As it may happen (_saith he_) that the Engin of Mr. _Hook_ may, by using all necessary precautions, succeed in the making of _Eye-Glasses_, or _small_ Optick-Glasses, but not in making _great_ ones; as we see, that an instrument composed of two Rulers, wherewith are traced Portions of Circles, succeeds well enough in _small_, but when there is no more than half a Line, a quarter of a Line, or less convexity, it will be no longer just at all, as he tells us to have made the proof of it in Circles drawn by the means of one of these Instruments, made by one of the best Workmen in his time, who, whilst he lived, esteemed them above price, although they be not just; as others and my self (_saith he_) have by tryal found, when we endeavoured to make _Moulds_ by their means, & as those, who by the like Instrument laboured to trace portions of Circles of 80 or 100 foot, _&c. Diameter_, can attest. But, notwithstanding all this, he hath thought upon two or three things, which he thinks may remedy some inconveniencies of Mr. _Hook_ his _Turn_. The _first_ is, to invert the Glass, and to put it under the _Ring_, that so not only the Glass may be placed more _Horizontally_, and not slide upon the _Cement_, but that the _Sand_ also, and the _Putty_ may stay upon the Glass. The _other_ is, that there must be two _Poppetheads_, into which the _Mandril_ must pass, where the _Ring_ is to be fastned; and the _Mandril_ must be perfectly _Cylindrical_, that so it may advance upon the Glass as it wears away by the means of its weight, or by the means of a spring, pressing it, without wrigling from one place to another, as it would presently happen in the fashion, {62} as the _Turn_ is composed. For, when the Glasses do wear, especially when they are very _convex_, it cannot be otherwise, but the _Mandril_ will play and wrigle, before the _Scrue_ be made firm. But he doubts, whether all can be remedied, which he leavs to the industry of Mr. _Hook_, considering what he saith in the _Preface_ of his _Micrography_, touching a Method, he knows, of finding out as much in _Mechanicks_, as can be found in _Geometry_ by _Algebra_.