133 
Specula. 
most equally well, without contracting tarnish, is a far- 
ther encouragement to prosecute its improvement to per- 
fection. 
Among others, I had formerly, from admiration at its 
contrivance, bestowed some attention on the mechanism 
of this instrument : and, as it would have spared me 
some expence of time and trials, if any other person had 
previously suggested to me the hints, which I am to re- 
late, I imagine they will be of use to others, in direct- 
ing or assisting the course of their labour, in the same 
pursuit. I had also taken some pains, to understand the 
merits of the different constructions of this telescope : 
but, as this enquiry ended in a conviction, that the New- 
tonian form of it is the most perfect that can be hoped 
for ; (it being the nature of its great author to persevere 
in his researches, till he had arrived at a complete solution 
of his doubts, and comprehension of the subject;) so I 
have only to report what resulted from my experience in 
the mechanical fabrication of it, as to the method of cast- 
ing the mirrors, and communicating to them the proper 
figure. 
Before I had heard of the improvements of the Rev. 
Mr. Edwards, in the composition of the specula for tele- 
scopes, I had made many experiments myself with that 
view ; which lead me to give full credit to his report of the 
superior excellence of that composition which he recom- 
mends : because I had found, that the qualities of hard- 
ness, whiteness, and indisposition to contract tarnish, ne- 
cessary to a speculum, could not, by any admixture that 
1 could hit upon, be produced, unless the metal were so 
highly saturated with tin, as to be excessively brittle ; and 
because I found that this brittleness,. however inconve- 
nient in some respects, was necessary to render it suscep- 
tible of the highest polish : for no metal yet known, ex- 
cept steel, (which, from its disposition to rust, is unfit for 
