the Cause of coloured concentric Rmgs. 191 
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 
17-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 arrange- 
ment 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 of 
rings ; one 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-convex or 
plano-concave lens, and the same variety which has been ex- 
plained in the third method, by using any incumbent lens that 
will make a central contact, either with the convexity or con- 
cavity of the subjacent glass, will always produce two sets of 
rings. 
Fourth Method. A more refined but rather more difficult 
way of seeing two sets of rings, is to lay a plain slip of glass 
on a piece of black paper, and when a convex lens is placed 
upon the slip, there may be perceived, but not without 
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