124 



HF.RSCHEL ON COLOURED RINGS. 



Generaliza- 

 tion. 



3d method. 

 Double convex 

 lens on a pia- 

 no convex. 



4th. The samp 

 on convex me 

 tal. 



Generaliza- 

 tion. 



5th. Double 

 convex lens in 

 a double con- 

 cave glass. 



same lens is laid on a surface of glass reflecting- but littt* 

 light, this may however be accounted for by the brilliancy 

 of the metalline ground, on which these faint rings are seen, 

 the contrast of which will orFuscate their feeble appearance. 



Generalization. On the same metalline surface every va- 

 riety of lenses may be laid, whatever be the figure of their 

 upper surface, whether plain, concave, or convex, and what- 

 ever be their focal lengths, provided the lowest surface re- 

 mains convex, and concentric rings will always be obtained ; 

 but for the reason mentioned in the preceding paragraph, 

 very small lenses should not be used, till the experimentalist 

 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, I have 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, which 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. 



Generalization. These two cases admit of a much greater 

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

 bent glass may have not only one, but both its surfaces of 

 any figure whatsoever; whether plain, concave, or convex ; 

 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 them 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 con- 

 centric 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 



