C3h3 
Powers of Magnifying, muft be as the Biquadratc 
Roots of the Cubes of their Lengths (See his 
Opics, 2 d Edition, p. 97-)- 
It is likewife a confiderable Advantage in this Con- 
Orudion, that the Refledion from the concave Side 
of the Speculum will do no fenfible Prejudice ; be- 
caufe the Image of any Objed made thereby, is re- 
moved to fo vaft a Diftance from the principal Image, 
formed by the convex Surface, as to create no man- 
mer of Confufion or Difturbance in the Vifion } 
which nccch'arily happens, in fome Degree, from 
the Vicinity of thofe Images, when the Glafs is 
ground concave on one Side, and as much convex 
on the other ; according to the Method propounded 
by Sir Ifaac Newtoriy in his moft excellent Book of 
Optics. 
It may be imagined, perhaps, at firft View, that 
(if our Reafoning is juft) the Errors of refrading 
Tdefcopes, occafioned by the different Refrangibility 
of Light, may be correded by a like Artifice : But 
the Aberration of the Rays from the principal 
Focus is there fo great, and bears fo confiderable a 
Proportion to the Focal Length of the Telefcope, 
that the Error cannot be redified by the Interpofition 
of any Lens, until the Rays are, by a contrary Re- 
fradion, colleded again at an infinite Diftance, which 
renders this Expedient quite ufelefs 5 however, there 
is no need to defpair of accomplifhing even this, by 
other Methods : And, by the way, we may obferve, 
if it were worth while to feek a Remedy for the 
Errors occafioned by the fpherical Figure of the 
Objed-glafs only, in Dioptrical Tdefcopes j that 
might be obtained by the proper Application of a 
fuitable 
