III.RSCHEI. ON COLOURED RINGS. JQ\ 



the very principles of Sir. I. Newton, to have an existence, 

 will find a ready admittance. I propose, therefore, now to 

 give some arguments, which will remove an obstacle to the 

 investigation of the real cause of the formation of the con- 

 centric rings; for after the very plausible supposition of the 

 alternate lits, which agrees so wonderfully well with a num- 

 ber of facts that have been related, it will hardly be at- 

 tempted, 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 modifi- 

 cations arising from the action of those surfaces, which have 

 been proved to be the only essential ones in the formation of 

 rings, 



XXX. Concentric Rings cannot be formed by an alternate 

 Reflection and Transmission of the Rays of Light. 



One of the most simple methods of obtaining a set of con- They cannot 

 centric rings is, to lay a convex lens on a plain metalline , e ormed b >' 

 mirror; but in this ease we can have no transmission of rays, flection and 

 and therefore we cannot have, an alternate reflection and transmission of 

 transmission of them. If to get over this objection it should ',' 

 be said, that, instead of transmission, we ought to substitute- 

 absorption ; since those rays, which in glass would have been 

 transmitted, will be absorbed by the metal, we may admit 

 the elusion: it ought however to have been made a part of 

 the hypothesis 



XXXI, Alternate Fits of easy Reflection and easy Trans* 

 ?nissiofi 9 if they exist, do not exert themselves according to 



various Thicknesses of thin Plates of Air. 



■ 



In the following experiment, I placed a plain well polished If fits of easy 

 piece of glass 5'ti inches long, and 2'3 thick, upon a plain reflection and 



_i li- • m 1 , , • , , , »• transmission 



metalline mirror of the same length .with the glass; and in exist, they do 



order to keep the minor and glass at a distance from each not exert 



• " themselves ac- 



otlier, I laid between them, at one end, a narrow strip of cording to va- 



sueh paper as we commonly put between prints. The thick- rious tl »cktus- 



c i • I r* l n ' ses of thin 



ness or that which J used was the 040th p&rt ot an inch ; plates of air. 



for 128 folds of it laid together would hardly make up 



two tenths. Upon the glass I put a 39-inch double convex 



lens ; 



