( 9*4 ) 
vex, of Glafs, and the Theorem will (land thus 
T~ 1 ffjfi whichfliews that in this cafe 
~dr-~d f— i r p 
the Focus is always affirmative. 
If the Lens were a Menifcus of Glafs, expofed to di- 
verging Beams, the Rule is -— ^^ r ^ f*±kh 
is affirmative when zr p is lets than d r—d p, other- 
wife negative : But in the cafe of converging Beams fal- 
*t*z dr p 
ling on the fame Menifcus, 'twill be ^JJZJ^ZTf-r 
and it will be whilft dp— dr is lefs than z r p, but 
if it be greater tnan xrp, it will always be found ne- 
gative or — /. If the Lens be double Concave, the fo- 
cus of converging Beams is negative, where it was affir- 
mative in the cafe of diverging Beams on a double Con- 
vex viz. - ~~ — ^ = f which is affirmative on- 
- fyd rfyd p—z rp 
ly when z r p exceeds dr&Jp- But diverging Beams 
paffiog a double Concave have always a negative focus, 
i d r p r 
t viz. -~, •— , — — - /• 
The Theorems for Converging Beams are principally 
of ufe to determine the focus refulting from any fort of 
Lens placed in a Telefcope, between ^ focus of the Od- 
fc<a-glafs and the Glafs it felf ; the diftance between thg 
faid focus of the ■ Objed-glafi and the interpofed Lem 
being made=— d* \ . _ " 
I here fuppofe my Reader acquainted with the Rules 
of Analytical Multiplication and Divifion, as that p 
multiplied by ii makes the produft ^by — vaefcm 
- and — by - makes fc dividing * by ^ makes- 
the Quote 4- by - makes and by - makes 
which will be neceflary to be underftood m the pre- 
ceding Examples. ^ 
