28 . Mtfcellanea Curiofa, 



larly pretended to explain the Phenomena of 

 the Rainbow - y I was willing to fupply what I 

 thought was wanting in this Do&rine, and 

 from the Proportion of the Refratlion given, Geo- 

 metrically to determine the Angle of its Di- 

 ftance from the Point oypofite to the Sun: Or 

 contrary wife, from the Irisgiven^ to determine 

 the refractive Power of the Liquid. 



What the Celebrated Mr. Newton has done 

 upon this Head, the Reader will find (with 

 much greater Advantage) in his Book of Light 

 and Colours, when he fhall think fit to be^ 

 ilow thofe excellent Difcoveries upon the Pub- 

 lick. 



But to proceed. 'Tis plain from what Cartes 

 has demonft rated, that the Primary Ids is 

 form'd by fuch Rays of the Sun, where the 



Excefs of Two ref ratted Angles, above one 

 Angle of Incidence, is the Create ft of all fitch 

 Excejfes pojfible. And that the Secondary Iris is 

 form'd by thofe Rays only, where the Excefs 

 of Three refracted Angles, above one of Inci- 

 dence, is in like manner the Greateft. And fo 

 we may go on to a 3d, 4th, or any other Wk\ 

 which are form'd, where the Rays emerge af- 

 ter 3? 4? or more Reflexions. But thefe can 

 jiever be feen in the Heavens, becaufe of the 

 $mh Light which is ftill more and more debi- 

 litated by each Reflexion and Refraction : 

 vi 7 hence it comes to pafs alfo, that the Secon- 

 dary Iris, is painted with Colours, fo much 

 fainter than the Primary one. But in all thefe 

 the general Rule ' is, that the Excefs of 4, or 

 or more rcfr ailed Angles, (viz,, the Number of 

 Reflexions being. increafed by Unity) above one 

 Single of Incidence, is of all the Greateft. 



Now 



