350 Mifcellanea Curio/a* 



As d to r, fb the Angle at G to the Angle 

 at D, and d-+r will be as the Angle of Inci- 

 dence GAD; and again as m to «, fo d-+r to 



dn + rn 9 w hich will be as the Angle GAH= 



iZAty This being taken from ACD which 



is as d* will leave m ~~ nd ~ nr analogous to the 



Angle A^D j and the Sides being in this Cafe 

 proportional to the Angles they fubtend, it will 

 follow, that as the Angle A 9 D is to the An- 

 gle AD?, fo is the Side AD or BD to A 9 



mdr 



or B 9 : That is, B 9 will be = , 



u r m — n d — nr 



which (hews in what Point the Beams proceed- 

 ing from D, would be collected by means of the 

 firft Refraction ; but if n r cannot be fubftracted- 



from m n d, it follows that the Beams after 



Refra&ion do ftill pafs on diverging, and the 

 Point f is on the fame fide of the Urn beyond D. 

 But if n r be equal to m— nd, then they proceed 

 parallel to the Axis, and the Point 9 is infinite- 

 ly diftant. 



The Point 9 being found as before, and 

 B9— Bj8 being given, which we will call A it 

 follows by a Procefs like the former, that 

 $F, or the focal Diftance fought, is equal to 



£if — /, And in the room of f 



m — ^ m % 



mdr 



fubftituting Bg— ■ , — *,put 



tins p for > after due Rcdudion this foil 



lowing 



