a window, and a piece of wire be held over it, there will be seen two 

 shadows of the wire, one from each of the surfaces ; if a second plate 

 be laid upon the former, there will be three shadows, one from the 

 uppermost surface, one from the lowest, and a third between them 

 from the contiguous surfaces which together form but one image. 

 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 sha- 

 dow of the wire. At the same time may also be seen other rings, 

 occasioned by light transmitted through the surfaces in contact and 

 reflected back to the eye from the undermost surface ; but as these 

 rings are not the subject of Mr. Knox's experiments, he painted the 

 under surface of the lower plate black, in order to prevent their in- 

 terference with his observations. Under these circumstances, a se- 

 cond image of the rings, formed by reflected light, is seen by means 

 of two reflections from the parallel surfaces of the upper plate of 

 glass ; and in a favourable light further repetitions of the same rings 

 may be seen by several successive reflections. Under these circum- 

 stances, in addition to these appearances, Mr. Knox observed certain 

 fringes to proceed on each side from the points of intersection of a 

 set of primary circles, with their reflected images appearing as parallel 

 lines at right angles to the line joining their centres, and divided into 

 two sets, coloured in opposite directions from a central line. 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 cir- 

 cular 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. To these curves, 

 which have not been observed before, Mr. Knox gives the name of 

 intersectionary rings. 



In the same manner as one set of rings is produced by the inter- 

 section 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. And if 

 a third plate of glass be placed upon the uppermost, with a slight 

 inclination situated transversely to the former, the bands thus pro- 

 duced are at right angles to the preceding ; and by their intersections 

 present a set of intersectionary fringes parallel to each other, and 

 bisecting the angle between the primaries from which they originate. 

 These fringes, as in the former cases, are divided by a central band 

 into two sets, of which the colours are oppositely placed, and on this 



