of Metals for reflecting felef copes . 527 
and confequently the whole bufinefs did not take up 
above ten minutes; under thofe circumftances, I fay, I 
always ufed to pleafe myfelf with the expe (Station of a 
correct figure, at lead as much fo as the metal had re- 
ceived from the hones, where the furface was but juft 
and equally taken off by the putty ; but in this I con- 
ftantly found myfelf deceived, and the metal turned out 
good for nothing. In fhort, at this time, though I fpe- 
culatively knew that a parabolic figure was neceffary to 
a perfect image, I yet confidered it as of little practical 
confequence. 
From the foregoing experiments, and a number of 
fueceeding trials, I at length difcovered a certain way of ' 
giving a correct parabolic figure, and an exquifite polifli 
at the fame time. This, which I have ftrong reafons to 
believe was Mr. short’s method, I will now defcribe in 
as few words as I can.. 
How to potijh the Speculum . 
It is firft neceffary to obferve, that, in order to avoid 
the detrimental intrufions of any particles of emery, it 
would not be right to polifh in the fame room. where the 
metal and tools were ground, nor in the fame deaths 
which were worn in the former proceis ; at leaft it. would 
