John Knox

“ If either of the surfaces in contact be slightly curved, so as to be a portion of a large sphere, then rings of colours observed by Newton are seen, and are found to arise from reflection at the surfaces of contact, as is proved by their being intercepted by the middle shadow of the wire. ”

“ In the same manner as one set of rings is produced by the intersection of primary and secondary reflected circles with each other, they may also be produced by the intersection of other sets, either of transmitted or reflected rings, and may be rendered numerous by a number of surfaces of various forms; or, on the contrary, may be exhibited in their most simple state by the intersection of primary fringes with each other. For if two slips of plane glass be applied to each other at a small angle, the fringes of colour then appear as straight bars of prismatic colours parallel to each other. ”

“ When a piece of glass is laid upon a convex surface, the secondary set is equal to the primary, and in that case the fringes are straight lines; but when by due combination of surfaces the primary and secondary sets differ in size from each other, then these fringes assume a circular form, coloured according to the same law as the straight fringes, being divided into two classes by a middle curve, towards which the violet edges of the curves on each side are turned. ”