3 $2 Mifcellanea Curio fa. 



To bring this to the other Cafe?, as of con- 

 verging Beams, or of Concave daffes, the Rule 

 is ever compofed of the fame Terms, only chang- 

 ing the Signs of -H and — ; for the diftance of 

 the Point of Concourfe of converging Beams, 

 from the Point B, or the firft Surface of the 

 Lens, I call a negative Diftance or — d ; and the 

 Radius of a Concave Lens I call a negative Radius, 

 or — r if it be the firft Surface, and ■ — ?, if it be 

 the fecond Surface. Let then converging Beams 

 fall on a double Convex of Glafs, and the Theo- 

 rem will ftand thus ; % ==f "+/» 



— d r — - d ? — 2 r t 



which (hews that in this Cafe the Focus is always 

 affirmative. 



If the Lens were a Menifcus of Glafs , 

 expofed to diverging Beams , the Rule is 



~~~ ^ ^ r g — fj which is affirmative 



■ — dr d ? H- ir ? 



when x r ? is lefs than dr — d?> otherwile negative: 



But in the Cafe of converging Beams falling on 



? -ii i_ -h 1 dr $ 



the fame Menifcus, twill be 



~h dr — • d ? H-2 rp 

 z=zf, and it will be -+/, whilft^ — dr is left than 

 zr%\ but if it be greater than ar?, it will always be 

 found negative or—/. If theLewj be doubleConcave, 

 the Focus of converging Beams is negative, where 

 it was affirmative in the the Cafe of diverging Beams 



on a double Convex, vi K . + ^l lr ( 



-= f, which is affirmative only when z r ? ex- 

 ceeds' d r ■+ d § : But diverging Beams pafling a 

 double Concave, have always a negative Fo- 



