Thus, if X, fig. 10, represents a section of the hair, and AB, CD, EF, GH, &c. rays passing at different distances from X, the ray AB will be more deflected than CD, and will cross it at _m_, the ray CD will for the same reason cross EF at _n_, and EF will cross GH at _o_. Hence the curve or caustic formed by the intersections _m_, _n_, _o_, &c. will be convex outward, its curvature diminishing as it recedes from the vertex. As none of the passing light can possibly enter within this curve, it will form the boundary of the shadow of X. The explanation given by Sir Isaac of the coloured fringes is less precise, and can be inferred only from the two following queries. 1. “Do not the rays which differ in refrangibility differ also in flexibility, and are they not, by these different inflections separated from one another, so as after separation to make the colours in the three fringes above described? And after what manner are they inflected to make those fringes? 2. “Are not the rays of light in passing by the edges and sides of bodies _bent several times backwards and forwards_ with a motion like that of an eel? And do not the three fringes of light above mentioned arise from three such bendings?” The idea thus indistinctly thrown out in the preceding queries has been ingeniously interpreted by Mr. Herschel in the manner represented in fig. 11, where SS are two rays passing by the edge of the body MN. These rays are supposed to undergo several bendings, as at _a_, _b_, _c_, and the particles of light are thrown off at one or other of the points of contrary flexure, according to the state of their fits or other circumstances. Those that are thrown outwards in the direction _a_A, _b_B, _c_C, _d_D, will produce as many caustics by their intersections as there are deflected rays; and each caustic, when received on a screen at a distance, will depict on it the brightest part or maximum of a fringe. [Illustration: _Fig. 11._] In this unsatisfactory state was the subject of the inflection of light left by Sir Isaac. His inquiries were interrupted, and never again renewed; and though he himself found that the phenomena were the same, “whether the hair was encompassed with air or with any other pellucid substance,” yet this important result does not seem to have shaken his conviction, that the phenomena had their origin in the action of bodies upon light.