MifceUanea Curiofa. 55 



&ions, in which thefe Species are feparated 

 from one another, and the Blue or Purple 

 Light, (even in the fame Diaphanous Body) 

 is more Refradted than the Ye How or Red. But 

 let the Reader confult this incomparable Per- 

 fon's Letters (N°. 80. and the following of the 

 Philofophical l*r an factions) from which Speci- 

 men he will be able to judge, how nobly this 

 Argument of Light will be managed by him. 



To my purpofe 'tis fufficient, that all kinds 

 of Blue Light, are fomething more refracted 

 than Red , from which difference ariles the 

 Latitude of the hides, which is hardly to be 

 determined by Obfervation, becaufe of the 

 uncertain Limits of the Colours. But by how 

 much the Proportion between CA and CD, is 

 of greater Inequality, or by how much the Re- 

 fra&ion is greater, fo much the greater is the 

 diftance of any Iris from the Sun, and confe- 

 quently thofe borders that are remoter from 

 the Sun, fhine with a Purple Colour, but thofe 

 that are nearer, with an intenfe Red. 



This may always be feen in the Primary 

 Iris, which vanilhes in the part oppofite to 

 the Sun, if the Sine of Incidence be to the 

 Sine of the Refraded Angle, as CA to CE* 

 or as 2 to 1. But if that Ratio be greater, 

 there can be no Primary Iris feen at all. 



As for the Secondary Iris, 'tis to be noted, 

 that this vanilhes into a Point, in the part 

 oppofite to the Sun, when the Ratio of the 



Refradion is as 1 to V— 4- V- , or as t 



3 1 2 7 



to 0,847487... and from thence it returns 

 back to the Sum it felf, where it vanifh^s, if 

 the faid Ratio be as 3 to 1, or as CA to Cc. 



D % But 



