the Cause of coloured concentric Rings. 75 



from the window most conveniently when they fell upon the 

 lens in an angle of a^bout 30 degree^ from the perpeadicu^ 

 jar, the eye being placed on the opposite side at an equal 

 angle of elevation to receive the reflected rays. 



Gmeralization. Instead of a plain slip of glass, the plain 

 side of a plano-concave, or piano-convex lens of any focal 

 length whatsoever may be used : and when the convex side 

 ofanylensis 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 rincrs" 

 will always be obtained. I have seen rings with lenses of all 

 varieties of focus, from 1 70 feet down to one quarter of an 

 inch. Even a common watch-glass laid upon the same plain 

 surface will give them. 



To ensure success, it is necessary that the glasses shoulxl 

 be perfectly well cleaned from any adhering dust or soil, 

 especially about the point of contact ; and in laying them 

 upon each other a little pressure should be used, accom- 

 panied 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 1 always use for vicwino; 

 the rings, is rather troublesome, in which case a small spot 

 of ink made upon the lens will serve as an object for a suf-r 

 ficient adjustment to find the rings. 'iviixvi,^ 



Second Method. Instead of the slip of glass, I laid down 

 a well poiished plain metalline mirror; and placing upon it 

 the same i;'6-inch double convex lens, I saw again a conj^ 

 plete 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 littlo 

 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 ofluscate their feeble appearance. 



Generalization. On the same metalline surface every va- 

 riety of lenses may b« laid^ whatever be the figure of their 



uppe 



