320 Mr. mudge on the ConJlruBlon 
perfect figure of the metal by the fharpnefs and diftindt- 
nefs with which the image appears. From hence it is 
very evident, that as the object and image are both diftant 
from the metal by exactly its radius, nothing but a true 
fpherical figure of the fpeculum can produce a fharp 
diftindt image.; and that the image could not be diftinct 
if the figure of the fpeculum were parabolic. Confe- 
quently, if the fame fpeculum ufed in a telefcope were 
to receive parallel rays, there would neceffarily be a con- 
fiderable aberration produced, and a confequent imper- 
fiedtion in the image. Accordingly, there never was a 
good telefcope made in this manner; for if the number 
of degrees, or the portion of the fphere of which the 
great metal is a part, were as confiderable as it ought to 
be, or as great as Mr. short allowed in his metal, the 
inftrument would bear but a very low charge, unlefs a 
great part of the circumference of the metal were cut off 
by an aperture, and the ill effedts of the aberration by 
that means in fome meafure prevented. 
If ever a finifhed metal turned out without this defied!, 
and has been found perfedtly lharp and diftinct, it muft 
have been owing to an accidental parabolic tendency, no 
ways the natural refult of the procefs, and therefore quite 
unexpedted, and moft probably unknown, to the work- 
man. 
Without 
