184 Dr. Herschel's Experiments for investigating 
metalline ground on which these faint rings are seen, the con- 
trast of which will offuscate 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 surface 
upon which many sorts of glasses have been placed ; in order 
therefore to obtain a still greater variety, I 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 metal- 
line 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 ; pro- 
vided 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, 
tyhen it is glass, may also be plain, concave, or convex ; and 
