefs 

 dilated and coloured will they be. Nor will they have the fame appearance at all diftances 

 from the point of incidence. Very near it they will be all in the form of fringes acrofs the 

 flreak, the breadth being greater than the length (if I may ufe the exprelFion) ; but as we 

 recede from it they will become diftended, as before defcribed, the length increafing fafter 

 than the breadth, and at one point or diilance they will be juft as long as broad ; all which 

 agrees with experiment. And it is needlefs to fhow by particular demonllration, the manner 

 in which one image is divided from another, the reafon obvioufly being the manner in which 

 tlie fibres on the refle£ling furface are arranged and inclined to one another. 



3. A number of phenomena involved in that of the images are explicable by what has 

 been faid on them. If a piece of metal be fcratchcd, and then expofed in the funfliine, 

 a number of broken <:olours will be formed by the fcratches, as may be feen eitlier by let- 

 ting them fall on the eye, or by receiving them on a white obje£l. This is evidently owing 

 to the different reflexibility of the rays incident on the fcratches, which are fo many irregu^ 

 lar fpecula, of great curvature ; the images are therefore diftorted and broken, juft as a can- 

 dle, &c. appears broken and coloured when viewed through a piece of irregular cryftal, 

 fuch as the bottom of a wine glafs. If we look attentively at any object expofed in tlie 

 light of the fun, provided it be not poliflied, we fhall fee its furface mottled with various 

 points of colours from the fpecular nature of its minute particles. If we look towards the 

 fun with a hat on our head, held down, fo that the fun's direft light may not fall on our 

 eyes, but on the hairs of the hat, and be reflcfted, we fhall fee a variety of lively colours 

 darting in all dire£lions from thofe hairs ; and we may eafily fatisfy ourfelves that they are 

 not the confequence of flexion, by trying the fame thing with unpolifhed threads ; in which 

 cafe they do not appear, provided the threads be not very fmall. In the fame manner we 

 may account for the colours of fpider's webs, of different cloths which change their colours 

 when their pofition is altered, and of fome fofhls which appear of different flrcaks of co- 

 lours when held in the light ; fuch as the (ire marble of Saxony, &c. All thefe bodies hav- 

 ing furfaces of a fibrous flruflure, each fibre reflefls and decompounds the rays. 



4. The confideration of the foregoing phenomena inclined me to think that, upon the 



3 prmciples 



