j(j() Hir.jiscni;i. oft vohcmiv.n king*. 



be reflected from the defective i>i;ice of the lowest surface. 



This being obtained, tlie rings of tins set were again as well 



formed, and as free from distortions* as those of the primary 



set. 



■Refraction of Upon a plain metalline mirror I laid down two lenses, one 



ihe4thsur%» a planoconvex, the other a pluno-coneave, both of 2*9 inches 



has little or no . . 



rtiect. 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 ati effect in altering the size of the rings that it 



cannot be perceived. 



The result oi^ the foregoing- experiments, relating to the 



action of the several surfaces, is, 



General re- i. That only two of them are essential to the formation of 



sulls * concentric -rings. 



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

 tion, and so as to form a central contaet. 



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

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

 surfaces about the same point to the other to be reflected 



. from it. 



I V. And that in all these cases a set of rings will be formed, 

 having their common centre in the place where the two sur- 

 faces toueh each other. 



XXIX. Considerations that relate to the Cause of the for- 

 mation of concentric Rings 



Inquiry con- 

 cerning the 

 cause of the 



It is perfectly evident, that the phenomena of concentric 

 rings must have an adequate cause, either in the very nature 

 or motion of the rays of light, or in the modifications that 

 are given to them by the two essential surfaces that act upon 

 them at the time of the formation of the rings. 



This seems to reduce the cause we are looking for to an 

 alternative, that may be determined; for if it can be shown, 

 that a disposition of the rays of light to be alternately re- 

 flected and transmitted eannot account for the phenomena, 

 which this hypothesis is to explain, a proposition of account- 

 ing for them by modifications that may be proved, even on 



tbe 



