VOL. LXXXVI.] PHILOSOPHICAL TRANSACTIOiVS. 749 



unless the curvature be considerable; and then kc and ic are both repelled, and 

 an image formed in the caustic by reflection. In Obs. 3, we found, that certain 

 irregularities in the surface of the reflector caused the images to be in the in- 

 verted order of colours. How does this happen ? In fig. 13, let gf, fe, er, ri, 

 and ih, represent the sections of the convex fibres on the surface of the reflec- 

 tor, and let the ray ab be reflected from ef, separated into Br red, and Bf violet ; 

 then if AB was so inclined to ef, that Br and ^v fell upon er, the side of the fibre 

 next to ef, and a little larger than ef, it is evident that -rv will be reflected into 

 vv, and Br into rR, and an image vr will be formed, having the violet outermost 

 and the red innermost, the intermediate colours being in their order, from v to 

 R. Lastly, it is evident that the greater the angle of incidence is, the longer 

 will be the image, and the farther separated its colours ; for which reason the 

 farther the images are from the shadow, the less dilated and coloured will they 

 be. Nor will they have the same appearance at all distances from the point of 

 incidence ; very near it, they will be all in the form of fringes across the streak, 

 the breadth being greater than the length, if I may use the expression, but as 

 we recede from it, they will become distended, as before described, the length 

 increasing faster than the breadth, and at one point or distance they will be just 

 as long as broad ; all which agrees with experiment ; and it is needless to show 

 by particular demonstration, the manner in which one image is divided from ano- 

 ther, the reason obviously being the manner in which the fibres on the reflect- 

 ing surface are arranged and inclined to each other. 



3. A number of phenomena, involved in that of the images, are explicable 

 by what has been said on them. If a piece of metal be scratched, and then 

 exposed in the sunshine, a number of broken colours will be formed by the 

 scratches, as may be seen either by letting them fall on the eye, or by receiving 

 them on a white object. This is evidently owing to the different reflexibility of 

 the rays incident on the scratches, which are so many irregular specula, of great 

 curvature ; the images are therefore distorted and broken, just as a candle, &c. 

 appears broken and coloured when viewed through a piece of irregular crystal, 

 such as the bottom of a wine- glass. If we look attentively at any object exposed 

 in the light of the sun, provided it be not polished, we shall see its surface 

 mottled with various points of colours, from the specular nature of its minute 

 particles. If we look towards the sun, with a hat on our head, held down, so 

 that the sun's direct light may not fall on our eyes, but on the hairs of the hat, 

 and be reflected, we shall see a variety of lively colours darting in all directions 

 from those hairs ; and we may easily satisfy ourselves that tliey are not the con- 

 sequence of flexion, by trying the same thing with unpolished threads, in which 

 case ihey do not appear, provided the threads be not very small. In the same 

 manner we may account for the colours of spider webs, of different cloths 

 which change their colours when their position is altered, and of some fossils 

 which appear of different streaks of colours when held in the light, such as the 



