ike Cause of coloured concentric Rings. 201 



Considerations t/tat relate to the Cause of the Forma- 

 tion of concentric Rings. 



It is perfectly evident that the phseiiomena oF concentric 

 rings must have an adequate cause, either in the very 

 nature or motion of the rays of light, or in the modifica- 

 tions that are given to them by the two essential surfaces 

 that act upon ihem 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 cannot account for the phaenomena 

 ■which this hypothesis is to explain, a proposition of ac- 

 counting for them bv modifications that may be proved, 

 even on the very principles of Sir I. Newton to have an ex- 

 istence, will find a ready admittance. I propose, therefore, 

 now to give some arguments, which will remove an obsta- 

 cle to the investigation of the real cause of the formation of 

 the concentric rings ; for after the very plausible supposi- 

 tion of the alternate fits, which airrees so wondcrfullv "weli 

 with a nuniber of facts that have been related, it will hardly 

 be attempted, if these should be set aside, to ascribe some 

 other inherent property to the rays of light, whereby we 

 might account for them ; and thus we shall be at liberty to 

 turn our thoughts to a cause that may be found in the mo- 

 difications arising from the action of the surfaces which 

 have been proved to be the only essential ones in the forma- 

 tion of rings. 



XXX. Concentric Rings cannot he formed Ij/ an alternate 

 Reflection and Transmission of i lie Rays of Light. 



One of the most simple methods of obtaining a set of 

 concentric rings is to lay a convex lens on a plain metalline 

 mirror ; but in this case we can have no transmission of 

 rays, and therefore we cannot have an alternate reflectioo 

 and transmission of them. If to get over this objection it 

 should be said that, instead of transmission, we ought to 

 substitute absorption 5 since those rays which ifi glass woulii 



have 



