C 355 }^ 
fuitable Lens, between the Focus and the Vertex of 
the Objedt glafs; which is much more eafy and 
pradicable, than the grinding of Glaifes to Hyper- 
bolical or Elliptical Figures. 
For a further llluftration of what is gone before, 
it may be proper to exhibit the feveral Parts and Pro- 
portions of a Telefcopc in Numbers computed ac- 
cording to the Theorems already delivered; and in 
Pradice we judge it will be moft convenient, that 
the Radii of the Spheres to which the concave and 
convex Sides of the Speculum are ground^ be nearly 
in the Ratio of 6 to 5 ; as in the following Example; 
where (fee Fig. 2. p. 337.) 
ABCDRFy reprefents the great Speculum of Glafs, 
ground concave on one Side, and convex on the 
other; quickflvefd over the convex Side, and of 
an equal Thicknefs all round its Circumference. 
The Radius of Concavity ^a—^l Inches. 
The Radius of Convexity =^>=40 Inches. 
Then putting' the Sine of Incidences too ; 
the Sign of Refradion of the lead refrangible Rays, 
out of Glafs into Air, r=: 154; and ^4, the Sine of 
Refradion of the moft refrangible Rays, = 1 5 6 ^ as 
Sir Ifaac Newton found them by Experiments ; we 
fhall have, 
the Focal Length of the Speculum with regard 
to the mod refrangible Rays— 1 8.2926 which 
will be fomewhat increafed by the Thicknefs of 
the Glafs, when that is condderable. 
RFlj the greateft Aberration of the Rays, occafi- 
oned by their different Degrees of Refrangibility, 
= •05 594-}-, which Quantity, in Pradice, fhould 
be 
