C ] 
whofe Radius is 
d — X f ' tyi') 
mi — (/>t — m)d ’ 
; and HR^ the 
Diftance of the given Point //, from R, the Point to 
which all the Rays will tend, after Refradion at the 
faid concave Surface, (whofe Radius being found, as 
upon the Point R thus obtained, as a Centre, with 
an Interval a little lefs than HRy deferibe the Cir- 
cumference KLMy and the Figure GHIMLK will 
denote the Sedioii of a double concave Lens, which, 
placed at the given Point in the Axis //, (taken never- 
thelefs within the Limits above-mentioned) will 
colle<5l all Sorts of Rays proceeding from the Spe- 
culum, into one and the fame Focus, or Point of the 
Axis, i?, as was required j for the Surface GHI, 
which firfl; receives thofe Rays, will refradl the moft 
refrangible Sort converging to the Point and alfo 
the leaft refrangible converging towards ^ fo that 
both Sorts, after fuch Refraction, will concur in the 
Point R s but the Rays tending to R, tis manifelt, 
will fuffer no RefraCtion at their Emergence from the 
Superficies KLM, becaufe R is the Centre thereof, 
by ConftruCtion ; which Point, R, where a perfect 
Image of an ObjeCt infinitely diftant will be formed, 
we call the Focus of the Telcfcope, to diltinguifh it 
from the Point, ‘P, which we have before called the 
Focus of the Speculum. 
In this manner a Lens, (or inflead thereof a trian* 
gular Prifm with two of its Sides ground concave, 
and the third plain, if that be found as practicable) 
may be formed and fituated, fo as to correCt the 
Errors of the Speculum arifing from the different 
above, we call v) will be 
U u 
Rc^ 
