VOL. LXXXVI.] PHILOSOPHICAL TRANSACTIONS. "43 



greater than 1 or 2 feet, its flame appears surroLinded by 2 bows of faint oolonrs, 

 the innermost of them terminating in a white which continues to the flame; 

 and the colours are red outermost, and green and blue innermost: the appear- 

 ance is most remarkable if we look at a small hole in the window-shut, the room 

 being otherwise dark; and if the eye be pressed on, and then opened, the co- 

 lours are more lively than before, as Des Cartes observed; from which both he 

 and Newton concluded, that the appearance was owing entirely to wrinkles 

 formed on the surface of the eye by the pressure. But this could neither form 

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

 be in the order above-mentioned from the different refrangibility of the rays; it 

 will also be obvious to any one who tries the thing, that the pressure only in- 

 creases the brishtness and breadth of the bows, but does not form them. The 

 true solution of the difficulty seems to be this: the rays which enter the pupil, 

 are inflected in their passage through the fibres, which extend over the cornea, 

 and which are very minute, but opaque; by these they are decompounded into 

 fringes, having the red outermost, and the violet innermost ; and the fringes 

 formed by each fibre being joined together, form the bow. How then does the 

 pressure enlarge and vivify them ? The fibres are naturally extended over the sur- 

 face of a spherical segment; when this surface is compressed into a plane circle, 

 they are condensed into a much less space, and consequently brought nearer to 

 one another; the rays are therefore more inflected and separated than before. If 

 this explanation be true, it will follow, that the like bows may be produced by 

 small hairs, like fibres, placed near one another: and this I found perfectly con- 

 sistent with fact; the bows are in this case brighter than the other; and the 

 small hairs on a hat, or the hand, made them brighter than any other I have 

 tried: a circumstance which I observed in both cases seems to show clearly the 

 identity of the causes; the white space, which reached from the interior bow to 

 the flame, was speckled or mottled, in a manner which cannot be easily described, 

 but which any one will perceive on trying the experiment. 



8. The last of these phenomena, which I shall mention, is the celebrated one 

 observed by Sir Isaac Newton, namely, the rings of colours with which the 

 focus of a coiicave glass mirror is surrounded. Sir Isaac made several most in- 

 genious and accurate experiments to investigate their nature; and finding their 

 breadth to be in the inverse subduplicate ratio of the mirror's thickness, he con- 

 cluded that they were of the same nature and original with those of thin plates, 

 described by him. The Due de Chaulnes pursued these experiments with con- 

 siderable success ; he found that the rings were brighter the nearer to the per- 

 pendicular ihe rays were incident; and that if, instead of a concave glass mirror, 

 a metal one was used, with a small piece of fine cambric, or reticulated silver 

 wire stretched before it, the colours were no longer disposed in rings, but in 

 streaks, of the same shape with the intervals between the threads: hence he 

 concludes that they are owing to inflection; that in passing through the first 



