C 159 3 
how this Error may be redified by one or more 
Glaffcs, placed in the Axis, at a Diflance farther 
from the Vertex than the Focal Point Pi but the 
former Speculum is fo much preferable to this, for 
the conftruding of Telefcopes, that we think it not 
worth while to profecute this Matter farther. To 
conclude this ElTay ; 
Whoever fhall think fit to put the Method here 
propofed in Execution, we dare venture (from a 
Trial that has been made) to affure him of Succefs j 
provided the fame Diligence, Care, and Accuracy, be 
applied, in choofing, figuring, polifhing and foiling, 
the Glafs, that has of late been employed for the 
forming Speculums of Metal 5 and let none be dif- 
couraged, though the firft and fecond Attempt fliould 
fails for that muft be expeded, if the ordinary way 
of grinding and polilhing be ufed : Greater Exadnefs 
is here required, than is ufually thought fuflicient 
for the Objed-glalTes of refrading Telefcopes : Let 
it be alfo confidered how many Effays, for a long 
Term of Years, were made by Mr. Gregory^ Sir 
Jfaac Newtorii and others, to reduce their Conftru- 
dions of the refleding Telefcope into Pradice, 
without anfwering, in any tolerable Degree, what 
their Theories proraifed : The Workmen they em- 
ployed were chiefly Optical inftrument-makers, and 
had it been left to fuch Perfons only to perform by 
themfelves, we have reafon to think, that it would 
have been pronounced impradicablc to this Day, to 
make a refleding Telefcope that fhould equal or 
excel refrading ones of Ten times its Lengths 
though we now fee, that moft of thefe Artificers arc 
X X capable 
