338 Mr. mudge on the ConJlruBion 
the parabolic, and hath affumed an hyperbolic form. 
When the latter is the cafe, the circular figure of the 
metal muff be recovered (after having fixed on the han- 
dle with foft pitch) by bold crofs ftrokes upon the po- 
liiher, finifhing it again in the manner above defcribed. 
If the fpeculum be not yet brought to the parabolic form, 
it muff cautioufiy have a few more round ftrokes upon 
thepolifher; indeed a very few of them in the manner be- 
fore defcribed make in effect a greater difference in the 
fpeculum than would be at firft imagined. If a metal of 
a true fpherical figure were to be tried in the above men- 
* 
tioned manner in the telefcope (which I have frequently 
done) the difference of the foci of the tw T o fegments of 
the metal would be fo confiderable, as to require two or 
three turns of the fcrew to adjuft them ; fo very great is 
the aberration of a fpherical figure of the fpeculum, and fo 
improper to procure that fharpnefs and precifion fo ne— 
eeffary to a good reflecting telefcope. 
This is by no means the cafe with the object glaffes of 
refractors ; for befides that they are in fa£t never fo dif- 
tinft as weli-finifhed refleftors, the apertures of them 
are fo exceedingly fmall, compared to the latter, and the 
number of degrees employed fo very fmall, that the in- 
convenience of a fpherical figure is not fo much per- 
ceived. Accordingly we obferve in the generality of 
a reflectors 
