HERSCHEL ON COLOURED RINGS. \<*§ 



convex lens of 20 inches focal length, and found that this 

 arrangement gave me a set of beautiful concentric rings. 



I viewed them with a double convex eye lens of 2f inches 

 focus mounted upon an adjustable stand, by which simple ap-. 

 paratus I could examine them with great ease; and as it was 

 not material to my present purpose by what obliquity of in-» 

 eidence of light I saw the rings, I received the rays from the 

 window most conveniently when they fell upon the lens in an 

 angle of about 30 degrees from the perpendicular, the eye 

 being; placed on the opposite side at an equal angle of eleva* 

 vation to receive the reflected rays. 



Generalization. Instead of a plain slip of glass, the plain Generalize 

 side of a plano-concave, or plano-convex len6 of any focal ti0n » 

 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 1?0 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 Necessary p t p, 

 be perfectly well cleaned from any adhering dust or soil, cautl0U ^ 

 especially about the point of contact; and in laying them 

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

 ied 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 sufhv 



cient adjustment to find the rings. 



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



v & 2d method. 



a well polished plain metalline mirror; and placingupon it Double con- 



the same 26-inch double convex lens, I saw again a complete vex lens nn * 



,. ° L metallic miir 



set ot concentric rings. ror< 



It is singular that, in this case, the rings reflected from a 



bright metalline surface will appear fainter than when the 



I same 





