of Metals for reflecting I’eleJ. copes. 343' 
from the former caufe, that it is never attended with any 
practical confequence. 
Magnifying very minute objects, and particularly read- 
ing at a diftance, have been generally confidered as the 
fureft tefts of the. goodnefs of a telefcope ; and indeed 
when the page is placed at a great diftance, fo that the let- 
ters fubtend but a very fmall angle at the eye, if then they 
appear with great precifion and fharpnefs, it is mod: pro- 
bable that the inftrument is a good one. But we are, ne- 
verthelefs, fometimes apt to be deceived by this method ; 
nor is it always poffible to determine upon the different 
merits of two indruments of equal power, by this mode 
of examination ; for when the letters are removed to the 
utmoft extent of the powers of the two indruments, the 
eye is apt to be prejudiced by the imagination. If tw r o or 
three w r ords can be here and there made out, all the reft 
are gueffed at by the fenfe; infomuch that an obferver, 
zealous for the honour of his inftrument, is very apt to 
deceive himfelf in fpite of his intentions. The furer teft is 
by figures, where you can procure no aid from this fort 
of deception. In order to examine my refledting tele- 
fcopes, I made upon a piece of copper and on a black 
ground, fix lines .confiding of about twelve pieces of gold 
figures, and each line of figures differing in magnitude, 
from the fmalleft that could be diftindtly made to thofe 
