(57) 
'Conji derations of Monfieur Aiizout upn Mr. 
HookV Nem Injirument for Grinding of 
Optick-GIaffes. 
la the above-mentioned French Trad: , there are, be» 
fides feveral other particulars, to be reprefented in due place , 
contained (ome Confiderations of Monfieur Au%out upon Mr* 
Hqo\% New Engine for grinding Optith^Glajfes. Where he pre- 
mifes ill General his thdughts touching the working of Great Op» 
tick-Glaffes, and that by the help of aTurn lathe-, alBrming 
firft of all, that not only the E?2^i?2 is to be confidered for give- 
ing the Figure, but the Matter alfo, which ought to be brought 
to greater perfection , than it hath been hitherto. For, he 
finds it not fo eafie ( at leafl^ mhere he is ) to procure Great pie- 
ces of Glafs without Veins, and other faults, nor to get fuch, as 
are thick enough without Bkbbsi which, if they be not, they 
will yield to the prefTure and weight, either when they are fitted 
to the or wrought. 
Secondly, He finds it difficult to work thefe Great Glafles of 
the/^^^thicknefsj which yet is very necefTary 3 becaufe, that 
the leafl: 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^ 
the length of time, which is required to wear and to fmooth 
them, may fpoil the beft Mould, before they be finiflied. Be- 
fides, that the ftrength of Man is fo limited, that he is unable 
to work GlafTes beyond a certain bignefs, fo as to finifli and 
poliih them all over fo well, as fmaU GlafTes s whereas yet, the 
bigger they are, the more compleat they ought to be : And if 
any weight or Engine be ufed to fupply flrength^ there is then 
danger of an unequal preffi-ire^ and of wearing away the En- 
gine ^ In the mean time, the precifenefs and dehcatenefs is 
greater^ 
