GRINDING. 
giving them the laft and fineft polifh, we muft refer to Smith’s 
hap. i. paffim 
Optics, book iii. chap. i. 
The late ingenious Mr. James Short, in a paper left with: 
the Royal Society, to be opened and printed after his death, 
gives the following direétions for working object-glafles of 
refra¢ting telefeopes truly fpherical. 
Prepare two s or tools, of brafs, the one convex, 
and the other concave, being both portions of a {phere of 
the fame radius as the focal length of the sje glafs you 
want to have, or rather of a radius fomewhat longer than 
the focal length you want, for a dioptrical reafon ; het thefe 
plates or tools be between two and three times the breadth 
of the objeét-glafs defired ; or, in long focal lengths, twice 
_ the breadth will be fufficient: let thefe teols be of a fuffici- 
ent thicknefs. in proportion to their breadth or diameter, 
and let them be ground with fine emery exaétly true to one 
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of the goodnefs of the gla 8s, you are then to prepare a 
lafs to. 
for fear of bending your glafs by the handle ; Mr. 
Short advifes to take a flat piece of brafs, or rather of the 
icavity of | » whi glafs is to be ground ; 
not be thicker than two-thirds o 
the thicknefs of the glafs, of a circular form, lefs in breadth 
brittle : or you may fhorten this operation, by melting equa 
uantities of pitch and rofin, and then there is no occafion to 
let it boil fo long. Your pitch being thus prepared, you 
again melt it, and take it off the fire, and let it ftand till the 
pitch becomes pretty cold, or of a thickifh confiftence ; and 
aving warmed the polithing tool a little, to make the pitch 
ftick to it, you pour out of the ladle upon the polifhing tool 
as much pitch as you judge will cover the whole tool, when 
{pread out, to about the thicknefs of one-eighth of an inch; 
you then invert this tool with the pitch upon it, and pre 
it upon the convex tool, which muft be quite dry, clean, and 
cold, in order to give it the figure of the convex tool; in 
cafe it has not fpread out fo as to cover the whole furface of 
the polifhing tool, you may warm the pitch by holding it 
before the fire, and prefling it upon the convex tool, as 
before, till it has entirely covered the furface of the polifh- 
ing tool ; you then plunge it into cold water, till the brafs 
is quite cold. 
kandle but peek you"then lay down the glafs and handle you then lay the glafs down to cool, and when quite cold, 
upon fomet ag oe taking care that t ¥ le is in you drop fome fpirits of wine upon it; and this, with a 
thiddle of the glafs till it is entirely cold. It is very material cloth, wil wipe off the reft of the pitch. . 
rior nielted ; for any other 
~ You then grind your glafs in the concave tool with emery, 
and give it the proper figure and fmoothing for the laft Sole, 
in the common manner. 
_ In order to give 
difficult part of the w 
for. Lt alt oe ye 
i$ done in this manner: take | 
ladle, and let it boi 
by this boiling, the pitch, when cold; will 
polifhing concave tool. which 
me pitch, sod ale vin in iron 
] for a quarter of an hour or thereabouts ; 
en cold; will become hard and 
_ other fide from the fpot as t 
_ your glafs; you then, by heat, re 
You then examine the centre of the fu 
rfaces of your 
afs 
its centre over the fpot of 
before, till the circular remai 
ground is as m 
centre of 
