GRINDING. 
Bdge of the perfect {pherical figure of the metal and tool, 
yhen there is a perfect correfpondence between the furfaces, 
free from all jerks and inequalities. Having proceeded thus 
far, you may put the laft finifhing to this figure of the metal 
by bold crofs ftrokes, only three or four in the dire@ion of 
each of the eight diameters, turning the metal atthe fame 
time : this mult be done quickly ; for, if the tool{be fuffered 
to grow quite dry, you will never be able, with all your force, 
to ental that and the metal, without.deftroying the po- 
lifher by heat. The metal will now have a beautiful polifh, 
and a true {pherical figure, but will by no means makea.fharp lefeope 
diftinét image in the telefcope.; for the fpeculum will-not be 
found to make parallel rays converge without great aberra- 
tion:: and, indeed, the deviation will be fo at, as to be 
very fenfibly perceived by a great igdidtdalBbel tes the image. 
‘Suppofing then the metal to be now truly {pherical, ftop . 
the hole in the polither, by forcing acork into it underneath, 
about an inch, fo that it do not reach quite tothe furface; and 
having wafhed off any mud that may be.on the furface of the 
tool with a wet foft piece of {ponge, whilft the furface of it 
is a little moift, place the centre of the metal upon the middle 
of the polither ; then havin g, with the wet brufh, lodged as 
much water roun e edge of the metal as the projecting 
edge will hold, fill the whole of the metal and its handle 
with water, to prevent the evaporation .of the moitture, and 
the confequent adhefion between the fpeculum and polifher, 
and let the whole reft in this ftate for two or three hours: 
this will produce an intimate conta between the two, and by 
pes: with any degree-of warmth they may have acquired 
y the nearnefs of the operator, they will become perfeéily 
‘ he: y this time you may pufh out the cork 
from the polifher to difcharge the water, and give the metal 
othe parabolic figure, in the pian manner : move the me- 
gently and flowly at firft, a very little round the centre 
of the pélifher, then increafing by degrees the diameter of 
ithefe ftrokes, and turning the metal frequently round its axis, 
give it a larger circular motion, without any preflure, but its 
own weight; this manner of working may fafely be conti- 
and confequently no frefh putty can be adde 
be fafe to continue | h 
mentioned ; for if the parabolic tendency be din the 
leaft too far, it will be impoflible to recover a true & of 
a eee 
that kind, but by going eee. the whole procefs for the 
lone in the manner betore defcribed, by the crofs- 
Bhp be x At about twenty 
ards a watch-paper, or fome fuch objeét, on which there are 
up. 
Jead muft be taken off from the fpeculum, the {peculu 
placed on the cell of the tube, the in belie dcaae 
part of tk t 
equal in breadth to about an eig! 
he fi ve 
. of the telefcope, 
ment, ror the ' 
ence, 
part of the diameter of 
: — fixed in the mouth 
» ane remain fo during the whole experi- 
of the metal covered by it is fuppofed to 
therefore unemployed, = 3 beta 
by the fine equable feel there is in working, which is totally _ 
‘defcribed annular piece ; and the other, fuch around. piece 
as fhall exactly fit into the tube, and io broad, as that the 
inner edge may jult touch the outward circumference of the 
middle annular piece. It would be convenient to have thefe 
two lait pieces fo fixed to an axis, that they may be put in | 
their places or removed from thence fo eafily as not to dif- 
place or fhake the initrumeat. All thefe pieces there 
fore together will completely {hut up the mouth.of the te. 
° 
e 
nular one, by which means the circumference only of the 
fpeculum will be expofed, and the image now formed will 
he from the rays reflected from the outfide of the metal. If 
there be no occafion to move the fcrew and little metal, and 
the two images formed by thefe two portions of the metal 
be perfectly fharp and equally diftin€, the {peculum is per- 
feét, and of the true parabolic curve ; or at lealt the errors 
of the great and little {peculum, if there be any, are cor- 
retted by each other. 
If, on the contrary, under the laft circumftances, the 
image from the outfide of the -metal fhould net be diitin@, 
and it fhould become neceflary, in order to make it fo, that 
the little {peculum be brought nearer, it is plain that the me- 
tal is not yet brought to the parabolic figure ; but if, on the 
other hand, in order to procure diftin@tnefs, you be obliged 
to move the little fpeculum farther off, then the figure of 
great fpeculum has been carried beyond the parabolic, 
and hath affumed an hyperbolic form. n ter is 
the cafe, the circular figure of the metal muft be recovered 
(after having fixed on the handle with foft pitch) by bold 
crofs itrokes upon the polither, 
manner above defcribed. I 
: erent parts of the 
perfectly coincide, and that by the union of them, 
one 
-diftin@ fituation each time the eye-piece is ferewed 
on: when, by trying and. j or : rou! 
a | 
