The Form and Pofition of the Prifm i\T, and the 
Contrivance of the other Parts neceflary, will be 
much the fame as in the Ne wtoni an 
If the Focal Length of the Eye-glafs be ^ of an 
Inch, the Telefcope will magnify about 200 times. * 
This Telefcope may be contrived in the Gregorian 
way, by ufing, inftead of a Lens and Prifm, a fmall 
Speculum fpherically concave on one Side, and con- 
vex on the other; but we think; it not worth while 
to attempt this Conftrudion, as an Inveftigation of 
the Proportion between the two Surfaces necelTary, 
in this fmall Speculum, to unite the Rays proceeding 
from the great one, into one Point, would be in- 
tricate, and the Pradtice alfo very difficult ; by reafon 
that a little Inaccuracy wrlU in this Cafe, occafion 
Errors much more conliderable than a like Imper- 
fcdlion in the refrading Lens. 
We have hitherto fuppofed the Radius of the 
Concavity greater than that of the Convexity ; as being 
moft convenient and ufeful, on feveral Accounts, in 
forming this kind of Telefcopes; however, it may 
be proper to remark, that the fame Method may be 
ufed for correding the Errors of the Speculum, 
when the Radius of its Concavity is lefs than that 
of the Convexity j only the refrading Superficies of 
the Lens, placed between its Vertex and Fcciis, will 
be 
