of Metals for reflecting felej 'copes. 347 
In the annexed plate are figured the lhape of the 
leaden tool for rough-grinding; the hones; and the ap- 
paratus to be applied to the mouth of the telefcope, to 
afcertain the true figure of the fpeculum. 
POSTSCRIPT. 
It was fome time after I had written the above account 
that I faw Mr. short’s method of polifhing object glaffes 
for refracting telefcopes, which is publiflied in the Tran- 
faCtions. By that paper I find that what I before 
ftrongly fufpeCted is really the cafe, viz. that he knew 
how well pitch was calculated for purpofes of this kind. 
Only it may be remarked, that as glafs is much harder, 
polifhes much flower, and confequently does not wear 
away and alter its figure fo foon as the metal of which 
the fpeculum is made ; and as at the fame time (on ac- 
count of the very fmall apertures allowed to telefcopes 
of this fort) nothing more than a fpherical figure is pro- 
poled ; he is therefore obliged to ufe pitch in a hard, 
friable, and ftubborn ftate : whereas, confidering the de- 
licate fubftance of the metal fpeculum, and the figure 
intended to be given to it, the foft pitch of the common 
fort, by fuffering the putty to bed itfelf in its fubftance, 
Y y 2 produces 
