76 Experiments for investigating ^''■ 



upjjtr surface, whether plain, concave, or convex, and 

 whatever be their focal lengths, provided the lowest surface 

 renoains convex, and concentric rings will always be ob- 

 tained ; but for the reason mentioned in the preceding para- 

 graph, very small lenses should not be used till the experi- 

 mentalist has been familiarized with the method of seeing 

 these rings, after which lenses of two inches focus, and 

 gradually less, may be tried. 



Third Method, Hitherto we have only used a plain sur- 

 face upon which many sorts of glasses have been placed ; in 

 order therefore to obtain a still greater variety, 1 laid down 

 a plano-convex lens of 15 inches focal length, and upon the 

 convex surface of it I placed the 26-inch double convex 

 lens, v/hich produced a complete set of rings. 



Fourth Method. The same lens placed upon a convex 

 metalline mirror of about 15 inches focal length gave also a 

 complete set of rings. j 



Generalization. These two cases admit of a much greater 

 variety than the first and second methods ; for here the in- 

 cumbent glass may have not only one, but both its surfaces 

 of any figure whatsoever ; whether plain, concave, or con- 

 vex ; provided the radius of concavity, when concave lenses 

 are laid upon the convex surface of glass or metal, is greater 

 than that of the convexity on which they are laid. 



The figure of the lowest surface of the subjacent substance, 

 when it is glass, may also be plain, concave, or convex ; and 

 the curvature of its upper surface, as well as of the mirror, 

 may be such as to give ttiem any focal length, provided the 

 radius of their convexities is less than that of the concavity 

 of an incumbent lens; in all which cases complete sets of 

 concentric rings will be obtained. 



Fifth Method. Into the concavity of a double concave 

 glass of 8 inches focal length I placed a 7-inch double con- 

 vex lens, and saw a very beautiful set of rings. 



Sixth Method. Upon a 7-feet concave metalline mirror I 

 placed the double convex 26-inch lens, and had a very fine 

 set of rings. 



Generalization, With these two last methods, whatever 

 may be the radius of the concavity of the subjacent surface, 



provided 



