265 



the upper surface was plain and polished, but the under surface either 

 plain, concave, or convex. These were placed before a window, in 

 such a position that the light fell upon the lens at about 30 from 

 the perpendicular, and was received by the eye, at an equal eleva- 

 tion, on the opposite side. 



Instead of the surface of glass, a metallic surface was next substi- 

 tuted ; and the same lens placed upon it, gave the same appearance 

 of similar concentric rings. 



Moreover, it is not necessary that the surface of the under sub- 

 stance should be plain. It may be either concave or convex ; so also 

 may the upper surface, in contact, be either convex or concave, pro- 

 vided that when a concave surface is applied to another that is con- 

 vex, the radius of concavity be greater than that of the convexity to 

 which it is applied. 



The second section treats of seeing the same rings by transmission, 

 which, of course, admits the same variety in the forms of the surfaces 

 in contact, but will not allow either of them to be metallic. 



The third section distinguishes the several images, of any object, 

 that are reflected from the different surfaces of several plates of glass, 

 laid one upon another, on account of the use that may be made of 

 these images, in assisting to discern the complicated phenomena pro- 

 duced in succeeding sections. 



In the fourth section a second series of rings is produced, by placing 

 the lens upon a piece of looking-glass, which occasions the primary 

 set to be seen a second time by reflection. But as this is less bright, 

 the primary set must first be obscured, by bringing the second re- 

 flected image of a pen-knife, or other pointed body, over it. In this 

 case there are three images of the pen-knife. The second obscures 

 the primary set of rings ; the third shows them to the greatest ad- 

 vantage. 



The same varieties of contact which were found to make one set 

 of rings, may, of course, be applied to make a secondary set, if there 

 be a reflection beneath sufficiently bright to render it visible. 



The fifth section treats of three sets of rings, produced by increas- 

 ing the number of reflecting surfaces, as when a slip of glass is in- 

 terposed between the lens and the looking-glass of a former experi- 

 ment, or when the lens, laid upon two slips of glass, is placed on a 

 plain metallic reflector. 



The sixth section pursues the same complicated appearance, as far 

 as four sets of rings, and shows how they may be discerned, by 

 means of the reflected images of the pen-knife. 



In the seventh section the size of rings is considered, so far as it 

 depends on the curvature of the surfaces; but Sir Isaac Newton 

 having already treated this part of the subject at large, Dr. Herschel 

 does not think it necessary to enter further into it. 



In the eighth section, the species of contact requisite for exhibiting 

 the rings is mentioned, the size of them being considerably affected 

 by pressure. They grow larger when the two surfaces that form 

 them are pressed closer together, and diminish in proportion as the 



