( 57 ) 
Confederations of Monfeur Auzout upon Mr. 
HookV Netp Injirument for Grinding of 
Optick-Glafles. 
Iii the above-mentioned French Trad:, there are, be- 
fides feveral other particulars, to be reprefented in due place a 
contained fome Considerations of Monfieur Auzout upon Mr. 
Hookas New Engine for grinding Optic l^GlaJps. Where he pre- 
mifes in General his thoughts touching the working of Great Op- 
:ick-Glaffes , and that by the help of a Turn lathe', affirming 
irft of all, that not only the Engin is to be confidered for give- 
ng the Figure , but the Matter alfo, which ought to be brought 
o greater perfedion, than it hath been hitherto. For, he 
inds it not fo eafie ( at leaft, where he is ) to procure Great pie- 
;es of Glafs without Veins, and other faults, nor to get fuch, as 
ire thick enough without Blebbs ; which, if they be not, they 
vill yield to the preffure and weight, either when they are fitted 
io the Cement , or wrought. 
Secondly, He finds it difficult to work thefe Great Glafifes of 
:he fame thicknefs, which yet is very neceffary , becaufe , that 
he leaft difference in Figures fo little convex , can put the Center 
out of the Midle, 2 or 3 Inches s and if they be wrought in Moulds , 
he length of time, which is required to wear andtofmooth 
:hem, may fpoil the beft Mould, before they be finiffied. Be- 
sides, that the ftrength of Man is fo limited , that he is unable 
:o work Glaffes beyond a certain bignefs, fo as to finilh and 
roolifh them all over fo well, as /mail Glaffes ; whereas yet, the 
oigger they are, the more compleat they ought to be : And if 
any weight or Engine be ufed to fupply ftrength^ there is then 
danger of an unequal preffure, and of wearing away the En- 
gine > In the mean time, the precifenefs and delicatenefs is 
greater^ 
