150 |8 0|7 0|5 11| -------------|-------------|-------------|-------------| 200 |9 6|8 0|6 9| 250 |10 6|9 2|7 8.| 300 |11 6|10 0|8 5| 350 |12 *6.|10 9|9 0| 400 |13 4|11 6|9 8| -------------+-------------+-------------+-------------+ {57} * * * * * _Considerations of Monsieur _Auzout_ upon Mr. _Hook_'s New Instrument for Grinding of _Optick-Glasses_._ In the above-mentioned _French_ Tract, there are, besides several other particulars, to be represented in due place, contained some _Considerations_ of Monsieur _Auzout_ upon Mr. _Hook_'s New _Engine_ for grinding _Optick-Glasses_. Where he premises in _General_ his thoughts touching the working of _Great_ Optick-Glasses, and that by the help of a _Turn lathe_; affirming first of all, that not only the _Engin_ is to be considered for giveing the _Figure_, but the _Matter_ also, which ought to be brought to greater perfection, than it hath been hitherto. For, he finds it not so easie (as least, _where he is_) to procure _Great_ pieces of Glass without _Veins_, and other faults, nor to get such, as are thick enough without _Blebbs_; which, if they be not, they will yield to the pressure and weight, either when they are fitted to the _Cement_, or wrought. Secondly, He finds it difficult to work these _Great_ Glasses of the _same_ thickness, which yet is very necessary, because, that the least difference in _Figures_ so little _convex_, can put the _Center_ out of the _Midle_, 2 or 3 _Inches_; and if they be wrought in _Moulds_, the length of time, which is required to wear and to smooth them, may spoil the best _Mould_, before they be finished. Besides, that the strength of Man is so limited, that he is unable to work Glasses beyond a certain bigness, so as to finish and polish them all over so well, as _small_ Glasses; whereas yet, the bigger they are, the more compleat they ought to be: And if any weight or Engine be used to supply strength, there is then danger of an unequal pressure, and of wearing away the Engine; In the mean time, the preciseness and delicateness is {58} greater than can easily be imagined. Wherefore he could never, having some experience of this preciseness, conceive, that a _Turn-lathe_, wherein must be two different, and in some manner contrary motions, can move with that exactness and steddiness, that is required, especially, for any considerable length of time.