Mifcellanea Curiofa. 39 



Greater Cube o, 93472007, whole fide o, 9777486 



t o, 2278063 



T=Tang. Incid. 51 0 . 32'. i, 2586322 

 i T=Tang. Refr. 52 0 . 11'. 0,6293161 



LafHy, AsVtT+ 4 : VtT+i :: r : s : : 

 1 : 168026. Which Proportion comes very 

 near to that, which Experience ihews to be 

 In Glafs and moft pellucid Solids. The Dia- 

 mond indeed, exceeds all tranfparent Bodies, 

 not only in refpeft of its hardnefs and value, 

 but alfo its Refractive Power, the Propor- 

 tion here being as 5 : 2, nearly, or more ac* 

 curately as 100:41. But of this, perhaps 

 more in another place. 



While I was writing thefe things, that 

 skillful Geometrician Mr. De Moivre, at my 

 requeft, found a like Equation for deter- 

 mining the Ratio, from the Semidiameter of 

 the Secondary Iris^ given. By which, the Ratio 

 is indeed fomething more exactly determined, 

 but that Equation being a Biquadratical one, 

 the Calculation is not fo eafily performed. 

 This Equation is T 4 + % T 3 t — 2 T* r 2 — f 

 r 4 =o ; where Tis the Tangent of the Re- 

 fracted Angle, the Tangent of J the di- 

 ftance of the Iris from the Point oppofite to 

 the Sun, to the Radius r— 1. And this Equa- 

 tion is of that Form , as to be always expli- 

 cable, by an Affirmative and one Negative 

 Root, the one and the lefs of which, is the 

 Tangent of the Refra&ed Angle, in the Re- 

 grefs to the Sun, yit. when the Pur fie Colours 

 are nearer to the Sun. The greater Root is 

 the Tangent of the Refra&ed Angle in an 

 D 4 Iris 



