[ ] 
Rcfrangibility of the Rays of Light. But, in order 
to render this kind of Telefcopes abfolutely perfed 
in their Conftrudion, the Errors alfo that refult from 
the fpherical Figure, muft be redifiedj and with 
regard to this, we aflert, that it is poffible to affume 
a Point in the Axis, between the Focus of the Spe- 
culum and its Vertex, (as we have taken the Point Hy 
in the following Example, fee Fig. 2 .p. 3 37-) at which, 
if a refrading Superficies, or Lens, be conftituted, 
according to the Method already delivered, it will 
not only corred the Errors occafioned by the un- 
equal Refradion of the Rays of Light, but alfo 
redify fuch as proceed from the fpherical Figure of 
this Speculum, to a much greater Degree of Exad- 
nefs than is requifite for any Phyfical Purpofe (mean- 
ing always the Errors of thofe Rays which refped the 
Axis). Now to find or determine this Point, affords 
a Problem not eafy to be folved; and we recom- 
mend it, as worthy of the Confideration of Geome- 
tricians. 
Seeing therefore it is poffible, and we believe alfo 
pradicable, to remedy the Imperfedions of this kind 
of Speculums, (from whatfoever Caufe they arife) by 
the Method we have here propofed ; it feems to 
follow, that Catadioptrical Telefcopes may be carried, 
by this means, to as great a Degree of Perfedion, 
as they are capable of receiving; provided fpherical 
Figures can be truly communicated, with an exquifite 
Polifh, to Glaflcs of a large Aperture, and a Foil of 
C^ickfilver made alfo to retain that Figure accurate- 
ly, and without any Inequality ; for the Objcd-glafs 
or Speculum being rendered perfed, fo as that all 
forts of Rays, proceeding from one lucid Point in its 
Axis, 
