(UPD ETpetiments for investigating ' ^ 



a 56- inch lens was laid upon it, the rings of the set it pro- 

 duced were much distorted. The lens was then put upon a 

 perfect slip of glass, and both together were laid upon the 

 defective place of the coach-glass. The rings of the secon- 

 dary set reflected by it were nevertheless as perfect as those 

 of the primary set. It occurred to me that these rings might 

 possibly be reflected from the lowest surface of the perfect 

 slip of glass, especially as by lifting it up from the coach- 

 glass I still continued to see both sets. To clear up this 

 point, therefore, I took away the slip, and turning the de- 

 fective place of the coach -glass downwards, produced a set 

 of perfect rings between the lens and the upper surface of 

 the coach-glass, and brought it into such a situation that a 

 secondary set must be reflected from the defective place of 

 the lowest surface. This being obtained, the rings of this 

 set were again as well formed and as free from distortions as 

 those of the primary set. 



Upon a plain metalline mirror I laid down two lenses, 

 'one a plano-convex, the other a plano-concave, both of 

 2*9 inches focus, and having the plain side upwards. When 

 two 21 -inch double convex glasses were laid upon them, 

 the secondary sets of both the combinations were of equal 

 size, and perfectly like their primary sets ; which proves 

 that the refraction of the fourth surface is either not at all 

 concerned, or at least has so little an effect in altering the 

 size of the rings that it cannot be perceived. 



The result of the foregoing experiments, relating to the 

 action of the several surfaces, is, 



I. That only two of them are essential to the formation 

 of concentric rings. 



II. That these two must be of a certain regular construc- 

 tion, and so as to form a centra! contact. 



III. That the rays from one side or the other, must either 

 pass through the point of contact, or through one of the sur- 

 faces about the same point to the other to be reflected from it. 



IV. And that in all these cases a set of rings will be 

 formed, having their common centre in the pl^ce where the 

 two surfaces touch each other. 



XXIX. Coa- 



