Mifcellanea Curiofa. 35^ 



any Colour may be calFd Faults • the one, an 

 over-careful acknowledgment of every** Trifle 

 the Author had receiv'd from others ; and 

 the other that he labours to make eafie this 

 curious Subject, fo little underftood by moll, 

 in a manner perhaps too familiar for the Learned 

 Critickjt and which demonftrates that it was writ 

 cum ammo docendi^ both which require but very 

 little Frienfliip or good Nature in the Reader, to 

 pafs for Vertues in an Author. 



But to return to our firft Theorem, which 

 accounting for the thicknefs of the Lens, we will 

 here again refume, viz* 



m p d r g — ndft-l-nprft 



m d r ■+ m d $—m p r %—m-+-n d t-^nr t 



And let it be required to find the Focus 

 where a whole Sphere will collect the Beams 

 proceeding from an ObjecT: at the diftance d : 

 Here t is equal to % r, and r equal to?. And after 

 due Reduction, the Theorem will ftand thus, 



-~ — : — = /j but if d be 



2 n a ~r Zn r — m p r 



Infinite, it is contracted to m $ r . — r ~ 



z n 



r t== f, wherefore a Sphere of Glafs 



Z m — zn 



collects the*Sun-Beams at half the Semi-diamete 1 " 

 of the Sphere without it, and a Sphere of Wa- 

 ter at a whole Semi-diameter. But if the 

 Hfltio of Refraction m to n be as z to i, the 

 Focus falls on the oppofite Surface of the Sphere j 

 but if it be of greater Inequality it falls within. 



A a % Another 



