of Metals for refledling TeleJ copes, 335 
added. It will not be fafe to continue this motion longer 
than the time above-mentioned ; for if the parabolic ten- 
dency be carried the leaft too far, it will be impoSTible ta 
recover a true figure of that kind but by going through the 
whole procefs for the fpherical onein the manner before 
defcribed, by the crofs Strokes upon the polilher, which 
takes a great deal of time. However, when there is occa- 
sion, it may be done; and I have myfelf feveral times 
recovered the circular figure, when I had inadvertently 
gone too far with the parabolic and ultimately finished 
thejnetal on the polilher without the ufe of the hones. . 
To try the true figure of the metal. 
It will now be proper to try the figure of the Specu- 
lum, and that is always heft done by placing it in the 
telefcope it is intended for. In order to this, I ufe the in- 
strument as a kind of microfoope, placing theobjeff, 
however, at fuch a distance that the rays may be nearly 
parallel. At about twenty yards a watch -paper, or feme 
fuch object, on which there, are fome very fine hair 
Strokes of a graver, is fixed up. The lead muft be then 
taken.off from the back of the- lpeculuin y which is belt 
done by placing the edge of a knife at the junction of 
the lead and metal, when, by Striking the back of it with a 
Slight 
