C 9<M ) 
Optick GlafTes, and the Conftruttion and Ufe of Micro- 
fcojpfs and Telefcopes; and though fome nicely Cntictl 
have endeavoured tofpy faults, and to traduce the Bock, 
yet having long fince examined it with care, I affirm, 
that ifl can judge, it hath but two things that with any- 
Colour may be call'd faults ; the one an over-careful ac- 
knowledgment of every Trifle the Author had received 
from orhtr ; and the ocher, that he labours to make ea- 
fie this curious Suljeft, fo little underftood by mod, in 
a manner perhaps too familiar for the Learned Critick^ 
and which demonftrates that it was writ cum ammo do- 
ttndi, both which require but very little Friendfhip or 
good Nature in the Reader, to pafs for Vertues in an 
Author. 
But to return to our firfl Theorem, which account- 
ing for the rhicknefs of the Lens, we will here again re- 
iimie, ?m. 
mpdrp—ndpt^npjr p / , 
mdrfymd — mp r nd t-\- n r t V 
And let it be required to find the faus where a whole 
Sphere will colled the Beams proceeding from an Obje& 
at ibediftance d; Here / is equal to z r and r= ? . 
And after due Reduftion the Theorem will ftand thus, 
m p d r — z k d r^* z n p rr 
- r -j~ = f> but if d be iflfi- 
z n d 2. #r-~ m p r 1 
• ■ • n i rn p r zn — m 
mte, it is contracted to ~ J ~ — r= r—f t 
%n z m—z » 
wherefore a Sphere of Glafs coHeds the Suns Beams at half 
the Semi-diameter of the Sphere without it, and a Sphere 
of Water at 2 whole Semi-diameter.But if the rath of Rc- 
fra&ion m to n be as 2 to 1, the focus falls 00 the op- 
pofite furfe.ee of the Sphere, but if it be of greater ine- 
quality it falls within, 
Ano- 
