Mifiellanea Curiofa. 3 5 5 



The Theorems for converging Beams, ar e 

 principally of ufe to determine the Focus refulting 

 from any fort of Lens placed in a Telefcope, be- 

 tween the Focus of the Object-Glafs and the Glafs 

 it felf ; the diftance between the (aid Focus of 

 the Object-Glafs, and the interpofed Ltns being 

 made == — d. 



I here fiippofe my Reader acquainted with the 

 Rules of Analytical Multiplication and Di- 

 fion, as that H- multiplied by makes the 

 Product -+• by — makes and — by 



makes ■+ ; fo dividing -+• by -f* makes the 



Quote ~h, -H by +r~ makes and — by — makes 

 4*75 which will be neceflary to be underftood in 

 the preceding Examples. 



In cafe the Beams are parallel, as coming 

 from an infinite diftance, (which is fuppofed 

 in the Cafe of Telefcopes) then will d be 

 fuppofed Infinite , and in the Theorem 



g r the Term pre vanifhes, as 



d r d ?—prf ^ r 



being finite, which is no part of the other infi- 

 nite Terms j and dividing the Remainder by 

 the infinite Part d 9 the Theorem will Hand 



thus^-^ ==/, or in Glafs, ==£ 

 r *+ £ r ■+ j 



In cafe the Lens were PUno-Convex expofed 



to diverging Beams, inftead of— — 



d r . -+■ d f •— p r $ 

 r being infinite, it will be = or 



if the Lens be Glafs. 



If the jLro* be Double-Convex, and r be 

 equal to & as being formed of Segments of 

 A a 



