Mifcellanea Curioja. 351 



lowing Equation will arife, 



m pd r <j*—>nd ft H- np r ? t 



mdr-t-mdf — mprf — m — ndt -+ nrt ~f* 

 Which Theorem, however it may teem opero/e 

 is not fo, confidering the great Number of Data 

 that enter the Queftion ; and that one half 0 f 

 the Terms arife from our taking in the thicknefs 

 of the Lens, which in moft Cafes can produce 

 no great ErTe& ; however it was neceffary to con- 

 fiderit, to make our Rule perfect. If therefore the 

 Lens confift ofGUfi, whofe Refraction is as 3 to a 

 'twill be 6 '^:z2ill±4?^_ - f jc 



or 0W,who£ Refra&ion is as 4 to 3, theTheorcm 

 will ftand thus - l%d ^SZjJjL±9^Sl_ 

 * if. 4^-1-4^-- ixrr— ^-f.j r , 

 g= / f It it could be made of DUmant % whofe 

 Retraction is as f to 2 , k would be 



$dr-+ $d!—^r?—3dt-i-zr~t ~ f ' Ani 

 this is the univerfal Rule for the Foci of double 

 Convex Glaffes expofed to diverging Rays. But 

 if the thicknefs of the Lens be rejected, as not 

 fenfible, the Rule will be much fhorter, 



j — — ~ /, or in Glafs 



dr-irdi—frt dr-\'df—2r? 

 stts /, all the Terms wherein t is found being o- 

 mitted, as equal to nothing. In this Cafe, if d 

 be fo. /mall, as that 2r? exceed dr~h d? 9 then 

 will it be — /, or the Focus will be Negative, 

 which fliews that the Beams after both Refracti- 

 ons ft ill proceed diverging. 



To 



