C 177 ] 
middle t>f them. I fent for two to London, both of 
which were faulty ; I therefore return’d them, and 
had two fent me again, as eccentric well nigh as the 
former ones. 
Harris’s Lexicon Techn. Vol. I. (fee Optics ) gives 
a rule for centering optic- glades ; but I think the foL 
lowing may be more fure and handy for a glafs- 
grinder’s ufe, and foon try whether a convex lens is- 
well center’d. 
Fig. 8. • ; 
Reprefents a round plate of brafs, conveniently 
thick, and well harden’d by hammering (were it not 
for the ruft, harden’d fteel would be better), hav- 
ing many notches round it, one a little wider than 
that which is next to it, and number’d 1, 2, 3, &c. 
in their proper order, each of them wider at the bot- 
tom than at the entrance. I fitted luch a notch to 
the thickeft fide of one of the glades I had from 
London, fo as the edge enter’d it but a little way, 
not half the depth thereof ; but, on trying the op- 
podte lide, it went in, the whole depth thereof, and 
would have gone deeper, if the notch had been fo 
cut : I then ground the lens narrower on that fide 
which was thinned:, until I found it was at that 
place as thick as where I fird try’d it in the notch. 
After this manner I reduc’d the glafs to an equal 
thicknefs on its four quarters, and then ground off 
from other places what was needful to bring it cir- 
cular. I alio took care, when I tried it in the notch, 
that the lens diould not be warmer on the one 
fide than on the other by grinding, but tarried till I 
thought it thoroughly cold y and was alfo careful not 
to thrufl it in harder on the one dde than on the 
Z oppodte 
