[ so? ] 
-LXVTIT. A Method of working the OhjeSi 
Glajfes of RefraSiing "Telef copes truly fphe^ 
rical. By the late Mr, James Short, 
B.R.SA 
Read Jan. 2^; R' E P A R E tWO platCS OF tOOls ofi 
1770.. brafsj.the one convex,. and the other 
concave, being both portions of a fphere of the fame 
radius as the focal length of the objea glafs you want 
to have, or rather of a radius fomewhat longer than' 
the focal length you want, for a dioptrical reafonj. 
let thefe plates or tools be between two and three 
times the breadth of the objed* glafs defired ; or, in 
long focal lengths, twice the breadth will be fuffi- 
cient: let thefe tools be of a fufhcient thicknefs in 
proportion to their breadth or diameter, and let them 
■ be ground with fine emery exadly true to one an-r 
other, working them alternately,, the one above the 
other, to preferve the fame focal length; or, if it is 
defired longer, you muft work the convex above the 
concave; or, if defired fliortcr, you muft grind the 
concave above the convex. 
After this, you prepare another brafs-plate or tool,, 
of the fame breadth and thicknefs as the two former, 
* This paper, which was delivered, fealcd up, by Mr. Short,, 
at the Society, on the 30th of April, 1752, was, after his death, 
opened by the Council, and ordered to be printed. 
X t t 2.. 
and. 
