ITERSCIIEL OX COLOURED RINGS. 129 



Gvops. This will increase the distance between the centres of , 



the primary and secondary sets, and at the same time occa- 

 sion a more copious reflection of light. 



Instead of a common looking-glass a convex glass mirror With glasses 

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

 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 set Generalization. 

 of rings was proposed to be obtained may be easily applied 

 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- 2d. Lens on 

 rallel slip of glass, and placed upon it the convex surface fg l ass and metal. 

 a 17 -inch plano-convex lens, by which means two sets of 

 rings were produced. 



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

 glass maybe 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 ar- 

 rangement two sets of rings will be obtained. 



Third Method. Upon a small well polished slip of glass 3d. Lens'on 

 place another slip of the same size, and upon them lay a 39- fc ^° s,1 t )S '^ 

 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 dip of glass we mav place 

 upon the lowest slip the plain side of a plano-convex or 

 piano-conrave lens, and the same variety which has been ex- 

 plained in the third method, by using any incumbent lens that 

 Vol. XIX. Feb. 1S08. K will 



