E ??8 3 
be convejTj and hot concave, as in the former Cafe. 
And there is another thing worthy of Remark, that 
the Focus, or Point vs^here the moft refrangible 
Rays are colleded, will fall farther from the Vertex 
of this Speculum, than the Focus of the leaft refran- 
gible ( i a Circumftance which never happens by 
Refradion alone, in Glaffes of any Figure whatfo;^ 
ever, or howfoever they be difpofed. 
Hg. 3 , 
R KG 
E 
DUG 
B 
■V. 
And 
Now all things being put as before, and making 
(Fig. 3 .) I fay the convex Superficies GHI 
of a Lens placed at i7, that fhall corred the Errors 
arifing from the different Refrangibility of Rays, in 
this kind of Speculum, will be part of a Sphere, 
{fj i^m)x{dd-\-dB) _ 
HRy the Diftance of the Point i?, where the Rays 
of all forts will unite, after this Refradion, from H 
the given Point in the Axis, will be = , j J 
which Point R being taken as a Centre, defcribe 
thereon the Arch ALM, and the Figure A 
will rcprcfcnt the Sedion of a Menifcus-glafs, or 
Lens, which, placed at the Point Hy aflumed between 
the Vertex and Focus of the Speculum, wilLcolled 
all forts of Rays proceeding therefrom into one and 
the fame Point, or Focus, R, We might alfo (hew, 
how 
whofe Radius is 
