204 T xperimcnt$ for investigating 



found that the breadth of a rin^ of a mean size was about 

 the 306th part of an inch. 



Now, according to Sir I. Newton's calculation of the ac- 

 tion of the fits of easy reflection and easy transmission in 

 thick glass plates, an alternation from a reflecting to a trans- 

 mitting fit requires a difference of yj-^Vrr part of an inch in 

 thickness*; and by calculation this diflfercnce took place in 

 the glass plate that was used at every 80th part of an inch 

 of its whole length : the 12 rings, as well as the central 

 colour of the secondary set, should consequently have been 

 broken by the exertion of the fits at every 80th part of an 

 inch ; and from the space over which these rings extended, 

 which was about '13 inch, we find that there must have 

 been more than ten such interruptions or breaks in a set of 

 which the 308th part was plainly to be distinguished. But 

 when I drew the glass plate gently over the small mirror, 

 keeping the secondary set of rings in view, I found their 

 shape and colour always completely well formed. 



This experiment was also repeated with a small plain glass 

 instead of the metalline mirror put under the large plate. In 

 this manner it still gave the same result, with no other dif» 

 ference but that only six rings could be distinctly seen in 

 the secondary set, on account of the inferior reflection of 

 the subjacent glass. 



XXXIII. Coloured Rings may he completely formed without 

 the Ass'istance of any thin or thick Plates, either of Glass 

 or of Air, 



The experiment I am now to relate was at first intended 

 to be reserved for the second part of this paper, because it 

 properly belongs to the subject of the flection of the rays o£ 

 light, which is not at present under consideration ; but as it 

 particularly opposes the admission of alternate fits of easy 

 reflection and easy transmission of these rays in their pas- 

 sage through plates of air or glass, by proving that their 

 assistance in the formation of rings is not rcijuired, and also 

 throws light upon a subject that has at diflferent times been 



* Newton's Optics, p. 277. 



considered 



