the Cause of coloured concentric Rings. 183 
Generalization. Instead of a plain slip of glass, the plain side 
of a plano-concave, or plano-convex lens of any focal length 
whatsoever may be used : and when the convex side of any 
lens is laid upon it, whatever may be the figure of the other 
surface, whether plain, concave, or convex, and whatever may 
be its focal length, a set of concentric rings will always be 
obtained. I have seen rings with lenses of all varieties of 
focus, from 170 feet down to one quarter of an inch. Even a 
common watch glass laid upon the same plain surface will 
give them. 
To insure success, it is necessary that the glasses should be 
perfectly well cleaned from any adhering dust or soil, especi- 
ally about the point of contact ; and in laying them upon each 
other a little pressure should be used, accompanied at first 
with a little side motion, after which they must be left at rest. 
If the surface of the incumbent lens, especially when it is of 
a very short focal length, is free from all imperfection and 
highly polished, the adjustment of the focus of the above 
mentioned eye-glass, which I always use for viewing the rings, 
is rather troublesome, in which case a small spot of ink made 
upon the lens will serve as an object for a sufficient adjustment 
to find the rings. 
Second Method. Instead of the slip of glass, I laid down a 
well polished plain metalline mirror; and placing upon it the 
same 26-inch double convex lens, I saw again a complete 
set of concentric rings. 
It is singular that, in this case, the rings reflected from a 
bright metalline surface will appear fainter than when, the 
same lens is laid on a surface of glass reflecting but little light ; 
this may however be accounted for by the brilliancy pf the 
B b 2 
