c m I 
Another Example fiiall be when a Hemifphere is ex* 
pofed to parallel Rays, that is d and ^ bsing infinite*' 
and / = r, and after due Redudton the Theorem re- 
futes — ~ r = f. That is , in Giafs it is at 
I r, in Water at | r y but if the Hemifphere were Dia- 
mant, it would collect the Beams at A of the Radius be* 
yond the Center. 
Laflly, As to the effed of turning the two fides of a 
Lens towards an Objed; it is evident, that if the thick- 
■nefs of the Lens be very froall, foas that you negleft it, 
or account t = o, then in all cafes the focus of the fame 
Lens, to whatfoever Beams, will be the fame, without 
any difference upon the turning the Lens : But if you 
are fo Curious as to confider the thicknefs, (which is fel- 
dom worth accounting for ) in the cafe of parallel Rays 
falling on a Piano-Convex of Glafs, if the plain fide be 
towards the Objeft, t does occafion no difference, but 
the focal diflance /= z r. But when the Convex fide 
k towards the Objeft, it iscontraded to z r~ f fothat 
the focus is nearer by I t. If the Lens be double Con- 
vex the difference is iefs ; if a Menijcus greater. If the 
Convexity on both fides be equal, the focal length is a- 
bout 1 1 fliorter than when t — o. In a Menifcus the 
Concave fide towards the Objed encreafes the focal 
length, but the Convex towards the Objed diminishes, 
it. A General Rule for the difference arifing on turn- 
ing the Lew, where the Focus is Affirmative, is this 
zr t — z%t £ m ^j® Convexes of differing; 
Spheres. But for Menifci the fame difference becomes 
z r t 41 z g t j. w j 1 - c j 1 ... j neec j gj ve no ot j 1€r ^mon- 
3 r — 3?* F & 
ftration, but that by a due Redu&ion it will fo follow 
from what ispremiled, as will the Theorems for all forts 
of Problems relating to the/kiof Optick Giaffes. 
V, An 
