748 PHILOSOPHICAL TRANSACTIONS. [anNO I/QS. 



or at lea-t by the moisture adlicring to tlie under ciliary process, and those which 

 appear from the bottom of the body, by the upper eye-lid ; which could not be, 

 either if they were formed by reflection from the processes, or by inflection 

 through the lashes. I have however observed another kind of streaks, mottled 

 with broken colours of all kinds, and formed by reflection from the moisture on 

 the processes : in these the under streak corresponds to the under process, and 

 vice versa : they may be formed by any polished body held in the proper position 

 between the pupil and luminous body. The colours are very beautiful 

 when made by the sun, and resemble, in form and irregularity of arrangement, 

 some of the streaks made by large half-polished bodies, as described in part 2 of 

 this paper. '2. The next object of attention is one of the greatest importance 

 to our theory, namely, the formation of images by reflection : 3 things here 

 require explanation, the number of the images, their colours, and their variations 

 in point of size. 



Olis. 15. I have uniformly found that no reflecting surface forms them, except 

 it be curve, and its surface of a structure somewhat fibrous. A plain mirror, 

 nor a concave, nor a convex one do not make them, unless they are of that 

 structure ; and, for the same reason, quicksilver, when held so as to reflect the 

 light incident on it, forms them not, but by triturating it, so as to divide it into 

 small particles, and by placing these in the beam of the sun's light, each particle 

 formed an image, with the colours in the regular order and very bright : on hold- 

 ing a cylinder in the rays, and observing the lengths of the images, I found 

 that if the curvature was increased, the images were also increased in size, being 

 more distended, and highly coloured. These things immediately suggest the 

 explanation. Each of the small fibres forms an image, which, from the different 

 reflexibility of the rays, is divided into the 7 primary colours. But why does not 

 a plain mirror form one of these on the same principles ? In fig. 1 2, let ae be 

 the curve surface of a very convex mirror, that is of a small fibre ; gc a ray 

 reflected by the small surface dc ; it will be separatetl into ci red, and ck violet, 

 by the unequal action of fc on its parts. But if dc be continued to l in a 

 straight line, then lc's sphere of reflection extending a little way beyond it, to 

 KC, the part nearest to c, and not to ic, will drive kc, and also the indigo and 

 part of the blue, nearer to the perpendicular: then ic being within lc's sphere 

 of inflection, will, together with the orange, yellow, and part of the green, be 

 brought nearer to kc ; so that ic and kc will both be brought to an angle equal 

 to that of incidence, and will be reflected in a parallel white beam. If lc be 

 removed a little, or the surface become more convex, ic is attracted, and kg 

 repelled, but not so much as to reduce them to parallelism and whiteness, an 

 image being formed narrower and less coloured tlian when lc is moved so far 

 round that kc is attracted, and ic deflected or repelled. If lc be moved round 

 so that the mirror be concave, then kc is repelled, and ic attracted^ as before. 



