the Cause of coloured concentric Rings, SI 



may be used, on which may be placed either a plain, a con- 

 cave, or a convex surface of any lens or glass, and two sets 

 of rings will be obtained. 



In the same manner, by laying upon a concave glass mirror 

 a convex lens, we shall also have two sets of rings. 



The generalizations that have been mentioned when one 

 get of rings was proposed to be obtained, may be easily ap<^ 

 plied with proper regulations, according to the circumstances 

 of the case, not only to the method by glass mirrors already 

 mentioned, but likewise to all those that follow hereafter, 

 and need not be particularized for the future. In the choice 

 of the surfaces to be joined, we. have only to select such as 

 will form a central contact, the focal length of the lenses 

 and the figure of the upper surface being variable at pleasure. 



Second Method. On a plain metalline mirror I laid a pa- 

 rallel slip of glass, and placed upon it the convex surface of 

 a 1 7-inch plano-convex lens, by which means two sets of 

 rings were produced. 



Upon the same mirror the plain side of the plano-convex 

 glass may be laid instead of the plain slip, and any plain, 

 convex, or concave surface being placed upon the convexity 

 of the subjacent lens, will give two sets of rings. 



The plain side of a plano-concave glass may also be placed 

 upon the same mirror, and into the concavity may be laid 

 any lens that will make a central contact with it, by which 

 arrangement two sets of rings will be obtained. 



Third Method. Upon a small well polished slip of glass 

 place another slip of the same size, and upon them, lay a 

 39-inch double convex lens. This will produce two sets bf 

 rings ; bne of them reflected from the upper surface of the' 

 first slip of glass, and the other from that of the second. 



Instead of the uppermost plain slip of glass we may place' 

 upon the lowest slip the plain side of a plano-oonvex 

 or plano-concave lens, and the same variety which has 

 been explained in the third method, by using any incum- 

 bent lens that will make a central contact, either with the 

 convexity or concavity of the subjacent glass, will always 

 produce two sets of rings. 



Vol. 30, No. U7. Feb. 1808. F Fourth 



