( 9«3 ) 
is not fb , conftdering the great number of data 
that eater the QueQicn, and that one half of the terms 
arife from our taking in the thicktiefs of thei^i, which 
in moft cafes can produce no great effed, however it was 
neceffiryto confider it, to make our Rub perfect If 
therefore the Lens cdnfift of G/afs, whofe Refraction is 
, . 6 dr 2 2 f/+4r p f 
as no i twill be — r— ^ 7-- — -rr, — /. 
If of * Water, whofe Refir act ion is as 4 to 3 the Theorem 
will ftand thus ^7— T / r 7*3 ; =/. 
If it could be made o\D'tamant, whofe RefraSion is jis 5 to 
x \dr p—idp t*b\ r p t 
a, it would *fj^~ 5 dJ-^T^dT^Tr^ 
And this is the univerfal Rule for the foci of 
double Convex GlafTes expofed to Diverging Rays. 
But if the thicknefs of the Lens be rejected as not 
fenfible, the Rule will be much ftorter, viz. 
^^.t£yiA-~~- —f orinGIafs — ~£ 
drfydp—prp 1 drfydp — irp J 
all the terms wherein t is found being omitted, as equal 
to nothing, In this case, if d be fo fmall, as that z r p 
exceed drfyd p, then will it be — /, or the focus will 
be Negative, which (hews that the Beams affef both Re- 
fractions {till proceed Diverging. 
*To bring this to the other Cafes, as of Converging 
Beams, or of Concave Glaflbs, the RuJe is ever com- 
pcfedofthe fame terms, only changing the figns of t$ 
and — ; for the diftance of the point of Concourfe of 
converging Beam?, from the point B, or the firft furface 
or ther 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 furface, and — p, if it be the fecond fur- 
face, Let then converging Beams fall on a double Con- 
vex 
